Choosing a Strut or Spring From a Metric Catalogue
Gas struts come in 100 N, 150 N, 200 N, 400 N. Compression springs are listed by a rate in newtons per millimetre. Meanwhile the job in front of you is described in the other language entirely — a hatch that needs "about twenty pounds of help", a lever that must not take more than five pounds of finger effort, a lid that currently drops shut. Matching the two is a single multiplication, and getting it wrong by a factor of four is easy because 200 N sounds much bigger than the 45 lbf it actually is.
What the Catalogue Figure Is Telling You
A gas strut quotes one point on a curve
A coil spring quotes a slope
Geometry sits between force and feel
Gas force drifts with temperature
Sizing a Mechanism Without Reaching for a Calculator
Most of this work is trying values: what does 150 N feel like, what would 30 lbf be in the catalogue's units.
Type the catalogue rating
Put 100, 150, 250 or 400 in the left field and read the pounds-force straight off. Nothing is submitted, so stepping through a whole product range takes as long as it takes to type the numbers.
Reverse it once you know the target
If the requirement came to you in pounds-force, press the swap button (↔) and work lbf → N instead. Enter 25 and you get 111.21 N, which tells you the 100 N part is light and the 150 N part is the one to try.
Drop to ounce-force for small springs
Search the right-hand dropdown for ounce-force or gram-force when the part is a detent, a switch or a light return spring. A 0.6 N keyswitch reads as 2.16 ozf, a figure that means something at that scale.
Copy the number into your notes
The copy button hands over the value alone with no unit or spacing, so it drops cleanly into a parts list, a supplier enquiry or a spreadsheet comparing candidate parts. Ctrl + C in a field works the same way.
Forces Inside Everyday Mechanisms
Typical figures, arranged from the lightest touch to the heaviest lid, so a catalogue number can be placed against something you have actually felt. Real parts vary with size, stroke and manufacturer.
| Mechanism | Newtons | Pounds-force | Ounce-force | How it feels |
|---|---|---|---|---|
| Mechanical keyswitch, actuation | 0.6 N | 0.13 lbf | 2.16 ozf | A fingertip, barely noticed |
| Push-button detent or small latch | 5 N | 1.12 lbf | 17.98 ozf | A deliberate click |
| Cabinet catch or light return spring | 20 N | 4.50 lbf | 71.94 ozf | A firm tug with two fingers |
| Small lid strut, tool chest or locker | 100 N | 22.48 lbf | — | Holds a light hinged lid open |
| Cabinet or seat-base strut | 150 N | 33.72 lbf | — | Noticeable resistance when closing |
| Bonnet or hood strut | 400 N | 89.92 lbf | — | Takes the whole panel off your arm |
| Tailgate strut, one of a pair | 700 N | 157.37 lbf | — | Needs a pull to bring the tailgate down |
| Heavy hatch or canopy strut | 1 000 N | 224.81 lbf | — | Two hands and body weight to close |
Notice how compressed the middle of that range is. Almost every domestic and automotive strut lives between 100 N and 700 N, which in pounds-force is a span from 22 to 157 — narrow enough that picking one step too high turns a lid that opens gently into one that fights you.
What Suits This to Mechanism Work
Try a whole product range in seconds
Live fields mean stepping 100, 150, 200, 250 through the input is four keystrokes and four answers, which is how strut selection actually happens.
Ounce-force and gram-force for small parts
Both dropdowns are searchable and hold every force unit in the app, so a light detent or switch spring can be read in the unit its data sheet uses.
Requirement-first direction
Swap flips the page to lbf → N, so a target given in pounds-force can be turned into the metric size you have to order.
Values ready for a parts list
Copy returns the bare number, so a shortlist of candidate struts can be pasted into a comparison sheet or an enquiry without tidying.
Strut and Spring Questions From the Workbench
What exactly does the newton figure printed on a gas strut refer to?
It is the nominal extension force, taken close to full extension — the standard measuring point sits a few millimetres short of the rod being fully out, with the strut at room temperature and the rod pointing down. That is the weakest point of the stroke. As the rod is pushed in, the gas volume shrinks and the force climbs; a typical strut is perhaps a quarter stronger fully compressed than the number on its label. So the printed value, 400 N or 89.92 lbf, is a floor rather than an average, and it is the figure to use when you are asking whether the strut will still hold the lid up at the very end of its travel.
Why does a 400 N strut not lift a panel that weighs 400 N?
Because the strut and the panel act about the same pivot with different lever arms, and only the moments have to balance. A strut mounted close to the hinge has a short arm and must push far harder than the panel's weight; mounted further out it needs much less. The panel's own arm also shortens as it rises, which is why a lid can be heavy to start and light to finish. Add a second strut and the required force per unit halves, which is exactly why tailgates use pairs. The honest way to size the job is to work out the moment the panel produces about its hinge, divide by the strut's own arm, then convert — 89.92 lbf of strut is not 89.92 lbf of lifting at the panel edge.
My spring is specified in N/mm — how does that become lbf/in?
Multiply by 5.7101. The factor comes from converting both halves: 0.22481 lbf per newton multiplied by 25.4 mm per inch. So a 7 N/mm spring is about 39.97 lbf/in, and a 70 N/mm coilover spring is roughly 400 lbf/in — a figure suspension people recognise instantly. This page converts plain forces, so use it once you have turned a rate into a force by multiplying by a deflection: 7 N/mm compressed 30 mm gives 210 N, which is 47.21 lbf of push at that point. Confusing rate with force is the classic error here, and it is out by whatever the deflection happens to be.
When is ounce-force the better unit to work in?
Whenever the number in pounds-force starts with a zero and a decimal point. Switch springs, detents, contact pressures, tape tensions and light return springs all sit under a newton or two, where 0.13 lbf tells you nothing but 2.16 ozf is a figure you can compare against a data sheet. Keyboard switches are a good example: the trade quotes actuation in grams — which is really gram-force — and 45 g, 60 g and 80 g springs correspond to 0.44 N, 0.59 N and 0.78 N. Both of those units are in the dropdowns, so the small end of the scale is a search away rather than another calculation.
Why do catalogues only offer round metric steps rather than the exact force I need?
Because the parts are built around a small number of cylinder bores and charge pressures, and stocking every intermediate value would be uneconomic. The practical steps are typically 50 N apart in the low range and 100 N apart higher up, so a requirement that converts to 118 lbf — 525 N — is going to be served by a 500 N or a 600 N part. Choose deliberately: on a lid that must not slam, err high; on anything a person has to pull closed, err low, since the extra force is felt in the hand every single time. Some suppliers will also charge a strut to a specified value, which is worth asking about when the geometry leaves no margin either way.
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