Page 302 - Gear Technology Solutions

21 Cross-Axis Helical Gears - Misalignment and
Transmission Error

21.1 Basics of Cross-Axes Helical Gears

Cross helical gears are manufactured by a generating process using a trape-
zoidal generating profile. The question if the pinion can be referred to as a ZI
worm has to be answered with “no”. ZI refers to a straight profile in a worm
which is not manufactured by a generating process but merely by turning on a
lathe, like the trapezoidal threads of an ACME screw. The tool for the worm
gear replicates the worm, having a straight profile and generating the worm
gear just like the hobbing of a cylindrical gear. This means, the ZI worm is the
generator of the worm gear, which also means that the pair rolls conjugate with
no crowning [1, 2, 3]. The cross axes helical gearset has no relationship to a
worm gear set except the fact that both include a shaft angle different from ze-
ro degrees.

Figure 1: Spur gear pair (left) and 90° cross angle helical gear pair

An example of a spur gear pair (left) and a cross-axes helical gearset (right)
with a 90° shaft angle is shown in Figure 1. Due to the cross-axes angle, the
transverse involute profiles are in different planes and rotate about axes with
different directions. In their theoretical position, the generating trapezoid can
be placed between both members. As the generating element shifts, both
members rotate without transmission error. This is a result of the kinematic
coupling condition and not a result of the involute based gearing law. Changes

287