Automotive Power Transmission Systems

3Transmission Gear Design

3.1 Introduction

Gears are one of the most important components for mechanical systems. Although gears are applied in all machinery, the automotive industry is by far the largest user of gears because of the huge number of vehicle sales. In automotive powertrain systems, gears are indispensable, regardless what type of transmission is used for the vehicle. Different gear designs are applied for different types of transmissions. Lay‐shaft gears are applied mostly for manual transmissions and sometimes also for automatic transmissions, where all gears rotate about fixed axes and multiple transmission ratios are achieved by multiple gear pairs rotating about parallel shafts, as discussed in Chapter 2. Most of the conventional automatic transmissions use planetary gear trains to transmit power and achieve multiple gear ratios, with multiple clutches to control the power flows. Different from lay‐shaft gear systems, not all gears rotate about fixed axes in planetary gear systems. Continuously variable transmissions do not need as many as gears as manual transmissions or conventional automatic transmissions, but they still use gears in the final drive to increase the overall drive train ratio.

The history of gears dates back thousands of years in human civilization [1], but it was not until the eighteenth century that the gear industry started to take shape. The vast majority of gears in the gear industry today are involute gears (i.e. gears with involute ...