Automatic and Manual Positively Engaged Power-Dividing Units
Locked up power-dividing units, or positively engaged PDUs, provide for identical angular velocities of the wheels that they connect, or a constant ratio of angular velocities, independently of the conditions of the vehicle’s motion. Automatic power-dividing units may control the ratio of angular velocities. A classiﬁcation of positively locking drives of wheels and driving axles was provided in Figure 1.33. Consider now certain typical designs of such drives. A classical example of an interwheel positive engagement may consist of the locked
differential of the driving axle. Irrespective of whether the axle moves in a straight or a curved path, the angular velocities of the left and right wheels will always be the same. If a free-running differential was installed in the driving axle (for more detail on such differentials see Chapter 5), then the axle’s wheels would rigidly be coupled to one another, that is, they would rotate at the same angular velocities. When the vehicle takes a turn, the freerunning differential may disengage the overtaking wheel and the entire power will then be transmitted to the lagging wheel. Both examples pertain to a part-time locked drive. It should be emphasized that a full-time locked interwheel drive is not used in ordinary vehicles because the difference between the paths traveled by the wheels when making a turn is extremely different; this will cause the wheels to slip and skid relative to the road. As a rule, a locking positive interwheel drive is also not used on off-road vehicles. Even if the tires have a high tangential ﬂexibility, their slip, particularly when taking a turn, may cause damage (break) of the grass layer, sinking of wheels into the soil, and loss of mobility. The full-time locked drive is used in multiwheel drive vehicles as the interaxle drive of
the driving tandems. Figure 1.40 shows the kinematic layout of a 12 12 vehicle with a full-time positive engagement of three axles of the rear tandem and full-time locked interaxle drive between the second and third axles. Figure 3.1 shows a design example of a full-time positive engagement of two tandem axles. If a free-running differential is installed between two of the tandem’s axles, then this
results in a part-time positive engagement drive, since such a differential is capable of automatic disengagement of the overtaking axle (Figure 3.2). The part-time positive engagement drive is extensively used in 4 4 vehicles in which
one of the driving axles may be engaged and disengaged manually (see Table 1.10, items 1 and 2) and automatically (Table 1.10, items 3, 6, and 7). Several more examples are given in Tables 1.11, 1.12, and 1.14. Figure 3.3 shows kinematic layouts of transfer cases that
provide for engagement=disengagement of the front axle while the power is supplied without interruption to the rear axle. Rotation is continuously transmitted from input shaft 1 to shaft 2 of the rear axle drive.
The output shafts of rear axle drive 2 and of the front axle drive 3 are not coaxial. The front axle is supplied with power from shaft 3, upon engagement of clutch 4. Figure 3.4 shows an example of a design of a transfer case, in which the front axle driven by shaft 1 is engaged simultaneously with the engagement of the lower stage by means of clutch 2. Transfer cases designed as shown in Figure 3.5 differ from those shown in Figure 3.3 by
the absence of direct transmission (the input shaft is not coaxial with any of the output shafts 2 and 3).