# Hertz Contact Stress for static condition

1. Jan 11, 2010

### servaa

Hi all,

I am designing a set of side guide rollers for a vertical gate. There are four rollers on the gate (two on each side), and the rollers are designed for jamming load condition (they are not loaded most of the time). When jamming happens, one top roller and one bottom roller (on the opposite side of the jammed top roller) will be in contact with the rail, resisting tilting moment caused by hoist load. I was able to calculate hertz contact stress on rollers from Roark's as follows:

stress = 0.798*sqrt(p/(Kd*Ce))

where p is applied load, Kd is equal to roller diameter, and Ce is some constant based on poisson's ratio and young's modulus of roller/rail material.

I am not sure though, what my allowable stress is in this case. I was able to find some criteria and standards that define allowable contact stress for wheels such as DIN 19704 (german), US Army Corps Engineering Manual EM 1110-2-2703, but the allowable stress defined in these standards seem to be for dynamic contacts.

The rollers that I'm designing is not going to be loaded repeatedly (maybe once in 5 years or so) and I think I need allowable static contact stress. Does anyone know how to approach this...??

Thanks a million!

2. Jan 11, 2010

### nvn

servaa: Assuming you do not want the contact surface to yield, I think you could use the allowable bearing yield stress, which you could assume to be 1.5 times the allowable tensile yield stress. Or if you want to be more certain to not yield the contact surface, I think you could use the tensile (or compressive) yield strength.

3. Jan 12, 2010

### servaa

nvn,

Thanks for info. Could you tell me what your source is..? Using allowable bearing stress seems to be a bit too conservative, as the allowable stresses defined in the standards that I mentioned are quite higher than those (2~2.5 times yield strength for USACE and 1.8 times ultimate strength for DIN standard).

Thanks!

4. Jan 12, 2010

### nvn

I don't have a good reference for that.

5. Dec 7, 2010

### lin99

may I know where I can find the 2.5 factor in USACE I mean section.....

thanks