12. Design of Brakes and Clutches free study material

1. What is Brakes and Clutches

Brakes and clutches are essential machine elements used in power transmission systems. Both work on the principle of friction between contact surfaces

• Brake: Device used to stop or slow down motion by absorbing energy
• Clutch: Device used to engage or disengage power between shafts

2. Difference Between Brake and Clutch

Feature	Brake	Clutch
Function	Stops motion	Transmits power
Energy	Dissipates energy as heat	Transfers energy
Application	Vehicles, machines	Gearboxes, engines

3. Types of Brakes

2. Working Principle

The brake works on the principle of friction. When a force is applied to the brake lever:

1. The brake shoe moves toward the rotating wheel or drum.
2. The friction lining on the shoe comes into contact with the wheel/drum surface.
3. A frictional force develops between the shoe and the rotating member.
4. This friction opposes the motion of the wheel/drum.
5. The kinetic energy of the rotating system is converted into heat energy.
6. As a result, the wheel/drum slows down and eventually stops.

The braking torque produced by a shoe brake is:$$T_B = \mu \times R_N \times r$$Where:

• $T_B$TB​ = Braking torque (N·m)
• $\mu$μ = Coefficient of friction
• $R_N$RN​ = Normal force between shoe and drum (N)
• $r$r = Radius of brake drum (m)

4. Types of Clutches

(a) Single Plate Clutch

• One friction plate
• Common in cars

(b) Multi Plate Clutch

• Multiple plates → higher torque capacity

(c) Cone Clutch

• Conical friction surfaces

(d) Centrifugal Clutch

• Engages automatically with speed

5. Materials for Friction Surfaces

• High coefficient of friction
• Good wear resistance
• Heat resistance

Common materials:

• Asbestos (older, now replaced)
• Sintered metals
• Ceramics
• Organic friction materials

6. Design Considerations

For Both Brakes and Clutches

• Torque to be transmitted
• Coefficient of friction
• Heat generation and dissipation
• Wear resistance
• Operating conditions (dry/wet)
• Safety and reliability

7. Design of Brakes

(a) Braking Torque

$T = \mu W R$T=μWR

Where:

• $\mu$μ = coefficient of friction
• $W$W = normal force
• $R$R = effective radius

(b) Energy Absorbed

$$E = \frac{1}{2} I \omega^2$$E=21​Iω2

• Must be dissipated as heat

(c) Heat Dissipation

• Heat generated = heat dissipated
• Prevents brake fading

8. Design of Clutches

(a) Torque Capacity (Uniform Pressure Theory)

$T = \frac{\mu W}{2} (R_o + R_i)$

(b) Torque Capacity (Uniform Wear Theory)

$T = \mu W R_m$

Where:

• $R_m$ = mean radius

(c) Power Transmitted

$$P = \frac{2\pi N T}{60}$$

9. Uniform Pressure vs Uniform Wear

Aspect	Uniform Pressure	Uniform Wear
Assumption	Pressure constant	Wear uniform
Application	New clutch	Worn clutch
Accuracy	Less realistic	More realistic

10. Failure of Brakes and Clutches

• Overheating
• Excessive wear
• Slippage
• Material failure
• Poor lubrication (for wet clutches)

11. External Sources

• NPTEL Lecture on Design of Brakes and Clutches

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