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tween the resistance of the member on the hard surface and that in the mud that the differential gear will permit the member on the hard surface to remain absolutely stationary while the wheel in the mud hole revolves at a high rate of speed without producing movement of the vehicle. With ordinary forms of light and medium weight pleasure cars, this condition is not so apt to materialize as with heavier vehicles.

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Fig. 182.-Spur Gear Differential Assembly Adapted for Bevel Gear

Drive.

Q. Is there any way to put the differential gear out of action in such an emergency?

A. Some vehicles are provided with a mechanism for locking the differential gear together to form a solid unit and deliver the power to both wheels so these must turn in unison. A construction of this kind is outlined at Fig. 181 where it is mounted as a part

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 differDrive 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.

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