Durability Of Automobile Gears

Author:  Almen, J. O. and Straub, J. C.
Source:  Automotive Industries, Sept. 25 and Oct. 9, '37
Doc ID:  1937001
Year of Publication:  1937
Abstract:  
The cause of failure of gears in service varies with the type of gears and with the service they perform but, in general, failure is due to tooth breakage, to the destruction of the tooth surfaces by wear of several kinds, and to pitting. For the past eight years, General Motors Research Division has made an organized effort to determine the causes of failure in automobile gears and to find practical remedies. In the course of this investigation, laboratory breakdown tests have been run on some four hundred automobile and truck rear axles, and thousands of service records have been examined for the purpose of correlating the laboratory tests and the performance of the gears in normal service. This study has led to some definite conclusions, particularly for spiral-bevel rear-axle gears, but it is probable that many of the factors that influence the service life of such gears will apply also to gear sets in general. It has been found that gear-tooth breakage in service is due to fatigue, and that this fact provides data for more accurate tooth stress calculation than has been possible heretofore. Destruction of rear-axle gear-tooth surfaces by scoring is due to welding of small areas of the mating teeth under the influence of high pressure and high temperature. Pitting results from fatigue of the tooth surface due to repeated high compressive stresses, and is of the same nature as is encountered in ball and roller bearings. Of the alloy steels and heat treatments that have been used for carburized automobile rear-axle gears, one cannot be shown to be superior to another, except for warping tendencies and consequent variation in stress concentration. The potential load-carrying capacity of automobile gear teeth is not realized, due to stress concentrations resulting from deflection of the gears and their supports, unequal tooth spacing, eccentric and wabble mounting, etc.