Hydraulics: force from oil

An excavator tears several tonnes of soil out of the ground with its bucket, and if you look closely you will find no gears or ropes anywhere: only slim steel cylinders with shiny piston rods sliding out of them. Inside those cylinders, oil is working under high pressure. The same technology sits in your car's brake pedal, in the jack and in the workshop car lift. Hydraulics is the name of the trick by which a liquid not only passes force along but amplifies it enormously on the way.

Pressure passes it on

Liquids have a property that sets them apart from air: they can hardly be compressed at all. If you press on an enclosed liquid, it does not shrink away, it passes the pressure on undiminished in every direction. The French scholar Blaise Pascal recognised this back in the 17th century. You can test it with a tightly filled water bottle: poke several holes in it and squeeze, and the water shoots further out of every hole than before, no matter whether the hole points up, down or sideways. Your push arrives everywhere with the same strength. Pressure here is force divided by area, $p = F/A$, measured for example in newtons per square centimetre. Engineering usually counts in bar: 1 bar is exactly 10 N per square centimetre.

Small piston, big piston

Now for the actual magic trick. Connect two cylinders with oil and fit one with a small piston of 5 cm², the other with a big one of 100 cm². Push the small piston in with 50 N and a pressure of 10 N per square centimetre builds up in the oil. That same pressure acts under the big piston too, on every one of its 100 square centimetres: it pushes upwards with 1000 N, twenty times as hard as you pushed. This is not free, the golden rule of mechanics collects its price as always: whatever you gain in force you pay for in distance. For the big piston to rise one centimetre, you must push the small one in twenty centimetres, because the volume of oil displaced is the same on both sides.

Your car's brakes work in exactly this way: your foot pushes a small piston in the master cylinder, the pressure runs through thin lines to all four wheels and pushes larger pistons forward there, which press the brake pads against the discs. Every line carries the same pressure, so all four wheels bite at the same moment. The excavator drives its cylinders with a pump that raises the oil to several hundred bar, which is why a cylinder as thick as your arm effortlessly shoulders tonnes.

One last question remains: why oil instead of air? Air can be compressed and would bounce like a sponge, while oil passes every bit of pressure on instantly and completely, lubricates the moving parts along the way and protects them from rust. There is a practical bonus too: a hose carries force around any corner, where no rod and no rope could ever reach.