Figure 13-14.-Cross section of a two-speed transfer case.of a common type of two-speed transfer case isshown in figure 13-14. Compare it with the actualinstallation in figure 13-13.This same type of transfer case is used for asix-wheel drive vehicle. The additional propellershaft connects the drive shaft of the transfer caseto the rearmost axle assembly. It is connected tothe transfer case through the transmission brakedrum.Some transfer cases contain an overrunningsprag unit (or units) on the front output shaft. (Asprag unit is a form of overrunning clutch; powercan be transmitted through it in one direction butnot in the other.)On these units the transfer is designed to drivethe front axle slightly slower than the rear axle.During normal operation, when both front andrear wheels turn at the same speed, only the rearwheels should lose traction and begin to slip. Theytend to turn faster than the front wheels. Asslipping occurs, the sprag unit automaticallyengages so that the front wheels also drive thevehicle. The sprag unit simply provides anautomatic means of engaging the front wheels indrive whenever additional tractive effort isrequired. There are two types of sprag-unit-equipped transfers, a single-sprag unit transferand a double-sprag unit transfer. Essentially, bothtypes work in the same manner.POWER TAKEOFFSPower takeoffs are attachments in the powertrain for power to drive auxiliary accessories. Theyare attached to the transmission, auxiliarytransmission, or transfer case. A common type ofpower takeoff is the single-gear, single-speed typeshown in figure 13-15. The unit bolts to anopening provided in the side of the transmissioncase as shown in figure 13-12. The sliding gear ofthe power takeoff will then mesh with thetransmission countershaft gear. The operator canmove a shifter shaft control lever to slide the gearin and out13-12
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