Abstract
Many studies on tooth surface strength have been reported but they are mainly reports on tempered steel. So, it can be said that sufficient data to design the case hardened steel for tooth surface strength has not been published. Therefore, spur gears were tested for tooth surface strength under several lubricating conditions and several surface treatments of test gears. The tooth surface strength of case hardened gears is not so affected by the difference of the viscosity of lubricating oils, as are the tempered gears. The EP additive is effective for the tooth surface strength of case hardened gears but it is not effective for the tempered gears. The difference of the chemical structure of the lubricating oils has some effects on the surface strength of gears. The effective case depth of case hardened gears is an important factor to determine the tooth surface strength.
