Why both springs get replaced together
On a dual-spring door, both springs were fitted on the same day and have done exactly the same number of cycles. When one fails, the other is not healthy — it is simply a few weeks behind.
Fitting one new spring against one aged spring also unbalances the door, because the two no longer pull with the same force. The door lifts skew, one cable takes more load than the other, and the rollers grind against the track on the loaded side. Replacing the pair costs marginally more once and avoids a second call-out, a second labour charge and a door that never quite ran right in between.
How a torsion spring actually works
A torsion spring sits on a steel shaft above the door opening. As the door closes, the cables wind onto drums at each end of that shaft and twist the spring, storing energy. When you open the door, the spring unwinds and returns that energy, lifting the weight for you.
Balance is the whole point. On a correctly sprung door you can stop it at waist height with the motor disconnected and it will simply stay there — neither falling nor rising. That single test tells a technician more about a door's condition than any electronic diagnostic.
Torsion springs versus extension springs
Torsion springs are the modern standard on sectional doors: mounted centrally, wound under controlled tension, and safer in failure because the spring stays captive on its shaft.
Extension springs stretch along the horizontal tracks and are still found on tip-up doors and older installations. They do the same job less elegantly, and when one breaks it can travel — which is why safety containment cables through the centre of an extension spring are not a nice-to-have.
Both types are fitted and replaced. What we will not do is fit a torsion conversion to a door that does not have the headroom for it, or leave an extension spring without its containment cable.