Front Axle and Steering System

Front Axle and Steering System

Front axle carries the weight of the front part of the automobile as well as facilitates steering and absorbs shocks due to road surface variations. The front axles are generally dead axles, but are live axles in small cars of compact designs and also in case of four-wheel drive. The steering system converts the rotary motion of the driver's steering wheel into the angular turning of the front wheels as well as to multiply the driver's effort with leverage or mechanical advantage for turning the wheels. The steering system, in addition to directing the vehicle in a particular direction must be arranged geometrically in such a way so that the wheels undergo true rolling motion without slipping or scuffing. Moreover, the steering must be light and stable with a certain degree of self-adjusting ability. Steering systems may also be power assisted. The chapter discusses the front axle construction and its align­ment, and steering geometry and steering systems.

27.1.

Front Axle

The front axle (Fig. 27.1) is designed to transmit the weight of the automobile from the springs to the front wheels, turning right or left as required. To prevent interference due to front engine location, and for providing greater stability and safety at high speeds by lowering the centre of gravity of the road vehicles, the entire centre portion of the axle is dropped. As shown in Fig. 27.1, front axle includes the axle-beam, stub-axles with brake assemblies, u ack-rod and stub-axle arm.

Front axles can be live axles and dead axles. A live front axle contains the differential mechanism through which the engine power flows towards the front wheels. For steering the front wheels, constant velocity joints are contained in the axle half shafts. Without affecting the power flow through the half shafts, these joints help in turning the stub axles around the king-pin.

The front axles are generally dead axles, which does not transmit power. The front wheel hubs rotate on antifriction bearings of tapered-roller type on the steering spindles, which are an integral part of steering knuckles. To permit the wheels to be turned by the steering gear, the steering spindle and steering knuckle assemblies are hinged on the end of axle. The pin that forms the pivot of this hinge is known as king pin or steering knuckle pin. Generally dead front axles are three types. In the Elliot type front axles the yoke for king spindle is located on the ends of I-beam. The axle ends are forked to hold the steering knuckle extension between them. The reverse Elliot front axles have hinged spindle yoke on spindle itself instead of on the

axle. The forked portion is integral with the steering knuckle. This type is commonly used as this facilitates the mounting of brake backing plate on the forged legs of the steering knuckle. In the Lemoine type front axle, instead of a yoke type hinge, an L-shaped spindle is used which is attached to the end of the axle by means of a pivot. It is normally used in tractors.

Fig. 27.1. Front axle.

The axle beam in use is of I or H-section and is manufactured from alloy forged steel for rigidity and strength. As compared to dead front axles, a totally different type of swivelling mechanism is used on the live front axle. To connect the wheel hub axles with driving axle shafts, constant velocity joints are used for the vehicles fitted with the front live axles. Tracta, Rzeppa (or Sheppa) on Bendix constant velocity or universal joints are normally used.

Front axles are subjected to both bending and shear stresses. In the static condition, the axle may be considered as a beam supported vertically upward at the ends i.e. at the centre of the wheels and loaded vertically downward at the centres of the spring pads. The vertical bending moment thus caused is zero at the point of support and rises linearly to a maximum at the point of loading and then remains constant.

Thus the maximum bending moment = Wl, Nm

where, W = The load on one wheel, N

I = The distance between the centre of wheel and the spring pad, m

Under dynamic conditions, the vertical bending moment is increased due to road roughness.

But its estimate is difficult and hence is general­ly accounted for through a factor of safety. The front axle also experiences a horizontal bending moment because of resistance to motion and this is of a nature similar to the vertical one but of very small magnitude and hence can be neglected except in those situations when it is comparatively large.

The resistance to motion also causes a torque in the case of drop type front axle as shown in Fig. 27.2. Thus the portions projected after the spring pads are subjected to combined bending and torsion.

Fig. 27.2. Loads on front axle.

The shear stress in the axle is due to braking torque and its magnitude (as shown in Fig. 27.2)