A sectional door, the most common variety, rolls up and down on steel tracks.
1. Torsion Spring
Counterbalances door's weight so that it can be lifted easily.
Guides door up to ceiling and back.
Rides inside track. Steel ones are noisy; nylon is quieter.
Connects horizontal sections.
6. Lock Bar
Engages with track to prevent door from opening.
Seals door edges against air infiltration.