Monday, June 13, 2011

Rear-wheel Drive Arrangements

2:50 AM

Rear-wheel Drive Arrangements
The statement "every action has an equal and opposite reaction", means that every component that produces or changes a torque also exerts an equal and opposite torque tending to turn the casing. To understand the torque reaction consider the Fig. 26.25A, which represents a tractor with its rear driving wheels locked in a ditch. In this situation torque reaction is likely to lift the front of the tractor rather than turn the rear wheels. When the above principle is applied to rear axles, some arrangement must be provided to prevent the axle casing turning in the opposite direction to the driving wheels.
A torque (t) applied to the wheel, which may be considered as a lever (Fig. 26.25B), produces a tractive effort (Te) at the road surface, and an equal and opposite forward force at the axle shaft. This driving thrust must be transferred from the axle casing to the frame in order to propel the vehicle. The maximum tractive effort is limited by the adhesive force (P) of the type on the road. This force depends on the coefficient of friction (u) and the load (W) on the wheel.

Fig. 26.25. Lever action of a wheel

Types of Drive
A drive system is an arrangement, which transmits the driving thrust from the road wheels to the vehicle body and also incorporates a means to resist the movement of the main components due to torque reaction.

In the early days only leaf springs were used for the rear suspension of a vehicle, and hence these springs were often utilized to provide the drive thrust and torque reaction functions of the drive system. Since 1950, three notable drive systems are in use on motor cars and these basic systems have undergone several modifications to meet modern requirements.

 The drive systems include:

Although these systems are not used today in their original form, the objective of describing them is to make the reader familiar with the arrangement used to resist the various forces associated with the propulsion of a vehicle. However, this knowledge may help in the diagnosis of many of the faults associated with drive systems.


Hotchkiss Open-type Drive
This type of drive is commonly used on passenger cars and heavy commercial vehicles. This arrangement (Fig. 26.26) uses two rear leaf springs, which are longitudinally mounted, and are connected to the frame by a 'fixed' pivot at the front, and swinging shackles at the rear. A universal joint is mounted at each end of the exposed or 'open' type propeller shaft, with provision for accommodating change in shaft length due to the deflection of the springs. This drive, therefore, incorporates an open propeller shaft with two universal joints and a slip joint.

Fig. 26.26. Hotchkiss open-type drive (light commercial vehicle).

To resist torque reaction the axle is clamped to the springs using 'U' bolts. Under heavy driving conditions the springs deflect up at the front and down at the rear and vice versa during braking. This movement helps to damp driving shocks and improves transmission flexibility. A universal joint is installed at rear to accommodate continuous up and down motion of the axle. Driving thrust is transferred from casing to the spring by the friction between the two surfaces, and then transmitted through the front section of the springs to the vehicle frame. If the 'U' bolts become loose, the spring centre bolt (axle location bolt) has to take the full driving thrust, so that early failure of the bolt takes place due to the high shearing force.

Fig. 26.27. Forces acting on Hotchkiss drive.

In the Hotchkiss drive, rear axle torque and the propelling and retarding forces are taken up by the rear springs. The rear springs, which are generally half-elliptic type, are shackled to spring seats on the axle housing at one end and are pinned to the chassis frame at their forward end. The shackles may be vertical or may be arranged at some angle (a) as shown in Fig. 26.27. Thus the springs act as both torque and thrust members in this case.

Fig. 26.28. Free body diagram of Hotchkiss drive



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