What Is Clutch - a Mechanism Designed to Disconnect and Reconnect Driving

CLUTCHES

• A clutch is a mechanism designed to disconnect and reconnect driving and driven members.
•  To obtain a smooth engagement, the clutch has to be progressively engaged to take up the drive until the torque transmitted from the engine equals that required to propel the vehicle.
• Also clutch enables to disconnect the engine from transmission to change gears.
• Thus, the clutch takes up the drive smoothly and also disengages the drive whenever necessary.

CLUTCH FUNDAMENTALS:

1. Main functions:

1. To enable smooth transmission without snatch.
2. To provide a positive linkage for transmitting the maximum engine torque without stalling.
3. To enable rapid engagement and disengagement of the engine from transmission for gear changing and emergency stops.

1. Operation:

• Friction type coupling – accomplished by bringing both the driving and driven members into gradual contact.
• Contact established by strong spring pressure – spring pressure must be sufficient to prevent slippage.
• Therefore, transmission of power depends on: 1. Material in contact (which causes friction), 2. The contact pressure.
• Clutch must provide smooth engagement – no grabbing or chattering.
• Clutch operation depends on: 1. The co-efficient of friction surface, 2. Along with operation of cushion and damper springs.
• The clutch is usually designed to have capacity of 125% to 140% of the maximum engine torque to handle the expected loads.
• Clutch torque requirements become highest during running of the engine with the drive line stationary.
• Slippage occurs during engagement – heat is produced during slippage – heat must be absorbed by clutch material and then to dissipate to atmospheric air.
• The amount of heat absorbed by the clutch is proportional to the time required to increase output speed to input speed.
• Clutch temperature is the major limiting factor in clutch capacity.
• Inertia is another important factor – if the out put member is too heavy, its inertia keeps it spinning after releasing of the clutch, causing hard shifting and gear clashing.
• Centrifugal forces on the clutch increase at a ratio of 4 times the engine speed – if the centrifugal forces greater than the stregth of the parts, the clutch can fly apart and shatter.
• Clutch burst speed is designed at twice the expected maximum speed.

1. Desired features:

• The force required by the clutch to separate the drive must not be excessive.
• The clutch friction surface should maintain a reasonable co-efficient of friction under all operating conditions.
• The rubbing surfaces of the clutch must be correctly machined and be hard enough to resist wear but not so hard to cause scoring.
• The rubbing surfaces must provide adequate surface area and mass to transfer and absorb the heat generated.
• It should have provision for adequate cooling or ventilation to dissipate generated heat.
• The clutch material should have reasonable thermal conductivity to dissipate the heat so that distortion of the flywheel and pressure plate is avoided.
• It should use a friction material, which must withstand high temperatures and clamping loads without crushing.

1. Main Components:

• Divided into 3 main parts: 1. Driving member, 2. Driven member and 3. Operating member.
• 1. Driving member: uses pressure-plate assembly and the flywheel both bolted together such that the assembly rotates at the speed of the engine.

Explain construction of pressure plate: coil pressure spring type and diaphragm spring type.

Maximum driving force or driving torque occurs just before the disc slips while it is clamped between the pressure plate and flywheel.

• 2. Driven member: consists of a disc or plate assembly and the clutch shaft.

Explain the construction and working.

Cushioning device and a torsional vibration damping unit.The torsional device absorbs the torsional vibrations of the crankshaft so that they are not transmitted further.

• 3. the operating member: is consisted of release bearing, release lever, foot pedal, linkage and the spring with adjuster.

Explain the construction and working.

PLATE CLUTCH:

        Fig: to refer.

• Explain with fig.
• The driven assembly contains one clutch disc, the clutch is known as single plate clutch.This has an advantage of producing a quick disengagement.
• The double plate clutch is basically same as the single plate clutch except that another driven disc and an intermediate driving disc are added.
• Multi plate clutch contains more than three discs. Due to higher frictional area through many number of plates, this clutch has a greater capacity to transmit the power. It consists of alternate driving and driven discs.

TORQUE TRANSMITTED:

        Fig: to refer.

• Ref hand written sheets for derivations.

[pic 1]

Problem 1:

An automobile power unit gives a maximum torque of 13.56 Nm. The clutch is of a single plate dry disc type, having effective clutch lining of both sides of the plate disc.The coefficient of friction is 0.3 and the maximum axial pressure is 8.29x104 Pa, and external radius of friction surface is 1.25 times the internal radius.

Calculate the dimensions of the clutch plate and the total axial force that must be exerted by the clutch springs.

Problem 2:

A motor car engine develops 5.9kW at 2100 rpm. Find the suitable size of clutch plate having friction linings riveted on both sides to transmit the power, under the following conditions:

1. Intensity of pressure on the surface not to exceed 6.87X104 Pa.
2. Slip torque or losses due to wear etc. is 35% of engine torque.
3. Coefficient of friction on contact surface is 0.3.
4. Inside diameter of the friction plate is 0.55 times the outside diameter.

[pic 2]

[pic 3]

[pic 4]

[pic 5]

[pic 6]

      [pic 7]         

CLUTCH CONSTRUCTION:

1. Coil spring Clutch
2. Diaphragm Clutch

[pic 8]

[pic 9]

CLUTCH OR DRIVEN PLATE:

• It consists of hub and a plate, with facings attached to the plate.

[pic 10]

Main types of clutches:

1. Friction clutches
2. Fluid flywheel.

General dry friction clutches are:

1. Cone clutch
2. Single plate clutch
3. Multi plate clutch
4. Semi centrifugal clutch
5. Centrifugal clutch.

CONE CLUTCH:

• The contact surface between the driving and driven members forms a part of the cone.
• The effectiveness of the clutch is increased due to wedging action of the cone.
• So normal force on the lining increases.

Advantages:

The only advantage of the cone clutch is that the normal force acting on the contact surfaces 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.