Determining required surface hardness

[DaveJFT]

Hi folks,

I have a roller riding on a surface. Our system analysis has yielded a contact pressure at the interface. Would anyone be able to point me in the direction of a text explaining how to specify a required surface hardness to resist a given rolling contact pressure?

Many thanks,

Dave

 

[metengr]

This is no easy task because you have surface roughness and lubrication that must be accounted for in rolling contact fatigue life. Design of roller bearings is very involved.

Here is a slide presentation that would get you started on understanding the effect of hardness and case depth on rolling contact fatigue life.

http://www.slideshare.net/DavePalme...ling-contact-fatigue-behavior-of-alloy-steels

 

[Tmoose]

there was a nice INA bearing catalog with a bunch of useful design info. Required surface hardness and depth were covered in detail, using loads, rolling element and race/ring diameters.
"Recent" attempts to find it online have not ended well.

Highest Hertzian stresses are likely below the surface a little bit.

what is your contact pressure?

How long does this device need to survive?

 

[tbuelna]

The surface hardness of the mating parts is not the only concern. The geometry/curvature of the mating parts matters, the number of load cycles each parts experiences matters, the relative MoE of the parts matters, and the sub-surface shear strength of the parts matters. With Hertzian contacts, the typical failure mode is spalling due to a sub-surface shear fracture that propagates to the local surface. Roark's gives a simple calculation for the depth and level of max sub-surface shear stress in a Hertzian contact.

 

[DaveJFT]

Hi,

Thanks for the replies ...and there was me hoping to just plug the numbers into a formula!

1400MPa and 10,000 hours are what (/all) I have to work with.

Dave

 

[tbuelna]

Like I noted in my previous post, you can get the basic equations you need from Roark's. All you need to do is "plug the numbers" into these equations and you will obtain a fairly accurate result for Hertzian contact and subsurface shear stress in your roller body. However, in order to determine if your roller has adequate fatigue life you will need a better definition of your roller load conditions. Is the load constant over the 10K hours? What is the total number of load cycles over the 10K hours? 1400MPa contact stress is not a problem, even at a fairly high number of load cycles, given the right materials and heat treatment. But unless you can provide more details I can't give you much more help with this problem.