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Transmission
Gear Ratios
Input Output
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Gear Ratios
Input Output
Gear Ratios
Input Output
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Gear Ratios
• To produce one full rotation of the driven or
output gear we have to turn the driving or input
gear 1.66 times.
Speed Versus Torque
• When a force of 10 pounds is applied perpendicular to the
centerline of a bolt at a distance 1 foot from the centerline,
10 pound-feet of torque is generated at the centerline of
rotation.
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Speed Versus Torque
• Meshed gears use this same leverage principle to transfer
torque.
• If two gears in mesh have the same number of teeth, they
will rotate at the same speed, and the input gear will
transfer an equal amount of torque to the output gear.
• When a driven gear is larger than its driving gear, output
speed decreases.
• But how is torque affected?
Speed Versus Torque
• Figure shows a small gear with 12 teeth driving a large gear
with 24 teeth. This results in a gear ratio of 2:1, with the
output speed being half of the input speed.
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Speed Versus Torque
• In this example, torque at the input shaft of the driving gear
is 10 pound-feet.
• The distance from the centerline of this input shaft to the
gear teeth is 1 foot.
• This means the driving gear transfers 10 pounds of force to
the teeth of the larger driven gear.
• The distance between the teeth of the driven gear and the
centerline of its output shaft is 2 feet.
Speed Versus Torque
• The result is that torque at the output shaft is:
• In this instance, torque has doubled. The torque increase
from a driving gear to a driven gear is directly proportional
to the speed decrease. When speed is halved, torque
doubles.
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Engine Requirements
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The Need For A Transmission
• If a vehicle did not have a gearbox, the following
disadvantages would soon become apparent.
• Poor acceleration from rest
• Poor hill-climbing ability
• Vehicle cannot be driven at low speeds
• No neutral or reverse
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The Need For A Transmission
• Today, the type of engine fitted to a light vehicle generally
requires a gearbox that gives five forward speeds and a
reverse (i.e. a set of five different forward gear ratios in
addition to reverse gear).
• It should be noted that a high gear is one that has a ratio
with a low numerical value (i.e. a ratio of 1 : 1 is higher
than the ratio of 2 : 1.)
• The lower the gear, the greater the reduction between the
engine and the road wheels; this means that, for a given
engine speed, the road wheel speed is lower.
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Under-gear and over-gear
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Ideal tractive effort
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Conventional stepped transmission
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Conventional stepped transmission
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The Need For A Transmission
• A vehicle requires a lot of torque to start off and to climb
hills, yet it does not require as much torque to move on
level ground.
• An engine produces increasing torque as its speed
increases up to a certain point where the torque output
starts to decrease.
• Therefore, to get a vehicle moving or to accelerate up a hill,
it is desirable to use a transmission that allows the engine
speed to be increased even though the vehicle speed may
be low.
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The Need For A Transmission
• Using gears allows the engine speed to increase at low
vehicle speeds yet still permits the engine speed to drop at
higher speeds to save fuel and reduce exhaust emissions.
• For example,
– First gear: Vehicle speed is low, engine speed is high.
– Second gear: Vehicle speed increases, engine speed decreases as
shift is made.
– Third gear: Vehicle speed continues to increase, engine speed is
kept in a narrow range.
– Fourth gear: Again, the vehicle speed is increasing, yet engine
speed is about the same as in third gear.
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Gear Ratios
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Gear Ratios
• When one gear turns another, the speed that the two gears
turn in relation to each other is the gear ratio.
• Gear ratio is expressed as the number of rotations the drive
gear must make in order to rotate the driven gear through
one revolution.
• To obtain a gear ratio, simply divide the number of teeth on
the driven gear by the number of teeth on the drive gear.
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Gear Ratios
• Gear ratios, which are expressed relative to the
number one, fall into three categories:
– Direct drive
– Gear reduction
– Overdrive
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Gear Ratios: DIRECT DRIVE
• If two meshed gears are the same size and have the same
number of teeth, they will turn at the same speed.
• Since the drive gear turns once for each revolution of the
driven gear, the gear ratio is 1:1; this is called a direct drive.
• When a transmission is in direct drive, the engine and
transmission turn at the same speed.
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Gear Ratios: GEAR REDUCTION
• If one gear drives a second gear that has three times the
number of teeth, the smaller drive gear must travel three
complete revolutions in order to drive the larger gear
through one rotation.
• Divide the number of teeth on the driven gear by the
number of teeth on the drive gear and you get a 3:1 gear
ratio (pronounced three to one).
• This type of gear arrangement, where driven gear speed is
slower than drive gear speed, provides gear reduction.
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Gear Ratios: GEAR REDUCTION
• Gear reduction may also be called underdrive as drive
speed is less than, or under, driven speed.
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Gear Ratios: OVERDRIVE
• Overdrive is the opposite of a gear reduction condition and
occurs when a driven gear turns faster than its drive gear.
• For the gears shown in fig , the driven gear turns three
times for each turn of the drive gear.
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Gear Ratios: OVERDRIVE
• The driven gear is said to overdrive the drive gear.
• For this example, the gear ratio is 0.33:1.
• Overdrive ratios of 0.65:1 and 0.70:1 are typical of those
used in automotive applications.
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IDLER GEARS
• A gear that operates between the drive and driven gears is
called a floating gear, or idler gear.
• Idler gears do not affect the speed relationship between the
drive and driven gears, although they do affect the direction
of rotation.
• Reverse gear on a manual transmission often uses an idler
gear to change the direction of rotation.
• When a drive and driven gear mesh directly, they rotate in
opposite directions.
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IDLER GEARS
• When an idler gear is installed between the drive and driven
gears, both drive and driven gears rotate in the same
direction.
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Types of gearboxes
Types of
gearboxes
Manual Automatic
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Types of gearboxes
Manual
sliding-mesh
constant-mesh
synchromesh
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Manual Gearbox
sliding-mesh
• The sliding-mesh gearbox was popular on cars up to about
1930, but today it is rarely used.
• Nevertheless, many modern gearboxes have been
developed from its configuration.
• The basic layout of a four-speed and reverse gearbox is
shown in Figure
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sliding-mesh
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Manual Gearbox
sliding-mesh
• The various spur-type gears are mounted on three
shafts:
– Primary shaft (alternative names: input, clutch or first
motion shaft)
– Layshaft (countershaft)
– Mainshaft (output or third motion shaft)
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sliding-mesh
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sliding-mesh : 1st
Main shaft
Primary shaft
Lay shaft
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sliding-mesh: 2nd
Main shaft
Primary shaft
Lay shaft
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sliding-mesh: 3rd
Main shaft
Primary shaft
Lay shaft
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sliding-mesh: 4th
Main shaft
Primary shaft
Lay shaft
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sliding-mesh: Reverse
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Gear Ratio
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Gear Ratio
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Gear Ratio
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Gear Ratio
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Manual Gearbox
Disadvantages : sliding-mesh
• Although the mechanical efficiency of the sliding mesh
gearbox was high, it suffered from two great disadvantages:
– Gear noise due to the type of gear
– The difficulty of obtaining a smooth, quiet and quick
change of gear without the application of great skill and
judgement
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Constant Mesh Gearbox
Devender Kumar,MED,Thapar University
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DOG Teeths
308 Transmission systems
dog teeth
engaged
Devender Kumar,MED,Thapar University
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bearing dog clutch
splined
to shaft
Constant Mesh Gearbox
first gear – sliding-mesh type
second gear – dog clutch locks gear wheel to mainshaft
third gear
to road wheels
top gear – direct drive by connecting
primary shaft to mainshaft
primary shaft
mainshaft
Figure 3.39 Constant-mesh gearbox
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the cones adjusts the speed of the gear wheel to suit a longer time and noise will probably be heard.
the hub and mainshaft. Extra pressure on the lever This time factor has presented problems for the
will allow the sleeve to override the spring-loaded lubrication specialist, since the high-viscosity oil 24
balls and positively engage with the dogs on the gear. required by the gears takes a considerable time
to disperse from the cones. The solution to this
If the gear change is rushed, there will not be enough
problem was to use a lower viscosity oil (similar to
time for synchronisation, and the change will be
medium engine oil SAE 30) and provide a series of
noisy. The time taken for the speed to be equalised
grooves on the cone face to cut through the oil film
is governed by the frictional force that exists at the
and disperse the lubricant. Until recently, it was
cone faces. This force is controlled by:
considered essential to drain and refill the gearbox
! total spring strength every 8000 km (5000 miles) in order to remove the
! depth of groove in sleeve particles worn from the cones and gear teeth. With
! angle of cone extended service schedules now in operation this
en
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Constant Mesh Gearbox
bearing d
s
t
first gear – sliding-mesh type
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second gear – dog clutch locks ge
third gear
Constant Mesh Gearbox
top gear – direct drive
primary shaft to main
prim
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Constant Mesh Gearbox
308 Transmission systems
dog teeth
engaged
bearing dog clutch
splined
to shaft
first gear – sliding-mesh type
51 second gear – dog clutch locks gear wheel to mainshaft
third gear
to road wheels
top gear – direct drive by connecting
primary shaft to mainshaft
Synchromesh Gearbox primary shaft
mainshaft
Figure 3.39 Constant-mesh gearbox
the cones adjusts the speed of the gear wheel to suit a longer time and noise will probably be heard.
the hub and mainshaft. Extra pressure on the lever This time factor has presented problems for the
will allow the sleeve to override the spring-loaded lubrication specialist, since the high-viscosity oil
balls and positively engage with the dogs on the gear. required by the gears takes a considerable time
to disperse from the cones. The solution to this
If the gear change is rushed, there will not be enough
problem was to use a lower viscosity oil (similar to
time for synchronisation, and the change will be
52 medium engine oil SAE 30) and provide a series of
noisy. The time taken for the speed to be equalised
grooves on the cone face to cut through the oil film
is governed by the frictional force that exists at the
and disperse the lubricant. Until recently, it was
cone faces. This force is controlled by:
considered essential to drain and refill the gearbox
! total spring strength every 8000 km (5000 miles) in order to remove the
! depth of groove in sleeve particles worn from the cones and gear teeth. With 26
! angle of cone extended service schedules now in operation this
! coefficient of friction between cones. mileage has been increased considerably, and some
manufacturers have recommended that after the first
Therefore, if, because of mechanical defects, any of
change no further changes are necessary.
these factors is reduced, synchronisation will take
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Devender Kumar,MED,Thapar University
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Devender Kumar,MED,Thapar University
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and equalise speed the dog clutch engages
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Synchronizer Parts And Operation
friction cone groove for selector fork
primary shaft mainshaft
Figure 3.40 Principle of the synchromesh gearbox
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3.10.5 Baulk ring synchromesh
The baulk ring system, which is sometimes called
Synchronizer Parts
blocker ring or inertia lock, is a later development of
And Operation
clearance to allow ring to rotate
a small amount
the constant load system, used in the manufacture
of modern gearboxes, and is designed to overcome
the main disadvantage of the earlier design – noise or 1
crashing of the gears due to a quick change.
Two main features are incorporated in the baulk ring 2
system:
1 The cone pressure or load is proportional to the
speed of change.
2 An interception device prevents positive gear
engagement until the speed of the two members
is equal. 3 4 5 6
Various constructions are 56 used to produce these
features. Figure 3.41 shows one system in common use.
Operation
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Three spring-loaded shifting plates, which push out
from the hub into a groove in the sleeve, fit into slots
in the baulking cone. Each slot is wider than the plate; cones contact ring rotates
the clearance on each side is equal to half the pitch of
the splines on the sleeve. The baulking cone, which 1 shifting plate 4 gear dog teeth
is made of phosphor-bronze, has specially chamfered 2 circlip spring 5 baulking cone and ring
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Devender Kumar,MED,Thapar University
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Synchronizer Parts And Operation
Constant-mesh and synchromesh gearboxes
Constant-mesh 309 gearboxes
and synchromesh 309
friction cones contact friction cones contact when speeds are equal when speeds are equal
and equalise speed and equalise speed the dog clutch engages the dog clutch engages
friction cone groovefriction cone fork groove for selector fork
for selector
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primary shaft primary shaft mainshaft mainshaft
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Figure
Figure 3.40 Principle of the 3.40 Principle
synchromesh of the synchromesh gearbox
gearbox
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Synchronizer Parts And Operation
• The shifter fork fits into the groove of the synchronizer
sleeve. When a shift is made, the sleeve is moved toward
the speed gear.
• The sleeve presses the stop ring (synchronizer ring) against
the cone area of the speed gear.
• The friction between the stop ring and the speed gear
causes the speed of the two to become equal, permitting
the sleeve to engage the gear clutch teeth of the speed
gear.
• When this engagement occurs, the shift is complete.
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Synchronizer Parts And Operation
Clutch
Teeth of
Speed Gear
Synchronizer
Sliding Sleeve
Synchronizer
Ring
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Synchronizer Parts And Operation
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Synchronizer Parts And Operation
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Synchronizer Parts And Operation
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Synchronizer Parts And Operation
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Synchronizer Parts And Operation
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Synchronizer Parts And Operation
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Synchromesh
1–2 synchronizer sleeve
2 1
4 3
5
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Synchromesh
3–4 synchronizer sleeve 1–2 synchronizer sleeve
5 synchronizer sleeve
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Synchromesh
2 1
4 3
5
1st Gear
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Synchromesh
2 1
4 3
5
2nd Gear
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Synchromesh
2 1
4 3
5
3rd Gear
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Synchromesh
2 1
4 3
5
4th Gear
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Synchromesh
2 1
4 3
5
5th Gear
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Synchromesh
2 1
4 3
5
Revarse Gear
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A patient in ambulance
can tell you the
value of
time
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