There are two different types of garage door springs, although both involve big wire coils that are vital to lifting or lowering garage doors. The two types are torsion springs, which are used with sectional roll-aways, and extension springs, which are used with swing-ups. The torsion garage door springs will be located on either side of the center of the door frame, while extension springs are on swing-out arms placed on either side of the door itself.
Garage door springs can be easily broken, making the garage door difficult to open, or even putting the vehicle or driver at risk (by dropping on them). The best way of preventing this kind of danger is to check your doors regularly for corrosion, loosening bolts, or worn-out parts which can suddenly snap. It is also a good idea to check the tension in the spring, as a poorly-adjusted spring can break unexpectedly.
Step 1 - Check the Length
Measure the old and new springs to ensure that they are the correct size.
Make sure that you don't touch the winding cone, as a torsion garage door spring can revolve at the slightest touch. Be extremely careful while working!
Step 2 - Get the Springs Right
Place the new garage door springs against the garage door, position the springs so that the ends are facing you, and coming from opposite directions.
The left spring should have the wire coiling from the left, the right spring should have the wire coiling from the right. Take up the left garage door spring, and put it by the left side of the door. Take the right spring and place it by the right side of the door.
Step 3 - Take off the Old Spring
Remove the old springs. This is exceptionally dangerous; make sure that you have a properly fitting bar placed in the winding cone, and use the recommended wrench. Do not use a screwdriver under any circumstances. Unwind the coil slowly, keeping a tight grip on the wrench. When the spring is loose enough, remove the spring completely from the garage door.
Step 4 - Put on the New Spring
Slide the new spring onto the shaft: the bigger hole in the cone goes at the stationary end of the shaft. Check that the wire is tight enough. If the wire starts to spin after a few turns, you have placed the wrong garage door spring on the shaft. Remove the wrong spring and place on the correct one.
Step 5 - Finish Up
Place the cable drum back in position, and screw in tightly.
Replacing a torsion garage door spring is hard work, and can be quite dangerous: you may be better off asking a local professional to perform this kind of home improvement, as they are more experienced in handling torsion garage door spring mechanisms than the average DIYer.