Depending on the design, you can know in advance how many turns are going to have to be unwound. Lifting a 7-foot door by winding a cable on a 4-inch diameter (about 1 foot circumference) drum requires about 7 turns, plus one extra turn to maintain cable tension at the top-of-travel. Maintaining tension at the top-of-travel is critical; without it the cable will jump off the drum, requiring a serious repair.
Since their invention in the 1920s, electric garage door openers have come a long way. Garage door openers work by using a trolley connected to an arm that attaches to the top of the garage door and slides back and forth on a track, which opens and closes the garage door. When operating the motor, a chain or belt turns and pulls the trolley along the track. A good garage door opener will have a horsepower of 1/2 HP, 3/4 HP, or 1-1/4 HP. Garage door openers have the ability to open and close a limited number of times in power outage emergencies. Security is something else to consider when purchasing an opener. It's helpful to have sensors that will stop the operation of the garage door when a person, vehicle, or other obstacle is in the way.
Prices, like the garage doors themselves, run the gamut. You can pay as little as $400 for a door that you install yourself, or several thousand dollars for a high-end premium door that includes all the bells and whistles, including installation. A mid-range, 16-by-7-foot door will typically cost from $750 to $1,500, installed. Premium insulated steel doors run from $750 to $3,500. According to Remodeling Magazine’s 2017 Cost vs. Value report, the average national cost of a garage door replacement is $3,304.
Most electric garage door openers have two lights: one in front of the opener and one behind or sometimes they sit side by side. They should provide enough light for you to get into your home from your car. Generally, people don’t leave the garage light on when it’s not in use, so it’s nice to have a temporary light when you arrive home. The length of time the light stays on varies according to how it is programmed. In most instances, you don’t need more than a few minutes from the time you leave the car to when you enter your home.
Right-hand versus left-hand winding: Springs are chiral, that is, wound or "laid" in either a left- or right-hand orientation. This is a critical property of their design and specification; you cannot substitute a left for a right or vice versa. If you were to grasp the spring in your hand, and if your right hand orients the tips of your fingers like the ends of the coiled wire when your thumb points "out" of the core of the spring, then you have a right-hand spring; likewise left (which end you grasp does not matter). (This also happens to match the "right hand rule" of magnetic polarity, if you happen to be knowledgeable in such esoteric subjects.) Another way to identify the winding is to examine the spring vertically in front of you; if the coils facing you rise going to the right, it is right-hand (thus you can remember, "rise to the right is right-hand"), and likewise left indicates left-hand. Another way is to view the coil axially; a right-hand spring winds in a clockwise direction as it recedes away, and a left-hand spring counter-clockwise. Yet another way, not so easy to remember, is to hold the spring vertically and compare the coil shape to the letter "Z" (indicates right-hand lay) or the letter "S" (indicates left-hand lay).
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The replacement springs in my case proved to be 0.2253 wire size, 2.0 inch (inside) diameter, and 24 inches long, in a pair of one left- and one right-hand winding. Actually, the old springs in these pictures were a slightly smaller size, but another similar door on this garage was better balanced by that size. Whoever installed the old springs didn't quite get the weight and size just right; it is not unusual to find a repair service installing a slightly off-balance spring size that happened to already be on the truck during the service call. My electric opener had no trouble handling the small imbalance. But since it is safer to reduce the electric operating force as much as possible through careful balancing, I chose the size that was working better on the other door. The Chamberlain brand electric openers (also sold by Sears) I have incorporate a plastic worm gear that tends to wear out after some years of use, requiring a disassembly and installation of a $20 repair kit; this wear is minimized by a properly balanced door.

Resetting the drums, if needed: If the drums were incorrectly set in their old positions, one must reset both drums in new positions on the shaft. This is complicated by the presence of old dimples in the torsion shaft from previous setting(s), which must be avoided lest they improperly influence the new setting of the drums. To begin this process of resetting the drums, the door must first be lowered and resting level on the floor, the spring(s) must be in the unwound condition with their set-screws loosened, and the lift cables wrapped around the drums. If for some reason the door does not rest level on the floor, such as the floor being uneven, then insert temporary shims between the door bottom and the floor to bring the door up to level. Loosen the set-screws on the drums, and turn the torsion shaft to avoid the old dimples from the set-screws in the old drum position. Tighten the set-screw on the left drum (that is, on your left as you face the door from in the garage), creating a new dimple, and apply tension to its cable with the locking-pliers technique, enough tension to keep the cable taut but not enough to start to move the door up. Attach and wind the cable on the opposite (right) drum by hand until the cable is similarly taut, and set the screw, remembering that tightening the screw will tend to add a bit of extra tension to the cable. Both drums should now be fixed on the torsion shaft, with the cables about equally taut (listen to the sound when you pluck them like a guitar string) and the door still level on the ground. Setting the left drum first, and the right drum second, will allow you to take up any slack in the cable introduced by the left drum rotating slightly with respect to the torsion shaft as you tighten the set screws. This alignment and balance of the cables, drums, and door is critical to smooth operation and proper closing. If you have a single-spring assembly, the distance along the torsion tube from the spring cone to one drum is longer than to the other drum, which allows a bit more twist to one side than the other, and you may have to compensate with the setting of the drums.

Garage Door Service Co