The auxiliary transmission allows a rather small truck engine to move heavy loads by increasing the engine-to-axle gear ratios. The auxiliary transmission provides a link in the power trains of construction vehicles. This link diverts engine power to drive four and six wheels and to operate accessory equipment through transfer cases and power takeoff units. (See fig. 13-12).

Trucks require a greater engine-to-axle gear ratio than passenger cars, particularly when manufacturers put the same engine in both types of equipment. In a truck, the auxiliary transmission doubles the mechanical advantage. It connects to the rear of the main transmission by a short propeller shaft and universal joint. Its weight is supported on a frame crossmember as shown in figure 13-12. The illustration also shows how the shifting lever would extend into the drivers compartment near the lever operating the main transmission.

In appearance and in operation, auxiliary transmissions are similar to main transmissions, except that some may have two and some three speeds (low, direct, and overdrive).

Transfer cases are put in the power trains of vehicles driven by all wheels. 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, such as the one shown in figure 13-13, serve also as auxiliary transmissions.

Some transfer cases are 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. A cross section

of a common type of two-speed transfer case is shown in figure 13-14. Compare it with the actual installation in figure 13-13.

This same type of transfer case is used for a six-wheel drive vehicle. The additional propeller shaft connects the drive shaft of the transfer case to the rearmost axle assembly. It is connected to the transfer case through the transmission brake drum.

Some transfer cases contain an overrunning sprag unit (or units) on the front output shaft. (A sprag unit is a form of overrunning clutch; power can be transmitted through it in one direction but not in the other.)

On these units the transfer is designed to drive the front axle slightly slower than the rear axle. During normal operation, when both front and rear wheels turn at the same speed, only the rear wheels should lose traction and begin to slip. They tend to turn faster than the front wheels. As slipping occurs, the sprag unit automatically engages so that the front wheels also drive the vehicle. The sprag unit simply provides an automatic means of engaging the front wheels in drive whenever additional tractive effort is required. There are two types of sprag-unit-equipped transfers, a single-sprag unit transfer and a double-sprag unit transfer. Essentially, both types work in the same manner.