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5-1  TRANSFER CASES

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 many 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  1 1/4-ton trucks 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.

CONSTRUCTION OF A TYPICAL TRANSFER

To understand how a transfer case works, it is best to first become familiar with the location of the internal parts.

The typical transfer assembly has one input shaft and three output shafts. Other transfers may have one input shaft and two output shafts: one shaft for the front axle and one for the rear axle or axles. Their operations will be much the same.

We will discuss the components as they are listed.

FIGURE 26. TYPICAL CONVENTIONAL TRANSFER ASSEMBLY FOR 6X6 VEHICLES.

At the top of the illustration, look at gear (1). It is mounted on the main input shaft (3) but is free to rotate without turning the shaft. It has external teeth on the outside that are in constant mesh with gear (14). It also has internal gear teeth on the inside. Gear (2) is also mounted on the main input shaft (3). However, notice that this gear is splined to the shaft and must turn anytime the input power shaft turns. It can slide back and forth on the shaft between gears (1) and (4). This gear has external teeth and acts as a dog clutch between gear (1) and shaft (3).

Gear (4), the rear axle (rear unit) drive gear, is fastened to and turns item (5), the rear axle (rear unit) drive shaft assembly. Gear (4) is in mesh with gear (6), the idler shaft constant-mesh gear.

Gear (6) has both external and internal teeth. It is mounted on, and is free to turn on, item (7), the idler shaft. The idler shaft constant-mesh gear is in mesh with gear (8), the drive shaft constant-mesh gear.

The drive shaft constant-mesh gear has external and internal teeth. It is mounted on item (9), the rear axle (front unit) drive shaft. An external tooth gear, fastened to item (9), is in mesh with the internal teeth of the drive shaft constant-mesh gear (8). This gear turns item (9), the rear axle (front unit) drive shaft.

Gear (8) is in mesh with gear (10), the drive shaft constant-mesh gear. This gear has external and internal teeth and is mounted on, and free to turn on, item (11), the front axle drive shaft.

Gear (12), the drive shaft sliding gear, has external teeth and internal splines and can slide on the splines of the front axle drive shaft. When the external teeth of gear (12) are in mesh with the internal teeth of the drive shaft constant-mesh gear (10), the front axle drive shaft is engaged and will turn at the same speed as gear (10).

Gear (13), the idler shaft low-speed gear, is mounted on the idler shaft. One end of gear (13) is in mesh or engaged with gear (6), the idler shaft constant-mesh gear. The other end is in mesh or engaged with item (14), another idler shaft constant-mesh gear. This locks gears (6), (13), and (14) together so they turn as a unit. The idler shaft constant-mesh gear (14) is in mesh with the main shaft constant-mesh gear (1).

OPERATION

Now that you have seen how the parts of a transfer fit together, we will take a look at what goes on when the transfer is in operation. When the transfer is operating in high range, with the front-wheel drive engaged, all wheels are pulling.

Power comes into the transfer from the transmission on the main shaft (3). When the main shaft turns, the main shaft sliding gear (2) turns because it is splined to the main shaft. In high range, the main shaft sliding gear is in mesh with the internal teeth of gear (1), the main shaft constant-mesh gear. This causes gear (1) to turn.

The external teeth of gear (1) are in mesh with the external teeth of gear (14), the idler shaft constant-mesh gear. When gear (1) turns, it drives gear (14).

One end of gear (13), the idler shaft low-speed gear, is in mesh with gear (14). The other end is in mesh with gear (6), the other idler shaft constant-mesh gear. This means that when gear (14) turns, all gears on the idler shaft turn.

Gear (6) is in mesh with gear (4). When gear (6) turns, it drives gear (4); gear (4) turns shaft (5) which sends power to the rear axle rear unit.

Gear (6) is also in mesh with and drives gear (8). The internal teeth of gear (8) are in mesh with the gear fastened to shaft (9). When gear (8) is turned, power is sent to the rear axle front unit.

Gear (8) is also in mesh with and turns gear (10). When the front-wheel drive is engaged, gear (10) is in mesh with and drives gear (12), the drive shaft sliding gear. Gear (12) is splined to shaft (11), the front-axle drive shaft. When gear (12) is turned, power is sent through shaft (11) to the front axle. The driver, by means of a shift lever and linkage, can slide gear (12) away from gear (10). When gear (12) is not in mesh with gear (10), no power is being sent to the front axle.

When more torque is needed to move the vehicle, the driver can shift the transfer to low range. The shifting linkage on some vehicles is made so that the vehicle must be in front-wheel drive before shifting to low range. This prevents the driver from sending full torque to the rear wheels only, which might cause damage.

When the transfer is shifted to low range, the mainshaft sliding gear (2) is disengaged from the constant-mesh gear (1). It is then engaged with the idler shaft low-speed gear (13). Gear (2) is the driving gear and is smaller than driven gear (13). This gives less speed and more torque or a gear reduction. From here on, the power flow is the same as in high range. The transfer is in neutral, while gear (2) is not in mesh with either gear 1 or gear

Learning Event 2:
DESCRIBE THE CONSTRUCTION AND OPERATION OF

5-2  SPRAG UNITS

Some transfers contain an overrunning sprag unit (or units) on the front output shaft. The operation of these units is much like that of an overrunning clutch in the starting motor.

On these units, the transfer is made to drive the front axle a little slower than the rear axle(s). During normal operation on good roads, when both front and rear wheels turn at the same speed, only the rear wheels drive the vehicle. However, if the rear wheels start to slip, they turn faster than the front wheels. If this happens, 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-wheel drive whenever the back wheels start to slip. There are three types of sprag units used with transfers. They are a single sprag unit, a double-sprag unit, and an air-operated, double sprag unit. They all work in almost the same way. This type transfer is much like the one we just discussed. The main difference is that a sprag unit is used instead of the hand-operated sliding gear or clutch on the front output shaft.

The sprag is a steel block shaped to act as a wedge in the complete assembly. In some units, there are 42 sprags assembled into an outer race and held in place by two springs. The springs fit into the notches in the ends of the sprags and hold them in place. The outer race is in the driven gear on the front output shaft. The inner race is on the front output shaft.

Now, let's see how a single-sprag unit operates. During normal operation, the front and rear wheels are turning at the same speed. The outer race of the sprag unit (in the driven gear) turns a little slower than the inner race (on the front output shaft). This prevents the sprags from wedging between the races. No lockup takes place, and the front wheels turn freely; they are not driven. However, if the rear wheels start to slip and turn faster than the front wheels, the outer race tries to turn faster than the inner race. When this happens the sprags wedge or jam between the two races. The two races now turn as a unit and provide driving power to the front wheels. As soon as the rear wheels quit slipping and start to turn the same speed as the front wheels, the sprags release.

FIGURE 27. TRANSFER ASSEMBLY SPRAG UNIT.

In reverse, it is necessary to lock out the single-sprag unit. The turning is reversed; this means that we cannot drive through the sprag unit. The lockout is done through a linkage to the transmission that shifts a reverse shift collar in the transfer. As the reverse shift collar is shifted, internal splines in the collar mesh with external splines on the reverse shift driven gear and on the front output gear so there is a solid drive around the sprag unit.

The double-sprag unit operates the same way as the single-sprag in forward speeds. The difference between the two units shows up in reverse. In the double-sprag unit, a second sprag unit has been added which operates only in reverse.

When the shift is made to reverse, the forward sprag unit is locked out, almost exactly the same as the single-sprag unit. However, the reverse-sprag unit now comes into operation. The front wheels drive, in reverse, when the rear wheels start to slip. The shift from one sprag unit to the other is done by a linkage to the transmission. This shifts a reverse shift collar in the transfer. As the reverse shift collar is shifted, internal splines (or teeth) in the collar unmesh from the external splines on the outer race of one sprag unit. It then meshes with the external splines on the other sprag unit.

Another type sprag unit is the air-controlled, double-sprag unit. This unit does the same job as the double-sprag unit we just discussed. The main difference is an air valve on the transmission low and reverse shifter shaft. This valve automatically shifts the sprag unit to forward or reverse whenever the transmission is shifted to low or reverse.

 

FIGURE 28. AIR-CONTROL DIAGRAM OF TRANSMISSION AND TRANSFER ASSEMBLY USING AN AIR-CONTROLLED, DOUBLE-SPRAG UNIT.

This illustration shows an air-control diagram of the transmission and transfer assembly using an air-controlled, double-sprag unit. This type is used in the 5-ton truck. Now let's take a look at what happens when there is compressed air in the system, and the transmission is in a forward or neutral position. The air cylinder control valve will admit air under pressure from the compressed air system into the shift air cylinder assembly. The air is then admitted to the forward-shift side of the spring-balanced piston in the cylinder. This moves the piston and causes engagement of the forward sprag unit. When the transmission is shifted into reverse, the air cylinder control valve admits air to the opposite side of the piston. This causes engagement of the reverse sprag unit. When the forward sprag unit is engaged, the front wheels will freewheel or turn only in a forward direction. If the reverse sprag unit is engaged, the front wheels cannot be turned in a forward direction. A parked vehicle, with air pressure in the system and the transmission in neutral, cannot be pushed backward until the transmission is shifted to reverse.

The transfer uses the dip or splash type of lubrication system. The gears are in the transfer case, and the case is filled with gear oil (GO 90 in warm weather). As the gears operate, they turn in the oil and no pressure is used. The turning gears will throw the oil onto the shafts and into the bearings. The oil is kept from leaking from the case by seals around the input and output shafts.

David L. Heiserman, Editor

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All Rights Reserved

Revised: June 06, 2015