Garage door springs are designed to constantly counteract the effects of gravity on even the largest of garage doors. Properly adjusted, garage doors only require a slight amount of pressure to open or close them despite weighing several hundred pounds or more. Adjusting the tension of garage door springs is not normally required, but will make a significant difference in the operation of the doors and is not a hard task to accomplish.
Determine what type of garage door springs you have, twisting or pulling. Torsion springs (twisting) are generally mounted on a fixed metal shaft attached above and parallel to the top of the garage door track assembly. Side mounted springs (pulling) are installed in pairs, and are relatively long springs mounted transversely on either side of the garage door track assembly.
Torsion springs need to be adjusted with the door in the closed position, so be sure to have your tools with you before closing the door. If you have additional access to the garage (such as a window) that is even better. Secure the door from opening by putting a clamp on the garage door track above the bottom door roller. This will keep the door closed as you apply more tension to spring.
Locate the torsion spring adjustment collar that holds the spring in place on the spring centre shaft. It will be on the end of the spring that twists. The adjustment collar will have holes around its perimeter, which will allow you to move it. A set screw holds this collar in place, so find the correct tool (screwdriver or wrench, etc.) that will loosen it when necessary.
Insert a steel rod into an available hole on the adjustment collar, and apply slight pressure to the spring. Unscrew the set screw enough to allow the adjustment collar to move freely. Twist the torsion spring with the steel rod one quarter turn to add or subtract power from the spring. Tighten the set screw firmly. Remove the clamp from the door and test the door operation several times. It may be necessary to repeat this step several times until you find the proper amount of preset spring tension.
Locate both side mounted springs if torsion springs were not used on your garage door. Side mounted springs cannot be adjusted and must be replaced if they are worn out. Open the garage door to the fully open position, and secure the door to garage door track with a clamp. Disconnect and remove the safety cables from the springs, remembering how they were installed. Side mounted springs are attached at two points: 1) the garage door track and 2) a pulley through which the garage door cable is routed. Disconnect both points and reattach the new springs in the same manner as they were removed. Reattach the garage door safety cables after reattaching the springs at either end.
Always test the door operation several times after adjusting or replacing the springs to ensure correct performance.
Garage door springs are very powerful. Use caution when attempting to adjust or replace them.