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Transmission
Functions of Transmissions
The main functions which are performed by the transmissions are:
1. The torque produced by engine varies with speed only with narrow limits. But under
practical considerations running of automobile demands a large variation of torque
available at the road wheels. Hence the main purpose of the transmission is to provide
a means to vary the torque ratio between the engine and the road wheels as required.
2. The transmission also provides a neutral position so that the engine and the road
wheels are disconnected even when the clutch is in engaged position.
3. A means to back the car byreversing the direction of rotation of the drive is also
provided by transmission.
Total Resistance to the vehicle motion
It consists of:
(i) Resistance due to wind-This is taken to be proportional to the square of the vehicle speed.
(ii) Resistance due to gradient-This remains constant at all speeds. This is the component of
the vehicle weight parallel to the plane of the road.
(iii) Miscellaneous-Apart from the above two types, various other factors also contribute
towards the vehicles resistance. These are: type of the road, tire friction, etc. This may also be
taken approximately to remain constant with the speed.
Necessity of Transmission:
By now we understand the variation of total resistance to the vehicle motion and the tractive
effort of the vehicle with speed. It is obvious that whenever the tractive effort exceeds the total
resistance, the vehicle will accelerate to a speed where tractive effort becomes equal to the total
resistance.
The graph below shows the various resistance curves I, II, III and IV. The gradient of the road
increase from curve I to IV thus increases the total resistance offered to the vehicle. The curves 1, 2
and 3 show the available tractive effort at the 1st, 2
nd and 3
rd gear respectively.
Let the vehicle be in the top gear and suppose the vehicle is travelling on a gradient which gives
total resistance curve I. Then from graph below, it is seen that OA is the stabilizing speed. If the
speed at any instant is less, say, OB, the excess of tractive effort will accelerate it to speed OA.
Similarly if the speed at any instant is OC, the excess of resistance will decelerate it to OA.
Now let the vehicle, go on next gradient of curve II. In this case it is noticed that the stabilizing-
speed has decreased. Next consider further the curve Ill. At this gradient, we see that nowhere does
the curve 3 cross curve III. Therefore the vehicle will not be able to go at this gradient in the top gear.
However, if we pass on to second gear, we get a stabilizing speed OD. Similarly in second gear also
the vehicle will not be running on gradient IV for which we shall have to shift to first gear.
Again at start more acceleration is needed to gain speed quickly. This can best be done in
first gear because in this gear the maximum tractive effort is available for acceleration.
However, when the necessary speed has been obtained, we may shift into higher gears,
because then the vehicle speed needs to be simply maintained and no acceleration is required.
Necessity of Gearbox:
In addition to many advantages of internal combustion engine, such as high power to weight
ratio, relatively good efficiency and relatively compact energy storage it has 3 fundamental
disadvantages
1. Unlike steam engines or electricmotors the combustion engine is incapable of
producing torque from the rest.
2. An IC engine can produce maximum power at a certain engine speed.
3. The efficiency of the engine, i.e.its fuel consumption is verymuch dependent on the
operating point in the engines performance map.
The graph above show the variation of tractive effort (torque) required by a conventional
vehicle at various velocities. FZ,A shows the available traction by the IC engine. If the engine
produces more tractive effort than FZ,Ae the tyres will only slip and won’t propel the vehicle.
At rest if the tractive effort applied on the wheels is greater than the adhesion limit the tyres
will again just slip. Thus the top curve which outlines the shaded area gives the maximum
tractive effort which can be applied on the wheel without tyre slip.
The shaded area cannot be completely covered by a conventional IC engine. It can be seen
that when the velocity is close to zero the car will not have the required tractive effort to
propel the vehicle. Thus the engine cannot be directly coupled to the wheels of a car for this
reason.
With the use of a transmission we see how we can multiply the tractive effort or torque
produced by the engine. After suitable selection of gear ratios the whole shaded area can be
covered by the vehicle powertrain thereby getting faster acceleration to a desired speed and
keeping the engine in its operating range.
The point at which the fractional resistance curve intersects the available traction curve will
determine the maximum speed of the vehicle.
Types of Transmission
Manual Transmission
Sliding Mesh Gearbox
This is the simplest type of gear box. The figure gives a simplified view of the gear box. The
power comes from the engine to the clutch shaft and thence to the clutch gear which is
always in mesh with a gear on the lay shaft. All the gears on the lay shaft are fixed to it and
as such they are all the time rotating when the engine is running and the clutch is engaged.
Three direct and one reverse speeds are attained on suitably moving the gear on the main
shaft by means of selector mechanism. The change in direction for reverse is done by using
an idler gear; the sole purpose of this gear is in changing the direction of rotation of the main
shaft. By moving the gears on the main shaft to mesh with their corresponding gear on the
layshaft various gear ratios are achieved. In this mechanism on top gear the main shaft simply
directly meshes with the clutch shaft.
Constant Mesh Gearbox
In this type of gear box, all the gears are in constant mesh with the corresponding gears
on the lay shaft. The gears on the main shaft which is splined are free (Fig. 4.9). The dog
clutches are provided which are free to slide on the main shaft. The gears on the lay shaft are,
however, fixed.
When the left dog clutch is slid to the left by means of the selector mechanism, its teeth
are engaged with those on the clutch gear and we get (the direct gear. The same dog clutch,
however, when slid to right makes contact with the second gear and second gear is obtained.
Similarly movement of the right dog clutch to the left results in low gear and towards right in
reverse gear.
Double Declutching
In the constant mesh box, for the smooth engagement of the dog clutches it is necessary
that the speed of main shaft gear and the sliding dog must be equal. Therefore to obtain lower
gear, the speed of the clutch shaft, lay shaft and main shaft gear must be increased. This is
done by double declutching. The procedure for double declutching is as given below:
The clutch is disengaged and the gear is brought to neutral. Then the clutch is engaged
and accelerator pedal pressed to increase the speed of the main shaft gears. After this the
clutch is again disengaged and the gear moved to the required lower gear and the clutch is
again engaged. As the clutch is disengaged twice in this process, it is called double
declutching.
For changing to higher gear, however, reverse effect is desired i.e., the driver has to wait
with the gear in neutral till the main shaft speed is decreased sufficiently for a smooth
engagement of the gear.
Advantages
Compared to the sliding mesh type, the constant mesh gear box has the following
advantages:
I. As the gears have to remain always in mesh, it is no longer necessary to use straight
spur gears. Instead, helical gears are used which are quieter running.
2. Wear of dog teeth on account of engaging and disengaging is reduced because here all
the teeth of the dog clutches are involved compared to only two or three teeth in the case of
sliding gears.
Synchromesh Gearbox
This type of gear box is similar to the constant mesh type in that all the gears on the main
shaft are in constant mesh with the corresponding gears on the lay shaft. The gear on the lay
shaft is fixed to it while those on the main shaft are free to rotate on the same. Its working is
also similar to the constant mesh type, but in the former there is one definite improvement
over the latter. This is the provision of synchromesh device which avoids the necessity of
double declutching. The parts which ultimately are to be engaged are first brought into
frictional contact which equalizes their speed, after which these may be engaged smoothly.
The figure shows the construction and working of a synchromesh gear box. In most of the
cars, however, the synchromesh devices are not fitted to all the gears as is shown in this
figure. They are filled only on the high gears and on the low and reverse gears ordinary dog
clutches are only provided.
It is this speeding up and slowing down of the input side of the gearbox that the synchromesh
does. The work done by the synchromesh assembly is to change the speed of the inertia on
the layshaft and input shaft, which includes the clutch driven plate. The large majority of the
inertia is found in the clutch plate.
The figure above shows a small cross sectional portion of the synchromesh gearbox with the
engine shaft A and gear B on the main shaft. The gear B is in constant mesh with the
corresponding gear on the layshaft. Thus all gears on the main shaft as well as on the layshaft
continue to rotate as long as shaft A is rotating. Member F1 is free to slide on the main shaft
with internal teeth which mesh with the splines on the main shaft. G1 is a ring gear having
internal teeth which mesh onto the external teeth of F1. K1 have dog teeth and are present on
B, they also fit onto the teeth of G1. The ring gear G1 is moved using a fork S1 . TF are balls
supported by springs. They are present to prevent G1 from sliding off F1. However if the force
applied is greater than a certain value the balls are overcome and G1 begins to slide over F1.
There are usually six of these balls symmetrically placed circumferentially in one
synchromesh device. M1 and M2 are frictional surfaces.
For engaging a gear, G1 is slid with the help of the fork S1 and hence member F1 is also slid.
The cones M1 and M2 rub and friction makes their speed equal. Further pushing G1 will cause
it to override the balls and get engaged to the dog K1. There are synchronizer rings present
between K1 and F1. These rings prevent the movement of the gear G1 onto K1 until the speeds
of the two shafts have equalised. The speed is equalised with the help of the frictional
surfaces M1 and M2. Thus smooth transitions between gear changes are achieved using a
synchromesh gearbox.
Clutches
In vehicles there is a requirement for a device to provide a coupling from the engine
crankshaft to the transmission. This allows the engine to be started and run without the
vehicle moving and the vehicle to be started from rest under control at various rates of
acceleration. In manual gearboxes, the drive from the engine also has to be disconnected
during gear changes. Clutches are associated with manual gearboxes and are normally
operated by the driver. The spring pressure clamps the pressure plate onto the driven plate
and the flywheel; with the assembly like this, the drive is passed from the engine to
transmission as seen in the figure. When the driver depresses the clutch pedal, the movement
is passed to the release bearing by either hydraulics or cable and the release bearing then
pushes or pulls the diaphragm spring (depending on whether the clutch is a ‘push’ or ‘pull’
design). The outer part of the release bearing is held (by the release lever) so it does not
rotate, and the inner race of the bearing rotates with the diaphragm spring and clutch cover.
The load has to always be sufficient to clamp the driven plate and not allow any slip, and yet
fully clear to allow the plate to rotate freely when the clutch pedal is depressed. By
depressing the diaphragm spring, the pressure on the driven plate is released. This actuation
of the spring is achieved by hydraulics, cable . When the load on the cover plate is released,
the driven plate is allowed to rotate freely inside the assembly and the drive to the
transmission is disconnected.
Automatic Transmission
Torque Converter
A torque converter is used in an automatic transmission instead of a frictional clutch. It is a
fluid couplings contain two rotating elements – impeller and turbine – within a toroidal
casing. Both these elements have radial vanes and the cavity is filled with hydraulic fluid.
The impeller and casing are driven by the input, and fluid trapped between the rotating vanes
must also rotate and this in turn causes flow outwards to the largest diameter as a result of
centrifugal action. This outward radial fluid flow is directed by the curvature of the impeller
shroud back to the turbine section where the rotational component of velocity gives a torque
reaction on the turbine blades as the fluid flow direction is changed. The fluid returns towards
the centre line of the assembly and re-enters the impeller at a smaller diameter.
The transmitted torque will depend on the relative speed of the impeller and turbine. It will
reduce to zero if they are rotating at the same speed and will reverse if the turbine rotates
faster than the impeller. The relative speed may be expressed either as a speed ratio (angular
velocity of output /angular velocity of input ) or by a relative slip s, defined by:
The power transmission efficiency η, is also related to speed ratio as follows:
The torque converter is like the coupling in having a turbine and impeller but, in addition,
uses a third vane element called a reactor or stator that does not rotate. To prevent it from
rotating, it is connected via a tube concentric with the turbine output shaft to an internal part
of the gearbox casing such as a bearing housing. The stator vanes redirect the flow as in the
figure and add to the torque provided by the engine input to give a multiplying effect on the
output torque (despite the apparent sequence implied by the flow path). The torque balance
then becomes:
Epicyclic Transmission
An epicyclic single-stage gear train consists of an internally toothed annular (ring) A
band brake encircling it. In the centre of this gear is sun gear S, which forms part of the
input shaft. The sun gear and the annular gear are connected by a number of planet
(pinion) gears P which are mounted on a carrier C and is integral with the output shaft.
For transmission of torque, either the sun gear, the carrier, or the annular gear must be
held stationary.
Let,
TA = number of teeth on annular, internal or ring gear
TS= number of teeth on sun or centre gear
TP= number of teeth on planet gear
TC= effective number of teeth on arm or planet carrier
Thus,
TA = TS+2 TP
TC=TS+TA
For first gear, the input is given to the sun gear keeping the annular gear band brake
fixed and thus preventing its rotation. The planet gears simultaneously rotate around
their axes and revolve around the input sun gear axis along the inner circumference of
the annular gear. The carrier is the output shaft, which also rotates because of the
rotation of the planets but at a slower rate than the input shaft.
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For second gear, the sun gear is held stationary by a friction surface such as a brake or clutch.
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For reverse gear, the planet carrier is held stationary. The input is given to the sun gear and
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Continuously Variable Transmission
The idea of a variable pulley system is a logical extension of a conventional V-belt fixed ratio
drive. The figure shows the principle, where there are two pairs of conical pulley sheaves and
a fixed length V-belt. One half of each pulley pair is moveable and their movement is
synchronized. This allows the belt-rolling radius to be changed such that a relatively low
output speed is obtained when its radius at the output is large. The output speed can be
increased as this radius is made smaller and the rolling radius on the input is increased. As in
a conventional V-belt, there is a frictional force between the pulley sheaves and the angled
belt face that provides the transfer of tractive effort. There must be sufficient normal force
between them to prevent gross slipping and the clamping forces that hold the pulley sheaves
together also produce a tension load in the belt. The relative magnitude of these clamping
forces can then also be used to control the belt position and overall ratio.