Gas Spring Force Explained: Understanding How Force Varies with Stroke and Temperature

Gas springs are widely used for lid, hatch, and counterbalance applications because they provide smooth, controllable force. However, a common misunderstanding is that gas springs deliver a constant force throughout their stroke. In reality, the force output varies depending on several factors, and selecting the wrong spring based on a static force rating can lead to performance issues or premature failure. This article breaks down the key variables that influence gas spring force, how to measure them, and what to consider when specifying a spring for your design.

What determines the force of a gas spring?

The force of a gas spring is set by the internal nitrogen pressure, the rod diameter, and the tube volume, with variations caused by stroke position and temperature.

A gas spring consists of a pressure tube filled with nitrogen gas and a small amount of lubricating oil. The force is generated by the nitrogen pressure acting on the cross‑sectional area of the rod. When the rod is compressed, the gas volume decreases and pressure rises slightly, causing the force to increase as the spring compresses. Conversely, when the rod extends, the volume increases and pressure drops, reducing the force. This relationship creates a force curve that is not flat but rises toward the compressed end.

The baseline force (often quoted as the mid‑stroke value) is calculated using the formula: F = P × A, where P is the nitrogen pressure (in bar or psi) and A is the rod’s cross‑sectional area (in mm²). For example, a 10 mm rod (area ≈78.5 mm²) charged to 150 bar produces a theoretical force of about 1178 N. Real‑world ratings are lower due to friction and oil damping, but the principle holds.

Mark Arrington observation: “I’ve seen engineers specify a gas spring by reading the force rating on the label and assuming it’s constant. When the lid doesn’t stay open at the top of the stroke, they blame the spring—when in fact they missed the force curve. Always check the force at the extended position if you need hands‑free hold‑open.”

How nitrogen pressure sets baseline force

Higher nitrogen pressure increases force proportionally to the rod area, but also reduces the available stroke because more gas is required to fill the tube.

The nitrogen charge is the primary lever for adjusting force. Increasing pressure raises the force across the entire stroke, but it also reduces the usable volume for gas expansion, which can limit the maximum stroke. Manufacturers balance pressure, rod diameter, and tube length to achieve a target force‑stroke curve. Changing the rod diameter while keeping pressure constant alters force because the area changes—doubling the rod diameter quadruples the area and thus the force.

In practice, off‑the‑shelf springs are offered in standard force increments (e.g., 50N, 100N, 150N). Custom springs can be tuned by adjusting the fill pressure, but this requires specialized equipment and is usually reserved for OEM volumes.

Effect of stroke position on force (force curve)

Force is lowest at full extension and highest at full compression, typically varying by 15‑25% between the two extremes for a standard spring.

The force curve is approximately linear for many gas springs, though the exact shape depends on the tube‑to‑rod ratio and the oil volume. A spring with a large tube relative to the rod will have a flatter curve because the gas volume change per millimeter of stroke is smaller. Conversely, a small tube with a thick rod shows a steeper rise in force as it compresses.

For applications where the spring must hold a lid open with minimal effort, the force at the extended position is critical. If the extended force is too low, the lid will drift closed. If the compressed force is too high, the lid may snap open or be difficult to close manually. Designers should request force‑at‑extension and force‑at‑compression values from the supplier when precision is required.

Mark Arrington observation: “In automotive boot applications, I always look for the force at 80% extension. That’s where the driver’s hand typically releases the lid, and if the spring can’t hold there, you’ll get a slow droop that’s annoying but not dangerous. For safety‑critical hatches on industrial equipment, I require the extended force to be at least 80% of the compressed force to prevent sudden closure.”

Temperature effects on force

Gas spring force decreases approximately 3‑4% for every 10°C drop below the nominal 20°C rating, and rises similarly above that temperature.

Because the internal nitrogen pressure follows the ideal gas law (PV = nRT), temperature directly influences pressure—and thus force. A spring rated at 100N at 20°C will produce only about 82N at 0°C and roughly 118N at 40°C. This variation can be significant in outdoor equipment, cold storage, or environments near heat sources.

Manufacturers offer low‑temperature grades that use a different gas charge or lubricant to maintain performance down to −40°C. High‑temperature versions are less common but exist for applications near engines or ovens. Always verify the operating temperature range with the supplier, especially if the equipment will see seasonal swings.

How to measure gas spring force in practice

Force is best measured with a calibrated force gauge at the desired stroke position, while the spring is mounted in its intended orientation.

To measure force, compress or extend the spring to the target position (using a jig or the actual application) and read the force gauge aligned with the rod axis. Take readings at multiple points—fully compressed, mid‑stroke, and fully extended—to map the force curve. Ensure the gauge is zeroed and that any friction from guides or brackets is minimized; otherwise, the measurement will include side loads.

If a force gauge is not available, you can estimate force by weighing the load the spring holds in a static test. For example, if a vertically mounted spring supports a 10 kg mass at full extension, the force is approximately 98N (10 kg × 9.81 m/s²). This method works only for simple gravity‑loaded setups and ignores dynamic effects.

Using a force gauge

A digital force gauge with ±1% accuracy and a resolution of 0.1N provides reliable readings for spring validation and troubleshooting.

When using a gauge, attach a clevis or eye adapter that matches the spring’s end fitting to avoid introducing bending moments. Pull or push steadily at a rate of 50‑100 mm/min to mimic quasi‑static conditions; rapid impacts can cause oil‑damping effects that skew the reading. Record the average of three trials at each position to account for minor variations.

Mark Arrington observation: “I keep a handheld force gauge in my toolkit for on‑site checks. It’s saved me hours of guesswork when a customer says a strut ‘feels weak.’ A quick measurement often shows the spring is within spec but the mounting geometry has changed—perhaps a hinge has sagged or a bracket has loosened.”

Calculating from spec sheet

If the supplier provides force values at two stroke points (e.g., compressed and extended), you can estimate the mid‑stroke force and the slope of the force curve.

Many datasheets list force at 0% stroke (fully extended) and 100% stroke (fully compressed). Assuming a linear curve, the force at any stroke percentage can be interpolated. For more accurate predictions, ask for the force at 25%, 50%, and 75% stroke, or request a full force‑stroke curve. This data is essential when simulating the mechanism in CAD or calculating the required motor torque for powered lids.

Common misconceptions about constant force vs gas spring force

Gas springs are not constant‑force devices; their force varies with stroke, unlike true constant‑force springs made from pre‑stressed ribbon.

Constant‑force springs (also called clock springs or power springs) are made by winding a flat strip of material around a drum. They deliver nearly the same force regardless of extension because the material’s bending stress remains constant. Gas springs rely on compressible gas, so their pressure—and thus force—changes as volume changes. Attempting to use a gas spring where a constant‑force spring is needed will result in increasing resistance as the lid opens, making manual operation harder at the top.

However, in many lid‑assist applications, a modest force increase is acceptable or even beneficial—it provides a gentle push to close the lid. The key is matching the force curve to the lid’s weight distribution and the desired feel. If you need near‑constant force over a long travel, consider a mechanical constant‑force spring or a gas spring with a very large tube‑to‑rod ratio to flatten the curve.

Why gas springs are not constant force

The compressible nature of nitrogen means that pressure increases as volume drops, so force rises during compression—a fundamental difference from elastic ribbon springs.

From a physics standpoint, the gas spring’s force equation includes a term for changing volume (V), while a constant‑force spring’s force depends only on material geometry and strain, which stays nearly flat. This difference also