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Engine Main bearing bolt yielding help
9

Engine Main bearing bolt yielding help

Engine Main bearing bolt yielding help

(OP)
Hi

I am new to this forum, but used to be in ROM (Read only mode). I am new to the fastening field but have quite a bit knowledge. My problem is we are yielding some bearing bolts right after installation on the cylinder block and crank case joint. I will provide you guys the info on bolt and joint parts and environment.

Bolt – carbon and low alloy steel flanged head 3/8-16 grade 8 bolt.
Joint – both cyclinder head and crankcase are made of Aluminum – copper alloy casting (soft joint compare to the bolt)

We also use a bead of gel seal in between the mating parts (cylinder head and crank case), when in contact, the gel seal uniformaly gets distributed on the mating surfaces. (making the joint even more soft?)

We make two engines V4 and V6. for v4 we have 6 bearing bolts and for v6 we have 8 bearing bolts. The tool used is two spindle dc electric torque angle monitor. For v6 We do bolts 1-2 then 3-4 then 5-6 then 7-8 and again do 1-2 (re torque due to elastic interaction) (I will try to upload a picture for better understanding)

Torque used is 31-15 lbs-ft on each bolt.Min proof load of the bolt is 9300 lbs.

Do you guys know why we are yielding bolts on a soft joint (if it is?)

RE: Engine Main bearing bolt yielding help

3
Hi preload

When you say after installation do you mean during engine running? is there any temperature involved at failure point? how are the bolts failing? information regarding yield stress of castings? and finally when you make the bolted connection is there any lubricant involved on the threads?
I would of thought that the softer materials may have yielded and taken a set before the bolt fails, perphaps when we see a picture and more info we can help further.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
What I mean installation is during assembly (engine not running). So no temperature involved at failure point, except for the temperature produced while torquing. Right now I don’t have any info on yield stress of casting, but will try to get that.

Regarding lubrication, yes we apply outboard lube oil to the bolt and also some gel seal which is applied to the mating surfaces will be in contact with the bolt (please see the pic, the pic despcription: casting shown is cylinder head and u can see the blue gel seal layer on the surface which is also in contact with the bolt and hole. You can see the fasteners in the pic and the crank case (not in pic) will be installed on that and assembled. The yielding occurs right in the thread area u can see.)

http://img410.imageshack.us/img410/6853/dscf0016ef7.jpg

Fyi – the pic is the brand new cyclinder head with bolts ready for assembly not the failed one.

RE: Engine Main bearing bolt yielding help

(OP)
\\\\ I would of thought that the softer materials may have yielded and taken a set before the bolt fails,\\\\\

But with out any external loads acting on the joint , why would the bolt yield after the joint yielded?

RE: Engine Main bearing bolt yielding help

2
Low friction (from intentional or unintentional lubricants on the parts) can cause high preload from the tightening torque, leading to bolt plastic deformation.

High friction can cause torsional yielding of the bolt.

Are you sure the applied torque is correct?  Measured by an accurate means?

Your characterization of the joint as "soft" is not appropriate.  "Soft" is a misnomer - a soft joint is elastically compliant, which is not related to indentation hardness, but rather to elastic modulus and joint clamped length and joint cross-sectional area.  For your engine, the bearing joints' cross sectional area is large enough that the joint compliance is much less than the bolt.

The missing information here is friction coefficient (we can assume ~ 0.1), as well as clearance hole size.  Also, is that supposed to be 15 lbf-ft or 51 lbs-ft for the torque?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

(OP)
We are using torque control and angle monitoring with dc electric which has +/- 5% variation. Torque is 31-35 lbs-ft (I am sorry about the typo 15 lbs-ft).

when u say friction coeff , do I need to get the thread friction and bearing friction from tables? The lube oil we use decreases the friction coffecient. The gel seal which comes in contact with bolt (u see in pic), does that make any effect as it is not applied to the threads of the bolt?

RE: Engine Main bearing bolt yielding help

(OP)
while doin angle monitoring, we see some fasteners achieve thier clamp in less than 20 deg of turn , but some take 260 deg of turn.we dont know if those 260 deg turnd fasteners are the one which are yielding but we think those are the ones.

we dont know why they get that much high rotations on some?

RE: Engine Main bearing bolt yielding help

The fasteners in your picture appear to be studs with no heads.  Do you use a nut to tighten these?

If you are using liquid engine oil as your lubricant, you shouldn't need to worry about the gel as it likely won't have a significant effect.

If you have a graph of torque as the ordinate and angle as the abscissa, are you saying that some of the fasteners have a much smaller slope?  Are you accounting for the variation in angle at the start (the free running portion of the assembly)?  You can use a non-zero torque (something like 5 lbf-ft) as the snug torque, and then count angle from there.  Do you still see a difference?

Based on your information, I would look strongly at thread dimensions of the bolt and tapped hole, and bolt material strength.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

(OP)
cory

we are doin torque only but monitoring the angle.

we re-hit/ retorque two fasteners (please see new picture attached).

http://img64.imageshack.us/img64/8936/seqpt2.png

first we torque 1-2 then those two spindle move and tighten 3-4 and then 5-6 and then 7-8 and at last spindles re torque 1-2 again (just to make sure we dint lose any torque on elastic interaction)

sequence 5 is re-hit of 1.

in angle monitoring we see some fasteners get to 260 deg of turn to achieve 32 ft-lb torque and some only less than 20 deg to get the same torque. I dont know why?

why r we acheiving very high deg of rotation some factors in the same joint?

RE: Engine Main bearing bolt yielding help

(OP)
If we are yielding the bolt , then why there is no damage to parts? are the parts stronger than the bolt?

RE: Engine Main bearing bolt yielding help

(OP)
One thing I noticed from my data is if we have high degree of rotation then the torque achieved is low always.

RE: Engine Main bearing bolt yielding help

As Cory said, the objects in the photo are studs, not bolts.  We get confused enough; don't make it worse by using inaccurate terminology. You are probably torquing flanged nuts over them to retain the crankcase halves.

And that may be what's confusing your nutrunner.

By which I mean, and this is a wild-ass guess, that it's twisting some of the studs farther into the block while it thinks it's actually tensioning the studs.  

I think I remember that studs are actually stronger if they're not fully bottomed in a blind hole ... but that leaves them free to rotate, which could confuse a torque angle monitor.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

(OP)
Mike,

I am sorry for the confusion. The pic I uploaded is a different engine in which we use studs and nut.

But the problem is with the other engine in which we use just the flanged bolt torqued down into crank case and into the cylinder head tapped hole. No nut used here

RE: Engine Main bearing bolt yielding help

I agree with Mike regarding studs, confusion, flange nuts, wild ass guessing, studs not fully bottomed, free rotation and confusing a power tool monitor.

Any time I see high angle with low torque, I think deformation of parts (bolt threads, tapped hole threads, embedment of fasteners into parts).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

(OP)
\\\Any time I see high angle with low torque, I think deformation of parts (bolt threads, tapped hole threads, embedment of fasteners into parts). \\
Agreed. So what do u think would be the reason for this high angles and yielding?

RE: Engine Main bearing bolt yielding help

Non-conforming bolt thread geometry

Non-conforming tapped hole thread geometry

Non-conforming bolt material strength

Lower than expected tapped hole material strength

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Are you sure you've got enough engagement so that you're nut just stripping the threads out of the block?

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

Hi preload

How do you know what tensile force you have in the bolts
how are you calculating that ie what formula are you using?
What is the engagement length of the bolt?
How have you determined that the bolts have yielded are you measuring bolt lengths ?

Regards
desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertforx,
Visually we can see the bolt yielded.
After the installation we are measuring the length of the bolt using ultrasonics. Then calculating the clampload using T=KDF.
Is engagement length same as grip length?
In our tool, we set 260 deg as max turn and when ever the tool shows 260 then the tool shows a red light saying something wrong. Then we take out the bolt see visually for any deformation and also measure the length using ultrasonics. We have a threshold/snug torque too.
Today I brought digicam, will take the pics of exact assembly I am talking about and upload them in an hour

RE: Engine Main bearing bolt yielding help

(OP)
I have a question, when the bolt just starts yielding does the clamp load drops immediately?

RE: Engine Main bearing bolt yielding help

Hi preload

yes to your last question and yes grip length=engagement length

RE: Engine Main bearing bolt yielding help

(OP)
Godddd, I realised just now that I dint bring my data tranfer cord for my digicam. I cant upload the pics now. I will do them as soon as I go home today.

I will get you all other details on monday as we are going to do some test rundowns.

I can paste the load and torque for previous rundowns for one engine

T(ft-lbs)   F(lbs)
32.50        12180
32.10        12880
32.10        7550
32.20        8860
32.10        11970
32.50        7550
32.30        12180
32.10        8860

above is one v6 engine (8 bolts) worth sample data. tension numbers are calculated by measuring elongation and torque from the tool.u can see the scatter of preload and torque pretty contant. I dont know why

RE: Engine Main bearing bolt yielding help

I disagree with desertfox (that doesn't happen often).  

Preload does not drop during assembly if the bolt yields.  Bolts are made with strain hardening steels, so the preload increases when the bolt yields.  The preload can decrease if a service load yields the bolt and then is removed, resulting in less preload.

Engagement length in this context is the length of bolt threads in contact with tapped hole threads.  The grip length is the length from under the bolt head to the start of the tapped hole.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

(OP)
I can't tell you guys the engagement length because I don’t see any part of the bolt except for the the head as everything will be in the housing

RE: Engine Main bearing bolt yielding help

(OP)
\\\so the preload increases when the bolt yields.\\\\

Cory,

so u saying that the >12,000 lbs (my prev post numbers) are the ones that are yielding?

this bolt 3/8 - grade 8 bolt has a min proof load of 9300 lbs

RE: Engine Main bearing bolt yielding help

You have to calculate the engagement length from the design documents or by measuring the disassembled components.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

(OP)
Mike I will do that on Monday for sure

RE: Engine Main bearing bolt yielding help

My first thought is some bolts have proper hardness and others missed heat treat, wrong material, or some other strength problem.  You have two groups of data; the 12,000 group and the 8,000 group.  The question "Are the parts to print" often leads to the root cause here where I work.

32.50        12180
32.10        12880
32.10        7550
32.20        8860
32.10        11970
32.50        7550
32.30        12180
32.10        8860

RE: Engine Main bearing bolt yielding help

(OP)
This is set of data from another engine rundown

32.40    10666
32.00    8855
32.20    7245
32.20    11571.875
32.10    9358.125
32.00    7245
32.50    6238.75
32.20    5131.875


RE: Engine Main bearing bolt yielding help

The torque-tension data won't help find low strength parts.  It will have huge variation due to friction variation, and relies only on length change from the ultrasonic measurement.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Hi cory

Your right on both counts I was leaving work in a hurry and wasn't thinking straight.Yes the bolt tension would increase due to work hardening then drop off before final bolt failure.
Anyway I was going to suggest that from the 9300lb proof load quoted in the first thread and using the formula "preload" as provided we can theoretically work out
the friction factor to achieve 9300lb.

K = T/FD

which based on an average 33lb-ft would be 0.1135 friction factor.

We can see from the figures now posted by "preload" that
most of the bolts have seen more than 9300lb.
If we assume the bolt engagement is sufficient for the joint design which it appears to be if only the bolts are failing, then the problem lies with the friction factor in practice and possibly in the original design of the joint, or screw thread geometry etc not conforming to standards.
Preload I assume somebody did a joint analysis and established a torque figure for the working conditions of
this joint?
It seems to me that you need to do a lot of experimental work and get your fastening sequence consistent in terms of bolt lubrication, fastening rotation and method of measuring bolt tension force and not least of all certified
materials if the joint is so critical.
There are quite a few methods of measuring and controlling
bolt pre-load all of which have varying degrees of accuracy
I will provide a few links which give some information on this topic:-
http://www.boltscience.com/pages/tighten.htm
http://www.lawrencepumps.com/newsletter/news_v03_i11_nov06.html
http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html
Finally you may find that you either need to increase your number of 3/8" bolts or increase the size of the bolts you have already, or reduce the existing pre-load if the design can stand it.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Cory, Thanks for your analysis. I am new to this job (plant quality eng) and my manager says we get specs  (only torque) from our engineering group and for some reason they almost always go to proof load on all joints be it critical or non critical. So we in the plant, when problems arises, find the root cause of the issue and if the spec is too high we need to prove it some how and tell the engineering group to reduce the spec.
Now on this problem, we need to find the rooot cause and analyse why the bolts are yielding and if torque is the culprit we got to let them know. We need to prove things before we complaint to engineering dept.
This problem arised even before I joined the company and I dint see the assembly of this joint. Will see it on Monday. I can give u guys a very good information and calculations the present technician used.
My concern is,
Bolts is steel material and joints are aluminum-copper casting(so the tapped hole thread are aluminum threads). If the bolt yields, then the aluminum threads in the joint should yield even before that right? I "think" it is not happening, only the bolt is yielding at the threads. How come a high strength bolt yield first even before aluminum threads?

RE: Engine Main bearing bolt yielding help

(OP)
I am sorry, my prev post is addressed to Desertfox.

RE: Engine Main bearing bolt yielding help

If the thread engagement is large enough, then the bolt (even a high strength steel one) will fracture before stripping of the bolt or nut threads (even if they are weaker Al alloy ones).  This is due to the differences in areas (bolt stress area vs. bolt thread shear area vs. tapped hole thread shear area).

Designing the fastener to be at proof load is common for highly stressed powertrain joints like bearing caps, etc.

You need to measure dimensions, material properties, and torque-tension performance.  If you don't have the skills and facilities to do this yourself, use an outside lab to do it for you.  One such company is Archetype Joint:

http://www.archetypejoint.com/index.cfm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Have you verified that the bolts are really grade 8? The reason is that there are still a lot of counterfeit and improperly heat treated fasteners out there.

It is unusual, though not unheard of to see double ended studs made to the grade 8 standard.

If you can find a Skidmore-Wilhelm see if you can get some testing done. You will probably have to make a few fixtures but it will save a lot of testing on real components.

If you find that you need a little stronger stud you might want to contact ARP.

http://www.arp-bolts.com/index.html
http://www.skidmore-wilhelm.com/sw_industrial_torque.asp

RE: Engine Main bearing bolt yielding help

Hi preload

Well to find the root cause start at the beginning first look at the original design ie calcs etc.
Then look at the joint materials and get the material properties for the crankcase and head.
If the thread engagement is designed correctly it is possible that he bolt thread will fail first this is because
the female thread fails in shear and it has a bigger shear area than the shear area of the bolts, in addition the bolt can fail in tension as it is subject to tensile loading whereas the female thread which is acting like a nut is subject to compression. The criteria now is to establish what pre-load the joint actually needs with a saftey margin built in to enable the joint to function in service.
Having done this you need to then to get experimental evidence by building a few units and testing them to ensure
theory and practice mate up.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desert,

I agree with you 100%. cylinderhead is acting like a nut and the grip length here is just the length of the crank case.

But,

I dont find any kind of pattern, for example, 6 bolts in the joint exhibit totally different clamploads varying from 5000lbs in one bolt to 12000 lbs in another. If torque is very high then why am I getting a very less (5000lbs) clamp load in another bolt in the same joint?

RE: Engine Main bearing bolt yielding help

Hi preload

Thats because all pre-load methods are not highly accurate and you get a spread even when you torque to a specific figure.
the roymech site gives the following accuracy for various
pre-load methods as:-

                    Preload Setting Error
Operator "Feel" +/- 35%  
Torque Wrench +/- 25%  
Angle Torquing (Turn of nut) +/- 15%  
Load Indicating Washer +/- 10%  
Measuring Bolt elongation +/- 5%  
Hydraulic Bolt pretension +/- (1% to 10%)  
Strain Gauges / Ultrasonics +/- 1%  

so your angle torquing is +/- 15% at least this can also be affected by bolt class (fit like 6H etc)and geometry of thread, pre-loading bolts is not an exact science more of a black art.
Maybe you need to use one of the last 2 methods to achieve better results.
Your biggest problem I think is the friction factor, I had a similar problem once a few years back, I had specified some torque figures for joints which had no lubrication and should have been fine, however I got a call to go and look as the fitters were stripping threads.When I arrived the fitters said the first one they did was fine but after that they all started failing. I asked them what there system was and found that after the made the first joint okay they were using electrical contact grease on the assembly as it was a (conductor bolted joint) consquently this grease was on their hands tools etc and yes on the threads too.
I told them to make all the joints dry before they went near any grease and that cured the problem and running a rough calc on the bolts I estimated the grease reduced the friction factor by half and doubled the pre-load and hence
thread stripping followed.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desert thanks a lot for ur suggestion.

Here I am attaching two snaps of cylinder head and crank case joints.in the first snap u will see only 4 holes of cylinder head and second snap u can see all 6 holes of crank case where the bolt is tightened. I know these photos are notreally valued added but uploading them for reference.

http://img180.imageshack.us/img180/7151/dscf1647ew7.jpg

http://img248.imageshack.us/img248/4562/dscf1648zi9.jpg

I approximately measure the engagement thread length and it is more than an Inch for sure (almost to 1.5 inch). I can get you guys the exact length on monday.

RE: Engine Main bearing bolt yielding help

Ah.  That helps clarify your situation.

Those threaded holes are truly blind, right?  They don't go 'thru', and they don't intersect other holes.  So as you run bolts into them, you are trapping air, and some oil.  Probably more oil as you work faster to squirt the oil and insert them, and variable depending on who is doing the squirting.

At some point, you may have enough oil in there to hydraulically stop the bolts from entering the holes any farther, at which point the heads may not even be seated, and the shank may yield in pure torsion.

Did you save any of the bolts that visually yielded?  Examine them under magnification.

;---

Try applying the outboard lube oil to the bolts, and then setting them down to drain for a little while before inserting them, or find another way to make sure the amount of oil on the bolts is uniform, and not more than you absolutely need for lubrication.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

Hi preload

Thanks for the images, If your thread engagement depth is at least an inch or more then that would probably be about right and explains why only the bolts appear to be failing.
Usually if your tapping a hole for a bolt and it as a lower strength than the bolt material you make the tapped hole depth about 3or4 x "bolt dia" and make sure that the bolt screwed portion engages the majority of this tapped length.
The machinery's handbook as formula for checking strength of threads for both similar and disimilar materials, however you need the mechanical properties of all the materials as mentioned previously.
I'll try and respond on monday but its difficult because I work away from home all week and can only access the net after working hours. Friday evenings and weekends its easier
as I have unlimited access.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Hi again

One further point try to establish the forces the joint as to withstand in service that way we can advise you better
regarding the bolt pre-load.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Cory,

Yes those threaded holes are blind, It dosent go "thru".
Yes I have the yileded bolt with me and the failure is in the threaded portion of the bolt.I will get the image of yielded bolt too on monday. What do u want me to check them on magnification?

Regarding the oil lubrication, I think its a very good Idea.I will check how the operators doing that on monday.

Desertfox,

I will get the material properties of joint and bolt.Bolts are not failing on the serivce loads but they are failing during assembly.But its still a good idea to find out the service loads on that joint as it wil be usefull if the joint fails after the durability test.But for now the problem is they are yielding during assembly itself.

RE: Engine Main bearing bolt yielding help

Hi preload

I understand that the bolts are failing during assembly but unless you know what the forces the joint see's in service
you cannot revise the torque setting or more importantly
the pre-load for the assembly of the crank case and head.


regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

u r absolutely right, I wasn't thinking in that point of view.I will do that. Thanks

RE: Engine Main bearing bolt yielding help

What you're looking for under magnification is an indication of relative rotation across the yielded area, so you can guess if there was any tension in it when it yielded.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

(OP)
Mike, your point sounds intersting. could u please explain it more, like how to do that? what do u mean my relative rotation here? and how to check that?

Does yielding occur when bolt is not in tension too?????

RE: Engine Main bearing bolt yielding help

There are normally natural axial striations in a bolt.  In the case of cut threads, there will be witness marks where the die stopped, but there may be other more subtle markings that were in the original wire.  

If you can track one of those across the yielded area, it may provide clues about what was going on when the bolt yielded.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

1. Assemble all the joints
2. Thread on until finger tight in the prescribed sequence
3. Mark the head of the bolt extending to the crankcase
3. Turn 45 degrees with a wrench
Without even knowing the length of the bolt it's just an educated guess
Cheers

RE: Engine Main bearing bolt yielding help

Hi preload

Just some further points to add in case you had not considered them already:-

When you get the case and head material info also get the coeff of linear expansion for them as well and of the bolt
material also.
When you look at the design analysis of the joint it should
include stresses due to thermal expansion of the materials as well as external loading.
Depending how hot the joint gets you may find the point at which the material reaches its yield stress drops due to the
running temp again this should have already been looked at.
Although now I am wondering why your assembling with such a high intial pre-load ie:- bolt material reaching proof load
before service conditions are encountered.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Checked the process.

They got a small container of lube oil and the operator is dipping the threaded portion of the bolts in the oil (absolutely no pattern/consistency, just dippin them and placing) and placing them in to the crank case holes into the cylinder head threaded holes. I can see the oil drippin off the bolt end while placing in to the hole.

After placing the bolts, they don’t hand start the bolts (I don’t know if this makes much difference in my issue)

RE: Engine Main bearing bolt yielding help

I am wondering how the load numbers were obtained?  Was it ultrasonics or a load cell?
High angle and low torque is indicative of something in the joint yielding, the question is what is the component that is yielding.  Measure both the male and female threads to see if they are in print.  The tap drill size might be over on some of the holes, resulting in thread yeilding.
One thing to look for is any kind of a pattern for the location of the bolts that are exhibiting this result.  Often times it turns out that a problem is located only in certain hole locations and if that is the case it becomes much easier to trouble shoot.
One question that may seem dumb: are you sure it's not an equipment problem with the programing of some of the spindles on the gun?  I've seen that before several times.

RE: Engine Main bearing bolt yielding help

(OP)
Screwman,

Load is obtained from measuring the elongation (ultrasonics) and then the load.

Bolt is yielding in the threaded area. Bolt threaded area is almost fully engaged in to the cylinder head thread hole.

What do you mean by print?

Could you please explain this more " The tap drill size might be over on some of the holes, resulting in thread yeilding."

Regarding the pattern, I will check into that. I think the equipment is calibrated recently but I will have them check it again to make sure.

RE: Engine Main bearing bolt yielding help

Hi preload

Any news on design and material information?

regards

desertfox

RE: Engine Main bearing bolt yielding help

Preload,  Have you checked the hardness of the studs yet?  Specifically a good one versus a bad one?

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

Bad news is that I cant get the material properties of the joint.I am here in the plant quality group (I just get torque spec from engineering group) and my manager said that our engineering design dept cant help me get the material properties as they don’t know. This design is a carryover since 20 yrs.

So from the plant end I can only work with the bolt point of view. What all calculations I need to do, I can do that only using bolt specs. So now the problem boiled down to friction.

As you guys said, we all believe the problem is with the friction (lube oil) and I got two options now to address the issue.

1)    Don’t use lube oil and run some tests and see if I can reduce the scatter of clamp load.
2)    Use angle control strategy and see if I can reduce the cloam load varaiation.

The variation now is

5000 lbs – 9000lbs – 12000 lbs, 9000 being the proof load

Our goal is to reduce the variation to 6500 – 9500 lbs

RE: Engine Main bearing bolt yielding help

Uh, no.  Option 1 should be to use a controlled amount of lube oil, not none.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

preload,

You have not proven that this is a friction problem.  Screwman and I have both mentioned the possibility of deformed parts.  If you were simply overstressing the bolt due to low friction, you wouldn't have high angle and low torque.

You need to do fundamental quality control checks on your parts.  Measure part dimensions and material properties.  You must do these steps before trying to re-engineer the joint.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Hi preload

Using no lube will probably increase your scatter and not to mention reducing the clamping load of the joint.
In addition without the material info you run the risk of failure in service if you reduce the preload without knowing the service conditions.
Someone somewhere will have access to the info how about the foundry were there cast for instance.
Advice from Cory & Mike should also be followed.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Please post a picture of a yielded fastener.

RE: Engine Main bearing bolt yielding help

Hi preload

The definitve steps for trying to find the cause of a failure I would take:-

1/ check design

2/ inspect components to drawing

3/ check material conformity.

What is aconcern is what control on quality and material conformity you have if you can't obtain material data.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Mike, I want to see if the inconsistency of applying the lube oil is the main reason? So planning to do some non lube oil tests and see the variation. If that improves then we gonna look for applying the oil consistently.

If this dosent work, then I am gonna do torqe-angle control strategy. As I have no chance to get the material properties and dimensions of the joint the best bet would be doing a torque-angle signature tests on the actual joint and deduce the threshold and angle from several rundowns.

What do u guys think?

Cory,

I asked my boss again, and he said there is no way engg dept can help me because they have to search for 20 yrs back records for the material properties. What all I can get is the material properties and dimensions of the bolt. If I want to do any improvement to the joint then I have to do using the existing numbers.
When u say dimensions of the joint, u mean the grip length? Or u need the area of the joint?

RE: Engine Main bearing bolt yielding help

(OP)
MintJulep,

I will upload the yielded fastener pic today

Desertfox,

I AGREE with you . I am new to the job/company. My boss said this is not a problem joint in the recent past.This is not a problem joint in the recent past. We have no problems from customers or auditors on tha joint. But recently in the last 3-4 weeks we saw some problems, so he says that the joint strength and material strength is good. There is some other variable causing the problem.

Our main aim is to reduce the scatter. They beilieve the joint is good and torque is good(as we never had problems with the joint before).So they asked me to do some studies on the line and try couple of other strategies and see if we can reduce the scatter.
Our req preload for this joint is 9300 lbs max. we are getting to 12000 on some bolts (we believe those bolts yielding). So goal is to keep the clampload below 9300 lbs for every fastener and make the joint process capable.

RE: Engine Main bearing bolt yielding help

(OP)
Observed the yielded bolt and as Mike said, I found 3-4 striations across the bolt (along the whole minor diameter) in the yielded area. I will try to take a picture of the bolt with full magnification.

RE: Engine Main bearing bolt yielding help

Hi Preload

Well maybe as others have said its a material quality
problem or a thread geometry problem just keep us posted.

regards

desertfox

RE: Engine Main bearing bolt yielding help

preload,

Sounds like the torque applied is inconsistent.  Have you tried using a torque wrench to verify the applied torque?  Nothing like using old-fashioned technology.

Next, are all bolts hand tightened before they are torqued?  Aluminum is relatively soft, and what could be happening is that the high turn angle results because of the fastener not being initially tightened.  My thoughts is that this nee be done with each fastener prior to torquing it.  When I installed the skins on my son's drums set, all bolts were initially finger tightened, and as I when in star pattern tightening sequence, some were found to have a large initial gap again, requiring finger tightening.

If all that is not causing the problem, I would purchase a fastener load cell, or load indicating washers, and setup a test piece and install several fasteners using the turn of angle and torque wrench methods to verify the bolts are good.

hope this gives you some ideas.

jetmaker

RE: Engine Main bearing bolt yielding help

Hi preload

I can't see the images on this comp have to wait till friday.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Your plating seems to be junk and the thread form doesn't look very good to me.

I suspect you have garbage fasteners.

It is also possbile that whatever the plating is, it is reducing the coefficient of friction below what you are assuming.

The one picture looks like a crack, although I can't tell if it only the plating that is cracked, or the base material.

RE: Engine Main bearing bolt yielding help

(OP)
MintJulep

If the plating is junk and coeff of friction is less then I should get high preload in every bolt right? But its not the case here, I am getting preloads anywehere between 6000lbs to 12000 lbs , and 9300lbs being the proof load. Do u think this big variation is due to "inconsistency" in the plating and thread form?

Jetmaker,

We don’t do hand start of the bolts. May be I can ask them to include this in the process. Thnaks for the suggestion. I wil definetly look into your ideas too

RE: Engine Main bearing bolt yielding help

I agree with MintJulep, this does not appear to be a high quality bolt suitable for engine applications.  It looks like it was picked off the shelf from Home Depot.  A high quality bolt for main bearing or cylinder head applications requires excellent dimensional control (not just the thread shape, but also cylindricity/runout), careful control of the coating/lubricant (usually phosphate + lubricant not zinc plating + yellow chromate), and proper microstructure (quenched and tempered martensite, no decarburization, etc.).  You need to at least verify that the fasteners meet the design requirements as stated on the appropriate engineering drawing.  I would also recommend sending these images, data, etc. to the design group responsible for this joint.  I know if this were my responsibility I would want to see the fasteners, both unused and cracked.

RE: Engine Main bearing bolt yielding help

This smells like it might be the unintended result of a Supply Management Department cost reduction effort.

Have you checked the hardness of the bolts yet?

RE: Engine Main bearing bolt yielding help

I wonder how much control you have on the
tapped drilled hole before tapping?
Looks like your length of engagement is at
least 2 times the diameter.  What is the
strength of the castings?  I know that was
asked before, but I did not find it as I
went down the replies.  Has the casting
material been changed?

RE: Engine Main bearing bolt yielding help

Something is not adding up here.  I think your torque calibration is way off on your guns.
I went back to your torque tension results and when you are getting 12,000lbs. of clamp at 35ft.lbs. it would give you a nut factor (K factor) of .093.  That is lower than you are going to see with any type of commercial lube, fastener and nut material combination.  Since the two variables are torque and clamp in that equation and you appear to have visual yielding, my guess is that your torque sensors in your drive gun are whacked out. my guess would be the one of the strain gages is becoming unglued and giving you crazy readings from hole to hole.
Do a torque study using a calibration standard and I think you will find your problem.

RE: Engine Main bearing bolt yielding help

(OP)
Strategy : Torque control – Angle "monitor" from threshold

Torque installed : 31-35 ft-lbs and 32 ft-lbs is the target
 
Angle filter: 260 deg (device is set up for any bolt that sees more than 260 deg after the threshold will show a red light and operator will back off the bolt and scrap it and use new one)

Problem: when a operator sees 260 angle and red light,we examine the bolt , some of them shows signs of yield and some of them dosent (may be jusssst started to yield?)
 I am uploading the data of the bolts which saw more than 260 angle and corresponding torque we saw in the controller. The bolts which saw less than 260 deg all attained the target torque of 31/32 ft-lbs. If u example the data at 261 deg of turn u get a torque of around 31 ft-lb but when u see the data for 262 deg the torque significantly drops (whats happening there?)

http://img244.imageshack.us/img244/8646/highangle1yp3.png
http://img467.imageshack.us/img467/1277/highangle2do1.png

Proposal from management: they asked me to develop a angle strategy instead of angle monitor to reduce the scatter.

I asked them about the calibration and bolt strength concern (as many here suggested) and they replied the guns are very well calibrated (checked last week) and all the bolts are to the print and design. Material properties are upto the mark.

Machine is a 2 spindle machine and I have the data of 5000 rundowns per spindle
Spindle 1 – 144 rundows showed angle more than 260 deg out of 5000 rundowns
Spindle 2 - 89 rundows showed angle more than 260 deg out of 5000 rundowns

I just attached the data for 89 rundowns of spindle 2 which are red lighted due to high angle.

Please look at the data and give some suggestions.

RE: Engine Main bearing bolt yielding help

(OP)
Please magnify the image if u guys dont see then numbers clearly. Thanks

RE: Engine Main bearing bolt yielding help

I'm still not clear on how you come up with 12,000 pounds load.  Early on you wrote:

Quote:

After the installation we are measuring the length of the bolt using ultrasonics. Then calculating the clampload using T=KDF

which doesn't make much sense.  More sensible would be to calculate load based on extension using DL = F((Lb/EAb) + (Ls/EAs)).

However, this only works in the ELASTIC region.  Once you get into plastic deformation calculating load based on change in length gets way more complicated.

RE: Engine Main bearing bolt yielding help

I'm not sure I'd want to 'reduce the scatter'.  

The bolts in the photo appear to have cracks at the thread roots.  The presence of cracks suggests that it would be possible to fracture the bolts by continued twisting, at a torque level below the desired range, so when the angle monitor stops the line, it's preventing continued use of a bolt that's basically already failed, and stopping the operation soon enough that you don't have to split the cases to exract the pieces of the bolt.

So the wrenching system appears to be working in a desirable way, perhaps even as someone intended.

It goes without saying, so I'll say it, that bolts flagged by the wrenching system should never be re-used, i.e. they should be cut in half immediately, and discarded.  You might want to hold onto a few until you find out why a substantial  fraction are failing at assembly.

Something about the topology of the cracks reminds me of grade 5 bolts torqued to failure, not grade 8 bolts.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

(OP)
Mintjulep,

We are using the same formula. I meant to say T=KD(EA*deltaL/L)
DeltaL being the elongation. When I say 12000 I agree we are really not seeing that clampload but according to alongation we get that number. But those reached 12000lbs are yielded. Because 12000lbs is more than the yield strength of the bolt. We are trying to keep it simple.

Desertfox,

I will confirm with my supplier quality guy reg the hydrogen embrittlement

Mike,

Exactly. When we are seeing 260 deg they are visually necked. So our angle filter is doing good job but at the same time we need to lower the angle filter to 200 deg and see if any fastener hit that mark are yielding??????? By doing this we are scrapping more fasteners and my management don’t like it. So they want to reduce the scatter by using angle monitor and revising torque specs (reduce the target torque). What do u think Mike?

But I dint understand one thing in my data. At 261 deg of turn we almost see 31.xxx of torque but at 262 deg we always see a big drop in torque. Whats goin on in that 1 deg of difference?????????

RE: Engine Main bearing bolt yielding help

Hi preload

Its not a good idea to just reduce torque specification!
thats just a knee jerk reaction without proper technical thinking, all your going on so far is people saying that the materials okay, the joint design is ok, just look at the fasteners, you need to convince yourself these things are okay.
So having said that, if you have got to look at the bolt start by checking the material properties of the bolt have them confirmed by test, in addition you could torque some bolts to failure and compare them with that picture of a bolt failure due to hydrogen embrittlement on that site I posted previously.
Hopefully I will get to see the pics you have posted on friday.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

I believe in you guys but my management has limited resources. I was arguing with them yesterday regarding my concerns on bolt and joint (material,geometry,strength) rather than angle control or torque change. I don’t know, for some reason they feel the preload scatter is due to inconsistent application of lube oil

What is the simple method to prove them wrong? My plan is to do some sample studies with no lube or consistent lube and see if we still have scatter. If we still have scatter then that means the preload scatter is not really due to lube oil. What do u guys think? Or gimme some ideas to prove them wrong.

RE: Engine Main bearing bolt yielding help

Ok, once again.  Once you yield, there is no longer a linear relation between bolt length and preload.  If your 12,000 lb figure is based on ultrasonic length measurement, and calculation of load based on elastic deformation then it is WRONG.  No simple.  WRONG

Quote:

At 261 deg of turn we almost see 31.xxx of torque but at 262 deg we always see a big drop in torque. Whats goin on in that 1 deg of difference?????????

Answer:  The bolt yeilding.

RE: Engine Main bearing bolt yielding help

As many have said, the most likely problem is bad bolts.  The simplist way (and probably the least costly) is to buy new bolts from a known good source. If, say, 100 of these work OK, you've got your answer.

RE: Engine Main bearing bolt yielding help

Looking at the pictures of the failures I don't like the cracking that you are having. I concur that something is going on with the fastener itself, as mention H2 embrittlement, heat treating cracks, etc.
If dimensions, clearances, or friction  are involved you will normally get a shear or torsion failure. This type failure is normally across on thread and break is characteristically flat/round.
Failure from tension is normally a jagged break across sevral threads, as in you case.

My first inclination would be to toss this lot of fasteners.

If possible I would get a failure analysis of one of the failed fasteners.

The grade 5's that I've worked with all have had considerably more necking prior to failure with a clean break.

Grade 5's will crack from H2 embrittlement contrary to what some vendors will tell you.

RE: Engine Main bearing bolt yielding help

For fifty bucks or less, you can buy a Hamilton gas-tight syringe that will allow you to apply very small controlled volumes of oil to the bolts.  I would suggest buying the 100ul size and dispensing about 10ul for a start.  That requires about 6mm of stroke on the plunger.  Get the luer or luer-lok tip.  You don't need a needle.

Here's a 250ul that will work okay for $17:
http://cgi.ebay.com/Hamilton-Gastight-Glass-Syringe-250uL-Luer-Lock-HPLC_W0QQitemZ190094675100QQihZ009QQcategoryZ26414QQcmdZViewItem
That will require ~2.4mm of stroke to dispense 10ul, but you don't need extreme precision for screwing around with the idea.

Buy two; the barrels are made of glass.  They're not delicate, but they can be broken.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

(OP)
Thanks Unclesyd and Mike.

What is the procedure of doing a failure analysis of one of the failed fasteners.

RE: Engine Main bearing bolt yielding help

Here are some things you can accomplish with little or no equipment.
If you can get your management to agree I would send one or two yielded fasteners to a Metallurgical Laboratory for failure analysis.

I would by getting a hardness reading on the failed studs. The sample for the hardness test should be cut from one end of the stud. The faces of cut stud should be carefully ground parallel with no heat input from the grinding. Take hardness readings, discard the the first reading.

I would also take a small longitudinal sample, just split a short section of the bolt. Grind and polish the flat surface to get as smooth as possible. I would lightly swab this surface with a 2% HNO3 in Alcohol, laboratory ethanol, after say 15-20 seconds rinse in cold water dry, dipping in some pure ethanol, and look at the edges of the threads to see if they are lighter or darker than the body of the stud.

To take a look at the fracture surface I would take one of the elongated samples with cracks and tighten it until it breaks. Maintain the integrity of the fracture surface and if possible take a picture of the entire surface and post it on Image Shack for all to see.  

RE: Engine Main bearing bolt yielding help

(OP)
Unclesyd

Hey, we have a lab and I can get this metallurgical testing done.Do I have to do all the three testings you specified?

Hardeness
Longitudinal sample test
Fracture surface

Or they are all one test?

RE: Engine Main bearing bolt yielding help

Do a tensile test on the bolts using a lairly large sample size and I bet you will find that you have parts that are under strength.  In my experience you are much more likely to have a component quality problem or a drive gun issue than a need to change the torque setting.
One quick thing that you could do would be to take some EP grease and coat the threads on a few bolts and see what results you get. You should yeild 100% of the bolts if this is a lube related issue and break some if it is a QC issue.
For a high load critical joint you should switch to angle control and torque monitoring, which will help to reduce your clamp load scatter (once you get the rest of your parts and process under control).  Using a snug fit torque threshold and then using angle control will take much of the clamp variation caused by frictional changes out of the picture and you can still monitor torque at the angle shut-off to see if it is within limits.  (low torqeu at angle means something is stretching or compressing unusally and high torque at angle means excessive friction (thread nicks or cross threading ususally)).  Angle control is a very useful tool and I highly reccoment it for critical, rigid joints.

RE: Engine Main bearing bolt yielding help

(OP)
Screwman,

The lube oil we are using is applied inconsistently. Some times we get some lube oil to the threads and also under the head and some time only to the threads. My management thinks this is the reason for some bolts having high clamploads up to yield point and some normal clamploads or low clamp loads.

The tensile load test studies done before on the same bolt has an very good avg of proof load. they did a 24 sample tensile test and found out that the proof load for this bolt is 9300 lbs and which is exactly right to the print. This study got confidence in my management to believe that bolts are doing good.

But my plan is instead of using EP grease, can I use the same lube oil we are using now but will make sure to apply consistently only to the thread area of the bolt and see the results? And if I still see this yielding problem and large scatter then I can argue with my management that the bolt or drive gun is the issue. And if I get comfortable results then I will agree with my management and develop an angle control stategy.

What do u think?

RE: Engine Main bearing bolt yielding help

That sounds like resonable approach.  You may also want to apply oil under the head since almost half of the friction is under the head and if you lube both it would truly give you the worst case situation.
Were the bolts that the tensile pulls done on from the same lot as the suspect parts?  I just have a feeling that the bolts are playing a role in this problem.....

RE: Engine Main bearing bolt yielding help

(OP)
Screwman,

I can get the tensile load test studies on the recent (the one we are using on the line now) on Monday. It seems our supplier has done the tests and the bolts are pretty good regarding the tensile strength and yield strength. But I will get the data from them to make sure again.

RE: Engine Main bearing bolt yielding help

Hi preload

I looked a the fastener pics you posted showing the cracks and came to the conclusion I would have liked to have seen
a pic of a bolt that had been completely destroyed ie:-
torqued till it broke in two.
The reason being if you look at the two fracture surfaces it can tell you where the crack started and also whether it was a complete tensile failure or tensile/shear stress combined.
This brings me to my next point the bolts are failing during tightening and therefore the yield stress of the material is being reached by a combination of tensile and shear stresses and not by tensile stress induced alone.
This tensile stress is known as a principle stress and would occur at some angle relative to the bolt central axis.
I surfed the net for some information and found that for a grade 8 material for your thread size the proof stress was
equal to:-

             9300lb/0.0775"^2  = 120000 psi

the 0.0775" is the stress area for your bolt.
see this site:-

http://www.almabolt.com/pages/catalog/bolts/proofloadtensile.htm

Now with that info and a figure of 33lb-ft as the torque on
a plated bolt of your size from this link:-

http://www.almabolt.com/pages/catalog/bolts/tighteningtorque.htm

I used a Mohr stress circle and found that the principle stress acting at an angle of 14.24 degrees below the horizontal was about 136500 psi which is in excess of the proof stress of 120000.

Now it is interesting to note that the torque figure I got from the above site is the recommended one for joints that are not critical and (although it doesn't say so) the torque figure I believe is for an unlubricated joint.
It also shows an expected clamping force or pre-load for that joint to be 6975lbf
In your application you are using that torque figure with lubricant which is obviously reducing the friction greatly
and generating a high a pre-load.
Whilst I accept that the torque figure stated on the above site is for cap head screws and not flanged headed bolts which may well have some effect I now believe your main problem is one of friction coefficient.
If you really need a pre-load of 9300lb as you stated in an earlier post you need to get a better grade of fastener.
Finally it is still possible that the other points raised about material propeties and control, thread geometry etc
are still playing a part in your situation.
I will look forward to seeing the tensile results of the current fasteners next week and if you could torque a bolt in two and post pics of the fracture faces also that would be helpful.

regards

desertfox

RE: Engine Main bearing bolt yielding help


  1 independent(truly,anonymous)testing of bolts
  2 source aftermarket(race)engine bolts and test
  3 wait for screaming(pissing contests) to die down and
   assemble realible engines
  4 the whole thing seems like a dollar waiting on a dime
                            Regards,Ed

RE: Engine Main bearing bolt yielding help

Hi preload

I found a site that confirms that the 33lb-ft figure is without lubricant look here:-http://www.morbark.com/Service/belttorque.pdf

Also look at the notes at the bottom of the page it talks about reducing the torque setting if oil lubricant is used
and a further reduction for new cap screws which are plated.
I haven'nt full access to the document they quote:-

IFI 5th Edition Technical Data N-12/N-16, using Equation (1)

as I am based in the uk but if you can get hold of it,it might help.

Now if I calculate the principle stress based on there torque and a 6975lbf clamp force then the principle stress
is:-
          111102.56 lbf/in^2

which is below the proof stress of 120000 by about 7.5%

Reading back through earlier posts it is clear that this joint was not a problem untill recently so I wish to ask the following:-

1/ Was lubricant always used?

2/ Has the lubricant changed recently?

3/ Were the bolts always plated with the same coating?

4/ Were there any changes to pre-load or torque setting
   methods?

The reason for my questions is that assuming everything else
is okay with thread geometry, material properties etc I cannot see how this joint would not give the problems your seeing now with the torque setting and lubricant unless something changed significantly.

regards

desertfox

RE: Engine Main bearing bolt yielding help

The test I recommended are primarily use to quickly eliminate some factors that may be contributing to your problem.

The hardness test will indicate if the heat treatment of the studs has went awry. It also gives a tidbit of information for future reference.

The longitudinal section should show whether you have either carburization or decarburization which can throw you tightening data into a mess.

It is always nice to look at a fracture surface. Somethings it will point to the smoking gun while at other times it becomes a chore to pin down the mode of failure.

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

I greatly appreciate your help.

I will get the tensile test data soon. I will also get the picture of a broken bolt.Regarding your 4 questions in the recent post , I wil surely answer them on Monday.

regarding the math,

33 ft-lb torque - 6975 bf according to the document and he said take 10% less if the engine oil is used. so in our application :

29.7 ft-bs generates 6975 lbf right? and I am using 32 ft-b as  the target torque which shoyld generate 7500lbf, which is way below the proff load (9300lb).Thne how am I yielding?Am I using way too much of lubricantion? thats generating more than 9300 lbf for the same 32 ft-lb torque than what the document says?

Could you please explain the following statement more
"I used a Mohr stress circle and found that the principle stress acting at an angle of 14.24 degrees below the horizontal was about 136500 psi which is in excess of the proof stress of 120000."

I am not that good at mohr circe.

RE: Engine Main bearing bolt yielding help

Hi preload

The Mohr stress circle can be used to find the resultant stresses acting on a plane, in your case the bolts are failing during tightening which means they are failing when they have both shear and tensile stresses present.
Only when you stop tightening the bolt are the shear stresses removed.

Have a look at these sites for Mohr's

 http://www.sunybroome.edu/~biegen_j/strength/examples/chap7/ch7_cl1_mohr2.htm

http://physics.uwstout.edu/StatStr/statics/spec1/spec85.htm

http://en.wikipedia.org/wiki/Stress_%28physics%29#Principal_stresses_in_2-D

Now regarding your other points the 6975lbf at 33lb-ft torque is quoted using a friction factor of 0.15 without any lubricant,for a joint that is not critical and is subject to +/- 25% on pre-load.
Bear in mind that torque figures are only a rough estimate
and if your joint is critical then you need to monitor your pre-load by a more accurate method like measuring elastic stretch of bolt during assembly. I cannot put an accurate figure of pre-load against a torque because the friction factor which can vary considerably is dominating the equation.Having said that the point which I am trying to make is that you're using a torque figure which is recommended for a joint without lubrication,therefore in a joint without lubrication I would expect the friction factor to be higher and for a given torque you would achieve a smaller pre-load resulting in lower tensile stress in the bolt during and after tightening and less likely to fail during assembly.Because you are lubricating the bolt your friction factor is much lower causing the bolts to fail during tightening.
Now the only way to solve this problem is to reduce your torque figure for the lubricated joint or more importantly your pre-load, however this puts us back to square one because we need to know what the pre-load needs to be in order to comply with the original design which as yet has not been confirmed.
The other point I am making is that if in the past the assembly was being made then as it is now I beleive that the same problems would have occurred unless there has been a major change in material supply or lubrication, pre-load procedure of recent.
Not sure how you got from 32lb-ft to 7500lbf depends on your friction factor.
First thing to do is define your joint pre-load with a tolerance, then we can estimate a torque to achieve the pre-load, from there you need to make practical measurements with a consistant pre-load procedure to ensure that goal.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

I am gonna call a meeting with the design engineer of the joint soon. He should give me a clamp load spec.

And regarding the 32ft-lb - 7500lbf figure, the math goes

from document,

without lubrication 33 ft-lb gives 6975 lbf for my bolt.

with lube oil (which is my situation) the document says to decrease 10% of the torque used to get the same 6975 lbs.So 10% of 33 is 29.7 lb-ft. so 6975 lbs clamp will be generated from a 29.7 lb-ft in a lubricated state. right?

so in lube state,
29.7 lb-ft. - 6975 lbs
then 32 lb-ft (my target torque) should produce 7500 lbf right?

but, my bolt has a capacity of 9300 proof load,then how am I exceeding the proof and yield strength?

May be I am over lubricating the bolt...



RE: Engine Main bearing bolt yielding help

Two things bother me.
Were did you get the 10% reduction for the torque value.
The values I have say that for oil you reduce torque values 35%-40%. .2K to .12K is a 40% reduction.

The values when using 70% UTS are 9250 min strength using 39 ft lb dry torque. The very max.

Using 75% proof load there is a clamp load of 4940 lb using 30 ft lb dry, 23 ft lb(K=.15) lubricated. This will give around 60,000 bolt stress.

85000 psi proof load for grade 5.
85000*.0775= 6587 psi proof load for 3/8-16 grade 5
Tensile area = .0775 in^2

Where do you get the 9300 psi proof load for your bolt?

RE: Engine Main bearing bolt yielding help

Hi preload

Your exceeding the proof load & stress because yes the friction factor is to low or as you say your lubricating the bolts too much.
However the bolts are failing during tightening when both
shear stress and tensile stress are applied to the bolt at the same time as I have mentioned in my earlier posts. When you cease tightening the shear stress due to the applied torque drops off and your left with tensile stress in the bolt, but at that point its too late the bolt as already yielded.
I think I see were your coming from with 7500lb I don't quite get that figure but fairly close but that assumes a friction factor of 0.15 which is much higher then what your getting in practice.

Do you understand the Mohr circle now or not?

Finally if you get the pre-load from the designer we can go from there.
If you post any pics of bolt fractures I won't get to see them before next friday so maybe you can describe the surfaces in your post.

Hi unclesyd

The figures come from the links to sites I posted earlier
ie:-
http://www.morbark.com/Service/belttorque.pdf

http://www.almabolt.com/pages/catalog/bolts/proofloadtensile.htm
http://www.almabolt.com/pages/catalog/bolts/tighteningtorque.htm

The bolt used is a grade 8 with 16 threads per inch and the 9300lbf is a force not a stress in psi.

I mentioned earlier also that the figures are not for flange head bolts as that may have an effect but I think the
priciple cause of this failure is friction factor being to
low for the pre-load torque there trying to achieve.

regards

desertfox

         

RE: Engine Main bearing bolt yielding help

Another senior moment with my last post.
Your 9300 psi proof load is correct for the grade 8.

My information uses .2K for dry steel and .11K-.12K for various lubricants. It uses .15K for Cadmium plating and Aluminum Plating. Both would reduce torque by 25%. I'm trying to find a paper on various combinations of plated and lubricated surfaces.
My tables show a clamp load of 7000 psi with a dry torque of 45 ft lb(k=.2) and a lubricated torque of 35 ft lb (K=.15) for a 3/8-16 grade 8.

As mentioned in my earlier post it would be a great help to get on Skidmore-Willhiem tester.

http://www.skidmore-wilhelm.com/sw_industrial_torque.asp
 

RE: Engine Main bearing bolt yielding help

Hi unclesyd

The site I had earlier said .15k for just plated threads unlike your last figure in your post just shows how arbitrary these friction factors are.

This table is based on IFI 5th Edition Technical Data N-12/N-16, using Equation (1) and a torque coefficient, K=0.20 for non plated steel fasteners and K=0.15 for plated fasteners.


RE: Engine Main bearing bolt yielding help

desertfox, preload,
You are correct the fine print got me again. I concur that something is amiss in what value of K to use if the fasteners are good.

The numbers I have come from Fastening Reference, Machine Design, Nov 1977.

I'm kinda like POGO, "I have found the enemy they is me"

RE: Engine Main bearing bolt yielding help

Hi preload

Just correcting my figures if you take the proof load of 9300lb and combine it with a torque of 33lb-ft the principle stress would be 148459lb/in^2 not 136500 as previously stated acting on a plane at 23.645 degrees above the horizontal and not 14.24 degrees below.
Similarly for the 6975 clamp force at 33lb-ft I obtain a principle stress of 124056lb/in^2 at an angle of 27.65 degrees above the horizontal note also that this is just slightly above the proof stress of 120000 lb/in^2 which is to be expected.
Sorry for any confusion this may have caused but it still clearly shows why those bolts are yielding.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox, Thnks for those links. I "kind" off understood mohrs circle.

RE: Engine Main bearing bolt yielding help

Hi preload

your welcome

RE: Engine Main bearing bolt yielding help

A statement and and two question;

1) A torque of 33 lb-ft should be quite reasonable for a grade 8 bolt.

2)A grade 8 bolt should have a minimum yield strength of 160,000 PSI?

3) A proof load is a load which should show no evidence of yielding?

RE: Engine Main bearing bolt yielding help

Hi sreid

I believe your right with your statement about proof load.
Looks like I misunderstood and took proof load to be related to proof stress.
definition of proof stresssurprised.2% proof stress. Stress at which the material undergoes a 0.2% non-proportional (permanent) extension during a tensile test.

33lb-ft is the recommended torque for a grade 8 bolt when unlubricated in a joint, if you lubricate it then friction
reduces significantly and your  pre-load can go above that which is required for yield.I believe this is the cause of this problem. Also during tightening the bolt see's maximum stress which is a combined stress due to tension and shear loads caused by the torque.

This site shows a grade 8 bolt having a minimum 130000lb/in^2

http://www.americanfastener.com/technical/grade_markings_steel.asp

regards

desertfox

RE: Engine Main bearing bolt yielding help

Desertfox,

Thanks.  I guess I get spoiled using Unbrako cap screws.  I'll bet that most of the time, bolt yield is well above the 130 ksi spec for grade 8 bolts. Now with a batch (say) at the lower limit, some will strech oiled at 33 lb-ft.

RE: Engine Main bearing bolt yielding help

Hi preload

Anymore information?

You could try this for an experiment:- make a joint without
any lubrication, torque all bolts to 33lbs-ft and I bet you don't have any bolts fail.

regards

desertfox

RE: Engine Main bearing bolt yielding help

desertfox,

By fail, do you mean exhibit visually detectable yielding?

I don't think there is enough information to conclude there won't be high angle results with your proposed experiment.

If this indeed is a friction related problem, I believe it likely would be due to variation at the bearing surface (some parts have no oil, some parts have oil) rather than in the threads (since all parts likely will have oil in the threads).

Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Hi cory

I think the main problem is friction being to low because of the lubricant and then torquing the bolt to 33lbs-ft
which is the recommended torque figure without lubricant.
If you do a combined stress problem ie:- torque loading plus tension in the fastener, I calculate that the shear stress at 33lbs-ft in the bolt is about 65170lb/In^2.
Using a friction factor of 0.1427 and calculating the tensile load from this info you get 7595lbf.
That equates to about 98000lb/in^2 tensile stress in the bolt.
At the point of tightening if you combine 98000lb/in^2
with 65170lb/In^2 shear stress you get a principle stress of 130536.6lb/in^2. this latter figure is just over minimum yield stress for the bolt.
However if you consider lubricant the friction factor would be a lot lower than 0.1427 maybe less than 0.1 in which case if you do the calc again your well into the yield stress for the bolt.
My idea of doing a dry joint was just to show that friction was the problem and if failures occured in the dry joint then theres something else wrong.

regards
desertfox

RE: Engine Main bearing bolt yielding help

Why not just use angle control and be done
with this.  It is no surprise to anyone that
torque is not a good method to tighten bolts
for all of the reasons posted so far.  I would
expect any bolt to fail or yield after turning
it 260 degrees.

RE: Engine Main bearing bolt yielding help

(OP)
Dimjim,

I am trying to develop a angle strategy on this issue (that is my mnagments goal to improve the capability), but before that I am just trying to prove that it is a friction problem. And with desertfox's caluculaions we can see that.

Please see the failed bolts pics,

http://img442.imageshack.us/img442/4011/dscf1701kp1.jpg
http://img477.imageshack.us/img477/7199/dscf1703ho8.jpg

Please let me know if the pics are ok. The other part of the bolts are in the tapped hole,as they are difficult to remove.

I am starting a new thread on another problem , It would be great if you guys help me on that issue and continue helping me in this thread too

RE: Engine Main bearing bolt yielding help

Hi preload

Can't see the pics till Friday pm but thanks for posting them.
Whats happening regarding my questions posted last week ie:- joint design pre-load, change of lubrication etc?

No point in trying to develop an angle stratergy till you get the designed pre-load for the joint.
The ony way currently to prevent this bolt failure is reduce bolt pre-load by reducing torque or increasing friction and you can't do that till you know what the joint design needs.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

lubrication is not changed,proces is also not changed,but we are seeing the problem since 2-3 yrs.no one exactly dont know what changed,but to thier knowledge nothing has been changed.

regarding joint design preload,my meeting with design guy is on 2nd may,he is gonna let me know the design preload.

RE: Engine Main bearing bolt yielding help

Hi preload

Thanks for the response, when you say you are seeing the problem since 2 or 3yrs, do you mean that you have not had this problem for 2 or 3 years or you have had this problem for 2 or 3 years?

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

we had this problem from 2004 or 2005.I and my managaement dosent know why this problem started from 2005.

or it should be there since many years but may be we found the problem in 2005.

but nothing has changed in lube,process or materials

RE: Engine Main bearing bolt yielding help

Hi preload

Well I am surprised your companies had it this length of time.
The problem I am convinced is friction factor to low as previously stated and the calcs I have done seem to confirm this in my mind at least.
Try what I said earlier make some joints without lube I don't think any bolts will fail

regards

desertfox

RE: Engine Main bearing bolt yielding help

And you've always dipped the bolts in the exact same brand and type of 'outboard lube oil', and its formulation hasn't changed  ...  right?

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

desertfox,

Were you calculating shear stress on the fastener shank using the entire applied torque of 33 lbf-ft?  It appears that you were.  This is not realistic (since the head friction torque opposes the applied torque) and not typical for bolted joint calculation.  

When I do calculations, I find that 33 lbf-ft should not yield the screw.

Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Hi Cory

Yes cory I was using the torque of 33lb-ft on the shank
but combining it with the tensile stress to get the principle stress of the 134,000 I quoted previously.
The shear stress alone I agree would not yield the material
but combine it with the tensile stress and it will.
I just used a plane stress model with torsion and tension.
What value of friction do you use under the head so I can reduce the torque on the shank?
If theres oil under the head the friction would be quite low I thought, so I used the friction factor of .152 something to get the tensile force and just went from there.
I think its a fairly good representation of whats happening when you consider that there using the maximum recommended torque for a dry bolt and putting lubricant on it.
The reason some bolts aren't failing is the spread of friction and the fact not all the bolts will have minimum yield of 130000.
What calculation are you doing? are you just considering the shear due to torque?

regards

desertfox

RE: Engine Main bearing bolt yielding help

desertfox,

I used the last equation in FAQ725-536: How do I calculate an assembly torque?.  It accounts for tension loading and torsion loading, but the torsion loading is only due to the pitch torque and thread friction torque.  The sum of these two is equal to the difference of applied torque and head friction torque.  It is widely documented that the rotating fastener's contact surface creates a torque equal to approximately 50% of the applied torque.  Here is a link to one source:

http://www.boltscience.com/pages/casestdy.htm

Your analysis is incorrect in that it uses the entire applied torque on the shank.  This is why I have stated multiple times that, with the data provided, friction variation has not been proven as the cause of this problem.

Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Hi Cory

I will post later just starting work at the minute.
I'll look at that faq link.
I still think that its a friction problem.
One of the sites I posted states a friction factor of 0.15,
recommended torque of 33lbs-ft and a preload of 6975lb

using p= T/(.15*d)

now if the torque is 33lb-ft and I reduce friction by 50%
get 2*6975lb tension in my fastener which is greater than the proof load of 9200lbs it was this that started me thinking that it might be friction.
Bear in mind the 33lbs-ft is for an unlubricated joint.
I will post properly later.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
?M = Rp0.2 / [1 + 3(3/2 · d2/d0 · {P/(? · d2) + 1.115 ?G})2]0.5

MA = FM (0.16 · P + 0.58 · d2 · ?G + ?K · DKm/2)

Cory,
In the above equations from the thread you referred to calcluate assembly torque..

Is d0 - shank cross section dia same as Nominal dia?
how can I get the ?G,?K,DKm values?
Is there any chart which can give us these values for any type of fastener?

Thanks

RE: Engine Main bearing bolt yielding help

d0 is the diameter of smallest shank cross section.  This is thread minor diameter for your parts.

μG & μK are measured with laboratory equipment (like the Skidmore-Wilhelm machine mentioned by unclesyd above).  If you can't obtain measurements, then assume 0.1 for the low end and 0.2 for the high end.  There are many published sources of friction coefficients (Internet sites like RoyMech, books like Handbook of Bolts and Bolted Joints, and standards like VDI 2230).

DKm is the effective contact diameter for the rotating fastener.  You will need to measure your parts or look at your part drawings.

Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

Hi Cory, preload

Firstly I was incorrect in using all the torque in my analysis and only a proportion should have been used as rightly stated by Cory.

Now going back to one of my earlier posts which had a link
to this site:- http://www.almabolt.com/pages/catalog/bolts/tighteningtorque.htm

It gave figures for the 3/8" bolt with a recommended torque
of 33lbf-ft with a clamp load of 6975lbf and using a friction factor K of 0.15,associated with the formula P= T/(K*D) which we have seen on other posts here.
Now with reference to The Machinery's Handbook 25th Edition.
I was able to calculate the torque req to overcome thread friction, bearing friction under the head and the torque req to produce the axial load or pre-load in the above bolt.
I started with the pre-load 6975lbf and the dry friction factor .15 given which according to the Machinery's handbook split into 0.12 for thread friction and 0.1 for the bearing friction.I calculated the torque from the formula and found it to be in good agreement with the 33lbs-ft torque quoted on the website I mentioned earlier (my calculated figure was 32.871196 lbs-ft).
I then began to reduce the friction factors for both the threads and bolt head bearing keeping the torque constant and working out the increased pre-load that this created.
From the pre-load figures and the torque required to achieve it, I worked out the maximum principle stress.
I found that if the friction factors for thread and bearing
friction reached 0.07 then the principle stress would exceed the 130000lbs/in^2 minimum yield quoted for that bolt grade during assembly.
On this last calculation the axial load would be about 9894lbf which would generate a stress of 127664.5 lb/in^2 just below the minimun yield but when combined with the torsional stress it just fails.
I have uploaded a file which shows the calculations and formula I used it can be found at:-
http://www.uploaddownload.eu/download.php?file=353014b5cbd05ec1c20b31370f3276ce

One thing I didn't show on the above file was the K factor
that would be used in the formula P= T/KD for this last calculation where the bolt reached yield, you can work it out from the formula that are there but for ease it will be
0.106.
I still believe that this problem is caused by reduced friction given that the bolts seemingly fail during tightening which is when they're most stressed.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

Thanks a ton for all the effort and time you put in those calculations. I really appreciate you help.

RE: Engine Main bearing bolt yielding help

Hi preload

Your welcome, hows it going with this problem any more info yet?

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

meeting on this problem tomm with the design guys. Will let you gusy know the minutes and what we have decided.

Most probably,everyone is gonna agree upon angle strategy, and eventually I am the one to develop the strategy.

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

I got couple of general questions (not on this specific problem though)

1) How reliable is clutch tool compared to dc electric? Is it ok using a cluth tool on critical joints even if we have very good confidence on the tool?

2) Which one gives less scatter of preload? Slow Hand tightening or clutch/pulse tool? For a given torque, which tightening method gives less clamp load compared to other (taking friction into consideration?)

3) I am gonna ask you guys a real stupid question.

All the tables give torque and tension numbers for different fasteners.

For a given torque, if the grip length changes, tension in the joint changes right? Then how come the tables give a tension number with out specifying the grip length?

RE: Engine Main bearing bolt yielding help

1) Pulse tools are not reliable and should not be used on critical joints.
2) DC electric gives less scatter than the others.  I would assume hand tightening has less scatter than pulse tools.  The problem with pulse tools is variation, not that it is always less clamp load for a given torque.  Hand tightening would probably result in the lowest consistent clamp load.

RE: Engine Main bearing bolt yielding help

Hi preload

I posted earlier regarding setting accuracy ie:-

Preload Setting Error
Operator "Feel" +/- 35%  
Torque Wrench +/- 25%  
Angle Torquing (Turn of nut) +/- 15%  
Load Indicating Washer +/- 10%  
Measuring Bolt elongation +/- 5%  
Hydraulic Bolt pretension +/- (1% to 10%)  
Strain Gauges / Ultrasonics +/- 1%  

However when you get the info ie:- preload etc you need to ask them what tolerance is on that preload then we can look at methods etc.

regards
desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,
thanks for the info. What I am asking for is if tightening speed is more, then clampload decreases or increases?

And also
All the tables give torque and tension numbers for different fasteners.

For a given torque, if the grip length changes, tension in the joint changes right? Then how come the tables give a tension number with out specifying the grip length?

RE: Engine Main bearing bolt yielding help

Increasing the grip length increases the strain energy stored in the joint for a given torque.  If you think of the bolt as a spring, longer bolts and longer grip lengths have a lower spring rate.

Changing the spring rate in turn changes the 'feel' of the joint.  A short grip length will come up to torque very rapidly, whereas it will take more rotation to achieve a given torque for the longer grip length.  One side effect is that the extra tension per unit of extra rotation is dependent on grip length, so if you were using shorter bolts, you would have a smaller rotation limit, and you would have to measure it more precisely.

In theory, the tension produced for a given torque should be independent of grip length.  Because of the nonideal way in which the various wrenching machines work, there may be some accidental correlation.  

To give a contrived hypothetical example, suppose a given wrenching machine actually stopped applying torque at some fixed time or some fixed angle after detecting the correct torque level.  In the former case, the actual torque and tension would be speed- sensitive, and in the latter, it would be grip- length sensitive.

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

Hi preload

As Mike as already said grip length and tension should be independant and the strain in the bolt will change dependant on length.
Not sure about effect of speed although it may effect how the joint relaxs after initial tightening I will try to find some info.
Can you use longer bolts in your current joint without them bottoming?


Regards

desertfox

RE: Engine Main bearing bolt yielding help

Here is an outstanding paper on bolts, metric. But the discussions are relevant to the ongoing discussions in both of preload's threads.
Some very good information on the spread expected from the various tightening regimes.

http://www.kamax.de/pdf/Bolt_and_Screw_Compendium.pdf

RE: Engine Main bearing bolt yielding help

(OP)
Thnaks Unclesyd for the paper.Thanks Mike and DesertFox for the answers.

I got a small update on this issue.

I have two 4 inch length bolts and one has a shank length of 2.75 in and thread length of 1.25 inch.

The other bolt has a shank length of 2.5 in and thread length of 1.5 inch.

We get a mix of these bolts for this assembly; can we attribute the yielding problem to this change?

RE: Engine Main bearing bolt yielding help

If you can correlate the yielding with the threaded length, you've got a problem other than threaded length.  It shouldn't make a significant difference for bolts that meet the spec.

Are you by chance buying the bolts offshore?

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

(OP)
Mike,

No I am not corelating the issue, but one of my technician is doing that. I am positive that this shouldnot make a significant difference if the parts meet the spec. I just want to make sure with u guys and go a ahead with my thoghts.

RE: Engine Main bearing bolt yielding help

Hi preload

My terminology was incorrect the strain will remain the same for longer and shorter bolts but the elongation of the bolts will be different.
By the way the bolt you are using what class is it? I assumed class 3A but is it 2A?.
Have you seen evidence of the the bolts being to spec yet ,or just taking managements word.
You could take one of the bolts and have your lab do a tensile test on them perhaps.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

Its fit class 2A not 3A.

How does it make a difference in your calculations?

Reg the tensile data I am still awaiting the info from my supplier.

RE: Engine Main bearing bolt yielding help

HI preload

Yes it will slightly effect the stress for a given torque
but I don't think its very much.
Have you had your meeting yet with the designers?


regards
desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

If the fit is 3A then the stress is going to be more than 2A right? because of the tight fit for 3a.

Yes, I met the designer and he dosent have info, he said he has to Dig in to it and will give me the range. He is also gonna give me the tensile data.I am also awaiting the data from my supplier too.

Did u see my recent update?

Though the major problem is friction, we also found something yesterday. A year back we used bolts which has less thread length (so for the same griplength, the joint sees less threads). Now we are using the same bolt but more thread length (so for the same grip length joint has more threads)

I also got the history load data

For the bolts we are using present (more thread length) avg clamp load for a 24 pc sample is 9300 lbs for the same torque

For the bolts we used 1 yr back(less thread length) avg clamp load for a 24 pc sample is 7800 lbs for the same torque

What do u think? Because the threaded length is more now, are we seeing more stretch and more clamp load?

RE: Engine Main bearing bolt yielding help

Proload,
Your clamp load variation is more likely due to frictional variation between the two lots of bolts.  For critical engine bolts the auto industry requires each lot to pass a torque tension test into a referee nut and bearing surface to assure that the friction is consistent from lot to lot.
The change in thread length that you have will cause a variation but it will be very small: less than 10%.
  
I second the comments about pulse guns and critical joints.  They are not prefered.  Best is DC using angle control and torque monitoring. That method will catch parts or joints that are yielding during assembly or that are cross threaded or stripped out.  Hand torquing will have more variation than a well maintained power tool and clutch because of the slip stick effect going from static to dynanic friction as the final torque is applied with the torque wrench. (more good joints have been messed up by 'inspectors' and their torque wrenches, than you can imagine).

RE: Engine Main bearing bolt yielding help

(OP)
Screwman,

I have clamp load and elongation data for both bolts (ultrasonic). I have different load factor values for both as the goemetry change.

one bolt lot has 7800lbs and other has 9300 lbs. Its 15% difference.

With the data I have , how can I say that all this 15% difference is not due to thread length change? By taking the load factor percentage?

 

RE: Engine Main bearing bolt yielding help

If the thread engagement stays the same you will have your percentage difference in the effective length of the bolt.

To calculate the effective length.

http://www.riverhawk.com/boltloading.php

RE: Engine Main bearing bolt yielding help

(OP)
the load factors are as follows: For the current (short shank) fastener the Load Factor is 1.00625 and the (long shank) is 1.07224.

RE: Engine Main bearing bolt yielding help

(OP)
Will explain it clearly.
The thread engagement length remains same, but we have two different bolt cross sections in the effective length (grip length). In one case we have 1/4th of the effective length as threaded length and in other case we have 1/3rd of the effective length as threaded length. This is because our supplier can supplier can supply different bolts for the same application until he stays with in the spec.

Will I get more clampload for the same torque on a bolt which has more threaded part in the effective length? If yes why is that?

RE: Engine Main bearing bolt yielding help

Using only torque as the measurement and keeping the same thread engagement length you should get the same clamp load or the same bolt stress. They parameters that affect the torque will stay the same, that is the under head friction and the thread friction.

RE: Engine Main bearing bolt yielding help

(OP)
Unclesyd,

so are u saying that it shouldn't matter if I use a fully threaded bolt or half threaded bolt as long as I use the same size of bolt.

so,

if the engagement length is same,torque is same,effective grip length is same, then I should get the same clamp load for a fully threaded bolt and non-fully threaded bolt.

But my question is,

for fully threaded bolt, the area is different and for half threaded bolt the area is different (shank area + threaded area).If area is different clamp load changes right?If the answer is no, then where does that calcluated area difference go?

I am thinking, if the bolt is fully threaed, then for the same torque, the stretch will be more,so the clamp load will be more compared to the half threaded bolt.I am really confused.

RE: Engine Main bearing bolt yielding help

If you use the information available in the Riverhawk site posted above and use your elongation measurements and measurments to calculate the bolt stress based on elongation you can see the difference between the thread lengths if the thread engagement stays the same. This is why you want to use the minimum thread length to get the job done.

Again using torque you really don't know what the clamp force is until you measure it experimentally or by elongation of the fastener and calculate it. If you know the bolt stress you can calculate the expected elongation.

A given torque is just a value that will get you in the ballpark for the required bolt stress to preload the joint to do it's intended function. The torque values given in the tables only use the fastener pitch not the number of threads on the fastener.

In critical joints the length of the threaded section is always kept the same, at a minimum. That is why there are standard dimensions for threaded fasteners to eliminate one of the variables in any joint.   




RE: Engine Main bearing bolt yielding help

Hi preload

I will run some numbers when I get home later today.
Regarding thread length none of the formula I have for working out the tensile load contain anything to do with thread length, the only time thread engagement length comes in is when you are considering thread stripping.
As Mike said earlier the longer bolt as a lower stiffness than a shorter bolt.

I'll post later

regards

desertfox

RE: Engine Main bearing bolt yielding help

Hi preload

What exactly is the thread engagement of the 2 different length bolts?

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Unclesyd,

Exactly.

What I am doing now is the same. I have two different threaded length bolts. After tightening them to the same torque, I measure the elongation using ultrasonics and that instrument will calculate the load for me. Inorder to caluculate the load for me I need to do some inputs to the ultrasonic instrument and then it calculates a "load factor".

So,

Ultrasonics instrument asks me to enter a area of cross section for the bolt. So when I enter those numbers I get a different load factor for these two different bolts because the area of bolt cross section changes for these two bolts as the threaded length is different.

So when I measure elongation, for the same elongation I get different load values for these two different bolts because load factor is different.

But as Desertfox says,

When we do T=kdf, the d we enter here is same for different threaded length bolts as long as the size of the bolts remains same. And so we don’t see any difference in the final load.

So, finally,

Is T=kdf an approximate and what I am doing now is accurate? Does the different threaded lengths have different clamploads for the same torque if the thread engagement is same. This subject is so complicated. lol

RE: Engine Main bearing bolt yielding help

(OP)
My thread engagement length is 0.814 In
Effective grip length is 2.77 in

For bolt 1: 2 inch is the shank length and 0.77 in is the threaded length in the whole grip length
For bolt 2: 2.4 inch is the shank length and 0.37 in is the threaded length in the whole grip length

Why we have these different threaded lengths is because, our engineering drawing says a min thread length, but it dosent specifies a max thread length, so our supplier has authority to supply different threaded lengths as it will be per our chart spec.

So when I develop a new strategy I have to develop the torque keeping in mind this variability in threaded length.

RE: Engine Main bearing bolt yielding help

Whoa!  If the thread engagement length is .814,
how does the .37 and .77 thread length engage
the .814 depth?

Why isn't the wrench angle cutoff point  
set to 180 degrees?

RE: Engine Main bearing bolt yielding help

(OP)
Dimjim,

You got me wrong. When I say .37 and .77 thread length, its not the whole thread length.
It is the length of the thread left over after the thread engagement.

So the thread length for

Bolt 1 is .37+.814
Bolt 2 is .77+.814

If we set 180 as angle monitor, many of the non-yielded bolts might fail for angle. And we also not sure how much angle monitor we need to keep. So to avoid all this problems, we want to go to angle control.

RE: Engine Main bearing bolt yielding help

>>>Ultrasonics instrument asks me to enter a area of cross section for the bolt. So when I enter those numbers I get a different load factor for these two different bolts because the area of bolt cross section changes for these two bolts as the threaded length is different.<<<

Huh?

Preload,
How are you computing the cross section area that you enter into the ultrasonic instrument?

Mike Halloran
Pembroke Pines, FL, USA

RE: Engine Main bearing bolt yielding help

(OP)
Mike

Mike,

F=E*A*DL/L

A/L changes for different cross sections of the bolt.

For 1 bolt : As/Ls + At/Lt
For 2 nd bolt : Ls and Lt are different

As is area of shank
At is area of threaded portion

RE: Engine Main bearing bolt yielding help

Your effective thread length should be:

Your shank length (l1)
        +
Your exposed thread length + 1/2D (l2)  (you use only 1/2D of your thread engagement.

The stress area of the above two have to be considered independently.

y = stretch

F=   yE/L/A   L/A = l1/A1 + l2/A2

Areashank= .7854(D)2
Areathreaded poertion= .7854[D-(D-.9743/n)]2


RE: Engine Main bearing bolt yielding help

Hi preload

Firstly I am confused you stated in one post:-


"I have two 4 inch length bolts and one has a shank length  
 of 2.75 in and thread length of 1.25 inch."

"The other bolt has a shank length of 2.5 in and thread
 length of 1.5 inch."

Then in another the shank lengths changed to:-

"For bolt 1: 2 inch is the shank length and 0.77 in is the  
 threaded length in the whole grip length"
"For bolt 2: 2.4 inch is the shank length and 0.37 in is
 the threaded length in the whole grip length"

Which are the correct shank lengths?

Next question you state in a very earlier post that the tapped holes are blind and therefore for a given joint configuration you surely cannot have the same thread engagement for each bolt but you quote 0.814, depending
which bolt you use one must have more thread engagement than the other, can you please sketch the joint and upload it so we can see exactly what your trying to say.

Regarding Class 2A threads, in the threads calcs I uploaded
I assumed that it was 3A but if you substitute the pitch dia
of the 2A thread in the formula it actually reduces the torque req to overcome friction on the threads and would therefore generate more tensile load and stress in the screw
although I don't think its very significant.

The stress calculations in a given bolt or screw are based on bolt or screw stress area which is normally equivalent to the mean of the minor and pitch diameters and is therefore an assumed area for calculations.

However with a bolted joint, the components bolted together
usually have far more stiffness than the bolt itself, so when you tighten a bolt it stretches more than the components compress, if you know the relative properties of the components you can draw a force deflection diagram of the joint, examples of which can be seen at:-

http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf

Sadly you haven't been able to get the material info of the component parts so we are stuck with the bolt spec.
The different length bolts you have however can be viewed  like two different springs lets say one as a stiffness of 2lb/in and the other 4lb/in and you wish to compress them individually to give a force of 6lb in order to do this your given a screwed stud mounted on a fixed plate, in addition you have a loose plate and a nut.
If you place the 2lb/in spring over the stud and rest it on the fixed plate, then place the loose plate on top and screw the nut down on the thread till you achieve the 6lb force, at this point you will have compressed the spring by 3" :-
          3" * 2lb/in =6lb

Consquently if you do the same with the 4lb/in spring the deflection to achieve the same force is only 1.5".
What I am trying to convey is that which Mike Halloran and uncle syd stated that even if your bolts vary in stiffness and stretch differently the outcome for a given joint configuration and a target force should be the same.
In simple terms if your aim is a given preload and your bolts vary in stiffness then you have to stretch each bolt
by a different amount, so it is no surprise if you stretch
the bolts by the same amount as you have stated, you will get different loads but what is the difference that you have measured? Looking at the load factors you quote the difference between them is just under 7% does that reflect the load readings?
For us to help you further we need correct and consistant information. I find it quite amazing that a designer with at least a weeks notice of a meeting to discuss a problem joint turns up and says I'll have to go and find the design pre-load.

regards

desertfox
turns up

   

RE: Engine Main bearing bolt yielding help

(OP)
Sorry for any confusion.

Firstly, the bolt lengths are not different, length is same but the threaded length is different. Yes I agree because of the load factor difference the difference in clamp load is just under 7%.But we made sure with our supplier last friday, from now onwards we will be getting a consistent threaded length.

this is the exact dimensions
2.77 is the grip length for both bolts
"For bolt 1: 2 inch is the shank length and 0.77 in is the  
 threaded length in the whole grip length"
"For bolt 2: 2.4 inch is the shank length and 0.37 in is
 the threaded length in the whole grip length"

0.81 is the thread engaement length (as the bolt length is same, the thread engagement length will be same).

thanks for the explanation and regarding the joint material properties, the only thing I know is the joint materials are Aluminum and bolt is low alloy carbon steel. Yes Its reallyy amazing to me that he dint turn up with exact number and said "I can get you the info some time next week, but if I remeber that correctly load spec should be around 4500 lbs"

RE: Engine Main bearing bolt yielding help

When you are saying grip length are you taking into account the 1/2 D that should be added to the threaded section to account for the thread engagement?

RE: Engine Main bearing bolt yielding help

(OP)
I dont think we are adding that. I will make sure to add that.

is it 1/2D or D/2? D being the nominal diameter of the bolt.

RE: Engine Main bearing bolt yielding help

Hi preload

So if the bolts are both 4" long in total and they both have
0.81" of engagement length then the height to the underside
of the bolt head should be:-

     4"-0.81= 3.19"

according to this site I found the effective length for the
bolt in a tapped hole should be:-

                  3.19"+0.4*.375= 3.34"

http://www.co-design.co.uk/dpg/bol/bol3.html

The effective joint length according to the above site would
be:-

                    3.19"+.375= 3.565"

preload can you please confirm that this is the joint
configuration?

If you can confirm above and based on the 4500lb pre-load I can do some more calcs?

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

will explain again clearly,

Bolt length (from head to bottom) - 3.78 in(ultrasonic)
thread engagement length - 0.814 in
effective grip length - 2.77 in (we din't consider the 0.4*D of the bolt in the grip length, I know its a mistake, but I will make sure we will change the grip length value)

the above values are common for both the bolts we are using.but what different is

"For bolt 1: 2 inch is the shank length and 0.77 in is the  
 threaded length in the whole grip length"
"For bolt 2: 2.4 inch is the shank length and 0.37 in is
 the threaded length in the whole grip length"

But, In future we made sure to get only bolt 1 from our supplier.

RE: Engine Main bearing bolt yielding help

Hi preload

The original 4" length came from your earlier post
now your saying that between the bottom of bolt to underside of head is 3.78" inches long so therefore :-

2"+0.77"+0.814"= 3.58" I thought it would add to 3.78"

and 2.4"+0.77"+0.37"= 3.58" again not 3.78"

Have you made a typo error?

Lets just try to get this right firstly:-

1/ are the tapped holes blind or not?

2/ are you including the 0.814" in your effective grip
   length or not? I assume not

3/ what is the thickness of the flange the bolt passes   
   through before entering the tapped hole as it stands at  
   the moment I am subtracting 0.814 off 3.58 which leaves
   me a flange thickness of 2.766"

I know that the .77 and .37 are the tapped portions not in the region and therefore if the holes are blind I know they are in the grip length as you call it.
Might it be easier to upload a sketch of the joint so we can all see it.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

when I say 3.78 in, its not from the underside of the bolt to the bottom of the bolt.It is from the very top of the bolt to the bottom.

yes 3.58 in is from the underside of the bolt head to the bottom. which you have calcluated right i.e 2"+0.77"+0.814"= 3.58" and 2.4"+0.77"+0.37"= 3.58".

answrs for ur questions
1) yes blind hole.
2)no I am not including 0.814 in. We are just taking the length of the top joint plate as the grip/effective length, which is wrong.I have to add 0.4*D to the grip length.
3)yes u r right the flange thicknes is 2.766 which is our grip length.

RE: Engine Main bearing bolt yielding help

I have to agree with Desertfox's that a posting a a drawing or sketch would be very nice.

I'm still at a loss as to the exact form of this fastener if it's a hex head you will have two entities, one threaded section and one shank section, in your calculations and if this is a set-in stud you will have three unties to consider, two threaded sections and one shank section.

RE: Engine Main bearing bolt yielding help

Hi preload

Thanks thats cleared that up except to say that normally when you talk about a length of a bolt or screw in engineering its from the underside of the head.

unclesyd from posts earlier if my memory serves the bolts are hex flange head.

preload if we are now talking (4500lb pre-load) with the infomation I now have,I calculate that to achieve this pre-load you would need to stretch a bolt with the largest threaded length by 0.00453385" and 0.00429564" for the one with the shortist threaded length. The difference in these extensions is just over 5%.
Going back to my earlier calculations which I uploaded, I now calculate that to achieve a 4500lb pre-load you will need about 21lbs-ft of torque with an assumed K of 0.1497
which is unlubricated.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Desert Fox,
Are your stretch values off by a multiple
of 10?  .00453386 would only require a
26 degree angle to achieve that amount of
stretch?  Maybe I am off by 10 factor.

RE: Engine Main bearing bolt yielding help

Hi dimjim

I will check, but I am only considering a 455lb preload not the previous figure of 9300lb

regards

desertfox

RE: Engine Main bearing bolt yielding help

Hi dimjim

I checked my figures they seem okay I used 30*10^6 for E
modulus of elasticity.
My last post should have said 4500lb not 455lb


preload if your design department only wanted the loading in the bolts to 4500lb on assembly which means your well inside the yield stress how did these figures you quoted earlier and have records for, ever came into being:-

"I have clamp load and elongation data for both bolts (ultrasonic). I have different load factor values for both as the goemetry change.

one bolt lot has 7800lbs and other has 9300 lbs. Its 15% difference."

regards
desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

I think plant is using high torque. The only thng plant gets from engineering is torque spec. They gave a spec of 33 ft-lb target torque for this joint.

When I talked to the design guy , he said "if he remember that correctly, it is 4500lb" if he confirms the number, then the spec specified by engineering is wrong.

I am awaiting his conformation regarding the load.

RE: Engine Main bearing bolt yielding help

Desertfox,
Normally for a bolt with a length of 8 times
the diameter, the turn of the nut method
recommends 180 degree of rotation to reach
75 percent of the proof load which would be
equivalent to .03125 stretch equal to 7000
pounds clamping force.  From this I would
expect .020 stretch for 4500 pounds.
Maybe the turn of the nut method guideline
that I have from SSTC is wrong.

RE: Engine Main bearing bolt yielding help

Hi preload

Yes I am aware you're waiting confirmation and it is certainly not your fault that the company have had this problem.
Couple of points though:-

1/ Did the engineering specification specify using a
   lubricant with the 33lbs-ft. If they didn't specify
   lubricant then the joints should have been made dry.

2/ Whilst delving into this problem, I have to say, I have  
   learned a few things too regarding bolted joints which
   brings me to my next point. From what I understand with
   joints from within your industry torquing bolts upto 90%
   of the yield stress is quite common so I am a bit   
   surprised that your department was given only a torque   
   figure and not a pre-load, given the equiment you have
   to measure bolt elongation. Finally if the 4500lb figure
   is correct then your bolt is only seeing about 48% of   
   the proof load given in the sites I posted earlier:-

   http://www.almabolt.com/pages/catalog/bolts/tighteningtorque.htm
   http://www.morbark.com/Service/belttorque.pdf
   
which makes me think the 4500lb might be a bit low for your application; especially when the dry torque of 33lbs-ft quoted in the above sites results in an estimated 6975lb pre-load.

regards

desertfox
   
  
   

RE: Engine Main bearing bolt yielding help

Hi dimjim

I used the site given by unclesyd to get the equivalent lengths for the joint. ie:-

http://www.riverhawk.com/boltloading.php

However when you posted your question I just did a rough check like this:-   

             σ*L/E  = x (deflection)

where σ=stress= F/A approx  (4500*4)/(3.142*0.375^2)

             L=bolt length 3.625"

             E= modulus elasticity = 30*10^6 psi

punching this through my calc gives:-

             x= 0.0049225"

Maybe I have missed something but I can't see what, besides working in inches is alien to me, were metric in the uk.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

1)    Engg specified 33ft-lb with lube oil.But no where in the specification says about clamp load.
2)    Yes, we always go upto 95% of the proof load and sometimes go to 95% of the yield load. Everyone knows it’s a mistake but its been working like this for yrs. So they don’t want to change anything. I agree if 4500lb is the specified load then we are 50% of the proof load on a joint which is really critical.
I am learning things from you guys. Being from quality dept and moved to work on a new field of fastening, exciting yet challenging. May be in 3-4 month time I will be on my own. I started reading some introduction books by Bickford.

RE: Engine Main bearing bolt yielding help

Hi preload

Hope you enjoy your new venture.
Anyway my approach in your situation would be to get the design dept to give you the pre-load figure then abandon for the moment your torque figure and use the ultrasonic kit
to measure bolt elongation till you get the pre-load you're looking for, at the same time you can monitor torque, angle rotation etc after you get confidence in either the torque setting, angle rotation or whatever whilst using the ultrasonic measurement, after which you can dispense with the ultrasonic measuring once you achieve fairly consistant results, I suppose it will depend on the tolerance the designers give you to work with.
Alternatively you could use the same method to achieve the 90% proof load and send the info back to the design dept for approval and put the ball in their court.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Desertfox,
Thanks.  I cannot see anything wrong with the method
you used or the results. I am at a loss now to understand
the recommended turn of the nut angles.  Thanks for your patience.

RE: Engine Main bearing bolt yielding help

Hi dimjim

Thankyou, is there anywhere on here I can see your turn of nut reference?
Just a thought does the angle of rotation apply to a diameter of a thread as they will have different pitches.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Desertfox,
Structural Bolting Handbook
SSTC
Steel Structures Technology Center, Inc.
Publication No. SBH-2, 2st Edition
2nd Printing: June, 2001
ISBN 0-9707400-1-8

They reference RCSC Table 8.2.
Research Council on Structural Connections
I think I have seen the later on the internet
in the past.

RE: Engine Main bearing bolt yielding help

Hi dimjim

Yes I have seen a reference to it now and the figure quoted for extension doesn't tie up with the extension calculated the way I did it. Interesting to note though that a 1/2 turn
on a 7/8" UNC would be 0.055r" according to its pitch.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Desertfox,
I think that answers my question.  It also assumes all
parts are steel.  I guess that explains why the bolts
are failing if:

in angle monitoring we see some fasteners get to 260 deg of turn to achieve 32 ft-lb torque and some only less than 20 deg to get the same torque. I dont know why?

Thanks for the article.

RE: Engine Main bearing bolt yielding help

Hi dimjim

Thanks for your response, its the only thing a could see that might explain the difference, interesting though.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Hi preload

Some easy questions if you can answer please:-

1. what is the depth of the tapped hole and its tolerance.

2. what is the tolerance on length of the bolt from under the head

3. of the 2.766 grip length what is the tolerance on that too

regards

desertfox

RE: Engine Main bearing bolt yielding help

Dessertfox,

I too think the drilling and tapping
tolerance is suspect to have that much
difference in angle or pitch tolerance
on the bolts.  Go NoGo guages can quickly
verify this.  Chips in the threaded holes
if bottom tapped may also be a cause.

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

Grip length tolerance is +/- 0.02
Tap size tolerance is 1.12 inch +/- 0.02 and drill size tolerance is 0.16 in dia +/- 0.02
Bolt length tolerance is 3.5 inch + 0.04 and – 0.06

After seeing the numbers and if u feel there is some problem, could you please explain how these things effect the angle,clampload.....?

RE: Engine Main bearing bolt yielding help

Preload,
You have more than sufficient tap depth if .814 is the
length of depth that the threads are engaged in the
casting.  The 1.10 depth exceeds .814 by .286 inches.
You could screw in the bolts 15 full revolutions without
binding as a check for the proper depth and ensure
clearance.

The tapped hole size should be .318 to .325 diameter
before tapping for a 3/8-16UNC-2B thread which should be the minor diameter of the threads.

RE: Engine Main bearing bolt yielding help

Hi preload

I was trying to see whether the bolts were bottoming in the tapped hole this doesn't appear to happen,however I couldn't get to a thread engagement of 0.814 from your figures around 0.79 was the best and as low as 0.6?? from memory.
What is interesting though is that if your bolts in the joints were just snug ie no free play and you turn them by around 20 degrees that should generate a pre-load of about 4500lb and roughly 40degrees would the proof load.
Therefore at a rotation of 260 degrees the tensile force would be very large in comparison.
One other thing to check is that were the bolts that failed of the longer or shorter threaded lengths:- ie are the failures limited to one length of threaded bolt either longer or shorter if some can you tensile test some.


regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

I think I know why some of them take more angle (Correct me if I am wrong).

I have to make some things clear first. The 260 deg angle is just the monitor limit.
Maximum number of bolts fall in the range of 70 deg to 150 deg. But some of them touch the 260 deg mark. When the operator sees that angle, they will back off the screw and observe for any yielding. Even if they don’t observe visible signs of yielding at 260 deg we will scrap the screw. So bolts which sees 260 deg should be scrapped irrespective of yielding signs.

Our concern is if we see yielding at 260 deg, then yielding might have occured well below 260 deg, which we are saying OK because it passed the 260 deg angle monitor.

Basic point is we really don’t know when the bolt yields.

Now coming to your point, you said we should not see more than 40 deg of turn after a nice tight snug to reach proof. When u say nice snug, what would be the approx torque value?
Our process is, we snug to 3.2 lb-ft and then start monitoring torque from there. So at 3.2 lb-ft the joint is not really snug right? I think this is the reason we are seeing high angles to reach the final clamp. And also the joint is made of aluminum and bolt is carbon steel. When the operator runs the bolt down, I can visually see the Aluminum parts compressing like a soft joint.

So when we see more than 70 deg angle is because of the low snug torque. And when we see high angles, bolts are yielding. But one thing I don’t understand is why some of the bolts seeing 260 deg are not visually yielding as others?

RE: Engine Main bearing bolt yielding help

Hi preload

When I say snug I mean like finger tight but not putting any tension in the bolt which is very difficult to do and then tightening the 20 degress or so.
Now you have moved the goal posts here the parts were originally a copper-aluminium alloy and now aluminium?
Also what your seeing when the aluminium compresses is embedding of the screw which you haven't mentioned before.
Whats happening in this last case is the aluminium is yielding under the bolt head because the tensile load your putting into the bolt is to high for the alumium to withstand, it could be that that causes the high angle rotation,ie as you rotate the bolt instead of increasing tension the alumium gives way and therefore you need to keep turning the bolt to try and reach your torque figure.
Early on you said there was no damage to the parts see your quote:-

"If we are yielding the bolt , then why there is no damage to parts? are the parts stronger than the bolt?"

You need to get the pre-load from that designer and really now find the mechanical properties of the component parts.

I'll try looking into the embedding a bit further and post again.

regards

desertfox

RE: Engine Main bearing bolt yielding help

http://www.cisc.ca/files/publications/techpubs/design/bolting/HighStrengthBolting.pdf

Desertfox was kind enough to post this.  If you read page 14 to 20, it may explain the answer to many of your questions.

I would simply use the turn of the nut method and turn
the bolts to 60 to 90 degrees and forget the torque specs.
Though the clamp force remains the same beyond this, the
bolt is knecking at the threaded portion and putting greater strain on the bolts.  Almost all of the elongation is taking place thru the threaded portion which is obvious too when looking at the failed bolts that you have posted.
I also notice what appeared to be a small amount of c'bore
or c'sink in the castings before the threaded portion.  Is
this always specified and called out?

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

Thanks for explanation. Yes they are copper-Aluminum joint . When I say there is no damage to the joint, I mean there is no physical damage to the bearing surface of the joint. But I see the joint compressing (what I mean compressing is , I can see the joints pulling together when the bolt is running down, This says joint is soft right?).

If I see the joint compressing, does that mean its damaging the joint? I dint know that. I am sorry.
Regarding the load specification, I am bugging our design guy like anything..

Dimjim,

I will definetly go through the link.
Regarding the Counterbore, you are exactly rightttttt sir. Today in our meeting, we found out that our technician (who measures the grip length and do minimax studies) did not take into account the counter bore length. So there will be a change in grip length now. By how much??? I would know that soon and will post here.

Thanks a lot guys for helping.

RE: Engine Main bearing bolt yielding help

(OP)
Dimjim,

The article says

" However, for longer bolts, 1/2 turn may not be
sufficient to bring the pretension up to the desired level,
whereas for shorter bolts 1/2 turn might twist off the bolt.
Laboratory studies show that for bolts whose length is
over eight diameters but not exceeding 12 diameters, 2/3
turn of the nut is required for a satisfactory installation."

Length of my bolt is over 9 diameters, so I would fall under the 2/3 turn of the nut category. i.e is 240 deg of turn for satisfactory installation. And also he says this rule is acceptable only for steel joints. My joints are little soft Al-Cu alloy parts. So I need more turn.

If I go by the article, then I would end up again turning the nut over 240 deg and yielding the bolt. Am I missing something?

And also how to determine the snug torque? Is there any procedure or just an estimate?

I just got the counterbore difference on clamp load from my tech.The clamp load difference between the old block dimension and the new counter bore block id about 7% less.

RE: Engine Main bearing bolt yielding help

Quote (preload):

I can see the joints pulling together when the bolt is running down, This says joint is soft right?

Maybe.  As I said on 12 Apr 07 12:38, "soft" is a misnomer.  If your parts are deflecting elastically, that can be considered a soft joint.

Quote (preload):

If I see the joint compressing, does that mean its damaging the joint?

Again, maybe.  If the compression is all elastic, then you aren't damaging them.  If you are indenting under the screw or nut, or deforming the the entire volume of the joint members between the fasteners, then yes you would be damaging the parts.

Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Engine Main bearing bolt yielding help

hi preload,

If your joint is embedding, then it would mean that the material under the head is yielding. This will affect the preload, ie as the head slowly sinks into the joint material you will lose tensile load.
This can also occur in the threaded portion of the joint.
As I said earlier, this might explain the excessive angle you are having to turn some of the bolts through.
It seems to me the bottom line is that the preload for the joint is too high. Without actually seeing the joint it's hard for me to say whether you're damaging the material on the surface or not. I would have thought that there would have been a slight witness mark on the surface of the material, which should be visible to the naked eye or possibly with a magnifying glass.
I think the bolts are failing because you are exceeding the tensile stress in the bolt whilst trying to tighten it.
This needs to be reviewed by the design team. The only way forward now is to reduce the preload preferably by reducing the torque setting or by increasing the friction in the joint.
Either way your design office need to give you a preload for that joint which is suitable for its service conditions.
The best way, once they have given you a preload, is to monitor bolt elongation, torque and possibly angular movement of the wrench.
Once you have done a few and built up some confidence you can drop the bolt elongation measurement and then use either torque measurement or angular measurement depending on what tolerance the design office give you for the design preload.

Regards,
desertfox

RE: Engine Main bearing bolt yielding help

(OP)

"""The best way, once they have given you a preload, is to monitor bolt elongation, torque and possibly angular movement of the wrench.
Once you have done a few and built up some confidence you can drop the bolt elongation measurement and then use either torque measurement or angular measurement depending on what tolerance the design office give you for the design preload.""""

Desertfox, This sounds like a great idea. Will let you guys know the specified load as soon as I get the info.

Cory, Thanks for the help.

RE: Engine Main bearing bolt yielding help

The article is for high strength bolts used in structural
components where it is typical to take the bolts to the
yield levels into the elastic range.  These are basically static load applications like bridges etc.  

Most mechanical bolt applications will see other external loads being applied to the joint and require that only 75 percent of the proof load be the general guide for the joint assuming you might have a plus or minus 25 percent
error in your tightening method.  If you have only a plus or minus 10 percent error in your tightening method, you can use 90 percent of the proof load as the tightening spec.  Because you are not using steel on steel conditions the angles must be determined by tests to understand the deflection differentials that are taking place in the aluminum parts vs steel parts.

The only reason that I referenced the article was to show you that the upper proof load was reached and the clamping force remained relatively constant for greater degrees of rotation until the bolts finally broke.

I will have to review the c'bored holes to see if you have
enough support material around the holes at the interfaces to ensure proper contanct between the castings.
The dscf1648zi9.jpg drawing looks really suspect as to whether you have enough material around the interfaces where the castings meet?  Is there non supported material
as pictured?  If so, I can see you would have a very hard time analysing the 4 bolt application.

RE: Engine Main bearing bolt yielding help

(OP)
""""
I will have to review the c'bored holes to see if you have
enough support material around the holes at the interfaces to ensure proper contanct between the castings.
The dscf1648zi9.jpg drawing looks really suspect as to whether you have enough material around the interfaces where the castings meet?  Is there non supported material
as pictured?  If so, I can see you would have a very hard time analysing the 4 bolt application.""""

Dimjim,

I am sorry, I didn't quite understand the above lines. Could you please explain that again.The picture u quoted is the crankcase and the bolt head would sit on those 6 holes u seeing in the picture. I can get you more pics in different angles if you want.

Thanks

RE: Engine Main bearing bolt yielding help

It may have been the angle of the picture of the
4 bolt casting but the c'bored holes looked
strange like they were not flush or parallel
with the top surface of the casting.

RE: Engine Main bearing bolt yielding help

Preload,
It finally sunk in that those holes are not threaded
and simply clearance holes for the bolts.  Sorry about
that.  I assumed the heads were on the opposite side
and the bolts were coming thru those openings and
imposing a moment load on the bolts.
dscf1648zi9.jpg

RE: Engine Main bearing bolt yielding help

My appologies beforehand if I missed something but are there washers under the bolt heads?  

RE: Engine Main bearing bolt yielding help

(OP)
No washers. But the bolt head is a flange head.

RE: Engine Main bearing bolt yielding help

Hi preload

Any more info yet?

regards

desertfox

RE: Engine Main bearing bolt yielding help

Preload,
The photo that I am talking about does not have
6 holes in it.  It has 4 recessed holes in it.

RE: Engine Main bearing bolt yielding help

(OP)
General question.

Bolt 1 is a hex flange head and also indented head
Bolt 2 is a hex washer head and no indentation

everything else is same (plating,threads,size of bolt.....)

My question is,

if we use both these bolts in same application, will there be a significant change? if yes, why is that?

I feel indedntation shouldn't make a difference, but flange or washer head should make a difference, but is that difference real significant? and which is better choice? and why?


RE: Engine Main bearing bolt yielding help

The effects of an indented head are reduced fastener mass and increased fastener compliance (very small effect).  These have no real influence on joint behavior.

Your description of flange head and washer head are not sufficient to provide conclusive answers.  The head height and outer diameter influence fastener compliance, fastener mass (and cost), bearing friction torque, and allowable surface pressure.  If the washer head has a larger outer diameter than the flange head, then it will also generate a large bearing friction torque (less preload) and a lower surface pressure (less indentation of clamped parts).  The design engineer must balance these characteristics with the joint requirements.

Regards,

Cory

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RE: Engine Main bearing bolt yielding help

(OP)

Cory,

Flat washer head has same outer diameter as flange head bolt and the head height of the flange head bolt is lil bit more than the flat head bolt. I just measured them.

So now that the outer diameters are same, what is the flange head bolt effect vs flat washer head bolt effect on a joint.

RE: Engine Main bearing bolt yielding help

Is it a washer head or a bolt plus washer?

If it is a washer head, then there is no difference.

If it is a bolt plus washer, then the advantage is that you are not rotating the fastener against the clamped parts, which reduces coating damage (if applicable) and applied shear stress.

Regards,

Cory

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RE: Engine Main bearing bolt yielding help

Hi preload

According to the site I found for flat washer hex head screws the flat dia is 0.72- 0.78" compared with the flanged
hex head screw which is 0.81".
As CoryPad stated the larger flat face would reduce pressure
on the joint surface which leads to less pre-load lost to embedding but increases the frictional torque required to tighten the fastener. So if you take the proof load of 9300lbs and divide them by the area under the head it will
give you an idea of the pressure variation under the head for a given tensile load:-

         9300/(3.142*(0.72^2- .375^2)/4)= 31344.4 lb/in^2

         9300/(3.142*(0.81^2-.375^2)/4) = 22971.3 lb/in^2

http://www.smithfast.com/hexheadms.html
http://www.almabolt.com/pages/catalog/bolts/flanged.htm

Don't know what the tolerance is on the flanged head but the above illustrates possible pressures under screw head which according to the above is 27%.

typical wrought aluminium alloy for cylinder heads is
2018-T6 which has a yield stress of 43ksi which is higher
than the above stress values.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Cory,

Yes it is a washer head not a washer plus bolt.Thanks for the explanation

Desertfox,

Thanks for the calcluations. I measured the washer dia of both bolts and they are almost same. (0.74 and .75 in)

RE: Engine Main bearing bolt yielding help

hi preload

Then there should be no difference.

regards

desertfox

RE: Engine Main bearing bolt yielding help

Desertfox,
The minor diameter should not be .375 but should be
the hole size thru the casting.  The 9300 pound clamping load is probably exceeded when they are turning the bolts greater than 90 degrees.  The bearing stresses under the
heads must be exceeding the yield limit of the castings.

RE: Engine Main bearing bolt yielding help

Hi dimjim

Fair point you make about the hole in the casting; although it won't change the variation between them it does change the actual value of the stress.
To reach 43ksi according to me the hole for the screw would need to be 0.4929" dia based on a .72" flat washer hex head.
Preload what is the hole clearence size in the component? and we touched on embedding previously have you found any marks on the surface of the component under the screw head?

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

the dia of the hole is 0.4375" and regarding maks on the underhead surface, yes, but it dosent look like a damage though.

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

Finally I got the clampload specification.

6000 lbs - 8000 lbs and 7000 lbs being the target clamp load. we decided to do torque-angle signature. we also decided (not full confirmed though) not to use any lube oil (just the dry bolts with cad plating). will be doing some tests with dry bolts and get the torque angle graphs and then will go from there. will let you guys know how it goes

RE: Engine Main bearing bolt yielding help

Hi preload

Thanks for the update

RE: Engine Main bearing bolt yielding help

Hi preload

You might find without lube your clamping load spread will
be greater, and the method your using might not achieve
6000-8000lb clamp force i think the method you are going to use is about +/- 15%.

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
Desertfox,

I thought torque-angle control is independent of friction. The how lube or no lube matters?

And also we have some room on the higher limit as 9300lb is the min proof load and max spec is 8000 lbs.

RE: Engine Main bearing bolt yielding help

Hi preload

We mentioned using ultrasonics to measure bolt elongation
which is very accurate compared to the method your using.
I suggested earlier that you make some joints dry to see if the bolt failure stopped.
If you don't use lube you need a higher torque setting to overcome increased friction, using lube reduces the torque required to achieve clamp load, have a look at some of the earlier posts.
I agree now you have a clamping load you can now clarify a method to achieve that and that if you aim for 7000lbs with lube then yes you can reduce your torque.

regards

desertfox

regards

desertfox

RE: Engine Main bearing bolt yielding help

(OP)
yup I agree, we will play with torque for sure.
anyway I will et you know how consistent things show up after we do some experiments with dry bolts. and if we are consistently holding the clamp spec then we are good to go.

RE: Engine Main bearing bolt yielding help

Does this thread get the record for most posts ? Who said fasteners were simple ?

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