Load Carrying Capacity of Spur Gears : A New Gear-Load Testing Machine and Running-in and Failure of Gears Having a Lengthwise Tooth Error

Abstract

A new type of power-circulating, four square gear-load-testing machine in which the test gears can be loaded while they are rotating is introduced. This machine, having a capacity of 200 horse power at 1800 rpm, was designed by the authors and made in their laboratory. The results obtained were as follows : (1) The testing machine was simple in mechanism and gave no trouble during lengthy running tests of over 2×107 rotations because it was designed such that the gear load could be applied mechanically using a pair of circular-arc-tooth-trace cylindrical gears (having advantageous properties such as self-centering action, due to which the load applying mechanism became very simple), oil lubricated helical and spur splines, etc.(2) Test gears could be operated beyond 5×106 rotations producing only a very small number of pits (pitting area ratio less than 0.1%) at a Hertzian stress of 0.42 HB which was greater than the Hertzian stress 0.4 HB corresponding to the tangential load which produced the maximum reversed shear stress at the subsurface nearly equal to the endurance limit in reversed twisting (where, HB is Brinell hardness before running).(3) It seems that past discussions made on the mechanism of pitting occurring on gear tooth surfaces must be re-examined fully using the data obtained by the authors.(4) It was clarified that the equations given by D.W. Dudley for calculating the allowable tangential load on spur gears with tooth-end contact were very insufficient in practice and that new equations based on the authors' experiments gave good results because the effects of running-in were taken into account.