pinions and the side gears in the differential case. Thatis, the pinions do not turn on the trunnions, and theirteeth will not move over the teeth of the side gears.When the vehicle turns a comer, one wheel mustturn faster than the other. The side gear driving theoutside wheel will run faster than the side gearconnected to the axle shaft of the inside wheel. Tocompensate for this difference in speed and to remainin mesh with the two side gears, the differentialpinions must then turn on the trunnions. The averagespeed of the two side gears, axle shafts, or wheels isalways equal to the speed of the bevel drive gear.Some trucks are equipped with a differential lock toprevent one wheel from spinning. This lock is a simpledog clutch, controlled manually or automatically, thatlocks one axle shaft to the differential case and beveldrive gear. This device forms a rigid connectionbetween the two axle shafts and makes both wheelsrotate at the same speed. Drivers seldom use it,however, because they often forget to disengage thelock after using it.Several automotive devices are available that doalmost the same thing as the differential lock. One thatis used extensively today is the high-tractiondifferential. It consists of a set of differential pinionsand side gears that have fewer teeth and a differenttooth form from the conventional gears. Figure 13-19shows a comparison between these and standard gears.The high-traction differential pinions and side gearsdepend on a variable radius from the center of thedifferential pinion to the point where it comes incontact with the side gear teeth, which is, in effect, avariable lever arm. While there is relative motionbetween the pinions and side gears, the torque isunevenly divided between the two driving shafts andwheels; whereas, with the usual differential, the torqueis evenly divided always. With the high-tractiondifferential, the torque becomes greater on one wheeland lesson the other as the pinions move around, untilboth wheels start to rotate at the same speed. Whenthat occurs, the relative motion between the pinion andside gears stops and the torque on each wheel is againequal. This device helps to start the vehicle or keep itrolling when one wheel encounters a slippery spot andloses traction while the other wheel is on a firm spotand has traction. It will not work however, when onewheel loses traction completely. In this respect, it isinferior to the differential lock.With the no-spin differential (fig. 13-20), one wheelcannot spin because of loss of tractive effort andthereby deprive the other wheel of driving effort. Forexample, one wheel is on ice and the other wheel is ondry pavement. The wheel on ice is assumed to have notraction. However, the wheel on dry pavement will pullto the limit of its tractional resistance at the pavement.The wheel on ice cannot spin because wheel speed isFigure 13-19.-Comparison of high-traction differential gears and standard differential gears.13-16
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