Clutch: Functions, Types, Mechanism, Maintenance, and Common Faults

The clutch is located inside the flywheel housing between the engine and the transmission. It is a key component of the automotive drivetrain, responsible for connecting or disconnecting engine power to the gearbox. This allows smooth vehicle starts, gear shifting, and protects the transmission system from overload.

The clutch assembly is usually fixed to the rear plane of the flywheel, with its output shaft connected to the transmission input shaft. As a crucial part of mechanical power transmission, the clutch must meet the following requirements:

• Smooth engagement and complete disengagement;
• Easy and light operation;
• Good wear resistance and heat dissipation;
• Compact structure, light weight, and easy adjustment/maintenance.

1. Functions and Working Principle of the Clutch

(1) Functions

1. Ensure smooth vehicle start
   If the engine and transmission were rigidly connected, the vehicle would jerk violently at startup, potentially stalling the engine. The clutch allows gradual torque transfer through friction plates, enabling smooth starts.
2. Facilitate gear shifting
   When gears are constantly under load, they cannot easily disengage or mesh, leading to gear clash. Temporarily separating the engine from the gearbox reduces tooth pressure, making shifting smoother.
3. Protect the drivetrain from overload
   During emergency braking, wheel speed drops suddenly, but the drivetrain inertia remains high, risking component damage. The clutch will slip automatically once torque exceeds the friction limit, preventing overload.
4. Reduce torsional vibration
   Engine torque output is uneven. The clutch’s torsional damper springs absorb shocks, reducing adverse effects on the transmission shaft and prolonging gearbox life.

(2) Working Principle

Clutches transmit torque via friction, hydraulic fluid, or magnetic force. Modern cars predominantly use spring-loaded friction clutches.

• Engaged state: The pressure spring presses the flywheel, driven plate, and pressure plate tightly together, transmitting torque to the gearbox.
• Disengagement: Pressing the clutch pedal moves the release bearing and release lever, overcoming spring force so the driven plate separates from the flywheel.
• Re-engagement: Releasing the pedal allows gradual contact between flywheel, pressure plate, and driven plate, restoring power transmission.

A friction clutch should:

• Transmit maximum engine torque with reserve capacity;
• Disengage completely and engage smoothly;
• Have low driven mass to reduce gearshift shock;
• Operate quietly, reliably, and dissipate heat efficiently.

2. Types and Components of Clutches

(1) Types

Automotive clutches generally fall into three categories:

1. Friction Clutch – Uses friction surfaces between driving and driven parts; most widely used.
2. Hydraulic Coupling – Uses liquid as the medium for torque transfer; provides cushioning and damping; common in automatic transmissions.
3. Electromagnetic Clutch – Engages/disengages using magnetic force; widely used in auxiliary systems such as automotive air conditioning compressors.

(2) Basic Components

Taking the friction clutch as an example, the structure consists of:

• Driving parts: Flywheel, clutch cover, pressure plate.
• Driven parts: Driven plate and driven shaft, with friction linings contacting flywheel and pressure plate.
• Clamping mechanism: Pressure springs or diaphragm spring that ensure reliable torque transmission.
• Release mechanism: Release bearing, fork, and lever, enabling clutch separation.

Driven plates are made from high-friction, heat-resistant materials to ensure durability and stable performance.

3. Clutch Operating Mechanism

The clutch operating mechanism transmits and amplifies the driver’s pedal force to control clutch engagement/disengagement. It mainly includes:

(1) Mechanical Mechanism

• Structure: Uses linkage rods or cables to transmit pedal force.
• Advantages: Simple, inexpensive, reliable.
• Disadvantages: Low efficiency, higher friction losses, affected by frame/body deformation.

(2) Hydraulic Mechanism

• Structure: Includes pedal, master cylinder, reservoir, hydraulic lines, and slave cylinder.
• Principle: Depressing the pedal builds hydraulic pressure in the master cylinder, transmitted via fluid to move the release bearing.
• Advantages: Smooth operation, lightweight, flexible installation.
• Disadvantages: Requires airtight system, regular checks, and fluid maintenance.

(3) Pneumatic-Assisted Hydraulic Mechanism

• Features: Uses compressed air as the primary energy source, with driver input as auxiliary. Common in heavy-duty trucks and long-distance vehicles.
• Requirements:
  • System pressure ≥ 0.22 MPa;
  • Air reservoir pressure ≥ 450 kPa;
  • Good sealing and smooth movement of hydraulic components.
• 

4. Diaphragm Spring Clutch: Disassembly and Inspection

(1) Structure & Features

• Combines the functions of pressure spring and release lever; simplifies design.
• Advantages: Stable pressure, lightweight operation, uniform wear, long service life.
• Disadvantages: Complex manufacturing, high material precision, prone to cracks at slots.

(2) Inspection Points

• Driven plate: Check for cracks, rivet exposure, broken damper springs; face runout ≤ 0.4mm; rivet depth ≥ 0.3mm.
• Pressure plate: No warping or cracks; flatness ≤ 0.2mm.
• Diaphragm spring: Wear depth < 0.6mm, width < 5mm; finger height deviation ≤ 0.5mm.
• Release bearing: Should rotate smoothly without noise; self-aligning mechanism should move freely about 1mm.

(3) On-Vehicle Inspection

• Pedal travel: Free play 30–40mm; total 150–160mm; release stroke 25–30mm.
• Check results:
  • Gear clash when shifting → incomplete disengagement.
  • Engine speed rises but car speed doesn’t → clutch slip.

(4) Disassembly Procedure

1. Disconnect battery for safety.
2. Remove transmission-to-clutch housing bolts.
3. Support transmission, move it back to expose clutch.
4. Loosen clutch cover bolts evenly in diagonal order.
5. Separate driven plate and pressure plate for inspection.

(5) Installation Guidelines

• Use alignment tool to ensure driven plate spline and flywheel concentricity.
• Tighten bolts evenly and gradually to prevent distortion.
• Verify pedal free play and release stroke after installation.

5. Common Clutch Faults and Troubleshooting

(1) Clutch Slippage

• Symptoms: Engine revs increase without speed gain.
• Causes: Worn/oil-soaked friction plates; weak/broken springs; improper release lever adjustment.
• Fix: Replace or clean friction plates; adjust/replace springs; correct lever height.

(2) Incomplete Disengagement

• Symptoms: Gear engagement difficulty, grinding noise.
• Causes: Excessive free play; hydraulic leaks or air in lines; worn release bearing; uneven diaphragm spring fingers.
• Fix: Adjust pedal free play; bleed/repair hydraulic system; replace faulty parts.

(3) Abnormal Noise

• Symptoms: Sharp or knocking sounds during engagement/disengagement.
• Causes: Worn release bearing, damaged springs, loose driven plate rivets.
• Fix: Lubricate or replace bearing; repair/replace damaged components.

(4) Clutch Judder (Shuddering)

• Symptoms: Vehicle shakes during takeoff.
• Causes: Burned/uneven friction lining; warped pressure plate; loose splines.
• Fix: Replace lining or pressure plate; resurface flywheel; repair spline fit.

Extended Reading 

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• Brake Master Cylinder
  Precision casting and sealing design ensure stable and safe braking performance.
• Brake Wheel Cylinder
  Durable pistons and imported seals for efficient and reliable force transmission.
• Clutch Master Cylinder
  OE replacement for multiple brands, fast response, and long lifespan.
• Clutch Slave Cylinder
  Compact structure and stable thrust ensure smooth clutch disengagement.
• Brake Proportioning Valve
  Smartly adjusts front and rear braking force for enhanced vehicle safety.
• Brake Accessories
  Covers reservoirs, push rods, seals, and other key parts with customization support.

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Learn More About Brake Components

• What Is a Brake Master Cylinder? (Comprehensive Guide) — Detailed explanation of its role in converting pedal force into hydraulic pressure.

• What is a Brake Wheel Cylinder? Function, Symptoms, Repair & Cost — Understanding the key component of drum brake systems.

• Clutch Master Cylinder Full Guide: Definition, Symptoms, Repair & Cost — Everything you need to know about clutch hydraulics.

• What Is a Clutch Slave Cylinder? — How it works to disengage and engage clutch systems smoothly.

• What Is a Brake Proportioning Valve? — Why it’s essential for balancing braking force across axles.