THE CONSTRUCTION AND OPERATION OF 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 singlesprag unit, a double-sprag unit, and an air-operated, doublesprag 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 handoperated 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 springbalanced 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. |