How does a hydraulic jack work?

The principle

Hydraulic jack. Oil can hardly be squeezed. Push on it in a closed space and the pressure is passed on, the same everywhere, to every wall and every piston it touches.

Step 1: Pressure spreads everywhere. Your push on the long handle is multiplied by the lever, here about 7 times: about 30 kg on the handle gives about 200 kg on the small pump piston. Oil can’t be squeezed, so that push turns into pressure that reaches every corner of the oil, including under the big piston, the ram.

Step 2: Same pressure, bigger area. Look at the bottom of the two pistons. The oil under the small pump piston and the oil under the ram are at the same pressure, about 180 bar. One bar is about 1 kg pressing on every square centimetre (strictly 1.02 kg per cm², so 1 bar ≈ 1 kg/cm²). The small piston is about 1.1 cm² (about 12 mm across), so 180 bar on it is about 180 × 1.1 ≈ 200 kg. The ram is about 11 cm² (about 37 mm across), 10 times the area at the same pressure, so it pushes with about 180 × 11 ≈ 2000 kg, enough for one end of a car. Picture squared paper: the small piston covers about one square, the ram about eleven, and every square gets the same ~180 kg push.

Step 3: Two one-way valves. One stroke isn’t enough, so you pump. Each valve is a small steel ball on a seat. Handle up: the pump piston rises and sucks oil from the tank; that pushes the inlet ball off its seat, while the pressure under the ram presses the outlet ball shut. Handle down: the pump piston presses the oil; the inlet ball is pushed shut and the outlet ball opens, so the oil can only go under the ram. The balls never let it flow back, so the car stays up between strokes.

Step 4: Force for distance. Nothing is free. The small piston moves about 20 mm down for each stroke, and the ram only rises about 2 mm: 10 times the force, a tenth of the travel. So lifting a car about 10 cm takes roughly 50 strokes. It’s like a low gear on a bike: easy to push, slow to move.

Step 5: The release valve. To lower the car you turn the release knob. It opens a third path from under the ram straight back to the tank. The car’s weight pushes the ram down and the oil flows back. Open it slowly and the car comes down gently.

Step 6: Putting it together. Same pressure on a bigger area gives a bigger force, the one-way valves let you pump it up stroke by stroke, and the release valve lets it back down.

Bottle jack or trolley jack?

• Bottle jack (upright cylinder): Upside: small, cheap and lifts a lot for its size. Downside: tall to begin with, so it may not fit under a low car, and it has a narrow base.
• Trolley (floor) jack: Upside: very low, rolls under the car and lifts fast with a long arm. Downside: big and heavy to carry, and more expensive.

Safety: a jack is for lifting, not for holding. Oil seals can leak, so always put axle stands under the car before you work under it.

Try it yourself: join a thin syringe and a fat syringe with a tube full of water. Push the thin one: the fat one moves only a little, but you can barely hold it back with your thumb.