The determination of hob offset design values for nonstandard spur gears

Abstract

It is easy for a designer to design a set of spur gears to operate properly at a standard center distance. But the problem is more complicated when a predetermined center distance is necessary that is not considered standard. When the gears are cut, the hob can he advanced into or withdrawn from the blank to give teeth of a necessary thickness so that the gears will mesh properly at the nonstandard center distance. A derivation for determining the sum of the hob offsets for the pinion and gear has been shown by several authors, but there is no mathematical expression to separate the total hob offset into the individual hob offsets needed on the pinion and the gear. Several authors describe the advantages of designing gears with a substantial length of recess and a minimal length of approach. They describe the gears as operating with lower friction, less wear, less noise, and smoother action. Accordingly, the object of this investigation was to determine the individual hob offsets of the pinion and gear based on a high ratio of recess ·action to approach action. A computer program was used to produce design charts and tables to be used by the designer so that he can easily determine the individual hob offsets necessary for particular numbers of teeth and changes in center distance.