During straight-ahead operation, the differential
assembly operates like a standard differential; all internal
gears turn as a unit. When the vehicle is making a turn, or
when one drive wheel is slipping, the relative speed of the
drive wheels and, therefore, of the axles, changes. This
speed change is transmitted from the faster axle to the
slower one by the action of the meshing spur gears.
The axle gear on the faster axle can drive the respec-
tive worm wheels. This driving force is transferred from the
spur gears on the faster turning worm wheels to the spur
gears on the slower turning worm wheels. Engine power is
transferred from the faster to the slower worm wheels by
the interaction of the gears. The worm wheel on the slower
side still cannot drive the slower axle gear, but it can trans-
fer the increased power from the faster wheel as pressure.
This pressure increases the amount of power sent to the
slower axle gear and axle. It does not turn the axle gear,
but it does allow it to turn with more force.
Hydraulic Locking Differentials
Some late model SUVs have locking differentials that
are operated by hydraulic pressure. It may be called a
Hydra-Lock, Vari-lock, or Georotor system. A hydraulic
locking differential consists of a pump with internal and
external gears, a ring-shaped pressure diaphragm, and a
clutch pack that resembles the clutch pack used in a con-
ventional locking differential, Figure 16-24A. The gear oil
that operates the hydraulic system comes from the rear axle
assembly sump. Special oil is not needed. The same oil is
used for rear axle lubrication, and the hydraulic system does
not have to be sealed from the other rear axle components.
The pump resembles a rotor-type engine oil pump,
with a six-point external gear that turns inside of an inter-
nal gear with seven cavities. The spaces between the pump
internal and external gears are filled with gear oil at all
times. When the gears move in relation to each other,
spaces on the intake side of the pump open and draw in
gear oil. The fluid is carried around to the output side of
the pump, where the spaces begin to close. Closing the
Chapter 16 Rear Axle Assembly Construction and Operation 321
Both driven clutches and center drive
member travel at same speed.
Driven clutch elevated by cams and
travels at faster speed.
Driven clutch elevated by cams and
travels at faster speed.
Driven clutch and center drive member
are locked and rotate at same speed.
Driven clutch and center drive member
are locked and rotate at same speed.
R/H side
gear
L/H side
gear
R/H side
gear
L/H side
gear R/H side
gear
L/H side
gear
A B C
Figure 16-22. The ratchet differential uses matching sets of teeth on each side of the differential case. Teeth are engaged and
disengaged to transfer power. A—Differential is straight-ahead operation. Teeth are engaged on both sides of the case, and power
is transferred equally to each wheel. B—When the vehicle makes a left turn, the greater speed of the right wheel causes the internal
cam on the right side of the case to take the right-side teeth out of engagement. All power goes through the left axle and wheel.
C—When the vehicle makes a right turn, the greater speed of the left wheel causes the left-side cam to take the left-side teeth out
of engagement. All power goes through the right axle and wheel. (Ford)
Ring
gear
Axle shaft
Axle
shaft
Differential
case
Spur
gears
Worm
(axle gear)
Worm wheels
Figure 16-23. The Torsen differential uses a unique arrange-
ment of gears to transfer power. This differential has been
available as a high-performance aftermarket replacement for
about 25 years. It is now being offered as original equipment on
some European vehicles. The operation of this differential is
complex. (Torsen)