Urethane Covered Bearings Load Ratings

[nick22]

Hi Guys,

I am working on a project that requires the load ratings of polyurethane covered bearings (over-molded). Loading these types of bearings could be a very dynamic situation and take into account temperature, rpm, load, etc. My question is, do you think this can be derived using a formula that takes into account the thickness & width of the material, the compression modulus of the specific urethane being used, the load, and deflection to determine an appropriate load rating? Or should this only be determined through physical testing?

Even the "failure point" doesn't seem very clear. Obviously the urethane is the weak link so the bearing load rating will not be accounted here. Is it the urethane physically breaking or would it be something like if the urethane deflects more than 5-10% it risks separation from the over-molded metal bearing surface. 

Currently I am using the deflection theory as I think this would happen well before the material would physically tear or break. Here is what I am currently using to calculate the deflection per the load. 

 

U=[ (.75*W* (b-a))/(E*S* (8b)^1/2) ] ^ 2/3

 

U=Deflection in inches

W=Load in pounds

a=Inside radius of cover

b=Outside radius of cover

E=Compression Modulus of urethane

S=Cover width in inches

 

If the calculation shows more than 5-10% of deflection then the load maximum has been reach as the urethane risks separation from the bearing.

Any thoughts would be helpful.