The current standards and design relationships for the surface durability o
f gears are based on contact stress considerations. Thermal effects are con
sidered only for scoring failures resulting from the destruction of the lub
ricating film owing to temperature rise. This paper investigates the effect
of thermal shock and thermal stress cycling on pitting, micropitting and w
ear for different gear geometries, materials, operating conditions, machini
ng processes and surface treatments. Because of the current interest in app
lying surface coating technology to gears, special attention is given to il
lustrating the potential benefits of this technology in improving the perfo
rmance and extending the life of gears.