the internal parts can be inspected, removed, and
installed. Lubricant filler plugs are usually
incorporated in the housing inspection cover.
To prevent pressure buildup when the axle
becomes warm, a breather vent or valve is provided
atop the housing. Without this valve, the resulting
pressure could force the axle lubricant past the rear
wheel oil seals and damage the brake linings. The
valve is constructed so air may pass in or out of the axle
housing; however, dirt and moisture are kept out.
REAR DRIVE AXLE
The rear drive axle connects the differential side
gears to the drive wheels. The axle may or may not
support the weight of the vehicle. Rear axles are
normally induction hardened for increased strength.
There are several types of rear axle designs:
semifloating, three-quarter floating, and full floating.
However the semi- and full-floating types are the most
common. Most automobiles use the semifloating type,
whereas four-wheel drive vehicles and trucks use full
The semifloating axle is used in passenger vehicles
and light trucks. In vehicles equipped with this type of
axle, the shaft, as well as the housing, supports the
weight of the vehicle. The inner end of the axle is
carried by the side gears in the differential housing.
This relieves the axle shafts of the weight of the
differential and the stresses caused by its operation that
are taken by the axle housing. The inner ends of the
axle transmit only turning effort, or torque, and are not
acted upon by any other force.
The outer end is carried by a bearing located
between the shaft and the housing. A tapered roller of
ball-type bearing transfers the load from the shaft to
the housing. The axle shafts take the stresses caused by
turning, skidding, or wobbling of the wheels.
The axle shafts (fig. 5-23) are flanged or tapered on
the ends. When the tapered axle is used, the brake drum
and hub are pressed onto the shafts, using keys to
prevent the assemblies from turning on the shafts. In
some cases, the outer ends of the shafts may have
serrations or splines to correspond with those on the
drum and hub assembly. Should the axle break with
this type of axle assembly, the wheel can separate from
Figure 5-23.Semifloating axle installation.
The full-floating axle (fig. 5-24) is used in many
heavy-duty trucks. The drive wheel is carried on the
outer end of the axle housing by a pair of tapered roller
bearings. The bearings are located outside the axle
housing. In this way, the axle housings take the full
weight of the vehicle and absorb all stresses or end
thrust caused by turning, skidding, and pulling. Only
the axle shaft transmits torque from the differential.
Figure 5-24.Full-floating axle shaft.