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action is practically the same in either case, each driving axle being attached to one of the large gears while the pinions engaging with them are depended on to obtain the differential effect.

Q. Which type is more generally used and why?

A. The bevel gear form of differential has received the widest application because it is simplest in design and the gear teeth can be more substantial and stronger than the corresponding members in a spur differential assembly of equal size.

Q. How many bevel pinions are used and why is it better to use a number of these gears?

-Worm A. On light cars a three bevel pinion differential assembly is all that is required, but on heavier cars from four to six pinions are employed. It is advisable to use a large number of gears where the work is severe because the work on any one pinion is lessened as the number of pinions is increased.

Worm Q. Are there any practical

Gear substitutes for the differential gear?

Fig. 180.—Worm Driving Gears A. Various forms of ratchet Forming Supporting Member

for Differential Pinions. and slipping friction mechanisms have been evolved in attempts to replace the differential gear but none of these have proven commercially practical. Differential mechanism comprising cranks and eccentric arrangements have been tried but these are invariably more costly and cumbersome than the gear forms.

Q. Where is the driving sprocket or gear attached to the differential assembly?

A. The driving sprocket or gear, when this is a separate member, is always attached to the case which carries the studs on which

[graphic]
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Fig. 181.-Gearset and Differential Assembly of Motor Truck Show

ing One Method of Locking Differential Gearing So the Differential Assembly Will Turn as a Unit.

the differential pinions revolve. In some cases, as outlined at Fig. 180, where a worm gear is used for driving it is possible to carry the compensating pinions on studs inserted directly in the interior of the gear rim.

Q. Are there any conditions where differential gear action is not desirable ?

A. In some cases the differential gear action proves to be a disadvantage. For example, if one of the driving wheels is in mud or other material offering but little traction, and the other wheel is on dry hard road surface, there will be so great a difference be

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.

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