Contact stress of nylon roller on steel rail?

[MASSEY]

I am having a problem determining the contact stress of a Nylon 6/6 cylinder (roller) rolling a hardened steel flat rail.

In fact, finding a consistant value for yield strength of Nylon 6/6 is difficult and I am still not sure what to use.
I have concluded that 9000 psi is a 'safe' value to use for yield strength.

Given that, I need to determine the contact stress that the Nylon will see under a given load so I can determine what load the Nylon will carry.

The 'best' equations I have found require a selection of the 'half-width' that is formed under load and this would return a maximum load using a selected yield strength.

Or I can select the load and using selected yield strength I can determine the 'half-width' that will be formed.

What is an acceptable 'half-width' for a 2" diameter Nylon 6/6 roller for example?

Is there a 'better' way to determine a maximum load for above roller?

Where can I find the required information?

Thank you,
MASSEY

 

[patprimmer]

I am not sure I can find the data you require, but I will try.

The problem when trying to relate metal type data to plastics, is that plastics are actually a super high viscosity fluid, ans they flow with time under load.

Another problem with nylons in particular, is that their mechanical properties can be dramatically affected by the absorbtion of moisture from the atmosphere.

Many manufactur's only publish dry as moulded figures as these are the most reproducable, but most parts are used in various conditions, and designs must accomodate worst case scenarios.

Regards
pat

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[CoryPad]

You need to use the equations for Hertz contact stress. An introduction is shown here:

http://www.engr.usask.ca/classes/ABE/323/laboratory/abe323-lab3-lecture2.pdf

You only need the material properties (elastic modulus, Poisson's ratio, yield strength) and the geometry (curvature radius) to solve the problem.

Regards,

Cory

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