Garage Door Torsion Spring Torque, Turns, and Rate

The torque it must produce is the door's weight acting through the cable drum radius, the turns come from the door height, and the required spring rate is one divided by the other. A torsion spring balances a door by storing exactly as much torque at the closed position as the door weight exerts through the drum, so torque is weight times drum radius and that is the target. The number of turns is pure geometry -- the cable has to wind the full door height onto the drum, and one turn takes up the drum's circumference. Spring rate in inch-pounds per turn, IPPT, is what a supplier is given, and it is the target torque divided by the turns. Note what that means: two doors of the same weight but different heights need springs of DIFFERENT rate, because the taller door gets more turns to reach the same torque, and two springs share the rate with each carrying half. A 7 ft door weighing 150 lb on a 2 in effective cable radius wants 300 in-lb over 6.68 turns, which is 44.9 IPPT total or 22.4 IPPT per spring in a pair. Add an insulated panel that brings it to 190 lb and the required rate rises 27 percent with the weight; raise the door to 8 ft at the original weight and the rate FALLS to 39.3, because the same torque is reached over more turns. Weight and height pull in opposite directions. The balance is exact at only one position, because the spring is linear and the door's demand is not once it starts breaking over the radius, which is why a properly balanced door still needs a few pounds of hand force mid-travel. TORSION SPRINGS ARE STORED ENERGY AND THEY INJURE PEOPLE. Winding, unwinding, and replacing them is done with proper winding bars by someone trained to do it, and a broken cable or a slipping drum turns a wound spring into a projectile. This calculates what a spring must do; it does not tell anyone how to install one and no one should learn that here. It does not select a spring from wire size, inside diameter, and length -- that is the manufacturer's table, which also sets the cycle life most owners actually care about -- and the helical torsion spring rate calculation here is the one that goes the other way, from wire geometry to rate. It assumes a linear spring pair on a standard-lift door with matched drums; high-lift, vertical-lift, and low-headroom conversions change the drum geometry. The door and hardware manufacturers and a qualified installer govern.

required torque = door weight x cable drum radius; cable travel per turn = 2 pi x drum radius; turns = door height / travel per turn; required rate IPPT = torque / turns; per spring = that / number of springs.

The torsion-spring balance relation -- torque equals door weight times cable drum radius, turns equal door height divided by the drum circumference, and required rate equals torque divided by turns -- with the inch-pounds-per-turn (IPPT) rate convention used by overhead door manufacturers, by name. Torsion springs are stored energy: this states what a spring must do and is not installation instruction. The door and hardware manufacturers, their spring and cycle-life tables, and a qualified installer govern.

The balance relation and the drum geometry are public arithmetic; the spring itself is selected from the manufacturer's table, which is cited and not reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: door_weight_lb, door_height_in, drum_radius_in, springs, required_torque_inlb, travel_per_turn_in, turns, required_ippt, ippt_per_spring

• Balance at the closed position the spring is matched at one position only; a linear spring cannot follow the door's demand through the whole traveloverhead door practice
• Weight and height oppose heavier raises the required rate, taller lowers it, because the same torque is reached over more turnsoverhead door practice
• Not an installation method a wound torsion spring is stored energy and is handled with winding bars by a trained installeroverhead door practice