chapter 09

Chapter 9Reaction and Friction UnitsUpon completion and review of this chapter, the student should be able to:Classroom Manual Objectives Describe the purpose and operation of

the common reaction members. Explain how a brake band works and

what its purpose is. Identify the basic components in a

hydraulic servo and describe their function.

Describe the different types of one-way clutches used in automatic transmissions.

Explain how a roller-type or sprag-type, one-way clutch works.

Identify the components in a hydraulic multiple disc clutch and describe their function.

Describe the conditions required for automatic shifting.

List the conditions that may have an effect on shift feel.

Explain the purpose and operation of an accumulator and modulator valve.

Shop Manual Objectives Inspect and replace bands and drums. Adjust bands, internally and externally. Inspect and replace servo including

bore, piston, seals, pin, spring, and retainers and repair or replace as necessary.

Inspect an accumulator bore, piston, seals, spring, and retainers, while the transmission is in or out of the vehicle.

Inspect and service clutch drum, piston, check-balls, springs, retainers, seals, and friction and pressure plates.

Measure and adjust clutch pack clearance.

Air test the operation of the clutch pack and servo assemblies.

Inspect and service roller and sprag clutch, races, rollers, sprags, springs, cages, and retainers.

OverviewThis chapter in both manuals covers the construction, purpose, and service of the various clutches and brakes used in transmissions and transaxles.

Reading AssignmentsClassroom Manual, pages 281-308Shop Manual, pages 371-403

Terms to Know

CantileverClutch volume index (CVI)Coast

FulcrumOver-center springPressure plate

Split-bandSteel discsSteels

59

Dual band Selective servo apply pins Transmission tune-up

Lecture Outline and Notes

I. ObjectivesReview the chapter’s objectives.

II. Reaction MembersDiscuss the importance of reaction members with regard to gear ratio changes in the planetary gearset.

III. Friction and Reaction UnitsDiscuss the purposes of clutches and brakes and explain how each of them works to connect or hold members of the planetary gearset.

IV. Brake BandsA. Purpose and Construction

1. Describe the different types of bands used and explain their applications.

2. Describe how bands are used and their operation.B. Band Service

1. Describe general inspection of the bands.2. Describe band adjustment.3. Describe how drums should be checked.

V. Servo and AccumulatorA. Purpose and Construction

1. Describe the operation of a servo.2. Describe the accumulator and its use.

B. Servo and Accumulator Service1. Describe the disassembly of the accumulator.2. Describe how to disassemble the servo.3. Describe inspection of the servos and accumulators.4. Describe the reassembly procedure.5. Describe the method of measuring for the correct selective servo pin.6. Discuss seal removal and inspection.

VI. One-Way Clutches and BrakesA. Describe an overrunning clutch and its use.B. Describe and discuss the ways to check a roller clutch.C. Describe and discuss the ways to check a sprag clutch.

60 Instructor’s Guide to Accompany Today’s Technician: Automatic Transmissions & Transaxles 3E

VII.Multiple-Friction Disc Clutch and Brake Assemblies A. Multiple-Friction Disc Packs

Describe the purpose of the multiple-disc packs found in transmissions, emphasize that they can be used to connect and brake gearset members, and describe how their purpose can be identified by their placement in the transmission.

B. Construction1. Describe the individual parts that make up a multiple-disc pack.2. Describe the different types of spring used in multiple-disc packs.

C. Service1. Describe the disassembly of the multiple-disc pack assembly.2. Describe the general inspection of multiple-disc packs.3. Describe the reassembly procedure.4. Describe how to make clearance checks on a multiple-disc pack

assembly.5. Describe how to air test a multiple-disc pack assembly and servos.

D. Clutch Volume IndexDiscuss CVI in terms of efficiency.

VIII. Gear ChangesA. Shift Feel

Describe the necessity for proper shift feel and how it is accomplished.B. Shift Quality

1. Describe what is meant by shift quality.2. Describe the ways shift quality can be changed.

IX. SummaryReview the material covered, emphasizing the main points and key words.

Chapter 9 Answers to Review Questions

C L A S S R O O M M A N U A L , P A G E S 3 0 8 - 3 1 0

Short Answer Essays1. Reaction members are those parts of a planetary gearset that are held in order to produce an

output motion. Other members of the planetary gearset react against the stationary or held member.

2. Because of the location of the servo and band anchor, the band becomes self-energized and wraps itself around the drum in the same direction as drum rotation. This self-energizing effect reduces the force that the servo must produce to hold the band.

3. A multiple-disc pack also contains one or more return springs, return spring retainers, seals, one or more pressure plates, and snap rings. The seals hold in the hydraulic pressure during application of the pack. A pressure plate is a heavy metal plate that provides the clamping surface

Instructor’s Guide to Accompany Today’s Technician: Automatic Transmissions & Transaxles 3E 61

for the plates and is installed at one end or both ends of the pack. The snap rings are used to hold the parts in the pack together. In a typical pack, the apply piston at the rear of the drum is held in place by the return springs and a spring retainer secured by a snap ring. Hydraulic pressure moves the piston against return spring pressure and clamps the plates against the pressure plate. The friction between the plates locks them together, causing them to turn as a unit.

4. The multiple-friction disc pack can be used to drive or hold a member of the planetary gearset by connecting the member to the transmission’s case or to a clutch drum.

5. Two types of single-wrap bands are commonly used in transmissions today. One type is made of light and flexible steel and the other type is made of heavy and more rigid cast iron. The heavy bands are typically made with a metallic lining material that can withstand large gripping pressures. Light bands are lined with a less-abrasive material that helps limit drum wear.

6. A Belleville spring acts to improve the clamping force of the assembly, and as a piston return spring. The spring is locked into a groove inside the drum by a snap ring. As the piston moves to apply the pack, it moves the inner ends of the Belleville spring fingers into contact with the pressure plate to apply the assembly. The spring’s fingers act as levers against the pressure plate and increase the application force of the pack. When hydraulic pressure to the piston is stopped, the spring relaxes and returns to its original shape. The piston is forced back, and the pack is released.

7. Accumulators rely on the action of a piston or a valve to delay the delivery of high pressure to a clutch or band. An accumulator works like a shock absorber and cushions the application of servos and disc packs. An accumulator cushions sudden increases in hydraulic pressure by temporarily diverting some of the apply fluid into a parallel circuit or chamber. This allows the pressure to gradually increase and provides for smooth engagement of a brake or clutch.

8. “Shift Feel” is controlled by the pressure at which each reaction member is applied or released, the rate at which each is pressurized or exhausted, and the relative timing of the apply and release of the members. It is also affected by fluid type, the momentary engagement of a component in a different circuit, pulsed pressures, the clearance of the apply devices, and many more design features of the various transmission models.

9. A roller clutch utilizes roller bearings held in place by springs to separate the inner and outer races of the clutch assembly. Around the inside of the outer race are several cam-shaped indentations. The rollers and springs are located in these pockets. Rotation of one race in one direction locks the rollers between the two races, causing both to rotate together. When a race is rotated in the opposite direction, the roller bearings move into the pockets and are not locked between the races. A one-way sprag clutch consists of a hub and a drum separated by figure-eight-shaped metal pieces called sprags. The sprags are shaped in such a manner that they lock between the races when a race is turned in one direction only. Between the inner and outer races of the clutch are the sprags, cages, and springs. The sprags are longer than the distance between the two races. The cages keep the sprags equally spaced around the diameter of the races. The springs hold the sprags at the correct angle and maintain contact of the sprags with the races for instantaneous engagement. When a race turns in one direction, the sprags tilt and allow the races to move independently. When a race is moved in the opposite direction, the sprags straighten up and lock the two races together.

10. A band is applied hydraulically by a servo unit. The servo contracts the band when hydraulic pressure pushes against the servo’s piston and overcomes the tension of the servo’s return spring. This action moves an operating rod toward the band, which squeezes the band around the drum.


Fill-in-the-Blanks1. Sun, ring

2. Double-wrap, single-wrap

3. Governor, throttle, throttle

4. Centrifugal check ball relief

5. Coil, small coil, Belleville

6. Straight, lever, cantilever

7. Driving

8. Surface area

9. Driving device, splined, splined, together, the same speed

10. Reaction members

Multiple Choice1. C

2. A

3. A

4. C

5. A

6. A

7. C

8. B

9. C

10. C

S H O P M A N U A L , P A G E S 4 0 3 - 4 0 4

ASE-Style Review Questions1. B

2. C

3. C

4. D

5. C

6. B

7. D

8. A

9. C

10. B

Instructor’s Guide to Accompany Today’s Technician: Automatic Transmissions & Transaxles 3E 63

ASE Challenge Questions1. A

2. C

3. C

4. C

5. A


chapter 09

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