Differential gears were designed to allow for equalization of the power strain transmitted to the rear axles.

The rotary movement is transmitted to the axles joining the wheels by a bevel gear, which if simple would drive both wheels at the same speed. This is satisfactory on the “straight ahead” drive, but it is clear that in turning a corner the car is describing a portion of a circle, and the inner wheel having a smaller circumference to traverse, must go at less speed than the outer. The differential gear was devised to allow for this difference in power stresses.

It is perhaps the functional action more than the simple mechanism that one finds the most confusion about. The diagram given in Fig. 115 shows how the functional action is mechanically carried out.

In the first place, each wheel, W, is fixed firmly to an independent axle turned by pinions, D and E. These pinions are connected by another, C. Now if D turns, E will rotate in the opposite direction due to the action of C. If D and E are rotating in the same direction at the same speed, C will merely lock with them and not rotate. If now, D accelerates slightly, C will turn, slowly retarding E, while if E accelerates, C will turn slowly in the opposite direction retarding D. This is precisely what is required in turning a corner. Now fix these in a box, driven as a whole by the bevel or ring gear B driven by the driving pinion gear A. When the car is on the straight ahead drive D, C, E are locked. C does not rotate and the three act as a single axle. As the car turns, C turns slowly, acted upon by the outer wheel, and gives the differential action.

The Worm Gear Drive.—The worm gear drive differential action is practically the same as the bevel gear action, the only difference being that there is a worm gear on the end of the drive shaft which engages with a helical toothed gear, which takes the place of the bevel gear B.

Fig. 116 shows the differential gear assembly which is carried by a set of bearings. These bearings are held in place by a set of shoulders, or retainers which are built into the housing on each side of the differential assembly. These bearings may be of either the radial, roller, or ball type. However, when the ball or roller bearing is used for carrying the differential, an end thrust bearing must be used in conjunction to take the end thrust and for adjusting purposes. The differential assembly shown is known as the bevel gear and pinion drive. The pinion gear is keyed to the tapered end of the drive shaft and usually does not carry an adjustment. The bevel gear mesh adjustment is made by setting the bearing supporting the differential assembly backward or forward. This adjustment, however, applies mostly to the full floating axle, as the axle shaft in this case usually has a square end which slides into the small bevel gear of the differential. The shaft used in this type of axle may be drawn out through the wheel and replaced without disassembling the axle or removing the weight from the wheels.

When the Hotchkiss drive is employed in combination with the semi-floating or three-quarters floating axle, three adjusting points will be found. Fig. 117 shows the three points at which adjustments are made. The short drive shaft carries the pinion gear at the rear end, and a universal joint at the front end is supported by a set of radial bearings inside of the front and rear ends of the housing.

The adjustment on this shaft is made by turning the notched cone A1 to the right, which pushes the bearings farther upon the bearing cones and reduces the looseness. After the short shaft has been properly adjusted, remove the lugs B, which fit into the notches of the adjustment nuts, A2 and A3, and turn A2 to the left to loosen, now turn A3 to the right until the bevel gear is meshing properly with pinion gear, then replace the lugs, B, to hold the adjustment. It is only necessary to make this adjustment when play occurs from natural wear, which will happen probably once in every five to seven thousand miles.

Fig. 118 shows a cross-section of the Allen gearless differential. The main gearing is bolted to the casing. The wheel shafts are splined to ratchet rings. The two lugs of the pawl block are secured in slots in the casing so that the block turns with it. Eight pawls on the pawl block drive, the ratchet rings two on each side operate for forward, and two on each side for reverse. The pawls permit either ratchet ring to overrun them and move freely in the direction of motion, so long as it is moving faster than the pawl block. The lugs of the pawl block have a little motion, about ³⁄₁₆″, in the slots, so that the casing moves this distance before engaging them for forward or reverse motion. This operates the rocking cams by their heads inserted in slots in right angles to the lugs, having the effect of pressing on and disengaging the forward or reverse pawls according to the direction of the motion.

When the car is running by its momentum with the clutch out, the action is reversed and the ratchet rings drive the casing and driving gear through the pawl block.

The adjustment given above also applies to the setting of the Allen differential.

Lubrication.—See Chapter on Axles.