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M6 8.8 bolt fatigue failures (HELP!!!!)
4

M6 8.8 bolt fatigue failures (HELP!!!!)

M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
I am trying to improve a bolted connection (aluminum plate bolted to threaded aluminum case) which currently uses three M6 8.8 allen head cap screws.   I have had all three bolts fail in fatigue, presumably due to vibration.  I suspect that the problem may be that the bearing strength of the aluminum is lower than the tensile strength of the bolt, resulting in creep of the aluminum and loss of pre-load.  Unfortunately, I don't have the option of using a different size of bolt or more of them.  

I am looking for suggestions.  I was considering replacing the 8.8 fasteners with 10.9 hex head cap crews and a stainless steel fender washer.  I realize that the 10.9 fastener will require a higher torque to reach a working pre-load.  The purpose of the washer was to  spread this pre-load over a greater area in order not to exceed the bearing strength of the aluminum.  I'm no expert (Obviously!), so I'd really appreciate an expert opinion.  

Thanks for suggestions and direction!.

Dave C.
 

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

The first step would be stop suspecting what the problem is and identify the problem.

From there we can help with solving it.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

What evidence do you have to allow the fatigue failure conclusion?  What is the nature of the capscrew failure?  Through the threads?  Under the head?

The threads in the aluminum may not allow higher fastener preload without aluminum thread failure.

If you believe you are losing preload, reduce the diameter of the capscrew shank to the thread minor diameter to allow the capscrew to stretch more under preload and perhaps not unload as much if you suspect aluminum creep.  Are the capscrews short compared to their diameter?  Can you increase the capscrew length?
 

Ted

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

If there is room, add spacers to increase the free-length of the bolt.  This will allow for more stretch and can prevent a loss of preload in a similar manner as the suggestion by hydtools. I would not expect creep.  If there is vibration that could cause any relative motion between the two parts, it could quickly relax the strain from the bolt.  The greater free length prevents this.  Make sure that the mounting surfaces are smooth and have good contact. Blue check for contact between the surfaces.  The greater the contact area, the lower chance of wear sufficient to relax the strain.  If there is not room for spacers, consider Belville washers to prevent loss of pre-load.

Johnny Pellin

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Thank you for your replies.  I'll try to be as clear as I can in identifying the problem.  

The application is in the engine driving a pusher propeller in a small ultralight.  The plate in question attaches to the crankcase of the engine.  Mounted to the plate is one of two pulleys, the other pulley mounts to the crankshaft of the engine.  A belt between the pulleys provides approximately 2.1:1 speed reduction.    The propeller is mounted to the driven pulley.  The motor develops a maximum of 27hp at 6250rpm and a peak torque of 35 Nm at 4500rpm.

The plate is approximately 23mm thick.  There are three screw locations attaching the plate to the crankcase.  These are arranged in an equilateral triangle approximately 120mm on a side centered on the crankshaft.  There are 6mm deep recesses in the plate for the screw heads in each of the locations.  In the third location a bushing is used to fill the recess, and the screw at this location captures a mounting bracket for the electric starter motor.  The screws in the two recessed locations are 35mm long.  The screw in the bushed location is 45mm long.  

Facing the crankshaft of the motor as normally mounted, the driven pulley is directly above the crankshaft.  One of the two recessed mounting screws is also directly above the crankshaft.  The other recessed screw is in the lower left location.  The screw that holds the starter bracket is in the lower right location.   

Of three separate cases of failures that I know of the top screw failed first, the lower left failed next, and the 45mm screw failed last.   

Now, to try and answer some questions:

The reason I think it is fatigue failure is because of the relatively small load and the definite vibration due to the motor and slight propeller imbalances.  The static load is minimal, there will be some shear load due to the torque transmitted to the propeller, but since this is a pusher application there should be minimal strain on the screws   The one failure I examined in detail showed signs of fretting corrosion under the heads of the two 35mm screws.  the 45 mm screw location was bright and clean.

It appears the screws failed near the surface of the crankcase.  This would also be near where the transition from shank to threads would have been.  I did not recover any of the broken screw tops, so I can't say for sure.  In one other failure that was related to me  it was observed that the failure was in the un-threaded shank area.

Regarding increasing the pre-load, I am also concerned about the strength of the threads in the aluminum, however, the other crankcase screws which are also M6 are specified at 15Nm of torque, so I feel confident increasing the pre-load up to this level if it will help.

The capscrews are relatively long compared to their diameter, and it is possible to lengthen the two 35mm screws to 45mm as I was hoping to do.  It would be difficult to increase the length of the top screw much more, as the screw would interfere with the pulley.  Certainly it would seem that lengthening them would be a good idea, since in all cases I know of the longest one was the last to fail.

Belleville washers definitely seem like a good idea.  Would it be appropriate to use these on top of the stainless fender washers?  Again, I am wondering about the bearing strength of the aluminum.  

I understand the principle behind reducing the shank diameter to the thread minor diameter.  If such fasteners are commercially available in M6 x 45mm I would be willing to try this.  Would I be better off with 8.8 or 10.9 grade (each with their appropriate pre-load torque)?

Here are some of my thoughts (that certainly may be misguided!)

The location where the starter bracket mounts consistently fails last.  I felt that by going to 45mm screws with bushings and stainless steel fender washers in all locations I would be most closely replicating the starter mount location.

I felt that the more total pre-load I can apply to the plate, the better.  Since It would be a significant change to try and use larger diameter screws (or more screws), I felt that by using a higher grade of screw properly preloaded with a torque that I have evidence that the crankcase would withstand would be the next best thing.

One more question:  Should I use Loctite on these screws?  

Thanks again for your quick and helpful replies.  
 

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

2
Depending on the alloy, you may be critical on surface pressure and axial thread engagement for the aluminium parts.  Do you know what your preload is?  If you are using torque to estimate, then you need to measure or estimate the friction coefficient.  Here are some hints on calculations:

FAQ725-536: How do I calculate an assembly torque?

thread725-35222: Tensile capacity/minimum engagement

Your failure probably is fatigue.  You should consider increasing preload by using a 10.9 screw.  You should not consider using a hex head fastener (too small bearing area).  You should consider a hex flange fastener.  A waisted screw (shank diameter equal to thread minor diameter) is worth considering, but may not be available in small quantities.  

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
This is great stuff, I wish I wasn't so constrained by what I have to work with.  

Estimating the thread friction coefficient and bearing friction coefficient at between 0.25 and 0.5 (steel to aluminum) I get an estimate of pre-load of the original joint with a screw torque of 10Nm of 2.7 to 5.2kN.  With an equivalent 10.9 grade fastener at a tightening torque of 15Nm I get 4.04 to 7.75kN of pre-load.  

I hadn't considered the axial thread engagement before.  Examining the existing crankcase bolts I see that they have a thread engagement of 15mm and a tightening torque of 15Nm.  If I were to use 45mm screws to mount the plate they would have a thread engagement of 22mm.  As I result I feel fairly confident in using a tightening torque of 15Nm in these locations.

I am able to get M6 x 45 10.9 hex head bolts locally.  I can order M6 x 45  10.9  flange bolts with a 50 piece minimum.  I am not able to find waisted bolts.  

Based on suggestions so far I am considering using the 10.9 bolts torqued to 15Nm with stacked Belleville washers.  I would stack the washers to achieve just less than 100% deflection of the washers at 15Nm of torque on the fasteners.  If I am thinking correctly, this would give me maximum retention of pre-load.  Stated another way, if I were to deflect the Belleville washer 100% then I would lose some pre-load in the joint before the washers came in to play.  In this scenario, does it make sense to use flange bolts?  If the Belleville washer is deflected less than 100%, then it seems there is no advantage of flange bolts over hex head or allen head screws.  Am I thinking correctly about this?

Thanks again for the great help and suggestions.

Dave C.
 

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

You should reduce the friction to ~ 0.1 using a lubricant rather than rely on 0.25 to 0.5 range.  This will enable you to maximize joint preload.

I forgot to mention in the previous reply, but a thread adhesive appears to be a good idea.  If you use this, you don't need a lubricant.  With this, the friction coefficient likely will be between 0.15 and 0.25.

Why do you want to use Belleville washers?  You need as much preload as possible and don't need to worry about extra joint surfaces that can reduce preload due to settling at the surfaces.  Stacked washers is for low preload joints or joints with short clamping lengths, which doesn't apply here.

Go with the flanged screws tightened as high as possible, somewhere close to the proof load, which is 16.7 kN for an M6 with property class 10.9.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Thanks, CoryPad.  I'll order those 10.9 flanged screws.  

Just to summarize, I'm going to bush out the recesses in the plate, use M6 x 45 hex head flanged screws with medium strength thread adhesive (blue Loctite), and I will torque the bolts to 15Nm (The highest value that I have evidence that the aluminum casting will tolerate)

Thanks again for your help.

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

If I'm understanding the geometry correctly, the 'top' bolt is reacting all of the belt pull, in tension.  Which may be why it fails first.


 

Mike Halloran
Pembroke Pines, FL, USA

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Brilliant!  I hadn't even considered this.  Yes, I think you are understanding the geometry correctly.  Tension on the belt will create flex in the plate which will be countered by tension in the top screw, thus reducing the contact pressure between the plate and crankcase in this location.  Thus this location is the most susceptible to fretting, loss of pre-load and fatigue.  The question, of course; is there anything I can do to counter this?

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Longer, stronger bolt, more preload, as has been suggested.
High strength stud and nut with thick washer.
Same, necked stud.
Thicker plate.
Narrower belt and pulleys.
Smaller offset of belt and pulleys to reduce the moment.
...



 

Mike Halloran
Pembroke Pines, FL, USA

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Can you use a type of Loctite, if the problem is caused by loss of preload?

Dik

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Yes, I can (and will) use Loctite, although as I understand it Loctite by itself will not prevent this type of failure, since the loss of pre-load can occur without the screw ever turning.

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

How are you losing the preload?

Dik

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
I believe that the initial pre-load is insufficient to prevent relative motion due to vibration.  This causes fretting corrosion which causes loss of surface material and thus further reduces the pre-load.  As the pre-load drops the surfaces are more free to move putting more fatigue stress on the screws, which are then more susceptible to fatigue due to their lower loading.  This continues until the screws fail.  

That's how I understand it, anyway.

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Fretting corrosion caused by electrical contact? Is there a material that will isolate the two materials and also 'glue' the parts together?

Dik

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Quoted from The ASM Handbook on Fatigue and Fracture (as found on Wikipedia).  Fretting is:  

"A special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force."

I don't believe there needs to be an electric current or potential involved.  Certainly there is not in my case.

If there are surface treatments that would help I would like to know about them.  I am not aware of any.  Welding the parts together or some other form of permenanent bonding that would prevent motion would presumably work, but I would prefer to keep the motor in a configuration that would allow for periodic disassembly and maintenance.  

Dave C.  

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

I didn't think that that was considered as corrosion... if it is, then mechanical wear might be eliminated by using the Loctite material...  For mechanical wear, there has to be movement?

Just a thought...

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Fretting is mechanical wear, not electrochemical corrosion.  A thread adhesive like Loctite only prevents fastener rotation, but the clamped joint members can move at any time when the friction forces from preload are lower than the applied external forces.  Thus, fretting still can occur and result in preload loss and joint failure.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Uh, Trike, what carries the propeller thrust to the airframe?

The plate directly?

Or the plate through the motor?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Sorry about the confusion, looks like I used the term "fretting corrosion" rather than the correct term "fretting wear".  I'm trying to be careful (still trying not to call a screw a bolt).  I'm just a darned electrical engineer.  That's my excuse, anyway.

Mike, if I am understanding your question correctly the plate is mounted only to the motor.  The propeller thrust is carried through the plate to the motor, then through the motor to the airframe (via rubber mounts).  This is a pusher type propeller, and other than the effect of belt tension on the top bolt, I believe all connections are in compression.

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Thanks Cory... Is this caused by the difference in the hardness of the materials?  fasteners aren't my forte (I'm not sure what is at times <G>)... just adding my $.02... it's a learning process for me, too.  It appears to be a real problem, then... I would think the proper solution would be to increase the size of the bolt or change the material into something that be of similar hardness.  Is it possible to 'ultramachine' or try fitting several fasteners for the threads to better match and to reduce the clearances?

Dik

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

But the propeller axis is not coaxial with the motor shaft, right?  In that case, the plate is stressed in bending, and because it acts like a lever, the propeller thrust is _magnified_ at that 'top' bolt.

Or, I could be completely wrong, as often happens.

Could you post a picture of the assembly?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Mike, I just went out to take a picture, then I remembered that I have it all torn apart to fix it! HA! Oh well, I found a picture that shows the thing folded up. If you zoom in to the propeller area you can see the motor and reduction drive. Anyhow, in normal "unfolded" mode the prop is in the back.

You seem to have a pretty good idea of how it is configured. I'm still impressed by your insight regarding the belt tension taking pre-load off of the top bolt location. For sure the thrust is contributing a moment about that top bolt which is opposite the moment from the belt. I'm having a bit of trouble figuring out how to calculate the bending moment from the belt, and I'm wondering what the static vs vibration contribution is of each. The other factor is that the belt tension is more or less static, but the propeller thrust varies quite a bit depending on mode of flight (climbing, cruise, etc.) This is just a SWAG, but I'm thinking that for most of the time the moment from the belt tension dominates.

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Oooh.  That's not a plate anymore, not with that big hole around the engine crankshaft.  It's more like a pickle fork.

Okay, when I zoom in, it gets kind of fuzzy.  A close-up including just both pulleys and the plate might help.  If I'm seeing it right, the 'top' bolt is between the pulleys, and the other two bolts are nearer the engine cylinder, basically at the tips of the pickle fork.  

There's no idler, so even if the belt is a little tight at assembly, it will lose static tension as it stretches (permanently) in the first few minutes of operation.  

The good news is that the dynamic belt tension, the direct result of the power being transmitted, should produce a moment opposite to that produced by the propeller thrust, and because they both originate in torque transmitted, they should track each other.

The less good news is that the net moment is trying to rotate the metal being squeezed by the top bolt, and the legs of the pickle fork can't help much in resisting that, because they're very narrow relative to the sturdy remaining portion of the plate between the hole and whatever carries the prop, presumably some kind of flanged stub shaft.

The belt tension also tries to shear the bolts.  The traditional (terrestrial) solution for a situation like that is a hollow dowel around two of the bolts (in matching counterbores) or small short dowels next to the bolts.  Or a pilot diameter and counterbore around the big hole concentric to the crankshaft; I can't tell from the photo if a suitable feature in the crankcase is present, or available and unused.

It might be instructive to get a 'tenths' dial indicator and good adjustable base, and try to directly measure deflections of portions of the plate relative to the engine as you manually apply thrust to the propeller hub.  I suspect you'll find the plate is a lot more flexible than you'd like.
 

Mike Halloran
Pembroke Pines, FL, USA

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

The bending moment from the belt would be the product of the belt tension times the distance from the belt midline to the plate mounting surface.

My visualization of the plate is that it flaps back and forth as the thrust changes.

Ted

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Yeah, you are right, it is more of a pickle-fork.  

I just pulled it apart today, and with the standard bolts and installation torque after 5 hours of running time there is noticeable fretting corrosion near the top bolt.

The good news is I have another option which I am in the process of testing/implementing.  There is an alternative "plate" available that has the same offset, just down instead of up.  By flipping the whole motor over and using this other plate the thrustline stays the same.  It's a bit of a plumbing mess to do it, but the upshot is that the alternate arrangement then has the single bolt at the bottom and the two bolts on the sides sort of half way between the crankshaft and the propeller pulley.  Overall the plate is stouter in the area between the two top bolts and the propeller pulley.  It's still a pickle fork on the bottom part, but at least there are two bolts at the top where they are really needed.

I just realized that I can calculate the belt tension.  It is speced at 2mm deflection with 5kg applied to the center of the span.  I need to measure the span do the trig and convert that all to fig newtons, but it's getting late.  I'll do that as well as take some pictures tomorrow morning.  Oh, and for thrust, it's 27hp and my forward speed is around 35mph, so with a bunch of unit conversions I can do that too, I think.  I'm thinking its about 100lbs, but I'll calculate it tomorrow.

BTW, the pickle fork is about 0.8" thick and the fork tines are about 0.4" wide.

Great stuff, thanks guys for taking an interest.

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

hi trikeflyer

I realise I have entered this thread quite late on and not 100% sure I have the configuration correct in my head, maybe we can have a close up picture of the area concerned at some point.
Anyway I have sent a link showing a typical failure of a cap screw by fatigue.
In addition I agree MikeHalloran that the one bolt is seeing most of the load and I have uploaded a file on how you can calculate that based on the assumption that the plate doesn't bend and that it rotates about one edge.
This would confirm why the top bolt breaks first and then the other two bolts.
Incidently my calculation assumes all the bolts are in tension and that the bolt loads in the othr two bolts can also be calculated by multiplying the linear distance to the bolt centres from the pivot point.
If I have mis-understood your problem please let me know.

Regards

desertfox






http://books.google.co.uk/books?id=NaZwZK2xm-QC&amp;pg=PA700&lpg=PA700&;dq=cap+screw+failures+due+to+fatigue&amp;source=bl&ots=1BabrXOG3N&sig=NDN1NZRz2melVHXC-EYaxD7ET6k&;hl=en&;ei=C96YSvr6H9ugjAe4ppDDBQ&;sa=X&oi=book_result&ct=result&amp;resnum=8#v=onepage&q=&amp;f=false

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Thanks, desertfox, and thanks for joining the thread.  

I'm out of town for the weekend and having a hard time with my laptop, so I won't get a chance to carefully look at your analysis until I get home to my printer.  From a first glance, though It looks like you have the configuration correct.

Going back to the question of the M6 screws and specifically choosing 10.9 vs 8.8 grade, I'd like to ask another question.  Maybe corypad could provide the definitive answer.  Here's the scenerio:  Whenever I mention changing the 8.8 screws to 10.9, I invariably get the comment that although 10.9 are harder they are also therefore more brittle, and thus more likely to break.  

So what is the real story?  My simple minded thinking is that by going to 10.9 screws properly pre-loaded I am more likely to prevent relative motion than with the 8.8 screws tightened to their (lower) pre-load.  If there is no relative motion, then there is no fatigue, and the screws don't fail.  But what if the 10.9 screws do not provide enough pre-load to prevent motion?  Are they in fact then more or less likely to fail than an 8.8 screw?  Does that make sense?   A slightly different angle on the question might be to ask this:  If I can achieve a higher pre-load by either using a 10.9 screw or a larger diameter 8.8 screw which is the best approach?  

Thanks again, everyone.  It's going to take me a while to sit down and go through all the comments and calculations in detail, but I already can see that there are several insights that are immensely helpful.  For example, I wouldn't have considered changing from the 'up" plate to the "down" plate without the insights about how belt tension and thrust impact the load on the screws.  

I think I saw someone else on another thread comment that it was a bit like drinking from a firehose.  I have to agree; I am feeling a bit drenched here!  You guys are great!

Dave C.



 

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Firehose?  Nah.  More like a Wood Screw Pump.

Wait'll the clutch hooks up...

 

Mike Halloran
Pembroke Pines, FL, USA

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Property class 10.9 material has lower fracture strain than property class 8.8, but it isn't brittle.  Class 10.9 material is stronger than 8.8 material both for static and fatigue condition, but property class 8.8 and 10.9 screws have essentially the same fatigue strength due to the high stress concentrations present in threaded fasteners.  The fatigue improvement to the joint for 10.9 screws is that you can use a higher preload which reduces the oscillating load applied to the bolt.  Usually, a smaller diameter, stronger fastener is better than a larger diameter, weaker screw in fatigue because the smaller screw is more compliant, which reduces the oscillating load applied to the bolt.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Hi trikeflyer

Here's are a couple of sites to give you some info on blted joints:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

http://www.rbwmfg.com/images/HelpfulHints2.pdf

With reference to our question about 8.8 and 10.9 grade fastenings I came across a situation where one company prefered to use 8.8 over 10.9 wherever possible, their reason for it was explained to me as follows:-
"Most of our joints are tightened using a torque figure that generates 80% to 90% of the fastener proof stress (yield stress), given that torque settings can have an error of 20%-30% there is a very good chance that some of the bolts will be stressed over the yield figure.
Now once over the yield figure the stress/strain curves differ and as Cory stated the 10.9 as a lower fracture strain than grade 8.8 therefore they felt that when the bolt exceeded the yield stress at installation they had a little bit more margin against completely fracturing the bolt using a grade 8.8 as opposed to a grade 10.9.
Just looking at the Torques you are quoting for your application it is possible that you could be reaching or exceeding the yield stress of the 8.8 bolts particularly if there is any lubricant present on the threads at the assembly stage (whether lubricant is present delibrately or accidently)which could also contribute to the failure mode your seeing at present.

desertfox

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Mike, I had to look up a Wood Screw Pump.  HA!  I think you are right.  It's not the pressure that's going to drown me, its the sheer volume.  Thanks again everyone for taking an interest in this.

desertfox, I've had a chance to look at your analysis.  Thanks very much for that.    If I am interpreting your drawing correctly, it's not quite the situation that I have.  In my case the plate bolt pattern is centered on the crankshaft, and the pulley that is mounted to the plate is offset (to the top in your drawing)  I think the analysis is similar, except in my case I think the pivot point (assuming a ridgid plate) would be at the bottom of the plate rather than the top.  I am assuming that the pully that you are showing is the one on the crankshaft and the one that is out of the picture is mounted to the plate.  

I've attached a couple of pictures:  One shows the motor and the mounting surface for the plate, the other shows the plate and the eccentric to which the propeller pulley is mounted.  The plate has a rim approximately 3mm high at the top and near the mounting bosses.  For a reason I'm not sure of, the manufacturer machines the motor and plate so that that this rim is an interference fit on the motor.  I tried to get a decent picture showing the fretting wear:  It appears to be on the edge of that rim perpendicular to the plate surface, and mostly on the rim near the top bolt.

I've tried to catch up on some of my calculations.  Here's the results:


Bolt pre-load:

Assuming 0.15 to 0.25 coefficient of friction (screw lubricated with Loctite) a torque of 15Nm results in approximately 11kN of pre-load.  This is 66% of the proof load of the property class 10.9 M6 screw.  This should result in a maximally pre-loaded joint for the size of fastener and the strength of the crankcase threads.



Engine thrust and torque:

At full power:
    Torque: 32.6Nm
    RPM:  6000
    Engine HP:  27

Based on 35mph forward speed, 500fpm max climb rate, and power off glide of 10:1 the actual thrust delivered to the airframe is 106lbs.



Drive belt:

Center-center distance between pulleys:  110mm
Center of belt to plate/motor surface:  50mm
Belt tension:  1.348kN
Bending moment on plate:  67Nm
Unloading of top bolt assuming ridgid plate (original plate):  515N
Unloading of top bolt assuming plate pivots on bottom bolts (original plate):  644N
Unloading of each of two top bolts assuming ridgid plate (reversed  plate):  335N
Unloading of top bolt assuming plate pivots on bottom bolts (reversed plate):  322N


Thanks again,

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Hi trikeflyer

I may have the drawing the wrong way round however I was really just drawing a pulley with a plate with 3 holes at 120 degrees and showing the belt tension, I believe that my analysis is right for your situation though even if my configuration is slightly out.
The step for the motor and plate which you say as an interference fit is probably to take the direct shear load off the bolts and possibly reduce some of the bending load that were discussing now.
Any chance of a picture with it all assembled and close up?
Those bolt loads you have stated in your last post are they using my calculation method and also what is the pitch circle diameter of the bolts? The preload of the bolts on assembly it seems are far higher than those you have listed as external loads, there are presumeably more external forces on those bolts than were aware of at present? Finally it appears that your saying "unloading bolts at top etc" can you expand on that please or even better upload your calculations.
Your bolt preload of 11kN with the friction coeff for loctite gives a torque figures of 9.9 to 13.2Nm is that what your using?

desertfox

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

trikeflyer,
I would say the counterbore on the plate is to centralize it on the motor and take the shear load.  Is the counterbore/recess deep enough that the surfaces of the ears for the bolts do come hard together?  If the faces do not come together this may be your bolt loading problem.  Just asking because of the fretting on the diameter of the motor.  Is there fretting on the face of the bolt ear surface?

Ted

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
desertfox, thanks for the clarification.  It does seem that your drawing is an accurate schematic representation of what is going on, I just wanted to try and head off or clear up any possible confusion over the actual configuration.  This thread has evolved well beyond my original question about screws, so my original simplistic explanation of the configuration may be misleading.  

I am in the process of re-builing everything the other way up, so I don't have a complete assembly to take a photo of at the moment, but I will as soon as I can.

For the pre-load calculation I was using the FAQ that corypad linked to earlier (FAQ725-536: How do I calculate an assembly torque? How do I calculate an assembly torque?).

Here are the numbers I am using:

Assembly torque - 15Nm
Pitch - 1mm
Pitch diameter - 5.35mm
thread fiction coeff. - 0.15 to 0.25
bearing fiction coeff. - 0.15 to 0.25
hole diameter - 6mm
bearing surface outer diameter - 10mm for original allen head capscrews, 13.3mm for proposed flanged screws

I calculate a pre-load tension of between 10.42kN and 11.11kN for the flanged screws (depending on the friction coefficient)
I am using the numbers corypad suggested for friction coeff of a screw lubricated with Loctite.
I will try and generate a (readable) fascimile of my other calculations.  

When I say "unloading of top bolt" (dang it, I mean top screw; there I go again!) what I mean is that the belt tension has the effect of peeling the plate away from the motor in the top screw location.  I am assuming that nothing moves and that the tension in the screw stays unchanged, therefore the "peeling" action would reduce the contact force between the plate and motor in the top location.  Rather than "unloading of the top screw" I should have said "unloading of the joint in the top screw location"  This may not be relevant if the interference of the counterbore is preventing the flat surfaces from coming together.

Finally, I would agree that the static loads seem small compared to the pre-loads.  I think that the vibrational loads are the culprit here.  The motor manufacturer has indicated that this joint is quite sensitive to proper propellor balancing.  It also appears that in at least two and probably four of the failures that I know of rough idling of the motor has been implicated.  Excessivly rich idle mixture is causing this two stroke motor to "four stroke" at low rpms resulting in significant impulse torque.  

hydtools, I think that what you seem to be implying is correct.  The flat surfaces do not come together, as I saw no fretting wear there, only on the perpendicular surface of the counterbore.  The counterbore has a tapered wall thickness, approximately 4mm thick at the surface of the plate taperering to 0.5mm.  I am wondering if the purpose of this counterbore is to amplify the clamping force.  This would explain why it is machined to an interference fit.  If so, then it would seem that any wear or relaxation of this material would quickly cause the joint to lose pre-load.  On the original part in which the screw failures occurred this counterbore is almost completely worn away.


 

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

The flat surfaces of the ears should come together.  That is also how the machining would control the squareness of the plate to the motor.  Otherwise, you would have to carefully tighten each bolt/capscrew to maintain plate squareness to the motor and parallel alignment of the pulleys.

This area would be worth checking for machining or design error.  Just my opinion.

Ted

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Ted, that's a good catch.  I agree that it seems the "ears" should come together.  It would seem like the machining of the counter-bore would need to be very accurate in order to ensure that the ears come together while at the same time achieving the desired clamping action by the counter-bore.   I'm having a hard time imagining a way to unequivocally verify that these surfaces have come together.

What I am going to say next is really ugly, so just a warning to the squeamish....

Is there some kind of joint filling material that could be applied to these flat surfaces to fill the gap?  What I am thinking of is something that can be applied and then the whole assembly is clamped to some intermediate torque.  The filler is then allowed to set and then the screws are tightened to their final torque.

OK, that should firmly establish that I am open to any suggestions regardless of how hairbrained or repulsive!

Thanks again for the great insights,

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

That is a really ugly idea.  Bondo comes to mind.
Better yet call the maker to verify what that fit should be.  Best, compared to the Bondo idea, would be to have a machine shop machine the plate for a close slip fit over the dowelling diameter of the cranckcase.
Since you have another plate, try it for fit, too.

Ted

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Another idea would be to remove material from the face of the dowelling diameter on the crankcase.  Since it would not be a precision contact surface, you could file it away leaving some dowelling length for the location and shear function while allowing full contact of the bolt ears.

Ted

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Ted, funny you should mention Bondo.  I had a report from one pilot who had the top bolt location fail twice.  He filled it with JB Weld and clamped it with a 12.9 grade screw and hasn't had a problem since.  Unfortunately in spite of my prior claim I do find that a bit too ugly to try.

I'm a little nervous about machining the counter-bore/dowling for a slip fit as the manufacturer indicated to me that the interference fit was a design feature.  Unfortunately he bought the design from another company, and I'm not sure how much of the lore has been lost, or how much design expertise he has.  Certainly someone went to a lot of effort to incorporate the feature in the design.

Instead of taking on this part of the design myself, I'm thinking that I will build the motor up with the other plate and the 10.9 fasteners.  Rather than using Loctite I will check the torque at short intervals to see if the plate will settle into position.

I will also monitor it for fretting wear.  If I find any, I will tear it down, inspect it, and see where the contact surfaces are and attempt to make small changes and repeat the process.  WD-40 sprayed along the clamped joint seems to be a really good fretting wear detector,  if it starts seeping out black, there's fretting.  

  

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Hi trikeflyer

Reading these last posts it seems to me that the interference fit is important and if that intereference fit is no longer there for whatever reason then just bolting it together is not the answer because now those bolts are taking all the load which in the original design they weren't.
Also my calculation method is only valid if there is no interference fit.
I'll post later bit short on time now but I have a bit more to say.

desertfox

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

Looking closer at the last pictures there appears to be contact of the outer edge of the dowelling diameter on the cranckcase and a fillet radius in the corner of the matching diameter on the plate.  Relieving this contact may be enough.  File the edge on the cranckcase to a camfer to clear the radius in the plate.

I still think the ears need to be brought tight together.  

Ted

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Chamfering the edge of the crankcase seems like a good idea.  I also think that your earlier idea of removing some material from the face of the crankcase dowling is good.  Looking at the plate it appears there may be some contact in this area, and that would only prevent the ears from coming together and full engagement of the dowling into the counter-bore.  

On the other battle fronts: I seem to have isolated and corrected the carburation issue, and I have ordered  a dynamic propeller balancer (Balance Master) to help eliminate vibration from any slight propeller imbalance.

I think this one is just about nailed down.  It's just a matter of building it, running it and watching it.  Thanks again everyone for all the detailed work and analysis on this.  Your help has been unbelievably valuable.  

I've been a bit remiss in awarding stars, here.  Ted and Mike get one each for really delving into the issues beyond my original question and getting me to understand how this plate *really* connects to the motor.  And of course corypad gets two stars for so clearly and definitively answering my original fastener question and follow-ups.

Dave C.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

I realize this has nothing to do with the original question, but have you thought of applying a layer of machinist dye, (dykem blue) to the contact surfaces prior to assembly. Then during the next dis assembly, the wear points will be easily seen? Just a thought.

RE: M6 8.8 bolt fatigue failures (HELP!!!!)

(OP)
Thanks; yes, I think that might have even been suggested before.  It's just a question of how to best approach the analysis:  If I use the WD-40 test to look for fretting on the assembly then that would defeat the dyekem.  I think that I may need to allow the assembly to settle and re-torque a couple of times to really see where it settles out, and I'd really like to monitor it with the WD-40 during this process.  In any case I think that before I am done I will have tried a couple of different approaches, and using dykem should likely be in one or more of them.

Dave C.

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