Sheave

A sheave is a grooved pulley with a ball bearing in its hub, used in the cable routing of an extension-spring counterbalance system. One sheave mounts on each side of the door, typically at the end of the horizontal track or on a bracket at the back of the track. The lifting cable runs from the bottom bracket on the door, up through the vertical track area, around the sheave, and back to a fixed anchor point near the track. The sheave redirects the cable's direction of pull so the spring's stretching force translates into a vertical lifting force on the door.

The ball bearing inside the sheave is what distinguishes it from a plain pulley. Without a bearing, the cable would have to drag against a fixed post or a bushing under the full cable tension. Friction would be high, the cable would fray quickly, and the sheave body would wear a groove and eventually fail. The bearing allows the sheave to rotate freely under load, reducing friction to a minimum and extending both the cable and the sheave's service life.

Construction:

Sheaves for residential garage doors are typically made from nylon or cast metal with a sealed ball bearing pressed into the bore. The groove around the circumference is sized for the specific cable diameter used in the system, usually 1/8 inch for residential applications. A groove that is too narrow cuts the cable; a groove that is too wide lets the cable slip sideways and pile up on one side.

Sheaves wear in two ways. First, the groove surface wears as the cable cycles over it, eventually becoming rough and fraying the cable strands. Second, the ball bearing can fail from contamination or lack of lubrication, causing the sheave to seize and drag. A seized sheave causes rapid cable wear and can break the cable entirely.

Extension springs, sheaves, and cables function as a complete system. When the cable is replaced, the sheave condition should be checked. If the groove shows deep wear marks or the bearing feels rough when spun by hand, the sheave should be replaced along with the cable.