be spur, spiral, hypoid beveled, or worm gears, asillustrated in figure 13-17.The function of the final drive is to change by 90degrees the direction of the power transmitted throughthe propeller shaft to the driving axles. It also providesa fixed reduction between the speed of the propellershaft and the axle shafts and wheels. In passenger carsthis 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 isfound by dividing the number of teeth on the drive gearby the number of teeth on the pinion. In a worm gearfinal drive, you find the gear ratio by dividing thenumber of teeth on the gear by the number of threads onthe worm.Most final drives are of the gear type. Hypoid gears(fig. 13-17) are used in passenger cars and light trucksto give more body clearance. They permit the beveldrive pinion to be put below the center of the bevel drivegear, thereby lowering the propeller shaft. Worm gearsallow a large speed reduction and are used extensivelyin larger trucks. Spiral bevel gears are similar to hypoidgears. They are used in both passenger cars and trucksto replace spur gears that are considered too noisy.DIFFERENTIALSChapter 11 described the construction and principleof operation of the gear differential. We will brieflyreview some of the high points of that chapter here anddescribe some of the more common types of geardifferentials applied in automobiles and trucks.The purpose of the differential is easy to understandwhen you compare a vehicle to a company of sailorsmarching in mass formation. When the company makesa turn, the sailors in the inside file must take short steps,almost marking time, while those in the outside file musttake long steps and walk a greater distance to make theturn. When a motor vehicle turns a comer, the wheelsoutside of the turn must rotate faster and travel a greaterdistance than the wheels on the inside. That causes nodifficulty for front wheels of the usual passenger carbecause each wheel rotates independently on oppositeends of a dead axle. However, to drive the rear wheel atdifferent speeds, the differential is needed. It connectsthe individual axle shaft for each wheel to the beveldrive gear. Therefore, each shaft can turn at a differentspeed and still be driven as a single unit. Refer to theillustration in figure 13-18 as you studydiscussion on differential operation.the followingFigure 13-18.-Differential with part of case cut away.The differential described in chapter 11 had twoinputs and a single output. The differential used in theautomobile has a single input and two outputs. Its inputis introduced from the propeller shaft and its outputsgoes to the rear axles and wheels.The bevel drive pinion, connected to the pinionshaft, drives the bevel drive gear and the differential caseto which it is attached. Therefore, the entire, differentialcase always rotates with the bevel drive gear wheneverthe pinion shaft is transmitting rotary motion. Within thecase, the differential pinions (refereed to as spider gearsin chapter 11) are free to rotate on individual shaftscalled trunnions. These trunnions are attached to thewalls of the differential case. Whenever the case isturning, the differential pinions must revolve-oneabout the other-in the same plane as the bevel drivegear.The differential pinions mesh with the side gears, asdid the spider and side gears in the differential describedin chapter 11. The axle shafts are splined to thedifferential side gears and keyed to the wheels. Poweris transmitted to the axle shafts through the differentialpinions and the side gears. When resistance is equal oneach rear wheel, the differential pinions, side gears, andaxle shafts all rotate as one unit with the bevel drive gear.In this case, there is no relative motion between the13-15
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