Rear axle construction
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.
The axle shaft
(half shaft) transmits the drive from the differential sun wheel to the rear
types may be compared by considering the stresses the shaft has to resist. The
half shaft subjects to the following stresses:
stress due to driving and braking torque.
stress due to the weight of the vehicle.
stress due to the vehicle.
4- Tensile and
compressive stress due to cornering forces.
The types of
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:
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:
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.
floating rear hub:
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
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
(1st speed) = Te ig
(2nd speed) = Te ig
are the 1st and 2nd axle speeds respectively, and ηt
is the transmission efficiency.
b) Double reduction
axle; two reductions within the same unit
= Te ig
if = Te
ib are the 1st and 2nd
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
ih are the back axle
ratio and hub reduction respectively.