Two of the spring references specifically for the garage-door industry are the APCO Spring Manual by Bill Eichenberger, and the Torsion Spring Rate Book by Clarence Veigel; these give tables of spring sizes and torque constants. Spring engineering principles in general are described in the Handbook of Spring Design published by the Spring Manufacturer's Institute; the formulas allow you to calculate torque constants knowing only the geometry and the Young's modulus of the material. You can also find some brief spring information in standard references like Machinery's Handbook and Marks' Standard Handbook for Mechanical Engineers.
Uncentered center bearing plate: The center bearing plate need not actually be in the center. It doesn't much matter where it is, since the purpose of the bracket is to anchor the spring ends. This anchoring must be secure, since all the torsion is held together at that point. On a stud-framed wall, this bracket may be placed over the stud closest to the center rather than exactly at the center of the door opening.
Today's garage doors come with those door sensors, and if anything is in the path of the door the sensor detects it and raises the door back to the open position. These precautions are important for families with young children or pets. Even if you don't have a mini me or furry friend running around your yard, it's a good idea to have a garage door that is safe for you, your guests and your belongings. 
Measure springs only when relaxed: Measurements must be taken on a relaxed spring because the winding adds significant overall length while reducing the coiled diameter. If you have a paired design, and one is broken and one is intact, then don't try to measure the length of the intact spring with the door down. A wound spring has 7 or 8 turns adding to the overall length, and will therefore be about 2 inches longer than when relaxed. Measure the lengths of the pieces of the broken spring, which will be unwound, and add them together. As a check, one can measure the length of the intact spring after it is unwound in the procedure to follow below. Be sure also to observe whether the springs are originally of equal sizes, because it is quite possible that they are not.
For Sears Garage Door Installation and Repair Services, making your garage door safe is as important as making it function well. For this reason, all of our repair services and tune-ups feature a 20-point safety check. Our technician will carefully examine your garage door panels, springs, sensors, safety release, hardware, track, and more. This ensures that, not only has our work been done well, but that your garage door poses no danger to your vehicles or family. Due to the intricacy of garage door systems, only garage service professionals should attempt to adjust, repair, or service door equipment.
Electric Garage Door Openers – Service and repair of the electric garage door opener itself, including the lift mechanism that pulls the door up and guides it down. This is typically not part of the garage door itself and is serviced and repaired on its own interval. Typical service includes inspection, repair, adjustment, and lubrication if needed. Also, we typically inspect the mounting of the unit as well as its attachment to the door itself.
Having a mismatched pair makes it difficult to specify the correct matched-pair replacements. To obtain replacement springs for a mismatched pair, you can either specify the same odd pair, try to calculate the equivalent matched pair sizes, or (this is the best method:) measure an accurate door weight and calculate the right spring size(s) "from scratch". The spring seller should be able to do the calculations from your accurate measurements of weight, height, and drum size; or you can attempt the calculations yourself using my engineering formulas below.
Lift cable placement: On the standard residential door mechanism, the loops at the lower ends of the two lift cables loop over the two bottom roller shafts which project from the bottom bracket on the door. The upper cable ends fasten to the drums using one of the methods described above. The drums are positioned along the torsion shaft such that the inner edge of each drum is approximately over the edge of the door. The cable winds onto the drum from outside in, so at the top of travel the cable is winding onto the inner edge of the drum, vertical from the edge of the door where it is looped over the roller shaft. As the door is lowered, the cable winds out to the outer edge of the drum, and thus is a bit out from the vertical, but the cable still falls in the gap between the guide rails and door edge. My cables rub and slap on the rails a bit, but after 30 years and many 10,000s of cycles, they don't seem to have worn at all.
Safe automatic door openers. All automatic openers must now have an auto-reversing mechanism and photoelectric eyes located near the floor on both sides of the door (see photo). If the door is closing and the beam between the eyes is interrupted, the door will automatically reverse. If the eyes aren’t connected, the door won’t operate. For instructions on how to install a new garage door opener, see How to Install a Garage Door Opener.
This is the electric opener which operates this door. I'm picturing it here because you pull the rope to disconnect the trolley, run the trolley under power to the fully-open position, and then disconnect power before working on the door. Then you should lock the door down with either the security lock or with Vise-Grips or C-clamps. This avoids the door lifting when you don't expect it as you are applying spring adjustments. It you were to foolishly overwind the spring without the door locked down, you could possibly find the door trying to leap up to the raised position when you aren't prepared. That would likely knock your grip off the winding rods, with potentially disastrous results. I like to work under the safety principle that serious accidents should be physically possible only when you make two or more stupid mistakes at the same time.
Garage doors cause injury and property damage (including expensive damage to the door itself) in several different ways. The most common causes of injury from garage door systems include falling doors, pinch points, improperly adjusted opener force settings and safety eyes, attempts at do-it-yourself repair without the proper knowledge or tools, and uncontrolled release of spring tension (on extension spring systems).
First and foremost, a garage door, by design, contains springs designed to balance your door and make it easier to lift. Those springs are under incredible amounts of tension. If a spring breaks or is improperly released, it can cause incredible and potentially fatal injuries. Keep in mind, when working on a garage door spring, it is likely that your face and head will be close to it, meaning that your most sensitive area will be in the direct path of the released spring.

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