Analysis of the efficiency of the planetary gear sets

Abstract

Planetary gears, obtained by the kinematic combinations of toothed pairs with external and internal contacts, are able to provide considerably improved performance compared to the gear pairs with fixed axes, however if not properly designed they can exhibit poor performance regarding gear efficiency. In this regard, and given the fact that majority of research paper concerning this topic emphasize that planetary gears have generally better weight-to-power ratio and improved efficiency compared to the gear pairs with fixed axes, this paper examines the above statements. The main aim is to determine the domain of feasible gear ratios for which the planetary gear sets exhibit better performance, in terms of efficiency and weight-to-power density, than the classical fixed axes gears. In this work, the model of instantaneous gear efficiency is presented and applied to study the efficiency of both standard and high contact ratio involute spur gears. Approximate expressions for the friction power losses and for the efficiency are presented assuming the friction coefficient is changeable along the path of contact.