Clutch Drives by S Vijayarangan Dept of Mechanical
Clutch Drives by S. Vijayarangan Dept. of Mechanical Engineering, Dr. Mahalingam College of Engineering and Technology
INTRODUCTION A clutch is a mechanism (or a device) used to connect a machine shaft with a prime mover shaft so as to transmit power either fully or partially, at will. The role of a clutch is 1. To disconnect the load from the prime mover and allow the later to be started easily. 2. To enable gradual transfer of load on to the prime mover (and there by reduce transmission shocks). 3. To enable changing of machine shaft speed without switching off the prime mover. 4. To act as a mechanical fuse, whenever the machine is overloaded, safeguard the prime mover by way of slipping.
1. Positive engagement clutches: 8 Motor and machine shafts are either fully connected or fully disconnected. No partial engagement is possible. 8 Engagement of shafts involve light to heavy shock loads. 8 Generally preferred in m/c tools. Jaw clutches and Claw clutches are Toothed clutches, used to connect two shafts in-line. 1. a. Jaw or Claw clutches : Square Jaw Clutch Spiral Jaw Clutch Toothed Clutch or Claw clutch
Jaw or claw clutches are 8 Designed for bending strength. 8 Checked for shearing and crushing. Advantages: 8 Positive engagement No slip. Hence equal angular velocity. 8 Smaller in proportions. 8 Simpler in design. 8 Easy to manufacture & hence less cost. Disadvantages: Ø Can be engaged & disengaged only at low peripheral speeds and smaller loads. Ø Susceptible to tooth breakage because of shock loads.
Square Jaw Clutch Spiral Jaw Clutch Working of Jaw Clutch Design Dimensions of Jaw Clutch
1. b. Over – running clutches: Æ Preferred and used in places where automatic engagement & disengagement are required at pre-determined speeds. Æ Accuracy of manufacturing required is high. Expensive Direction of disengagement Direction of Rotation of Driver Driven Member Direction of Engagement Driving Member
An Over Running Clutch in Action Roller Type ORC Sprag Type ORC
2. Friction clutches: ÆFrictional contact established between the driving and the driven members enables power transmission. ÆTorque transmitted by the frictional contacting surfaces, T = P rm where is friction coefficient P is axial force rm is mean (frictional) radius. Friction clutches are further classified as: 8 Plate clutch 8 cone clutch 8 Block or shoe clutch 8 Band & spring clutch
A Diaphragm Spring Clutch A Coil Spring Clutch
Details of various elements in a coil spring clutch
Details of another type of coil spring clutch
Details of clutch plate in a coil spring clutch
Exploded view of a coil spring clutch Different types of clutch plates
Exploded view of a diaphragm spring clutch
Types of plate clutches based on the mode of transmitting the effort: 1. Manually operated clutch Single lever type and Toggle lever type 2. Automatic clutch Centrifugal clutch and Fluid clutch 3. Power operated clutch Electromagnetic clutch and Eddy current clutch Details of clutch actuating mechanism
Torque Transmitting Capacity of a Clutch The reaction pressure p depends on the nature of friction faces in contact; accordingly they are classified as :
a) Uniform Pressure clutch: 8 Reaction pressure is assumed to be uniformly distributed and hence p is constant. 8 This is possible only when the friction facings are soft like cork or paper and/or when the clutch is new. 8 Since p is constant, equation (1) becomes and total axial force, P, resulting because of pressure p will be
b. Uniform Wear Clutch: 8 The assumption of uniform pressure distribution is not valid after a certain time of usage of the clutch. Uniform Pressure Uniform Wear 8 In reality, the friction facings are sufficiently hard and hence they can not provide uniform pressure distribution. 8 Also the sliding velocity at the outer periphery will be larger
8 In reality, the friction facings are sufficiently hard and hence they can not provide uniform pressure distribution. 8 Also the sliding velocity at the outer periphery will be larger The rate of wear is dependent on the work done by the friction. i. e. if is constant then work αl to (p. V) i. e. W αℓ {p(w. r)} αℓ (pr) = a constant say c. w is constant. i. e. After initial wear, when equilibrium condition is reached, p. r = constant c will occur. Thus p=(c/r)
The uniform wear case gives higher values for the axial force P, which are on the safer side. Hence the usage of uniform wear equation is in common use.
Multiple plate clutch: à When the torque to be transmitted is more, or the radial space available is small, multiple plate clutches are preferred. Correction for variation of ‘ ’ with ‘r’: 8 In the derivation for equations of T, it was assumed that w is constant. i. e. friction is independent of sliding velocity and the radius r. 8 However, in practice, it is found that friction is a decreasing function of linear sliding velocity and is given by where o = static coefficient friction, w = relative angular velocity between the two rotating plates of the clutch and m is slope of friction – speed characteristics of the faces in contact. Because of this, the equation for torque capacity is getting changed as
A Multiple Plate Clutch
In general, the following values are recommended for use = 0. 2 to 0. 3 for dry asbestos facings = 0. 1 to 0. 01 for plates immersed in oil.
P = 1 to 1. 2 kgf/cm 2 for heavy duty vehicles with frequent starts = 1. 5 to 2 kgf/cm 2 for light commercial vehicles = 2 to 2. 5 kgf/cm 2 for passenger cars. Material of friction faces Operating condition kgf/cm 2 Max. operating temp o. C p Pressed asbestos Dry 0. 3 2. 0 to 3 250 Pressed asbestos CI or Steel Dry 0. 3 2 to 3 250 CI CI or Steel Dry 0. 15 2. 5 to 4 250 Powdered metal CI or Steel Dry 0. 4 3 550 Hardened Steel In oil 0. 08 6 -8 250 CI CI or Steel In oil 0. 06 6 -8 250 Bronze CI or Steel In oil 0. 05 4 150
Some points to be considered while designing a clutch: Clutch design includes determination of Clutch plate ri, ro & t, material Friction facing r. I, ro, t, material Coil spring, d, D, no. of coils, free length material. Pressure plate ri, ro, t, material Release lever no. of levers, Z, l, a, b Cover plate, shape and size
Addition features to be considered and incorporated: The moving parts of the clutch should be designed as light as possible to minimize the inertia load. Projecting parts are to be avoided as far as possible or else should be covered by appropriate guard/s. Provision for wear adjustment must be provided. No external focus should be required to maintain contact of friction surfaces. Adequate provision for heat dissipation should be made available.
# 1. A single plate dry clutch with both sides active is required to transmit a power of 22 k. W at 1500 rpm. Six springs are to be used to provide the friction contact between the mating surfaces. The friction facings provide a friction coefficient as 0. 25. An axial force of 10 k. N is generated by six coil springs placed around the facings. # 2. A friction clutch is required to transmit 25 k. W at 2000 rpm. The clutch is be of uniform wear type with two active friction surfaces. The working pressure is created by springs and is limited to 70 k. N/mm 2. The maximum space of 300 mm in diameter is available to accommodate the clutch. The friction coefficient of the lining is estimated as 0. 285.
A single plate Clutch is required
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