Motor-car principles; the gasoline automobile

by Roger B. Whitman

DIFFERENTIAL

When a car takes a corner, the outside wheels make a larger curve than the inside, and cover a longer distance. As the front wheels are loose on the axle, they accommodate themselves to this; but as both rear wheels are driven by the engine, it is necessary to apply a device that will permit them to rotate at different speeds without interfering with their driving the car. This is accomplished by means of a compensating and differential gear.

To understand the necessity for a differential, stand behind a wagon, with one hand on each tire; push, and if the vehicle is steered straight ahead, the hands will move ahead equally; but if the vehicle turns, the hand on the outside wheel will move ahead faster than the other. Now take a stick, and run it through the rear wheels so that it bears against the spokes; press it forward from its center, and if the vehicle moves straight ahead, the stick will go forward equally; but if the vehicle turns, the outside end of the stick will go ahead faster and farther than the other, although the pressure is being applied to its center.

Fig. 43.—Differentials.

In applying a simple form of differential the axle is divided into two parts, to the inner ends of which are fitted bevel gears, these being held at a fixed distance apart by the construction of the housing (Fig. 43). Between the bevel gears and in mesh with both of them are small bevel gears, or pinions, which may revolve on short studs carried on a ring so that they are a fixed distance apart. When the ring is revolved it carries with it the studs and pinions. The ring forms a housing that incloses the differential, and is driven by the single chain or driving shaft. To understand the action of the differential, imagine the rear wheels of a car to be jacked up clear of the ground so that they are free to revolve, and the housing to be revolved by hand. As it turns, the driving wheels will turn also, for the resistance to each is the same, and the pinions, being in mesh with both bevel gears, cannot revolve on their studs. If one of the wheels is now held stationary and the housing revolved, the bevel pinions will revolve on their studs, and roll around on the stationary gear; this will drive the gear of the free wheel at twice the speed of the housing. The revolving of the housing in the first instance caused the wheels to turn equally at the speed of the ring, and in the second permitted one to remain stationary while the other turned at twice the speed of the housing, the speed of the latter being unchanged. The first is the effect when the car moves straight ahead, and the second the result if the car could make so short a turn that it would pivot on one wheel.

With the wheels jacked up, hold one hand lightly against one of the wheels, so that while it may turn, there is more resistance to it than to the other. If the housing is now revolved, the bevel pinions will revolve on their studs, and roll slowly around the gear of the wheel that presents the resistance, the free wheel being revolved at a higher speed than the housing. This is the condition when the car takes a corner, for there is then more resistance to the inside than to the outside wheel, and it slows down; this will start the pinions revolving on their studs, and they will drive the outside wheel correspondingly faster.

With the housing revolving at a fixed speed, the outside wheel will revolve as much faster as the inner wheel is revolving slower; for an illustration, if the housing makes fifty revolutions a minute, and the inner wheel is slowed to forty, the outer will be driven at sixty revolutions.

If one wheel of a jacked-up car is revolved by hand, the other wheel will revolve in the opposite direction. This is caused by the housing remaining stationary and the pinions being revolved on their studs by the turning of the wheel, the movement being transmitted to the free bevel gear and wheel in the reverse direction.

The bevel gear differential described was the early type, but a more recent design employs spur gears. The axle ends carry spur instead of bevel gears, these being in mesh with other spur gears that are long, but of small diameter. These small gears are in pairs, as shown in Fig. 43, being in mesh with each other at their inner ends, and each member of a pair meshing with one of the axle gears. The small gears revolve on studs supported by the housing that is revolved by the drive, the studs in this case being parallel with the axle instead of at right angles to it, as are the studs in the bevel-gear type.

If the small gears meshed only with the axle gears, and not with each other, revolving the housing would cause them to roll around the axle gears, all rotating on their studs in the same direction, and the axle gears remaining stationary. Being in mesh with each other, they cannot revolve in the same direction, for when two gears are in mesh they must revolve in opposite directions. Thus the small gears cannot roll around on the axle gears when the housing is revolved, and if there is equal resistance to the turning of the wheels, the small gears will not revolve on their studs, but will carry the axle gears with them.

If the car is turning a corner, the greater resistance to the inner wheel will cause the small gears to revolve on their studs, rolling around the resisting gear and driving the other correspondingly faster.

On cars with double-chain drive, the differential is fitted to the jack shaft, and of course receives the drive from the change-speed mechanism through its housing.

Both the driving-shaft and double-chain drive have points of advantage and of weakness, and each type has its advocates. For the double chain, great strength can be claimed with light weight, as the axle is in one piece, and perfectly adapted to support the car. Against it is the difficulty of keeping the chains properly lubricated, and their consequent wear and stretching. The driving-shaft type has the advantage of the perfect lubrication of the parts, for all may be inclosed and running in oil or grease; the rear axle must be divided, however, which requires it to be heavily braced in order that the weight imposed on it may not bend or spring it out of line. Where a bent dead axle can be straightened by a blacksmith, a similar condition in a live axle requires the services of an expert mechanic; on the other hand, bevel gears make less noise than chains.