Pitting is a common damage form of closed gear transmission, which usually occurs on the tooth root surface near the pitch line. The reason is that the fluctuating cyclic contact stress on the tooth surface exceeds the ultimate stress of the material.

When the contact stress of fluctuating cyclic change at the tooth surface exceeds the limit stress of the material, the fatigue crack will appear on the tooth surface.

When the crack is meshed, it will close and increase the oil pressure in the crack gap, thus accelerating the crack growth.

In this way, the surface metal on the tooth surface will peel off and form pockmarks, i.e. pitting.

There are two types of pitting:

① The initial pitting (also known as convergent pitting) usually only occurs on the soft tooth surface (HB < 350). After pitting, it will not continue to develop or even disappear.

The reason is that the micro bulge becomes flat gradually, which enlarges the contact area and reduces the contact stress.

② Extended pitting occurs on the hard tooth surface (HB > 350). After pitting, the edge of the pit will not be flattened because of the high brittleness of the tooth surface, but will continue to crack until the tooth surface is completely damaged.

The fatigue crack on the tooth surface of the split gear is worn away before it forms or expands, so there is no pitting.

When the heat treatment of the hardened gear is not right, along the interface between the surface hardening layer and the core, the tooth surface sometimes flakes off, which is called flake corrosion.