In previous lessons, we discussed how transmissions could provide various gear ratios so that the engine could develop enough power for starting a vehicle, running at high speeds, and pulling heavy loads. We also mentioned that military vehicles had to be able to operate in mud, snow, sand, and other unusual terrains. To do this, driving power has to be available at the front wheels as well as the rear wheels so the vehicle will not get stuck. Therefore, tactical military wheeled vehicles include a second gearbox called the transfer case. Its purpose is to take the output power from the transmission and divide it so that it will drive the rear wheels at all times and drive the front wheels when needed.
FIGURE 25. TYPICAL DRIVELINE ARRANGEMENT WITH TRANSFER ASSEMBLY.
The transfer case can be mounted in several ways in a vehicle. It can be a separate component mounted to the rear of the transmission and driven by a propeller shaft connecting it to the output of the transmission. It can also be a part of the transmission and driven by a gear on the output shaft of the transmission. The transfer case performs one or more of the following functions:
• It transfers the transmission power to a point low enough so that a propeller shaft can be mounted under the engine and power the front axle.
• It provides an output to power one or more rear axles.
• It provides a high and low gear ratio for vehicles that do not have the necessary gear reductions in the transmission.
• It provides arrangements for engaging and disengaging front wheel drive and high and low ranges when applicable.
Because of its many functions, the shape, mounting, construction, and appearance of transfers for various vehicles will differ. In the following paragraphs, we will discuss the wheeled vehicle transfer cases.
The transfer case used with the M151A1, 1/4-ton vehicle is also mounted directly on the transmission. This transfer does not provide a separate gear reduction and has only a direct drive. It does have a shift lever for engaging and disengaging the front-wheel drive.
The transfer case of the M561, 1 1/4-ton truck is mounted at the output end of the transmission. It transfers power from the transmission to the propeller shafts and differentials. Selections are high- and low-operating ranges, neutral to disengage the case, and two- or six-wheel drive. Two-wheel drive disengages the front and rear axles, leaving only the center wheels to drive the vehicle.
The 3/4-ton and 1 1/4-ton trucks use the same type of transfers. They are mounted behind the transmission and connected to the output of transmission. They contain a high and low range. Engaging and disengaging of front-wheel drive and range selections are done manually with shift levers.
The 2 1/2-ton and 5-ton truck transfers are similar. They are mounted behind the transmission and powered by a propeller shaft. All have a high and low range which is selected with a manual lever. On some vehicles, the front-wheel drive engages automatically anytime the rear wheels start turning faster than the front wheels. This is done with a sprag unit that is similar to a bicycle coaster brake. On other models, the front-wheel drive is engaged and disengaged with a manual shift.
For our discussion in this lesson, we will refer to the transfer cases used on 2 1/2-ton trucks primarily. Most transfers work about the same. However, more unit level maintenance is required on 2 1/2-ton transfers than most other models. |