Auxiliary Transmissions

The auxiliary transmission is used to provide additional gear ratios in the power train (Figure 10-13). This transmission is installed behind the main transmission, and power flows directly to it from the main transmission when of the integral type, or by a short propeller shaft (jack shaft) and universal joints.

Support and alignment are provided by a frame cross member. Rubber-mounting brackets are used to isolate vibration and noise from the chassis. A lever that extends into the operator's compartment accomplishes shifting. Like the main transmission, the auxiliary transmission may have either constant mesh gears or synchronizers to allow for easier shifting.

This transmission, when of the two-speed design, has a low range and direct drive. Three- and four-speed auxiliary transmissions commonly have at least one overdrive gear ratio. The overdrive position causes increased speed of the output shaft in relation to the input shaft. Overdrive is common on heavy-duty trucks used to carry heavy loads and travel at highway speeds.

The auxiliary transmission can provide two-speed ratios. When it is in the direct drive position, power flows directly through the transmission and is controlled only by the main transmission. When the auxiliary transmission is shifted into low range, vehicle speed is reduced and torque is increased. When the low range is used with the lowest speed of the main transmission, the engine drives the wheels very slowly and with less engine horsepower.

In this constant mesh auxiliary transmission, the main gear is part of the input shaft, and it is in constant mesh with the countershaft drive gear. A pilot bearing aligns the main shaft output shaft with the input shaft. The low-speed main shaft gear runs free on the main shaft when direct drive is being used and is in constant mesh with the countershaft low-speed gear. A gear type dog clutch, splined to the main shaft, slides forward or backward when you shaft the auxiliary transmission into high or low gear position.

In high gear, when direct drive from the main transmission is being used, the dog clutch is forward and makes a direct connection between the input shaft and the main shaft. When in low gear, the dog clutch is meshed with the low-speed, main shaft gear and is disengaged from the main drive gear.