A Car Differentials and How Work

Before we get into it, lets start with the basics of the car differential; whether it be gas, diesel, hybrid, or electric. A cars power source delivers a certain amount of power to the wheels via a driveshaft, or a front differential transaxle in the case of a frontwheeldrive car. The power produced by this driveshaft needs to be split to drive the two wheels. Thats why differentials exist: to split the power between the wheels while allowing them to travel at different speeds. There isnt just one gear in a car differential or diff, but a few different parts.

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D Halderman

start

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• Prepare for ASE Manual Drive Train and Axles (A3) certification test content area “E (Rear

Axle Diagnosis and Repair).

torque to be applied to both drive wheels and

still allow a difference in the speed of the

drive wheels during cornering.

After studying Chapter 98, the reader should

be able to:

OBJECTIVES:

Continued

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determine corrective action.

pinion gear depth.

bearing preload.

to provide the proper backlash.

After studying Chapter 98, the reader should

be able to:

OBJECTIVES:

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axle ratio

coast (or concave) side • companion flange • crown

diff • differential case • drive (or convex) side • drive

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rear end • ring gear • ring gear runout • root

side gears • spider gears • straddle-mounted pinion

toe • turning torque • viscous coupling

KEY TERMS:

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PURPOSE AND FUNCTION OF A DIFFERENTIAL

The differential allows engine torque to be applied to both drive

axles, which rotate at varying speeds during cornering and while

traveling over bumps and dips in the road The differential also

changes the direction of engine torque 90° from the rotation of the driveshaft lengthwise with the vehicle

Continued

These two purposes of a differential can be summarized as follows:

To change the direction of engine torque.

See Figure 98–1

To allow the drive wheels to rotate at different speeds.

See Figure 98–2

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Figure 98–1 The differential assembly

changes the direction of engine torque and

increases the torque to the drive wheels.

Continued

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Figure 98–3 When the vehicle turns a corner, the inner wheel slows and the outer wheel

increases in speed to compensate This difference in rotational speed causes the pinion gears

to “walk” around the slower side gear

Continued

A differential is a mechanical addition and subtraction assembly

By splitting the engine torque to the drive wheels when the

vehicle is turning a corner, the torque forces cause the side gear

and pinion mate gears to subtract torque from one side and add

torque to the opposite side

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PARTS OF A DIFFERENTIAL

A differential is also called a rear end or abbreviated simply as a

diff Whenever any vehicle makes a turn, the outside wheel must

travel a greater distance than the inside wheel The driveshaft

applies torque to the drive pinion gear that meshes below the

center line of a ring gear as shown here.

Continued

Figure 98–4 A hypoid gear set uses a drive

pinion that meshes with the ring gear below

the center line of the ring gear

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This type of gear set is called a hypoid gear set and requires gear lubrication specifically designed for this type of service.

The ring gear is attached to a differential case that also contains small beveled spider gears or

pinion gears.

A pinion shaft passes through the two pinion gears in the case.

In mesh with the pinion gears are two side gears that are splined

to the inner ends of the axles.

See Figure 98–5

Continued

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convex side While decelerating, the drive pinion contacts the ring teeth on its coast, or

concave side See Figure 98–7 The intermediate position between drive and coast, when

neither the ring gear nor the pinion is driving each other, is called the “floating” position

Continued

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The Pinion and Ring Gears During Operation During operation, the

position of the drive pinion gear on the ring gear changes.

The ring gear mounts onto the differential case.

Each slanted ring gear tooth has two ends Its toe is closest to the ring gear

center; its heel, closest to the outside circumference.

The tooth root is the depression lying between two teeth, and the crown is the

very top of each tooth.

See Figure 98–6.

Continued

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Figure 98–7 The drive side is the convex side of the ring gear except for some front axles used

in four-wheel vehicles, and they often use the concave side on the drive side they often use the concave side on the drive side.

While a vehicle accelerates, the drive

pinion contacts the ring teeth on its

drive, or convex side

While decelerating, the drive pinion

contacts the ring teeth on its coast,

or concave side.

The intermediate position between

drive and coast, when neither the

ring gear nor the pinion is driving

each other, is called the “floating”

position.

Continued

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DIFFERENTIAL GEAR RATIOS

The final gear reduction in the drive train occurs in the differential

assembly The amount of torque a gear set delivers depends on the

gear ratio between the drive pinion gear and the driven ring gear.

The ratio of the final drive (differential) is called the axle ratio

Continued

To determine the axle ratio, divide the number of teeth of the ring gear (driven gear) by the number of teeth of the drive pinion gear

(driving gear):

The higher the axle ratio number, the faster the engine rotates for

each rotation of the drive wheels

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Ring and Pinion Gear Set Types The final drive gear ratio

determines how many times a drive pinion tooth will make contact with a particular ring gear tooth during one revolution This

affects final drive gear set manufacture and service.

Continued

Final drive gear sets may be divided into three types, depending on the final drive gear ratio

1 Hunting gear sets are sets with final drive ratios expressible

in a fraction that cannot be reduced to any lower terms.

An example is one that has 41 teeth on the ring gear and 11

teeth on the drive pinion This combination creates a 3.73:1

axle ratio This type of gear set requires no timing marks or

alignment during assembly As the pinion gear drives the ring

gear, each pinion tooth will hunt for, or seek, contact with

every ring gear tooth.

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2 Nonhunting gear sets are gear sets with final drive ratios

expressible as a whole number.

Nonhunting gear sets require timing marks As the pinion gear

drives the ring gear, each pinion tooth contacts only a few ring

gear teeth during each revolution.

3 Partially nonhunting gear sets are sets with final drive ratios

expressible as a reducible fraction not equaling a whole number.

Continued

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Partially nonhunting gear sets also require timing marks During

final drive operation, each pinion tooth contacts only some of the

ring pinion teeth.

For the pinion teeth to make contact with the highest number of

ring gear teeth, the pinion gear must drive the ring gear more than

one revolution.

On nonhunting and partially nonhunting gears, manufacturers lap

the contacting gear teeth to decrease wear These gear sets are

marked to ensure proper alignment during assembly procedures.

To preserve wear patterns, the gear sets should be reassembled

using the same alignment This prolongs the life of the gear set

and decreases operational noise.

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While a vehicle travels straight ahead, the speed of each driven wheel must be allowed to vary slightly as they go over bumps, potholes, railroad tracks, and other road surface irregularities.

While cornering, the wheels must be able to turn at much greater differences in speed Without some

mechanism to allow for a difference in speed between the wheels, the left wheel would skid through the

turn.

Inside the differential gear housing four to six bevel gears help drive the axles In most rear axles, two of

these bevel gears are smaller pinions mounted on a shaft They drive two side gears splined with each

inner axle end.

Continued

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Torque Flow Through a Standard Open Differential

Open differentials deliver equal torque to both wheels at all times As the case rotates (driven by the engine through the ring and pinion gears), the cross-shaft applies drive force to the spider gears.

The two side gears apply reaction forces that counter this drive force Because the spider gear is free

to rotate about the cross-shaft, the two reaction forces are equal As the side gear applies a force to

the spider gear, the spider gear applies an equal and opposite force to the side gear.

It is this force, on the side gear, that supplies the torque to the axle that drives the wheel Because the force on each side gear is equal, the torque supplied to each wheel is also equal This is true

regardless of whether one wheel is rotating faster than the

other or at the same speed See Figure 98–8

Continued

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Figure 98–8 A close-up view of the side gears and spider (pinion) gear Note the ridges on the

gear teeth These ridges are manufactured into the gear teeth to help retain lubricant so that no metal-to-metal contact occurs

Continued

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gear, carrier, and the drive axles are traveling at the same speed; they rotate as a unit.

Traveling Around Corners The ability of differential pinion gears to spin on their shafts allows

each axle to rotate at a different speed.

Case speed is always equal to the average speed of the two side gears Since the ring gear rotates

with the case and each side gear rotates with its axle; when a vehicle corners, the outside wheel

gains the same number of RPMs that the wheel on the inside loses, while ring gear RPM remains

constant.

Continued

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Limited Slip Differentials When a vehicle equipped with a standard differential spins a tire, the

opposite wheel does not receive enough torque to move the vehicle.

To solve this problem, most manufacturers use differentials that direct more power to the side gear attached to the spinning axle Many differentials do this by forcing the side gear against the

revolving case This bypasses differential action, allowing the

case to drive the side gear directly.

A limited-slip differential distributes torque to both wheels equally or unequally, allowing the

wheels to turn at the same or at different speeds.

See Figure 98–9.

Continued

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Figure 98–9 (a) A two-wheel-drive vehicle equipped with an open differential (b) A two-wheel

drive vehicle equipped with a limited-slip differential.

Continued

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Preloaded clutches Use two mechanisms to accomplish this action First, a coil, Belleville, or leaf spring cocks the two side gears

Figure 98–10 Trac-loc limited-slip

differential This type of limited-slip

differential uses the preload force from a

spring and the torque generated by the

side gears as the two axles rotate at

different rates to apply the clutches and

limit the amount of difference in the

speed of two axles

Second, a multidisc clutch

pack or cones lie behind

one or both of the side gears

Manufacturers refer to these

differentials using brand

names such as Positive

Traction, Sure-Grip,

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When forces inside the differential push the side gear against the case, the clutch pack allows the gear

to lock smoothly without jerking The pack, lying between at least one of the backs of the side gears

and the case, consists of two types of alternatively stacked steel discs with holes through their middle

sections.

The first is coated with friction material and the second is not The interior splines of the friction disc

holes mesh with the splines on the center hub protruding from the backside of the side gear The holes

of the steel discs are not splined They have two or three tabs along their outside diameter that fit into

slots in the case.

As the side gear is pushed further against the case, each friction plate rubs against the steel plate in

front of and behind it, gradually slowing the side gear’s rotation until it smoothly locks against the case.

Continued

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Two forces help to push the side gear out of mesh from the

differential pinions and to lock it against the differential case

Continued

1 The rotating differential pinions apply pressure against the

side gear The amount of pressure depends on the traction

beneath the tire connected to the side gear.

2 The spring applies the second force that allows the preloaded

clutch to work This spring, whether a coil, leaf, or Belleville, preloads the clutch discs, narrowing the clearances between

them Tighter springs cause the differential to lock up sooner

than looser springs.

To vary the amount of traction a tire must have before its axle’s

side gear locks up, many differentials use adjustment shims.

Available in different thicknesses, manufacturers install them

between the rear of clutch packs and differential case

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The thicker the shims, the sooner the differential locks up Some differentials rely on

the preload spring alone to determine when the side gear locks up They use no shims.

During operation, the clutch discs slide against each other every time the vehicle turns

and corners This action creates friction, making it important to add the

manufacturer-recommended lubricant that contains a friction modifier.

This lubricant may already be included in the gear oil or may be a special additive

Using improper lubricants may cause the discs to wear prematurely and vibrate during

turns.

Continued

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Cone-Type The second type of preloading clutch differential

uses two tapered cones instead of multidisc clutch packs

Positioned between the side gears and the case, the cone’s

tapered end fits inside a dished receptacle machined into

the case.

The same forces that push the side gears against the case in the

clutch pack differential push the side gears against the case in the

tapered-cone differential.

When these forces press the cones into their dished receptacles,

they come to a stop smoothly, locking the side gears to the case.

Continued

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Viscous Coupled Limited-Slip Units Differentials can be designed

to work on the principle of a viscous coupling This design uses a

series of closely positioned plates, which do not physically touch

one another.

Half of the plates are splined to the case, and the other half are

alternately splined to each side gear The plates are housed in a

sealed chamber, which is filled with a thick and viscous

silicone-based fluid The silicone allows normal speed differences between two shafts, resisting high-speed differences associated with wheel spin on one shaft.

This type of viscous-coupled differential is used in the front

differentials of some Japanese front-wheel-drive vehicles and in

four-wheel-drive vehicles

Continued

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The Eaton “Locker” Differential The Eaton Locker’s sturdier design makes it more efficient in transferring torque from the driveline to the axles while in the locked position than the clutch-type multidisc differential.

When a vehicle spins a rear wheel, both the preloaded multidisc differential and the Eaton “locker” compress their clutch packs lying between the side gears and the case This allows the case

to directly drive the axles, bypassing the effect of the rotating differential pinions.

The Eaton and preloaded multidisc differentials differ in how they compress their clutch packs As the rotating pinion gears push against a slower moving side gear, it is forced against the case,

thereby compressing the discs in the clutch pack together.

Continued

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A governor with two flyweights

A cam plate and cam side gear on the left side

A latching bracket

A thrust block

The Eaton differential, on the other hand, uses a unique mechanism

to collapse the clutch packs It has four parts:

Continued Figure 98–11 An Eaton locker differential.

These parts allow the

Eaton to momentarily

shift into the locked

mode when side gear

and case rotational

speeds differ by more

than 100 RPM.

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I Used to Have a Limited-Slip Differential

An owner of a Chevrolet S-10 pickup truck equipped with a V-6 and

five-speed manual transmission complained that he used to be able to spin

both rear tires on dry pavement, but lately only one tire spins.

The service technician assigned to the repair order was very familiar with what might have occurred Many General Motors pickup trucks are

equipped with an Eaton locking differential that uses a torque limiting disc.

The teeth of this disc are designed to shear to prevent the possibility of

breaking an axle See Figure 98–12.

The service procedure to correct the customer’s concern is to replace the left-hand clutch plates Usually, the shearing of the torque-linking teeth is associated with a loud bang in the rear axle The differential will continue to operate normally as a standard (open) differential

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Figure 98–12 This Eaton design differential uses a torque-limiting disc to prevent the

possibility of breaking an axle in the event of a high-torque demand When the disc tangs shear, the differential will continue to function but as an open rather than as a limited-slip differential

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Torsen Differentials The Torsen differential is a torque sensing, locking design that uses a set of worm side gears in mesh with individual worm wheel pinions that are supported by the differential case.

The pinions have spur gear sections machined on at each end, which form the connection between left

and right side pinions Because of the worm and wheel configurations, the side gears

can turn the pinions, but the pinions cannot turn the side gears.

The result is a complex operation, which meets all drive requirements without excessive wheel slip This unit has been available as original equipment on several vehicles as well as for many aftermarket

applications.

See Figure 98–13

Continued

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Figure 98–13 A Torsen differential This type of differential provides torque to both drive

wheels even if one tire is on ice The complex system of gears allows this smooth transfer of

torque without the use of clutches.

Continued

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Drive Pinions The drive pinion is driven by a flange often called a

companion flange The heavy bulk of the companion flange helps

dampen vibrations and absorb shocks in the driveline

Continued

Figure 98–14 This pinion flange is equipped with

a damper weight to help dampen driveline vibrations

The final drive pinion shaft

may be supported by bearings

using one of two methods.

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two inner races to support an overhung pinion This term means all

of the bearings supporting the pinion shaft lie on one side of the

pinion gear The bearings are preloaded, a state that minimizes

wobble and endwise movement while the drive pinion shaft rotates.

Continued

Figure 98–15 A collapsible

spacer-type drive pinion shaft

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The second method uses two opposed taper roller bearings to support a straddle-mounted

pinion, but the distance between them is smaller than in the overhung design.

Most noticeably, a third smaller bearing attaches to a stemlike machined pilot protruding from

the gear-end of the drive pinion shaft This third bearing, usually a straight roller type, fits into

a bore in the carrier Unlike the overhung design, bearings supporting a straddle-mounted

pinion lie on both sides of the pinion gear.

Like the overhung design, a straddle-mounted pinion shaft usually has a compressible

preloaded spacer positioned between the two larger differential case bearings

Straddle-mounted pinions are usually Straddle-mounted in a pinion housing that can be removed from the carrier.

Continued

Tiêu đề	A Car Differentials and How Work
Tác giả	James D. Halderman
Trường học	Pearson Education, Inc.
Chuyên ngành	Automotive Technology
Thể loại	sách
Năm xuất bản	2008
Thành phố	Upper Saddle River

Định dạng
Số trang	98
Dung lượng	8,08 MB
File đính kèm	Differentials.rar (7 MB)