Motor-car principles; the gasoline automobile

by Roger B. Whitman

FINAL DRIVE

From the change-speed mechanism the power is passed to the driving wheels by the final drive.

Fig. 37.—A, Propeller or driving-shaft drive; B, single-chain drive.

In the most usual construction the engine is so placed that the crank shaft is at right angles to the axle, and it is therefore necessary to change the direction in which the power acts, which is done by means of bevel gears. In ordinary spur gears the teeth are parallel to the shaft, and the two shafts that carry them are parallel, while in bevel gears the teeth are at an angle, and the shafts may be at right angles to each other. In Fig. 37 the diagram of the single-chain drive illustrates a car in which the engine is in the center of the frame, and as the crank shaft is parallel to the axle, the power may be directly applied. In the illustration of the propeller or driving-shaft drive the crank shaft is at right angles to the axle, and the power is turned by means of the bevel gears at the rear axle.

The single-chain drive can only be used for light cars, and is usually applied in connection with a change-speed mechanism of the planetary type.

Fig. 38.—Typical Universal Joint.

The propeller-shaft drive requires the use of universal joints, which are devices that permit one shaft to drive another, even though they are at an angle with each other. A typical universal joint is illustrated in Fig. 38. The ends of the shafts bear yokes, the ends of which are pivoted to a block of metal of + shape. When the two shafts are in line, the joint will force one to rotate with the other, and this will not be prevented if the two are out of line, for then the pivots will act, the + swinging on its pivots in the yokes.

Fig. 38A.—Types of Shaft Drives.

The change-speed mechanism is carried on the frame of the car, and is therefore supported by the springs, but the axle end of the driving shaft follows the axle as that follows the inequalities of the road. One end of the propeller shaft is therefore comparatively stationary, while the other is in constant motion, and if the shaft were inflexible it would be jammed in its bearings and twisted out of line. This is prevented by the universal joints with which the shaft is provided, there being one and often two in the shaft, and usually one between the clutch and change-speed mechanism.

Fig. 39.—Live Axle—Non-Floating Type.

Fig. 40.—Live Axle—Full Floating Type.

The single-chain and driving-shaft drives require the use of a live axle, which is an axle that revolves with the wheels. The simple type of live axle consists of the shaft to which the wheels are attached, and the housing that contains and supports it (Fig. 39). This axle is continuous, and usually has square ends that fit into the square hubs of the wheels so there may be no slipping. The second diagram in Fig. 40 shows a live axle of the floating type, in which the revolving part serves only to turn the wheels. The housing is extended, and the wheels run on its ends, the driving part projecting beyond the housing and having square ends that are secured to the outside of the hubs by square caps. The wheels thus run on the housing, which takes the weight of the car from the driving part. A live axle must be divided into two parts in order that a differential gear may be fitted, and the housing must therefore be strong enough to support the weight and prevent sagging. The efficiency of a bevel gear is greatly reduced if the teeth are not in their exact mesh, and sagging of the axle will throw them out to such an extent that they will be noisy, and wear rapidly. The floating type of live axle, in relieving the driving part of the weight, has a great advantage over the simple type, and is in general use.

Fig. 40A.—Dead Axle with Driving Shaft. Axle supports Bevel Gear and Differential. The Driving Shaft is supported in Bearings at the Differential end, and drives the wheels through clutches in the hubs.

With the driving-shaft drive it is necessary to use a torsion rod, which extends from the gear case, or a crosspiece of the frame, to the rear axle. The necessity for this is the tendency of the driving bevel gear to roll around on the driven bevel gear rather than to revolve it. If it were not for the torsion rod, there would be a continual strain on the parts because of the tendency of the axle housing to revolve around the axle, instead of the axle being revolved inside of the housing. The torsion rod has a flexible joint at one end, that permits it to give as the axle follows an uneven road surface, but it retains the housing in the correct position, preventing the bevel gears from getting out of line (Fig. 41).

Fig. 41.—Torsion Rod.

Fig. 42.—Double Side-Chain Drive.

In Fig. 42 is shown a car with double-chain drive, in which the bevel gears that change the direction in which the power is applied are contained within the gear case that incloses the change-speed mechanism. As will be seen from Fig. 34, the bevel gears connect the square shaft with the jack shaft, which is a shaft passing across the car, and bearing on its ends the sprockets by which the wheels are driven. This type of drive requires the use of a dead axle, which is stationary with the wheels running loose on its ends, like the axle and wheels of a coach. An axle of this type may have great strength with light weight, and is usually a manganese-bronze or steel forging. The sprockets on the rear wheels are bolted to the spokes, and should be, of course, exactly in line with the sprockets on the jack shaft in order that the chains may run true.