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Rear Axle Operation
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| Rear Axle Operation |
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| Updated |
Jul 7, 2004 20:29:14 |
| Rating |
 201 ( -19 -9.45% ) |
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Description:
Below is an overview of rear axle operation
The Differential
The differential is the thing that works both drive axles
at the same time, but lets them rotate at different speeds so that the car
can make turns. When a car makes a turn, the outer wheel has to turn
faster than the inner wheel, due to the difference in the length of the
paths they take. The differential is located between the two wheels, and
is attached to each wheel by a half-shaft rotated through a bevel gear.
Four-wheel drive cars have a separate differential for each pair of
wheels.
A grooved, or splined, axle side gear is positioned on
the splined end of each axle. The side gears are driven by "spider" gears,
which are little gears mounted on a shaft attached to the differential
case. As it is supported by the differential case, the side gear can turn
inside the case.
The differential case can be turned, revolving around the
axle gears. The differential pinion (a pinion is a small gear that either
drives a larger gear or is driven by one) shaft turns the ring gear, which
is fastened to the differential case. The propeller shaft (drive shaft)
connects the transmission output shaft to the differential pinion shaft.
The turning differential case is mounted on two large bearing holders.
These bearings are called carrier bearings.
The propeller shaft rotates the ring gear pinion, and the
pinion turns the ring gear. The ring gear then turns the differential case
and pinion shaft, but the axle side gears will not turn. By passing the
differential pinion shaft through two differential pinion gears that mesh
with the side gears, the case will turn and the axle side gears will turn
with it. During turns, the side gears turn at rates dictated by the radius
of the turns, and the spider gears then turn to allow the outer wheel to
turn faster than the inner one.
Differential Fluids
For lubrication fluid, a very heavy oil, must be used in
rear axle housings. Special hypoid oils are used in the differential case.
Even another type of fluid, or oil must be used in a positraction type
differential.
The oil is circulated by the ring gear, and flung all
over all the parts. Special troughs, or gullies are used to bring the oil
back to certain spots, like the ring and pinion area and the piston
bearings. The fluid is kept in with gaskets and oil seals. The bottom of
the housing has a drain plug, and another filler plug is located part way
up the housing. The housing must never be filled above this plug.
The housing fluid lubricates some of the outer bearings,
but others have lubrication fittings for the injection of wheel bearing
grease. A hand gun, not a pressure grease gun must be used to grease these
bearings (sparingly). A pressure grease gun could inject grease into the
brakes-- greasy brakes are inefficient at best!
Finally, some bearings are filled with grease at the
factory and are sealed. These never require attention unless they are
defective.
Positraction Differentials
A positraction differential is a special traction
differential. Its purpose is to improve the way your differential performs
under adverse conditions. When one wheel starts to slip, these
differentials transfer the torque to the wheel that is not slipping. The
car can then continue to go forward. There are several different kinds of
positraction differentials, but all of them are based on a friction device
to provide resistance to normal differential operation.
A positraction differential provides better traction,
which is handy when roads are slippery. It also lends itself to fast
acceleration.
One type uses four differential pinions instead of two,
with two pinion shafts. It also uses a series of four clutch discs. The
differential pinions run into resistance when they try to turn the axle
side gears. The resistance gets transferred to the pinion shafts driving
the pinions. The shafts are forced to slide up little ramps. This action
moves both shafts outward. The pinions cause the clutches to lock.
Other types use cone clutches, or disc clutches under
pressure from coil springs. By restricting the differential action, torque
is delivered to the slipping wheel.
The Drive Shaft
The drive shaft, or propeller shaft, connects the
transmission output shaft to the differential pinion shaft. Since all
roads are not perfectly smooth, and the transmission is fixed, the drive
shaft has to be flexible to absorb the shock of bumps in the road.
Universal, or "U-joints" allow the drive shaft to flex (and stop it from
breaking) when the drive angle changes.
Drive shafts are usually hollow in order to weigh less,
but of a large diameter so that they are strong. High quality steel, and
sometimes aluminum are used in the manufacture of the drive shaft. The
shaft must be quite straight and balanced to avoid vibrating. Since it
usually turns at engine speeds, a lot of damage can be caused if the shaft
is unbalanced, or bent. Damage can also be caused if the U-joints are worn
out.
There are two types of drive shafts, the Hotchkiss drive
and the Torque Tube Drive. The Hotchkiss drive is made up of a drive shaft
connected to the transmission output shaft and the differential pinion
gear shaft. U-joints are used in the front and rear. The Hotchkiss drive
transfers the torque of the output shaft to the differential. No wheel
drive thrust is sent to the drive shaft. Sometimes this drive comes in two
pieces to reduce vibration and make it easier to install (in this case,
three U-joints are needed).The two-piece types need ball bearings in a
dustproof housing as center support for the shafts. Rubber is added into
this arrangement for noise and vibration reduction.
The torque tube drive shaft is used if the drive shaft
has to carry the wheel drive thrust. It is a hollow steel tube that
extends from the transmission to the rear axle housing. One end is
fastened to the axle housing by bolts. The transmission end is fastened
with a torque ball. The drive shaft fits into the torque tube. A U-joint
is located in the torque ball, and the axle housing end is splined to the
pinion gear shaft. Drive thrust is sent through the torque tube to the
torque ball, to transmission, to engine and finally, to the frame through
the engine mounts. That is, the car is pushed forward by the torque tube
pressing on the engine.
The Universal Joint (U-joint)
The Universal joint (U-joint) is used to connect the
drive shaft to the transmission output shaft and the differential pinion
gear shaft. This joint must be flexible enough to allow changes in the
driving angle (road incline) and the drive shaft. This way, the torque is
constantly transmitted when the rear axle is moving up and down. Smaller
U-joints are used to route the turning motion of the steering wheel
through the steering column to the steering box.
There are two types of U-joints, the cross and roller
type and the ball and trunnion type. The cross and roller type is used the
most; it allows the drive shaft to bend. The ball and trunnion type less
frequently used; it allows the drive shaft to bend and also permits
backward and forward motion of the drive shaft.
Constant Velocity Joints (CV
Joints)
Front wheel drive cars need u-joints which not only allow
up and down motion, but steering motion as well. the angle at which they
turn requires a different design than the standard U-joint.
Constant velocity, or CV joints are universal joints that
are able to transfer torque at large angles efficiently. These joints
transfer power very smoothly. They are comprised of four basic parts: 1.
The outer section, which has grooves machined on its inner surface, 2. the
bearings, which are usually in a "cage", 3. the inner ball, which has
grooves on its outer surface for the bearings to ride in, and 4. a rubber
boot to protect the unit from dirt and moisture.
A common cause of CV joint failure is cracks in the CV
boot. As dirt enters the CV joint, its parts grind themselves until a
clicking noise is heard when turning, or until they fail completely. The
boots should be replaced as soon as cracking is visible in their rubber
folds. |
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