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dition to the usual countershaft carried below the main shaft, a quill is mounted below this which carries a differential gear. From this differential two drive shafts provided with universal joints transmit the power to two auxiliary differentials located on the front and rear axles respectively. The differentials on the axles in turn drive the wheels and as the wheels are movable to steer the vehicle, as well as rotatable to drive it, it is apparent that the final drive shafts which transmit power to the wheels must have universal joints to compensate for wheel movement when set at an angle to direct the vehicle.

A section through the universal joints employed at the wheels is clearly shown at B, Fig. 177-c. Here it will be seen that the universal joint is a regular double yoke and cross type enclosed in a housing that protects it from the dirt and grit and which permits of considerable movement of the drive shaft without allowing the lubricant with which the universal joint is packed to escape. One end of the universal joint is attached to the drive shaft which runs to the differential on the axle while the other yoke drives a spur pinion which is well supported by a taper roller bearing placed at each side. This spur drive pinion engages with an internal spur gear carried within the body of the wheel and applies the power near the wheel rim, thereby gaining the greatest driving leverage possible. The internal gear of the wheel and the pinion driving it are well housed to exclude grit and retain lubricant and this housing at the same time serves as a brake drum.

The wheels, one of which is shown at Fig. 177-d, are interchangeable. The two drive axles with their universals are duplicates of each other and the three differential gears, consisting of that in the gear box and those on the axle, are all alike. The differential is a special Wayne type which differs from the common construction in that it automatically drives the wheel which has the greatest resistance and which has traction and this feature coupled with the use of four driving wheels makes it possible to propel the truck if any one of the four wheels has traction. It is claimed that while the truck has 120 inches wheel base it will turn in a circle of twenty-two feet radius, which is accomplished by the fourwheel steering feature. There are four ample brakes, one on each

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Fig. 177-d.-Type of Wheel Used on Rambler Four Wheel Drive Truck.

wheel, and it is cla'med that either set of two brakes is sufficient to hold the loaded truck on a 30 per cent grade. The wheel revolves on taper roller bearings carried by the wheel spindle. The steering knuckle is a special form of the inverted Elliot type and not only carries the wheel spindle but the frame supporting he spur drivin pinion as wel. The loaded front axle is supported by a taper roller bearing at the lower end of the steering knuckle which makes it possible to turn the wheel with minimum exertion on the part of the operator. When the wheel is moved by the steering knuckle the frame carrying the spur drive pinion turns as we'l but owing to the universal joint the drive is not interrupted and the wheels are all

driven regardless of whether they are n a straight ahead position or at an angle to steer the car. Other vehicles have been evolved and applied as tractors for fire department service where the drive is through the front wheels only, these members being very similar in construction to the form described.

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NOTE. For further information relative to power transmission see The Modern Gasoline Automobile." By Pagé.

LESSON TWENTY

DIFFERENTIAL GEAR CONSTRUCTION AND

OPERATION

Q. What is a differential gear?

A. A differential gear is a compensating mechanism interposed in the automobile transmission system to permit the traction members or driving wheels to turn at different speeds when describing

curves.

Q. Is a compensating arrangement needed on a wagon?

A. The ordinary forms of horse drawn vehicles do not need any compensating arrangement because all wheels are independent of each other and revolve on fixed axles.

Q. What would be the result if no differential gear was fitted to a four wheel automobile?

A. Without a differential gear the ordinary form of motor vehicle would be difficult to handle on curves and all parts of the driving mechanism as well as the tires would be subjected to severe stresses that would produce rapid depreciation.

Q. Where is the differential gear carried?

A. The ordinary form of differential or compensating gear may be incorporated as part of the gear set where this member is attached to the countershaft; as part of a countershaft assembly, even if the gearset is a separate unit, and as a part of a rear live axle if the direct drive system is employed.

Q. Can a practical automobile be made without a differential gear?

A. The only method by which a practical automobile can be constructed and dispense with a differential arrangement is to use but one rear driving wheel mounted in such a way that its point

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