Rear Drive Axle

In cases where the rear suspension is non-independent, the type of axle used is either a dead axle or a live axle. The former only has to support the weight of the vehicle, where the latter has to fulfill this task and, in addition, contain a gear and shaft mechanism to drive the road wheels.
Axle shafts:
The axle shaft (half shaft) transmits the drive from the differential sun wheel to the rear hub.

The various types may be compared by considering the stresses the shaft has to resist. The half shaft subjects to the following stresses:

1- Torsional stress due to driving and braking torque. 2- Shear stress due to the weight of the vehicle. 3- Bending stress due to the vehicle. 4- Tensile and compressive stress due to cornering forces.

The types of axle shaft:
In addition to their other features of general construction, driving rear axles are classified into three groups depending on the type of bearing mounting used to support the hubs. The three arrangements classified as follows:

• Semi-floating • Three-quarter floating • Fully floating

Semi-floating rear hub:
A single bearing at the hub end is fitted between the shaft and the casing, so the shaft will have to resist all the stresses (shear, bending, torsional). In this arrangement, if the axle shaft breaks, the driving wheel comes away from or out of the axle housing.

Three-quarter-floating rear hub:
In this construction the single bearing is located between the hub and the outside of the axle casing. This relieves the shaft of shear load and bending loads due to the vehicle’s weight, but it is still subject to bending loads due to cornering side thrust, and torque. If the shaft fails, the wheel will still be supported but side loads may cause it to rock on the bearing.

Fully floating rear hub:
In this construction the axle is supported by double taper-roller bearings on the outside of the axle casing. In the frilly floating construction the axle shaft transmits driving torque alone. The axle removal or failure does not affect the road wheel, and the disabled vehicle can be towed to a service area to for replacement of the axle shaft. This system is generally used with heavy vehicles.

Special transmission units:
a) Two speed axle; This is a convenient method of providing a large number of gear ratios and retaining a light gearbox. The torque applied to the final drive unit is also moderate as the extra reduction and the increased torque are mad after the crown wheel and pinion
Tw (1st speed) = Te ig ηt i1, Tw (2nd speed) = Te ig ηt i2
where: i1, i2 are the 1st and 2nd axle speeds respectively, and ηt is the transmission efficiency.
b) double reduction axle; two reductions within the same unit
Tw = Te ig ηt if = Te ig ηt ia ib
where: ia, ib are the 1st and 2nd reductions respectively.
c) Hub reduction; a reduction made at the wheel hub reduces the stresses which would be applied to the final drive unit having a large reduction ration. Fore example if a 2:1 hub reduction is made this halves the torque to be transmitted to the axle shafts, differential, crown wheel and pinion, propeller shaft and gearbox.
          Tw = Te ig ηt if ih
where: if, ih are the back axle ratio and hub reduction respectively.