Friction Clutch

What is a Clutch?

As name implies, there’s some grasping going on here. The clutch is the connection point for the engine and the transmission; the torque converter of manual cars.

A clutch is a mechanical device that provides for the transmission of power (and therefore usually motion) from one component (the driving member) to another (the driven member) when engaged but can be disengaged.

A Friction clutch is a special type of clutch which is used in vehicles to allow the transmission input shaft and engine to run at the same speed when rotating. The friction that is created between the engine and the transmission is what provides the force required to move the vehicle. When the clutch is engaged, the friction plate is sandwiched between the engine flywheel and a steel pressure plate that is bolted to the flywheel. If the pressure between the flywheel and pressure plate is not sufficient, it will allow the friction plate to slip and thus the vehicle will not perform correctly.

What Materials are preferred in designing a clutch?

Various  types of materials have been used for the disc friction facings, including asbestos in the past. Modern clutches typically use compound organic resin with copper wire facing or a ceramic material. Typical coefficient of friction used on a friction disc surface is 0.35 for organic and 0.25 for ceramic. Ceramic materials are typically used in heavy applications such as trucks carrying large loads or racing, though the harder ceramic materials increase flywheel and pressure plate wear.

Desirable properties of material in friction clutches:

• It should have a high coefficient of friction
• Coefficient of friction should not vary with temperature
• It should have good thermal conductivity
• It should divide pressure equally in contacting surfaces
• To should have high resistance to wear
• It should remain unaffected by dirt and moisture

How are the generated vibrations taken care of?

In addition to the damped disc centres which reduce driveline vibration, pre-dampers maybe used to reduce gear rattle at idle by changing the natural frequency of the disc. These weaker springs are compressed solely by the radial vibrations from an idling engine. They are fully compressed and no longer in use once drive is taken up by the main damper springs.

Types of Friction Clutch:

        i.Disc Clutch

a.      Multiple Plate Clutch:

This type of clutch has several driving members interleaved or "stacked" with several driven members. It is used in race cars including F1, IndyCar, World Rally and even most club racing, motorcycles, automatic transmissions and in some diesel locomotives with mechanical transmissions. It is also used in some electronically controlled all-wheel-drive systems.

b.      Single plate clutch 

it is the most common type of clutch plate used in automobiles. It consists only one clutch plate which is mounted on the splines of the clutch plate. The Flywheel is mounted on the engine crankshaft and rotates with it.

       ii. Centrifugal 

A centrifugal clutch is used in some vehicles (e.g., Mopeds) and also in other applications where the speed of the engine defines the state of the clutch, for example, in a chainsaw. This clutch system employs centrifugal force to automatically engage the clutch when the engine rpm rises above a threshold and to automatically disengage the clutch when the engine rpm falls low enough. The system involves a clutch shoe or shoes attached to the driven shaft, rotating inside a clutch bell attached to the output shaft. The shoe(s) are held inwards by springs until centrifugal force overcomes the spring tension and the shoe(s) make contact with the bell, driving the output. In the case of a chain saw this allows the chain to remain stationary whilst the engine is idling; once the throttle is pressed and the engine speed rises, the centrifugal clutch engages, and the cutting chain moves.

     iii.   Cone clutch

As the name implies, a cone clutch has conical friction surfaces. The cone's taper means that a given amount of movement of the actuator makes the surfaces approach (or recede) much more slowly than in a disc clutch. As well, a given amount of actuating force creates more pressure on the mating surfaces.

    iv.   Slip Clutch: (Safety Clutch)

Allows a rotating shaft to slip when higher than normal resistance is encountered on a machine. An example of a safety clutch is the one mounted on the driving shaft of a large grass mower. The clutch will yield if the blades hit a rock, stump, or other immobile object. Motor-driven mechanical calculators had these between the drive motor and gear train, to limit damage when the mechanism jammed, as motors used in such calculators had high stall torque and were capable of causing damage to the mechanism if torque wasn't limited.

Advantages of friction clutch:

• Its engagement is smooth
• No heat generation unless the operation requires frequent starts and stops
• Once engaged there is no slip
• In some cases it works as safety devices because it gets disengaged when torque crosses safety limit 

Disadvantages of a friction clutch

• Slip occurs at the start and stop
• Due to slip wear and tear can happen

Working of Clutch:

                    

The pressure plate is bolted to the flywheel through clutch springs and is free to slide (Move) on the clutch shaft when the clutch pedal is operated (Engage and Disengage).
Engaged: When the clutch is engaged ( means the pedal is not pressed) the clutch is gripped between the flywheel and pressure plate.
The friction linings are on both sides of clutch plate. The clutch plate revolves with the flywheel because of friction between flywheel, clutch plate and pressure plate. The clutch shaft also revolves with the clutch plate. Clutchs shaft is connected to the transmission earbox. Now the engine powet is transmitted to the crankshaft and then to clutch shaft and gearbox.

Disengaged:
When the clutch pedal is disengaged (means you press clutch pedal), the pressure plate moves back against force of springs , and clutch plate becomes free between flywheel and pressure plate.
The pressure plate pulls away from clutch disc, disengaging the clutch, thus interrupting power flow.
The flywheel is always rotating with the crankshaft, Then, the clutch shaft speed reduces slowly and stops rotating.

Applications:
1. Engagement is smooth
2. Slip occurs only during engaging operation and once the clutch is engaged, there is no slip between contacting surfaces. Therefore, power loss and consequent heat generation do not create problems, unless operation requires frequent start and stops.
3. In certain cases, the friction clutch serve as a safety device. It slips when the torque transmitted through it exceeds a safe value. This prevents the breakage of parts in the transmission chain.