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Mechanics Behind Fasteners
7

Mechanics Behind Fasteners

Mechanics Behind Fasteners

(OP)
After reading a brief description on fasteners, their nature of loosening etc etc, its incredible the volume of science behind the simple bolt and nut. I have a few questions regarding this.

On the topic of bolt/nut loosening (of which is a problem on our machinery), I asked myself why cannot castellated nuts together with cotter pins be used wherever possible. After reading up, this configuration is to only be used in "low torque" applications. I ask, why low torque only?

Secondly, I do realize that a big problem is lining up the hole in the bolt so that the cotter pin slips in. But surely this can be corrected by incrementally adding/removing shim washers until a perfect fit is found. However I hear doing this compromises bolt strength as it creates a "weak point" due to numerous shims. TBH, im more concerned about bolt loosening than strength reasons. Also the bolt may stretch in its life or shims compress thus the torque tightness will drop off after time.

Ultimately, how true are the above points and the reasons behind it. I guess castellated nuts cant be a solution or else we would see them ubiquitously on truck hubs, all fasteners etc...

RE: Mechanics Behind Fasteners

I think you touched on the key reason when you said the cotter pin slips in.  'Slips in' implies clearance.  Clearance which will allow the nut to loosen if the preload is not sufficient to prevent loosening.  The cotter pin then only prevents losing the nut after it is loose.

Ted

RE: Mechanics Behind Fasteners

2
Cotter pins should only be used for zero/very low prestress conditions, as should Loctite.

For high preload conditions where there is (and must be)a solid metal-to-metal contact with rigid members, the preload must be higher than any service tensile stresses.

Think of an engine connecting rod bolt. Severe alternating stresses on the rod itself, but almost nothing on a well-torqued bolt.

Alternating/varying stresses are required for starting fatigue cracks, and the number one cause of such cracking in fasteners is insufficient or loss of preload.  Bolt head/nut imbedment can cause loss of preload, and such things as Loctite do not prevent it.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein  

RE: Mechanics Behind Fasteners

Everything said is true... But main reason so much study has been made on this subject - SPEED and ECONOMY.  When GM installs about a billion bolts a month ---- try putting cotter pins on all those!!  Or for that matter a NASCAR racing tire where 2/100s of a second can win or lose a race.

Most installations using cotter pins are typically what my dear old Dad used to call a "jezuz" bolt.  If it let go - you said "Jesus" because he was the next person you were probably going to meet.  Typically, the bolt(s) that hold the helicopter rotor in place or the prop on your airplane.

RE: Mechanics Behind Fasteners

Quote
""Most installations using cotter pins are typically what my dear old Dad used to call a "jezuz" bolt.  If it let go - you said "Jesus" because he was the next person you were probably going to meet.  Typically, the bolt(s) that hold the helicopter rotor in place or the prop on your airplane.""

  Critical installations like this  on aircraft are typically not cotter pinned, they are safety wired. The bolts are properly torqued, then a twisted wire is inserted in drilled holes in the head of the bolt and led to the next bolt or an anchor plate. The sole purpose of the wire is to prevent the bolt backing out if it does lose its clamping force.
B.E.

RE: Mechanics Behind Fasteners

Berkshire - true but the idea is still the a same....

RE: Mechanics Behind Fasteners

   I can see another issue with cotter pins.

   If I install a castle nut and drilled bolt and I tighten them to the maximum allowed torque, I must back them off anywhere up to 120° so that I can insert the cotter pin.  An alternate model of this would be that I would torque them down approximately where I wanted them, then adjust the nut ±60°

   On shorter bolts, a 60° turn is enough to unclamp the bolt.

               JHG

RE: Mechanics Behind Fasteners

drawoh,

Most castellated nuts have cotter pin openings every 60°.
You don't torque to the Max.Torque.
You torque to the Min., then tighten to the next opening without going over Max. torque.

Rerig
 

RE: Mechanics Behind Fasteners

regig,

   Oops.  My bad.  I should have said 60° and ±30°.

   This is still a lot of tension.  On a 1/4-20UNCx1 screw, a 60° turn works out to .008in extension, and a strain of .008.  If the screw has an elastic modulus of 29,000,000lb/in2, the resulting stress is 232,000lb/in2.

   Compression in the joint will reduce this a bit.

               JHG

RE: Mechanics Behind Fasteners

Pretty high strength screw there!  Even an SAE Grade 8 bolt will go past yield ~130ksi, which is an elastic strain of ~4 mils/in.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein  

RE: Mechanics Behind Fasteners

Metalguy,

   One hopes it will be a strength screw. smile
 

               JHG

RE: Mechanics Behind Fasteners

The company I work for has invested thousands of dollars per machine in Nord-lock style washers and have seen the return on this investment.  We have found it works better than anything else when it comes to keeping a properly torqued nut/bolt in place including chemical, spring washer, and wire tieing.

They are expensive (on the order of magnitude of half the cost of a bolt) and must be replaced every time you remove/reinstall the bolt but they are well worth it.

And make sure your maintenance people are using the torque wrench every time they do something.  We find our biggest fasterner issues now are with under/over tightened bolts.  
 

 

RE: Mechanics Behind Fasteners

Unfortunately, torque can be a very poor indicator or the all-important preload on a fastener for rigid connections, and critical applications should not/do not use them.  Very large preload errors are frequently experienced when using them.

Something as simple as the turn-of-the-nut method can be more accurate, and bolt stretch measured directly or via ultrasonics are far superior.  Many engine builders have given up on torquing con rod bolts and use direct measurement of stretch.

Loctite, etc. and the various lockwashers/trick nuts have their place for non-rigid connections, but for true rigid connections they can lead to fatigue fractures if the preload is insufficient.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein  

RE: Mechanics Behind Fasteners

Metalguy,

I can agree with most everything you say, but how does Loctite make a bolted connection more prone to fatigue?

RE: Mechanics Behind Fasteners

Loctite etc. doesn't actually cause fatigue cracking, but if the preload is lost via bolt head/nut embedment, the bolt/nut doesn't back out/fall off.  In a multibolt assembly where a loose piece isn't a problem by itself, the missing bolt can be observed, thus alerting people of a problem.

We had an embedment problem with some 1.5" main steam support bolting, and the heavy nuts were falling down ~20 feet.  Without telling engineering (me), maintenance decided to fix it themselves.

Too hot for any Loctite, they WELDED the back side of the nuts to the bolt--ultra high-temp "Loctite".  For a while nothing happened, then a nut fell down WITH a piece of the bolt inside!  Happened twice before I found out--classic fatigue cracking right across the bolt.

Sure glad one of those things didn't hit anyone in their hard-hat!

Moral:  It's better to know you're losing preload when you are, and things like Loctite can hide what's happening.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein  

RE: Mechanics Behind Fasteners

Fair 'nuff, good story, glad I asked.

RE: Mechanics Behind Fasteners

One consideration might be to use one of the Bellville washer designs.  These have been successful in many petrochem applications to avoid loosened nuts.

RE: Mechanics Behind Fasteners

I just recently researched 'thread locking' for a recent failure analysis I did.  Loctite (I use the term generically) is apparently an accepted form of locking in environments where vibration might loosen the bolt.  HOWEVER, it is no substitute for correct preloading of the bolt.  

Too tight, and more commonly too loose, are by far the most common causes of fastener failure.   

RE: Mechanics Behind Fasteners

Can you share details of the joints whose bolts are "loosening?" Pictures of failed joint surfaces might be interesting. Details about materials and operating temperatures are important too. If the clamp load is simply inadequate for the applied load, or other features conspire to allow embedment, micromotion of the joint, then fretting and  and loss of preload will occur, even if the nut is not allowed to "back off" at all.

I'd be surprised if anything but carefully controlled tightening against gorgeous mating surfaces is used to retain main bearing or con rod caps on Indy or Formula cars.  Is your application that severe?  

RE: Mechanics Behind Fasteners

How about PAL nuts? Thin, stamped, sheetmetal nuts installed on top of a torqued up nut.

What's the science behind them?

RE: Mechanics Behind Fasteners

When dealing with nuts and bolts, it's all about maintaining the all-important preload that the engineer had dutifully used to calculate the joint separation margin, etc.
Primary bolt retention is always friction, but in any vibratory environment a secondary retention feature is recommended. Multiple methods exist and each has its benefits and drawbacks.

Thread locker (glue) is a pretty good animal, but makes it impossible to go back and check the torques on bolts after assembly.

Locking nuts and bolts are available in many varieties, and work well, but tend to wear out if reused.

Mixed reviews on locking washers; some folks say they work, some say they don't do a thing.

Safety wire and cotter pins do little to maintain preload, but ensure that the bolt will never fall out.   

RE: Mechanics Behind Fasteners

The thing that is obvious when you know it and difficult to intuit on your own is that preload can be lost without the nut/bolt ever moving.

When I was a fresh young engineer and the world was still full of unicorns and rainbow's and kittens, I was under the impression that if a bolt is being held in place (like castle nuts, pal nuts, lock-tite, wire ties, nord-lock wasthers, cotter pins, however you please) then it will never loosen up.  I mean how could it.

But the earth shakes, machines vibrate, joints heat up and cool down, materials are not perfectly plastic, parts live and breathe, and all these things can induce creep into threads.

RE: Mechanics Behind Fasteners

(OP)
Okay. I see many, including Tmoose, want to know the real deal behind my scenario.

When I say "machinery", I mean underground mining machines. So yes, the environment is *that* severe. This mining machine has

a standard Berco undercarraige system. i.e. It has the tension wheel, tension cylinder, track set, and finally, "dolly

wheels/or road wheels". In either case, refer to my photograph attachment depicting the seven "road wheels" at the bottom of

the Dozer.

The problem is that the entire machine is undamped so we are talking about a 4 ton machine being tossed and turned like a toy

with the bottom road wheels (more specifically the front road wheel) taking the full impact. Before you know it, at best the

first wheel breaks off at 400 hours and at worst breaks off at as little as 100 hours of operation. The cost of one a piece

is pretty hefty. And that is the problem. Before I move onto my solutions, let me say that by me implementing a solution to

this would be a massive achievement in my junior engineering career. It really is a challenge, and its relevant to my

studies.

Nevertheless, onto my thinking of how to go about this... Any criticism you have, please dont be afraid to hammer home some

points, I will love any help.


I see many problems:

1) Bearing surface embedment. Being in the mines, it is inevatible that dust grit or flakes of paint (from new road wheel)

will be ever present between the mate. So that when you tighten the bolts, the joint will "relax" quickly after that? Apart

from the obvious solution to clean and make sure no grit is on bearing surface, I see no other way around this. And just by

the way, Im not too sure as what literature means by which mating surface. They only seem to mention bolt head- part

interface. What about the part 1, part 2 interface? And the nut-part interface? I will provide a sketch if anybody requires

to understand.

2) Undertorquing. Using simple spanners, how can the technician "know" what is the right tightness. If they undertorque, the

preload will be low and the bolt may loosen quicker. Yes, I know I know, even without the bolt loosening, the preload may

drop but ill get to that now.

3) Overtorquing resulting in tapped hole deformation/stripping. They may at times use a pipe extension and tighten so much

that the thread on the machine strips. By the way, the road wheel bolts are bolted into a tapped hole and therefore do not

have a nut on the other side. If the thread is damaged (deformed or stripped), it may loosen quicker, or it may deform and

thus loosen pre-load without ever loosening.

4) Overtorquing resulting in bolt necking. In a desperate attempt to keep it as tight as possible, the bolt may neck. To be

honest I have not seen or heard of this. I even wonder if this is a possibility because the bolts used are 10.9, and the

thread into which they bolt into (directly into the machine) is mild steel. So the thread will always be the weaker point and

the thread will strip due to overtightening before the bolt necks?

5) My worst fear is the invetable and what maybe gadkinsj was pointing out. The tapped hole into which the bolt goes into,

simply gets deformed over time no matter how the hole is torqued. Due to the severe impacts, the hole plastically deforms

each time. Its mild steel afterall. This worst nightmare of mine may be true. Upon inspection of undercarraige with road

wheels broken off, the holes looked simply terrible. The steel almost looked like chewing gum on the surface. If this is

inevatable, I can surrely to some degree mitigate this?

Ultimately the bolts break due to loss of preload which is a combination of bolt loosening and surrounding deformation.

One of my solutions, and correct me here, is to use a torque wrench and adjust the bolt to a value *JUST* high enough before

the bolt necks OR before the surrounding thread strips or deforms. Like I mentioned before its one of these and most prob the

thread because its mild steel. But what value to use? Well... solely judging on the bolt (M12 and 10.9) the literature says

that a guideline value of 129 N.m ("dry thread") is to be used considering failure to shear and tension combined with a 90%

of the actual yield value (safety factor... if you will). But my problem with this is that I cannot only consider the bolt yeild strength. What about the thread which should give way first??? Infact the bolt science website (very comprehensive by the way, I recommend this site to all of you... its simply biblical in its breakdown of bolt mechanics) for some reason never

mentions how to take thread strength into account. If I can know what the thread can stand, then I know the limiting factor we can then tighten it to that limiting factor. Further, if it is indeed inevitable that even this value loses pre-load after many hours (initial pre-load loss due to embedment), we can pick it back up to that value after, I dont know, say, 50 hours.

It is at this very point that I had a heated debate with one of my colleagues. It was the typical "young engineer" (me)

versus the "old veteran" (colleague) of the company.
I argued my torque wrench idea that it may have *some* help with this and he flat out rejected it. His argument is that there

are too many variables. By this he consistently pointed to the fact that "we cannot know the value" "too much dirt" "what

about deformation over time?" etc etc. Im sure he has a point to a degree. But I find it hard to accept prefering the use of

a spanner over a torque wrench. Atleast with the wrench , you can consistently hit *a* value and not guess. Yes I understand

that thread fricion has play here and grit etc but this can be put under certain control. The colleague mentions that "with

nordlock washers, using a clean thread/surface, using loctite and tightening the hell out of the bolt, the road wheel WILL

NOT COME LOOSE". I cannot beleive this as im sure thats been done and never worked therefore I am to conduct this procedure

and see if it holds out. Ultimately, how true are points 1 through 5? And how does my line of reasoning hold? as you see, its very interesting and stimulating topic smile

 

RE: Mechanics Behind Fasteners

(OP)
Hmmm, so much white space in that post, I assure you I never hit "enter" so many times, so excuse the gaps. Unfortunately I cannot find the edit button. Old school forum layout uh?

RE: Mechanics Behind Fasteners

There are too many unknowns in any given bolt-nut-lube combination to calculate it empirically.  Get a Skidmore-Wilhelm tension device and 'ask the bolt' what torque is required for a proper preload.  Try about 80% of specified min. yield.  This procedure is well documented in RCSC's A325/A490 spec, and is available on-line, for free.

"Too much dirt" is a pitiful excuse.  The mechanics should be rinsing out the tapped holes with brake cleaner or other clean-drying solvent.  If they are gritty after rinse, use a bristle brush to clean the threads.  Soap & water will work fine.

Buy the mechanics a hydraulic or air-over-hydraulic powered torque wrench.  Easier to use than a spanner with pipe 'cheater', so they will be inclined to use it.  THEN, somebody from Eng/QC needs to go witness the torquing.  It is silly to expect a worker to perform a hard, exacting task correctly and without shortcuts when nobody considers it important to go witness & inspect the work.

RE: Mechanics Behind Fasteners

Did I miss the attachment?

RE: Mechanics Behind Fasteners

http://www.berco.com/wps/wcm/connect/bercositelibrary/it/home

Is this the manufacturer of the under carriage?  I would give them a call.  Are the depth of thread penetration at least 16mm deep?  There are calculations for designing or ensuring proper thread depths for different materials.  I think I would try using 85 percent of proof load to see if you can get better life.  100 hours to failure sounds like an underdesigned parts or extreme abuse of equipment. Are you certain that the bolts are certified and not Chinese copies?

RE: Mechanics Behind Fasteners

BERCO Deutschland - Process technologyBERCO, manufacturers of undercarriage components for chain driven earth moving machines and member of the ThyssenKrupp AG Group, have invested more than 400 million Euros over the past years in modern process technologies - which has also allowed enhanced production of mining components, products subject to extreme demands.

BERCO operate a total of seven fully automatic production lines in the four Northen Itally plants.

Why not give them a call?
http://www.berco.co.uk/pages/aunder.html
They did have a dealer here in the States and I think it was in the Carolinas.  Maybe it was only replacement parts.

RE: Mechanics Behind Fasteners

How many hours did the factory built machine survive before the first "loosening" occurred?  Was it after some field maintenance was done?

I expect a professional, conservative design would have plenty of thread engagement (over two bolt diameters) and grip length (5 or more diameters), and generous joint footprint".  IF the mating faces between bolted components are (still) flat, AND the fasteners bear on hardened washers AND the threades don't bottom in the tapped holes AND sufficient torque is applied to achieve clamping greater than the service loads, then the joint will be dead reliable without fancy ramped washers or desperate measures like safety wire, cotter pins, jam nuts, or tack welded nuts. If any of the four prerequisites are missing then the joint will loosen and cascading damage will occur.  

++ How many diameters of engagement do the failing fasteners have? Thread engagement is often described relative to bolt diameter.  A 14 mm tapped hole that engages a 28 mm long section of the bolt's thread has "two diameters" of engagement.  Generally 1.5 diameters engagement in mild steel allows the full strength of a Grade 8 fastener to be utilized (full torque, etc,  and thus clamping).  The torque applied to the 7/16" main bearing cap bolts of Many early Chevy V8 engines was over 60 lb-ft into cast iron. Main cap loosening is pretty much unheard of in any service.

Deformed tapped holes suggests significant damage to the mating faces of the components that are supposed to be clamped together. Plus the threads need to be restored with an insert of some kind. "High spots" adjacent to the wallowed out deformed tapped holes will prevent proper clamping and support.  Here is a picture of a VW crankshaft damaged when the flywheel worked loose. 4 hardened dowels were no match for the alternating forces the crank and flywheel endure.  A typical cause is the large bolt (called a nut) that secures the flywheel to the crank was not tightened correctly during a clutch replacement. A proper repair requires refacing the end of the crank and mating flywheel surface flat and smooth.

I'd say if the mating faces are not flat and smooth, and proper torque, etc is applied, your next assembly is doomed to failure by loosening.

RE: Mechanics Behind Fasteners

(OP)

Quote:

There are too many unknowns in any given bolt-nut-lube combination to calculate it empirically.  Get a Skidmore-Wilhelm tension device and 'ask the bolt' what torque is required for a proper preload.  Try about 80% of specified min. yield.  This procedure is well documented in RCSC's A325/A490 spec, and is available on-line, for free.

I know what you mean, but before I can even consider tightening that bolt to *its* maximum, what about the chassis maximum strangth? Last time I tightened a 10.9 bolt on another tap (not the ones in question), I felt the threads on the chassis stripping (it started to get easier to tighten the more I tightened). So first I need to know can the hole take it?


Quote:

"Too much dirt" is a pitiful excuse.  The mechanics should be rinsing out the tapped holes with brake cleaner or other clean-drying solvent.  If they are gritty after rinse, use a bristle brush to clean the threads.  Soap & water will work fine.

Thanks for this. We will definitely make cleaning the threads a bigger priority.

Quote:

Buy the mechanics a hydraulic or air-over-hydraulic powered torque wrench.  Easier to use than a spanner with pipe 'cheater', so they will be inclined to use it.  THEN, somebody from Eng/QC needs to go witness the torquing.  It is silly to expect a worker to perform a hard, exacting task correctly and without shortcuts when nobody considers it important to go witness & inspect the work.

You mean the guys on site torque it to some low set value and then used a torque wrench once QC comes by?


Quote:

Did I miss the attachment?
No you never, I forgot to attach it. See attachment on this post.

Quote:

http://www.berco.com/wps/wcm/connect/bercositelibrary/it/home

Is this the manufacturer of the under carriage?  I would give them a call.  Are the depth of thread penetration at least 16mm deep?  There are calculations for designing or ensuring proper thread depths for different materials.  I think I would try using 85 percent of proof load to see if you can get better life.  100 hours to failure sounds like an underdesigned parts or extreme abuse of equipment. Are you certain that the bolts are certified and not Chinese copies?

Yes thats them. And I would love for you to forward me to these calculations of depth penetration so I can check if its in range. Like I said I cant proof the bolt yet because I dont know if the chassis/tapped hole can handle what the bolt can handle. Like I said, the thread might strip. How do I calculate for that? What I know is that material is "mild Steel". and thats that. So how much can a mild steel thread take? And these bolts are most surely the real deal. BTW, how would I check if they were poor quality? I hear, examining the nature of the crack along the bolt reveals a story.

Quote:

How many hours did the factory built machine survive before the first "loosening" occurred?  Was it after some field maintenance was done?

HA! Why you ask? The one technician complained that after the first new road wheel is lost, after that, it NEVER performs as well after new ones. Secondary and tertiary replacement wheels etc etc always break off quicker! Apparently after the machine rolls out the workshop all new, they do NOT torque it to specific values, so its not the torquing thats an issue. It surely can only mean that the thread on the chassis gets deformed more and more after time. But how do I minimise this?

Quote:

I expect a professional, conservative design would have plenty of thread engagement (over two bolt diameters) and grip length (5 or more diameters), and generous joint footprint".  IF the mating faces between bolted components are (still) flat, AND the fasteners bear on hardened washers AND the threades don't bottom in the tapped holes AND sufficient torque is applied to achieve clamping greater than the service loads, then the joint will be dead reliable without fancy ramped washers or desperate measures like safety wire, cotter pins, jam nuts, or tack welded nuts. If any of the four prerequisites are missing then the joint will loosen and cascading damage will occur.  

Now we are talking. I will measure this as soon as possible and let you know. But roughly, the thread engagemnt is 2 bolt diameters. I will get back on this. Any calcs or standards on this by the way? What do you mean by "grip length"? I think what you mean is over the yellow portion of my attachment? If thats the case its about 2 bolt diameters aswell (but not sure what you mean. The joint footprint is something I cant change. Thats a function of the road wheel. Look at my attachent where I outlined in red. Thats the footprint. Although it seems like a lot, on the edges, around the bolt, its really bad. Very thin areas around there. while were at it, make it hard interface and clean for no embedment. Bearing on hardened washers im not too sure about, that depends on what the nord locks are. You mention torquing correctly but once again, like I said in the replies above, what can the mild steel chassis thread take? I cannot find a way to determine this.

Quote:

++ How many diameters of engagement do the failing fasteners have? Thread engagement is often described relative to bolt diameter.  A 14 mm tapped hole that engages a 28 mm long section of the bolt's thread has "two diameters" of engagement.  Generally 1.5 diameters engagement in mild steel allows the full strength of a Grade 8 fastener to be utilized (full torque, etc,  and thus clamping).

As above, ill check this value out, but it looks roughly like 2.
You said 1.5 d's allows grade 8 to be used. Any where I can see how you got this? big smile Seems very "out of the blue". You see thats what I mean, I dont know the calcs or if its too complex to calc then what standards to follow? Any literature? Also when I mention alot how "can the thread take it". By that I mean, overtightening the bolt resulting in hole tapping to strip so I dont see how this can be a funcion of how long the thread engagement is.

RE: Mechanics Behind Fasteners

Tsiolkovsky said - "You said 1.5 d's allows grade 8 to be used. Any where I can see how you got this?  Seems very "out of the blue". You see thats what I mean, I dont know the calcs or if its too complex to calc then what standards to follow? Any literature?"

=================

Not quite "out of the blue". More like, consistently found when ambling aimlessly all alone through the forest sniffing and foraging for scraps of knowledge under rocks and logs.

For charts of what UNBRAKO felt in 1996 were reasonable tapped hole thread strength to consider for a quality socket head capscrew into brass, aluminum, cast iron and mild steel, look at page 66/68 here -
http://www.dalecompany.com/pdf/UNBRAKO.pdf

Note their comment on page 64/66 about using figgerin' and cypherin' to come up with tapped hole thread strength being "not entirely satisfactory".  Thus what they chose to present in the technical section of their "engieering guide" were tables created using empirical data from tests done with non-digital threaded test pieces, real bolts, and torque wrenches of some type.  Seems like something I could even do at home if I put my mind to it.

For an online discussion with a rule-of-thumb allegedly from a machine design handbook, look 2/3 of the way down here-
http://store.curiousinventor.com/guides/Metal_Working/Screws/

Attached is an image of a mossy rock, I mean, the worn cover and a pertinent page from a hard cover machine design handbook I bought new from the Lowell Tech (now UMass Lowell) bookstore for a mech engineering class I was taking nights.  I paid the then shocking price of almost ~20 US Bucks for it.  Guess the copyright date.

26th Edition of Machinery's Handbook has some formulas for calculating such stuff, too.
 

RE: Mechanics Behind Fasteners

"HA! Why you ask? The one technician complained that after the first new road wheel is lost, after that, it NEVER performs as well after new ones. Secondary and tertiary replacement wheels etc etc always break off quicker! Apparently after the machine rolls out the workshop all new, they do NOT torque it to specific values, so its not the torquing thats an issue. It surely can only mean that the thread on the chassis gets deformed more and more after time. But how do I minimise this?"

=========================================

My suspicion is after the first failure the mating surfaces of the cap and frame are beat up deformed and nasty, and the threaded holes are all wallowed out. Timidly tightening the bolts to clamp the uneven surfaces together means the impacts quickly buff down the high spots, leaving the bolts loose and subject to motion and bending, doing even greater damage to themselves and the badly damaged frame and bearing housings.

If the original bolts were torqued tight enough, then they never would have loosened.

Is the contact surface of the bearing housing really a raised area around the perimeter, NOT a full contact flat surface?

This is what I think it should look like.  Big, flat and smooth, but without the dowels which serve only to locate it, not to handle loads.
http://glandpackings.com/image/main_bearing_oil_sal/LowerBearingCapSealIn.gif

I'm confused about

RE: Mechanics Behind Fasteners

what I meant to say is, I'm confused about the comment "Apparently after the machine rolls out the workshop all new, they do NOT torque it to specific values, so its not the torquing thats an issue."

What is the event the triggers the first field repair?  If it is the wheels "breaking off" then I'd work to understand the suitability of factory assembly method and the design.  Certainly some operators can break almost anything, but I don't think wheels routinely break off Caterpillar and Case off road equipment.

Until REAL info is available  might consider alternatives for a about 30 seconds. I REALLY think No official "torque" by the OEM  is unlikely, but I wonder if that could be a misconstrued understanding of a "turn of the nut" spec, or, that they just buzz the bolts down as hard as some generic impact wrench will tighten them.  

Regardless, unless high quality fasteners are tightened thoughtfully, the full potential of the design will not be available.  

RE: Mechanics Behind Fasteners

(OP)
Wow, great, great contribution here Tmoose, I am overwhelmingly grateful, you really are clued up on this. I will reply to your above suggestions in due time.
That UNBRAKO technical section is *JUST* what I was looking for.
Okay I will start going through that theory plus measure the parameters you mentioned and I will reply here soon to give feedback. Thanks again for that and everyone else. Heres hoping, solutions will be found!

RE: Mechanics Behind Fasteners

(OP)
Such colossal depth of information in this manual. I dont even know where to start. What a gem.

Anyway, refer to the charts on pg 65 onwards...

In the material lines, what do "SS"values refer to? Shear Strength? As a matter of interest, what does "Rc"mean?

Secondly; Does theory mean to say that (should the load along the thread be uniform) the longer the thread engagement is, the less tendency to strip meaning that if my thread engagement was infinitely long, no matter how soft the material, the bolt will break before thread strips? i.e. A very high grade bolt in the softest of Steel tapping with sufficient length of engagement would make the bolt break first from torquing?

RE: Mechanics Behind Fasteners

(OP)
Hi all.

I had another heated argument with the same party regarding the torquing of these road wheel bolts into the tapped hole. His message was this:
Some holes were drilled into to get broken bolts out in the past. Some taps are corroded. Some housed loose bolts and these loose bolts combined with the impacting road wheel, deformed the thread.
He also mentioned how its impossible to know the condition of the thread with respect to other tapped holes. And therefore you cannot apply one given torque to different taps even if you clean the taps. secondly he says after assembly, my theory of applying a predetermined torque doesnt work. After many hours the tap deviates from theory. I dont even understand whats going on here to be honest. So he mentioned to just tighten the bolt like you would any bolt because torque specifics means nothing in an old tap. I cannot find any theory or reasearch into old vs new threads/taps. To be honest, I thought the only thing that makes a tap "old" is corrosion and thats it. My thinking here is if you dont exceed the shear stress of the tap, it will behave elastic and wont plastically deform.

So basically, can threads get damaged (apart from corrosion) to the extent that torquing to specific value means nothing? even if you dont exceed shear stress? (i.e. overtightening the bolt until the thread strips). If so, how exactly does it get "damaged" if its neither of these. He said drilling out broken bolts in past damages the tap. I read about this and it seems true but in that case wont an insert fix things up and so torquing it in new insert will tend to approach what theory says.

RE: Mechanics Behind Fasteners

Tsiolkovsky, you are on the right track on everything.  No, threads cannot be so damaged that torque 'means nothing'.  The outer 1/3 of the width of your threads can be gone and they will still have most of their holding power remaining.  Torque such that you stay in the elastic rangs, and do not exceed the yield stress.

Buy your mechanic an "Easy Out", a broken-bolt extractor.  Unless you like ruining tapped holes, you never drill out a bolt.  You drill a hole in the broken bolt about half the diameter of the hole, screw in the extractor, and back out the bolt -- extractor has a left-hand thread.

Yes, threaded inserts work for severely damaged threads.  Google for "HeliCoil" and "Gardsert".

RE: Mechanics Behind Fasteners

(OP)
Maybe I forgot to clarify what his most important point was:
He was basically saying a torque wrench or the concept of torquing it to a specific (or atleast a remotely specific) Nm is useless because the values to set the torque wrench to, will deviate from thread to thread due to some of them being old etc... But he said it will deviate very large, not by five or ten nm's

RE: Mechanics Behind Fasteners

Concur that the torque value is impossible to calculate when the threads have been damaged.  Use the Turn-of-the-Nut procedure from RCSC's Bolting spec.  Google "RCSC" or just go to:

http://www.boltcouncil.org/files/2009RCSCSpecification.pdf

Second way is to use load-indicating washers.  These are also covered in the above spec.

RE: Mechanics Behind Fasteners

(OP)

Quote (duwe6):

Torque such that you stay in the elastic rangs, and do not exceed the yield stress.

Quote (duwe6):

Concur that the torque value is impossible to calculate when the threads have been damaged.  Use the Turn-of-the-Nut procedure from RCSC's Bolting spec.

wait we are getting somewhere here. You say a thread may be damaged say a third of the way. Do you have any research and literature papers graphing how it retains its strength? This means the thread strength is not compromised and we can focus on the bolt strength. But how to know its torque specification?

I want to torque the bolt to just before yield strength to maximize preload. Suppose I take a strain gauge and apply to a bolt and torque it to, say, 122 Nm for the bolt to reach yield strength. This is done on an old tap.
If I take this same spec bolt and apply it to a new tap, will 122Nm correspond to a bolt reaching yield strength aswell? In summary, will thread damage inside one tapped hole to another tapped hole effect the torque-strain relationship of the bolt?
If I apply 122Nm in one tapped hole that I measured with a strain gauge that corresponds to the bolt about to yield, can I do this to another hole (more/less deformed) and expect the same conditions of the bolt about to yield? If so, then I can use a torque wrench to that value on all taps?

 

RE: Mechanics Behind Fasteners

(OP)
The above post is basically the ultimate question of this whole scenario.

RE: Mechanics Behind Fasteners

From the OP's questions and replies I don't think they are going to (be allowed to?) devote time to repairing the threaded holes and flattening the mating surfaces, and assembling with reasonably tightened bolts.

A housing held against a fragile lunar surface littered with outcroppings and volcanoes by a few honest hardworking bolts tightened to some tentative value to preserve wallowed out threads is going to wiggle free in a week. Then the housing and the poor bolts will be yanked around like that swimmer at the beginning of "JAWS" and the surface destruction will increase to a new, even more gruesome level until the bolts fatigue and break, again.

RE: Mechanics Behind Fasteners

When thread damage causes increased friction between bolt thread and tapped thread, the same applied torque will not result in the same tensile stress in the bolt.  The increased thread friction will absorb more of the applied torque so that less applied torque is available to stretch the bolt.  Therefore, less tensile stress in the bolt.

Ted

RE: Mechanics Behind Fasteners

Tsilky, Threads are a pyramid -- The outer [top] 1/3rd has very little of the area, thus very little of the strength.  When about half the height is gone, I use an insert.  That's the point that I feel that too much engagement and strength is gone.  This is empirical and 'rule-of-thumb'.

As for torque, it cannot be used on damaged threads.  You have to go to turn-of-the-nut to estimate preload.  As noted in RCSC A-325, 1/3rd turn from "snug tight" will tension a hardened bolt with a typical length-to-diameter ratio and a standard [coarse] thread pitch.  

RE: Mechanics Behind Fasteners

Lets take a step backwards and try to see if there are other directions to go.

Do you have the room to design a bracket,what-have-you, that allows you to mount the wheel to it, and then it to the frame? I'm thinking in terms of a bearing cartridge on your modern front wheel drive car, so that even if you run your bearing to the point of shearing off,  rather than damage those threads in your main frame you can unbolt that unit and bolt up and new one?

Can you get a wheel that has more flexibility in the tire and absorbs more of the impact?

Can you go back to the manufacturer and ask them if they have seen this problem elsewhere and what others are doing to solve it?

 

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