Replacing Extension Springs
This style spring was used often for the do it yourselfer, they are not as common today, however some doors still have them and they work great, these springs are mounted next to the garage door track, and extend/stretch to do their job, they are called extension springs. Extension springs are not as difficult of a home repair. However, some safety measures are recommended. Remember, when these springs are extended (door down) they are stretched to a potentially deadly force, they may be lifting a 400 pound door. In the industry it is standard for a 7' high door to have 25" springs with an additional stretch of 42", and an 8' high door to have a 27" spring with add. 48" of stretch. ie: the spring needs to stretch half of the doors height for the door to fully close. Also we use a dasma color code to identify the weight pull of extension springs, however, this is just a guide. For example, a red painted spring could indicate 50#, 150#, or 250#, etc. We recomend weighing your door with a bathroom scale to ensure you order the correct springs. One last note; it takes 2 extension springs to pull the total weight of the door. So, a 100# door, requires 2 100# extension springs. Do not devide the weight of the door as you would with torsion springs. If you have any questions or would like assistance, please call 800-997-7606 or email firstname.lastname@example.org
Below is an instructional guide on how to replace an extension spring;
From: Doorman (email@example.com)
Subject: Re: Help Replace Garage Door Spring
1.0 With the help of a friend, lift the door (it will be very heavy) to the fully open position, and place a pair of locking pliers (ViseGrips) on the door track directly under the bottom most roller which runs in the track. (Broken spring side of door).
2.0 Remove what's left of the broken spring. Check the sheaves [pulleys] and replace them if there is significant wear in the bearings, and or if the sides of the sheaves are worn were the cables rub.
3.0 Take the broken spring to your local overhead door dealer and purchase two new springs, identical in length, diameter, and [gauge/wire size] as the broken sample. It's a good idea to pick up new lift cables as well, particularly if you notice any sign of kink or fraying. Remember to check those sheaves and replace if worn.
4.0 Don't disturb the unbroken spring yet as you will need to see how the cable is woven through the sheaves as you replace the broken side. Install the new spring, cable etc. ensuring that the spring is stretched no more than about 1" (door fully open). You may have to adjust this later.
5.0 Move the locking pliers over to the other side of the door and replace the old unbroken spring, cable, etc. in the same manner.
6.0 ENSURE THAT NOBODY IS STANDING NEAR THE SPRINGS. Remove the locking pliers and carefully lower the door with an eye on all that you have replaced. As the door nears the fully closed position, the sheaves located in the end of the spring, must not touch the other fixed sheave, and they might if you stretched the spring out too far when you installed it. Observe the anchor points of the spring to ensure there is no danger of them pulling free from their moorings.
7.0 If the door is heavy to lift and providing there is adequate space between the fixed sheave and the spring sheave, open the door, reposition the locking pliers, and stretch the spring out another 1" or 2". DO NOT EXCEED THE AVAILABLE DISTANCE BETWEEN THE TWO SHEAVES THAT YOU OBSERVED WHEN THE DOOR WAS FULLY CLOSED.
8.0 Finally, make sure that there is safety cable installed, which runs through the springs, and anchored past the further most travel points of the springs. This is important to help retain the springs if they should ever break. I have seen these types of springs when unrestrained go right out through the roof, or wall on occasion, and I have also heard of cases of serious injury to people standing near a spring that has suddenly broken.
Here's my way of saying step 8 above. Add a safety cable if there isn't one already. A safety cable is a roughly 3/16" braided steel cable threaded through the center of the spring (Thinking of the spring as a tube, the cable is threaded through the tube.) and anchored securely at each end (often to some part of the track support) and, positioned so it cannot interfere with opening and closing the door. This cable shouldn't be taut. If the spring breaks this cable will hold the parts, keeping them from shooting
into something or somebody.
Here's some more helpful information from Clopay Doors.