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?
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
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
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
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
RE: Bolted joint slippage and implications
RE: Bolted joint slippage and implications
RE: Bolted joint slippage and implications
is the torque "seen" by the joint the static slip amount, or something less?
RE: Bolted joint slippage and implications
interfaces? Sounds like a creep
phenomenon.
RE: Bolted joint slippage and implications
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
and
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
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RE: Bolted joint slippage and implications
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
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
RE: Bolted joint slippage and implications
RE: Bolted joint slippage and implications
RE: Bolted joint slippage and implications
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
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
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
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
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
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RE: Bolted joint slippage and implications
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
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
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
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
http://www.sciencedirect.com
Regards,
Cory
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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
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