01 — Your lid
Advanced — measured installation ▾
03 — Hand force through the whole movement
Above the line you are lifting, below it you are pushing down. We pick the highest gas spring force that still keeps the lid closed on its own, because that is the setting that asks for the least effort while opening.
- Angle
- 0°
- Gas spring length
- –
- Rod out
- –
- Moment arm
- –
- Hand force here
- –
04 — Calculated recommendation / preliminary selection
| Order notation | – |
|---|---|
| Gas spring series, rod / tube | – |
| The travel your lid actually uses | – |
| Stroke to order | – |
| Extended length, centre to centre | – |
| Workable force window | – |
| The lid you entered | – |
| Crossover angle | – |
| Mounting | – |
| Assumptions used | – |
The frame bracket drop is the most sensitive input on the page. Below is the same lid solved with that bracket moved a little up and a little down, everything else left the same.
Tick the acknowledgement above to enable these. The report is a PDF: the drawing at three positions, the hand force table in five degree steps, the force window, the bracket positions we used and the assumptions behind them. We send the report by email rather than generating it in the browser, so you have a copy you can pass to your fabricator.
What sits behind this figure
A gas spring calculator that solves one angle with one formula gets you close and then leaves you guessing. This tool solves the whole movement step by step and accounts for the factors that actually change the result.
Perpendicular distance, not bracket spacing
The moment is taken about the hinge using the perpendicular distance from the hinge to the line of action of the gas spring. Using the straight distance between the brackets instead is the most common error in free calculators and can put the result out by a factor of three.
Every degree, not just one
From closed to open in one degree steps. The lid has to stay closed at one end and stay up at the other, and those two conditions squeeze the force from both sides together. A single angle shows you only half of that.
Force rises as the gas spring compresses
The value printed on the tube is F1, measured fully extended. Push the rod in and the force rises to F2. The ratio F2 over F1 is the K-factor, and it is not a single number. It comes from the gas law, so it depends on the ratio between the volume the rod sweeps and the gas volume in the tube. Even within one series a longer stroke means a higher K. On our own test bench we see roughly 1.3 to 1.4. We model the rise as a curve rather than a straight line, so the figure we give you is the F1 you order and not the force somewhere else in the stroke.
Opening and closing are not the same
Seal friction shows up as a difference between pushing and pulling, and it is an absolute value in newtons, not a percentage. It is about 20 N on a small gas spring and 30 to 70 N on a larger one, set by the seals and not by the force. That is why it is barely noticeable on a 1000 N spring and decisive on a 60 N one. It only acts while the rod is being pushed in, which is why the two lines on the chart do not sit on top of each other.
Cold weather and service life
The force changes by roughly 3.4 percent for every 10 degrees of temperature, which is why an outdoor lid behaves differently in February. A gas spring also loses some nitrogen over its life, and we allow for that too.
Lengths we can actually build
The stroke takes up space both inside and outside the tube, so the lower limit of the extended length is roughly twice the stroke plus the dead length of the series. We check that and lengthen the gas spring rather than quoting something that cannot be built.
Which side of the lid does the gas spring go on?
This question comes up in almost every enquiry, so it is worth writing down plainly.
The gas spring must be extending as the lid opens. A push type gas spring pushes outwards, and it is that push while extending that holds your lid up. If your spring shortens as the lid opens, the brackets are on the wrong sides and no force value will rescue it.
Lids that lift upwards
Tool boxes, storage lids, engine covers, machine enclosures, bench hatches
The hinge is at the back edge and the lid opens upwards. The gas spring sits inside the box, anchored below the hinge line and reaching up to a bracket on the underside of the lid. That is the layout drawn above, and it is the layout this tool sizes.
Flaps that fold upwards
Folding worktops, wall counters, serving hatches
Hinged at the top edge on a wall. The panel hangs down when closed and comes to horizontal when opened. A van tailgate works the same way. The rule that makes it behave is a single inequality: place the wall anchor at least twice as far below the hinge as the panel bracket sits along the panel. That one condition does three jobs at once. It keeps the gas spring manufacturable, because the stroke roughly matches the bracket distance and the extended length roughly matches the anchor drop. It keeps the panel bracket above the anchor at every angle, so the rod points down from start to finish. And with the panel hanging down it carries the spring past over centre, where it holds the panel closed instead of opening it, so you need no catch. Choose Wall flap above and the calculator builds the layout for you.
Side hinged doors
Cabinet doors, service panels, enclosure doors
On a vertical hinge there is no gravity moment at all, so there is nothing to balance. What people usually want here is a controlled close, and that is a damper's job, not a gas spring's.
Tube up, rod down
Every manufacturer says the same thing about this. The small amount of oil that lubricates the rod seal sits at the bottom of the tube, so the tube has to be the upper end. Mounted the other way up the seal runs dry, the gas spring loses pressure early and the end of stroke damping disappears. The drawing above shows the correct orientation.
Over centre works both ways
Once the line of action passes over the hinge, the gas spring stops helping and starts pulling the other way. In the middle of the movement that is a fault and the lid slams. At the fully closed position it is the whole point of a wall flap, because that is what holds the panel down. The calculator finds the crossover angle in both cases and tells you where it falls. Ten to thirty degrees from closed is the typical target: you break the lid off its seal by hand and the spring carries it from there.
Two matched springs by default
On a lid, two matched gas springs mounted symmetrically, one on each side, share the load evenly and reduce twisting. A single spring is possible but must be mounted centrally, and the panel must be stiff enough not to flex or go out of line. mounting brackets cover the common pin and ball fittings for both.
Frequently asked questions
Gas spring terms, in plain language
Gas spring, gas strut, gas prop, lift support and damper are names used for the same part depending on the industry. Here is what the figures on a quotation mean.
Get in touch
A question about a fitting, a series, or something this tool does not cover yet? Write straight to our engineering team.
Send us what you have
The figure above is only as good as the bracket positions behind it, and most people do not know those until they are halfway through fabrication. That is fine. Send us the specification this page produced, a photo of the lid, or a picture of the gas spring you are replacing with the printing on the tube readable. Any one of those is enough for us to start.
We build gas springs ourselves from 4-12 to 25-55 and from roughly 25 N to 7500 N. You can see the whole standard gas spring range or read how our custom and OEM manufacturing works; our catalogues list every size we hold. What comes back to you is what we can actually build, not the nearest catalogue item.