Mechanism which enables rotary motion of shaft to be transmitted to the second shaft axis, which is coincident with the first. OR
Clutch is a device to connect driving and driven shafts of a machine, where the driven shaft can be disconnected almost instantaneously from the driving shaft as desired by the operator or driver.
Requirements of clutch:
1. Torque Transmission
2. Gradual Engagement: Clutch should take the drive gradually without occurrence of sudden jerks.
3. Heat Dissipation: During clutch application large amount of heat is generated, the rubbing surfaces should have sufficient area and mass to absorb the heat generated. The design of clutch should ensure proper ventilation or cooling for adequate dissipation of heat.
The friction clutches work on the fact that friction is caused when two rotating discs come into contact with each other.
Principle for friction clutches:
Figure 1: Principle for friction clutch
Let the shaft A and Disc C be revolving at some speed say N rpm. Shaft B and disc D keyed to it are stationary, initially when the clutch is not engaged.
Now apply some axial force W to disc D, so that it comes in contact with Disc C. As soon as the contact is made the force of friction between C and D will come into play and consequently the disc D will also start revolving.
The speed of D depends upon friction force present, which in turn is proportional to the force W applied.
If W is increased gradually, the speed of D will be increased correspondingly till the stage comes when speed of D becomes equal to speed of C.
Then clutch is said to be fully engaged.
Let W = axial load applied. µ= coefficient of friction R= effective mean radius of friction surface. Then, T = W R
Clutches are usually designed based on uniform wear. Since the uniform wear assumption gives a lower torque capacity clutch than the uniform pressure assumption. Designing the clutch using the uniform wear assumption to carry a maximum engine torque will need bigger disc clutch dimensions.
In fully centrifugal type clutches, the springs are eliminated altogether and only centrifugal force is used to apply the required pressure for keeping the clutch in engaged position.
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, as shown in Fig. The outer surface of the shoes is 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 mass 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.
An internal expanding brake consists of two shoes S] and S2 as shown in Fig. The outer surface of the shoes are lined with some friction material (usually with Ferodo) to increase the coefficient of friction and to prevent wearing away of the metal.
Each shoe is pivoted at one end about a fixed fulcrum O1 and O2 and made to contact a cam at the other end. When the cam rotates, the shoes are pushed outwards against the rim of the drum. The friction between the shoes and the drum produces the braking torque and hence reduces the speed of the drum.
Fig. 3: Inner Expanding Brake
Let r = Internal radius of the wheel rim,
b= Width fo the brake lining
p1=Maximum intensity of normal pressure,
pN=Normal pressure,
F1=Force exerted by the cam on the leading shoe, and
F2=Force exerted by the cam on trailing shoe.
Consider a small element of the brake lining AC subtending an angle δθ at the centre. Let OA makes an angle θ with OO1 as shown in fig.
The pressure distribution on the shoe is nearly uniform, however the friction lining wears out more at the free end. Since the shoe turns about O1, therefore the rate of wear of the shoe lining at A.
The following assumptions are implied by the preceding analysis:
External expanding brakes contract to make contact with the rotating drum.
The notation for external contracting shoes is shown in Fig.
Both these equations give positive values for clockwise moments (Fig 16-11) when used for external contracting shoes. The actuating force must be large enough to balance both moments
The horizontal and vertical reactions at the hinge pin are found in the same manner as for internal expanding shoes. They are
By using Eq.(16-8) and Eq. (c) from Sec. 16-2, we have
If the rotation is counter clockwise, the sign of the frictional term in each equation is reversed. Thus Eq. (16-11) for the actuating force becomes
And self-energization exists for counterclockwise rotation. The horizontal and vertical reactions are found, in the same manner as before, to be
The moments of the frictional and normal forces about the hinge pin are the same as for the internal expanding shoes.
When external contracting designs are used as clutches, the effect of centrifugal force is to decrease the normal force.
Thus, as the speed increases, a larger value of the actuating force F is required.
A band brake is a primary or secondary brake, consisting of a band of friction materials that tightens concentrically around a cylindrical piece of equipment either to prevent it from rotating or to slow it.
A band brake consists of a flexible band of leather, one or more ropes, or a steel lined with a friction material, which embraces a part of a circumference of the drum. It is also termed as simple band brake.
In this one end of the band is attached to a fulcrum of the lever while the other end is attached to the lever at the distance b.
When the force P is applied to the lever C, the lever turns about the fulcrum O and tightens the bands and hence the brakes are applied. Band is the wrapped part round a rotating drum.
4.7.1: Construction of band brake:
Brake band is made of a rope or belt band which is lined with a friction material.
Band is wrapped partially around a drum with its free ends to a lever. An external force can be applied to the free ends of this lever for braking.
Due to the external forces, there is a friction between the bands. Due to this friction force, the band is tightening and a tangential force acts on the drum.
This tightness in the band will create the tension in the band, as a result it stops the wheel connected.
Let T1= Tension in the tight side of the band
T2= Tension in the slack side of the band
Θ= Angle of lap, µ= coefficient of friction between the band and the drum.
r= radius of the drum
t= thickness of the band
re= effective radius of the drum=r+t/2
Key takeaway:
Various geometric configuration of drum brakes are illustrated above Drum Brakes are classified based on the shoe geometry. Shoes are classified as being either short or long. A short shoe is one whose lining dimension in the direction of motion is so small that contact pressure variation is negligible.
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