---
title: "Single plate clutch and diaphragm spring clutch"
subject: "Automotive Technology"
chapter: 5
key: "semester_5/automotive_technology:single-plate-clutch"
source: https://www.bctengineer.com/q/semester_5/automotive_technology/single-plate-clutch
site: BCT Engineering
---

# Single plate clutch and diaphragm spring clutch

Automotive Technology · Chapter 5: Transmission Systems

- Asked in: year not given, 8 marks
- Video: [SINGLE PLATE CLUTCH ASSEMBLY | PART 4  | MODULE 1 | AUTOMOBILE ENGINEERING](https://www.youtube.com/watch?v=thWIEhmAlok) — PRAVEEN KP
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/automotive_technology/single-plate-clutch
- Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-5

## Question

What is the purpose of a clutch in an automobile? With a neat sketch describe the construction and working of a single plate dry friction clutch. How does a diaphragm spring clutch differ from a coil spring clutch?

## Short answer

A clutch connects engine and transmission for smooth starts and gear changes. A single plate dry clutch has a friction disc clamped between flywheel and pressure plate by springs. Diaphragm spring clutch uses a dished disc with fingers, giving constant clamp load, lower pedal effort and compactness, unlike coil spring clutches.

## Answer

#### Purpose

The clutch connects and disconnects the engine from the transmission. It lets the driver (1) start the engine without load, (2) move the vehicle smoothly from rest, (3) change gears, and (4) protect the drive line from shock.

#### Single plate dry clutch

```text
 Engine flywheel   Clutch plate     Pressure plate  Cover
 |==============|  |====lining===|  |=======|=======|
 |  (friction   |<-| disc, hub   |<-| springs push  |
 |   face)      |  | on splined  |  |  plate        |
                    | gearbox in- |  [release lever] <- release
                    | put shaft   |                    bearing
```

**Main parts**
- **Flywheel:** drives the clutch cover and provides one friction surface.
- **Clutch (friction) plate:** a steel disc with friction linings riveted on both sides, and a hub that slides on the splined input shaft of the gearbox. Torsional (damper) springs absorb shocks.
- **Pressure plate:** a heavy cast-iron ring pressed against the clutch plate by springs.
- **Springs, release levers, release (throwout) bearing, and clutch pedal linkage or hydraulic cylinder.**

**Working**
1. *Engaged (pedal released):* the springs clamp the clutch plate between the flywheel and the pressure plate, and torque is transmitted by friction to the gearbox shaft.
2. *Disengaged (pedal pressed):* the release bearing pushes the levers, which pull the pressure plate back; the clutch plate is free and no torque is transmitted.

The torque transmitted is $T = \mu W R_m n$, where $\mu$ is the coefficient of friction, $W$ the axial force, $R_m$ the mean radius and $n$ the number of friction surfaces (2).

#### Diaphragm spring vs coil spring clutch

| Point | Coil spring | Diaphragm spring |
|---|---|---|
| Spring | Several coil springs with separate release levers | One dished steel disc with radial fingers (acts as both spring and release lever) |
| Parts | More | Fewer, compact |
| Pedal effort | Higher; rises as the springs are compressed further | Lower; the spring load falls after a point during release |
| High-speed behaviour | Springs may bow under centrifugal force and lose clamping load | Not affected by centrifugal force |
| Clamp load as lining wears | Falls | Remains almost constant |
| Use | Older and heavy vehicles | Modern cars |

## Check yourself

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

1. Which is a purpose of the clutch?
   A. Increase engine power
   B. Connect and disconnect engine from transmission for smooth starts and gear changes
   C. Cool the transmission
   D. Differentiate wheel speeds
   Answer: B. The clutch lets the engine run without load, engages smoothly, allows gear changes and protects the drive line from shock. (Key point: Purpose of clutch)

2. In the engaged state of a single plate clutch, how is torque transmitted?
   A. Springs clamp the plate between flywheel and pressure plate; friction carries torque
   B. Release bearing pushes levers
   C. Hydraulic fluid couples the shafts
   D. Pressure plate pulled back
   Answer: A. With the pedal released, spring force squeezes the friction plate and torque passes by friction. (Key point: Engaged state)

3. In T = mu W Rm n, what is n for a single plate clutch?
   A. 1
   B. 2
   C. 4
   D. Number of springs
   Answer: B. The plate has linings on both sides, so there are two friction surfaces. (Key point: Torque capacity)

4. What advantage does a diaphragm spring clutch have?
   A. Clamp load stays almost constant as lining wears and pedal effort is lower
   B. More parts
   C. Pedal effort rises with speed
   D. Used on heavy vehicles
   Answer: A. One dished spring acts as spring and lever, so it is compact, with near-constant clamp load and effort, unlike coil spring clutches. (Key point: Diaphragm vs coil)

## Related in Automotive Technology

- [Automatic transmission and torque converter](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aautomatic-transmission-torque-converter) — Automotive Technology, chapter 5
- [Battery ignition system and spark timing calculation](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abattery-ignition-spark-numerical) — Automotive Technology, chapter 3
- [Manual gearbox types and transfer case](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Amanual-gearbox-types-transfer-case) — Automotive Technology, chapter 5

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