Extension Spring

An extension spring is a steel coil spring that works by stretching rather than twisting. On a residential sectional garage door, one spring mounts above each horizontal track, attached at one end to a stationary bracket near the back of the track and at the other end to a sheave (a cable pulley) via an S-hook. A cable runs from the bottom bracket of the door, up to the sheave, and back to an anchor point. When the door is closed, the spring is fully stretched. As the door rises, the spring contracts and its stored energy pulls on the cable, helping lift the door.

Extension springs are the older, simpler system. You will find them most often on doors with 8-foot or less ceiling height where the lower headroom makes it difficult to run the shaft and cable drums of a torsion system. One practical giveaway: if you see springs running parallel to the horizontal tracks with visible coils, those are extension springs. If there is a thick coiled spring mounted horizontally above the center of the door on a steel shaft, that is a torsion spring.

How they compare to torsion springs:

Extension springs apply force through a pulley-and-cable geometry, which means each spring can only counterbalance one side of the door. A broken extension spring leaves the door unbalanced and can cause it to fall or travel crooked. By contrast, a torsion spring system with two springs on one shaft keeps the shaft turning even if one spring breaks.

A safety cable threaded through the interior of the extension spring is a critical companion part. If the spring snaps under tension, the safety cable contains the broken spring rather than letting it fly across the garage. Many building codes and UL 325 safety guidelines require or strongly recommend safety cables on extension-spring installations.

Common residential extension springs stretch to about 100 inches of open-door travel and are rated for roughly 10,000 cycles. They are sized by the door's weight and height, with heavier doors requiring stiffer springs (measured by their inch-pound rating per inch of stretch).

For a two-car door weighing 200 pounds, for example, each extension spring must provide about 100 pounds of lift. A technician confirms the rating by checking the spring's color-coded end-coil markings or the stamped part number.