Gas springs

Last updated: October 6, 2008.

Have you ever tried lifting the trunk lid (sometimes called "tailgate" or "boot") of your car with just one finger? How come you can lift a heavy piece of metal and glass with so little force? The answer, if you didn't know already, lies in those clever piston-like hinges that support the lid either side. They're called gas springs (or gas dampers) and they make our lives a whole lot easier in all sorts of ways.

If you're sitting on an office chair right now, there's probably a gas spring underneath your body. Release the height lever and you'll feel (and may even hear) the gas in the spring being compressed as the seat gently falls down. Gas springs have loads of other uses too. Let's take a closer look at these handy gadgets and find out how they work.

Photo: A sturdy gas spring (the thin black cylinder and the silver rod that slides in and out of it like a bicycle pump) supports the tailgate of this car during loading and unloading.

Why do we need gas springs?

Suppose there were no springs on the trunk lid of your car. It would be really heavy to lift, for one thing. There'd be nothing to hold it up in the air when you wanted to load in your shopping, which would be a real nuisance. And, if you let the lid go, it would crash down onto your car's bodywork, probably doing a lot of damage in the process. Now we could put a normal metal spring on the lid, but that wouldn't help so much. It would need to be a very stiff and heavy spring, so it would take a huge amount of effort to lift the lid high in the air. The higher you lifted it, the harder it would get to lift any further. With the lid opened up fully, the spring would be stretched out so much that it would pull straight back down again!

Photo: A vertically mounted gas spring supports the seat of this "gas-lift" office chair, making it easy to adjust the height.

How a gas spring works

A gas spring solves these problems. It's like a bicycle pump filled with nitrogen gas (the major constituent of the air around us) and then sealed up. Just like a bicycle pump, a gas spring has a tight-fitting piston that can slide back in forth inside a cylinder that has gas at one end and a small amount of lubricating oil at the other. When you push on the spring, you make the piston compress the gas. Stop pressing and let go and the pressure of the gas pushes the piston back out again.

You can choose a gas spring with just the right size of cylinder and piston to give just as much force in the spring as you need to do a particular job. To support the trunk lid of a car, you need the two gas springs either side to provide roughly as much force when they're compressed as the weight of the lid. For a gas-lift office chair, you need the spring to provide a little bit more force than the weight of the seat. In most chairs, the spring doesn't actually support the person's weight; there's a separate locking device that does that. The spring is simply designed to let the seat move up and down very gently without your having to supply much force.

Gas springs are energy reservoirs

A gas spring's job is to make your life easy—and it does it by storing energy (when there's plenty available—usually when you're lowering something heavy) and releasing that energy (when you need extra help—usually when you're lifting something up). Think of a gas spring as a kind of mechanical battery that stores and releases energy by squeezing and releasing a gas and you can see why it's so useful.

What's happening with energy when you lift a heavy trunk lid that has no springs of any kind? There's a lot of mass in the steel and glass lid so it takes a lot of energy to raise it up against the force of gravity, which is constantly trying to pull it back down. Once the lid is high in the air, it has stored potential energy: you can release the lid and it'll crash straight back down again. If that happens, the potential energy is instantly converted into kinetic energy, as the lid accelerates, and then heat and sound energy when the lid smashes onto the car's body. What a waste!

With a couple of gas springs on either side of the lid, it works a different way. Now, when you gently lower the lid, the weight of the metal and glass compresses the gas inside the springs. As you lower the lid, the potential energy it had when it was up in the air is slowly converted into potential energy inside the gas springs and stored there. Next time you want to raise the lid, that potential energy is waiting inside the spring ready to help you. Release the lid catch, lift the lid gently, and the potential energy stored in the gas springs is slowly released. The pistons push out form the gas springs and help you lift the lid back up again.