---
title: "Clutch versus brake and self-energising"
subject: "Machine Design II"
chapter: 7
key: "semester_7/machine_design_ii:clutch-brake-differences-self-energizing"
source: https://www.bctengineer.com/q/semester_7/machine_design_ii/clutch-brake-differences-self-energizing
site: BCT Engineering
---

# Clutch versus brake and self-energising

Machine Design II · Chapter 7: Clutches and brakes

- Asked in: year not given, 5 marks
- Video: [Self Locking and Self Energizing Brake - Brakes and Dynamometers - Theory of Machine](https://www.youtube.com/watch?v=kVyDOZu07Rs) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/machine_design_ii/clutch-brake-differences-self-energizing
- Whole chapter: https://www.bctengineer.com/md/semester_7/machine_design_ii/chapter-7

## Question

Differentiate between a clutch and a brake. Explain the terms self-energising and self-locking in brakes, and the difference between the leading and trailing shoes of an internal expanding brake.

## Short answer

A clutch transmits torque between rotating shafts; a brake dissipates kinetic energy to stop a member, with one side fixed. Self-energising: friction helps apply the shoe, increasing torque. Self-locking: friction moment ≥ normal moment, causing lock-up without force. Leading shoe is self-energising; trailing shoe is not, ensuring stability.

## Answer

#### Clutch vs brake
| Point | Clutch | Brake |
|---|---|---|
| Function | Connects or disconnects two rotating shafts to transmit power | Slows or stops a moving member by absorbing its energy |
| Members | Both members are free to rotate | One member is fixed (frame) |
| Energy | Energy is transmitted, small slip heat during engagement | Kinetic or potential energy is converted into heat |
| Design basis | Torque capacity, wear | Energy absorption, temperature rise, heat dissipation |
| Example | Plate, cone and centrifugal clutch | Drum, disc and band brake |

#### Self-energising
A brake is **self-energising** when the friction force on the shoe produces a moment about the pivot that helps the actuating force press the shoe against the drum. This increases the normal force and therefore the friction and the braking torque for the same actuating force. Self-energising occurs when the friction moment $M_f$ acts in the same sense as the actuating moment.

#### Self-locking
If the friction moment exceeds the moment of the normal force, $M_f \ge M_N$, the shoe pulls itself into the drum even with zero (or negative) actuating force and the brake **locks** the drum. It is dangerous: it gives jerky braking or cannot be released. It occurs for a large coefficient of friction or when the pivot is badly placed.

#### Leading and trailing shoes
In an internal expanding brake:
- The **leading (primary) shoe** has its pivot at the trailing end, so the drum drags the shoe into contact. It is self-energising: more braking torque for the same force.
- The **trailing (secondary) shoe** has the pivot at the leading end, so the drum rotation tends to drag the shoe away from the drum. It is non-self-energising: friction reduces the pressure.

Hence the leading shoe does more work and wears faster; a brake with a leading and a trailing shoe is stable, which avoids the self-locking risk of two leading shoes.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is the correct difference?
   A. A clutch absorbs energy as heat while a brake connects shafts
   B. A clutch connects two rotating shafts; a brake slows a member against a fixed frame
   C. A brake has both members free to rotate
   D. Both are designed only by energy absorbed
   Answer: B. A clutch joins two rotating members; in a brake one member is fixed and kinetic energy is converted to heat. (Key point: Clutch vs brake)

2. A brake is self-energising when
   A. Friction moment about the pivot acts opposite to the actuating moment
   B. Friction moment about the pivot helps the actuating force press the shoe
   C. The actuating force is zero
   D. The drum is stationary
   Answer: B. The friction force then increases the normal force and braking torque for the same actuating force. (Key point: Self-energising)

3. Self-locking occurs when
   A. M_f <= M_N
   B. M_f >= M_N, so the shoe pulls itself in even with zero actuating force
   C. mu is very small
   D. The shoe is a trailing shoe
   Answer: B. If the friction moment exceeds the normal-force moment the shoe locks the drum. (Key point: Self-locking)

4. The leading shoe differs from the trailing shoe in that
   A. It is non-self-energising
   B. The drum drags it into contact (self-energising), so it does more work and wears faster
   C. It has its pivot at the leading end
   D. It wears slower
   Answer: B. The leading shoe has its pivot at the trailing end; the trailing shoe is dragged away from the drum. (Key point: Leading vs trailing shoe)

## Related in Machine Design II

- [Internal expanding shoe brake analysis](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Ainternal-expanding-shoe-brake) — Machine Design II, chapter 7

## Related in other subjects

- [Disc brakes and drum brakes](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Adisc-brake-vs-drum-brake) — Automotive Technology, chapter 7
- [Self GMD of 4-strand conductor in terms of r](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aself-gmd-four-strands-r) — Power System Analysis I, chapter 4