Choosing a gas spring is not complicated, but a 10 mm error in length or 100 N in force means the lid won't open, won't close or breaks the hinge. In this guide we explain, in four steps, how to identify the gas spring you need, whether you are replacing an existing one or designing a new application.
1. Measure the extended length and the stroke
The extended length is the distance between the centres of the two fixings with the gas spring fully open. The stroke is how far the rod travels between open and closed. If you have the old gas spring, measure it extended from the centre of one end fitting to the other; if it is removed and has lost its gas, pull it out by hand.
Rule of thumb for new applications: the extended length is usually around 55–60% of the lid length, and the stroke about 2/5 of the extended length. For example, for an 800 mm lid, a 450–480 mm gas spring with a 180–200 mm stroke is a good starting point.
2. Identify the series: 6/15, 8/18, 10/22 or 14/28
The figures are the diameters in millimetres of the rod and the cylinder. The larger the diameter, the more force the gas spring can take:
- 6/15: 50–400 N. Light lids, cabinets, caravan windows.
- 8/18: 100–700 N. Van tailgates, canopies, bus hatches.
- 10/22: 100–1,200 N. Truck bonnets, tilting cabs, bus luggage lockers.
- 14/28: 500–2,500 N. Trailer side panels, platforms, heavy machinery.
If you are replacing a gas spring, the cylinder diameter tells you the series: measure it with a calliper and compare with 15, 18, 22 or 28 mm.
3. Calculate the force in newtons
The force printed on the cylinder (for example "350N") is the extension force with the rod out. To calculate it for a new application you need the weight of the lid, the distance from the centre of gravity to the hinge and the distance from the gas spring mounting point to the hinge. The simplified formula for two gas springs is:
F (N) = (weight in kg × 9.81 × distance to centre of gravity) ÷ (mounting point distance × 2) × 1.1
The 1.1 factor gives a 10% margin so the lid opens by itself at the end of its travel. If you would rather not calculate, send us the weight, measurements and a photo and we will get back to you with the exact part number.
Watch out: a gas spring that is too strong is as much of a problem as a weak one: the lid slams open, is hard to close and the hinges suffer. If you are torn between two forces, choose the lower one and, if needed, increase it with our refill service.
4. Choose the end fittings and the fixing
Gas springs are supplied with a thread at each end (M6 on 6/15, M8 on 8/18 and 10/22, M10 on 14/28) and the end fitting screws onto it: ball socket (the most common, it clips onto a 10 or 13 mm ball stud), eyelet (pin mounting) or clevis. Choose the same end fitting as the original gas spring and check the ball diameter on the bracket. All end fittings and brackets are in gas spring accessories.
Fitting tips
- Always fit the gas spring with the rod pointing downwards in the closed position: this keeps the seal lubricated by the oil inside.
- Do not paint or grease the rod; any particle will damage the seal.
- In environments with salt, pressure washing or chemicals, use the AISI 316 stainless steel version.
- If the lid weighs more than 20 kg or people work underneath it, add a safety kit that holds the lid if the gas spring fails.
Do you have the part number of the gas spring you want to replace? Look it up in gas springs or email us at sales@mcnaughtans.es with a photo of the cylinder.