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.

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

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?

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.

Q. Are there any practical substitutes for the differential gear?

A. Various forms of ratchet 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

Fig. 181.-Gearset and Differential Assembly of Motor Truck Showing 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