The Hertzian pressure in the small contact band between mating flanks, calculated from load, face width, the curvatures of the two involutes at the contact point and the elastic properties of the materials. It rises only with the square root of load, so doubling torque raises contact stress by about 40 percent. It is compared with an allowable contact stress that depends heavily on surface hardness, which is why case hardening increases pitting capacity so much.
Contact stress is the limiting factor on this set, through-hardened won't cut it, it has to be carburized.
Also heard as
- Hertzian stress
- surface stress
- pitting stress
Do not mix it up with
- Bending stress: Bending stress is in the root fillet and causes tooth breakage, not surface pitting.
Related terms
Pitting
Surface fatigue of gear teeth that breaks out small craters, typically on the flank just below the pitch line.
Bending stress
The tensile stress in the root fillet of a loaded tooth, which decides whether teeth break off.
Carburizing
A heat treatment that diffuses carbon into the surface of low-carbon steel gears, then quenches for a hard case.
Lambda ratio
Oil film thickness divided by combined surface roughness, showing how well the lubricant separates the teeth.