Free-Running Differentials and Viscous Clutches
By virtue of circumstances, the term ‘‘free-running differential’’ came into use in the automotive terminology of the countries of the former Soviet Union as a designation of mechanisms that were known in the United States as the ‘‘NoSPIN Differential’’ and later on as the ‘‘Detroit Locker.’’ There are several design concepts of free-running differentials. Most often, these mechanisms consist of two-sided free-running geared clutches. They are capable of operation in automotive drives and are frequently used instead of other differentials. These mechanisms, similar to the positively engaged drive, rigidly lock the wheels of the vehicle on straight segments of the road, ensuring that the wheels grip the surface in a satisfactory manner, and consequently ensuring the high mobility of the vehicle. In the case when the vehicle takes a turn at a high traction load, the torques are continuously redistributed by decreasing the torque at the advancing wheel (at the advancing axles in the case when free-running differential is used in the interaxle drive) and increasing the torque at the lagging wheel (axle). Here the free-running differential will not be actuated, that is, the wheels will remain rigidly coupled until the torque at the advancing wheel (axle) will not drop to zero. When zero torque is attained, the freerunning differential will deﬁnitely disengage this wheel (axle), thus eliminating the circulation of parasitic power and the appearance of negative circumferential forces on the leading wheels. The wheels (axles) are disengaged, as a rule, in turns with good gripping of the road and moderate traction load on the vehicle. The automatically disengaged advancing wheels move in the driven mode. This means that the power coupling between the left and the right wheels of the driving axle (or between the axles) is lost. The free-running differential in this case maintains only a kinematic coupling between the disengaged advancing wheel (axle) and with the driving wheel of the axle (axles) that has not been disengaged from the driveline system. The property of this mechanism to disengage the advancing wheel (axle) and thus to
ensure the needed difference in the rpm of the wheels when taking a turn is implemented when the vehicle moves both forward and in reverse, in turning in either direction, and in deceleration and coasting modes. Consider a design of the free-running differential, as shown in Figure 5.1. In this differential, the ﬁnal drive driven gear 1 and housing 2 of the differential itself
comprise together with driving clutch 7, the driving link of the mechanism. Four pins to the differential’s housing rigidly fasten the driving clutch. Radial rectangular teeth are arranged rigorously one opposite the other at both ends of the clutch. Mechanisms with 12 and 18 teeth at each end of the clutch are most extensively used. Two teeth (each at one side) are longer and form key-like tooth 8 on the inner cylindrical surface of the clutch.
Two driven half clutches 9 that have similar teeth are in engagement with the teeth of the driving clutch. The width of the tooth space of the gear rings of the driving clutch and of the driven half clutches is much greater than the thickness of the teeth. This is made in order that the driven half clutches should be able to rotate relative to the driving clutch by some angle. In specimens of differentials with 18 teeth, the angular pitch of the tooth is 782000 and the angular pitch of the tooth space is 1284000, whereas the relative turning angle is 582000. Spaces between the teeth of the driving clutch and the driven clutch halves are needed in
order to facilitate the withdrawal (disengagement) of these same teeth when one of the clutch halves is uncoupled. Moving in the axial direction upon disengagement, the clutch half slides over the teeth of the side spur gear 4 that is rigidly splined to the half axle, as in an ordinary bevel-gear differential, and bears by its end through washer 3 on the wall of the differential’s housing 2. Each clutch half has, in addition the previously mentioned straight teeth, another inner
gear ring with trapezoidal end teeth (these are also known as cams). These teeth are arranged coaxially with the straight teeth, due to which their number is the same as the number of the straight teeth. The slit-spring lock ring 11 with trapezoidal teeth similar to those of the gear ring is seated with some interference on the gear ring. In this arrangement, each tooth of this ring serves as a continuation of the tooth of the clutch half. The slit in ring 11 allows seating this ring with interference and facilitates the placing of elongated teeth or the key of the driving clutch. Central ring 12 in Figure 5.1 and 2 in Figure 5.2 is located inside the driving clutch. This ring is held in the middle position by a spring retaining ring (items 13 and 3 in
Figures 5.1 and 5.2, respectively). Radial apertures are provided in journals over the circumference of the driving clutch to allow the disassembly of the retaining ring. The central ring has at both of its ends teeth that mesh with the teeth of both half clutches and both retaining rings. The length of each tooth of the central ring is equal to the sum of the lengths of the teeth of the clutch half and of the retaining ring.