Stresses developed by normal force in a photo-elastic model of gear tooth as per Dolan and Broghammer are shown in the figure. The highest stresses exist at regions where the lines are bunched closest together. The highest stress occurs at two locations:

• i. At contact point where the force F acts

• ii. At the fillet region near the base of the tooth

Gear failure can occur in various modes. If care is taken during the design stage itself to prevent each of these failures a sound gear design can be obtained.

The gear failure types are explained as follows.

A. Scoring

The excessive heat is generated when there is an excessive surface pressure, high speed or supply of lubricant fails. It is a stick-slip phenomenon in which alternate shearing and welding takes place rapidly at high spots.

This type of failure can be avoided by properly designing the parameters such as speed, pressure and proper flow of the lubricant so that the temperature at the rubbing faces is within the permissible limits.

B. Pitting of gears

Pitting is a surface fatigue failure of the gear tooth which occurs when the surface contact stresses are higher than the endurance limit of the material.

The failure starts with the formation of pits which continue to grow resulting in the rupture of the tooth surface. To avoid the pitting, the dynamic load between the gear tooth should be less than the wear strength of the gear tooth.

C. Bending Failure

Every gear tooth acts as a cantilever. If the total repetitive dynamic load acting on the gear tooth is greater than the beam strength of the gear tooth, then the gear tooth will fail in bending, i.e. the gear tooth will break.

To avoid such failure, the module and face width of the gear is adjusted so that the beam strength is greater than the dynamic load.

D. Abrasive Wear

The foreign particles in the lubricants such as dirt, dust or burr enter between the tooth and damage the form of the tooth. This type of failure can be avoided by providing filters for the lubricating oil or by using high viscosity lubricant oil which enables the formation of the thicker oil film and hence permits easy passage of such particles without damaging the gear surface

E. Corrosive Wear

The corrosion of the tooth surfaces is mainly caused due to the presence of corrosive elements such as additives present in the lubricating oils. In order to avoid this type of wear, proper anti-corrosive additives should be used.