A sprag unit is a steel block shaped to act as a wedge in the complete assembly. In the sprag unit there are 42 sprags assembled into an outer race and held into place by two energizing springs (fig. 5-31). The springs fit into notches in the ends of the sprags and hold them in position. The outer race is the driven gear on the front output shaft. The inner race is on the front output shaft itself.

On these units, the transfer case is designed to drive the front axle slightly slower than the rear axle. During normal operation, when both front and rear wheels of the vehicle are turning at the same speed, the outer race of the sprag unit (in the driven gear) turns slower than the inner race (on the output shaft). This design prevents the sprags from wedging between the races. No lockup occurs and the front wheels turn freely; they are not driven (fig. 5-32). However, if the rear wheels should lose traction and begin to slip, they tend to turn faster than the front wheels; the outer race tends to turn faster than the inner race. When this happens, the sprags wedge or jam between the two races and the races turn as a unit to provide power to the front wheels (fig. 5-32).

Figure 5-31 .- Transfer case sprag unit.