Differentials

Figure 2-15.-Gears used in final drives. cars, this reduction varies between 3 to 1 and 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 with bevel gears is frond by dividing the number of teeth on the driven or ring gear by the number of teeth on the pinion. In a worm gear final drive, the gear ratio is found by counting the number of revolutions of the worm gear for one revolution of the driven gear. Most final drives are gear type. Hypoid differential gears permit a lower body design. They permit the bevel-driven pinion to be placed below the center of the ring gear, thereby lowering the propeller shaft, as shown in figure 2-15. Worm gears allow a larger speed reduction and are sometimes used on large trucks. Spiral bevel gears are similar to hypoid gears and are used in both passenger cars and trucks to replace spur gears that are too noisy. DIFFERENTIALS Another important unit in the power train is the differential, which is a type of final drive. As shown in figure 2-16, the differential is located between the axles and permits one axle shaft to turn at a different speed from that of the other. At the same time, the differential transmits power from the transmission/transfer case to both axle shafts. The variation in axle shaft speed is Figure 2-16.—Differential operation. necessary when the vehicle turns a corner or travels over uneven ground. As a vehicle travels around a curve, the outer wheel must travel faster and further than the inner wheel. Without the differential, one rear wheel would be forced to skid when turns are made, resulting in excessive tire wear as well as making the vehicle more difficult to control. Some trucks have a differential lock to keep one wheel from spinning. This is a simple dog clutch, 2-11