Bearing Contact 1

[prost]

This month's Machine Design had an article about modeling bearing contact in a crankshaft. The author first made the point that a "...widely used technique to simulate crankshaft bearing load is with a simple linear arc of loads, say 60 degrees around a reaction-force vector." This seems like a terrible idea, by the way, especially if you care at all about the stresses near this hole where you applying the load. Nevertheless, here's the rest of the article.

http://machinedesign.com/ContentItem/71865/FEUpdateUsesurfacecontactformoreaccuratesimulations.aspx

He further makes the point that actually modeling the connection directly with a pin and contact surfaces produces more realistic results. Unfortunately, he added some twist to the model, so I couldn't make a direct comparison to the results using our standard technique for modeling bearing of pins against holes.

My question to this forum is: is this '60 degree constant arc of loads' (which I take to mean constant pressure, distributed 60 degrees around the hole) widely used by FEA forum participants? It would seem very easy to apply a distributed normal traction (AKA 'pressure' since it's in compression) of some form, A*cosine(theta), or B*cos(theta)*cos(theta). Of course maybe that 60 degree constant pressure is used because it's the easiest to implement, and users do not have capability to apply a pressure with a functional distribution like A*cosine(theta).

For comparison, our standard practice is to use one of 3 methods: 1) bearing traction, cosine distributed, 180 degrees on the hole surface or 2) normal springs, 180 degrees around the hole--allow the springs to react out an applied load or 3) modeling the pin-hole contact directly with contact surfaces between pin and hole.

The preferred method is 1) above, mainly because a) seems to work very well, and b) is really fast computationally! 2) above is pretty good too.

 

[prost]

Say you picked a particular distribution--what defines 'works best'? What is your metric; that is, what information are you using to decide that a particular distribution works best, or at better than others? I like the idea of using photoelasticity; unfortunately, from what I read about photoelasticitiy and other non destructive techniques, these seem to perform relatively poorly where you need the information the most, such as, at edges of notches, at hole edges, etc. But at least photoelasticity gives you SOMETHING to hang your hat on--reasonably accurate stress distributions almost all the way up to the hole edge.

 

[gurmeet2003]

Prost,

You have asked a good question. What does one mean when one
says something works best? There are different ways to answer this question:
1. If something you design works and does not fail during service. (off course you could be too conservative).
2. Do some kind of strain gage testing to verify your results. This is easier to do for static tests.
3. If somoething you design fails, and does not behave as expected. This is a bad way of finding out, but happens.

Gurmeet