Tooth breakage is the most common fault of gear. The fracture of gear teeth usually occurs at the root of gear, because the bending stress at the root of gear is the largest and the source of stress concentration.
There are three cases of broken teeth:
① The bending stress at the root of the tooth is fluctuating cyclic alternating stress, and the fatigue crack will be produced under the combined action of the root fillet, machining tool mark, material defect and other stress concentration sources.
The crack propagates gradually, which leads to the fatigue fracture of gear teeth.
② For the gear made of brittle materials such as cast iron or high hardness alloy steel, the stress on the dangerous section of the tooth root will exceed the limit value due to serious overload or impact load.
③ When the machining accuracy of the tooth surface is low, or the quality of gear maintenance and installation is poor, the phenomenon of partial load will occur along the contact line of the tooth surface.
The partial load makes the stress at the tooth root of the local contact increase obviously, and the partial tooth break occurs when the stress exceeds the limit value.
Local tooth breakage always occurs at the end of the tooth.