Lesson 2/Learning Event 2
Breather valves are installed in the axle assembly to allow excess pressure to escape.
Pressure will build up in the gearbox as the unit heats up during operation. The breather
valve must be able to perform this job without allowing dirt or water to enter from the
The 1 1/4-ton utility truck, M998-series, has a final drive that differs from other vehicles.
Each truck of this model is equipped with individual (independent) wheel suspension.
This means that each wheel has its own separate mounting and is not solidly connected to
any of the other wheels. Each of the wheels may move up and down as the vehicle
follows the road surface without affecting any of the other wheels.
This type of construction does away with the solid axle that extends across the vehicle
below the body and frame. The differential assembly is made as a separate unit. Both
the front and rear differential assemblies are mounted to the frame of the vehicle rather
than in an axle housing.
The HMMWV uses axle drive shafts (half shafts) to accommodate the independent
suspension system. The purpose of the half shafts is to transfer torque to the wheels from
the differential through the geared hub. The unit is basically a one-piece assembly with
boots on both the inboard (differential) and outboard (geared hub) ends. The inboard end
is bolted to the differential side flange, and the outboard end is splined to the drive gear
of the geared hub. The inboard boot encloses a tripot joint which accommodates the in-
and-out and angular motion of the axle drive shaft with no change to either system.
The outer boot encloses a constant velocity joint which transmits torque through various
steering angles to the geared hub. The constant velocity joint end of the shaft assembly is
held in place by the axle shaft retaining bolt located opposite the pipe plug in the geared
hub. The tripot housing is held in place by both the differential output flange bolts and
the caliper mounting brackets.
The geared hub is a gearbox, located at the wheel ends, that serves as the front wheel
spindle. It can be considered the final drive unit. It permits up to 16 inches of ground