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Machine Design - Tips & Tricks: Designing for Hertzian Contact

Sometimes in machine design it is very difficult to avoid line or point contact with fairly high loads. Rather than redesign your machine to avoid this situation, a bit of analysis can go a long way to deciding whether this is a problem, and if there are some easy things that can be done to reduce or eliminate spalling.

If one or more of the contact surfaces is curved, such as in the hook example below, the fairly simple Hertzian contact formula can be used to calculate the contact half-width:hook

eqn2

The area of these contact patches can be larger than you'd expect, and can be capable of transferring high loads, especially if you can select materials with low Young's modulii or Poisson's ratios.

Sharp edge contact should be avoided, but adding a simple round to the edge will reduce the contact stress and allow this equation to be used.

[Equation from Shigley. J.E "Mechanical Engineering Design"]