Gas Springs for Lids and Flaps: How to Size Them Right

A lid that slams shut, a hatch that is too heavy to lift or a flap that creeps open on its own: almost always the cause is a gas spring with the wrong force or the wrong mounting points. The good news is that sizing gas springs for a lid follows a clear logic.

This guide explains what decides the force, where to mount the gas springs, how temperature changes the result and how to check your design before you order.

Why Lids Need a Calculation, Not a Guess

The weight of a lid does not act on the gas spring directly. What matters is the moment around the hinge: the weight multiplied by how far its centre of gravity sits from the hinge. And that moment changes as the lid opens.

• Closed (horizontal): the centre of gravity is furthest from the hinge horizontally, so the lid is at its heaviest.
• Fully open (vertical): the centre of gravity is almost above the hinge, so the lid weighs next to nothing.

The gas spring has to follow this curve. Its lever arm around the hinge also changes with the angle, and that depends entirely on where the two mounting points are. Two gas springs with the same force can give a lid that floats perfectly or one that refuses to stay open. That is why the force and the mounting points are always chosen together.

Step 1: Work Out the Lid Moment

Start with three values:

• Weight m of the lid in kg, including handles, frames and fittings
• Centre of gravity: the distance from the hinge to the centre of gravity. For an even panel it is half the lid length.
• Angles: the lid angle when closed and when fully open

The moment in the closed, horizontal position is:

M = m × 9.81 × centre of gravity distance

Example: a lid of 650 × 800 mm in 18 mm plywood weighs about 5.6 kg. Its centre of gravity sits 325 mm from the hinge, so the closed moment is 5.6 × 9.81 × 0.325 ≈ 18 Nm. If the gas springs act with a lever arm of 60 mm at that point, they need to push with about 18 / 0.06 = 300 N together, or around 150 N each with two springs. The real lever arm depends on the mounting points, which is why a calculation tool saves a lot of trial and error.

Step 2: Decide How the Lid Should Behave

There are two common goals:

• Stays closed and stays open: the lid rests by itself in both end positions. You need a small hand force to open it and a small hand force to close it. This is the usual choice for machine covers, cabinets and storage boxes.
• As light as possible: the gas springs carry almost all the weight, so the lid feels almost weightless. Good for heavy hatches, but the lid may drift if the force is not well matched.

A useful way to set the target is the force at the handle. For frequent use, keep the maximum hand force low, for example 40 to 60 N at the outer edge. A minimum of 10 to 20 N keeps the lid shut when closed and stops it drifting when open.

Step 3: Choose the Mounting Points

Each gas spring has one mounting point on the lid and one on the body. Together they decide the stroke, the installed length and the lever arm in every position.

• The distance between the points when the lid is closed is the compressed length of the gas spring. When it is fully open, it is the extended length. The difference is the stroke.
• Mounting points close to the hinge need short strokes but high forces. Points further out need longer strokes but lower forces.
• Mount with the piston rod pointing down where possible. The oil inside then stays at the seal, which helps the gas spring last longer.
• Use two gas springs, one on each side, so the lid does not twist. Narrow lids can manage with one.
• Choose end fittings that suit the movement. Ball sockets accept some misalignment, eye ends need a straight, parallel pin.

Step 4: Allow for Temperature

A gas spring is filled with nitrogen, and its force follows the temperature. As a rule of thumb, the force changes by about 0.34 % per °C. Catalogue forces apply at 20 °C.

For outdoor equipment or unheated buildings, check the worst cases: opening at the lowest temperature, when the gas springs are weakest, and closing at the highest temperature, when they push hardest.

Step 5: Leave Room for Tolerances and Ageing

Real forces vary from part to part, and the force of a gas spring increases slightly as it is compressed. Over its service life you can also expect a force loss of about 10 %. Two simple ways to stay safe:

• Do not design right at the limit of your hand force range.
• If you are unsure of the exact force, choose a gas spring with a valve (V or X series). The force can then be reduced on site, but never increased, so start slightly high.

Fixed-force N and S series gas springs are filled at the factory and cannot be adjusted afterwards. Choose stainless steel (S or X series) for damp, outdoor or food environments.

Flaps, Inclined Lids and Floor Hatches

The same principle applies to other movements, but the curve is different:

• Hanging flaps that open downwards are often fitted with traction gas springs, which pull instead of push, so the flap does not drop open.
• Inclined lids start at an angle, so the closed moment is lower than for a flat lid.
• Floor hatches are often heavy and open against gravity from flat, so the force at the start of the opening is decisive.

Let the Calculator Do the Geometry

Working out lever arms by hand for every angle takes time. The free gas spring calculator for lids and flaps does it for you:

• Enter the lid size and material, or the weight directly
• Choose how the lid should behave and the hand forces you want
• Set the lowest and highest temperature

The calculator suggests mounting points, shows the hand force over the whole movement and lists matching gas springs that are in stock, with end fittings. You can lock a mounting point that is fixed in your design and let the tool find the rest, then print a data sheet or share the result with a link.

If you already know the length, stroke and force you need, go straight to the gas spring configurator or browse all gas springs and end fittings.

Summary

• Size gas springs from the moment around the hinge, not from the weight alone
• Choose the force and the mounting points together
• Set clear targets for the hand force when closed and open
• Mount with the piston rod down and use two gas springs on wider lids
• Check the coldest and warmest conditions, and allow for tolerances and ageing
• Unsure of the force? Choose a V or X series gas spring with a valve

Need help with a specific lid or hatch? Ask Springy in the bottom right corner, or contact us with the dimensions and weight.