Design of Brakes and Clutches

1. Introduction

Brakes and clutches are essential machine elements used in power transmission systems.

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

Both work on the principle of friction between contact surfaces

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

(a) Block or Shoe Brake

• A block presses against a rotating drum
• Simple and commonly used

(b) Band Brake

• Flexible band wraps around a drum
• Types:
  • Simple band brake
  • Differential band brake

(c) Disc Brake

• Friction pads act on a rotating disc
• Widely used in automobiles

(d) Internal Expanding Brake

• Shoes expand inside a drum

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:

• $R_m$Rm​ = 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)

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