CLUTCH

 Clutch Introduction

 A Clutch is ia machine member used to connect the driving shaft to a driven shaft, so that the driven shaft may be started or stopped at will, without stopping the driving shaft. A clutch thus provides an interruptible connection between two rotating shafts

Clutches allow a high inertia load to be stated with a small power.

A popularly known application of clutch is in automotive vehicles where it is used to connect the engine and the gear box. Here the clutch enables to crank and start the engine disengaging the transmission Disengage the transmission and change the gear to alter the torque on the wheels. Clutches are also used extensively in production machinery of all types

Mechanical Model

Two inertia’s and traveling at the respective angular velocities ωIand I1₁ and ω₂, and one of which may be zero, are to be brought to the same speed by engaging. Slippage occurs because the two elements are running at different speeds and energy is dissipated during actuation, resulting in temperature rise.

 

To design analyse the performance of these devices, a knowledge on the following are required.

1. The torque transmitted

2. The actuating force.

3. The energy loss

4. The temperature rise

FRICTION CLUTCHES

As in brakes a wide range of clutches are in use wherein they vary in their are in use their working principle as well the method of actuation and application of normal forces. The discussion here will be limited to mechanical type friction clutches or more specifically to the plate or disc clutches also known as axial clutches

Frictional Contact axial or Disc Clutches

An axial clutch is one in which the mating frictional members are moved in a direction parallel to the shaft. A typical clutch is illustrated in the figure below. It consist of a driving disc connected to the drive shaft and a driven disc co9nnected to the driven shaft. A friction plate is attached to one of the members. Actuating spring keeps both the members in contact and power/motion is transmitted from one member to the other. When the power of motion is to be interrupted the driven disc is moved axially creating a gap between the members as shown in the figure.

METHOD OF ANALYSIS

The torque that can be transmitted by a clutch is a function of its geometry and the magnitude of the actuating force applied as well the condition of contact prevailing between the members. The applied force can keep the members together with a uniform pressure all over its contact area and the consequent analysis is based on uniform pressure condition

Uniform Pressure and wear

However as the time progresses some wear takes place between the contacting members and this may alter or vary the contact pressure appropriately and uniform pressure condition may no longer prevail. Hence the analysis here is based on uniform wear condition

Elementary Analysis

Assuming uniform pressure and considering an elemental area dA

dA = 2Π.r dr

The normal force on this elemental area is

          dN =2.r.dr.p π

The frictional force dF on this area is therefore

                                                   dF= f.2.r.dr.p π

A single-Surface Axial Disk Clutch

Now the torque that can be transmitted by this elemental are is equal to the frictional force times the moment arm about the axis that is the radius ‘r’

i.e. T = dF. r = f.dN. r = f.p.A.r

= f.p.2.π.r. dr .r

The total torque that could be transmitted is obtained by integrating this equation between the limits of inner radius ri to the outer radius ro

Integrating the normal force between the same limits we get the actuating force that need to be applied to transmit this torque.

Equation 1 and 2 can be combined together to give equation for the torque

Equation 1 and 2 can be combined together to give equation for the torque

Uniform Wear Condition

According to some established theories the wear in a mechanical system is proportional to the ‘PV’ factor where P refers the contact pressure and V the sliding velocity. Based on this for the case of a plate clutch we can state

The constant-wear rate R_w is assumed to be proportional to the product of pressure p and velocity V.

R_w= pV= constant

And the velocity at any point on the face of the clutch is V r. = ω

Combining these equation, assuming a constant angular velocity ω

pr = constant = K

The largest pressure p_max must then occur at the smallest radius r_i ,

Hence pressure at any point in the contact region

In the previous equations substituting this value for the pressure term p and integrating between the limits as done earlier we get the equation for the torque transmitted and the actuating force to be applied.

I.e.The axial force F_a is found by substituting     for p and integrating equation dN= 2prdr = π

Similarly the Torque

Substituting the values of actuating force Fa

The equation can be given as

Single plate-dry Clutch – Automotive application

The clutch used in automotive applications is generally a single plate dry clutch. In this type the clutch plate is interposed between the flywheel surface of the engine and pressure plate.)

Single Clutch and Multiple Disk Clutch

Basically, the clutch needs three parts. These are the engine flywheel, a friction disc called the clutch plate and a pressure plate. When the engine is running and the flywheel is rotating, the pressure plate also rotates as the pressure plate is attached to the flywheel. The friction disc is located between the two. When the driver has pushed down the clutch pedal the clutch is released. This action forces the pressure plate to move away from the friction disc. There are now air gaps between the flywheel and the friction disc, and between the friction disc and the pressure plate. No power can be transmitted through the clutch.

Operation Of Clutches

When the driver releases the clutch pedal, power can flow through the clutch. Springs in the clutch force the pressure plate against the friction disc. This action clamps the friction disk tightly between the flywheel and the pressure plate. Now, the pressure plate and friction disc rotate with the flywheel.

As both side surfaces of the clutch plate is used for transmitting the torque, a term ‘N’ is added to include the number of surfaces used for transmitting the torque

By rearranging the terms the equations can be modified and a more general form of the equation can be written as

T is the torque (Nm).

N is the number of frictional discs in contact.

f is the coefficient of friction

F_a is the actuating force (N).

R_m is the mean or equivalent radius (m).

Note that N = n1 + n2 -1

Where n1= number of driving discs

n2 = number of driven discs

Values of the actuating force F and the mean radius for the two conditions of analysis are summarized and shown in the table

ELECTROMAGNETIC CLUTCHES:

They operate electrically, but transmit torque mechanically. This is why they used to be referred to as electro-mechanical clutches. The electromagnetic clutch is most suitable for remote operation since no linkages are required to control its engagement. It has fast, smooth operation. However, because energy dissipates as heat in the electromagnetic actuator every time the clutch is engaged, there is a risk of overheating. Consequently the maximum operating temperature of the clutch is limited by the temperature rating of the insulation of the electromagnet.

FIGURE 2: An electromagnetic clutch

Engagement

When the clutch is required to actuate, current flows through the electromagnet which produces a magnetic field. The rotor portion of the clutch becomes magnetized and sets up a magnetic loop that attracts the armature. The armature is pulled against the rotor and a frictional force is generated at contact. Within a relatively short time, the load is accelerated to match the speed of the rotor, thereby engaging the armature and the output hub of the clutch. In most instances, the rotor is constantly rotating with the input all the time.

Disengagement                                                                                                                                                      When current is removed from the clutch, the armature is free to turn with the shaft. In most designs, springs hold the armature away from the rotor surface when power is released, creating a small air gap.

Cycling                                                                                                                                                                                  Cycling is achieved by interrupting the current through the electromagnet. Slippage normally occurs only during acceleration. When the clutch is fully engaged, there is no relative slip, assuming the clutch is sized properly, and thus torque transfer is 100% efficient.

Clutch Construction

Two basic types of clutch are the coil-spring clutch and the diaphragm-spring clutch. The difference between them is in the type of spring used. The coil spring clutch shown in left Fig 3.2.6 uses coil springs as pressure springs (only two pressure spring is shown).

The coil-spring clutch has a series of coil springs set in a circle.

At high rotational speeds, problems can arise with multi coil spring clutches owing to the effects of centrifugal forces both on the spring themselves and the lever of the release mechanism.

These problems are obviated when diaphragm type springs are used, and a number of other advantages are also experienced.

Clutch or Driven Plate

More complex arrangements are used on the driven or clutch plate to facilitate smooth function of the clutch

The friction disc, more generally known as the clutch plate, is shown partly cut away in Fig. It consists of a hub and a plate, with facings attached to the plate.

First to ensure that the drive is taken up progressively, the centre plate, on which the friction facings are mounted, consists of a series of cushion springs which is crimped radially so that as the clamping force is applied to the facings the crimping is progressively squeezed flat, enabling gradual transfer of the force

On the release of the clamping force, the plate springs back to its original position crimped (wavy) state
This plate is also slotted so that the heat generated does not cause distortion that would be liable to occur if it were a plain plate. This plate is of course thin to keep rotational inertia to a minimum.

Plate to hub Connection

Secondly the plate and its hub are entirely separate components, the drive being transmitted from one to the other through coil springs interposed between them. These springs are carried within rectangular holes or slots in the hub and plate and arranged with their axes aligned appropriately for transmitting the drive. These dampening springs are heavy coil springs set in a circle around the hub. The hub is driven through these springs. They help to smooth out the torsional vibration (the power pulses from the engine) so that the power flow to the transmission is smooth.

In a simple design all the springs may be identical, but in more sophisticated designs the are arranged in pairs located diametrically opposite, each pair having a different rate and different end clearances so that their role is progressive providing increasing spring rate to cater to wider torsional damping

The clutch plate is assembled on a splined shaft that carries the rotary motion to the transmission. This shaft is called the clutch shaft, or transmission input shaft.

This shaft is connected to the gear box or forms a part of the gear box.

Friction Facings or Pads

It is the friction pads or facings which actually transmit the power from the fly wheel to hub in the clutch plate and from there to the output shaft. There are grooves in both sides of the friction-disc facings. These grooves prevent the facings from sticking to the flywheel face and pressure plate when the clutch is disengaged. The grooves break any vacuum that might form and cause the facings to stick to the flywheel or pressure plate. The facings on many friction discs are made of cotton and asbestos fibres woven or molded together and impregnated with resins or other binding agents. In many friction discs, copper wires are woven or pressed into the facings to give them added strength. However, asbestos is being replaced with other materials in many clutches. Some friction discs have ceramic-metallic facings.

Such discs are widely used in multiple plate clutches.

To minimize the wear problems, all the plates will be enclosed in a covered chamber and immersed in an oil medium. Such clutches are called wet clutches.

Typical characteristics of some widely used friction linings given in the table below

		Table  Properties of common clutch/Brake lining materials
Friction Material Against Steel or Cl		Dynamic Coefficient of friction			Maximum Pressure KPa		Maximum Temperature
	dry		In oil
Molded	0.25-0.45		0.06-0.09	1030-2070		204-260
Woven	0.25-0.45		0.08-0.10	345-0.10		204-260
Sintered metal	0.15-0.45		0.05-0.08	1030-2070		232-677
Cast iron of hard steel	0.15-0.25		0.03-0.06	690-720		260