Rear Axle [Automobile]

Rear Axle [Automobile]

The vehicle with non-independent rear suspension uses either a dead axle or a live axle. The dead axle only supports the weight of the vehicle, but the live axle besides fulfilling this task, contains a gear and shaft mechanism to drive the road wheels. The arrangements for supporting the road-wheels on live axles and providing the driving traction use an axle-hub mounted on to the axle-casing and supported by ball or roller-bearing. The two main components installed inside the axle of a rear-wheel drive vehicle are the final drive and differential.

26.6.1.

Axle Casing

The casing used now a days is either a banjo or carrier-type. In the past a split (trumpet) casing was occasionally used. These three types are shown in Fig. 26.51. The type of axle casing used decides the method for the removal of the final drive.

Banjo Type

The tubular axle section of this casing is built up of steel pressings, which is welded together and suitably strengthened to withstand the bending load. The center of this casing with the axle tube on one side resembles a banjo. The final drive assembly is mounted in detachable malleable iron housing and is secured by a ring of bolts to the axle casing. The axle shafts are slid into this assembly from the road wheel end of the casing. On some banjo axles a domed plate is bolted to the rear face of the casing. Removal of this plate provides excess to the final drive gears and in cases where the axle shaft is secured to the differential; this enables the axle shaft to be unlocked from the sun gear (side gear).

Fig. 26.51. Types of rear axle casing.

A lubricant level plug is screwed into the domed cover or the final drive housing at a height about one third up the crown wheel, which is normally just below the axle tubes. This allows lubrication of the hub bearings by splash caused due to rotation of the crown wheel. Overfilling of the lubricating oil swamps the oil seals causing the oil to enter the brakes and hence this should be avoided. The final drive becomes hot during operation, hence some form of air vent is provided to release the pressure in the axle casing. Consequently the possibility of oil being forced past the seals is prevented.

Carrier Type

This type of casing is more rigid than a banjo type and is often employed to support a hypoid gear. The final drive assembly is installed in a rigid malleable cast iron carrier, into which the axle tubes are pressed and welded. For extra rigidity reinforcing ribs extend from the pinion nose to the main carrier casing. A domed plate is fitted at the rear of the casing to provide access to the final drive gear.

26.6.2.

Axle Shafts and Hub Arrangements

The axle shaft transmits the drive from the differential sun wheel to the rear hub. The various types of shafts may be compared based on the stresses they resist. A simple automobile shaft has to withstand

(i) torsional stress due to driving and braking torque, («) shear and bending stresses due to the weight of the vehicle, and (lit) tensile and compressive stresses due to cornering forces.

Fig. 26.52. Loading of different axle-hub arrangements.

 A. Semi-floating axle hub.

 B. Three-quarter floating axle hub.

C. Fully floating axle hub.

Axle shafts are divided into semi-floating, three-quarter floating and fully floating depend­ing on the stresses to which the shaft is subjected. Axle half-shafts are situated on each side of the final drive and convey motion to the road-wheels. There are basically three different arrangements of supporting axle wheel hubs on the rear-axle casing.

These include :

(i) Semi-floating axle hub (commonly used on cars).

(ii) Three quarter floating axle hub (rarely used today).

(m) Fully floating axle hub (commonly used on heavy vehicles).

Figure 26.52 demonstrates how loads are resisted with different axle-hub arrangements. A tough, hard material is used for the axle shaft to withstand the various stresses, resist spline wear and provide good resistance to fatigue. Medium carbon alloy steel containing nickel, chromium and molybdenum is generally used to manufacture axle shafts.

Semi-floating Axle Hub

The road-wheel is attached to the axle hub, which is an extension of the axle half-shaft. A single bearing inside the tubular axle-casing supports the outer end of the shaft. The inner end of the shaft is splined and supported by the final-drive unit, which itself is mounted on bearings within the axle casing (Fig. 26.52A).

The semi-floating axle along with its overhanging hub is subjected to the driving torque as well as to both vertical and horizontal loads. The vertical load produces a shearing force, and the distance between the wheel and the suspension-spring seat on the axle causes a bending moment, the reaction of which is shared between the axle bearing and the final-drive-unit bearings. The horizontal load due to tilting of the vehicle, cornering centrifugal force, or side wind gives rise to both side-thrust and a bending moment. This bending moment may add to the vertical bending moment or may oppose it, depending on the direction of application of the side-force.

A semi floating axle, suitable for small and medium sized cars, is illustrated in Fig. 26.53. The axle half shaft and flanged hub are forged from a single piece of nickel chrome steel. The hub end of the shaft is provided with a larger diameter than the rest of its length, which resists the vertical and horizontal loads. The outer face of the flanged hub is shouldered so that it centralizes accurately the brake drum. The flange is provided with evenly spaced holes around it for wheel studs.

Fig. 26.53. Semi-floating ball-race-bearing axle.

A pre-greased and sealed deep grooved ball-race bearing is pressed over and along the shaft up to its shoulder. The bearing is sandwiched on one side by the axle-casing and on the other by the brake back-plate and the retaining plate by four nuts and bolts. To prevent excess oil leakage to the end of the axle-casing, a radial-lip oil-seal is pressed into a recess in the casing. Oil level of the final-drive is considerably lower than the axle shaft. However, the large amount of splash may cause the lubricant to spread along the shaft and enter the brake drum. An oil retainer is fitted at the hub end and the lip of the seal is positioned towards the final drive in the sealing arrangement.

A semi-floating axle shown in Fig. 26.54 uses a taper-roller bearing, which is suitable for larger and higher-performance cars because of its greater load-carrying capacity. A separate hub is wedged on to a keyed and tapered half-shaft and a castellated nut holds it is position. The taper-roller-bearing inner cone fits with a light force inside the mouth of the casing. The exact position of the bearing in the casing is provided by shims packed between the casing flange and the brake back-plate. Increasing the thickness of the shims on one side and decreasing it on the other shifts both half-shafts further to one side relative to the axl casing. On either road-wheel the outward thrust is absorbed by the adjacent hub bearing, while inward thrust is transmitted to the opposite bearing through the axle half-shafts and a slotted axle-shaft spacer (not shown). Therefore, each hub bearing takes thrust in one direction only.

Fig. 26.54. Semi-floating taper-roller-bearing axle.