In this blog post, we’ll explain the structure and operating principles of gas springs, as well as typical examples of their use in everyday life, in simple terms.
Many readers have probably been fascinated by the height-adjustable chairs in their father’s office or at school. If you recall sitting in a chair as a child and pulling a lever to lower or raise it, you might have wondered why the chair would lower easily when an adult sat in it but wouldn’t lower as easily when a child sat in it. That height-adjustment mechanism is a gas spring. In this post, I’ll explain how it works.
A gas spring is a very simple device consisting of a piston inside a sealed cylinder, where the air inside acts like a spring. When the piston is pushed inward, the volume of air inside the cylinder decreases, compressing the air and storing energy. Conversely, when the force applied to the piston is released, the compressed air expands, pushing the piston outward and generating a force that tries to return it to its original position—which is exactly how a spring behaves.
To understand air compression and energy storage, consider the example of a syringe. If you block the syringe’s opening with your finger and press the plunger, the air is compressed, and you feel a force pushing back against your hand. By the same principle, when the piston of a gas spring moves inward, the internal air resists with pressure, and that pressure is stored as energy, which then pushes the piston back out.
Although the piston has one or more holes that allow air to flow in and out on both sides, the key to making a gas spring behave like a spring lies in how the rod attached to the piston changes the space it occupies inside the cylinder. As the piston moves further into the cylinder, the volume of air remaining inside the cylinder decreases, causing the air pressure to rise.
Pushing the piston inward reduces the space available for air, causing the internal pressure to rise. This pressure stores energy, and when the force is released, the stored energy is released, pushing the piston back out. Thus, instead of providing elastic force like a metal spring, a gas spring performs the same function using the force of compressed air.
Office chairs use this principle to adjust their height. When you pull the chair’s lever, an internal valve opens, allowing air to flow in and out of the cylinder. Depending on the change in pressure inside the cylinder, the piston is either pushed outward on its own or pushed inward by body weight, thereby adjusting the chair’s height. When a person pulls the lever while seated, their body weight pushes the piston inward, and the air inside escapes through the valve, causing the chair to lower.
You may notice that when a lightweight child sits in the chair and pulls the lever, the chair does not lower easily or may even rise; this is because air generates high pressure in response to changes in volume. Especially when the piston rod is designed to nearly fill the interior volume of the cylinder, even a small change in volume can create a very large pressure difference; some gas springs are reported to generate a force approximately 170 times atmospheric pressure. Therefore, office chairs are designed based on an adult’s weight and may offer excessive resistance to children.
Powerful gas springs are useful in many ways in everyday life. For example, a car trunk is heavy and difficult to lift by hand, but if a gas spring is installed, the trunk can be lifted easily with minimal effort. In addition, they are widely used to support doors or covers on heavy equipment such as excavators, and to smoothly control the moving parts of various machines.
Although they are not usually very noticeable, gas springs have been quietly helping us for a long time; in terms of their functionality, they can be described as a kind of “unsung hero.”
