Clutches | Mechanical Engineering SSC JE (Technical) PDF Download

CLUTCHES

• A clutch is a machine member used to connect a or disconnect driving shaft to a driven shaft so that the driven shaft may be started or stopped at will, without stopping the driving shaft.
   
• Positive Clutch 
  • The positive clutches are used when a positive drive is required. The simplest type of a positive clutch is a jaw clutch. 
  • 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.
    
    
     
  • The spiral jaw may be left handed 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.
    
     
• Friction Clutches 
  • A friction clutch has its principal application in the transmission of power of shafts and machines which must be started and stopped frequently. In automobiles, friction clutch is used to connect the engine to the drive shaft. 
  • Material for friction surface. The material used for the friction clutch have following characteristic :
    • It should have a high and uniform coefficient of friction.
    • It should not be affected by moisture and oil.
    • It should have the ability to with no space stand high temperature caused by slippage.
    • It should have high heat conductivity.
    • It should have high resistance to wear and scoring.
       
• Single Disc or Plate Clutch 
  • A single disc or plate clucth 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.
    The axial pressure exerted by the spring provides a frictional force in the circumferential direction when the relative motion between the driving and driven members tends to take place. 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.
• Multiple Disc Clutch 
  • A multiple disc clutch 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 multiple disc clutches are extensively used in motor cars, machine tools etc.

Design of Disc or Plate Clutch

Let, T = torque transmitted by the clutch
p = intensity of axial pressure with which the contact surface are held together,
r₁ and r₂ = External and internal radius of friction faces
r = Mean radius of the friction faces
µ = coefficient of friction.
Frictional force on the ring acting tangentially at radius, x
F_x = µp.2px.dx Frictional Torque acting on the ring,

T_x = F_x.x = 2pµ.p.x².dx,

• Considering Uniform Pressure (Used for new clutches)

where, W = Axial thrust with which the frictional surfaces are held together.

Total frictional torque acting on the friction surface or on the clutch,

T = µ.W.r
where, r = mean radius of the friction surface

• Considering Uniform Axial Wear (Used for worn - out clutches) 
  • The basic principle in designing machine parts that are subjected to wear due to sliding friction is that the normal wear is proportional to the work of friction. The work of friction is proportional to the product of normal pressure (p) and sliding velocity (V). Therefore Normal wear µ work of friction
    µ p.V
    or pV = K (a constant) 
  • 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 velocity is maximum and this will reduce the pressure between the friction surfaces. This wearing in process continues until the product p.V is constant over entire surface.

Let, p, is the pressure intensity at a distance x from the axis of the clutch. p.x = c (a constant)
Total force acting on the friction surface,

Total frictional torque acting on the friction surface


mean radius of the friction surface. 

• In general, total frictional torque acting on the friction surfaces.

T = n µW.r
where, n = Number of pairs of frictional surfaces.
r = mean radius of friction surface 

• For a single disc or plate clutch, normally both sides of the disc are effective. Therefore a single clutch has two pairs of surfaces in contact, (n = 2).  In case of a multiple disc clutch, if there are n₁ number of discs on the driving shaft and n₂ number of discs on the driven shaft, then the pairs of contact surfaces,
  n = n₁ + n₂ – 1
• In case of a new clutch, the intensity of pressure is approximately uniform, but in an old clutch the uniform wear theory is more approxmate.  The uniform pressure theory gives a higher friction torque than the uniform wear theory. Therefore in case of friction clutches uniform wear should be considered, unless stated.
   

Cone Clutch 

• A cone clutch was extensively used in automobiles, but 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 by a sunk key and has an inside conical surface or face which exactly fits into the outside conical surface of driven.
  The material of the clutch faces depends upon the allowable pressure and coefficient of friction.
   
• For uniform pressure, total frictional torque produced by the clutch

• For uniform wear

• The above equations are valid for steady operation of the clutch and after the clutch is engaged. 
• If the clutch is to be designed for free disengagement, the value of tan a must be greater than µ. In case the value of tan a is less than µ, the clutch will not disengage itself and axial force required to disengage the clutch is, 

Centrifugal Clutch 

• The centrifugal clutches are usually incorporated into the motor pulleys. It consists of a number of shoes on the inside of a rim of the pulley The weight of the shoe, when revolving causes it to exert a radially outward force. 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. The increase of speed causes the shoe to press harder and enables more torque to be transmitted