Figure 13-19.-Comparison of high-traction differential gears and standard differential gears.

pinions and the side gears in the differential case. That is, the pinions do not turn on the trunnions, and their teeth will not move over the teeth of the side gears. When the vehicle turns a comer, one wheel must turn faster than the other. The side gear driving the outside wheel will run faster than the side gear connected to the axle shaft of the inside wheel. To compensate for this difference in speed and to remain in mesh with the two side gears, the differential pinions must then turn on the trunnions. The average speed of the two side gears, axle shafts, or wheels is always equal to the speed of the bevel drive gear. Some trucks are equipped with a differential lock to prevent one wheel from spinning. This lock is a simple dog clutch, controlled manually or automatically, that locks one axle shaft to the differential case and bevel drive gear. This device forms a rigid connection between the two axle shafts and makes both wheels rotate at the same speed. Drivers seldom use it, however, because they often forget to disengage the lock after using it. Several automotive devices are available that do almost the same thing as the differential lock. One that is used extensively today is the high-traction differential. It consists of a set of differential pinions and side gears that have fewer teeth and a different tooth form from the conventional gears. Figure 13-19 shows a comparison between these and standard gears. The high-traction differential pinions and side gears depend on a variable radius from the center of the differential pinion to the point where it comes in contact with the side gear teeth, which is, in effect, a variable lever arm. While there is relative motion between the pinions and side gears, the torque is unevenly divided between the two driving shafts and wheels; whereas, with the usual differential, the torque is evenly divided always. With the high-traction differential, the torque becomes greater on one wheel and lesson the other as the pinions move around, until both wheels start to rotate at the same speed. When that occurs, the relative motion between the pinion and side gears stops and the torque on each wheel is again equal. This device helps to start the vehicle or keep it rolling when one wheel encounters a slippery spot and loses traction while the other wheel is on a firm spot and has traction. It will not work however, when one wheel loses traction completely. In this respect, it is inferior to the differential lock. With the no-spin differential (fig. 13-20), one wheel cannot spin because of loss of tractive effort and thereby deprive the other wheel of driving effort. For example, one wheel is on ice and the other wheel is on dry pavement. The wheel on ice is assumed to have no traction. However, the wheel on dry pavement will pull to the limit of its tractional resistance at the pavement. The wheel on ice cannot spin because wheel speed is Figure 13-19.-Comparison of high-traction differential gears and standard differential gears. 13-16