28-02-2013, 10:07 AM
WHEELED VEHICLE DRIVE LINES, AXLES, AND SUSPENSION SYSTEMS
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INTRODUCTION
Automotive drive lines and suspension systems have changed quite a bit since the first automobile was
built. At first, automobile axles were attached directly to the main frame of the vehicle. This caused
many problems. For example, the vehicle produced a very rough ride. Also, rigid construction did not
work well on rough ground because sometimes one of the wheels would not touch the ground. If the
wheel off the ground was a drive wheel, the vehicle lost traction and stopped. This problem proved a
need for a more flexible vehicle.
The problem was corrected by using springs between the axles and the frame. The early springs were
the same type used on the horse-drawn buggy. They allowed the wheels and axles to move up and down
separate from the body. The body moved very little compared to the wheels and axles, and the ride was
much smoother.
Allowing the axles to move separate from the body also kept the wheels on the ground over rough roads,
but this caused a new problem. The old drive train between the engine and the axle would not work.
The train had to be made to move more. This was done by adding movable joints in the drive shaft
known as universal joints. Some early vehicles used only one universal joint on the drive shaft, while
later vehicles used two universal joints on the drive shaft. Drive shafts are now usually called propeller
shafts. Some long-wheel-base trucks now use as many as four propeller shafts between the transmission
and the drive axle. These propeller shafts are connected by universal joints.
PROPELLER SHAFTS
On vehicles equipped with a transfer case, power is transferred from the vehicle's
transmission to the transfer case to the axle assemblies by propeller shafts.A propeller shaft is also called a drive shaft. However, propeller shaft is the more
common name, and some repairers shorten this to "prop shaft." The illustration shows
the power transmission system, including the propeller shafts in one type of wheeled
vehicle. Notice that four propeller shafts are used to drive the vehicle. One transmits
torque from the transmission to the transfer case, one delivers rotary motion from the
transfer case to the intermediate differential, another delivers rotary motion from the
intermediate differential to the rear differential, and the fourth delivers torque to the front
differential.
Construction and Operation
Propeller shafts are made in many different sizes, shapes, and strengths, depending on the
needs of the different types of vehicles. One end of the shaft is built to house a universal
joint. The other end is usually splined to a slip joint. The shafts may be made of solid
steel or may be hollow (tubular). A hollow propeller shaft is usually preferred.
The twisting force (torque) applied to one end of a shaft is transmitted through the shaft
to its opposite end. The strain (stress) created within the shaft ranges from a minimum at
the shaft's rotational center (axis) to a maximum at its outside surface. Since the center
part of a shaft carries only a small portion of the load, tubular (hollow) propeller shafts
are used whenever possible. A solid shaft is stronger than a tubular shaft of the same
thickness (diameter). A tubular shaft, however, is much stronger than a solid shaft of the
same weight and length.
MAINTENANCE OF PROPELLER SHAFTS AND UNIVERSAL JOINTS
Inspection Procedures
Serviceability of propeller shafts, slip joints, and universal joints can be determined by
inspection while they are installed on the vehicle. In general, these parts are inspected in
the same manner on all vehicles. If a rear propeller shaft is to be inspected, place chocks
in the front and rear of the front wheels. Raise the wheels connected to the propeller
shaft being examined so the wheels are free to turn.
Look the propeller shaft over carefully for big dents that would cause it to be bent or out
of balance. Inspect for breaks or cracks at the welded seams at each end. Clean off
excess dirt or tar that may be stuck to the shaft causing it to be unbalanced. Check for
proper mounting, making sure that the propeller shaft has been installed with the slip
joint end nearest the transmission. Also, make sure the yokes on each end of the
propeller shaft are aligned.
Examine the slip joint oil seal and the universal joints. Make sure that the slip joint oil
seal cover is in place and is tight. The universal joint mounting bolts must be tight and
must have no stripped threads. Check each universal joint bearing for breaks or cracks.
Cracks usually occur in the bearing at the inside of the yokes. At this time, inspect for
missing and cracked bearing grease seals, which are also at the inside of the yokes. Look
for broken or missing bearing retaining snap rings.
DEAD AXLES
Dead axles are used on the front of trucks that do not have front-wheel drive. They are
also used on trailers. When used in the front of a vehicle, they must provide for the
steering system. When used on a trailer, they need to hold the wheels in an upright
position and must be strong enough to support the trailer load. Dead axles are usually
held in line with the vehicle frame by the springs that support the vehicle load. There are
several types of dead axle designs.