Bearing Contact
Bearing Contact
(OP)
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://ma chinedesig n.com/Cont entItem/71 865/FEUpda teUsesurfa cecontactf ormoreaccu ratesimula tions.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.
http://ma
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.





RE: Bearing Contact
corus
RE: Bearing Contact
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Bearing Contact
RE: Bearing Contact
corus
RE: Bearing Contact
Regardless of which method you use, however, which one is better than the others? corus seems to think that 'it depends' is the most reasonable answer; I tend to agree. We do a lot of lap joints, which generally have a few small holes in large plates, located a couple of diameters away from edges, so that cosine() seems to work pretty well. But if you have a rod end with a pin, cosine() might be a terrible distribution to use.
Better question yet--how do you know which method is better? I have tried to locate some experimental measurements of stress fields around loaded holes; I managed to find a limited set of photoelasticity results by Hyer, and got mixed results. I've uploaded the two comparison figures to Engineering.com
RE: Bearing Contact
RE: Bearing Contact
Hyer, M.W. and Liu, D. "Stresses in a Quasi-isotropic
Pin-loaded Connector Using Photoelasticity"
Experimental Mechanics, Vol. 24, pp. 48-53, March 1984
RE: Bearing Contact
RE: Bearing Contact
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Bearing Contact
RE: Bearing Contact
RE: Bearing Contact
Having studied it more closely it is very complex shape since they've plotted the polar plot of the force vector during one cycle of operation - so it isn't what you wanted. Sorry.
Having said that, www.pump-zone.com/articleimages/283_6.jpg
is in line with my memory - a sine wave is a reasonable approximation.
However www.me.berkeley.edu/PREM/hydrodyn.htm would make me cautious about such a simplistic assumption.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Bearing Contact
1) Prost, it's no surprise that a full contact schema reproduces experimental data in a much more consistent manner than any equation-based schema. The important thing with Gencoz form is that it reproduces experimental data much better (in this case) than the cosinusoidal form. At this point, I have two questions:
- how must the Gencoz series be read? I mean, which is the meaning of the "comma-separated double value" for the index "n" in the summations? Never seen anything like that in my math volumes...
- when is the Gencoz schema more appropriated, and when instead the cosinusoidal? Because, in the basic engineering books, what "thet" give you for a direct bearing-like contact is the cosinusoidal form...
2) GregLocock, the first reference you give relates to a deviated, eccentric bearing-like contact and is effectively a cosinusoidal schema, corrected in order to take into account eccentricity.
The second reference, instead, has in my opinion nothing to do with the original question, because it's a Reynolds-based pressure distribution which is valid for fluid-film hydro-dynamic bearings only.
Regards
RE: Bearing Contact
Here is a fortran function to evaluate gencoz:-
FUNCTION GENCOZ(A)
IMPLICIT DOUBLE PRECISION (A-H,O-Z)
GENCOZ=COS(A)
IF (GENCOZ.LT.0D0.0D0) THEN
GENCOZ=0.0D0
RETURN
END IF
OLD=-1.0D0
J=1
K=-1
10 J=J+4
K=K+4
RJ=FLOAT(J)
RK=FLOAT(K)
GENCOZ=GENCOZ-COS(A*RJ)*5.0D0/((RJ-8.0D0)*(RJ-1.0D0)*14.0D0)
* -COS(A*RK)*2.0D0/((RK-4.0D0)*(RK-4.0D0)*5.0D0)
IF (GENCOZ.LT.1.0D-8) RETURN
DIFF=ABS((GENCOZ-OLD)/OLD)
IF (DIFF.LT.1.0D-6) RETURN
OLD=GENCOZ
GO TO 10
END
RE: Bearing Contact
IF (GENCOZ.LT.0.0D0) THEN
in the fourth line of code !
RE: Bearing Contact
ah, OK, the meaning should be, if I understand well, "for n=5 to infinity step (9-5)" and "for n=3 to infinity step (7-3)"; i.e. the index "n" grows "step 4" in both summations, starting from 5 in the first one and from 3 in the second one... Not really a standard mathematical notation, I guess...
Thank you, regards !
RE: Bearing Contact
This creates a difficulty. Because if one applies the oil pressure obtained from the study to the bearing without the pin, then the conrod bore is too flexible. On the other hand
if pin contact is included then there is no way of applying
the actual oil pressure. The best study would be what is called the elasto-hydro dynamic study. But that is computationally very expensive.
Gurmeet
RE: Bearing Contact
RE: Bearing Contact
Gurmeet,
yes, this is a very special case and I have no knowledge on how to consider it.
In general, however, if you want to simplify, it's either one of these two situations:
- fluid-film hydrodynamic bearing: Reynolds equation applies, at least until the pressure of the fluid-film is not high enough to "strip" the film itself (overload), until the relative speed is not so low as to make the film creation impossible (lack of lift), or until the relative speed and load are not such as to create whirls in the film (instability) - this is however out of the original question
- dry contact between bodies: you can choose from cosinusoidal, Gencoz, or whatever pressure distribution is best appliable to your problem. The question is still: which distribution goes better for which situation?
Regards
RE: Bearing Contact
RE: Bearing Contact
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