Two types of rear axles are being used by the manufacturers of automobiles—the live axle, and the dead axle. The live axle which carries the weight of the load and transmits the power of rotation to the wheels, is built in two distinct designs called the semi-floating axle, and the full-floating axle. The semi-floating design is used extensively in manufacturing cars of light weight, while the full-floating design is favored more by the manufacturers of cars of medium and heavy weight. Both designs give equally satisfactory results.

The dead axle carries the weight of the car and load in much the same manner as a horse drawn vehicle. The power is conveyed to the loose wheels on the axle, by means of a chain which operates on a sprocket attached to the hub of the wheel, or by an internal gear drive arranged and housed in the brake drums.

The Semi-floating Axle.—In the semi-floating design of axle, the axle shaft carries the weight and transmits the rotation power to the wheel, which is keyed and locked to the outer end. The axle shaft is provided with a bearing at each end which operates on the inside of a closely fitted housing. The inside end of each axle shaft is bolted directly to the differential. The housing is split or divided into two halves, and bolts together in the center over the differential. This design of axle gives excellent service, but has one disadvantage in that it is somewhat difficult to disassemble, as the rear system must be disconnected from the car to take the housing apart. Fig. 119 shows a part sectional view of a semi-floating axle used by the Detroit Taxicab Co. The wide series of S. K. F. ball bearings used on this axle are self aligning, which prevents any binding action from shaft deflection.

The Full-floating Axle.—The full-floating design of axle serves the same functional purpose as the semi-floating design, but is constructed differently and operates on a widely different plan. In the full-floating design of axle, the axle shaft does not support any of the weight of the car or load, but serves simply as a member to transmit the power rotation to the wheels. The wheels are mounted on separate bearings, which operate on the outside of the outer end of the housing. The inner ends of the axle shafts are squared, or splined and slide into slots or seats in the differential gears. The differential assembly is in a separate unit, and is floated on bearings held by retainers extending from the forward end of the large ball-shaped center of the housing. The outer end of the axle shaft extends through the hub of the wheel, and has an umbrella-shaped plate on the end which bolts to the outside face of the wheel, as shown in Fig. 120, thus transmitting the power directly to the outside of the wheel, without the axle shaft taking any bearing. The axle shaft may be drawn out through the wheel, by removing the nuts which secure the umbrella plate, without removing the weight of the car from the wheels. The differential unit can also be removed without disassembling the housing, by removing a large cover plate from the center of the housing. Fig. 121 shows a typical full-floating axle, with a spiral bevel gear drive. The wheels in this case are mounted on a set of double series radial and thrust ball-bearings. The Hotchkiss type of short shaft final drive is carried in the forward extended part of the housing.

Two types of front axles are used by the manufacturers of automobiles. The I-beam type, which is a one piece drop forging, and the tubular or hollow type, which is round and has the yoke fitted into the ends. Both types operate on the same principle and plan, the only distinction between the types is that one type has the I-beam cross member and the other type has a pipe or tubular cross member.

The front axle consists of an I-beam or tubular cross member, which is yoked at each end as shown at A, in Fig. 122. A steering knuckle B is held between the ends of the yoke by C, a king pin, which allows the knuckle to swing in a half circle. D, the spindle or short axle, is provided with a set of radial thrust bearings. The wheel is adjusted snugly to the bearings E by a castillated nut F. The adjustment is held by a cotter pin which extends through the spindle and head of the nut F. A short arm extends backward from each steering knuckle, shown at G, in Fig. 122, and are connected together by an adjustable tie or spread rod shown at H. A half circle ball arm extends from the knuckle and circles over the axle. A rod or drag link forms the connection between the ball arm and the steering arm of the steering gear. Fig. 123 shows the location of the parts assembled on a typical drop forged I-beam front axle. A section of the hub has been removed to show the location of the double row radial end thrust ball bearings. This type of bearing is becoming very popular for automobile uses.

Adjustments of the Semi-floating Type of Axle.—The short shaft carried in the forward part of the housing has a center nut adjustment between the universal joint and the pinion gear; moving this notched nut to the right facing the rear axle draws the shaft backward and meshes the teeth of the pinion gear deeper with the teeth of the ring gear. After this adjustment is made, examine the teeth for even mesh; it may be necessary to shift the differential unit to secure an even bearing. (See chapter on differential gears for detailed instructions in regard to differential adjusting.)

Adjustments on the Full-floating Axle.—The adjustments on the full-floating axle are usually made by shifting the differential unit, although a pinion gear adjustment is usually provided as described above.

Care.—The housing of both the semi-floating and the full-floating axle should receive a fresh supply of medium fiber or graphite grease every thousand miles. To grease, remove the plug on the large part of the housing and force in grease with a dope gun until it begins to bulge out of the hole.

Wash out the housing every five thousand miles, and replace the lubricant, as small metallic particles are worn off the gear teeth and this grit, which is destructive to the gears and bearings, mixes with the grease making it necessary to remove it that often.

A grease cup will be found located at the outer end of each half of the axle housing, which supplies the lubricant for the outer bearing. This grease cup should be filled weekly with a medium cup grease and given a half turn each day.

Care of Front Axle.—Pack the space between the bearings in the hub of the wheel every thousand miles. Use a heavy cup grease. The king bolts which hold the steering knuckles between the ends of the yokes are hollow and carry a grease cup on the head, which forces the grease out through finely perforated holes, and lubricates the bushings on which the pins take their bearing. This cup should be filled weekly and given a half turn each day.