final drive with provisions for two speeds to be
Two-Speed Final Drive
The two-speed or dual-ratio final drive is used to
supplement the gearing of the other drive train
components and is used in vehicles with a single drive
axle (fig. 5-15). The operator can select the range or
speed of this axle with a button on the shifting lever of
the transmission or by a lever through linkage.
The two-speed final drive doubles the number of
gear ratios available for driving the vehicle under
various load and road conditions. For example, a
vehicle with a two-speed unit and a five-speed
transmission, ten different forward speeds are
available. This unit provides a gear ratio high enough
to permit pulling a heavy load up steep grades and a
low ratio to permit the vehicle to run at high speeds
with a light load or no load.
The conventional spiral bevel pinion and ring gear
drives the two-speed unit, but a planetary gear train is
placed between the differential drive ring gear and the
differential case. The internal gear of the planetary
gear train is bolted rigidly to the bevel drive gear. A
ring on which the planetary gears are pivoted is bolted
to the differential case. A member, consisting of the
sun gear and a dog clutch, slides on one of the axle
shafts and is controlled through a button or lever
accessible to the operator.
When in high range, the sun gear meshes with the
internal teeth on the ring carrying the planetary gears
and disengages the dog clutch from the left bearing
adjusting ring, which is rigidly held in the differential
carrier. In this position, the planetary gear train is
locked together. There is no relative motion between
the differential case and the gears in the planetary drive
train. The differential case is driven directly by the
differential ring gear, the same as in the conventional
single fixed gear final drive.
When shifted into low range, the sun gear is slid
out of mesh with the ring carrying the planetary gears.
The dog clutch makes a rigid connection with the left
bearing adjusting ring. Because the sun gear is integral
with the dog clutch, it is also locked to the bearing
adjusting rings and remains stationary. The internal
gear rotates the planetary gears around the stationary
sun gear, and the differential case is driven by the ring
on which the planetary gears are pivoted. This action
produces the gear reduction, or low speed, of the axle.
The rear wheels of a vehicle do not always turn at
the same speed. When the vehicle is turning or when
tire diameters differ slightly, the rear wheels must
rotate at different speeds.
If there were a solid connection between each axle
and the differential case, the tires would tend to slide,
squeal, and wear whenever the operator turned the
steering wheel of the vehicle. A differential is designed
to prevent this problem (fig. 5-16).
Driving Straight Ahead
When a vehicle is driving straight ahead, the ring
gear, the differential case, the differential pinion gears,
and the differential side gears turn as a unit. The two
differential pinion gears do NOT rotate on the pinion
shaft, because they exert equal force on the side gears.
As a result, the side gears turn at the same speed as the
ring gear, causing both rear wheels to turn at the same
When the vehicle begins to round a curve, the
differential pinion gears rotate on the pinion shaft. This
occurs because the pinion gears must walk around the
slower turning differential side gear. Therefore, the
pinion gears carry additional rotary motion to the faster
turning outer wheel on the turn.
Differential speed is considered to be 100 percent.
The rotating action of the pinion gears carries 90
percent of this speed to the slowing mover inner wheel
and sends 110 percent of the speed to the faster rotating
outer wheel. This action allows the vehicle to make the
turn without sliding or squealing the wheels.
LIMITED SLIP DIFFERENTIALS
The conventional differential delivers the same
amount of torque to each rear wheel when both wheels
have equal traction. When one wheel has less traction
than the other, for example, when one wheel slips on
ice, the other wheel cannot deliver torque. All turning
effort goes to the slipping wheel. To provide good even
traction even though one wheel is slipping, a limited
slip differential is used in many vehicles. It is very
similar to the standard unit but has some means of
preventing wheel spin and loss of traction. The
standard differential delivers maximum torque to the
wheel with minimum traction. The limited slip
differential delivers maximum torque to the wheel
with maximum traction. Other names for a limited slip