Transmission errors in precision worm gear drives

Transmission error is a measure of the positioning accuracy of a gear system. This
has been widely documented in gearing for many years as the source of problems in
noise and vibration. It is a result of errors in the contact conditions which affect the
driven gear with respect to the rotation of the driver gear. This research aims to
present a better understanding of the basic kinematics of worm gear systems by
identifying the significant influences which determine the contact conditions.

A literature review of existing theory is described which determines the major areas
considered in worm gear contact analysis. Formulae are derived which quantify the
effect of component parameter variation on contact. An investigation of the design,
manufacture, and operating processes is recorded which identifies error sources
relative to the theoretical contact condition. A computer program is developed which
calculates contact characteristics such as worm and wheel component form,
transmission error and contact marking pattern for a given design including any
contact error sources. Computer calculations are validated by comparing direct
measurements of these characteristics from several manufactured gear sets with
synthesised results produced using the design information, machine settings and error
sources detected during production. The behaviour of these gear sets during operation
under a torque load has been investigated experimentally. Measured transmission
error data from a test rig is used to develop a basic model of worm gear deformation
under load. This model has been added to the computer program to improve and
extend the analysis capability. The test rig has also been used to investigate the effect
of initial wear on contact characteristics.

The good correlation between calculated and experimental results shows that the
characteristics of a worm gear set can be predicted once all elements of the design and
manufacture are known. The results also validate the software as a useful design tool
for academic and industrial applications. Important conclusions are drawn on design
techniques, the manufacturing process, and the effects of operating under load.
Further areas of investigation are identified which offer future research an opportunity
to expand upon these conclusions.