Bolted joint slippage and implications 1

[ivymike]

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?

 

[CoryPad]

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

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[Screwman]

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.

 

[ivymike]

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.

 

[JimCasey]

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.

 

[ivymike]

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

 

[CoryPad]

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

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[Fabrico]

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?

 

[desertfox]

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