Bustion Engines

In all constructions, the torque is transmitted from the pro peller shaft to the final drive and from the final drive to the axle shafts ; it is equalized between the shafts by means of the differ ential.

Axle Differential.—The axle differential divides equally between the driving wheels the effort supplied by the engine and at the same time allows one wheel to overrun the other when turning corners. The most common method of doing this is by means of a combination of bevel gears. Spur gears, worm gears and dif ferent combinations of pawls and ratchets have been employed at various times, but because of its low cost and satisfactory opera tion, the bevel gear differential is most generally used.

The bevel gear differential may be described in the following manner (see fig. i8) : The gears are carried in a differential case A which has fastened to it the driven final drive gear B. Two bevel side gears C and C' are placed opposite each other and attached one to each axle drive shaft D and D', and are free to rotate in the case. Two (or more) equally spaced bevel pinion gears E and E', also free to ro tate, are carried by means of a stub-shaft F fastened in the case and meshed with the side gears C and C'. The turning effort is transmitted to the case A and stub-shaft F, and in turn to the pinion gears E and E', which divide it between the two side gears C and C' in the manner of a simple beam loaded in the centre and restrained at each end. The only serious disadvantage of the bevel and spur gear type appears when one rear wheel loses traction; i.e., starts to slip as in deep sand or mud; the wheel without traction will spin and the other will stand still, giving no motion to the vehicle. This can be overcome by introducing internal friction or locks. but in most cases the disadvantages of these methods outweigh the advan tages.

Rear Axles.

The three types of rear axles are classified ac cording to the shaft construction, as the semi-floating, the three quarter floating and the full-floating.

Semi-Floating.—In the semi-floating type of rear axle the axle shaft is supported on two bearings, one at the outer end of the housing and one at the tial end, and the wheel is keyed rigidly to the shaft, which in ad dition to transmitting the driving effort sustains the weight of the car and resists all external thrusts, such as are due to skid ding or to turning corners (see fig. 29).

Three-Quarter Floating.—In the three-quarter floating type of rear axle the inner end of the axle shaft is supported in the same manner as in the case of the semi-floating axle. The outer bearing, however, supports the wheel, which in turn is rigidly keyed to the outer end of the axle shaft. In this type of axle the car weight is carried _by the housing, but the side thrusts due to skidding and to turn ing corners, in addition to the transmission of the driving effort, are still carried by the axle shafts (see fig. 20).

Full-Floating.—In the full-floating type of rear axle the wheels are carried by double bearings on the axle housing, which, there fore takes all stresses except that of the driving effort. The wheel is not keyed rigidly to the axle shaft but is driven through splines by means of a splined flange on the axle shaft (see fig. 22).

Brakes.

The brakes are called upon to perform two duties, one of which is to bring the car to rest or to decrease its speed, and the other to hold the car in its place while unattended. These two functions are ordinarily performed by two separate braking systems, the former function by a foot pedal operated system, called the "service" brake, and the other by a hand operated brake, called the "parking" or "emergency" brake. The modern car moves so fast that the demands for rapid stopping ability have forced every American make of motor car to have a separate brake for each of the four wheels, and a modern motor car can stop from a speed of 6o m. per hour in less than 200 ft. (the distance depending on the con dition of the road and the tires). Maximum deceleration rates run as high as about 25 ft. per sec. per sec., but depend to a large extent on the condition of the road surface and the tires.