When a spring breaks on a garage door an immense amount of energy being displaced. Torsion springs are mounted over the garage door and they have a shaft running through the middle. When they break, the spring unwinds in less than a second and creates a loud noise due to the coils spinning on the shaft. Based on customer feedback, it can be quite startling.
Professionals can replace garage door springs in an hour or two. When you hire a pro, make sure to ask about the quality of the springs they will install. They may well offer several grades of springs to choose from, at a range of costs. Top of the line springs may be guaranteed for life, while economy springs can be expected to last perhaps five years under normal use.
Snap a locking pliers onto the torsion tube to lock it into place while you tighten the drums. Rotate the drum to wind the cable into the winding grooves. Pull the cable as tight as possible before tightening the setscrews. Leave the locking pliers in place and repeat the tightening procedure on the other side. You want equal tension on both sides. Otherwise, the door will open unevenly.
In most cases, only one spring breaks or wears out at a time, and you can get away with replacing just the failed spring. But this is a little like replacing old car tires one at a time. You'll get the best performance if all the springs are new and have the same strength. Extension springs are replaced individually, making it more tempting to replace just one. With torsion springs, you have to disassemble everything to replace either spring, so it makes sense to swap out both springs during the repair.