Quick answer: Case lid stay choice starts with the lid moment: 4.2 kg at 210 mm gives 8.6 N·m, so a strut holds mid-angle and a strap stops at 95 degrees.
Table of contents
- Case Lid Stay Restores the Lid Moment
- Work Out the Moment First: 4.2 kg at 210 mm
- Gas Strut or Strap: Who Does What
- Assembly Order and the Belt-and-Braces Setup
- FAQ
1. Case Lid Stay Restores the Lid Moment
A lid that drops the moment the hand lets go, or one that has to be held at the top, is not a loose hinge. The lid's own weight is generating a moment that nothing balances, and that is the single job of a case lid stay.
The moment is lid weight times the horizontal distance from the centre of gravity to the hinge axis. A 4.2 kg lid with its centre 210 mm from the axis produces 8.6 N·m, and the figure peaks when the lid sits close to horizontal.
Stand the lid at 90 degrees and the centre of gravity sits almost over the axis, so the moment drops to nearly zero. Open another 5 degrees and the centre crosses behind the axis, leaving about 0.8 N·m that keeps pushing the lid backwards. That residual load is why 95 degrees needs a hard stop rather than friction.
Without a stay, every opening is a fight with the lid.
2. Work Out the Moment First: 4.2 kg at 210 mm
Weigh the lid first. Put the whole case on the scale, then weigh the body alone; the difference is the lid, and 4.2 kg is typical for a mid-size model.
Find the centre of gravity next. Lay the lid on a round bar and slide it until it balances; the distance from that point to the hinge axis is the arm, usually 210 mm on this class of case.
Multiply last. 4.2 kg times 9.81 times 0.210 m gives 8.65, rounded to 8.6 N·m. Add a 3 kg monitor in the lid and the lid becomes 7.2 kg with a 14.8 N·m moment, which moves the selection up one class.
One geometric value can be changed. Raising the hinge axis 12 mm shortens the arm from 210 mm to 187 mm and cuts the moment by 11%, leaving a 120 N strut with spare capacity.
3. Gas Strut or Strap: Who Does What
A gas strut delivers constant thrust and can park the lid at any intermediate angle. A 120 N strut mounted 150 mm from the hinge produces 18 N·m at full lever, more than twice the lid moment, though real geometry only approaches that figure through part of the stroke.
Too much thrust and the lid springs upward, forcing you to push it shut. Too little and it still drops halfway. Sizing at 1.5 to 2 times the lid moment is the rule that holds up on the floor.
A nylon strap is an end stop, nothing more. At 25 mm wide with 1800 N breaking strength and four stitch rows, it catches the residual moment plus rebound at 95 degrees, where the working load is around 200 N and the safety factor is 9.
| Option | Lid moment, N·m | Holds mid-angle | Lid weight |
|---|---|---|---|
| Nylon strap | 0.8, end stop only | no | under 3 kg |
| Gas strut | 8.6, balanced mid-stroke | yes | 3 to 6 kg |
| Strut plus strap | 8.6 plus 0.8 | yes | over 6 kg or side wind |
The third option is not belt-and-braces for its own sake. Above 6 kg, or where the case is opened in side wind, a strut still has spare thrust at 95 degrees and keeps pushing the lid back. Only the strap can catch that.
4. Assembly Order and the Belt-and-Braces Setup
Assembly order causes more failures than selection: the same strut with the mount 10 mm off feels different, and no drawing shows it.
Follow this order and two rounds of rework disappear:
- Open the lid to 95 degrees, prop it with a wooden stay, and measure the arm to the hinge axis
- Set the lower strut mount 150 mm from the hinge, clear of the seal compression zone
- Tack the upper mount, cycle the lid 3 times by hand, and look for a dead point
- Fit the strap with the stitched end against the body and all 4 stitch rows on the boss
- Remove the prop, cycle 20 times, then measure sag after 10 minutes at 95 degrees
Sag beyond 5 mm means the thrust is short or the mount is off; fix the mount before ordering a stronger strut.
Strap length sets the end angle: 10 mm short and the lid stops at 92 degrees with the strut still pushing, 10 mm long and it runs past 95 degrees.
| Lid moment, N·m | Lid weight, kg | Recommended |
|---|---|---|
| under 4 | under 2 | single nylon strap |
| 4 to 9 | 2 to 4.5 | 120 N gas strut |
| 9 to 14 | 4.5 to 7 | 150 N strut plus strap |
| above 14 | above 7 | twin struts plus strap |
Twin struts go in pairs, one each side, sharing the thrust. A single strut offset to one side twists the lid, and the 95 degree stop ends up higher on one edge.
Three checks close the job. Measure the height difference across the stop and reset the mounts if it passes 3 mm. Measure the lid-to-body gap at full opening, since anything under 5 mm catches fingers. Press the strap, which should give about 10 mm.
Conclusion
A case lid stay is selected from the lid moment, not from thrust. A 4.2 kg lid with its centre 210 mm from the axis gives 8.6 N·m, and a 120 N strut 150 mm out offers 18 N·m, enough to park the lid mid-angle. Above 6 kg or in side wind, add a 25 mm strap as the 95 degree stop.
Related Reading
Related Products
- 8 Series — Gun Cases; Military Equipment Cases; Camera Equipment Cases; Drone Cases; Watch Cases; Knife Cases; First Aid Cases; Electronics Equipment Cases
- WL8-2812 PP Gun Case — model WL8-2812
- All protective cases and toolboxes: 7 series, 158 models