Most residential garage doors use just one of these. It is mounted on a shaft, which sits right above the door. During installation, the spring is wound. This is necessary for loading it with enough tension to counterbalance the door given the unit’s weight. Generally, in order for a spring to match the door it has to have specific lift, wire size and inside diameter. The component is connected to drums mounted on the sides of the shaft. There is a lift cable going over each drum. Its bottom end is attached to the bottom of the door. During door opening, the spring unwinds and passes its torque to the drums and then to the cables. This is how the door is lifted up. During closing, it winds and gets loaded once again. The tension held by the spring can be adjusted through winding and unwinding.
These come in a pair of two. They are typically installed over the horizontal tracks. There is a special bolt holding the back end of each spring. The front end is connected to a set of pulleys via a special fork component. The lift cables go over the pulleys. The extension springs are stretched when mounted. This is how they are loaded with tension. When the door is opening, they contract to release the energy held inside them and it is used for lifting the unit. These components usually have a shorter useful life compared to their torsion counterparts. Furthermore, the tension which they hold cannot be regulated.