Transmission

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Transmission
The internalcombustionengineusedona vehicleoperatesoveralimitedeffective
spee range o - 5000 rpm. At low engine speed, a reciprocating-piston engine
d°es not deveiop sufficient turning-effort or torque to propel a vehicle forward from
standstill. Even the greater torque produced at higher engine speed would be insuf-ficient to accelerate the vehicle at a reasonable rate. The gearbox provides a way of
varying the engine s output torque and speed to match the vehicle’s speed and load.
Main Topics :
• Gearing System Fundamental
• Spur Gearwheels
• Gear Trains
• Sliding-mesh Gearbox
• Constant-mesh Gearbox
• Synchromesh Gearbox
• Epicyclic Gear Train
• Over Drive
• Gearbox Lubrication
• Automatic Transmission Fundamentals
•Torque Converter
•Unidirectional Clutch
•Automatic Transmission Gearbox
•Hydraulic System
•Driving and Holding Devices
•Control System
•Transmission Fluid
•Transmission Seals
•Continuously Variable Transmission
15.1. Need for a Gearing System and Gear Ratios
In order to achieve a high maximum vehicle speed, combined with good acceleration and
economy over the whole speed range, a gearing system is required, which permits the engine to
operate at the speeds corresponding to its best performance. Maximum engine power, torque
and economy all occur at different engine speeds. As a result it becomes difficult to match the
gear ratio for best performance, especially when variable operating conditions and driver
demands are also to be considered. The engine requirement to suit a given operating condition
is as follows.
Engine requirement
Maximum engine torque
Maximum engine power
Maximum engine torque
Engine at mid-range speed and under
light load with a small throttle opening
The type of engine fitted nowadays to a light vehicle generally requires a gearbox capable
°f providingfour forward speeds and a reverse.This provides a reasonable perfonnance to suit
a
"thedrivingconditions except economy,which normally needsanextra ratio, afifthgear, that
Operating condition
Maximum traction
Maximum vehicle speed
Maximum acceleration
Maximum economy

AUTOMOBILEMECHANICS
is higher than the conventional top gear.
A high gear ratio means the lower is the
reduction between the engine and road
wheels. Conversely the lower the gear
ratio means the greater is the reduction
between the engine and road wheels.
Maximum Vehicle Speed.
Maximum vehicle speed is attained
when the gearisset in top and the throttle
is held fully open. A ratio of 1 : 1 (direct
drive) is chosen for top gear to keep the
friction losses to minimum value. Conse-
quently, the setting of top gear becomes
the choice of a final drive ratio to suit the
diameter of road wheel and engine char-
acteristic.
Figure 15.1 illustrates the balance be-
tween the power required and the power
available. Data for the power required are
obtained from the brake power curve of
the engine, and for the power available
are based on the calculation of the power
needed to overcome the tractive resis-
tance of the vehicle when it is moving
along a level road.
The tractive resistance, sometimes
called total resistance, includes:
(a) Air resistance which is due to
movement of the vehicle through the air.
(b) Rolling resistance which is due
to friction between the tyre and road, and
largely influenced by the type of road sur-
face.
200
ROAD SPEED, km/h ( A)
MAXIMUM
POWERMAXIMUM
TORQUE
150
—
cc
ISo
Iuj
*
5CD
I
I
POWER AVAILABLE
FOR ACCELERATION
(c) Gradient resistance occurs
when the weight of the vehicle acts
against the vehicle motion during move-
ment up a hill.
The power needed to propel a vehicle
(Fig.15.1A) increases with the cube of the
C. Balance between power available and power required, speed. In this example, a power of 150 kW
needed to drive the vehicle at 200 km/h.The power output curve of the engine installed in this vehicle (Fig 15 IB) indicates that theengine produces a peak brake power of 150 kW at 5000 rpm. To attain maximum road speed,theoverall gear ratio (,.*gear box ratio x final drive) of this vehicle must be set so that the peakof the power 150 kW occurs at a road speed of 200 km/h and an engine speed of 5000 rpm.
Once the relative positions of the two curves have been established, the vertical differencebetween the two curves gives the surplus power available for acceleration This can be plottedas a separate curve to show the speed at which maximum acceleration is achieved
0 200
ROAD SPEED, km/h (C)
Fig. 15.1. Power balance.
A. Power required driving of the vehicle.
B.Power available to drive the vehicle.

TRANSMISSION
brakepower"iims
^fertfre ThT*f
transn?ission5ystemis similar t0 the en«ine
££ii
=a
599
PA PB Pc150
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£ ur
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Pi
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POWER AVAILABLE
FOR ACCELERATION
(OPTIMUM)< SP*5
I
OVER I
ae°
9° V0
200
ROAD SPEED, km/h
Fig. 15.2. Under-gear and over-gear.
In both of these gearing conditions the maximum possible speed is reduced. But, when
compared with the optimum gearing needed to obtain the ideal maximum speed, the advantages
of these two situations are as follows.
(i) Since more power is available for acceleration in under-gearing, vehicle is livelier.Top
gear performance being flexible, less gear changing is necessary when the vehicle
encounters higher tractive resistances.
(ii) Due to lower engine speed for a given road speed, better economy, lower engine noise
level and less engine wear are achieved in over-gearing.
A comparison of these two conditions indicates that under-gearing is more suitable for the
averagecar, and hence under-gearing to the extent ofabout 10-20% is quite common.Therefore,
the engine power peak occurs during 10-20% prior to the attainment of the maximum possible
vehicle speeds.
Maximum Traction.
Once the overall top gear ratio in set, the bottom gear (1st gear) is then decided. This gear
is used when vehicle starts and is also needed when maximum tractive effort is required. Since
tractive effort depends on the engine torque, the maximum tractive effort in a particular gear
occurs when the engine delivers its maximum torque. The top gear performance, which was
Previously plotted as a difference in power in Fig. 15.3 A, now indicates as a balance of forces.
The driving force curve is similar in shape to the engine torque curve. The peak of the tractive
effort curve occurs at a road speed controlled by the overall gear ratio and effective diameter of
the road wheel. The difference between the effort and resistance curves represents the force
available for acceleration.
. Pigure 15.3B represents the effect of lowering the gear ratio on the tractive effort
bottom gearbox ratio of 4-1is used to produce sufficient tractive effort to meet the hill-climbing
requirement. The gradual engagement of the clutch is necessary for sufficient building up of
Active effort. Once the clutch is fully engaged, and the engine is operating in the region of
curve. A

AUTOMOBILE MECHANICS
maximum torque, a small acceleration is possible provided the engine speed does not drop too
low. The bottom gearbox ratio is obtained by the ratio of the maximum effort required and the
maximum effort available in top gear.
600
MAXIMUM TRACTIVE
EFFORT REQUIRED
2
id TRACTIVE EFFORT
'
(BOTTOM GEAR 4:1)
2 TRACTIVE RESISTANCE s
ON LEVEL ROAD
Ui 1ft
% CQ
1s k>UJTRACTIVE EFFORTCO ccco Q
!UJ
£cc AY
IQ
K
CC
okcc k I TRACTIVE EFFORT
( TOP GEAR 1:1)
UjO
k kik
ki MAXIMUM
ACCELERATION
£ “ ]T”
I
- rk
IIk
o IYMAXIMUM I
SPEED |
I I
£
110
4X
TRACTIVE RESISTANCE
ON LEVEL ROAD
200ROAD SPEED, knVh
**(
*) (S)
ROAD SPEED, km/h
Fig. 15.3. Tractive effort curves.
Intermediate Gear(s).
Once the top and bottom gear ratios are set, the intermediate ratios are then determined to
form geometric progression (GP). Therefore, all the individual ratios advance by common ratio.
For example, if the top and bottom overall ratios are 4 : 1 and 15 : 1 respectively, then the sets
of overall ratios for the 3 and 4 speed gearbox are 4, 8 and 15 (common ratio 2) and 4, 6.35, 10
and 15 (common ratio 1.59) respectively.
For optimum speed and acceleration performance, the engine should be operated in the
speed range between the limits of maximum torque and maximum power. The wider this
operating range, the smaller is the number of gear ratios required. Most modern car engines
have a narrow range, so gearboxes Fitted in conjunction with these engines normally have at
least four forward ratios.
Since most cars are under-geared, it is now common to use an extra gear, called a fifth gear
to offset some of the disadvantages associated with the under-gear condition. Normally, this
gear is an overdrive, which is a ratio that drives the output shaft faster than the engine.Typical
gear ratios for four and five speed gearboxes are as follows.
Five speed gear box: top1: 0.8, fourth 1:1, third.1:1.4, second 1: 2, first1: 3.5 and reverse
1: 3.5. Four speed gear box: top 1: 1, third 1: 1.3, second 1: 2.1, first 1: 3.4 and reverse 1: 3.5.
15.2. Spur Gearwheels
Tooth gearing is used for positive transmission of rotary motion from one shaft to another.
In spur gear the shafts are parallel, and the gearwheels are cylindrical discs with teeth on their
circumference. The gear wheels are usually manufactured from low-alloy nickel chromiummolybdenum steels. Figure 15.4 illustrates gear tooth profile terminology

'1
TRANSMISSION
601
TOOTH
PITCH
POINT
l 1FILLET
RADIUS
TOOTH
_THICKNESS %
W_v7
Fig.15.4. Terminology of gear-tooth profile. Fig. 15.5. Straight-tooth spur gear.
Straight-TOOTHL Spur Gear. In straight tooth spur gears (Fig. 15.5),
teeth are cut at right angles to the face and parallel to the axis of the
gear wheel. When the teeth profiles contact, the relative motion is a
rolling action at the pitch point, and this changes to sliding once contact
occurs on either the face or flank of the tooth. Radial forces between the
teeth in contact tend to separate the gears and that must be absorbed by
radial type bearings.
t i t
Helical-tooth Spur Gear. In helical tooth spur gears (Fig. 15.6),
teeth are cut at an angle both to the face and to the axis of the gearwheel.
^Thus, contact between meshing teeth takes place along a diagonal line L^
.
across the faces and flanks of the teeth. Since one pair of meshing teeth
remains in contact until the following pair engages, the load on the teeth
isdistributed over a larger area.This reduces tooth loadingand promotes
smoother and quieter running. Axial, or end thrust is felt at the shafts
and that must be absorbed by bearings. Side thrust may be reduced by Fig. 15.6. Helical-tooth
spur gear.
(A)
using double helical gears (Fig. 15.6B).
15.3. Simple Gear Trains
If two or more gearwheels are meshed in series, either in the same plane or in different
planes, the gearwheel assembly is said to form a gear train.
Simple Gear Train.If the gearwheels are sup-
ported on separate shafts and are in the same plane,
the gear train is known as a simple gear train.
Engine timing gears use these arrangements. When
only two gearwheels are involved then the gear train
is a single-stage simple gear train Fig. 15.7. This
arrangement is usually used with gearboxes as-
sociated with front wheel drive.
The gear ratio of a gear train is defined as the
input speed divided by the output speed, or it may be
obtained by using the following formula.
INPUT
7
]
OUTPUT
F
'9- 15.7. Simple single stage gear train.

602 AUTOMOBILE MECHANICS
Product of teeth on driven gears _Driven
Product of teeth on driver gears Driver
Compound or Multi-stage Gear Train. When two
pairs of gearwheels are connected in series and
the driven gearwheel of one gear train is connected by a
shaft to the driver gear wheel of the next gear
train, the gear wheel assembly forms a compound gear
train (Fig.15.8).If two gearwheels are joined together by
a single shaft, a double-stage compound gear train is
formed (Fig. 15.8). This layout is usually found in gear-
boxes used in vehicles with front mounted engines that
have a rear wheel drive.
Reverse Gear Train. If two gearwheels are con-
nected by a third or middle gearwheel, the additional
gearwheel does not affect the overall gear ratio, but chan-
ges the direction of rotation (Fig. 15.9). This additional
gear wheel is known as an idler gear.
Gear ratio =
or more
INPUT OUTPUT
common
P E5
c 3
i
Fig.15.8. Compound (double-stage)
gear train.
Driven Driven B C C
Driver Driver A B A.Gear ratio =
Torque Ratio. As applied to gearing,
INPUT f==3Output torque
Input torque
The torque acting on a pair of engaging gears is
inversely proportional to their speeds of rotation. In
other words, a decrease in speed in the output shaft of
a gearbox is accompanied by an increase in torque,
which is exactly what is required for driving heavy
loads up hills, moving a vehicle from rest.
If the efficiency is 100%, then Input torque
x Input speed = Output torque x Output speed.
Output torque _ Input speed
Input torque Output speed'
Therefore, Torque ratio = Speed ratio = Gear ratio.
In practice, however, there is always a certain amount of friction between the teeth of the
gears and also in the bearing, which support the shafts to which the gears are fixed, so that the
value of the output torque is reduced. The efficiency of the gearing is defined as :
Torque Ratio
Gear Ratio
Torque ratio = o
caOB
[ IDLER)
Oc
OUTPUT
Fig.15.9. Simple reverse gear train.
or
Efficiency = x (100),%.
15.4. Types of Gearboxes
Thereare two primarygroups ofgearboxessuch asmanual changeand automaticgearboxes.
Manual Change Gearboxes. In these gearboxes the driver has complete control of the
gear changing process and can select a gear ratio appropriate to the drivingconditions by means
of the manual control lever. Generally, these are four to five gear ratio options apart from the
reverse gear. There are three basic types of gearboxes: ’

TRANSMISSION 603
• Sliding-mesh.
• Constant-mesh.
Synchro-mesh.
Of these, the synchro-mesh type is
the other two types. Although sliding
Prevalent today.It is essentially a combination of
considered in the text for initial study! h
^g6ar b
°X 1S obsolete at Present-lt has been
rati
^automatic
^l
^ake
^d.
^Thedrivermerd
6111 ®6Veral sub-systems 80 that theSear
conditions. Most automatic gear box systems uZ 1 ' °T"u operatmf
fhVH rl/itrh t h n t i ° e
, , or
^ speec
* epicyclic gearbox. The torque converter is generally a
third clutch that replaces the conventional friction clutch. The two functions provided by the
converter include, ^
(a) automatic disengagement of the engine from the transmission when the engine speed
is less than 1000 rpm; and
(b) provision of an infinitely variable torque and speed ratio to bridge the steps between
the discrete epicyclic gearbox ratios.
15.5. Sliding-mesh Gearbox (Four-speed and Reverse)
A sliding-mesh gearbox (Fig. 15.10) is similar to a constant-mesh gearbox, but differs in the
way the individual gears are engaged. In the sliding-mesh gearbox, the individual gear ratio is
chosen by sliding the selected gearwheel axially along the splined main output shaft until it
meshes fully with the corresponding lay-shaft gear cluster. The sliding main shaft gearwheels
and their corresponding lay-shaft gearwheel clusters have to be of the spur straight-tooth fonn,
so that when engaged there is no side thrust unlike helical-cut teeth. The major problem with
this type of gear engagement is that, while attempting a gear change, the speeds of the input
and output shafts are matched first, otherwise the sliding teeth of the mashing gearwheels does
not align and hence crashes into each other.
Transmission shafts and gears are generally manufactured using low-alloy nickel-
chromium-molybdenum steels.This type ofgearbox is presently used only in certain commercial
vehicles where a large number of close gear ratios are required in a compact form.
The engine shaft (clutch shaft) contains the main drive gear A, which rotates at the speed
of the clutch shaft. The main drive gear is in constant-mesh with counter shaft (lay shaft) drive
gear B. Since all the gears on the lay-shaft are rigidly fixed, they also rotate along with the
dutch shaft. The main shaft is held in line with the clutch shaft. All the gears on main shaft
can beslid back and forth on the main shaft spines usingshiftingforks.The different gear ratios
°f sliding-mesh transmission may be obtained as follows. N and T with proper suffixes denote
j rpm and number of teeth respectively.
First or Low Gear Ratio, G.The position of gears to obtain this ration is shown in Fig.
15-10.Transmissionof power takes placefrom theengineshaft(clutch shaft) to layshaft through
gears A and B and finallv it is transferred from lay shaft to main shaft (driven shaft) through
i gears C and D.4
•
Speed of engine shaft _ NA NC _ TB TD
SPEED of main shaft NB ND
~
TA TQHence, Gi =lI
I

AUTOMOBILEMECHANICS
604
1srAND 2ND
SLIDING GEARS
SECOND
GEARFIRST
GEAR
TOP GEAR
DOG CLUTCH
AND 3RD
GEAR TAPER
BEARING
2
INPUT
(PRIMARY)
SHAFT
iDOG
TEETH A
$p-ii2d = JOUTPUT SHAFT
i4« mig
1v
CD
-J
-J
FIRST STAGE
CONSTANT r
MESH GEAR
oc
£8i 5C
^ 2CQ
LAY SHAFT
11v
I* 1
2RDLAY SHAFT
GEAR
CLUSTER
C
EGEAR . 1ST AND
REVERSE
GEAR
T B B
& 3RD
GEAR
srhlTHIRD
GEAR
FOURTH
GEAR
IPIp $2
I I
D
G1 IB
D D
REVERSE
IDLER
GEARS /iREVERSE
GEAR
m
OW s
=#- O. V
1 cc
RB
Fig. 15.10. Four-speed-and-reverse double-stage sliding-mesh gearbox.
Second Gear Ratio, G2. Figure 15.10 shows the second gear in action. Power from A goes
to B and from there it goes to E, which is on the same shaft, i.e.lay shaft. From E it goes to F,
on the main shaft.
Speed of engine shaft _ NA_ NE _ TB TF
Speed of main shaft NB NF
~
TATE
Third Gear Ratio, G3. When the third gear is in action as shown in Fig. 15.10, the drive
is from the engine shaft to lay shaft through the constant-mesh gear A and B and finally fro
lay-shaft to main shaft through gears G and H.
G2 =Hence,
A

ppANSMlSSION '
,, F
m605
G3 = engine shaft. _ NA NG TB THSpeed of main shaft “
NB NH
~
TA TG
Fourthor Top Gear Ratio, G4. The drive is direct from the engine shaft to main shaft by
^h^wnfnVig 15 10 The lay
^h^ft
0
^0
^
6®
^prov
êc
*on t*16111-The whole arrangement
GA'
Hence,
case revolves idly. The gear ratio is 1, i.e.
Th ayshau rotates in
^pposite
^'^0
^°^ro
â
ôn en&ine shaft and main shaft is the same.
‘1
ReverseGear, Gr.The reverse gear works as shown in Fig. 15.10. The idler is compound
type having two wheels Ii and I2 of different diameters mounted on a shaft, which is parallel to
the mam shaft.The idler is slid so that I2 engages pinion C and Ii comes in mesh with the gear
D.The reverse drive takes place through A to B, then C to I2 and finally from Ii to D.
Thprpfnre G.= -Peec*°f engine shaft NA Nc Nn
’ Speed of main shaft = N
~
BW2 ND
= TB T12 TD
TA TC Tn
(As NB = Nc and Nu = N11 )
15.6. Constant-mesh Gear box (Four-speed and Reverse)
The primary (input) shaft is splined at the flywheel end. It carries a first-stage constant-mesh helical gearwheel and a fourth-gear toothed dog clutch, formed on it at the gearbox end.
At the flywheel end it is supported by a small bush or ball-bearing and at the gearbox end by a
ball-bearing or taper-roller bearing (Fig. 15.11).
The lay-shaft holds cluster gears rigidly together. For small and medium sized gearboxes,
the gears are normally cast or forged as a one-piece unit. For larger heavy-duty gearboxes, the
gearwheels areseparately machined and then held together on a splined lay-shaft. Thelay-shaft
is generally force fit at its ends in the gearbox housing and supports the one piece lay-cluster
gears on needle-roller bearings recessed in the ends of the gear cluster. Thrust washers are
installed between the gear cluster and the gearbox housing to absorb any side-thrust generated.
In large heavy-duty gearboxes, the splined lay-shaft uses ball or taper bearings at its ends.
The main (output) shaft has sections with stepped diameter, some portions of which have
smooth polished surface so that various gears can revolve relative to this shaft, while other
portions are splined to cause power transmission from the drive path gears to the constant mesh
sliding-dogclutch inner hubs.This shaftcarries thefirst,second, third, and reversesfinal output
reduction gearwheels, which are free to revolve relative to this shaft and are in constant mesh
with the lay-cluster gearwheels. Additionally, this shaft supports the first/second, and
third/fourth-gear sliding-dog-clutch inner hubs, fixed to the shaft by spines. To facilitate the
assembly of main shaft, output gears, and hub, one end of the shaft has a reduced-diameter-
spigot plain bearing surface. This end carries a needle-roller bearing, which fits into a recess in
the primary-shaft gear end. The other (output) end of the shaft is supported by either ball or
taper bearings located in the gearbox housing.
The sliding dog clutch is a positive locking device, whose purpose is to allow the power flow
from the primary-shaft to the output shaft when the friction clutch has disengaged the gearbox
fromtheengine The dogclutch has an inner and outerhub.Theinner hubcontains both internal
êxternal splines and is fixed to the output main shaft through internal spines. The outer
hubcarries a single groove formed round the outside to position a selector fork and is internally
sPHned to mesh with the exterior spines of the inner hub.
iii
'1
>.:n

AUTOMOBILE
MECHANICS606
MAIN SHAFT
BALL
BEARING
DOG
SECOND GEARFIRST GEARTEETH
21
DOG
r«
INPUT
SHAFT /CLUTCH
A ea
Mm A
%iMAIN SHAFT
^
NEEDLE
( ROLLER
] BEARING!_FIRST
STAGE
CONSTANT
MESH
GEARS
i
t«=clTHRUST
WASHER
^i
tsLAY SHAFT
1 C
1sr GEAR
l8
X B
2ND GEAR
3RD GEAR
z. REVERSE
GEARLAY SHAFT /
GEAR
CLUSTER
FOURTH GEARTHIRD GEAR
liSsT
4
A
Ji
un ai-i
71
I
1 '
1<=1 I
'
1-
11G
iB
REVERRSE GEAR REVERSE IDLER
GEARJ
,4
m J
h 5 1 o
1 mm
i 3 Oc
11 'M 2m o /
1 /
R
—-N
Fig.15.11.Four-speed-and-reverse double-stage constant-mesh gearbox.
When a gear is selected, the speeds of both the input and the output shafts are initially
equalized either by allowing the engine speed to drop when changing up to a upper gear or by
revolving the engine slightly when changing into a lower gear. The outer hub is then slid over
thedogteeth of the particulargear chosen.This action provides a positive meansoftransmitting
power through the compound gear train.
First, Second, and Third Gear Selection. The power flow takes place from the input
shaft to the lay-cluster gear through the first stage constant-mesh gear. The power path then
follows three routes to the main shaft, through first, second, and third output gearwheels.In
the neutral position meshing lay-cluster gear drives these three output gearwheels, but mam
B

TRANSMISSION
shaft itseft does not revolve. To select individual gear ratio the outer dog-clutch hub is slid
towardsand over the dog teeth forming part of the required gearwheel. This engages and locks
theselectedoutput gearwheel to theoutput main shaft, therebycompletingthe power-flow path.
Top-gear Selection. In top or fourth gear, there is no gear reduction; instead a direct
power-flow path is established from the input to the output shaft. On the engagement of top
gear, the third/fourth-gear dog-clutch hub is slid over the dog teeth cut on the input shaft, thus
allowing direct power transmission from the input primary shaft to the output main shaft. All
the other constant-mesh gearwheels supported on the main shaft revolve about their axis at
their own speeds relative to the main shaft, when they are engaged.
Reverse-gear Selection. When the reverse sliding-mesh idler gear is slid into mesh, it
transmits motion from the lay-cluster reverse gearwheel to the reverse idler and then to the
reverse output gear, which forms part of the first/second-gear dog-clutch outer hub. This
provides a second stage gear reduction. The idler wheel changes the direction of rotation and so
provides a reverse gear train.
Referring Fig.15.11with Fig.15.10 it can be concluded that expression for all forward gears
remains same for both sliding and constant mesh gears. In case of reverse gear, the power flow
is from A to B and then from I to J through the idler gear. The idler changes the direction of
rotation of main shaft without affecting the gear ratio.
607
r _ NA Nr _ TB TJr
NB NJ TA TI
The essential difference between sliding-mesh and constant-mesh gear box is that in a
sliding-mesh gear box the gears are actually slide along the main shaft to engage or disengage
their respective mating gears on the lay-shaft. But in the constant-mesh gearbox the lay-shaft
and main shaft gears are in constant-mesh. The main-shaft gears are designed to revolve freely
and are engaged by sliding dog clutches splined to the main shaft, so that the particular gear is
locked to transmit power.
The problem ofside thrust on helical gears experienced with sliding-mesh gear transmission
is eliminated in constant-mesh gear transmission, as the gears do not slide. The difficulty in the
constant-mesh gearbox of bringing the input and output shafts to thesame speed when changing
gear has been overcome by the development of the synchromesh gearbox.
Thus
Example 15.1. In a gear box the clutch shaft pinion has 14 teeth and low gear main shaft
pinion 32 teeth. The corresponding lay shaft pinions have 36 and 18 teeth. The rear axle ratio is
3.7:1 and the effective radius of the rear tyre is 0.355 m. Calculate the car speed in the above
arrangement at an engine speed of 2500 rpm.
Solution. Speed of clutch shaft
Speed of main shaft
Teeth of lay shaft pinion v
“
Teeth of clutch shaft pinion Teeth of clutch shaft pinion
36 32
~
14
X
18
Gear ratio =
Teeth of main shaft pinion
= 4.57 : 1.
The rear ratio is 3.7 : 1
Hence overall gear ratio, G = 4.57 x 3.7 : 1 = 16.92 : 1.
Speed of the 2 n N r 2 n x 2500 x 0.355
m/min.V =car, 16.92f G
j 2 n x 2500 x 0.355 x 60
16.92 x 1000
km/h = 19.8 km/h. Ans.I
l

TRANSMISSION 613
15.8. Gear Interlocking Device
Changing the gear ratio involves two separate operations :
(a) Swinging the gear-lever lower end across the channel formed by the three selector
gates until the flats are aligned with the desired selector-rod gate.
(b) Sliding the selected gate and rod axially and parallel to the gearbox shafts towards
the desired gear until the dog-clutch and gearwheel dog teeth mesh and engage.
It is, however, possible for the rectangular-sectioned lower lever with its semi-rounded tip
to be placed or aligned between two selector gates while selecting the individual gates. In this
position sliding the lever parallel to the gearbox shaft forces both gates to engage two different
gears simultaneously, providing two power-flow paths. This
jam, so that the weakest gear teeth can smash and strip from their roots, if the vehicle is in
motion. To prevent such a situation, every gearbox incorporates some sort of safety interlocking
device such as :
the whole gear pack tocan cause
(i) Plunger-and-pin interlocking device, (ii) Caliper-plate interlocking device.
15.9. Synchromesh Gearbox
For wider ranges of engine speed (1000 to 6000 rpm), higher car speeds (150 km/h and more),
and high speed motorways, it is desirable, and even in some cases essential, to increase the
number of traditional four speed gear ratios to five where the fifth gear, and sometimes also the
fourth gear, have the overdrive ratio. Increasing in the number of ratio steps provide several
advantages. The extra gear provides better acceleration response, enables the maximum engine
rotational speed to be reduced while cruising in top gear, improves fuel consumption, and
reduces engine noise and wear. Typical gear ratios for four speed gearboxes are provided in
section 15.1. The following section deals with five speed synchromesh gearboxes used in
longitudinal and transverse mounted engines.
Five-speed and Reverse Gearbox.
In the five speed double stage gearbox layout, the power input to the primary shaft passes
to the lay-shaft and gear cluster through the first stage pair of meshing gears, so that motion
BAULK
RING
MAIN
OUTPUT
SHAFT
DOG
CLUTCH
SPIGOT
BEARING
C5,
PRIMARY
SHAFT
S
1
OUTER
HUB
SLEEVE
CRESCENT
OIL PUMP
LAY SHAFT
GEAR
CLUSTER
FIRST STAGE
LAY SHAFT
GEAR
Fig. 15.13. Five speed and reverse double stage synchromesh gearbox.

is relays to all the second stage lay-shaft and main-shaft gears (Fig.16.13). Each pair of second
stage gears has a different size combination, due to which complete range of gear ratios is
obtained.In neutral position each main-shaftgear revolves on the main-shaft at certain relative
speed to it.The output power flow is provided by locating the selected main-shaft gear with the
main-shaft,sothat the flow path from theinput shaftiscompleted to thefirststage gears,second
stage gears and finally to the main shaft.The fifth gear being an overdrive gear in this case, to
speed up the main shaft output relative to the input shaft, a large lay-shaft fifth gear wheel is
meshed with a much smaller main shaft gear.
A forced feed lubrication system is incorporated for heavy duty operations, which uses an
internal gear crescent type oil pump driven from the rear end of the lay-shaft (Fig. 15.13). The
oil is drawn from the base of the gearbox casing by this pump, and then pressurized and forced
through a passage to the main shaft. Subsequently the oil is transferred to the axial hole along
the centre of the main-shaft through an annular passage formed between two nylon oil seals.
The main-shaft gears are lubricated through radial branch holes.
15.10. Gear Synchronization and Engagement
The gearbox primarily contains an input shaft and an output shaft.The input shaft is driven
by the engine crankshaft through the clutch and the output shaft is coupled indirectly either
through the propeller shaft or intermediate gears to the final drive. Pairs of gear wheels of
different size are in mesh between these two shafts. In the neutral position of the gearbox only
one of these pairs of gears is actually attached rigidly to one of these shafts while the other is
free to revolve on the secondary shaft at some speed based on existing speeds of the input and
output drive shafts.
To engage any gear, first the input shaft is disengaged from the engine crankshaft. But the
angular momentum of the input shaft, clutch drive plate and gear wheels keeps them revolving.
Then the gear changing technique must judge the speeds of the dog teeth of both the gear wheel
selected and output shaft. When they rotate at a uniform speed, the dog clutch sleeve is pushed
over so that both sets of teeth engage and mesh gently without grating. The synchromesh
incorporated in the system applies a friction clutch braking action between the engaging gear
and drive hub of the output shaft to unify their speeds before permitting the dog teeth of both
members to engage.
Synchromesh devices utilize a multi-plate clutch or a conical clutch to equalize the speeds
of the input and output rotating members of the gearbox during the process of gear changing.
The conical clutch method ofsynchronization isgenerally used for producingsilent gear change.
In thismethod, the male and femalecone membersare brought together to produce a synchroniz-
ing frictional torque of sufficient magnitudes to automatically adjust speeds of both the input
and output members until they revolve as one. Once this speed uniformity is attained, the end
thrust applied to the dog clutch sleeve permits to mesh quietly the chamfered dog teeth of both
members into alignment.
15.11.Epicyclic or Planetary Gear Train
Epicyclic gear trains are generally used for automatic transmission, overdrives, and final
drives. The most commonly used gear trains in automatic transmission system are three-speed
Simpson geax train and two-speed Ravingeau gear train..The layout of a simple single-stage
epicyclic gear train is shown in Fig. 15.14. Epicyclic gears are very widely used in automatic
transmission because
(a) they are always in constant-mesh,
614
i
I

TRANSMISSION
Reverse Gear:
631
U -Nc T4 T2 TR
NT -Nc Ta T9 2y
Nl ~ 0 _ 60 x 60 x fin
A
^4 - 0 30 x 40 x 30 = “ 6or
(Since Nc = 0), Ns
N4
~ “ 6- A118
-
15.12. Gearbox Lubrication
The moving parts in the gearbox are lubricated by partially filling the box with the correct
grade oil though a level plug hole located on the side of the casing, until the oil starts to drain
back out of the ho e. he plug is then screwed on to prevent spilling out of oil during operation
of the gearbox. T e oi level submerges the lay-shaft or secondary-shaft cluster gears, so that
the oil is dragged around with the gearwheel teeth when the gears revolve.This helps the oil to
spread and flow between the individual gearwheels, output main shaft and primary shaft,
dog-clutch assemblies, and support bearings. The selector mechanisms are lubricated by oil
splashing up from the gear teeth. A drain-hole and a screw plug are usually provided at the
lowest point in the oil-bath casing, to drain used oil. Overfilling the gearbox creates a pumping
action, which builds-up pressure within the box and eventually forces oil past the input shaft
and output shaft oil seals. For smooth flow of oil between the shafts and the gears revolving
relative to them, two or three holes are drilled radially in each gearwheel.
Heavy-duty commercial vehicles in special cases uses a forced-feed lubrication system in
which a gear pump pressurizes oil along an axial hole in both the primary and main shafts.
Radial holes intersect this central hole and feed oil outwards between the shaft and the gears,
both when engaged and when disengaged. It is not necessary to pressurize the lay-shaft and
gear cluster.
In front-wheel-drive cars, a single oil supply is generally provided for both the gearbox and
the final drive. The lubricant used must be the one recommended, and its required viscosity
depends on whether it is to be used as a common oil for both the gearbox and the final drive
crown wheel and pinion or just for the straight or helical gear teeth.In the former case higher-viscosity oil may be selected. On the other hand, in synchromesh gearboxes, thinner oil is
generally preferred to provide sound and quiet gear changes.
15.13. Automatic Transmission
Althoughautomatic transmissionsare usuallylessfuel-efficient than their manual counter-parts, they do offer many driving advantages, especially in urban road conditions.
(a) Driver fatigue is reduced since there is no clutch or gear lever to manipulate.This is
specifically significant when driving in dense traffic.
(&) Both hands can remain on the steering wheel at all times, so there is increase in
drivingsafety.
(c) Since the transmission always engages the correct gear for the prevailing driving
conditions, the possibility of labouring ot over-reviving the engine is eliminated.
,.The automatic transmission systems are efficient, convenient, easy to operate, durable and
ehable, but they are relatively expensive to manufacture and service compared to standard
systems Current automatic transmission designs are lighter, smaller, and less
pensive to manufacture and have superior operating characteristics when compared with
*er versions. Automatic transmission system used in passenger cars has a three-member
4Ue
inverter driven through a two or three-speed automatic shifting planetary gear train.tor

IAUTOMOBILE MECHANICS
V
This combination provides smooth torque characteristics from starting to the designed peak
operating conditions. A typical modern three speed automatic transmission is shown in Fig
15.25.
632
i
:
1
I
TURBINE FRONT REAR PLANETARY
PLANETARY / GEAR SET
GEAR
IMPELLER
STATOR ! FRONT
CLUTCH
SET
f LOW AND
REVERSE BAND
/OVERRUNNING
/ CLUTCH
OIL
REAR
CLUTCHPUMP
GOVERNOR
OUTPUT
SHAFT
BEARING
SEAL
rp
<3
BUSHINGSPEEDOMETER
PINION 1
EXTENSION
HOUSING
rj
PARKING LOCK
ASSEMBLY> VALVE
BODY
^ SUN GEAR
DRIVING SHELL
KICKDOWN
BAND OIL
ENGINE
CRANKSHAFT
FILTERINPUT
SHAFT
FLEXIBLE
DRIVE PLATE
Fig.15.25. Atypical automatic transmission system.
Since a conventional gear train can not provide silent and smooth gear ratio changes,
automatic transmissions commonly adopt some sort of epicyclic gear arrangement. Different
gear ratios are selected by the application of multi-plate clutches and band brakes, which either
hold or couple various members of the gear train to provide the necessary speed variations. A
torque converter introduced between the engine and transmission gearing, automatically
reduces or increases the engine to transmission slip according to changes in engine speed and
road conditions.
Hydraulic pressure signals supplied by the governor valve and a throttle valve control the
actual speed at which gear ratiochanges occur.The governor valvesensesvehiclespeed whereas
throttle valve senses engine load. These pressure signals are directed to a hydraulic control
block consisting of valves and pistons, which translates this information in terms of pressure
variations.The fluid pressuresupplied by a pump then automaticallydirectsfluid tothe various
operating pistons causing their respective clutches or band brakes to be applied. Consequently
gear up-shifts and downshifts are automatically carried out takinginto account of the condition
of the road, the available output of the engine and the acceleration/speed requirements of the
driver.
)
Other than the continuously variable transmission (CVT) systems, most modern automatic
transmissions have two main units such as a fluid clutch and a main gearbox.
Fluid Clutch. This unit automatically disconnects the drive when the engine speed is low
and gradually connects it as the vehicle is moved from a stationary position. It is either a fluid

i •
TRANSMISSION
coupling or fluid converter. The latter is often caliph
*form, it can double the engine output torque A1
a a torclue converter because, in its simplenf the torque amplification as the output converter provides a gradual reduction
u
. u mu • ot the converter increase*
Mam gearbox. This automatically nr0viHp« « „ • , mcreases-vehicle to overcome road conditionsreauirino-* I Sfne.s of stePPed gear ratios to assist thetooperate over a wide road speed range
g
dnvingtor(
Jue-Also, it enables the vehicle
pjr„gto"
633
a positive
15.14. Torque Converter
proJdeVasmooth tmtoluc
^hide Tn?'m,echanicalgear transmission.performancecharacteristicsofa hydrokinetictornup
standstlll.and multiplication. Thethe gear train is illustrated in Fig 15 26 fbr hoht th
bet,weenthe engine 3nd
TlSnlJwflvfrnm
P
f?range-Theflgure indicatesthat the initial torque multiplication:6
•
res is
considerable.Also the largegear ratiosteps of the conventional
ransmi nee an smoothed out by theconverter’s response between automaticgear
si s. n irs gear 2g- • ), maximum torque multiplication is provided by the torque
converter at stall pull away conditions, which progressively reduces as the vehicle speed
increases until the converter coupling point is reached. At this point, the reaction member
freewheels. With further increase in speed, the converter changes to a simple fluid coupling so
>
T
I
r
FULL THROTTLE GEAR SPEED RANGE
T T T
* 1
2
*+« 3GEAR 4 >
5
4.8:1
100
KT
m TRANSMITTED
,, POWER
A (FULL THROTTLE)
Cf $TORQUE CONVERTER
EFFECT RESPONSE
y
2 Qy.
5 4
y
y
y
AHi yy
y
80i / y
£y
/ 1o y/ y
/ yy
oy
I
£ / yy y
i y
s / y
y
/ y
/I
ay y
/ / y y
I/ y y/
II yy
fcI
5 3
/ i»y
/
A $i //
i * co£ 60/
£
* TORQUE RATIOO /
%s
cc
2.4:1d /
d£ /
Uj
£25 /
OLIGHT -THROTTLE
/
dUJ 40
UJ/
/
£O FULL THROTTLE/
d o/
s a/
1.4:1/
/
a*/
1 / 1:1/
20
I /
/
0.7:1/
i LIGHT THROTTLEGEAR SPEED RANGE
/
/
i 1 2 3 4*+«+++
J
0 l ii i
00 8020 6040 100 120 140
i ROAD SPEED, km/h
^'9- 15.26. Torque multiplication and transmitted power performance relative to vehicle speed
for a typical four speed automatic transmission.J
I

that torque multiplication ceases. In second gear the converter starts to operate nearer the
coupling point so that it contributes far less torque multiplication. In third and fourth gears the
converter functions entirely as a fluid couplingoperating (beyond the coupling point) as a result,
there is no further torque multiplication.
The torque converter multiplies torque through theforce offluid movement.All of its moving
parts are submerged in lubricating oil. It transmits power silently and smoothly, without shock,
at various speed and torque ratios. Its operation is fully automatic and reliable, normally
requiring no service. The torque converter is enclosed in two-piece stamped steel shells or
housings, which are welded together and shaped like a tyre. It has three functional parts; the
driving member or impeller, the driven member or turbine, and the stator or reactor. The
impeller forms the back shell of the converter housing. The turbine drives the planetary gear
train and is mounted on the transmission input shaft. The reactor is connected to the transmis-
sion case through a one-way clutch mounted on a forward extension from the pump cover. The
front housing is connected to the engine crankshaft through a drive plate. On the back of the
converter, a rear hub is supported by a plain bearing located in the front of the transmission oil
pump housing. The outer diameter of the converter is approximately three times of its inner
diameter.
The hub of the converter housing drives the transmission oil pump, which produces oil
pressure for the transmission controls and for keeping the converter full of oil in the pressurized
condition (called charge pressure). Oil charge pressure is required to minimize formation of air
pockets (called cavitation) near the converter centre due to centrifugal action of oil during
rotation of the housing. The oil charge pressure ranges from 207 kPa to 1241 kPa for different
transmission models.
The impeller blades, rotating at engine speed with the housing, impart kinetic energy to the
oil and fling it towards outside of the housing. At the outer edge of the impeller the oil with high
kinetic energy leaves the impeller and is thrown into the outer edge of the turbine, which
provides a force to rotate the turbine. The turbine, which is connected to the drive wheels
through the transmission gears and drive line, causes the movement of the vehicle. During the
process as oil looses its kinetic energy, it moves towards the housing centre and consequently
direction of flow of oil changes opposite to that of the impeller. Oil leaving the turbine in a
backward direction hits the face of the stator blades. A free wheeling one-way clutch prevents
stator backward rotation and stator blades redirect the oil with little energy loss to enter the
impeller in the same direction of the rotation as that of the impeller. In this process the stator
acts like a fulcrum in a lever system to increase torque transfer and it is said to be reacting.
Within the coupling, oil flows in two directions simultaneously, producing a very rapid
spiraling oil flow that is like a coil spring with its ends brought together, called vortex flow. The
vortexflow provides torque multiplication within theconverter.Asthe turbinespeed approaches
the speed of the impeller, the amount of vortex flow is reduced, accompanied by a reduction in
torque multiplication.At the coupling point the turbine speed reaches 85 to 90% of the impeller
speed.At this point very little vortex flow occurs, because all the converter parts rotate at nearly
the same speed.
Some transmissions use variable pitch blades in the stator, where blade angle can be
changed over a range, from high to low angles. A high angle gives less oil flow restriction as the
turbine speed approaches impeller speed and it minimizes vehicle creep at idle. A low angle
high torque conversion is provided because of a higher differencein speed between impeller and
turbine. While variable angle blades result in substantially improved efficiency, they make the
stator very expensive.
The greatest amount of vortex flow and torque multiplication is attained at stall condition,
when the turbineisstopped and theimpelleris rotatingatits maximum speed.Maximum torque
634
SflEv
C
%
y
5s
C

15

TRANSMISSION
• lication ratioat stall variesfrom 2:1to
tful|JpThis ratio gradually and smoothly
as the turbine speed approaches i
^wspeed till the coupling point is reached.
P nuDling point, the oil leaves the turbine
&
Cforwarddirection, hitting the back of thein
tor bladeand the stator rotatesforward on
one-way clutch, moving with the oil flow
thereby producing minimum oil flow resis-tn
e Stall speed of a converter, which is
Ejectedto prevent creep at idle, is about 70%
Sfthe engines maximum torque speed at full
throttle. Operation at a high stall speed
auses excessive heat generation, fuel
sumption and noise. It also results in a high
coupling point, which causes the engine to
At high altitudes when engine output
lm-
con-
race.
reduces, converter stall speed also lowers
even with the same input torque. During
coasting, the turbine accelerates the oil flow
towardsits outsideand into the impeller where the oil’s energy is absorbed in trying to increase
enpnespeed due to which the stator is also forced to overrun.This type of operation, although
not efficient helps to slow the vehicle by transferring some of the vehicle’s energy to the engine.
Figure 15.27 illustrated the converter performance.
OUTPUT RPM/INPUT rpm
Fig. 15.27. Torque converter performance curves.
A clutch is used to minimize drive line
loss on some transmissions. It engages as
the converter coupling point is reached by
connecting the housing (which is bolted to
the engine crankshaft) to the turbine so no
slippage can occur between the converter
input and output members, and smooth
torque multiplication takes place during
acceleration and lock-up at all road speeds.
The transmission oil cooler and the
transmission gear train lubrication system
provide restriction in the outlet of the oil
flow to maintain converter charge pres-
sure.Transmission oil iswarmed as it flows
across warm mechanical parts while
lubricating and cooling them. Forced oil
circulation in the converter also heats the
oil rapidly, especially at low speeds. Under
severe operating conditions, oil tempera-
tures may reach as high as 423 K, but the
normal maximum limit is 408 K. The
recommended minimum operating
temperaturefor automatictransmission oil
is 361K and temperatures lower than this
produce sluggish action.
COUPLING
REGULATOR
PUMP
cr
^LUBRICATING
SYSTEMLOW TEMPERATURE
BYPASS
u
COOLER
SERIES SYSTEM
COUPLING
REGULATOR
o-PUMP
fry5
RFVLLUBRICATING
— J
I SYSTEM
TEMPERATURE
BYPASS
COOLER
PARALLEL SYSTEM
^•9- 15.28. Series and parallel transmission
oil cooling system.

w•
/ • - 1
i« ,i'sMME
HBHHHH-? ' V . • - - . •
0JWr -i*V
-I n
^J?
1BJ
V] /»{ A*
r j
//[
AUTOMOBILE
MECHANICS Itll‘Z
Transmission oil cooler is located in the radiator outlet tank where the engine
Coolant fVtemperature is lowest. This arrangement provides maximum transmission oil cooling. Plow of jA//Voil in the cooling system takes place in series and parallel arrangements. A series system sends W r hithe total quantity of oil through the cooler and then to the transmission system where as a
^parallel system sends part of the oil to the cooler and part to the lubrication system. Figure15.28 illustrates both the types of oil circulation
The arrangement of a three-element single-stage converter, shown in Fig. 15.29, is quite ll
common and is used in conjunction with many different types of automatic gearboxes.The fluid " A ‘
for the converter is normally supplied by the automaticgearbox and it is generally a low viscositymineral oil, which contains additives toimprove lubrication and resist frothing.Cavitation noise
caused by air in the converter is minimized by pressurizing the fluid to about 138 kN/m2
. It canbe seen in the figure that the free wheel is the only mechanical component, which can produce
faulty operation of the converter such as slipping and seizing of the stator.
/i'
•
636m
5t
!
iii:
t
Bv
i
j!
fi
u
1i
t
jtW*
V
V
$
i
i
I
C
B IMPELLER
{PUMP)
r.
Wos
i# or
TURBINE
COOLING FIN
{TO PUMP AIR)
STATOR
5tegeartrail
Wfir.Withp!
^byholdin
itoilti-pla
prcompc
^gearre
^iategea
FREE WHEEL
CASING {FIXED)
6 'i
Fig.15.29.Three-element single-stage converter.
Torque Converter Lock-up.
A lock-up friction clutch is incorporated between the input pump impeller and the turbine
output shaft to overcome the inherent problem of relativeslip, which always occurs between the
torqueconverter’s pumpimpellerand the turbine runner, even while drivingat moderate speeds
under light load conditions. To realize maximum benefit of this lock-up the torque converter is , •»,
^allowed to operate when light torque demands are made on the engine and only when the
^
1
converter is operating above its torque multiplication range that is beyond the coupling point. :
Consequently, converter lock-up is only permitted to be implemented when the transmission is
^^j
in either third or fourth gear.As a result the power transfer is bypassed through the circulating
fluid; instead transmitting the engine’s output directly to the transmission input shaft. This
eliminates drive slippage, thereby increases the power actually propelling the vehicle. Conse- V
JL 1
quently there is a net gain in power output, and the fuel wastage is reduced. &
man,
%
%

TRANSMISSION 637
15.15.Unidirectional Clutch (Free-wheel)
This device is also called a free wheel
bicycle. It transmits drive in one d’
r
.one'way dutch. Its action is similar to that used
Counted as a separate unit behind the
In the past a free'wheel unit was often
fITO
^cfthevehTdewasIn^31
s0 3
l feature thp unit u/ i t c
^anf>e lever. When the driver did not require the free
wheel featu e,
“nit was locked by a gear to provide a fixed-wheel condition. This was
116
Tse
^cted
C
’
S
°provislon was made to lock the unit automatically when thisgear
on a
Nowa ays e uni irectional clutch is used as a part of a automatic transmission and
overdrive uni s, o imi ho movement of a particular member to one direction. In these
examples, tec utx: works as a simple means for either driving or holding one part of an
epicyclic train so that it can only move one way.The two main types of unidirectional clutch in
use are roller type and sprag type.
15.16. Three Speed and Reverse Transaxle Automatic Transmission
A transaxle three speed automatic transmission is presented in Fig. 15.30. The planetary
gear train uses two sun gears, two sets of pinion gears ( three in each set), two sets of annular
(internal) gears and pinion carriers, which support the pinion gears on pins. Helical teeth are
used throughout. For all forward gears, power enters thegear train through the forward annular
gear and leaves the gear train by the reverse annular gear. Whereas in reverse gear, power
enters the gear train by the reverse sun gear and leaves the gear train through the reverse
annulargear. With planetary gear trains the gears are in constant mesh and gear ratios changes
areeffected by holding, releasingor rotatingcertain parts of thegear train by means ofa one-way
clutch, two multi-plate clutches, one multi-plate brake and one band brake.
First gear compounds both the forward gear set and the reverse gear set to provide the
necessary low gear reduction.Second gear only utilizes the forward planetary gearset to produce
the intermediate gear reduction. Third gear is achieved by locking the forward planetary gear
set so that a straight through drive is obtained. For better understanding of the operation of the
automatic transmission gear train Table15.2 may be referred, which represents the components
engaged in each manual valve selection position.
Selector Lever.
Theselector lever (Table 15.2) has a number of positions marked P R N D 2 1 with definite
functions as follows :
P-Park. In this position, there is no
actuated by a linkage causes a parking pawl to engage in the slots around a nng gear attached
totheoutputshaft (Fig 15 30) Thus the parking pawl locks the outputshaft to the transmission
«sing due to which backward or forward movementof the vehicle is arrested.The engine may
bastarted in this position.
drive through the transmission. A mechanical lock

TRANSMISSION
639
N-Neutral. When this position is selected 11
a result there is no drive through the transmit 68 and band brake are
êngaged, as
D-Drive. This position is used for all 1 •
& englne may be started in this position.
1-2, 2-3 up-shifts-and 3-2, 2-1downshifts conditions, automatically producing
the accelerator pedal. The engine does nnt cf
^. ]
lraadsPeeds or according to the position of
2-First and Second. This position Z n ^1-2 up-shifts and 2-1downshiftsonlv Thp 00i °S
+
6n t0 restnct &ear changes automatically from
km/h. The engine does not start in this range
ôshTon^^^P°Siti
°ned in 2 rangeS ab
°Ve 100
nd third gear A friction clntrh
^
Se
!ected» tbe transmission is not permitted toshiftinto second
nhtaine
^when travellincf
1
^^1
°CkS
1°Ut the one‘way ller dutch so that better control may
âvailable when descendingItee
^hflt” ^ ^r
°adS' Engine braking
°n
°VerrUn
First Gear (D-lst).
When the manual selector works in D range, engine torque is transmitted from the converter
through the applied forward clutch to the annular gear of the forward planetary gear train.The
clockwise rotation ot the forward annular gear causes the forward planet gears to rotate
clockwise, driving the double (compound) sun gear anticlockwise. The forward planetary carrier
is splined to the output shaft. This causes the planet gears to drive the double sun gear instead
of rolling walking around the sun gears.This counterclockwise rotation of the sun gears
the reverse planet gears to rotate clockwise. With the one-way clutch holding the reverse planet
carrier stationary, the reverse planetary gears turn the reverse annular gear and output shaft
in the clockwise direction producing a reduction ratio of around 2.71:1.
The power flow in first gear manual (1-lst) differs from the D range first gear (D-lst) in that
the first and reverse brake is applied to hold the reverse planet carrier stationary. Under these
conditions engine braking is provided on vehicle overrun.
Second Gear (D-2nd).
Whenin D rangesecond gear, theforward clutch and thesecond gear band brakeare applied.
Theforward clutch then transmits the engine torque from the input shaft to theforward annular
gear in a clockwise direction. The second gear band brake holds the double sun gear stationary.
Consequently the planet gears are compelled to revolve on their axes and roll walk around the
stationery sun gear in a clockwise direction.As a result the output shaft, which is splined to the
forward planet carrier, is made to turn in a clockwise direction relative to the input shaft with
a reduction ratio of approximately 1.50:1.
Third Gear (D-3rd).
causes
In this D ranee engine torque is transmitted through both forward clutch, and drive and
reverse clutch The driieand reverse clutch rotate the sun gear of the forward gear train
clockwise The fomard clutch turns the annular gear of the same gear set also clockwise Since
me wrwdiuu , forward gear tram revolve in the same direction at
both the annular gear and sun Saa^ locked in position, causing the forward gear train to
the same speed, *he Planet gear , being sphned to the forward planet carrier, also rotates
at the same speed as the input shaft with the drive ratio of 1.1.
i
Reverse Gear (R). .
ê R position, the drive and reverse multi-plate
When the manual selector valve
tothereversegear-setsun gear.Thereverse
wakeisapplied to transmitclockwiseengine
q
. ars are forced to revolve about their own
Planet gear carrier is held stationary. T P
^ output shaft
, ***,so that the reverse annular gear, which«.sphned to«o
l anticlockwise direction with a reduction ratio of about ^.4d
i
f , is also rotated in an
1

' I
AUTOMOBILE
MECHANICS640
15.17. Transmission Shift and Drive Line Features
; An automatic transmission gear ratio change is called a shift.Shifting requires the release
of one planetary member and the application of a holding device of another, both release and
£, application need to be properly timed. The reaction member of the planetary gear set always
i tends to turn backward while the gear set is carrying a torque load and t e reaction force is
proportional to the torque beingcarried. As the torque load transfersfrom one p anetary member
to another, the load on the reaction member changes from reverse to forward direction. Ideally,
the holding device should be applied or released at the instant torque reversal occurs. When a
holding or driving device becomes worn, it first become apparent to the driver when it slips and
fails to hold the required torque while it is being applied.
During up-shifts, the applied member must have a higher torque capacity than the released
member. This is required because engine inertia momentarily increases torque as the engine is
slowed to the new speed and this is added to the torque being produced by the engine. During
downshift, the engine speed must increase as the shift moves to a lower gear. The application
of force must be gradual before the holding force is released to prevent engine run away.
Shifting quality or smoothness is primarily dependent upon the characteristic output
torque, which varies during the shift. If one member is released before the second member is
applied the transmission momentarily remains in neutral and the engine tends to run away;
On the other hand, if application occurs before release, the transmission is momentarily locked
in two gears, producing a bump. Good shift quality transfers the load from one member to the
next by allowing a slight amount of slippage to occur during application as the new member
picks up the torque, which may last for 0.6 second. Longer application, though produces
smoother shifts, but reduces the service life of the unit.
Size and clearances in the automatic transmissions are required for correct operation and
hence are very carefully controlled during manufacturing. Because of the build-up method, one
part depends upon the accuracy of several other parts. The correct final axial movement or end
clearance is controlled by a selective fit thrust spacer somewhere in the assembly. Automatic
transmissions usually use pressure lubricated bushing-type bearings on their main rotating
parts. Most of these bearings are babbitt or copper-lead bearing materials on a steel backing.
All bushings are installed in bores located in either case or hub. The front of the gear train is
supported on a hub extending in back from the pump cover. Oil transfer rings are located on
this hub to minimize leakage as control oil transfers from the stationary hub to a rotating clutch
drum. The rear end of the gear train is supported by the rear of the transmission case. Shafts
and drum hubs support one another on these two main support locations. The transmission
; input shaft is splined between the converter turbine and a front clutch hub. On the front, the
l turbine is supported in a bushing within the torque converter cover or inside the front of the
stator shaft. The front clutch hub is mounted with a bushing on the rearward extension of the
I transmission pump cover. The front of the transmission output shaft rides on a bushing at the
I rear of the transmission case. The rear of the output shaft is supported by a bushing in the back
| of the transmission extension. The output shaft extends almost to the input shaft
intermediate shaft is used between them.
I Planetary gear-set members, clutch hubs, and brake drums are splined to these shafts for
| driving and riding on bushing when they are required to be free turning Non-rotating clutch
t and brake parts including oil transfer hub and seal rings in some transmissions are supported
I by the transmission case to minimize the load on the shafts. Transmission rotating membersspaced with thrust bearings, and needle roller bearings are used for high load conditions.
*
'
These bearings are normally made of babbitt on a steel backing. Sub-assemblies are held
‘ together with snap rings and retaining rings. Sub-assemblies, which rotate together, are
connected with drive lugs at their outer edges.
or an
t are
:i .

r
TRANSMISSION
front°^ss
^m|[)j-ng subassemblies
& Sna
^r*n
^s an
^retaining rings are to be removed completely
15.18.Driving and Holding Devices
An automatic transmission is fitted with a number of brake bands and multi-plate clutches.
Clutches are used to connect the gear train to the input shaft and band brakes hold a part of
the train stationery. In newer designs of gearboxes, band brakes are replaced by clutches to
obtain a compact and lighter gearbox. Additionally, it eliminates the need for periodic adjust-
ment of the brake bands to compensate for friction lining
A pump driven at engine speed from the fluid converter provides pressurized oil, which is
distributed by control valves to the appropriate clutch and brake for actuation of these parts.
Multi-plate Clutches.
Numerous wet type multi-disc clutches are used with automatic transmission and most of
these operate on the same principle. Figure 15.31 represents a typical construction of a
multi-plate clutch. Two sets of steel plates, inner and outer, are connected alternately by
protruding tabs to the hub and drum respectively.The faces of the inner plates are bonded with
a friction material having either a hard or comparatively soft texture. A hard facing is made of
a cellulose compound, or syntheticfibre, bonded together with a phenol resin to obtain a suitable
friction value. A soft facing, which is based on a compound of paper, is more porous and elastic.
Paper-based facings normally provide a smoother and quieter operation over a wider range of
temperature and pressure.
When the clutch is to be engaged, pressurized oil is supplied through a drilling, in either
the casing or the shaft, to the clutch operating cylinder. A number of synthetic rubber “0” rings
and square-section, cast iron seals are used to prevent leakage of oil between moving parts and
loss of pressure needed to operate the clutch. Torque transmitted by a given multi-plate clutch
BALL CHECK
VALVE
641
wear.
WRING
r///s///////??A
OIL SUPPLY
CASING
$
* SEALING
RINGS
CLUTCH
RELEASE SPRING
2
Fig. 15.31. Multi-disc clutch.
depends on friction value and operating pressure, thereforeone of theseis the cause when clutch
shp occurs.
When the clutch is disengaged the drag between the plates acts as an energy drain, so
suitable arrangement is incorporated to separate the plates. A large clutch release spring
^tracts the operating piston and in some cases the steel outer plates are slightly dished.
Immediatelyafter the disengagement of the clutch, the centrifugal motion of the residual oil in
the operating chamber acts on the piston and causes slight drag. This is prevented by releasing

TRANSMISSION 651
°nS
’
3 am0Unt 0f leakage is into the transmissions to aid inrings-
cooling .
tweer
^stationary and rotating members andas
^fl" ringS
’
USed aS oil transfer rinBs
hdps 0
Buna-N anH P l
0mP°Vn^s» which are oil-resistant synthetic rubber-like
materia s ,
hava °i r a r e mat
*e in a number of shapes for specialized
applications. Teflon seals have a self-lubrication property.
Elastomer seals are made in three basi
Fi,!5.47.Lathe cut and O-ring seals are »
lip seals are used only for dynamic sealing. Lathe-cut and O-ring seals fit loosely in their
pporting groove, but when assembled in the mating part the seal is squeezed from 0.3 to 0.6
mm to form the required seal. Lip seals are more expensive than other two so they are used
when cheaper versions do not function. The lip seals edge is deflected 0.76 to 1.66 mm when
installed.When pressure is put against the open end of theseal, the lipis pushed tightly against
the sealing surface to maintain its sealing function. Lip seals deflect and conform, as required,
to seal on moderately non-uniform surfaces. Dirt damages the dynamic seals by causing
leak-producing scratches. As seals age, they harden and do not function properly. Seals are to
be replaced when found faulty.
su
15.23. Continuously Variable Transmission (CVT)
The power output of a normal engine varies with the engine speed. At low speeds the output
is very less. For better vehicle performance, the engine must run at higher speed at which it
develops its maximum power. The same situation also repeats when the torque output and fuel
economy, the other two performance factors, are considered. Maximum torque occurs at a
different speed from that for maximum economy and also neither of these speeds coincides with
the point of maximum power.
The attainment of the constant engine speed required to achieve any one of the three
performance factors is not possible with a conventional gearbox, because the engine speed is
required to be continually changed to match the vehicle speed. Therefore, the engine only
performs its best at the vehicle speed appropriate to the point of maximum engine torque, power
or economy.
A continuously variable transmission
(CVT) is a particular automatic transmis-
sion capable of providing a smoothly vary- £ '
ing gear ratio. Unlike a conventional
automatic gearbox the CVT has
gears.It varies the drive ratio continuously
bychangingthe operating diameters of two
pulleys that are linked by a steel V-belt.
The transmission can alter its ratio imper-
ceptibly, without any interruption of drive.
Pigure 15.48 illustrates the basic principle
°f the CVT system.
HIGH
LOW
no fixed
3
*
Fig. 15-48. Principle of operation of CVT system.
this

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