chap101 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 101, the reader should
be able to:

•
•
•
•

Explain how apply drives function in an
automatic transmission/transaxle.

Describe how clutches and bands work.
Discuss one-way clutches and how they are
used.
Explain how shifts are controlled in an
electronically controlled automatic
transmission/transaxle.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
apply devices
balanced valve
check balls • clutch drum • clutch pack • clutch plates

default gear
force motors
governor valve
independent accumulator • integral accumulator
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KEY TERMS:
manual valve • multiple-disc clutch
one-way clutch • one-way sprag clutch • one-way valve
pressure control solenoids • pressure plate • pressure
regulator valve • primary spring
reaction member • rod

servo • shift valve • sprags • strut
transmission band • variable force solenoids (VFS)
worm holes • worm tracks
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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APPLY DEVICES
Apply devices are the mechanical assemblies that provide holding and driving forces. Automatic 
transmissions typically use transmission bands, multiple­disc clutches, and one­way clutches.
Transmission bands are holding devices. While bands always provide a holding force, they cannot 
provide a driving force.
The planetary gear set member held by the band is known as a reaction member.
A transmission band stops and holds one planetary gear set member so that another member can react 
against the held member and develop output motion. The reaction member has a control surface for the 

band to ride on known as a drum.

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The band provides holding force around the outside of the drum, 
closing tightly to keep it from turning. 
Transmission bands are made of cast iron or steel with friction 
material lining the inside surface. Transmission design and 
component size dictate the type of band used. 

Figure 101–1 Gear set
members are attached

to a drum and are held
stationary when the
band applies.

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Bands fall into one of two categories:
Single­wrap
Double­wrap 
The single­wrap band is a simple one­piece design, while the 
double­wrap band is a split band with overlapping ends.
Bands may also be classified as either flexible or fixed depending 

on how well they hold their shape when they are off the drum.

Figure 101–2 Transmission
bands come in several
designs and thicknesses.

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HYDRAULIC SERVOS
A hydraulically­operated piston that travels inside a machined cylinder bore applies the transmission band. This piston 
and cylinder assembly is known as a servo.
A piston return spring normally holds the servo piston in its unapplied position. To apply the band, hydraulic fluid 

under pressure enters the servo cylinder and acts on the piston.
The piston begins moving once hydraulic pressure overcomes spring force. A mechanical rod and linkage attaches to 
the piston and connects it to the band. As the piston moves, the linkage applies the band by tightening it around the 
drum.
See Figure 101–3. 

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Figure 101–3 A servo uses hydraulic pressure to move a piston, which applies a band.

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Servo Linkages  Servo rods and struts transfer the servo apply 
force to the transmission band. Rods are round metal bars and struts 
are flat metal plates. These may be used at either, or both, ends of 
the band. Linkages connect the band to the servo or an anchor. 

Figure 101–4 Notice that one
end of the band is held
stationary and the other end
is attached to the servo.

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ACCUMULATORS
An accumulator cushions, or dampens, hydraulic pressure surges by 
temporarily diverting part of the fluid flow in a hydraulic circuit into a 
parallel circuit or chamber.
The diversion allows pressure in the main apply circuit to increase 
gradually and provides a smooth engagement of the band or clutch. 
Accumulators fit into two classes: piston­ or valve­type.
Piston­type accumulators look and function much like servo pistons. 
In fact, some piston­type accumulators share a bore with one of the 
transmission servos. This common bore design is known as an 
integral accumulator.
See Figure 101–5. 

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Figure 101–5 An integral accumulator is combined with a servo in a single bore.

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Piston ­accumulators that install in a dedicated bore in the transmission case are known as 
independent accumulators.
As hydraulic pressure enters both circuits, the accumulator piston offers much less 
resistance than the servo piston because it represents a much lighter load. As a result, the 
accumulator piston moves first and absorbs any surges as the fluid fills the accumulator 
cylinder.
Once the accumulator piston reaches the end of its travel, cushioning is complete and 
pressure begins to build in the servo cylinder. This pressure acts on the servo piston to 
apply the band.

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MULTIPLE-DISC CLUTCHES
Like a transmission band, a multiple­disc hydraulic clutch is a type of apply device. Clutches have more 
friction area so they can develop more force and handle more torque than a band, and they are self­adjusting.
The multiple­disc clutch consists of plates, a piston, drum, and snap ring. The piston returns to an unapplied 
position via a ­return spring assembly.
Some transmissions use a special wave­type spring to return the piston and cushion clutch applications. The 
entire assembly is commonly known as a clutch pack.
See Figure 101–6.

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Figure 101–6 An exploded view of a multiple-disc
clutch pack assembly.

The plates of a multiple­disc clutch 
assembly consist of friction discs 
alternated with steel discs.
The friction discs have a rough surface 
friction material applied to both faces. 
The steel discs have a smooth, flat 
surface finish without any friction 
material.
Collectively, the friction and steel 
discs are called the clutch plates.
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The piston compresses the clutch plates against a thicker reaction plate known as the pressure plate. 
Pressure plates may be used at one or both ends of the clutch pack and are available in selective sizes 
for adjusting clutch pack clearance. 
The pressure plate and clutch assembly components fit into either a clutch drum or a machined bore 
in the transmission case. Snap rings retain all the clutch assembly components and are available in 
selective sizes for adjusting clearance. 
Although a multiple­disc clutch is an apply device similar to a transmission band, it differs from a 
band because it can be used to drive members of planetary gear sets as well as hold them.
See Figure 101–7. 

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Figure 101–7 A typical clutch pack assembly.

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There are two types of multiple­disc clutches:
Holding
Driving
Holding Clutches  An example of a holding clutch is shown in 
Figure 101–8. In this arrangement, splines on the inner edge of the 
friction discs engage matching splines on the outside of the clutch 
drum. The steel discs, alternated with the friction discs, have 
splines on their outer edge that engage matching splines machined 
into the transmission case.

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Figure 101–8 In a holding clutch,
one set of discs engages splines
on the transmission case and the
other set engages splines on the
drum. By applying the clutch, the
drum is locked to the case.

A holding clutch is an 
open design and  fits 
into either a machined 
area of the case or in a 
special support that 
bolts to the case.
Internal passages in the 
case route fluid to the 
apply piston.
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Driving Clutches  There are two types of multiple­disc driving clutches, and 
both types are commonly used in automotive transmissions.
In one configuration, splines connect a set of friction discs to the transmission 
input shaft so that these discs function as the driving member. The alternating set 
of steel discs spline to the inside of the clutch drum and serve as the driven 
member.
When the clutch is released, the drive discs rotate with the input shaft, but they 
do not drive the driven discs that spline to the drum.

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When the clutch applies, the piston takes up clearance and forces both sets of discs firmly together. 
Now, the input shaft and drum rotate together at the same speed and torque transfers from one gear 
set member to another. A passage inside the input shaft carries fluid to the clutch apply piston. 
With the second driving clutch configuration, the drum connects directly to the input shaft so the 
drum always rotates with the shaft.
Splines connect the steel discs to the inside of the drum and the friction discs to the outside of a 
clutch hub. When the clutch applies, hydraulic pressure compresses the clutch pack to lock the drum 
and clutch hub together.
The input shaft then drives the output shaft and both turn at the same speed. See Figure 101–9.

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Figure 101–9 Cutaway of a clutch housing on a GM 4T60 (440-T4) transaxle.

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Clutch Operation  To apply the clutch, pressurized fluid enters the drum through an internal passage 
in the input shaft. The fluid acts on the piston and moves it against return spring force to clamp the 
clutch plates together and hold them against the pressure plate. 
The friction between the discs then locks the clutch drum to the hub, causing them to turn as one unit.
To release the clutch, hydraulic fluid to the apply side of the piston is cut off and exhausted. 
The piston return springs, which were compressed when the clutch was applied, are now free to 
expand and move the piston back, allowing the clutch discs to disengage.
See Figure 101–10. 

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Figure 101–10 Hydraulic
fluid under pressure enters
the clutch housing and
exerts a force on the clutch
piston. The clutch piston
forces the steel plates and
the friction plates together,
creating a shift.

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ONE-WAY CLUTCHES
Another type of automatic transmission device in common use is the one­
way clutch, or overrunning clutch. Like a transmission band, a one­way 
clutch is always a holding device.
However, it can work together with a drive clutch to provide input. One­
way clutches are either roller or sprag clutches. However, the roller 
clutch is more common.

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