Serviceable Counterbalance / rolling steel door

The problem// why a $50 part triggers a $15k quote

On a commercial rolling steel door, the counterbalance spring lives inside the barrel — the pipe the curtain coils onto — on a fixed cross-shaft. The curtain sheets around the outside of that barrel. When the spring breaks, you can't reach it without taking the door apart around it:

1. Remove the hood.
2. Release curtain tension (or it's already slack from the break).
3. Unbolt the top of the curtain from the barrel.
4. Support the barrel across its span — typically on forklift blades.
5. Lower the heavy tube to the floor (the wound side drops harder).
6. Grind welds / pull the end plug, extract the internal spring.
7. Reverse all of it.

The goal: relocate the counterbalance so the curtain stays sheeted on the barrel and a broken spring becomes a bolt-level swap a two-person crew does from a ladder — no forklift, no barrel drop, no curtain removal. Header/barrel area only; retrofit first, then productize.

The physics you can't design around// the moving target

As the curtain coils up, the coil radius grows while the hanging weight shrinks — so required torque is a nonlinear moving target, highest near closed. A helical torsion spring gives torque linearly with wind (most when fully wound, least as it unwinds). That permanent mismatch is why field techs say rolling doors "don't balance in the middle." Any relocated-spring design inherits this exact curve — but a coupling ratio gives you a new knob to shape it.

The fix — Architecture A (retrofit, ship first)// the dual-bar idea

Put a second, accessible spring shaft up in the header, carrying standard stocked torsion springs, chain-and-sprocket coupled to the barrel. The curtain never leaves the barrel. A broken spring is a winding-bar swap techs already know — from a ladder.

The sprocket ratio N is the key insight: gear the counterbalance shaft to spin faster than the barrel, so a cheap small stocked spring delivers big reflected torque — and the ratio is a free lever to shape the torque curve toward the door's real demand. Gang 2–4 springs (normal commercial practice) and you cover heavy curtains with off-the-shelf parts.

White space: the mechanisms exist in adjacent patents (US 6,327,744 names the unmet need; US 8,590,209 jackshaft+chain; US 10,329,816 chain/sprocket torsion counterbalance) — but nobody has packaged it as a stocked-spring serviceability retrofit for rolling steel. That's the opening.