Spring Selection Guide: Gas Spring Force, Stroke and Mounting Explained
Gas spring selection requires calculating four critical parameters: required force, stroke length, compressed length and end fitting type. Getting any parameter wrong leads to installation failure, safety risks or premature wear.
⚡ In a Rush? Key Takeaways
- Required force = (Lid weight × Distance from hinge to CG) ÷ (Number of springs × Distance from hinge to spring mount)
- Stroke length must match lid travel geometry – measure compressed and extended eye-to-eye distance difference
- Gas springs lose approximately 1.5% force per °C below rated temperature – critical for outdoor applications
- Undersizing by 20N causes lid drift; oversizing by 20N makes lid difficult to close manually
- ✅ Always verify force calculation with our Gas Spring Force Calculator before ordering
How do I calculate the required force for a gas spring?
Required spring force equals (lid weight multiplied by centre of mass distance from hinge) divided by (number of springs multiplied by spring mounting distance from hinge).
The fundamental force calculation for a horizontally-opening lid is:
F = (W × Dcg) ÷ (N × Ds)
Where:
- F = Required force per spring (Newtons)
- W = Lid weight (Newtons)
- Dcg = Distance from hinge to lid centre of mass (metres)
- N = Number of springs supporting the lid
- Ds = Distance from hinge to spring mounting point (metres)
For example, a 500N lid with centre of mass 0.3m from the hinge, using two springs mounted 0.2m from the hinge:
F = (500 × 0.3) ÷ (2 × 0.2) = 150 ÷ 0.4 = 375N per spring
Always select the next standard force rating above your calculation (e.g., 400N in this case) to provide a 15% margin for temperature effects and wear. Never select below your calculated force – the lid will not hold position.
What units should I use in gas spring force calculations?
Use Newtons for force, metres for distances in calculations – convert millimetres to metres by dividing by 1000.
Gas spring force is always specified in Newtons (N). Convert lid weight from kilograms to Newtons by multiplying by 9.81 (e.g., 50kg × 9.81 = 490.5N). Convert all length measurements to metres for the formula, then convert the result back to Newtons. Using mixed units (e.g., mm with N) produces dangerously incorrect results.
How does lid weight distribution affect force calculation?
Measure the actual centre of mass – assuming it’s at the geometric centre introduces errors up to 40% for irregularly shaped lids.
For non-uniform lids (e.g., with hardware concentrated on one edge), find the true centre of mass by balancing the lid on a knife edge or using CAD software. An error of 50mm in Dcg for a 0.5m lid creates a 10% force error. In automotive applications, boot lids often have heavier latch mechanisms offset toward the rear, shifting the centre of mass significantly rearward.
How do I determine the correct stroke length for my gas spring?
Stroke length equals the difference between extended and compressed eye-to-eye measurements of the spring mounting points through the lid’s full travel range.
Do not guess stroke length from lid opening angle. Instead:
- Measure the distance between spring mounting points when the lid is fully closed (compressed length)
- Measure the same distance when the lid is fully open (extended length)
- Stroke = Extended length – Compressed length
- Add 10-15% preload to the calculated stroke when selecting a spring
For a lid opening from 0° to 90°, the spring mounting points typically move through a non-linear arc. Direct measurement eliminates geometry errors. A common mistake is measuring lid travel in inches and assuming that equals spring stroke – this underestimates requirements by 20-30% for most mounting configurations.
What happens if I specify insufficient stroke length?
Insufficient stroke prevents the lid from reaching full open position – the spring bottoms out before travel completion.
If the spring’s stroke is too short, it reaches full extension while the lid is still partially closed. This creates two problems: the lid cannot achieve its intended open position, and the spring experiences excessive stress at end-of-travel, accelerating seal wear. Always verify with physical measurement – never rely on lid angle alone.
How does mounting position affect effective stroke?
Mounting the spring closer to the hinge increases required stroke length – halving the mounting distance approximately doubles the stroke needed.
The relationship between spring mounting distance (Ds) and required stroke is inverse. If you move the spring mount from 200mm to 100mm from the hinge, the spring must travel twice as far to achieve the same lid opening angle. This often catches designers off-guard when retrofitting springs to existing mechanisms.
What mounting geometry errors cause gas spring selection failures?
Ignoring mounting angle introduces force calculation errors of 25-50% – always measure the actual angle between spring and lid at mid-stroke.
The basic force formula assumes the spring mounts perpendicular to the lid. When mounted at an angle, the effective force decreases by the cosine of the angle from perpendicular. A 45° mounting angle reduces effective force to 71% of the rated value. For example, a 300N spring mounted at 45° delivers only 212N of lifting force.
How do I account for mounting angle in force calculations?
Multiply the calculated force by the secant of the mounting angle (1 ÷ cosine angle) to determine the required spring rating.
If your calculation shows you need 200N of effective force and the spring mounts at 30° from perpendicular:
Required spring rating = 200N ÷ cos(30°) = 200N ÷ 0.866 = 231N
Select a 250N spring. Always measure the actual installed angle – theoretical angles from drawings often differ due to bracket flex or manufacturing tolerances.
What mounting errors cause premature gas spring failure?
Misaligned mounting points create side loads that bend the piston rod – exceeding 2° misalignment reduces seal life by 60-80%.
Gas springs are designed for axial loads only. Any lateral force causes the piston rod to rub against the cylinder bore, damaging seals and scoring the rod. Common causes include:
- Mounting brackets not parallel to each other
- Lid flex creating changing alignment during travel
- Using unsuitable end fittings that allow angular movement
Use clevis or ball socket mounts that accommodate minor misalignment, and verify alignment with a laser level or feeler gauges during installation.
How do temperature and cycle life affect gas spring selection?
Standard gas springs lose 1.5% force per °C below 20°C rating – select low-temperature variants for outdoor UK applications below 5°C.
Nitrogen pressure inside the spring decreases with temperature. A spring rated at 300N at 20°C delivers approximately:
- 255N at 0°C (15% loss)
- 210N at -20°C (30% loss)
- 165N at -40°C (45% loss)
For outdoor equipment in Scotland or Scandinavia, specify springs rated to -40°C or apply a 40% force margin. Cycle life is equally important – standard springs rate 50,000 cycles; for high-cycle applications (e.g., industrial machinery), select 100,000+ cycle variants.
How do I calculate temperature-adjusted force requirements?
Multiply your room-temperature force calculation by [1 + (0.015 × (20 − T_min))] where T_min is minimum expected temperature in °C.
If your calculation requires 200N at room temperature and your equipment operates down to -10°C:
Temperature factor = 1 + (0.015 × (20 − (-10))) = 1 + (0.015 × 30) = 1.45
Required spring rating = 200N × 1.45 = 290N
Select a 300N spring. This calculation prevents the dangerous scenario where a spring holds a lid open at 20°C but allows it to drop slowly at -10°C.
What cycle life should I specify for different applications?
Automotive boot lids: 50,000 cycles minimum; industrial access hatches: 100,000+ cycles; furniture: 30,000 cycles typically sufficient.
Estimate cycles per day: A car boot opened twice daily = 730 cycles/year. A factory machine guard opened hourly = 8,760 cycles/year. Match spring cycle life to your application’s expected lifetime. Paying 20% more for double the cycle life often reduces lifetime cost by avoiding mid-life replacements.
What are the most common gas spring selection mistakes?
Selecting by OEM part number without verifying force and stroke causes 65% of replacement failures – always measure your existing spring.
Based on 18 years of industrial engineering experience, these errors recur constantly:
- Assuming OEM replacements match without measurement (wear changes force requirements)
- Ignoring centre of mass shifts from added hardware or modifications
- Selecting stroke based on lid angle rather than physical measurement
- Overlooking temperature effects in outdoor or cold-storage applications
- Using incorrect end fittings that allow damaging side loads
The single most costly error is failing to verify the actual force requirement –
