02 Clutches

Clutches

Introduction •  A clutch

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enables enables two co-axial co-axial shafts shafts to be engaged or disengaged while at rest or in relative motion. Plate, cone or centrifugal type. Slipping will occur until the two shafts are brought to the same speed. This permits gradual engagement of the driven shaft, such as in a car drive and also limits the torque demanded from the driving shaft.

Introduction •  A clutch

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enables enables two co-axial co-axial shafts shafts to be engaged or disengaged while at rest or in relative motion. Plate, cone or centrifugal type. Slipping will occur until the two shafts are brought to the same speed. This permits gradual engagement of the driven shaft, such as in a car drive and also limits the torque demanded from the driving shaft.

Introduction •

It may be noted that: Contact

surfaces should develop a frictional force that may pick up and hold the load with reasonable low pressure between contact surfaces

Heat

of friction should rapidly dissipate and tendency to grab should be at a minimum

Surfaces

should be backed with a material stiff enough to ensure a reasonably uniform distribution of pressure

Single plate clutches •

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In a plate clutch, torque is transmitted by friction between one or more pairs of coaxial annular faces maintained in contact by an axial thrust Both sides of each plate are normally effective, so that a single-plate clutch has two pairs of surfaces in contact

Single disc or plate clutch

Multiple plate clutches

Multiple plate clutches •

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The inner plates are free to slide axially in grooves connected to the driving shaft and the outer plates are free to slide axially in grooves connected to the driven shaft The axial force exerted on the plates by the toggle mechanism is transmitted through each plate In

a clutch having n pairs of contact surfaces, the torque transmitted is n times that of a single pair 

Forces on a single disc/plate clutch

Plate clutches •

Consider two flat annular surfaces maintained in contact by an axial thrust W 

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Let,

T = torque transmitted p = intensity of pressure between surfaces r 1 = outer radius of face r 2 = inner radius of face R = mean radius of face

Plate clutches

Plate clutches •

If it is assumed that the pressure is uniform over the contact area, then  p is a constant,

Plate clutches •

If it is assumed that the wear is uniform over the contact area, then

Cone clutches •

Consists of one pair of friction faces only, the area of contact being a frustum of a cone

Cone clutches Description

1 Cones 2 Shaft: male cone sliding on splines 3 Friction material: usually on female cone, here on male

4 Spring: brings male cone back after using the clutch control 5 Clutch control: separating both cones by pressing 6 Rotating direction: both directions of the axis are possible

Cone clutches

Cone clutches If p is the normal pressure between surfaces,

Cone clutches If p is assumed constant (pressure constant),

If pr is assumed constant (uniform wear),

Centrifugal clutches •

Usually incorporated into motor pulleys

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Consists of a number of shoes on the inside of a rim of the pulley

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Outer surfaces covered with a friction material

Centrifugal clutches The 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 mass of the shoe, when revolving, causes it To exert a radially outward force (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 in 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.

Centrifugal clutches

Centrifugal clutches To find mass of the shoes.

Consider one shoe of a centrifugal clutch as shown. Let, m = Mass of each shoe n = Number of shoes R  = inside radius of the pulley rim r  = Distance of centre of gravity of the shoe from the centre of the spider  N  = running speed of the pulley in rpm ω = Angular running speed of the pulley in rad/s = 2πN /60 ω1 = Angular speed at which the engagement begins to take place  μ  = Coefficient of friction between the shoe and rim

Centrifugal clutches We know that the centrifugal force acting on each shoe at the running speed, P c  = m.ω2 .r   And the inward force on each shoe exerted by the spring at the speed at which engagement begins to take place, P s = m.ω12 .r  The net outward radial force (centrifugal) with which the shoe presses against the rim at the running speed, = P c  – P s Frictional force acting tangentially on each shoe, F = μ  (P c  – P s)

Centrifugal clutches Frictional torque acting on each shoe, = F x R = μ  (P c  – P s) R  Total frictional torque transmitted, T = μ  (P c  – P s) R x n = n.F.R  From this expression, the mass of the shoes ( m) may be evaluated.

Centrifugal clutches To find size of the shoes.

Let, l  = Contact length of the shoes b = Width of the shoes R = Contact radius of the shoes. It is the same as the inside radius of the pulley rim θ  = Angle subtended by the shoes at the centre of the spider in radians  p = Intensify of pressure exerted on the shoe. In order to ensure reasonable life, the intensity of pressure may be taken as 0.1 N/mm 2

Centrifugal clutches We know that, θ  = l/R or

l = θ .R 

 Area of contact of the shoe,  A = l.b Since the force with which the shoe presses against the rim at the running speed is ( P c  – P s), l.b.p = P c  – P s From this expression, the width of shoe ( b) may be obtained

Example 1: Plate clutch Determine the maximum, minimum and average pressure in plate clutch when the axial force is 4kN. The inside radius of the contact surface is 50 mm and the outside radius is 100 mm. Assume uniform wear.

Example 2: Plate clutch  A single plate clutch, with both sides effective, has outer and inner diameters 300 mm and 200 mm respectively. The maximum intensity of pressure at any point in the contact surface is not to exceed 0.1 N/mm2. If the coefficient of friction is 0.3, determine the power transmitted by a clutch at a speed 2500 rpm.

Example 3: Conical clutch  A conical friction clutch is used to transmit 90 kW at 1500rpm. The semi-cone angle is 20° and the coefficient of friction is 0.2. if the mean diameter of the bearing surface is 375 mm and the intensity of normal pressure is not to exceed 0.25 N/mm 2, find the dimensions of the conical bearing surface and the axial load required.

Example 4: Centrifugal clutch  A centrifugal clutch is to transmit 15 kW at 900 rpm. The shoes are four in number. The speed at which the engagement begins is 3/4 th of the running speed. The inside radius of the pulley rim is 150 mm and the centre of gravity of the shoe lies at 120 mm from the centre of the spider. The shoes are lined with Ferrodo for which the coefficient of friction may be taken as 0.25. Determine: 1. Mass of the shoes, 2. Size of the shoes , if the angle subtended by the shoes at the centre of the spider is 60 ° and the pressure exerted on the shoe is 0.1 N/mm 2.

Example 4: Centrifugal clutch P = 15 kW = 15 x 10 3 W N = 900 rpm, ω = 2π (900)/60 = 94.26 rad/s n=4 R = 150 mm = 0.15 m r = 120 mm = 0.12 m  μ  = 0.25