Machine Design - Design of Clutches

Brown Hills College of Engineering & Technology

Machine Design - 1

UNIT – 7______________________________________________ D e s i g n o f C l u t c h e s Various types of clutches in use, Torque transmitting capacity, Design of friction clutches – Disc, Multidisc, Cone & Centrifugal.  Introduction A Clutch is a 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 The driven shaft should be disengaged from the driving shaft. The engagement and disengagement of the shafts is obtained by means of a clutch which is operated by a lever.  Function of clutches: 1. To permit engagement or disengagement of a gear when the vehicle in stationary and the engine is running. 2. To transmit the power to the wheels smoothly without shock to the transmission system while selting the vehicle motion. 3. To permit the engine of the gears when the vehicle is in motion without the gear wheel. The amount of torque a clutch can transmit depends on the co-efficient of friction between the friction facings and their mating surfaces, the mean radius of the facings, the number of facings in contact, and the total spring force. Increasing the diameter of a clutch increases its torque capacity, as does increasing the spring force. Two or more clutch plates can be used to form a multi-plate clutch, increasing the number of facings, and torque capacity. They are useful where a reduction in diameter is advantageous or where increasing the spring strength is undesirable.  Main part of the clutch: 1. Driving member 2. Driver member 3. Operating member Sachin Chaturvedi Lecturer in Department of Mechanical Engineering Notes also available at www.sachinchaturvedi.wordpress.com For your assistance write an email on email ID. E-mail: [email protected]

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Machine Design - 1

1. Deriving member: it consists of a flywheel mounted on the engine crankshaft. 2. Deriver member: it consists of a disc or plate, called clutch plate. 3. Operating member: it consists of a foot pedal, linkages, bearing, release lever, spring etc.

 Chart for the types of clutches: Clutch

Positive Clutch

Diaphram Clutch

Hydraulic Clutch

Electromagnetic Clutch

Vaccum Clutch

Over Running Clutch

Gradual Eengagement Clutch

Dog or Spline Clutch Friction Clutch

Cone Clutch

Internal Clutch

External Clutch

Disc or Plate Clutch

Single Plate Clutch

Wet Clutch

Fluid Clutch or Fluid Flywheel

Centrifugal Clutch

Semi-Centrifugal Clutch

Multiple Plate Clutch

Dry Clutch

 Types of Clutches: The two main types of clutches commonly used in engineering practice: 1. Positive Clutches. 2. Friction Clutches. 1. Positive Clutches: The positive clutches are used when a positive drive is required. The simplest type of a positive clutch is a jaw clutch. The jaw clutch permits one shaft to drive another through a direct contact of interlocking jaws. It consists of two halves, one of which is permanently fastened to the driving shaft and the other half of the clutch is movable and it is free to slide axially on the driven shaft, but it is prevented from turning relatively to its shaft. The jaws of the clutch may be of square and spiral type as shown in Fig. (a, b). Sachin Chaturvedi Lecturer in Department of Mechanical Engineering Notes also available at www.sachinchaturvedi.wordpress.com For your assistance write an email on email ID. E-mail: [email protected]

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Machine Design - 1

(a) A square jaw type is used where engagement and disengagement in motion and under load is not necessary. This type of clutch will transmit power in either direction of rotation. (b) The spiral jaws may be left-hand or right-hand, because power transmitted by them is in one direction only. This type of clutch is occasionally used where the clutch must be engaged and disengaged while in motion. The use of jaw clutches are frequently applied to sprocket wheels, gears and pulleys. In such a case, the non-sliding part is made integral with the hub . 2. Friction Clutches: The friction force is used to start the driven shaft from rest and gradually brings it up to the proper speed without excessive slipping of the friction surfaces. In automobiles, friction clutch is used to connect the engine to the drive shaft. In operating such a clutch, care should be taken so that the friction surfaces engage easily and gradually bring the driven shaft up to proper speed. The proper alignment of the bearing must be maintained and it should be located as close to the clutch as possible. Types of Friction Clutches: There are three types of friction clutches as following: 1. Disc or Plate Clutches (Single Disc or Multiple Disc Clutches) 2. Cone Clutches 3. Centrifugal Clutches  Friction between two surfaces depends upon the following are: 1. Area of the surfaces 2. Pressure applied 3. Coefficient of friction of the surface material  Types of friction material are: 1. Leather: Coefficient of friction , µ = 0.3 2. Fabric: Coefficient of friction , µ = 0.4

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Machine Design - 1

Torque Transmitting Capacity (TTC): A friction disk of a single plate clutch is shown in figure: Let, D and d = outer and inner diameter of friction disk p = Intensity of axial pressure F = Total operating force T = Torque transmitted by the clutch r = Mean radius of the friction face, and μ = Coefficient of friction. Consider an elementary ring of radius r and radial thickness dr as shown in Fig. We know that for this ring:

Fig: c Area = 2π r.dr Axial force = Pressure × Area = p × 2π r.dr Frictional force = μ × F= μ.p × 2π r.dr Frictional torque = μ.p × 2π r.dr × r = 2 π μ.p r2 dr There are two criteria to obtain torque capacity as follows: 1. When there is a uniform pressure (Uniform Pressure Theory). 2. When there is a uniform wear (Uniform Wear Theory). 1. Uniform Pressure Theory: In case of new clutches employing a number of springs, the pressure can be assumed to be uniformly distributed over the entire surface area of the friction disk or plate as shown in Fig. (c), then the total operating fore is,

…………………… (1)

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Machine Design - 1 …………………… (2)

Divide eq. (2) by eq. (1) we get;

…………………… (3) The above equations have been derived for a single pair of mating surfaces. When there is a number of friction surfaces in contact, as in the case of the multi-disk clutch, eq. (3) should be multiplied by the number of pairs of contacting surface to obtain the resultant torque transmitting capacity. 2. Uniform Wear Theory: It is assumed that wear is uniformly distributed over the entire the surface area of the friction disk. The basic principle in designing machine parts that are subjected to wear due to sliding friction is that the axial wear is proportional to the friction work. The work done by the friction force is proportional to the product of frictional force (µp) and the sliding (rubbing) velocity (2 ) where N is speed in rev/min. Therefore, Wear

(µp).(2

Assuming wear, speed N and the coefficient of friction µ is constant for a given configuration then, Wear It may be noted that when the friction surface is new, there is a uniform pressure distribution over the entire contact surface. This pressure will wear most rapidly where the sliding (rubbing) velocity is maximum and this will reduce the pressure between the friction surfaces. Let p be the normal intensity of pressure at a distance r from the axis of the clutch. Since the intensity of pressure varies inversely with the distance r, therefore

The wear at the outer radius will be more, which will release the pressure at the outer edge due to the rigid pressure plate. This will change the pressure distribution.

Where pa is the intensity of pressure at the inner edge, which is also the maximum intensity of pressure. From eq. (1);

………………….. (4)

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Machine Design - 1 ….……… (5)

Divide eq. (5) by eq. (4) we get;

.…………………..…… (6) The above equation gives the torque transmitting capacity for a single pair of contacting surface. The uniform pressure theory is applicable only when the frication lining (A protective covering that protects an inside surface) is new. When lining is put into service, wear occurs. Therefore, the major portion of the life of frication lining comes under the uniform wear criterion. In the design of clutches the uniform wear theory if justified. TTC can be increased by three methods:

1. Increases the coefficient of friction ( 2. Increases the plate pressure (p) 3. Increases the mean radius of the friction disk  Single Disc or Plate Clutch A single disc or plate clutch, as shown in Fig d, consists of a clutch plate whose both sides are faced with a frictional material. It is mounted on the hub which is free to move axially along the splines of the driven shaft.

fig: d The pressure plate is mounted inside the clutch body which is bolted to the flywheel. Both the pressure plate and the flywheel rotate with the engine crankshaft or the driving shaft. The pressure plate pushes the clutch plate towards the flywheel by a set of strong springs which are arranged radially inside the body. Sachin Chaturvedi Lecturer in Department of Mechanical Engineering Notes also available at www.sachinchaturvedi.wordpress.com For your assistance write an email on email ID. E-mail: [email protected]

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When the clutch pedal is pressed down, its linkage forces the thrust release bearing to move in towards the flywheel and pressing the longer ends of the levers inward. The levers are forced to turn on their suspended pivot and the pressure plate moves away from the flywheel by the knife edges, thereby compressing the clutch springs. This action removes the pressure from the clutch plate and thus moves back from the flywheel and the driven shaft becomes stationary. On the other hand, when the foot is taken off from the clutch pedal, the thrust bearing moves back by the levers. This allows the springs to extend and thus the pressure plate pushes the clutch plate back towards the flywheel. If the torque due to this frictional force exceeds the torque to be transmitted, then no slipping takes place and the power is transmitted from the driving shaft to the driven shaft.  Multi-disk Clutch A multiple disc clutch, as shown in Fig. e, may be used when a large torque is to be transmitted. The inside discs are fastened to the driven shaft to permit axial motion. The outside discs are held by bolts and are fastened to the housing which is keyed to the driving shaft. The multiple disc clutches are extensively used in motor cars, machine tools etc.

Fig: e  Cone Clutch A cone clutch, as shown in Fig. f, was extensively used in automobiles, but now-a-days it has been replaced completely by the disc clutch. It consists of one pair of friction surface only. In a cone clutch, the driver is keyed to the driving shaft and has an inside conical surface or face which exactly fits into the outside conical surface of the driven. The driven member resting on the feather key in the driven shaft, may be shifted along the shaft by a forked lever provided at B, in order to engage the clutch by bringing the two conical surfaces in contact. Sachin Chaturvedi Lecturer in Department of Mechanical Engineering Notes also available at www.sachinchaturvedi.wordpress.com For your assistance write an email on email ID. E-mail: [email protected]

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Due to the frictional resistance set up at this contact surface, the torque is transmitted from one shaft to another. In some cases, a spring is placed around the driven shaft in contact with the hub of the driven. This spring holds the clutch faces in contact and maintains the pressure between them, and the forked lever is used only for disengagement of the clutch.

Fig: f The semi-cone angle α is always kept greater than the angle of static friction to avoid self engagement. The recommended semi-cone angle is 12.5°. The cone are easy to disengage and simple in construction. Their main drawback is the strict requirement for the coaxiality of two shafts. The equations for the torque transmitting, capacity of the cone clutch are derived in a manner similar to that of a single-plate clutch. The dimensions of the friction cone are shown in Fig. g. An elementry ring of the cone bounded by circles of radii rand (r + dr) is considered. For thll elementry ring:

Fig: g

 Design of a Cone Clutch: From fig. (A) Area, Sachin Chaturvedi Lecturer in Department of Mechanical Engineering Notes also available at www.sachinchaturvedi.wordpress.com For your assistance write an email on email ID. E-mail: [email protected]

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Brown Hills College of Engineering & Technology Normal Force = Friction Fore =

xp=2

Machine Design - 1

xp pxµ

Friction Tourqe =

pµ x r

……………………(1)

From fig. (B) Axial Force

=p

sin α = =

p (sin α)

Intregrating equation (1) and (2) with boundry conditions

……………………(2)

, we get

T=

……………………(3)

F=2

……………………(4)

There are two criteria to obtain torque capacity as follows: 1. When there is a uniform pressure (Uniform Pressure Theory). 2. When there is a uniform wear (Uniform Wear Theory).

1. Uniform pressure theory From this criterion p is constant, equations (3) and (4) intregrating separately with there limits/boundry conditions, and then results manipulates, we get T=

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Machine Design - 1

2. Uniform wear theory From this criterion pr is constant, put pr = p a (d/2) in equations (3) and (4), and then intregrating separately with there limits/boundry conditions, and then results manipulates, we get T=  Centrifugal Clutch The centrifugal clutches are usually integrated into the motor pulleys. It consists of a number of shoes on the inside of a rim of the pulley, as shown in Fig. (h). The outer surfaces of the shoes are covered with a friction material. These shoes, which can move radially in guides, are held against the boss (or spider) on the driving shaft by means of springs. The springs exert a radially inward force which is assumed constant. The weight of the shoe, when revolving causes it to exert a radially outward force (i.e. centrifugal force). The magnitude of this centrifugal force depends upon the speed at which the shoe is revolving. A little consideration will show that when the centrifugal force is less than the spring force, the shoe remains in the same position as when the driving shaft was stationary, but when the centrifugal force is equal to the spring force, the shoe is just floating. When the centrifugal force exceeds the spring force, the shoe moves outward and comes into contact with the driven member and presses against it. The force with which the shoe presses against the driven member is the difference of the centrifugal force and the spring force. The increase of speed causes the shoe to press harder and enables more torque to be transmitted.

Fig: (h) Sachin Chaturvedi Lecturer in Department of Mechanical Engineering Notes also available at www.sachinchaturvedi.wordpress.com For your assistance write an email on email ID. E-mail: [email protected]

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 Design of a Centrifugal Clutch In designing a centrifugal clutch, it is required to following: 1. To determine the mass of the shoe 2. To determine the size of the shoe 3. To determine the dimensions of the spring. fig: (i) 1. Mass of the Shoe: Consider one shoe of a centrifugal clutch as shown in Fig. (i). Let, m = Mass of each shoe, n = Number of shoes, r = Distance of centre of gravity of the shoe from the centre of the spider, R = Inside radius of the pulley rim, N = Running speed of the pulley in r.p.m., ω = Angular running speed of the pulley in rad / s = ω1 = Angular speed at which the engagement begins to take place, and μ = Coefficient of friction between the shoe and rim. We know that the centrifugal force acting on each shoe at the running speed, Pc = m.ω2.r Since the speed at which the engagement begins to take place is generally taken as 3/4th of the running speed, therefore the inward force on each shoe exerted by the spring is given 2 by: Ps = m.(ω1)2.r = m.( )2.r = m. .r Net outward radial force (i.e. centrifugal force) with which the shoe presses against the rim 2 at the running speed = Pc - Ps = (m.ω2.r) – (m. .r) Frictional force acting tangentially on each shoe, F = μ (Pc – Ps) Frictional torque acting on each shoe = F × R = μ (Pc – Ps) R Total frictional torque transmitted, T = μ (Pc – Ps) R × n = n.F.R From this expression, the mass of the shoes (m) may be evaluated. 2. Size of the shoes Let,

ℓ = Contact length of the shoes, b = Width of the shoes,

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Machine Design - 1

R = Contact radius of the shoes. It is same as the inside radius of the rim of the pulley, θ = Angle subtended by the shoes at the centre of the spider in radians, and p = Intensity of pressure exerted on the shoe. In order to ensure reasonable life, it may be taken as 0.1 N/mm2. We know that θ =

or ℓ = θ.R =

.... (Assuming θ = 60° = rad)

∴ Area of contact of the shoe = ℓ . b The force with which the shoe presses against the rim = A × p = ℓ . b . p Since the force with which the shoe presses against the rim at the running speed is (Pc – Ps), Therefore, ℓ . b . p = Pc – Ps From this expression, the width of shoe (b) may be obtained. 3. Dimensions of the spring We have discussed above that the load on the spring is given by 2 Ps = (m. .r) The dimensions of the spring may be obtained as usual.

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