What is a gas spring?

A gas spring is a sealed hydropneumatic device containing pressurized nitrogen, a piston and piston rod, seals and a small quantity of oil. It produces a controlled linear force for lifting, lowering, balancing or positioning a moving load.

How does a gas spring create force?

Pressurized nitrogen acts across unequal effective areas inside the cylinder. Because the piston rod occupies area on one side of the piston, the pressure creates a net force that pushes the rod outward.

Why does a gas spring get stronger when compressed?

The entering piston rod reduces the volume available to the sealed nitrogen charge. The gas pressure rises as the volume falls, increasing the reaction force. This increase through the stroke is called progression.

Are a gas spring, gas strut and gas shock the same thing?

Gas spring and gas strut are commonly used for the same compression-type component. Gas shock is also used informally, but it can be confused with a shock absorber, whose main purpose is damping rather than supporting a load.

What does the oil inside a gas spring do?

The oil lubricates the rod guide and seals. In designs with hydraulic end damping, the piston enters an oil zone near the end of travel so the final part of the movement slows instead of stopping abruptly.

Can a gas spring be opened, repaired or refilled?

Do not open, drill, heat or modify a gas spring. It is a high-pressure sealed component. Contact Gemini about replacement, approved force adjustment options or model-specific disposal instructions.

Technical guide

How Gas Springs Work

A gas spring is a sealed hydropneumatic device. Pressurized nitrogen acts across different effective piston areas, creating a net force that pushes the piston rod outward. As the rod is compressed, it occupies more cylinder volume, raising gas pressure and force. Oil lubricates the seals and can provide end-of-stroke damping.

Published September 17, 20268 minute read

The short answer

The main points to understand before you begin.

Nitrogen creates the force: Pressurized nitrogen acts on the internal surfaces. The piston rod creates an area difference that produces a net outward force.
Compression raises the force: Pushing the rod into the sealed cylinder reduces the available gas volume. Pressure rises, so the reaction force also increases.
Oil controls the final motion: A small oil volume lubricates the guide and seals and can slow the piston near the end of extension when the spring is oriented correctly.

Inside a standard compression gas spring

The cutaway shows the parts that turn stored gas pressure into controlled linear force.

Cutaway of a compression gas spring showing the piston rod, rod guide and seals, piston, oil, pressurized nitrogen and cylinder-end connection — Simplified construction diagram. Exact piston, seal, oil and damping arrangements vary by gas spring model and operating option. Select the image to enlarge it.

Piston rodTransfers the spring's linear force to the rod-end connection.
Rod guide and sealsGuide the rod and retain the nitrogen and oil inside the cylinder.
Piston and metering passageGuide internal flow and help control the speed of movement.
Hydraulic oilLubricates the guide and seals and can provide end-of-stroke damping.
Pressurized nitrogenStores energy and produces the pressure used to create extension force.
Cylinder-end connectionConnects the pressure cylinder to the fixed or moving structure.

How a compression gas spring creates force

This is what happens inside the cylinder as the spring extends and compresses.

Nitrogen is sealed under pressure
The pressure cylinder contains compressed nitrogen, the piston and piston rod, plus a small quantity of oil. The assembly is closed and is not intended to be opened or serviced internally.
Pressure acts on unequal effective areas
Gas pressure acts around the piston, but the rod occupies area on one side. That difference creates a net force in the extension direction.
The piston rod moves outward
When the external load is low enough, the net internal force extends the rod. In an application, the mounting geometry turns that linear force into lifting, balancing or positioning assistance.
The metering passage controls flow
As the piston moves, nitrogen and oil pass through or around a calibrated piston passage. The passage design helps control extension and compression speed.
Compression creates progression
Pushing the rod into the cylinder displaces volume and compresses the nitrogen further. The pressure rises, so force is higher near the compressed position than near full extension.
Oil lubricates and can add end damping
With the spring mounted rod down where the design permits, oil remains near the rod guide and the piston enters the oil zone near full extension, slowing the final part of the movement.

Why does a gas spring extend?

The extension force comes from gas pressure acting across the piston rod's effective area.

Inside the cylinder, the nitrogen pressure acts in every direction. The piston sees pressure on both sides, but the rod occupies part of the area on the rod side. The remaining area difference produces a net outward force. In a simplified model, that effective area is approximately the cross-sectional area of the piston rod.

Manufacturers set the nominal extension force by selecting a fill pressure for the chosen rod and cylinder size. Seal friction, temperature, piston design and the position in the stroke also affect the force measured at the rod, so the simplified relationship explains the principle rather than replacing product data or an application calculation.

F ≈ ΔP × Arod

F: approximate net extension force at the piston rod
ΔP: difference between internal gas pressure and the surrounding pressure
Arod: cross-sectional area of the piston rod

This relationship explains why a larger rod or a higher nitrogen charge can produce more extension force. Use the marked nominal force and the product specification for selection; do not calculate or alter the internal charge yourself.

Why force increases as the rod is compressed

The piston rod takes up space inside the sealed cylinder, leaving less volume for the nitrogen.

When an external load pushes the rod into the cylinder, the available gas volume decreases. Compressing the same nitrogen charge into a smaller volume raises its pressure. The gas spring therefore pushes back with more force as it approaches the compressed position. This force increase over the stroke is called progression.

Progression depends on the spring's internal volume, rod volume, stroke and construction. A larger reserve of gas volume generally produces a flatter force curve; a greater displaced rod volume produces more pressure change. The exact F1 and F2 values belong to the selected spring specification, not to a universal percentage.

How extension speed and damping are controlled

The piston passage controls internal flow, while oil can slow the final part of the extension stroke.

As the piston moves, nitrogen and oil must move through a calibrated passage in or around the piston. Passage size, piston design and fluid viscosity influence how quickly the rod can extend or compress. This is why a gas spring can provide smoother, more controlled motion than an undamped mechanical spring.

In a standard spring with hydraulic end damping, rod-down installation keeps oil near the guide and at the end of the piston travel. The piston enters this oil zone near full extension, increasing flow resistance and slowing the final movement. Other designs can provide different damping directions or orientation-independent behaviour, so the product label and model specification take precedence.

What determines gas spring behaviour

Force, speed and feel come from a combination of internal design and application geometry.

Factor	Primary effect	Why it matters
Nitrogen fill pressure	Nominal extension force	Higher pressure creates more force for the same effective rod area
Piston rod diameter	Force capacity and displaced volume	A larger rod increases effective area and changes progression
Cylinder volume and stroke	Force progression	They determine how much the gas volume changes through the stroke
Piston passage	Extension and compression speed	The passage meters internal gas and oil flow
Oil volume and viscosity	Lubrication and damping	They influence seal lubrication and end-of-stroke motion
Temperature	Gas pressure and force	Gemini's general specification states an approximate 3.5% force change per 10°C
Mounting geometry	Force delivered to the lid or panel	Lever arms and spring angle determine how linear force acts on the load

Gas spring compared with a mechanical coil spring

Both store energy, but they package and deliver that energy differently.

Characteristic	Gas spring	Mechanical coil spring
Energy storage	Compressed nitrogen in a sealed cylinder	Elastic deformation of metal
Force at the start of travel	Preloaded force is available immediately	Force normally builds from the installed deflection
Force curve	Usually relatively flat with defined progression	Typically rises in proportion to deflection for a linear spring
Motion control	Piston flow and oil can control speed and end damping	Usually needs a separate damper for controlled motion
Packaging	Compact linear unit with integrated end connections	Spring shape and separate guides or dampers may require more space
Selection	Force, length, stroke, progression, damping and mounting geometry	Spring rate, preload, deflection, dimensions and mounting

Selection and safety limits

Knowing how the spring works is only the starting point for selecting one.

The same nominal-force spring can behave very differently when its mounting points, panel weight, centre of gravity, opening angle or temperature change. Check the complete movement, including compressed and extended lengths, bracket loads, manual closing effort and the consequences of a failure. Gemini's calculator can provide a preliminary model for a rigid hinged panel, followed by an application review.

Gemini gas springs can contain internal pressure up to approximately 300 bar (4,350 psi). Never drill, cut, heat, bend or open one. Protect the piston rod from dirt, paint, scratches and side load. A gas spring supports or assists motion; it should not be treated as the only safety device where unexpected movement could cause injury or damage.

If a gas spring is used as a limit stop, Gemini states that the force must not exceed the force marked on the spring by more than 30%. Where possible, use a separate mechanical end stop.

Frequently asked questions

Concise answers to the questions that most often change a selection.

What is a gas spring?: A gas spring is a sealed hydropneumatic device containing pressurized nitrogen, a piston and piston rod, seals and a small quantity of oil. It produces a controlled linear force for lifting, lowering, balancing or positioning a moving load.
How does a gas spring create force?: Pressurized nitrogen acts across unequal effective areas inside the cylinder. Because the piston rod occupies area on one side of the piston, the pressure creates a net force that pushes the rod outward.
Why does a gas spring get stronger when compressed?: The entering piston rod reduces the volume available to the sealed nitrogen charge. The gas pressure rises as the volume falls, increasing the reaction force. This increase through the stroke is called progression.
Are a gas spring, gas strut and gas shock the same thing?: Gas spring and gas strut are commonly used for the same compression-type component. Gas shock is also used informally, but it can be confused with a shock absorber, whose main purpose is damping rather than supporting a load.
What does the oil inside a gas spring do?: The oil lubricates the rod guide and seals. In designs with hydraulic end damping, the piston enters an oil zone near the end of travel so the final part of the movement slows instead of stopping abruptly.
Can a gas spring be opened, repaired or refilled?: Do not open, drill, heat or modify a gas spring. It is a high-pressure sealed component. Contact Gemini about replacement, approved force adjustment options or model-specific disposal instructions.

Sources and supporting documents

Published material used to check the terminology, inputs and safety guidance.

Continue with the right next step

Use the guide to collect the inputs, then move into calculation, configuration or review.

Industrial Gas Springs

Compare 11 Gemini compression-spring models by rod diameter, cylinder diameter, stroke and force.

Compare gas spring models

Gas Spring Calculator

Model preliminary force and mounting geometry for a rigid hinged lid, hatch, door or flap.

Open the calculator

Calculate Gas Spring Force

Learn how load moment, centre of gravity, spring angle and mounting points affect force selection.

Read the force guide

How to Measure a Gas Spring

Record extended length, compressed length, stroke, diameters, force markings and end connections.

Read the measurement guide

Gas Spring Installation Guide

Check rod orientation, bracket alignment, end-fitting movement and side-load prevention.

Read the installation guide

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