Determination of the ISO tooth form factor for involute spur and helical gears

Abstract

To estimate the bending stress at the root of a loaded gear tooth, ISO/TC-60 introduced the tooth form factor and the stress concentration factor, which were both defined as a function of the tooth thickness at the critical section and the distance between this section and the intersection of the line of action and the tooth centerline, when load acts at the highest point of single tooth contact. Both factors depend on many parameters of both mating gears and generating tool, but for every tool geometry, the critical section depends only on two: the number of teeth and the rack shift factor on the considered gear. This paper presents a general method for the determination of the critical section parameter for nonstandard, external gear teeth, considering tool offsets, center distance modifications and backlash. Tables for the most usual sets of tooth proportions (addendum, dedendum, tool tip radius and pressure angle) are given, which are useful to make calculations by hand. For each set, only one table with the above two input parameters is necessary to evaluate the bending stress for any value of the rack shift factors, the number of teeth on both gears, the operating center distance and the helix angle, and it is also valid for load acting at the tip or at the highest point of single tooth contact. An approximate, accurate method to compute the above factors with no iterations, which is valid for any tool geometry, is also presented.