专业英语复习范围.docVIP

UNIT 10 THE DIFFERENTIAL P99 Most cars have a standard, or unlimited-slip, differential. (A limited-slip differential costs more and is not necessary in most normal driving conditions.) As mentioned, a differential must split power unequally when a car goes around a corner. Like the transmission, the differential has many gears and parts. 10.1 The Main Gears Suppose a shaft spins in one direction. You want that shaft to drive another shaft at an angle of 90° to the first shaft. You can do this by meshing two gears, each having teeth set at a 45° angle. There would be one gear on each shaft. Together, the two gears produce a 90° change in direction. Figure 10-1. Gears with teeth at such an angle are called bevel gears because they are beveled, or cut on an angle. The bevel gears in Figure 10-1 work well when one shaft must drive another. But in a final drive assembly, two axle shafts must be driven. (One shaft goes to each drive wheel.) Then one of the beveled gears must have a hole in its center through which the shaft extends. The large circular gear on the axle shafts is called the ring gear. Figure 10-1. The final drive assembly also provides gear reduction so that the drive wheels spin more slowly than the drive shaft. The gear reduction varies, depending on engine size and power, engine torque, and vehicle size and weight. In general most final drives produce gear reduction ratios of about 2.50: 1 to 3.50: 1. Thus, the drive wheels turn at about one-third the speed of the drive shaft. (If the drive shaft spins at 3,000 rpm, the drive wheels turn at about 1,000 rpm.) To give the necessary reduction, the driving gear in the final drive must be smaller than the driven gear, which is the ring gear. Thus, the driving gear must make several turns for every single turn of the ring gear. The driving gear is called the pinion gear or pinion. Figure 10-1. The drive shaft turns the pinion. The pinion rotates the ring gear and axle shafts. If the teeth of the be