# Hydraulic Clutch System

How a car's hydraulic clutch pedal sends fluid pressure through the master and slave cylinders to lift the pressure plate, and what trapped air does to it\.

By Thyurdin

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/8DSXAgpEE9az)

## Transcript

### 1. A clutch joins two spinning halves

A car engine never stops turning while it runs, but the wheels must stop at traffic lights\. The clutch sits between the engine and the gearbox\. Press the pedal and the two halves come apart; let it up and they join again\.

Drag anywhere to turn the model and look around it\.

### 2. A disc squeezed by a strong spring

The flywheel spins with the engine\. The clutch disc, covered in grippy friction material, is squeezed between the flywheel and the pressure plate by a big diaphragm spring\. Gripped tight, the disc and the gearbox shaft spin along with the engine\.

In a typical car that spring presses with the weight of several hundred kilograms\.

### 3. Try the pedal yourself

Slide the pedal down and watch the whole chain respond\. Your push travels from the pedal, through liquid, to the clutch\. When the plate lifts away, the disc and gearbox shaft slow to a stop while the engine keeps turning\.

### 4. The master cylinder makes pressure

The pedal is a lever that pivots near the top, so a long movement at your foot becomes a strong, short push on a rod\. The rod drives a piston into the master cylinder, a tube full of brake fluid\. Its first move seals off the reservoir so the fluid can only go one way\.

A clutch needs only a gentle push from your foot, about the weight of a 10 to 15 kg bag\.

### 5. Liquid carries the push

Liquids almost cannot be squashed, so pushing fluid in at one end pushes the same amount out at the other\. The fluid does not flow round in a loop; it just shifts a little along the line, back and forth, like a long row of marbles in a tube\.

Pressure in the line stays below about 50 times normal air pressure\.

### 6. The slave cylinder lifts the plate

At the clutch, the fluid pushes a ring\-shaped piston called the slave cylinder\. It moves the release bearing, which presses the middle of the diaphragm spring\. The spring tips like a see\-saw over its pivot ring and pulls the pressure plate away from the disc\.

Many cars put this slave ring right around the gearbox shaft\. Others use a small slave cylinder that pushes a lever called a fork\.

### 7. Why bubbles are bad news

Air, unlike liquid, squashes easily\. Switch on the air bubble and watch: the pedal goes down just as far, but the bubble shrinks instead of pushing the fluid along, so the clutch only partly lets go\. Mechanics remove trapped air by bleeding the system\.

### 8. Let go, and it grips again

Now the pedal is held down and the disc is still\. Let the pedal up: the diaphragm spring pushes the plate back onto the disc and shoves the fluid home\. The disc grips, catches up with the flywheel and the car moves off\.

Letting the pedal up slowly lets the disc slip a little first\. Drivers call that point the bite\.