Transmission gears operate through the meshing of teeth. The teeth are the direct working parts of the gear, hence gear failure primarily occurs on the teeth. The main forms of failure include tooth breakage, pitting on the tooth surface, wear on the tooth surface, bonding between teeth, and plastic deformation.

1. Tooth gear fracture

Gear tooth fracture typically occurs in two situations: one is fatigue fracture caused by repeated bending stresses and stress concentration; the other is overload fracture resulting from sudden severe overloading or impact loads.

Gears made from brittle materials such as cast iron and quenched steel are more prone to tooth breakage. Both types of breakages originate from the side of the tooth subjected to tensile stress. Increasing the root fillet radius, enhancing the mechanical properties of the material, reducing surface roughness to minimize stress concentration, and strengthening the root area through treatments like shot peening or rolling compaction can all improve the tooth's resistance to breakage.

2. Surface Pitting

During operation, under the repeated action of alternating contact stresses at the front meshing area, several small cracks will form on the tooth surface near the pitch line. As the cracks propagate, they will lead to the shedding of small metal fragments, a phenomenon known as tooth surface pitting.

Continued expansion of pitting on gear surfaces can affect the smoothness of the transmission, causing vibrations and noise, which leads to gears not functioning properly. Pitting is a common form of failure in well-lubricated closed gear drives. Increasing the hardness of gear surfaces and reducing the surface roughness values can both enhance the resistance to pitting, while open gear drives do not exhibit pitting as the gear surfaces wear down rapidly.

3. Tooth Surface Wear

During tooth engagement, surface wear on the gears occurs due to relative sliding, especially when hard particles from the outside enter the engagement area. As the gear surface wears gradually, the teeth lose their correct shape, which can lead to over-thin teeth and ultimately breakage. Gear surface wear is a primary form of failure in open gear drives. To reduce wear, important gear drives should employ closed drives and ensure proper lubrication.

4. Gear bonding

In high-speed, heavy-load gear transmission, the pressure between the gear surfaces is high, the temperature is high, and the lubrication effect is poor. When the instantaneous temperature becomes too high, it causes localized melting and metal adhesion between the two gear surfaces. As the two gear surfaces move relative to each other, the stuck areas are torn apart, forming strip-like or large-area scars along the sliding direction on the gear surface. In low-speed, heavy-load transmission, it is difficult to form an oil film. Although the frictional heat is not significant, cold bonding may occur due to the heavy load.

Utilizing lubricants with higher viscosity or good antigelling properties can reduce surface roughness to form optimal lubrication conditions; enhancing the hardness of the gear surface can also improve its antigelling capability.

5. Tooth Surface Plastic Deformation

Low-hardness soft gear teeth, under low-speed heavy loads, can lose their original tooth shape due to excessive tooth surface pressure and frictional forces causing plastic flow of the tooth surface metal. Increasing tooth surface hardness and using oil with higher viscosity can help prevent or reduce tooth surface plastic deformation.