Another form of differential which has been extensively used is known as the spur type illustrated in Fig. 105. The action of this can be best explained by comparison with the bevel type. The bevel gears B and C are here replaced by spur gears A and B fastened to the differential shafts C and D respectively. While the inner sides of the hubs of these spur gears come close together, the inner edges of the gears themselves are at some distance apart. Meshing with these two spur gears A and B are two, three or four sets of spur pinions, E, F. These spur pinions have a width of face almost twice that of the spur gears A and B. The outer portion of the face of each pinion meshes with one of the spur gears, and the inner portions of the faces of the pinions mesh together. It will be readily understood that if the casing of the differential G is held stationary and the spur gear is revolved by hand or otherwise in a clockwise direction, the pinion Emeshing with it is revolved in a counter-clockwise direction; the pinion F meshing with E is revolved in a clockwise direction and the gear B is revolved in a counter-clockwise direction. That is, gear B is rotated in an opposite direction to gear A, exactly as with the bevel gear dif-erential. In regular operation the turning effort is, of course, transmitted to the differential housing by means of gears and the turning effort thus received by the housing is equally divided by the sets of spur pinions between the spur gear A and B, that is, between the two driving road wheels. The properties of the spur-gear differentials are exactly the same as those of the bevel-gear differential. The problem of working out a neat and all around satisfactory differential lock for either of the above types presents considerable difficulty, which is probably the reason that this device is not more extensively used. Fig. 106, this lock which is in the form of a four-jaw clutch, one member being keyed to the drive shaft and free to slide upon it, while the other is keyed to the differential case. Meshing the two clutch members locks the differential, since one shaft is locked against the other through the differential housing.

A differential lock can also be provided with the spur type and an excellent example is illustrated in connection with Fig. 105. A large flange is so mounted on the differential housing that it can be moved endwise, and the stud of one of the spur pinions extends beyond the housing. Half of the end of this stud is milled off so that the sliding member can be moved in, thus preventing the pinion from turning, which locks the entire differential.

The ordinary type of differential mentioned above presents certain disadvantages, the principal objection being that it promotes skidding. Recently several designs of differentials have been developed in which this common objectionable feature has been eliminated. This type is known as the helical gear type.

In the Powrlok device, there are two or more pinions mounted in the differential housing which is rotated by the engine, and also two crown wheels are attached to either driving wheel. But, in addition, there are worm gears interposed between the pinions and the crown wheels, the teeth on which are shaped to correspond. These worms are mounted in the differential casing as shown in Fig. 107 with their axis at right angles to those of the pinions. It will be seen, then, that the rotation of the differential housing in the usual way causes both pinions and worm gears to be carried around bodily in rigid relation to each other, whilst at the same time both pinions and worms have a power of rotation upon their own axis, so that they can be moved rotationally, but not bodily, in relation to each other.

Spur Gear Type Differential.

Fig. 105. Spur-Gear Type Differential.

A Nent and Simple Differential Lock.

Fig. 106. A Nent and Simple Differential Lock.

When road resistance is sufficient to give adhesion to each driving wheel, both wheels are equally driven, the crown wheels to which they are attached being carried bodily round by the worms in which they are in engagement, just as, with an ordinary differential, they are carried round by the pinions. But when road resistance upon one wheel is reduced to a point at which it loses adhesion, and would, with the ordinary differential, start spinning, nothing of this kind happens, because the angle of the worms is such that whilst the crown wheels can drive the worms, the worms cannot drive the crown wheels, and, as a consequence, the differential is locked so far as any movement of the wheel in relation to the differential is concerned. The axle becomes for all practical purposes a solid one, and all the drive is taken by the wheel, which is for the moment supported on firm ground and can take advantage of its grip.