differential is a geared mechanism located between the driving axles of a
vehicle. Its job is to direct power flow to the driving axles. Differentials are
used in all types of power trains: rear-wheel-drive, front-wheel-drive, and
front-wheel-drive car or truck, the differential is normally an integral part of
the transaxle assembly located at the front of the vehicle. Depending on whether
the engine is mounted transversely or longitudinally, the transaxle differential
design and operation vary. With a transverse-mounted engine, the crankshaft
centerline and drive axle are on the same place. With a longitudinal-mounted
power plant, the differential must change the power flow 90 degrees.
rear-wheel-drive vehicles, the differential is part of the rear assembly. It is
located in the rear axle housing or carrier. A driveline running beneath the
vehicle from front to rear connects the transmission with the differential
gearing. Four-wheel-drive vehicles have differentials on both their front and
rear axles. A transfer case mounted to the side or back of the transmission
splits the power between two drive shafts or lines. One runs to the front drive
axle and the other runs to the rear drive axle. Differentials at the front and
rear axles redirect power flow to all wheels. Some full-time four-wheel-drive
systems employ a third differential. This interaxle differential is located in
the transfer case between the front and rear drive axles. The interaxle
differential allows the two drivelines to rotate at different speeds, which
improves vehicle handling and reduces component wear.
differential actually performs several functions. It allows the drive wheels to
rotate at different speeds when negotiating a turn or curve in the road and
redirects the engine torque from the drive shaft to the rear drive axles. The
drive shaft turns in a motion that is perpendicular to the rotation of the drive
wheels. The differential assembly redirects the torque so that the drive shafts
turn in a motion that is parallel with the rotation of the drive wheels.
differential also splits torque between the drive axles. Each drive wheel
receives an equal amount of torque. The torque delivered to the wheels is no
greater than the torque required by the wheel with the least amount of traction.
If one wheel begins to slip, it requires much less torque to turn it. Less
torque is then delivered to the wheel that is not slipping. In such a situation,
unless the differential is a limited-slip design, the vehicle loses traction.
drive gears of a differential are also seized to provide a gear reduction, or a
torque multiplication. Differentials with a low (numerically high) gear ratio
allow for fast acceleration and good pulling power. Differentials with high gear
ratios allow the engine to run slower at any given speed, resulting in better
are several other basic design arrangements. However, the one most commonly used
design features pinion/ring gears and a pinion shaft. The latter is a spiral
bevel gear that is mounted on an input (pinion) shaft. The shaft is mounted in
the front end of the carrier and supported by two or three bearings. An overhung
pinion gear is supported by two tapered bearings spaced far enough apart to
provide the needed leverage to rotate the ring gear and drive axles. A straddle
mounted pinion gear rests on three bearings: two tapered bearings on the front
support the input shaft and one roller bearing is fitted over a short shaft
extending from the rear end of the pinion gear.
pinion gear meshes with a ring gear. The ring gear is a ring of hardened steel
with curved teeth on one side and threaded holes on the other. The ring gear is
bolted to the differential case. When the pinion gear is rotated by the drive
shaft, the ring gear is forced to rotate, turning the differential case and axle
shafts. In most automotive applications, there are two pinion gears mounted on a
straight shaft. On heavier trucks, the differential contains four pinion gears
mounted on a cross-shaped spider. The pinion shafts are mounted in holes in the
case (or in matching grooves in the case halves) and are secured in place with a
lock bolt or retaining rings.
teeth on the drive pinion contact the ring gear at either the same, or
different, places after several revolutions of the ring gear. There are three
terms that describe the tooth contact: hunting gearset, nonhunting gearset, and
partial nonhunting gearset.
HUNTING GEARSET When one drive pinion gear tooth contact every ring gear
tooth after several revolutions, the gearset is hunting. In other words, the
drive pinion hunts out each ring gear tooth. A typical hunting gearset may have
9 drive pinion teeth and 37 ring gear teeth. The rear-axle ratio for this
combination would be 4.11:1.
NONHUNTING GEARSET When one drive pinion gear tooth contacts only certain
ring gear teeth, the gearset is nonhunting. A typical nonhunting gearset may
have 10 drive pinion teeth and 30 ring gear teeth. The rear-axle ratio for this
combination would be 3.00:1. For every revolution of the ring gear, each drive
pinion tooth would contact the same 3 ring gear. The drive pinion gear teeth do
not hunt out all ring gear teeth.
PARTIAL NONHUNTING GEARSET The difference between nonhunting and partial
nonhunting gearsets is the amount of ring gear teeth that are contacted. On a
partial nonhunting gearset, one drive pinion tooth contacts six ring gear teeth
instead of three. During the first revolution of the ring gear, one drive pinion
tooth contacts three ring gear teeth. During the second revolution of the ring
gear, the drive pinion tooth contacts three different ring gear teeth. During
every other ring gear revolution, one drive pinion tooth contacts the same ring
gear teeth. A typical partial may have 10 drive pinion teeth and 35 ring gear
teeth. The rear-axle ratio for this combination would be 3.50:1. The number of
teeth on the drive pinion and ring gear determine whether the gearset is
hunting, nonhunting, or partial nonhunting. Knowing the type of gearset is
important in diagnosing ring and pinion problems.
hypoid gear is similar to a spiral bevel gear. The hypoid gear contacts more
than one tooth at a time. The hypoid gear also make contact with a sliding
motion. This sliding action, however, is smoother than that of the spiral valve
gear, resulting in quieter operation. The biggest difference is that in a hypoid
gear, the centerlines of the ring and pinion gears do not match. A centerline is
an imaginary line that connects the axis of one part to the axis of another
part. For example, when the drive pinion is set in the middle of the right gear,
the centerline from the center, or axis, of the drive-pinion-gear shaft. Then,
locate the centerline of the ring gear. The centerlines should meet on spur-type
bevel gears. Using hypoid gears, the drive pinion gear is placed lower in the
differential. The drive pinion meshes with the ring gear at a point below
hypoid gears mesh during operation, they do so with a sliding action. The teeth
tend to wipe lubricant from the face of the gear, resulting in eventual damage.
Differentials require the use of extreme pressure-type lubricants. The additives
in this type of lubricant allow the lubricant to withstand the wiping action of
the gear teeth without separating from the gear face.
differential also contains two side gears. The inside bore of the side gears is
splined and mates with splines on the ends of the drive axle. The differential
pinion gears and side gears are in constant mesh. The pinion gears are mounted
on a pinion gear shaft, which is mounted in the differential casing. As the
casing turns with the ring gear, the pinion shaft and gears also turn. The
pinion gears deliver torque to the side gears.
the pinion the ring gear are manufactured, the faces of the gear teeth are
machined to provide smooth mating surfaces. The pinion gear and ring gear are
always matched to provide a good mesh. Pinion gears and the ring gear should
always be installed as a set. Otherwise, the mismatched geatset might operate
noisily. A matched gearset code is etched in each drive pinion and ring gearset.
Rear Axle Housing and Casing The differential in a rear-drive vehicle is
housed in the rear axle housing, or carrier. The axle housing also contains the
two drive axle shafts. There are two types of axle housings found on modern
automobiles: the removable carrier and the integral carrier. The removable
carrier axle housing is open on the front side. Because it resembles a banjo, it
is often called a banjo housing. The backside of the housing is closed to seal
out dirt and contaminants and keep in the differential lubricant. The
differential is mounted in a carrier assembly that can be removed as a unit from
the axle housing. Removable carrier axle housings are must commonly used today
on trucks and other heavy-duty vehicles.
integral axle housing is most commonly found on late-model cars and light
cast-iron carrier forms the center of the axle housing. Steel axle tubes are
pressed into both sides of the carrier to form the housing. The housing and
carrier have a removable rear cover that allows access to the differential
assembly. Because the carrier is not removable, the differential components must
be removed and serviced separately. In addition to providing a mounting place
for the differential, the axle housing also contains brackets for mounting
suspension components, such as control arms, leaf springs, and coil springs.
GEAR REDUCTION When a smaller gear drives a larger gear, the larger gear
turns slower. This is known as speed or gear reduction. The smaller drive pinion
gear may turn only about two-and-half times the ring gear speed. This difference
between the speed of the drive pinion gear and the ring gear is known as the
rear-axle ratio. Most rear-axle ratios appear with decimals, such as: 2.43:1,
3.08:1, or 4.11:1. In a differential, the smaller gear is the drive pinion gear
and the larger gear is the ring gear.
amount of power delivered to each driving wheel by the differential is expressed
as a percentage. When the vehicle moves straight ahead, each driving wheel
rotates at 100 percent of the case speed. When the vehicle is turning, the
inside wheel might be getting 90 percent of the case speed. At the same time,
the outside wheel might be getting 110 percent of the differential action.
flow through the differential begins at the drive pinion yoke or companion
flange. The Companion flange accepts torque from the rear U-joint and is
attached to the drive pinion gear, which transfers torque to the ring gear. As
the ring gear turns, it turns the carrier and the pinion shaft. The differential
pinion gears transfer torque to the side gears determine how much torque goes to
each driving axle. The pinion gears can move with the carrier, and they can
rotate on the pinion shaft.
drive shaft torque is applied to the input shaft and drive pinion, the shaft
rotates in a direction that is perpendicular to the vehicle's drive axles. When
this rotary motion is transferred to the ring gear, the torque flow changes
direction and becomes parallel to the axles and wheels. Because the ring gear is
bolted to the differential case, the case must rotate with the ring gear. The
pinion gear shaft mounted in the casing must also rotate with the case and the
ring gear. The pinions turn end over end. Gears do not rotate on the pinion
shaft when both driving wheels are turning at the same speed. They rotate and
over end as the differential case rotates. Because the pinions are meshed with
both side gears, the side gears rotate and turn the axle shafts. The ring gear,
differential gears, and axle shafts turn together without variation in speed as
long as the vehicle is moving in a straight line.
a vehicle turns into a curve or negotiates a turn, the wheels on the outside of
the curve must travel a greater distance than the wheels on the inside of the
curve. The outer wheels must then rotate faster than the inside wheels. This
would be impossible if the axle shafts were locked solidly to the ring gear.
However, the differential allows the outer wheels and axle shaft to increase in
speed and the inner wheels and axle to slow down. This prevents the skidding and
rapid tire wear that would otherwise occur. The differential action also makes
the vehicle much easier to control while turning. (The differential also allows
the wheels on opposite ends of the axle to rotate at different speeds to
compensate for eneven road conditions and slightly mismatched tires.)
example, when a car makes a sharp right-hand turn, the left-side wheels, axle
shaft, and side gear must rotate faster than the right-side wheels, axle shaft,
and side gear. The left side of the axle must speed up and the right side must
slow down. This is possible because the pinions to which the side gears are
meshed are free to rotate on the pinion shaft. The increased speed of the
left-side wheels causes the side gear to rotate faster than the differential
case. This causes the pinions to rotate and walk around the side gear. As the
pinions turn to allow the left-side gear to increase speed, a reverse action -
known as a reserve walking effect - is produced on the right-side gear. It slows
down an amount that is inversely proportional to the increase in the left-side