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).