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Bolted joint slippage and implications

Bolted joint slippage and implications

Bolted joint slippage and implications

(OP)
Suppose I have a pulley bolted to a shaft.  Clamp load between the bolt and shaft shoulder provide torque capacity (via friction).  I've performed static slip tests (clamp shaft, measure quasi-steady torque vs time while gradually increasing torque until the pulley slips) and found that the slip torque is very close to what I've calculated, and that it is reasonably well controlled (multiple units measured). I've match-marked the pulley to the shaft and used the assembly in its intended application.  After use, it is apparent that the pulley has slipped significantly on its shaft.  If I re-mark the bolt, loosen it, and measure torque while retightening to the new mark, I find that the torque required is the same as the installation torque within measurement error (ie bolt hasn't loosened significantly, if at all).  

The question / argument is:
  Position 1 (mine): Loads in use must be exceeding the quasi-static slip torque value often enough to cause the rotation observed (regardless of duration of high-torque events, they must exceed the quasi-static slip threshold)
  Position 2 (coworker): The gear will slip at a lower torque if the load is applied suddenly, so the loads required to explain the slippage may be much (~50%) lower than I think if there is any kind of "impact" going on in the system

Anyone care to weigh in?  Am I all wet - is the "dynamic slip torque" really much different from the "quasi-static slip torque" of a clamped joint?  Is there a third position that one might take?

RE: Bolted joint slippage and implications

Is there any way you could measure actual torque in use?  Of course, if loads are applied suddenly then you may get instantaneous torque loading that's far above your quasi-steady state.  Is the load applied suddenly as a rule?  If so, how is the load applied?  Let's say you're driving this shaft with a motor attached to a clutch.  Spin the motor at 1750 rpm and then drop the clutch.  Your motor's inertia will hit the shaft with significantly greater torque than the steady-state load.

I know that's an extreme example, but it's all I can come up with given the information you've posted.

RE: Bolted joint slippage and implications

(OP)
I expect that we are hitting the shaft with sudden torque increases.  I think the spikes are high.  What I'm wondering is whether I can draw a "minimum spike height" line at the static slip torque, or if the joint might slip with a lower torque impulse if the impulse happened fast enough.  

There is a great deal of disagreement currently about how high the spikes are - we've done shaft torque measurements, and they don't look that bad.  We don't slip every joint, though, and some joints will run for many hundreds of hours before they begin slipping (no identified system changes to speak of).  I suspect that there is something going wrong from time to time (which we haven't found) that increases the height of the spikes above what we've measured, and above the static slip torque level.  Others believe that the spikes are always as low as they were in the one test, and that there is something-or-other that makes certain joints decide it's time to slip at a low torque level.

RE: Bolted joint slippage and implications

Is there any chance of a thermal gradiant/transient being created during operation?  If there was, the bolt might loosen during the transient due to differential thermal expansion and then go back to 'normal' afterwards.

I've had similar problems with torqued fasteners, especially ones under a cyclic load even when that load is easily calculable and is much less than the frictional load.  Part of the problem might be explained due to thermal transients.

RE: Bolted joint slippage and implications

Would it be possible to change your clamp style pulley to a wedge-type keyless bushing?  I believe you'd get quite a torque capacity increase.

RE: Bolted joint slippage and implications

(OP)
I'm not sure a torque increase is what I want - if I start breaking shafts instead of slipping pulleys, I'm in a worse spot.  We're also sensitive to positiona and alignment, and taper joints are notoriously crappy on both counts.

RE: Bolted joint slippage and implications

So, basically you want to get torque enough to not slip but without potential shaft damage, right?  Would some sort of shear pin arrangement be an option?

RE: Bolted joint slippage and implications

(OP)
the question is simpler than that:
  is the torque "seen" by the joint the static slip amount, or something less?

RE: Bolted joint slippage and implications

Have you tried using loctite on the
interfaces?  Sounds like a creep
phenomenon.

RE: Bolted joint slippage and implications

I can't think of a mechanism by which a 'sudden' torque of a given magnitude would be more likely to cause slip than a steady torque of the same magnitude, with normal engineering materials (guessing you don't have a silly putty shaft).



Cheers

Greg Locock

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

RE: Bolted joint slippage and implications

Quote (ivymike):

If I re-mark the bolt, loosen it, and measure torque

and

Quote (ivymike):

bolt hasn't loosened significantly, if at all

How did you conclude this when you loosened the joint?  It would have been better to measure the new tightening torque before loosening the joint and compare to the original tightening torque.  Isn't it possible that the joint experienced vibration loosening?  This phenomenon can occur with torque inputs lower than the static slip torque.

Regards,

Cory

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

RE: Bolted joint slippage and implications

(OP)
I measured the retightening torque after loosening (as stated above).  Do you think that tightening farther would give a better answer than retorqueing to the same angle?

The only other thing I can think of is that if there is a substantial "tilting" load applied to the pulley, and the ratio of bolt stiffness to abutment stiffness is not low enough, I might reduce my friction capacity by separating the joing somewhat.  

RE: Bolted joint slippage and implications

Maybe you could examine the true capacity of the joint by applying a tangential force at the rim of the pulley.

Cheers

Greg Locock

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

RE: Bolted joint slippage and implications

Is milling a keyslot is out of the question?

RE: Bolted joint slippage and implications

(OP)
fixing the slip isn't the problem.

RE: Bolted joint slippage and implications

If you are getting slip immediately at start-up it is likely to be torque-rate dependant (effect of pully inertia causes slipping at lower loads than iniitally predicted).  If slippage occationally happens over time it is likely a case of loosening. Is the pully located by a precision fit over the shaft or a preciaion diameter bolt? If the pully simply butts to end of the shaft secured by a standard bolt and through hole, small imbalances in the pully during rotation will create shear forces wanting to create relative motion in the joint. These forces are additive to the forces you initially predicted and is a prime cause of vibration loosening. Also, is the direction of shaft rotation such that bolt will tend to tighten or loosen? Bike pedals have right and left hand threads to get rotaion in a tighening direction - not the same joint design, but illustrates the point.

RE: Bolted joint slippage and implications

(OP)
Slip seems to occur after hundreds or thousands of hours of use.  Slippage happens in either the loosening and tightening directions (both have been observed).  Rotation is primarily in the loosening direction.  Loading occurs alternately in both directions.  The pulley is located via a light press, then clamped stoutly in place via a flange-head bolt.  Total diametral runout is on the order of 0.020mm on this ~100mm diam pulley.

Again, the question was not in regard to stopping the slipping.  I would have thought that bolt loosening would be obvious during retorque measurements (is that assumption incorrect?).  I want to know "what I know" about the loads based on the slippage.

RE: Bolted joint slippage and implications

No, I don't think you can discount fastener loosening, because measuring residual torque with torque wrench (re-torque measurements) can be a very misleading test.  It is very operator dependant and can lead to higher that actual results because the peak measured is due to overcoming static friction, often when the head moves relative to the bearing surface.  The reading you want is the torque at the point when the male thread starts to rotate relative to the female threads(by the way this should be done in the tightening direction). This can be more accurately measured using a torque-angle transducer and a transient recorder.  The better method is to measure bolt tension directly. It is tension, not torque, that is keeping the joint together. This can be done most accurately using ultrasonics or strain-gaged bolts.
Regarding understanding the loads, your quasi-static test is an ideal case, because it nearly static.  If your pully loosens with time either the demand side of the equation (the loads applied to the joint increase)increases, or the supply side (clamp load and coeffcient of friction - and COF is constant)decreases .  My guess is that it's the latter which is causing slip.
Dave

RE: Bolted joint slippage and implications

(OP)
as above, the bolts were marked, loosened, and re-torqued while torque was being measured.  Torque vs time was plotted electronically as the bolt was brought back to alignment with its original (marked) position.

The bolt is too short for ultrasonic measurement (or so I'm told).  Strain gages don't seem to survive long enough in use.

It sounds as though you agree with my position (position 1, above), that the slip torque is close to the qs slip torque if the joint hasn't loosened.

RE: Bolted joint slippage and implications

Sounds like you have a good handle on the life of the joint.

How did you measure the in-service torques? Where were they measured?

This is beginning to sound like a case for a  classic 8D analysis, bad luck.

Cheers

Greg Locock

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

RE: Bolted joint slippage and implications

I just want to reiterate that fastener contact surfaces can slip at lower forces due to dynamic force application.  This has been confirmed in some recent journal articles, like these:

Engineering Failure Analysis 9 (2002) 383-402

Engineering Failure Analysis 12 (2005) 604-615

If your bolt is rotating in both the tightening and loosening directions, then it does seem like external forces that exceed the clamping force are responsible for the observed behavior.

It seems like you are getting consistent preload performance (which is a very good thing), so you should be able to do a systematic test to learn more (as Greg suggested).  Perhaps a little more preload would be good - do you have the ability to do that, either by tightening the current bolt more, or by using a higher strength bolt?

Regards,

Cory

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

RE: Bolted joint slippage and implications

I agree with Corey.
Slip conditions on pulley shafts subjected to clamp loads from fasteners are notorious for being difficult to have perform as expected.
I think that what you may be seeing is the result of a harmonic that is interacting at certain torques and speeds with the flange and the shaft.  
You are going to have to try to identify those particular conditions that are causing slippage; I don't think you are going to find a ready answer to this situation.

RE: Bolted joint slippage and implications

(OP)
in-service torques were measured using strain gages on several shafts within the system (including those with slipping joints).  Measurements were performed over a wide variety of load and speed conditions, and analytical models of the system were correlated (easily) to these measurements.

The tough thing is that the models and the measurements say we shouldn't slip.  Most of our running experience says we shouldn't slip.  Once in a long while, though, with (so far) no identified precipitating conditions, a pulley will decide it's time to slip, and away it goes.  


RE: Bolted joint slippage and implications

I will go 'way out on a limb and cite 2 previous responses that you have a creep or a harmonic situation.  I believe that the pulley is walking because of the cyclic loading it sees which is not only pure torque (which is essentially noncyclic) , but bending and shear loads  which cycle every revolution.  I fearlessly predict that your clamping mechanism is not completely aligned in the same plane as the groove on the pulley, so as the pulley rotates the clamping force is seeing a reversing shear component.

THis is much like if you spin a quarter on your desk, and watch the picture of Washington slowly rotate as the quarter nutates rapidly at a much higher frequency.  

RE: Bolted joint slippage and implications

(OP)
The clamping interface sees a shear load whether the plane is aligned or not.  The distance between the plane of loading and plane of clamping can also add an alternating normal load to the joint, which would be worse at greater distance, and have a greater impact if the bolt stretch ratio (over abutment compression) was inadequate.  Both are interesting points, and I'll have to see how the math works out.  

There is clearly a "harmonic situation": this is a multiple-DOF system with at least 5 complex excitations (fluctuations from driven equipment and somewhat unsteady input torque), and significant harmonic content that passes through several (somewhat minor) mode frequencies.  The particular shaft in question is very tame compared to some of the others.  I'm not sure that the "harmonic situation" is the source of the problem.  My inclination is that there is something that intermittently affects the loads at one of the other driven components, and that these loads are then carried through the system to slip this joint.   This is a contentious issue around here, because nobody wants to believe that his subsystem could be the source of the "mystery load."  There are a couple of subsystems I have in mind, but getting a thorough investigation done will require that I come up with adequate evidence to justify it (we're on a tight schedule, and they've got other fish to fry if I don't - such as other "mystery loads" that have already been documented, but which don't seem big enough to be the cause of my problem).

The question at hand was whether I know how much load it takes to slip the joint
  - the answer may be "no" if Cory's references pan out (haven't gotten copies yet)
  - the answer may be "not currently" if it turns out that shear in the interface plane is important (although I expect that the interference fit at the pulley ID will remove most of the shear from the clamped joint)
  - the answer may be "not currently" if the prying / tilting moment acting on the clamped interface turns out to be significant

RE: Bolted joint slippage and implications

I am not sure if you have access to a university library (which should have these available).  I should have provided a link for obtaining these ($30 apiece):

http://www.sciencedirect.com

Regards,

Cory

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

RE: Bolted joint slippage and implications


There can only be two things explaining the slip.

1) Torque exceeding the clamping capability.

2) Wabling or walking of the pulley.

The first has been addressed fairly well here.
Have you done a very careful check of pulley/shaft fit?

RE: Bolted joint slippage and implications

Hi ivymike

During production assembly of the pulley I am right in assuming that the bolt which holds the pulley in place is just tightened to a torque figure?
If so then your clamping force could be up or down by 30%-40% on each assembly couple this with the fluctuating loads you are now describing then it seems likely depending the actual clamping load for that assembly whether or not it works loose and slips over a period of time.
I agree with your intial post that it isn't an impact thats causing the problem but a external force that exceeds the friction force preventing slip, however whether thats due to
variation in the assembled preload or a higher external load than predicted I don't know.
A final point while the pulleys in service is there any substance the pulley or bolt might come into contact with that would reduce friction sufficiently enough to cause the pulley to slip?

Regards
Desertfox

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