Progress in the development of automotive worm-gearing is interestingly outlined. Previously tO 1912, American experience had been limited almost exclusively to the industrial form, generally of the single-thread type. Introduction of the motor truck required a worm for the final-drive but one having entirely different characteristics from that of the industrial gear. Experience in designing these was lacking, however, as was also the special machinery to produce them. In 1913, machinery was imported from England and since that time development has been rapid.
First efforts were devoted to simplifying the design of the axle as a whole, studying the problem of getting lubricant to the bearings, heat-treating the parts, and improving the materials of construction.
Passenger carrying brought new requirements in the way of higher speed, greater acceleration, less reduction in the rear axle to give more miles per hour with a given engine, and quietness; but, because the conditions under which they operate are better, motor-coaches allow the use of the underslung worm, the propeller-shaft brake and a lower chassis, with the accompanying lowering of the center of gravity of the vehicle and, consequently, an increase in stability.
In 1917 and 1918, study of the mathematical analysis of the principles of tooth contact and angular velocity brought new designs in tooth forms, which are discussed. The salient advantages of the worm-drive are said to be (a) silence, (b) the fact that it retains its silence throughout its life, (c) its ability to resist shock loads without damage, and (d) its contours, which may be generated with a grinding-wheel after all the other operations have been performed; consequently, a high degree of accuracy in tooth profile, lead angle and tooth spacing may be maintained. Freedom from knock is attributed to the obliquity of approach of the teeth, to the greater areas in contact, and to the low resonant qualities of the materials used.


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