Speed of a falling door:: Physics tells us that the transit time of a free-falling body is sqrt(2x/g), where x is the length of the fall and g is the acceleration due to gravity (32.2 ft/sec^2). If this typical 150 lb door were to fall an equivalent of 3.75 feet, this falling time would be sqrt(2*3.75/32.2) = 0.48 seconds (480 milliseconds). The terminal velocity is gt, or 0.48 seconds * 32.2 ft/sec^2 = 15.5 ft/sec = 10.6 mph.
See the Sectional Overhead Garage Door catalog (PDF file) from the Prime-Line replacement hardware company. Their brochure Sectional Garage Door Torsion Spring Installation Instructions (PDF file, part number GD-12280) is brief but informative. Apparently you can only obtain their torsion springs as special order items through mom-and-pop type hardware stores like Ace and True Value, and not the big-box Home Depot and Lowes.
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Here's a view of my door and its broken torsion spring. This door is 10 feet wide and 7 feet high, constructed of 3/4 thick hollow wood panels inside with 3/4 inch plywood siding outside to match the house exterior. This is original to the house which was constructed in 1978, and is much heavier (238 pounds, as I measured later as described below) than the steel doors most common today in new construction. The 10-foot width is a little larger than usual for a one-car garage; such doors are typically only 7 or 9 feet wide. The ceiling height is 9 feet, providing 18 inches clearance above the torsion shaft. This is in a 3-car garage with 3 separate extra-wide doors. Every man's dream! ('cept when the door is broke.)
The torsion shaft with lift drums on the ends is above the door. The standard residential door shaft is a 1-inch outside diameter hollow steel tube. The inside diameters of the bearings, drums, and winding cones are sized to loosely fit that 1-inch diameter shaft. At the center is a bearing plate, on either side of which are the torsion springs, or in some cases just one larger spring. The spring pictured on the left in the photo is broken about 1/4 of the way in from its left end. The black shaft with dangling rope and door bracket is the track for the electric opener.
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Single panel doors can also be installed with (one piece track type hardware) that folds the door back with a single horizontal track on each side (which is mounted at the top of the wood frame) and a roller, (mounted to the top of the door on each side. A hinge on each side that attaches to the bottom of each side of garage door. Using track hardware, a car can be parked much closer to the door, as the whole door, when in the open position, rests completely inside of the garage door header. Track type hardware has much less arc when raising and lowering the garage door as opposed jamb type hardware.[citation needed]
There’s another reason new doors are superior to old ones: energy efficiency. Keep in mind, garage doors are large, and when they open, they let a lot of outside air into your home. While you may have significant insulation separating your garage from the rest of your home, eventually that temperature differential will start to influence your energy bills.
A typical version of an overhead garage door used in the past would have been built as a one-piece panel.[1] The panel was mounted on each side with unequal parallelogram style hinge lifting mechanism. Newer versions of overhead garage doors are now generally built from several panels hinged together that roll along a system of tracks guided by rollers.[1] The weight of the door may be 400 lb (181.4 kg) or more, but is balanced by either a torsion spring system or a pair of extension springs.[2] A remote controlled motorized mechanism for opening garage doors adds convenience, safety, and security.[3]

The belt-driven Chamberlain B970 garage door opener is powerful and well-featured. With a 1.25 HPS motor, it's got all of the strength needed for just about any residential garage door, and the motor and belt are covered by a lifetime warranty. The feature line-up includes built-in Internet connectivity for those who want to be able to monitor and operate their opener from afar, and a battery back up in the case of a power outage.
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The inside of the door panels bear a few dings, since filled with Bondo, from my teenage boys shooting hockey pucks. The streaks on the right that look like dinosaur claw scratches came from operating the door inadvertently when the rear doors of a van were open. The responsibility for these scars is all mine; they have also been Bondo-filled. We tend to use a lot of Bondo around our house. The crud on the walls and ceiling are cobwebs and dust infiltrates.
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The prior clamping of the set-screws tends to have pressed a dimple into the hollow shaft and to have distorted the shaft's roundness into an eccentric shape. While releasing the set-screws, I was careful to loosen them enough to let the cone swing around any such distortions. I was also careful to observe any binding of the old cones on the eccentricity or burring on the shaft. The fit of the cone on the shaft is supposed to be loose enough to avoid binding, but if it were to occur one would have to be careful not to assume the spring was unwound when in fact the cone was just stuck on the shaft. If I had a stuck cone that I could not unwind with a little extra force, then I would have called in a technician to deal with it. In the worst case, I suppose the spring must be deliberately broken with some hazard, thus releasing it for a forceful disassembly, and the shaft and some other parts replaced. But this is an unlikely situation and in this case was not necessary.
If you have paired springs, you can take a shortcut here instead of using locking pliers. Simply apply a slight torsion to the bar by clamping one of the springs with an easy half-turn or so applied. This will hold the lift cables in slight tension while you wind the other spring. If you have a single spring design, you can't use this trick, and have to use the locking pliers.

Smartphone Control: Many newer garage door openers allow you to connect the device to your home's Wi-Fi network, according to RW Garage Doors. You then can open and close the door through a smartphone app. In fact, many of these apps will give you an alert on the smartphone when the garage door is open for a certain period. Some opener models even can connect to your existing Smart Home system, incorporating all your appliances on one system.

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