The 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 four-wheel-drive.

On a 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.

On 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 Functions

The 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.

The 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.

The 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 fuel conservation.

Differential Components

There 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.

The 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.

The 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.

A 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 its centerline.

When 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.

The 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.

When 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.

The integral axle housing is most commonly found on late-model cars and light trucks.

A 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.

Differential Operation

The 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.

Power 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.

When 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.

When 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.)

For 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 gear.