Pitting (Gear)
Pitting refers to small shell-shaped craters on the tooth flank caused by rolling contact fatigue. The cause is excessive Hertzian contact pressure, which through alternating compressive and tensile residual stresses below the surface eventually leads to crack formation and material spalling.
Formation Mechanism
In gear mesh, Hertzian contact stress compresses the tooth flanks together. Cyclic shear stresses develop below the surface (rolling contact fatigue). When these exceed the shear strength of the material, fine cracks form (typically at material inhomogeneities or grinding marks). Lubricant can penetrate the cracks and accelerate crack propagation through hydraulic pressure (hydraulic wedge effect). Finally, a particle breaks out of the surface and leaves a characteristic pit. Initially, pits tend to form preferentially in the pitch circle zone or slightly below it.
Countermeasures
Countermeasures against pitting include: reduction of flank pressure (smaller gear ratio, larger module, profile shift, larger center distance), hardening of tooth flanks (case hardening to 58–62 HRC), use of EP gear oil (extreme-pressure additives form protective layers), smoother flank topography (grinding, honing), reduction of speed/load, and optimum lubrication (correct oil viscosity, oil change intervals). Initial pitting (a few small pits) can stabilize at unchanged load (running-in process), while progressive pitting indicates excessive load and can lead to tooth failure.
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