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of the countershaft assembly of a motor truck. The locking device is very simple, consisting of a jaw or clutch member keyed to and revolving with an extension of the differential case. On the drive shaft extending through the differential member is mounted a sliding lock member which is keyed to the shaft so that it must turn with it. If it is desired to put the differential gear out of action, as might be desirable if one of the driving wheels became mired, the sliding lock member could be pushed into engagement with the fixed member on the continuation of the differential case and the differential mechanism would then revolve as a solid unit as there would be no possibility of the gears working.

Q. What types of vehicles are provided with differential lock?

A. Differential locking mechanism is usually provided only on the heavier classes of self-propelled vehicles, such as motor trucks, gasoline road rollers and agricultural tractors.

LESSON TWENTY-ONE

REAR AXLE TYPES

Q. What are the functions of the conventional rear axle?

A. The ordinary automobile rear axle must not only support the weight of the rear end of the car but usually carries the traction members or driving wheels and the brakes or retarding elements.

Q. What are the two common types of rear axle?

A. Rear axles may be divided into two general classes, termed "dead" and "live" constructions respectively.

Q. Describe the construction of a "dead" rear axle.

A. This form of axle which is outlined at Fig. 183 is practically the same in general construction as the rear axle used on horse drawn vehicles. It is composed of a stationary member, either of tubular or solid section, which is attached to the springs by suitable spring pads or chairs and which is provided with a spindle at either end on which the wheels revolve. The wheel hubs are independent of each other and are each driven by direct chain connection with a sprocket on a countershaft member carried by the frame.

Q. On what types of vehicles is a "dead" axle commonly used?

A. As the non-rotating axle construction may be made very strong without increasing its size to any extent, it is commonly applied to vehicles intended to carry heavy loads that need a double reduction drive:

Q. Where is the differential gear mounted when a "dead" rear axle is employed?

A. The differential gear must be used in connection with a stationary axle but it is usually incorporated in the countershaft assembly carried by the frame.

Q. How are the wheels driven when mounted on a stationary axle?

A. The wheel hubs are provided with sprockets which are connected to smaller sprockets carried by the axle shafts of the countershaft assembly by means of chains which transmit the power to each rear wheel.

Q. What is the advantage of this method of construction?

A. On heavy vehicles, such as motor trucks, which are designed to operate on solid rubber tires, it is desirable to suspend as much of the mechanism of the car as possible from the frame so the springs may absorb the larger portion of the jarring and jolting due to solid tires. As most of the heavier vehicles are designed to move slowly it is necessary to employ a double reduction system so the differ

Drive Sprocket

Stub Axle

Stationary Axle Tube
Spring Chair

Brake Rod

Ball Bearings

Fig. 183.-Simple Automobile Rear Axle of the "Dead" Type. ential and bevel gear drive assembly which provides the first speed reduction is part of the countershaft and the rear axle which must be strong and yet relatively light is called upon only to carry the car weight and not to transmit any of the engine power. Another reduction of speed is possible in addition to that obtained at the countershaft by making the driving sprockets smaller than the driven members carried by the wheel hubs. Owing to the use of a fixed rear axle member it is not necessary to carry any of the more or less delicate mechanism forming the differential and drive gearing as part of the axle and these portions of the drive system may be carried in a separate assembly attached to the spring supported frame members.

Q. Describe construction of simple type live axle.

A. A simple live axle of conventional design is outlined at Fig.

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Fig. 184.-"Live" Automobile Rear Axle of Bevel Gear Drive Type. 184. This member not only supports the weight of the car, but also incorporates the power transmission and differential gearing. The power is taken from the gearset through a drive shaft revolving in roller bearings. At the end of the drive shaft a bevel pinion is secured which meshes with a bevel or ring gear carried by the differential housing. The differential assembly is also supported on roller bearings and shafts extend from the differential gears to the wheel hubs. All the rotating parts are housed, the differential and bevel gear assembly being mounted in a housing of malleable iron while the drive shaft and axle shafts A and B are protected and supported by a tubular axle housing, which is securely attached

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Fig. 185.-"Semi-Floating" Automobile Rear Axle Construction With Shafts and Gearing Mounted on Double Row Ball Bearings.

to the differential housing at the center of the axle. In the simplest form of live axle construction the axle shafts not only transmit the power from the differential gears to the hubs but are called upon to carry the car weight at the rear end as well. This form of construction is suitable only for light and medium weight cars.

Q. How many types of live axle are used?

A. Live axles may be of a simple form, as previously described and shown at Figs. 184 and 185, or they may be of a semi-floating or full floating construction. The simple live axle shown at Fig. 185 is practically the same in construction and method of operation as that previously outlined with the exception that the axle shafts are carried on annular ball bearings instead of roller bearings. Q. What is the "semi-floating" axle?

.A. The term "semi-floating" is applied to those forms of axles where the axle shaft is depended on to drive the wheels as well as take part of the load caused by thrust reaction, when the wheel skids sideways. An axle of this general form is outlined at Fig. 186, and it will be noted by comparing it with the form shown at 184 and 185 that the wheel hub is carried by a large bearing that is mounted on the drive axle housing so the axle shaft that is fastened in the wheel hub is not called upon to carry any of the car weight. It is, however, depended on to steady the wheel against side thrust. This form of axle construction is often called "the one bearing floating axle" because for all practical purposes it is a floating type inasmuch as the axle shaft may be withdrawn from the interior of the axle housing or the differential assembly removed without relieving the wheels of the car weight.

Q. What is the "full floating axle" construction?

A. The full floating rear axle construction, which is depicted at Figs. 187 and 188, differs from the preceding form inasmuch as the wheel hub is carried by two bearings which are in turn supported by the fixed housing member through which the drive axle passes. The driving axle is a floating member that is called upon only to transmit power and not to take any of the car weight. In the simple live axle forms the axle shaft revolves in bearings carried by the interior of the housing and as the wheels are secured to the shafts

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