CHAPTER XXXI
THE DIFFERENTIAL GEAR

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.

Fig. 115. Differential Action Diagram

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.