When both wheels are on firm ground and the vehicle is traveling freely, the differential is enabled to act in the usual manner when turning corners, by reason of the fact, already alluded to, that the crown wheels can drive the worms. Each driving wheel is attached to its respective crown wheel, and when a curve in the road is followed, the outer wheel is forced by its contact with the road to travel a greater distance than the inner one. The outer wheel, therefore, revolving faster than the axle, turns the worm in connection with it and so enables the central pinions to act and react on the worms with a differential action and to distribute the power to each wheel in the usual manner.
The Walter differential (Fig. 108) is also of the irreversible worm-gear type, which drives both wheels regardless of the traction conditions of the other wheel and which still has a compensating differential action. It consists of two pairs of spiral gears mounted in a two-part housing, and meshing together and separately with worms in the housing and on the drive shafts. Both halves of the housing are alike except for the bevel or ring-gear flange. The two bolts which bolt the housing together set one-half ahead of the other so that the spiral-gear pairs mesh directly together.
The operation of this device is as follows: The driving resistance of the road wheels tends to rotate the spiral gears on their pins, but in opposite directions, but if one road wheel has a greater resistance the inequality of force cannot drive the other wheel faster as the spiral gear cannot drive the worm, so both wheels are positively driven. When turning the outer wheel rolls faster and so permits the inside wheel to turn correspondingly slower, giving a compensating differential action.
Lately there has been a tendency to substitute an equalizing gear for the differential which to certain extent eliminates its disadvantages, since this equalizing device can be so arranged as to limit the pull on one wheel to the amount required to slip the other. However, as the use of these has been limited to low-powered passenger cars they will not be considered in this chapter.
When all four wheels of a vehicle are used as driving wheels, it is generally necessary to provide three differentials; one in the transmission case to equally divide the turning effort between each pair of driving wheels and one for each front and rear drive.
Thus the differential is absolutely necessary in any form of final drive used on commercial vehicles and it presents considerable advantages in protecting all parts of the mechanism against stress when turning corners. While it also has the disadvantage of stalling a vehicle when one wheel gets into a mud hole.
This disadvantage may be overcome by providing a differential lock, this again places the responsibility upon the operator and should he attempt to round a curve with the differential locked, the tires and driving mechanism would be subjected to considerable wear. Should this be done quite frequently, the results would soon be noticeable.
In order to overcome the latter feature, this lock is operated by a foot pedal and so arranged that it must always be held in engagement. Removing the foot pressure disengages the lock, thus providing for the poor memory of the operator.
Fig. 107. Powrlok Differential.
Fig. 108. Exterior of the Walter Differential.