Clutch is a device, used to connect and disconnect the tractor engine from the transmission gears and drive wheels. Clutch transmits power by means of friction between driving members and driven members.

Why we need clutch:

The internal combustion engine as used in a tractor must be cranked manually or by a special starting mechanism. For cranking, the engine is disconnected from the rest of the transmission unit by a suitable clutch. After starting the engine, the clutch is engaged to transmit power from the engine to the gearbox. This type of engine must attain a certain speed before it will have any power.
Changing the transmission gears must be permitted for the purpose of securing different traveling speeds. The gearbox must be kept free from the engine power, otherwise the gear teeth will be damaged and engagement of gear will not be perfect. This is done by clutch.
Stopping the belt pulley must be permitted without having to stop the engine. This is done by placing a clutch between the engine and the transmission gears and belt pulley.

Requirements of clutch:

It should have good ability of taking load without dragging, grabbing and slipping.
It should have higher capacity to transmit maximum power without slipping.
It should be convenient, accessible and easy to operate, adjust and repair.
Friction surface should be highly resistant to heat effect. The control by hand lever or pedal lever should be easy.

Types of clutch

Clutches are mainly of three types:

Friction clutch
Dog clutch
Fluid coupling

Friction clutch is most popular in four wheel tractors. Fluid clutch is also used in some tractors these days. Dog clutch is mostly used in power tiller.

Friction clutch may be subdivided into three classes:

Single plate clutch or single disc clutch
Multiple plate clutch or multiple disc clutch and
Cone clutch.

Friction clutch

A clutch in which one part turns the other by the friction between them. Friction clutch produces gripping action, by utilizing the frictional force between two surfaces. These surfaces are pressed together to transmit power.

While starting the engine, the clutch pedal is depressed. After the start of the engine, the clutch pedal is slowly released to increase the pressure gradually on frictional surface until there is no slip. Thus the driven plate is gripped firmly to the driving plate. Transmission of power depends upon the kind of material used for the friction members and intensity of the force, pressing them together.

Single plate clutch:

This may be called single disc clutch.

It consists of:

Pressure plate

Clutch plate

Springs and

Release fingers

The single disc clutch is a plate type of clutch in which a single thick iron plate is coated with friction material on both sides. There is only one clutch plate in this type. The clutch plate is pressed against the flywheel of the engine by the spring loaded pressure plate. The pressure produced by a number of springs, located between the pressure plate and the housing, which is bolted to the flywheel, holds the friction surfaces firmly in contact. When the pedal of the clutch is depressed, the pressure plate is pushed back by the release fingers. This releases the pressure from the clutch plate and disengages the clutch. Then the clutch plate stops rotating but the fly wheel continues to rotate. When the clutch pedal is released, the pressure plate forces them to turn together as one unit. Thus the power of the engine goes to the gear box for onward transmission to rear wheels. This type of tractor clutch plate is usually foot operated.

Multiple plate clutch:

This may be called multiple disc clutch. It can be twin-disk or three-plate type of clutch. It differs from single-plate clutch is that the flywheel rims does not serve as a friction surface. It may also got a number of thin metal plates, arranged alternately to work as driving and driven members. One set is attached to the fly wheel and the other set is attached to the clutch shaft. If the plates are pressed together, the clutch is said to be engaged and the power is transmitted from the engine to the gear box for onward transmission to the rear wheels. This pressure is obtained by a set of heavy springs, fitted together in a housing.

Engagement and disengagement of this type of clutch is very smooth due to larger surface area of friction members.

Cone Clutch

The principal member of the cone-clutch assembly is a metal disk with a conical peripheral surface that engages with a similarly shaped recess in the flywheel . The cone is faced with ordinary brake lining. A heavy spring placed behind the cone, exerts sufficient pressure to insure its positive engagement. A sleeve fastened to the cone extends back and is bolted to the transmission shaft. Therefore, the engagement if the clutch connects the gears with the engine. Disengagement is produced by sliding the cone and sleeve backward on the flywheel shaft extension against the spring pressure.

Dog clutch:

It is a simple clutch having square jaws, which are used to drive a shaft in either direction. It is mostly used in power tillers.

Fluid coupling:

Fluid coupling consists of a driving member and a driven member - an impeller with radical vanes, housed in a suitable casing. A coupler is mounted on the engine crankshaft and is 3/4th filled with suitable oil. A spring loaded sealing ring is provided to make the driven shaft oil tight. At the rotation of the crankshaft, the oil is thrown out by centrifugal force from the center to the outer edge of the impeller, increasing the velocity and the energy of the oil. It then enters the runner varies at the outer portion and flows towards the center, causing rotation to the runner unit. As long as impeller and runner rotate at different speeds, the oil continues to circulate uniformly but when the impeller and runner start running at same speed, the circulation of oil stops. The coupling does not increase the applied torque but only transmits the torque in an uniform manner.

The main features of fluid coupling are:

Absorption of shock and vibration
Smooth starting and
Easy operation

Reason for Clutch problems

A clutch can be problematic and clutch slippage and dragging can occur because of any of these reasons

Failure to keep clutch fully engaged by keeping the clutch pedal or hand lever partly in the engaged position.
Failure to keep clutch adjusted to compensate for normal wear
Permitting oil and grease to get on friction surfaces
Failure to lubricate the throw out collar and bearings as directed.

If clutch surfaces are not firmly pressed together, there will be some slippage which will cause wear, heat and other effects which will produce slippage even with the lever fully engaged. So, always keep the clutch completely disengaged or completely engaged.. In case of tractor , the best practice if to put the gears in neutral and engage the clutch. Leaving the gears engaged and holding the clutch disengaged will the cause heating and wearing of the throw out collar and may wear the clutch facings. A dragging clutch is that does not completely disengage and is usually caused by incorrect adjustment f the clutch or clear lever. Dragging of the clutch causes failure of the tractor or pulley to stop and clashing of gear when shifting.