automobile module ii

Module II

Transmission System

Presented By

Anoop P

Dept. of Mechanical Engg - MITS 1

Transmission System

Dept. of Mechanical Engg - MITS

The mechanism that transmits the power developed by the engine of the

automobile to the driving wheel is called the transmission system or the power

train.

It is composed of the clutch, the gear box, propeller shaft, universal joints, rear axle,

wheels and tyres.

The vehicles which have front wheel drive in addition include a second set of

propeller shafts, universal joints, final drives and differentials for the fronts units.

2

CLUTCH A clutch is a mechanism which enables the rotary motion of one shaft to be

transmitted at will to second shaft ,whose axis is coincident with that of first.

Clutch is located between engine and gear box.

When the clutch is engaged, the power flows from the engine to the rear wheels through the transmission system and the vehicle moves .

When the clutch is disengaged ,the power is not transmitted to the rear wheels and the vehicle stops, while the engine is still running.

Clutch is disengaged when

a) Starting the engine,

b) Shifting the gears,

c) Idling the engine

clutch is engaged only when the vehicle is to move and is kept engaged when the vehicle is moving.


Functions


to permit engagement or disengagement of a gear when the vehicle is stationary

and the engine is running.

to transmit the engine power to the road wheels smoothly without jerks to the

transmission system while setting the vehicle in motion.

to permit the engaging of the gears when the vehicle is in motion without damaging

the gear wheels.

4

Characteristics of a good clutch


It must be simple to operate the clutch.

Power must be transmitted smoothly.

Once clutch is engaged, it must transmit power without slipping.

It must disengage accurately and quickly.

It must have good heat radiation properties.

It must be well balanced during rotation.

It must have long trouble-free life.

It must be easy to inspect, adjust and repair.

5

Requirements of clutch


Torque transmission

Gradual engagement

Heat dissipation

Dynamic balancing

Vibration damping

Size

Inertia

Clutch free pedal play

Ease of operation

6

Clutch Components


Friction clutch is used on most automobiles with manual transmission

It consists of three main parts namely:

Driving Members

Driven Members

Operating Members

7

Driving member


It is comprised of flywheel , pressure plate, pressure springs and releasing levers

Flywheel is the foundation on which the clutch assembly is mounted

Clutch side is machined smoothly to provide friction surface

Pressure plate is made of cast iron and is of the same size as of the clutch disc

9

Driven Member


It has a clutch disc/plate which is gripped between the flywheel and pressure plate

by the action of springs

It is constructed into two parts

Outer disc of high quality steel which is covered with friction material

Inner disc is reinforced and splined hub which engages with the clutch shaft

Clutch plates are perforated for quick and effective dissipation of heat generated

due to friction surfaces

10

Operating Member


It consists of

Foot pedal and linkage

Release or throw out bearing

Release levers and springs

11

Clutch - Types


Friction clutches

The friction clutches work on the fact that friction is caused when two rotating discs come into contact with each other

Cone clutches

Single plate clutch (coil spring & diaphragm type)

Multi-plate clutch

Semi centrifugal clutch

Centrifugal clutch

Fluid Coupling

Fluid flywheel Works on the transfer of energy from one rotor to the other by means of fluid pressure.

12

Friction clutches - Principle


Let, W = axial load applied μ =coefficient of friction T = torque transmitted

R = effective mean radius of friction surface.

Then, T= μWR

13


Torque transmitted by a friction clutch depends upon three factors i.e., μ, Wand R.

That is by increasing any or all of the above factors would increase the amount of

torque which a clutch can transmit

Coefficient of friction, μ - depends upon the materials comprising friction

surfaces

Axial Pressure, W - The maximum value of W is limited to that which a driver

can exert without undue strain.

Effective Mean Radius of contact surfaces, R - The value of R cannot be

increased beyond a certain maximum which depends upon the space available in the

particular type of vehicle.

14

Cone Clutch



Advantage

The only advantage of the cone clutch is that the normal force acting on the contact surfaces in this case is larger than the axial force, as compared to the simple single plate clutch in which the normal force acting on the contact surfaces is equal to the axial force.

Disadvantages

This type of clutch is practically obsolete because of certain inherent disadvantages:

If the angle of cone is made smaller than about 20° the male cone tends to bind or join in the female cone and it becomes difficult to disengage the clutch.

A small amount of wear on the cone surface results in a considerable amount of the axial movement of the male cone which cannot be tolerated.

Single Plate Clutch - Coil Spring Type


Single Plate Clutch – Diaphragm Spring type


ADVANTAGES


It is more compact than other designs.

It is easier to balance rotationally and is less subjected to unwanted effects due to centrifugal force at high rotational speeds.

It gives uniformly distributed pressure on pressure plate.

It needs no release levers.

Minimum effort is sufficient to disengage the clutch.

It provides minimum number of moving components and hence minimum internal friction is experienced.

This is very commonly used in cars, light Lorries and mini trucks but is not much used in heavy vehicles

Multi Plate Clutch



Multi-plate clutches are preferred in vehicles:

If the torque to be transmitted is high.

Space available is limited

Semi Centrifugal Clutch


Centrifugal Clutch



Advantages

This type of mechanical clutch is automatic, so no kind of control mechanism is

necessary.

Centrifugal clutch is fairly cheap compare to normal type on clutch.

It prevents engine from stalling in other words minimizes engine braking force.

Disadvantages

This clutch is not appropriate for transferring significant torque or power because

they can slip if loaded heavily.

Sometimes the centrifugal clutch do not engage or disengage reliably and therefore

can cause a safety hazard.

Fluid Coupling



A fluid coupling or hydraulic coupling is a hydrodynamic device used to

transmit rotating mechanical power.

It has been used in automobile transmissions as an alternative to a mechanical

clutch.

It has widespread application in marine and industrial machine drives, where

variable speed operation and controlled start-up without shock loading of the

power transmission system is essential.

Advantages


It provides acceleration pedal control to effect automatic disengagement of drive to

gearbox at a predetermined speed.

Vibrations from engine side are not transmitted to wheels and similarly shock loads

from transmission side will not be transmitted to engine.

The engine will not stall if it is overloaded.

No wear on moving parts and no adjustments to be made.

No jerk on transmission when gear engages.

Vehicle can be stopped in gear and move off by pressing acceleration only.

There is no direct firm connection between engines and wheels. So when engine is

overloaded, it will not stop. But it results in slip within coupling.

Unlike friction clutch, slip within coupling does not cause damage within working

components.

In case of FC, engine is not forced to operate at very low speeds when it is

overloaded.

No wear is experienced on impeller or turbine blades.

GEARBOX


The gear box is necessary in the transmission system to maintain engine speed (or torque) at the most economical value under all conditions of vehicle movement.

An ideal gear box would provide an infinite range of gear ratios, so that the engine speed should be kept at or near that the maximum power is developed whatever the speed of the vehicle.

Functions


Helps to select torque ratio between the engine and wheels to be varied for rapid

acceleration and for climbing gradients.

It provides means of reversal of vehicle motion.

Transmission can be disconnected from engine by neutral position at gear box

Types of gearbox


Selective type gear boxes

(i)Sliding mesh gear box

(ii)Constant mesh gear box

(iii)Synchromesh gear box

Progressive type gear box

Epi-cyclic type gear box

Selective type gear boxes


It is the transmission in which any speed may be selected from the neutral position.

In this type of transmission neutral position has to be obtained before selecting any

forward or reverse gear.

Sliding Mesh Gear box


Constant Mesh Gearbox


1. I speed gear

2. II speed gear

3. main shaft

4. III speed gear

5. top and III speed engaging

dogs

6. top gear

7. primary shaft or main drive

gear

8. counter shaft/cluster gear


Advantages

Compared to the sliding mesh type, the constant mesh gear box has the following

advantages:

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.

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.

Disadvantages

Dog clutch engagements are bit noisy

In constant mesh gear box the driver has to undergo double-declutching while

shifting gears.

Synchromesh Gearbox


Advantage


The synchromesh type of transmission has the big advantage of allowing smooth

type and quick shifting of gears without danger of damaging the gears and without

necessity for double clutching.

The synchromesh gear box is considered the most advanced and has been adopted

in most cars.

Automatic Transmission


It is the transmission which automatically provides varying gear ratios to suit

operating conditions.

In this case gear changes are effected automatically without manual assistance.

Components


Main Components of Automatic Transmission System

Torque Converter.

Gearbox of planetary type with friction brake bands and multiple disc clutch

operated by hydraulic system.

Hydraulic Control System This system has a source of hydraulic pressure servo

units and control valves.

ADVANTAGES


Ease of control, i.e. it relieves the driver from fatigue due to the elimination of

clutch and gear controls.

Smooth operation.

Numerous numbers of gear ratios are available.

Quick change of gear ratios effected automatically.

Minimum interruption of power during gear shifts.

Disadvantages


High cost.

Complicated design.

Possibility for oil leakage.

Torque Convertor



Most cars with automatic transmission use a form of fluid drive known as torque

converter

As the name implies, it converts the torque or tuning effort of engine into higher

torque needed by cars at low road speed.

An increase in torque has same effect as changing to a lower gear; so a torque

converter is also a gear reducer, acting like an extra set of gears before engine drive

reaches gear box.


A torque converter also does following jobs:

It serves as automatic clutch which transmits engine torque to the transmission

input shaft.

Absorbs torsional vibration of engine.

Acts as a flywheel and smoothens out engine rotation.

Drives oil pump.

Epi-cyclic (Planetary) Gear Box



The term planetary gear system comes from its similarity with the solar system

i.e. like planets revolving round the sun

Gearing provides different torque ratios by locking of 3 basic components

Sun gear

Planet gear

Annular ring


Gear reduction

Sun gear is held stationary

Ring gear drives the planet carrier

Ring gear is locked

Planet carrier drives the sun gear

Direct drive

Locking together the sun and planet gears

The entire planetary set rotates as a single unit

Reverse

Locking pinion carrier/arm

Sun gear is the driving member and ring gear the driven

Here planet gears are free to rotate about their own axes


Neutral

None of the driving members is coupled to planetary gear set

Overdrive

Sun gear is held stationary

Planet carrier is the driving unit and the ring gear is driven

Ring gear is locked

The planet carrier is the driving unit and sun gear is the driven unit

OVERDRIVE



Overdrive is a device interposed between the transmission (gear box) and

propeller shaft to permit the propeller shaft to turn faster than or-over drive the

transmission main shaft.

It is so called because it provides a speed ratio over that of the high speed ratio.

The overdrive permits the engine to operate only about 70 percent of the

propeller shaft speed, when the vehicle is operating in the high speed ranges.

Overdrive is usually, employed supplementary to conventional transmission.

It is bolted to the rear of the transmission between the transmission and propeller

shaft

A slightly higher rear-axle gear ratio is employed with an overdrive than without

one.


ADVANTAGES

This device permits the engine to operate at only about 70% of the propeller shaft speed when the car is operating in the higher speed ranges. i.e., over drive engine speed about 30%.

Because the engine is not required to turn over fast at high car speed, the use of over drive reduces engine wear and vibration and saves gasoline.

Usually a slightly higher rear –axle gear ratio is employed with an over –drive then without one.

DRAWBACKS

In descending long steep hills where the braking effect of the engine would be lost due to slip

The driving force available at the wheels is less in case of vehicles with over-drive.

Freewheel (one-way clutch)



A freewheel or overrunning clutch is a device in a transmission that disengages the

driveshaft from the driven shaft when the driven shaft rotates faster than the

driveshaft.

An overdrive is sometimes mistakenly called a freewheel.

The condition of a driven shaft spinning faster than its driveshaft exists in most

bicycles when the rider holds his or her feet still, no longer pushing the pedals. In a

fixed-gear bicycle, without a freewheel, the rear wheel would drive the pedals

around.

An analogous condition exists in an automobile with a manual

transmission going downhill or any situation where the driver takes his

or her foot off the gas pedal, closing the throttle; the wheels want to

drive the engine, possibly at a higher RPM.

Hydraulic control system


The hydraulic control system consists of an oil pan (which acts as fluid reservoir),

pump (which builds oil pressure)

Various valves (controls oil path to lock and unlock various clutches and band

brakes)


Manual shift valve

This valve is located in the hydraulic control unit, and actuated by drivers selector

unit.

It determines which gear ratio to be selected.

The position of the selector lever can be used to vary the working pressure of shift

valves.

Operation of the lever moves the valve and uncovers the fluid port that supplies

hydraulic pressure to clutches and brake bands to obtain the desired gear position.

Speed Sensing

Load sensing

Propeller shaft


The propeller shaft connects the transmission main or the output shaft of the gear

box to the differential of the rear axle.

The propeller shaft transmits the power from gear box to rear axle with the help

of universal joints.

Universal joints are fitted on both the ends of the shaft.


It propels the vehicle forward, so called propeller shaft

A Propeller Shaft connects a gearbox to the Differential.

It is strong enough to handle maximum low gear torque

It is provided with two Universal-joints to maintain constant velocity and

positioning of differential at different plane.

It is provided with a slip joint to take care of the change in length.

Shaft diameter and its thickness decide the torque carrying capacity and angle of

operation.

For long transmissions split type propeller shafts with central support are used.

Universal Joint



A universal joint allows driving torque to be carried through two shafts that are at

an angle with each other.

A simple universal joint consist two Y- shaped yoke, one on the driving shaft and

other on the driven shaft.

The four arms of spider are assembled in needle bearings in the two yokes. The

driving shaft and yoke force the spider to rotate.

The other two trunnions of the spider then cause the driven yoke to rotate.

When the two shafts are at an angle with each other, the needle bearings permit

the yokes to swing around on the trunnions with each revolution.

A simple universal joint does not transmit the motion uniformly when the shafts

are operating an angle. Because of this, two universal joints are used in a vehicle,

one between the gear box and the propeller shaft and other between the propeller

shaft and the differential pinion shaft.

Slip joint


Slip joint is attached to the driven yoke in order the increase or decrease the length

of propeller shaft.

It has outside splines on the shaft and matching internal splines in a mating

hollow shaft or yoke.

When assembled the splines cause the shafts to rotate together while they can

move back and forth. This changes the length of propeller shaft.

Final drive/ Final reduction



Final drive is a part of the transmission between the propeller shaft and the

differential gear assembly.

It consists of a combination of large sized crown wheel and a bevel pinion.

It serves the following functions:

Transmit the drive from propeller shaft through 900

Enables the driving wheels to rotate at different speeds

Reduce the drive line speed from that of engine to the drive wheels

Final drive in case of a motorcycle is a chain and sprocket.

Differential Assembly



Differential is the mechanism by means of which outer wheel runs faster than the

inner wheels while taking a turn or moving over upheaval road.

Differential is a part of the inner axle housing assembly, which includes the

differential, rear axles, wheels and bearings.

The differential consists of a system of gears arranged in such a way that connects

'the propeller shaft with the rear axles.

The purpose of the differential is to provide the relative movement to the two rear

wheels, while the vehicle is taking a turn.

The torque transmitted to each wheel is, however, always equal. Differentials are

used in rear drive axle of front-engine, rear-wheel drive vehicles.

Differentials are also used in the Trans axles on front-engine, front-wheel drive

wheels.

Also, four-wheel drive vehicles have differential at both the front and rear wheels. In

addition, some four-wheel-drive vehicles have a third differential in the transfer case.

Transaxle


In the automotive field, a transaxle is a major mechanical component that combines

the functionality of the transmission, the differential and associated components of

the driven axle into one integrated assembly.

Transaxles are used almost in all automobile configurations that have the engine

placed at the same end (transversely) of the car as the driven wheels:

the front-engine, front-wheel drive layout, rear-engine, rear-wheel drive layout

and rear mid-engine, rear-wheel drive layout arrangements.

FOUR WHEEL DRIVE (4WD or 4X4)


Axle


An axle is a central shaft for a rotating wheel or gear.

On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or

fixed to the vehicle, with the wheels rotating around the axle.

In the former case, bearings or bushings are provided at the mounting points where

the axle is supported.

In the latter case, a bearing or bushing sits inside a central hole in the wheel to

allow the wheel or gear to rotate around the axle.

Sometimes, especially on bicycles, the latter type axle is referred to as a spindle.

Dead axle (lazy axle)


Dead axle, also called lazy axle, is not part of the drive train but is instead free-

rotating.

The rear axle of a front-wheel drive car is usually a dead axle.

Many trucks and trailers use dead axles for strictly load-bearing purposes.

A dead axle located immediately in front of a drive axle is called a pusher axle.

A tag axle is a dead axle situated behind a drive axle.

Live Axle


A live axle is a type of beam axle in which the shaft transmits power to the wheels.

The drawbacks are that it does not allow each wheel to move independently in

response to bumps.

In case of vehicles having common axle for drive and steering will be employed with

constant velocity joints, so that no transfer losses are possible while steering.

Forces and torque on rear axle


Weight of the body

Rear axle behaves like a beam supported at the ends and loaded at two points.

The load coming on the axle is due to the weight of the body being transmitted

through the suspension springs.

Weight causes shear force and bending on the wheels.


Driving thrust

Torque produced by the engine causes the thrust on the wheels, which is

responsible for the forward motion of the vehicle.

The drive force from the wheels is transmitted to the body or chassis by means of

leaf springs or Radius rods (strut rod / thrust members).

These members are in longitudinal direction connecting axle casing and the body.


Torque reaction

If the road wheels are prevented from rotation, with propeller shaft rotating it is

seen that the bevel pinion tend to roll around the crown gear of the differential.

This tendency is also present when the vehicle is running so that bevel pinion

always tends to climb round the crown wheel.

Thus there is a force on axle casing to rotate. This is called torque reaction.


Side thrust

Side thrust comes mainly when the vehicle is taking a turn or when the vehicle is

moving along an laterally inclined surface.

The side thrust coming on to the axle can be taken by Panhard rod.

Types of Rear Axle Drives


Hotch Kiss Drive


Simplest and most widely used rear axle drive.


Propeller shaft with two universal joints and a sliding joint.

The spring is fixed rigidly in the middle onto the frame. The drive torque is transmitted through the front half of the springs.

The front end of the leaf suspension is rigidly fixed onto the frame while the rear is connected via a shackle.

Two universal joints are used to avoid the bending of the propeller shaft due to the torque reaction.

Sliding joint is provided to accommodate for the variation of the length in the transmission shaft.

Advantages


Torque reaction, driving thrust and side thrust are taken up by the leaf spring and

universal joints

Varying propeller shaft length can be accommodated without any inclination

Provides comfortable ride

Torque Tube Drive



Torque reaction, Braking torque and drive thrust are taken by Torque tube.

The suspension springs are takes only the side thrust and body weight.

One end of the torque tube is attached to the axle casing while the other end is

spherical and fits into the cup on the frame.

The torque tube encloses the propeller shaft.

Torque tube takes the torque reaction and centre line of the bevel pinion shaft always

passes through the centre of the spherical cup.

Single universal joint is used in the transmission drive because the universal joint is

situated exactly at the centre of the spherical cup.

No sliding joint is provided since the pinion shaft and the propeller shaft moves same

center ( spherical cup).

Radius rods


The side way forces are transmitted to the axle from the frame normally by the

springs.

The coil or torsion springs can’t be used with flexibility

In such cases a transverse radius rod is provided parallel to the wheel axis, which is

pivoted at one end to the axle and other end to the frame

The ball and socket joints are used at the end of the rods to provide relative

motion

Classification of Rear Axle



According to design of Axle Casing


According to the method of supporting

Fully Floating Axle


This is used in heavy vehicles

Axle carries only driving torque

Vehicle weight and end thrust aren’t carried by the axle

Wheels and the axle casing takes care of the vehicle weight

Axle is relieved from all strains due to load of vehicle and end thrust, hence its duty

is only to transmit rotary motion to the wheels

Also we can remove the axle shaft without disturbing the axle hub and wheel


Semi Floating Axle

In this type of axle, whole of the load of the rear portion of the vehicle, the end

thrust and the driving torque is carried by the axle.

The vehicle load is transmitted to each of the half shafts through axle casing and the

bearings.

The wheel hub is rigidly fixed to the outer end of the axle shaft.

For replacement of the axle shaft, the vehicle is jacked up. The wheel, the drum and

the bearing retainer plate must be removed in order to remove the axle shaft.


Three Quarter floating Axle

This design is a compromise between the fully floating axle and the semi-floating

type.

The bearing is carried on the axle casing, the hub is rigidly fixed to the outer end of

the axle shaft.

The axle shaft is not supporting any weight but will have to withstand the end

thrusts besides transmitting the driving torque.

Due to the reason that three quarter floating axle has only one bearing at the outer

end, but even then it takes some bending stresses, it, therefore, is not resembling

the fully floating type.

automobile module ii

Automotive