How Do Hydraulic Jacks Work?

When we want to lift heavy machinery or even loads of any kind, the first thing we’ll think about is hydraulics.

We live and breathe hydraulics, and we supply them for every kind of situation, but why are they so effective?

If you’ve ever lifted a car using a jack, you’ll have noticed that it’s quite easy using hydraulic based systems. Whereas trying to lift a car on its own would be impossible for most people, pop a small jack underneath and suddenly it’s easy.

The exact same principles come into force when lifting everything from cars to trains – a small amount of force at one point of the jack equals much more force at the other end.

So how does this work?

Pascal’s Law

The principle is described in Pascal’s Law, which states that a pressure changed occurring anywhere in a combined incompressible fluid is transmitted throughout the fluid such that the same change occurs everywhere.

Surely that means the same pressure in gives the same pressure out?

Well, no.

Think of a two cylinder system. One cylinder has an area of ten square centimetres, the other has an area of 100 square centimetres.

hydraulic jacks work

When the smaller one is pressed down, the same pressure is exerted across all areas, so the downward pressure on the smaller cylinder provides ten times that on the larger cylinder.

So if 1Kg of pressure down, will give 10kg pressure up at the other side.

Where does this extra power come from?

If we take the equation work = force x distance, then we can see the answer. The extra power to lift heavier objects doesn’t come from anywhere.

The fluid displaced by the smaller piston is dispersed over the larger area, so to move the large cylinder up one centimetre will mean the smaller one has to be pushed down ten.

You can’t win them all!