Hydraulics

Last updated: January 20, 2009.

What's the connection between a water pistol and this gigantic crane? On the face of it, no connection at all. But think about the science behind them and you'll reach a surprising conclusion: water pistols and cranes use the power of moving liquids in a very similar way. This technology is called hydraulics and it's used to power everything from car brakes and garbage trucks to motorboat steering and garage jacks. Let's take a closer look at how it works!

Photo: This crane raises its giant boom into the air using hydraulic rams. Can you spot the rams here? The main ones are shining silver in the sunlight in the center of the picture. There are also rams supporting the stabilisers: feet that extend out near the four wheels to support the crane at the base when the boom is extended (they're shaded with orange and grey stripes).

You can't squash a liquid

Gases are easy to squash: everyone knows how easy it is to squeeze a balloon. Solids are just the opposite. If you've ever tried squeezing a block of metal or a lump of wood, with nothing but your fingers, you'll know it's pretty much impossible. But what about liquids? Where do they fit in? You probably know that liquids are an in-between state, a bit like solids in some ways and a bit like gases in others. Now, since liquids easily flow from place to place, you might think they'd behave like gases when you tired to squeeze them. In fact, liquids are virtually incompressible—much like solids. This is the reason a belly flop hurts if you mess up your dive into a swimming pool. When your body smacks into the pool, it's because the water can't squeeze downwards (like a mattress or a trampoline would) or move out of the way quickly enough. That's also why jumping off bridges into rivers can be very dangerous. Unless you dive correctly, jumping off a bridge into water is almost like jumping onto concrete. (Find out more about solids, liquids, and gases.)

The fact that liquids don't compress easily is incredibly useful. If you've ever fired a water pistol (or a squeezy washing-up liquid bottle filled with water), you've used this idea already. You've probably noticed that it takes some effort to press the trigger of a water pistol (or to squeeze water from a washing-up bottle). When you press the trigger (or squeeze the bottle), you're having to work quite hard to force the water out through a narrow nozzle. You're actually putting pressure on the water—and that's why it squirts out with more force than you apply to the trigger. If water weren't incompressible, water pistols wouldn't work properly. You'd squeeze the trigger and the water inside would simply squash up into a smaller space—it wouldn't shoot out of the nozzle as we expect.

If water pistols (and squeezy bottles) can magnify force, that means they work just like tools and machines. In fact, the science of water pistols powers some of the world's biggest machines—cranes, tipper trucks, and diggers.

Hydraulics in theory

Turn a water pistol on its end and this is (crudely simplified) what's going on inside:

Photo: A simplified view of a hydraulic water pistol.

When you press on the trigger (shown in red), you apply a relatively large force that moves the trigger a short distance. Because the water won't squeeze into a smaller space, it gets forced through the body of the pistol to the narrow nozzle and squirts out with less force but more speed.

Now suppose we could make a water pistol work in reverse. If we could shoot liquid into the nozzle at high speed, the water would flow the opposite way and we'd generate a large upward force on the trigger. If we scaled our water pistol up many times, we could generate a big enough force to lift things. This is exactly how a hydraulic ram or jack works. If you squirt fluid through a narrow tube at one end, you can make a plunger rise slowly, but with a lot of force, at the other end:

Photo: How to magnify force with a water pistol working in reverse.

The science behind hydraulics is called Pascal's principle. Essentially, because the liquid in the pipe is incompressible, the pressure must stay constant all the way through it, even when you're pushing it hard at one end or the other. Now pressure is defined as the force acting per unit of area. So if we press down with a small force on a small area, at the narrow end of the tube on the left, there must be a large force acting upward on the larger area piston on the right to keep the pressure equal. That's how the force becomes magnified.

Hydraulics in practice

You can see hydraulics at work in this digger. When the driver pulls a handle, the digger's engine pumps fluid into the narrow pipes (shown in blue), forcing the hydraulic rams (shown in red) to extend. The rams look a bit like bicycle pumps working in reverse. If you put several rams together, you can make a digger's arm extend and move much like a person's—only with far greater force. The hydraulic rams are effectively the digger's muscles:

Photo: There are several different hydraulic rams at work in this digger. Photo by courtesy of US Navy.

Each ram is working like a diesel-powered water pistol in reverse:

Photo: Close-up of one of the digger's hydraulic rams. Photo by courtesy of US Navy.

The engine is pumping hydraulic fluid into the ram through the thin tubes on the right to move the plunger out toward the left with much greater force, like this:

Photo: How a hydraulic ram multiplies force.

Next time you're out and about, see how many hydraulic machines you can spot. You might be surprised just how many trucks, cranes, diggers, dumpers, excavators, and bulldozers use them. Here's another example: a hydraulic hedge-cutter on the back of a tractor. The cutting head needs to be sturdy and heavy to slash through hedges and trees and there's no way the driver could lift or position it by hand. Fortunately, the hydraulic controls do all that automatically: with several hydraulic joints, a bit like a shoulder, elbow, and wrist, the cutter moves with as much flexibility as a human arm:

Photo: A typical hydraulic hedge-cutter.

You can even find hydraulics on models of construction machines, though the model rams don't actually contain hydraulic fluid: they're just tight-fitting metal bars that slide in and out of plastic tubes:

Photo: The hydraulic rams on a Corgi model of a Mercedes Unimog dumper truck.