What it does
Torsion springs carry the weight of the door so the motor does not have to. A pole runs across the opening and the springs slide onto it, winding and unwinding as the door travels. Serviced once a year a set lasts up to ten years. Extreme swings in temperature are what break most of them, which is why they tend to go when nothing about your routine has changed.
Fitting notes
Sizing is measured, not looked up. The door has to be weighed and its height measured to establish whether it is a standard or a caravan height door. Where a door needs more than one spring it takes a left and a right, never two of the same hand. If one breaks we replace both, so the pair wears out together.