However, if the rear wheels should lose traction and beginto slip, they tend to turn faster than the front wheels. Whenthis occurs, the sprag unit automatically engages. Thisaction allows the front wheels to also drive the vehicle.The sprag unit simply provides an automatic means ofengaging the front wheels in drive for more traction.Power TakeoffsPower takeoffs, commonly known as the PTO, areattachments in the power train for power to drive auxiliaryaccessories.They are attached to the transmission,auxiliary transmission, or transfer case. A common typeof PTO is the single-gear, single-speed type that is boltedto an opening provided in the side of the transmission case,as shown in figure 2-10. The sliding gear of the PTOmeshes with the transmission countershaft gear. Theoperator can move a shifter shaft control lever to slide thegear in and out of mesh with the countershaft gear. Thespring-loaded ball holds the shifter shaft in position.On some vehicles, PTO units have geararrangements that give two speeds forward and one inreverse.Several forward speeds and reverse geararrangements are usually provided in PTO units used tooperate winches and hoists.PROPELLER SHAFT ASSEMBLIESThe propeller shaft assembly (fig. 2-12) consists ofa propeller shaft, commonly know as the drive shaft, aslip joint, and two or more universal joints. Thisassembly provides a path through which power istransmitted from the transmission to the drive axleassemblies or auxiliary equipment. Vehicles, having along wheel base, are equipped with a propeller shaft thatextends from the transmission or transfer case to a centersupport bearing and a propeller shaft that extends fromthe center support bearing to the rear axle (fig. 2-13).Figure 2-12.—Propeller shaft assembly.Figure 2-13.—Propeller shaft assembly with center support bearing.2-9
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