IPPT (inch-pounds per turn) is the torque a torsion spring delivers per full turn of wind — the spring's strength rating. A door is balanced when IPPT × turns × springs = door weight × drum moment arm.
Spring on the Shaft
IPPT by length — 0.25" wire, 2" ID
| Length | IPPT | Max Turns |
|---|---|---|
| 20" | 68.08 in-lb | 5.3 |
| 24" | 56.11 in-lb | 6.4 |
| 28" | 47.72 in-lb | 7.6 |
| 32" | 41.51 in-lb | 8.7 |
| 36" | 36.74 in-lb | 9.8 |
| 40" | 32.94 in-lb | 11.0 |
| 44" | 29.86 in-lb | 12.1 |
Longer spring = lower IPPT but more cycles at the same load.
References & Standards
- Formula: IPPT = E × d⁴ / (10.2 × D × active coils) — SSC Spring Engineering convention
- DASMA Technical Data Sheets — torsion spring references