A final drive is that part of the power train that transmits the power delivered through the propeller shaft to the drive wheels or sprockets. Because it is encased in the rear axle housing, the final drive is usually referred to as a part of the rear axle assembly. It consists of two gears called the ring gear and pinion. These may be spur, spiral, hypoid beveled, or worm gears, as illustrated in figure 13-17.

The function of the final drive is to change by 90 degrees the direction of the power transmitted through the propeller shaft to the driving axles. It also provides a fixed reduction between the speed of the propeller shaft and the axle shafts and wheels. In passenger cars this reduction varies from about 3 to 1 to 5 to 1. In trucks, it can vary from 5 to 1 to as much as 11 to 1.

The gear ratio of a final drive having bevel gears is found by dividing the number of teeth on the drive gear by the number of teeth on the pinion. In a worm gear final drive, you find the gear ratio by dividing the number of teeth on the gear by the number of threads on the worm.

Most final drives are of the gear type. Hypoid gears (fig. 13-17) are used in passenger cars and light trucks to give more body clearance. They permit the bevel drive pinion to be put below the center of the bevel drive gear, thereby lowering the propeller shaft. Worm gears allow a large speed reduction and are used extensively in larger trucks. Spiral bevel gears are similar to hypoid gears. They are used in both passenger cars and trucks to replace spur gears that are considered too noisy.

Chapter 11 described the construction and principle of operation of the gear differential. We will briefly review some of the high points of that chapter here and describe some of the more common types of gear differentials applied in automobiles and trucks.

The purpose of the differential is easy to understand when you compare a vehicle to a company of sailors marching in mass formation. When the company makes a turn, the sailors in the inside file must take short steps, almost marking time, while those in the outside file must take long steps and walk a greater distance to make the turn. When a motor vehicle turns a comer, the wheels outside of the turn must rotate faster and travel a greater distance than the wheels on the inside. That causes no difficulty for front wheels of the usual passenger car because each wheel rotates independently on opposite ends of a dead axle. However, to drive the rear wheel at different speeds, the differential is needed. It connects the individual axle shaft for each wheel to the bevel drive gear. Therefore, each shaft can turn at a different speed and still be driven as a single unit. Refer to the illustration in figure 13-18 as you study discussion on differential operation. the following

The differential described in chapter 11 had two inputs and a single output. The differential used in the automobile has a single input and two outputs. Its input is introduced from the propeller shaft and its outputs goes to the rear axles and wheels.

The bevel drive pinion, connected to the pinion shaft, drives the bevel drive gear and the differential case to which it is attached. Therefore, the entire, differential case always rotates with the bevel drive gear whenever the pinion shaft is transmitting rotary motion. Within the case, the differential pinions (refereed to as spider gears in chapter 11) are free to rotate on individual shafts called trunnions. These trunnions are attached to the walls of the differential case. Whenever the case is turning, the differential pinions must revolve-one about the other-in the same plane as the bevel drive gear.

The differential pinions mesh with the side gears, as did the spider and side gears in the differential described in chapter 11. The axle shafts are splined to the differential side gears and keyed to the wheels. Power is transmitted to the axle shafts through the differential pinions and the side gears. When resistance is equal on each rear wheel, the differential pinions, side gears, and axle shafts all rotate as one unit with the bevel drive gear. In this case, there is no relative motion between the pinions and the side gears in the differential case. That is, the pinions do not turn on the trunnions, and their teeth will not move over the teeth of the side gears.

When the vehicle turns a comer, one wheel must turn faster than the other. The side gear driving the outside wheel will run faster than the side gear connected to the axle shaft of the inside wheel. To compensate for this difference in speed and to remain in mesh with the two side gears, the differential pinions must then turn on the trunnions. The average speed of the two side gears, axle shafts, or wheels is always equal to the speed of the bevel drive gear.

Some trucks are equipped with a differential lock to prevent one wheel from spinning. This lock is a simple dog clutch, controlled manually or automatically, that locks one axle shaft to the differential case and bevel drive gear. This device forms a rigid connection between the two axle shafts and makes both wheels rotate at the same speed. Drivers seldom use it, however, because they often forget to disengage the lock after using it.

governed by the speed of the wheel applying tractive effort.

The no-spin differential does not contain pinion gears and side gears as does the conventional differential. Instead, it consists basically of a spider attached to the differential drive ring gear through four trunnions. It also has two driven clutch members with side teeth that are indexed by spring pressure with side teeth in the spider. Two side members are splined to the wheel axles and, in turn, are splined into the driven clutch members.