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CHAPTER IX

THE REAR AXLE AND DRIVING SYSTEM

Rear Axle Nomenclature-Semi-Floating Axles—Three-Quarter Floating Axles

-Full Floating Types—Taking Rear Axle Apart-Adjusting Bevel Drive Gears-Worm and Spiral Bevel Drive Gears—Two-Speed Axles-Double Reduction Axles—Internal Gear Drive-Four Wheel Drive_Spur and Bevel Gear Differential-Chain Drive Troubles-Trussing Weak Axle Housings—Axle Lubrication-Oil Retaining Means—Types of Ax’e Bearings-Care and Adjustment of Axle Bearings—Brake Forms and Adjustment.

Owing to the advances that have been made in metallurgy and a more general appreciation of principles of design by engineers, the rear axle is a part of the car that seldom gives trouble and which usually needs attention only when the car is thoroughly overhauled. Very few motor car manufacturers build their own rear axles and most of those used are the product of specialists who make nothing but front and rear axles. The result of this concentration upon one product means that the various details of proportion of parts have received careful attention which has been based on a wide experience. The material best adapted for the various parts have been carefully determined and practically the only condition that interferes with proper rear axle operation, barring occasional accidents, are those due to natural wear. Before describing the method of taking down rear axles it may be well for the reader to become familiar with the different axle types and their method of construction. The designs used vary widely. In some types it is possible to get at all the essential parts in a relatively short time without removing the rear construction from the chassis. In other forms it is necessary to take them completely apart before access may be had to the differential gears or the axle shafts and their supporting bearings.

Rear Axle Nomenclature.—The various types of axles that have been used in automobile construction as defined by the chief engirreer of the Weston-Mott Company, one of the largest axle manufacturers in the world, are illustrated at Fig. 372. This shows the four main classes of axles, which are termed semi-floating, three-quarter, seven-eighths and full floating types, these being designated by the letters A, B, C, D, respectively. While the illustrations are self explanatory to one well versed in automobile construction it may be well to describe the various types in detail for the benefit of those who have not had occasion to take all the various types apart. On the semi-floating axle, as shown at A, the entire weight of the car comes upon the axle shafts which also are depended on to transmit the power from the differential gearing to the wheel hubs. It is said that in time this would have a tendency to cause the shafts to crystallize and break unless great care is taken in proportioning the shafts so strong that they will resist the stresses imposed upon them. This type of axle is not generally recognized as the semi-floating form as most engineers call it a non-floating live axle. The reason for this is that the axle shaft does not even partially float as it is held in the hub of one of the differential bevel gears by threaded retention members. In order to be a semi-floating axle it would be necessary to utilize a bearing type at the wheel end that would take end thrust and keep the wheel shaft in place while the part of the axle that projected into the differential would not be held by any threaded nut.

A true semi-floating axle should be of such form that the axle may be readily withdrawn without necessitating the complete disassembly of the rear construction. The axle shown is semi-floating to a degree, however, because the differential gear is carried by bearings which are outside of the differential case bosses instead of bearing directly on the axle shaft, as is the case with light axles of such cars as the Ford. The three-quarter floating axle shown at B is a design in which the axle shaft is subjected only to torsional strains or to a twisting action due to the power applied to drive the car. The wheel bearing is mounted on the axle housing instead of inside of that member as shown at A. This brings the strain due to the weight of the car on the non-rotating axle hous

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Fig. 372.—Defining Principal Types of Weston-Mott Rear Axle

Construction.

ing instead of on the shaft as in the type shown at A. The type shown at C, which is termed the seven-eighths floating by some engineers and which is called a single bearing full floating hub by others, has many of the advantages of the three-quarter floating construction in that the drive axle tends to steady the wheel and also has the advantage of the full-floating type in that the wheel may be removed from the rear construction without taking the

housing apart. The axle shaft may be withdrawn from the differential, which is not possible in the form shown at A and B where the end of the axle is securely retained inside of one of the differential bevel gears by a nut.

The standard full floating type of axle, which is shown at D. does not depend on the driving shaft to steady the wheel, which is held against side movement by spacing the wheel hub bearings on each side of the spoke center line. The advantage of the fullfloating type of axle is that the driving shaft may be withdrawn without disturbing the wheels or jacking up the axle and the differential gearing may be removed from the rear construction by partially withdrawing the drive shafts and not requiring jacking up the axle inasmuch as the wheels still support the load.

Semi-Floating Axles.— The difference between the semi-floating and three-quarter floating axies may be readily understood by referring to the sectional views at Fig. 373. The complete assembly of the differential and driving gears with one of the axle shafts of a Weston-Mott three-quarter floating rear axle is shown at A, while a similar sectional view of the Reo axle is shown at B. The semi-floating axle used on some types of Pierce-Arrow cars is shown at Fig. 374. It will be observed that in the Reo construction the axle shaft must transmit the power and also support the portion of the weight of the car that comes on the rear wheel it carries. Beginning with the universal joint on the drive shaft, the power is transmitted through the pinion shaft to the bevel pinion which in turn imparts its motion to the ring gear or master gear riveted to the differential case. When the car is traveling straight ahead the power is transmitted directly to the two differential gears which are fastened to the axle shaft by keys and taper retaining pins and which turn the wheels forced onto the keys on the tapered outer axle end. When the car turns a corner the outer wheel travels faster than the other, suitable compensation for the difference in speed being made by the differential gears and pinions. As will be apparent the pinion shaft is carried by taper roller bearings as is the differential assembly. The axle is supported at the wheel end by a Hyatt, high duty type roller bearing

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Fig. 373.—Three-Quarter Floating Axle at A, Reo Semi-Floating

Axle at B.

There are two adjustments on this axle, one that allows the movement of the differential gearing so the ring gear may be brought into closer mesh with the bevel pinion, when depreciation occurs, or in the initial adjustment when the axle is assembled.

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