Automobile Transmission (Gearbox & Clutch)

Introduction to 'Automobile Transmission’

Transmission is the mechanism which is used to transfer the power developed by engine to the road wheels of an automobile. Simply, through the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source (IC Engine) to other devices of transmission. Transmission means 'gearbox' along with 'clutch' set.

The output power from 'Internal Combustion Engine' is available in the form of rotation of crankshaft. This rotary motion is transmitted to the road wheels with the help of "Transmission and its System". Therefor, the friction between road and the surface of the road wheel makes possible the movement of automobile. Transmission and its system performs this function. The transmission system consists of a number of components. These components work together to transmit the rotary motion of the crankshaft smoothly (without jerk) and efficiently to the road wheels. [1]

Now lets see about, why the transmission is necessary and what is core use of it? The answer is 'sudden change of state' from rest to motion or vice versa is not desirable. It may be uncomfortable, or even injurious and prone to damaged or accidents, to the occupants of the automobile. Therefore, the rotary motion of crankshaft should be transmitted gradually and not suddenly. Another aspect of transmission is that the motion from the crankshaft should not be transmitted as soon as the engine starts. It is not desirable that as soon as the engine starts the vehicle begins moving. The motion is required to be transmitted only when desired. [1]

'Transmission System' includes clutch, gear box, propeller shaft and CV joints, slip-joint, differential unit, axles and road wheels, etc. Only 'Transmission' means gearbox it is located between clutch and propeller shaft. While 'Automobile Transmission System' is complex term, it means whole 'transmission system' along with the method of drive or the arrangement of final drive, like 'front wheel drive' and 'rear wheel drive' etc.

An automobile or motor ( or wheeled) vehicle's transmission system comprises of the following main units; as shown in Fig 1.

1. Clutch
2. Gear box (manual or automatic)
3. Overdrive (on many recent vehicles)
4. Transfer box (in/for four wheel drive vehicles)
5. Propeller shaft and CV Joints
6. Universal joint
7. Final drive: differential gear assembly & rear axles

Fig 1: Transmission-System (Rear-wheel-drive)

Key features:

• understand the transmission system of automobiles,
• list out the components of the transmission system,
• describe the various functions, requirements and types of gear boxes and clutches

Purpose of Transmission (Gearbox):

Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox rotates at a slower rate than the input shaft, and this reduction in speed produces a mechanical advantage, i.e. increasing torque. The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. [2]

The purpose of the transmission is to provide high torque at the time of starting, hill climbing, accelerating and pulling a load. When a vehicle is starting from rest, hill climbing, accelerating and meeting other resistances, high torque is required at the driving wheels. Hence a device must be provided to permit the engine crankshaft to revolve a relatively high speed, while the wheels turn at slower speeds. This is obtained by a set of gears called a transmission or gear set. [3]

Automotive gearboxes are used to reduce load on the engine by manipulating torque and speed. They have the option to select one of several different gear ratios. It consists of gears and gear trains.

Once the engine has reached a number of revolutions per minute, it is advisable to increase the gear to reduce the engine rpm to reduce wear on the engine, allow more control, and greater speeds, better acceleration, and better fuel economy.

Automotive gearbox also have the provision to do the opposite i.e. provide an increase in output shaft speed with a reduction of torque (overdrive).

Simply, transmission has provides speed and torque conversions because of the limitations of internal combustion engines.

Functions of Transmission:

An automobile is able to provide varying speed and torque through its gear box. Various functions of a gear box are listed below:

• To provide high torque at the time of starting, vehicle acceleration, climbing up a hill.
• To provide more than forward speeds by providing more than one gear ratios. In modern cars, five forward gears and reverse gear is provided. For given engine speed, higher speed can be obtained by running in higher (4th and 5th) gears.
• Gear box provides a reverse gear for driving the vehicle in reverse direction.

Requirements of Transmission:

• Provide means of connection and disconnection of engine with rest of power train without shock and smoothly.
• Provide a varied leverage between the engine and the drive wheels
• Provide means to transfer power in opposite direction.
• Enable power transmission at varied angles and varied lengths.
• Enable speed reduction between engine and the drive wheels in the ratio of 5:1.
• Enable diversion of power flow at right angles.
• Provide means to drive the driving wheels at different speeds when required.
• Bear the effect of torque reaction, driving thrust and braking effort effectively.

Types of Transmission (Gear-boxes):

The most common transmission systems that have been used for the automotive industry are manual transmission, automatic transmission (transaxle), semi-automatic transmission, and continuously variable transmission (CVT). The first transmission invented was the manual transmission system. The driver needs to disengage the clutch to disconnect the power from the engine first, select the target gear, and engage the clutch again to perform the gear change.

An automatic transmission uses a fluid-coupling torque converter to replace the clutch to avoid engaging/disengaging clutch during gear change. A completed gear set, called planetary gears, is used to perform gear ratio change instead of selecting gear manually. Automobile or automotive transmission system consists of various devices that help in transmitting power from the engine through the drive shaft to the live axle of an automobile. Gears, brakes, clutch, fluid drive and other auto transmission parts work together for transforming the speed ratio between the engine and wheels of a vehicle.

• (a) Selective type gear boxes: (i) Sliding mesh gear box (ii) Constant mesh gear box (iii) Synchromesh gear box
• (b) Progressive type gear box
• (c) Epicyclic type gear box.

Description of various types of gear boxes and clutches shall be discuss in another articles.

Gear Train: A combination of two or more gears, which mesh in such a way that power is transmitted from driving shaft to driven shaft, is known as gear train.

There are three types of gear trains:

• (a) Simple gear train,
• (b) Compound gear train, and
• (c) Epicyclic gear train.

 

Introduction: Clutch and its Functions

Principles of Operations of Clutch

Purpose of Clutch

Requirement of Clutch

Main Parts of Clutch

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Gear-boxes with different speed gear

Three speed and Four speed Gear-box

Gear shifting

Gear box troubles and their causes

Lubrication of gear box

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Types of Clutches

Types of friction lining materials

Properties of good clutch lining

Clutch Maintenance

Clutch troubles and their causes

Factors Affecting the Power Transmitted by the Clutch, Propeller Shaft, Universal Joint, Rear Axle

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References:

[1] Automobile Engineering by Sudhir Kumar Saxena, University Science Press: Laxmi Publications, New Delhi, UAE-9356-195, C.17388/09/04,2009, “Transmission System” p.77.

[2] https://en.wikipedia.org/wiki/Transmission

[3] Objective Automobile Engineering by PK Mishra, Upkar Prakashan, Agra, C.N.981, “Part-B- Auto Vehicle Technology”, “Transmission, Differential and Propeller Shaft” p.73-88.