A rear axle is a complete component and comprises a casing, which carries the bevel pinion and shaft, crown wheel and differential gear assembly and the axle shafts. The axle also includes the road wheel hubs and brake assemblies. Axle casings are made of steel castings (machined where necessary) provided with inspection covers, oil level plugs or dipsticks and are made oil tight with oil retaining washers. Differential gear assemblies are mounted in a casing in a different ways with means of adjustment between crown and driving pinion. They can be removed from the casing but adjustment between crown wheels and driving pinion depends on the casing construction. The casings are described as:
(a) Split Casing. It is constructed in two pieces, both being jointed together by bolts and nuts with a gasket in between to obtain an oil tight joint. This type of casing must be removed from the chassis before the differential gear assembly can be dismantled from the casing.
(b) Banjo Casing. This is a one-piece construction with flanged faces on the enlarged centre portion. The rear flange carries an inspection cover.
(c) Built-up Casing. Two axle tubes of pressed steel are bolted to the differential housing which is a steel or aluminium alloy casting. Before the differential assembly can be removed from the built up axle the whole axle must be removed from the vehicle. Worms drive axles are made with built-up housing, as the pressed steel banjo housing is not sufficiently stiff for a worm drive, which requires very rigid support for the differential bearing.
Classification of Rear Axle
Live axles are classified as fully floating, semi floating or three quarter floating, according to the method by which wheels are mounted or are connected to the axle casing and the axle shaft. The axle bearings may be ball, roller or taper roller bearings and are lubricated by grease or oil.
(a) Semi-Floating. In this type, each wheel hub is rigidly secured by splines or keys to the end of the axle shaft and the shaft supports the axle casing through a roller or ball bearing. A special nut screwed to the end of the axle casing retains the ball or roller bearing and the axle in position. The axle shafts, apart from transmitting the drive, are subjected to severe bending stresses. They have to transmit much of the vehicle weight from the axle casing to the road wheels, and in addition they have to resist the side thrust to which the wheels are subjected when the vehicle is running round a bend.
(b) Fully Floating. An axle is said to be fully floating when the axle shafts transmit the drive from the differential gear and performs no other function. The wheel hubs are entirely supported on the ends of the axle casing by two ball or roller bearings. This type of axle is suitable for carrying heavy loads because the wheels support the axle casing directly. The axle shafts are splined at the inner end and engage with the sun wheel while the outer end carries a flange, which is bolted to the wheel hub. In this type, the axle shafts can be removed without jacking-up the vehicle and in the event of an axle shaft breakage the vehicle can be towed after removing the broken shafts.
(c) Three Quarter Floating. This is a compromise between the fully and semi floating type. The axle shafts are rigidly secured to the wheel hubs, but the ball or roller bearings are fitted directly between the hubs and axle casing. The weight of the vehicle is thus transmitted directly from the casing to the wheels, and is not taken by the axle shafts. The axle shafts in addition to transmitting the drive also hold the wheels in position. The drive transmitted from end of the axle shafts to the wheel hubs by splines or keys.
Oil retainers in various forms are usually fitted between rotating shaft and casing to prevent the leakage of oil e.g. adjacent to crankshaft rear main journal, gearbox shafts, axle shaft etc. There are various type of oil retainers used in transmission system like oil thrower ring, oil return screw, spring loaded leather/rubber washer, felt type oil retainer and oil catcher.
(a) Oil thrower ring. These type of oil retainers are used to deflect oil, into oil ways to the engine sump .These are the oil return threads cut into or raised on a shaft. The threads trap the oil and returns to oil sump. The normal oil return action fails, due to blockage or excessive wear out on the threads of the shafts and its bush or housing.
(b) Oil Return Screw. The oil return screw is widely used on axle shafts, where threads may be cut into the one end of shaft or may project above surface. When the axle shaft incorporates this type of oil returning device, the shafts are not interchangeable. To check the correct fitness, the best method is to hold the shaft in one hand and rotate it in same direction, as it will rotate when fitted to the vehicle, when the threads tend to force the fingers away from the hub; it shows that the shaft is correct match to the corresponding wheel hub.
(c) Spring Loaded Rubber Washer. This type of oil seals are widely used in modern vehicles, is a spring loaded built up in a steel cage with a coil spring creating pressure on the washer so that a close contact is made to the shaft. The oil seal is always replaced with new one, if damaged. While fitting the oil seal it should be coated with a sealing compound to ensure oil tight fit. Ensure that while fitting, the sharp edge of the rubber washer faces the source of oil supply.
(d) Felt Type Oil Seal. These type of seals are made of felt material and normally used as packing in different joints.
(e) Oil Catcher. In some of the hubs an oil catcher is spun on to the back of the driving flange of the axle shaft, so that during the operation, excess oil is caught in the catcher and leaks through a drilled passage in the wheel hub behind the nave plate. Ensure that the drilled passage is always kept clear, otherwise the catcher will over fill with oil which eventually reach the brake linings.