by Chris Woodford. Last updated: November 19, 2021.
What's the connection between a waterpistol and this gigantic crane? On the face of it, no connection at all. Butthink about the science behind them and you'll reach a surprisingconclusion: water pistols and cranes use the power of moving liquidsin a very similar way. This technology is called hydraulics and it'sused to power everything from car brakes and garbage trucks tomotorboat 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 a hydraulic ram. Can you spot the ram here? The main one is shining silver in the sunlight in the center of the picture.There are also rams supporting the stabilizers ("outriggers"): feet that extend out near the wheels to support the crane at the base when the boom is extended (they're highlighted with yellow and black warning stripes).
- You can't squash a liquid!
- Hydraulics in theory
- Hydraulics in practice
- Hidden hydraulics
- Find out more
You can't squash a liquid!
Gases are easy to squash: everyone knows how easyit is to squeeze a balloon. Solids are just the opposite.If you've ever tried squeezing a block of metal or a lump ofwood, 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 anin-between state, a bit like solids in some ways and a bit like gasesin others. Now, since liquids easily flow from place to place, youmight think they'd behave like gases when you tried 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 aswimming pool. When your body smacks into the pool, it's becausethe water can't squeeze downwards (like a mattress or a trampolinewould) or move out of the way quickly enough. That's also why jumping off bridgesinto rivers can be very dangerous. Unless you dive correctly, jumpingoff a bridge into water is almost like jumping onto concrete.(Find out more about solids, liquids, and gases.)
Photo: Why does water squirt so fast from a syringe? You can't really compress a liquid at all, so if you force the water up through the wide part of the syringe by pushing hard on the plunger at the bottom, where's that water going to go? It has to escape through the top. Since the top is much narrower than the bottom, the water emerges in a high-speed jet. Hydraulics runs this process in reverse to produce lower speed but more force, which is used to power heavy-duty machines. It's exactly the same in a water pistol (below), which is effectively just a syringe shaped like a gun.
The fact that liquids don't compress easily isincredibly useful. If you've ever fired a water pistol (or a squeezywashing-up liquid bottle filled with water), you've used this ideaalready. You've probably noticed that it takes some effort to pressthe trigger of a water pistol (or to squeeze water from a washing-upbottle). When you press the trigger (or squeeze the bottle), you'rehaving to work quite hard to force the water out through a narrownozzle. You're actually putting pressure on the water—andthat's why it squirts out at a much higher speed than you move thetrigger. If water weren't incompressible, water pistols wouldn't workproperly. You'd squeeze the trigger and the water inside would simplysquash up into a smaller space—it wouldn't shoot out of the nozzle asyou'd expect.
If water pistols (and squeezy bottles) can change force and speed, that means (in strict scientific terms) 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 waterpistol.
When you press on the trigger (shown in red), you apply a relativelylarge force that moves the trigger a short distance. Because the water won'tsqueeze into a smaller space, it gets forced through the body of thepistol to the narrow nozzle and squirts out with less force but morespeed.
Now suppose we could make a water pistol work in reverse. Ifwe could shoot liquid into the nozzle at high speed, the water wouldflow the opposite way and we'd generate alarge upward force on the trigger. If we scaled our water pistol upmany times, wecould generate a big enough force to lift things. This is exactly how ahydraulic ram or jack works. If you squirt fluid through a narrowtube at one end, you can make a plunger rise slowly, but with a lotof force, at the other end:
Photo: How to magnify force with a water pistolworking in reverse.
The science behind hydraulics is called Pascal'sprinciple. Essentially, because the liquid in the pipe isincompressible, the pressure must stay constant all the way through it,even when you're pushing it hard at one end or the other. Now pressureis defined as the force acting per unit of area. So if we press downwith a small force on a small area, at the narrow end of the tube onthe left, there must be a large force acting upward on the largerarea piston on the right to keep the pressure equal. That's how theforce becomes magnified.
What about energy?
Another way to understand hydraulics is by thinking about energy.
We've already seen that hydraulic rams can give us more force or speed, but theycan't do both at the same time—and that's because of energy. Look again at the water pistol graphic up above.If you press down quickly on the narrow pipe (with a little bit of force), the plunger on the wide piperises slowly (with a lot of force). Why would that be? A basic law of physics calledthe law of conservation of energy says wecan't make energy out of thin air. The amount of energy you use to move the plungeris equal to the force you use times the distance you move it. If our water pistolproduces twice as much force at the wide end as we supply at the narrow end, it can onlymove half as far. That's because the energy we supply by pushing down is carriedright around the pipe to the other end. If the same amount of energy now has to move twice the force, it can only move it half the distance in the same time. That's why the wider end moves more slowlythan the narrow end.
Hydraulics in practice
You can see hydraulics at work in this digger.When the driver pulls a handle, the digger's engine pumps fluid intothe narrow pipes and cables (shown in blue), forcing the hydraulic rams (shownin red) to extend. The rams look a bit like bicycle pumps working inreverse. If you put several rams together, you can make a digger'sarm extend and move much like a person's—only with far greaterforce. The hydraulic rams are effectively the digger's muscles:
Photo: There are several different hydraulic rams at work in this digger. The rams are shown by red arrowsand the narrow, flexible hydraulic pipes and cables that feed them in blue.
Each ram is working like a diesel-powered water pistol in reverse:
Photo: Close-up of a digger's hydraulic rams.
The engine is pumping hydraulic fluid through one of the thin pipes to move the thicker ram out with much greater force, like this:
Photo: How a hydraulic ram multiplies force.
You might be wondering how a hydraulic ram can move both inward and outward if the hydraulic fluid is always pushing it from one direction.The answer is that the fluid doesn't always push the same way. Each ram is fed from opposite sides by two separate pipes.Depending on which way the fluid moves, the ram pushes either inward or outward, very slowly and smoothly, as this little animation makes clear:
Photo: A hydraulic ram moves either inward or outward depending on which direction the hydraulic fluid is flowing.
Next time you're out and about, see how many hydraulic machines you can spot. You might be surprised just how manytrucks, 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. Red arrows indicate the hydraulic rams.
Not all hydraulic machines are so obvious, however; sometimes their hydraulic rams are hidden out of sight. Elevators ("lifts") keep their workings well hidden, so it's not always apparent whether they're working in the traditional way (pulled up and down by a cable attached to a motor) or using hydraulics instead. Smaller elevators often use simple hydraulic rams mounted directly underneath or alongside the lift shaft. They're simpler and cheaper than traditional elevators, but can use quite a bit more power.
Motors are another example where hydraulics can be hidden from view. Traditional electric motors use electromagnetism: when an electric current flows through coils inside them, it creates a temporary magnetic force that pushes against a ring of permanent magnets, making the motor shaft rotate.Hydraulic motors are more like pumps working in reverse. In one example, called a hydraulic gear motor, the fluid flows into the motor through a pipe, making a pair of closely meshing gears rotate, before flowing back out through another pipe. One of the gears is connected to the motor shaft that drives whatever the motor is powering, while the other ("the idler") simply turns freely to make the mechanism complete. Where a traditional hydraulic ram uses the power of a pumped fluid to push the ram back and forth a limited distance, a hydraulic motor uses continuously flowing fluid to turn the shaft for as long as necessary. If you want to make the motor turn in the opposite direction,you simply reverse the fluid flow. If you want to make it turn faster or slower, you increase or decrease the fluid flow.
Artwork: A simplified hydraulic gear motor. The fluid (yellow) flows in from the left, spins the two gears, and flows out to the right. One of the gears (red) powers the output shaft (black) and the machine to which the motor is connected. The other gear (blue) is an idler.
Why would you use a hydraulic motor instead of an electric one? Where a powerful electric motor generally needs to be really big, a hydraulic motor just as powerful can be smaller and more compact, because it's getting its power from a pump some distance away. You can also use hydraulic motors in places where electricity might not be viable or safe—for example, underwater, or where there's a risk of electric sparks creating a fire or explosion. (Another option, in that case, is to use pneumatics—the power of compressed air.)
Find out more
On this website
- Forces and motion
- Simple machines
For younger readers
These are particularly suitable for ages 9–12:
- Machines and Motors by Jon Richards and Ed Simpkins. Gareth Stevens/Wayland, 2016. A clearly written, illustrated guide to all kinds of powered machines. A good overview that will help children understand how simple machines power the bigger, real-world machines they see around them.
- The Way Things Work Now by David Macaulay. DK, 2016. Lots of hydraulic machines are taken apart and explained in this classic how-it-works tome.
- Can You Feel the Force? by Richard Hammond. Dorling Kindersley, 2007/2015. A fun-filled introduction to basic physics. (I was one of the consultants on this book.)
- Force and Motion by Peter Lafferty. Dorling Kindersley, 2000. Although this is quite old now and doesn't seem to have been updated, it's still easy to find in secondhand stores. One of the classic DK Eyewitness books, it has lots of fascinating history as well as modern science.
- How Things Work: The Power of Pressure by Andrew Dunn. Thomson Learning, 1993. A slightly dated but still very relevant children's book that connects the basic science of fluids and water pressure to such everyday machines as hovercraft, vacuum cleaners, jackhammers, car brakes, and elevators.
For older readers
- Hydraulic Machines and Energy by Giorgio Cornetti. Springer, 2022. A comprehensive general guide to hydraulic power.
- Understanding hydraulics by Leslie Hamill. Palgrave Macmillan, 2011. A huge and very popular introduction to hydraulics for college-level engineering students.
- Hydraulics and Pneumatics: A Technician's and Engineer's Guide by Andrew Parr. Butterworth-Heinemann, 1998 (reprinted 2013). A shorter book than Hamill's and one that compares the advantages of hydraulic and pneumatic power. Also for college-level.
- Hydraulic Actuators by Vickers Hydraulics. A dated but quite clear video that explains basic hydraulic actuators, including single- and double-acting rams and hydraulic motors.
- Make a Hydraulic Arm by Mist8K. A syringe-powered hydraulic arm with an electromagnetic pick up.
- How to Make Hydraulic Fighting Robots by Lance Akiyama. One of the projects covered in Lance's book Rubber Band Engineer.
- How a hydraulic scissor lift works by by DRHydraulics. This is quite a clear animation showing how a hydraulic pump makes a lift go up and down. It would be better if we could see a cutaway of the cylinder and how the fluid is flowing, but you get the idea.
- Watch the HyQReal Robot Pull an Airplane by Evan Ackerman. IEEE Spectrum, May 23, 2019. You might robots are mostly electromechanical, but hydraulic components are becoming increasingly popular.
- Disney Robot With Air-Water Actuators Shows Off "Very Fluid" Motions by Erico Guizzo. IEEE Spectrum, September 1, 2016. Exploring a robot that uses a combination of hydraulics and pneumatics.
- Hydraulics could enable fullscreen Braille display by Priya Ganapati. Wired, March 30, 2010. A newly designed hydraulic mechanism could make Braille displays cheaper, faster, and more accessible.
- Pressure on hydraulics: The Engineer, February 24, 2003. Why is hydraulics still such a popular way of powering machines when electric power is, on the face of it, simpler and easier to implement?