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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.

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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Fig. 178.-Parts of Typical Bevel Gear Differential and Their Relation in a Completed Assembly.

of contact with the ground corresponds to the apex of a triangle, the base line of which is the distance between the points of contact between the two front wheels and the ground.

Q. Describe construction of simplest type of differential gear.

A. The parts of a typical, simple differential gear are clearly outlined at Fig. 178. The gearing is carried in a case composed of two members having suitable openings between the halves of the case to carry the studs on which the bevel pinions revolve. All gearing is mounted inside of the differential casing, and as a rule the assembly is composed of two beveled gears, one being attached to each wheel and from three to six bevel pinions carried by the studs secured to the differential case.

P. Outline action of simple differential gear.

A. The arrangement of the gearing is such that when the resistance to both road wheels is the same the entire differential assembly rotates as a unit, though as soon as the resistance against one of the traction members is greater than that against the other then the gearing operates to permit the wheel that has the greatest resistance to turn at a lower rate of speed than the other member. When the resistance is the same against rotation of both wheels then the casing revolves at the speed determined by the driving gearing and the bevel pinions remain fixed on their studs so that both bevel gears attached to drive axles are carried around at the same speed. If the tendency of one of the wheels is to revolve slowly, such as the inner wheel does when turning the corner, then the differential pinions will not only be carried forward with the differential gear case but these will revolve on their studs and by running over the face of the bevel gear that tends to revolve slowly they will permit a difference in wheel speed without interruption of drive.

Q. Name two common forms of differential gear.

A. The two types of differential gear generally used are outlined at Fig. 179. That at A is the conventional form of bevel differential in which the compensating gearing is composed entirely of bevel gears and pinions. The form shown at B is termed a "spur gear differential” because all gearing is of the spur form. The

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Fig. 179.-Bevel and Spur Gear Differential Compared.

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