AUXILIARY TRANSMISSION

Auxiliary transmissions are mounted on the rear of the regular transmission to provide more gear ratios. Most auxiliary transmissions have only a L-low and a H-high (direct) range in a transfer assembly. The low range provides an extremely low gear ratio for hard pulls. At all other times, the high range should be used. Gears are shifted by a separate gearshift lever in the driver's cab (fig. 2-10).

Transfer Cases

Transfer cases are placed in the power trains of vehicles driven by all wheels (fig. 2-11). Their purpose is to provide the necessary _{offsets for} additional propeller-shaft connections _{to drive the} wheels.

Transfer cases in heavier vehicles have two-speed positions and a declutching device for disconnecting the ^{front driving wheels.} Two-speed transfer cases also serve as auxiliary transmissions.

Transfer cases are quite complicated. When they have speed-changing gears, declutching devices, and attachments for three or more propeller shafts, they are even larger than the main transmission.

Some transfer cases have an overrunning sprag unit (or units) on the front output shaft. A sprag unit is a form of a overrunning clutch; power can be transmitted through it in one direction but not in the other. During normal operation, when both front and rear wheels turn at the same speed, only the rear wheels drive the vehicle.

Figure 2-11.-Transfer case installed in a four-wheel

Figure 2-10.-Auxiliary transmission.

_{drive truck.}

However, if the rear wheels should lose traction and begin to slip, they tend to turn faster than the front wheels. When this occurs, the sprag unit automatically engages. This action allows the front wheels to also drive the vehicle. The sprag unit simply provides an automatic means of engaging the front wheels in drive for more traction.

Power Takeoffs

Power takeoffs, commonly known as the PTO, are attachments in the power train for power to drive auxiliary accessories. They are attached to the transmission, auxiliary transmission, or transfer case. A common type of PTO is the single-gear, single-speed type that is bolted to an opening provided in the side of the transmission case, as shown in figure 2-10. The sliding gear of the PTO meshes with the transmission countershaft gear. The operator can move a shifter shaft control lever to slide the gear in and out of mesh with the countershaft gear. The spring-loaded ball holds the shifter shaft in position.

On some vehicles, PTO units have gear arrangements that give two speeds forward and one in ^reverse. Several forward speeds and reverse gear arrangements are usually provided in PTO units used to operate winches and hoists.