Figure 2-15.-Gears used in final drives.cars, this reduction varies between 3 to 1 and 5 to 1. Intrucks, it can vary from 5 to 1 to as much as 11 to 1.The gear ratio of a final drive with bevel gears isfrond by dividing the number of teeth on the driven orring gear by the number of teeth on the pinion. In aworm gear final drive, the gear ratio is found by countingthe number of revolutions of the worm gear for onerevolution of the driven gear.Most final drives are gear type. Hypoid differentialgears permit a lower body design. They permit thebevel-driven pinion to be placed below the center of thering gear, thereby lowering the propeller shaft, as shownin figure 2-15.Worm gears allow a larger speedreduction and are sometimes used on large trucks.Spiral bevel gears are similar to hypoid gears and areused in both passenger cars and trucks to replace spurgears that are too noisy.DIFFERENTIALSAnother important unit in the power train is thedifferential, which is a type of final drive. As shown infigure 2-16, the differential is located between the axlesand permits one axle shaft to turn at a different speedfrom that of the other. At the same time, the differentialtransmits power from the transmission/transfer case toboth axle shafts. The variation in axle shaft speed isFigure 2-16.—Differential operation.necessary when the vehicle turns a corner or travels overuneven ground. As a vehicle travels around a curve, theouter wheel must travel faster and further than the innerwheel. Without the differential, one rear wheel wouldbe forced to skid when turns are made, resulting inexcessive tire wear as well as making the vehicle moredifficult to control.Some trucks have a differential lock to keep onewheel from spinning. This is a simple dog clutch,2-11
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