It is common for garage door extension springs to be used in light residential overhead units. Typically, one door system requires a set of two springs of this type. They are usually made from galvanized steel which is more resistant to rust and corrosion than plain steel. The size of these hardware components is selected in line with the height of the overhead door.
One spring is set on each side of the door. In residential door systems, these components are mounted above the horizontal tracks. On one side, each spring is attached to the track hander which is mounted on the ceiling. On the other, it is connected to a sheave. This is done with the use of a specific fork component. There is a lift cable that goesover the sheave. Its bottom end is attached to the lowest section of the door. There is another cable that runsthrough the spring. This is a safety cable whose job is to keep the door in place in case the spring breaks.
When the door is in closed position, the extension springs stretch above the horizontal tracks. They are loaded with force corresponding to the weight of the door. This is how they keep it balanced and stable. When the opener starts taking the door up, the springs contract and release the force by passing it to the pulleys and the lift cables. This force is used for lifting the door upwards and then above the floor. During the closing of the door, the springs stretch again and get loaded with force.
Garage door extension springs and the components connected to them require regular inspection. It is particularly important for the safety cables to be checked as part of this maintenance job.