Chapter 20 - The final drive and differential

Abstract

This chapter focuses on the final drive and differential of motor vehicle. The final-drive gears can be directly driven from the gearbox and incorporated in a front-mounted engine-transaxle unit. Alternatively, they can be mounted in a live rear axle or in a sprung rear drive unit. The bevel pinion is carried by taper-roller bearings supporting radial and axial loads. A shim adjustment allows the depth of engagement with the crown wheel to be varied, and shims or a collapsible spacer provide for the specified pinion-bearing preload torque. For heavy vehicles, worm drive provides silence, durability, a large gear reduction, and an easy facility for driving a second axle. It is costly and, with a purely sliding action, has a lower mechanical efficiency than bevel gearing. An alternative to providing additional ratios at the gearbox is the use of the two-speed epicyclic final-drive unit. The usual arrangement consists of an annulus formed on the inside of the crown wheel that drives four planet pinions mounted on a carrier formed as an extension of the differential cage. The differential provides an equal torque to each half-shaft or drive shaft. It allows the outer road wheel to revolve faster than the inner when cornering, while maintaining a positive drive to both wheels.