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A garage door is a large door on a garage that can either be opened manually or by a garage door opener. Garage doors are necessarily large to allow passage of automobiles. Small doors may be made in a single part, larger doors used in commercial or industrial garages are usually made in several jointed parts. To save space, garage doors normally raise or roll upward, instead of sliding or swinging. The operating mechanism is spring-loaded or counterbalanced to offset the weight of the door and reduce effort required for opening and closing. Doors are made of wood, metal, or fiberglass, and may be insulated to prevent heat loss.
A typical overhead garage door consists of several panels hinged together that roll along a system of tracks guided by rollers. The weight of the door is balanced by either a torsion spring system or a pair of extension springs.
The history of the garage door could date back to 450 BC when chariots were stored in gatehouses, but in the U.S. it arose around the start of the 20th century. As early as 1902, American manufacturers---including Cornell Iron Works---published catalogs featuring a "float over door." Evidence of an upward-lifting garage door can be found in a catalog in 1906. Development of the door progressed from there. The first one-piece wood door in the U.S. dates back to at least 1935, to a man named Rhead. Leno Martin is credited with inventing one of the first one- piece overhead type garage doors in 1936.
Single panel garage doors 
Single panel doors are constructed from one monolithic panel. From the closed position a single panel door will slide up and overhead on rails to the fully open position. A disadvantage of monolithic panel doors is that the swing up arc of the door occurs partially outside the garage. This means a vehicle must stop and park several feet in front of the door to avoid being hit by the garage door when it is opened.
Sectional garage doors 
Sectional doors are usually constructed of six to eight panels and slide up and overhead. Sectional doors occupy exactly the same amount of internal garage space as a monolithic door. Sectional doors have two distinct advantages over single panel monolithic doors:
- Sectional doors do not require any space outside the garage to open. A vehicle may park very close to the garage before opening the door.
- Each panel of a sectional door has its own connection to the door track. This increases reliability and robustness compared to monolithic doors which have only a couple of track connections for the whole panel.
Garage doors can be made out of many materials, but steel, aluminum, wood, cooper, glass and vinyl (polyethylene) are the most popular materials. A few manufacturers are making garage doors by putting foamed-in-place polyurethane insulation in monolithic panel and sectional garage doors.
Roller doors 
Roller doors are usually constructed of corrugated steel. They evolved from cover window and door coverings. Other materials can be used (e.g.; transparent corrugated fibreglass) where strong impact resistance is not required. Corrugations give the door strength against impacts. A typical single car garage roller door will have a preloaded spring inside the rolling mechanism. The spring reduces the effort required to open the door. Larger roller doors in commercial premises are not sprung and use a manual pulley and chain system or a geared motor to raise and lower (roll up and roll down) the door. Roller doors cannot be effectively insulated.
In the UK (and other parts of the EU), 'insulated' roller garage doors are available, using an aluminium lathe which is then foam filled, which not only adds thermal but acoustic insulation.
In terms of thermal insulation, the roller door has a typical U value of 4.9 to 5.2. A sheet steel garage door has a typical U value of 6.1 to 6.4. For 'real' thermally insulated garage doors, you would strongly consider the insulation values of a foam filled sectional garage door, which has a typical U value of 2.7 to 1.3.
Materials and insulation 
A common material for a new garage door is steel sheet formed to look like a raised panel wooden door. Steel doors are available in uninsulated, insulated, and double skin steel. A design mimicking carriage house doors has become popular since about 2002, and many manufacturers clad the exterior of a steel door with composite, vinyl boards, or DecamTrim to give it the appearance of wood. A more economical alternative for garage doors is steel stamped construction.
Torsion spring lift mechanism 
A torsion spring counterbalance system consists of one or two tightly wound up springs on a steel shaft with cable drums at both ends. The entire apparatus mounts on the header wall above the garage door and has three supports: a center bearing plate with a steel or nylon bearing and two end bearing plates at both ends. The springs themselves consist of the steel wire with a stationary cone at one end and a winding cone at the other end. The stationary cone is attached to the center bearing plate. The winding cone consists of holes every 90 degrees for winding the springs and two set screws to secure the springs to the shaft. Steel counterbalance cables run from the roller brackets at the bottom corners of the door to a notch in the cable drums. When the door is raised, the springs unwind and the stored tension lifts the door by turning the shaft, thus turning the cable drums, wrapping the cables around the grooves on the cable drums. When the door is lowered, the cables unwrap from the drums and the springs are rewound to full tension.
Life of torsion spring 
Most garage door manufacturers, garage door dealers, either produce and sell garage doors fitted with torsion springs that have a minimum of 10,000 - 15,000 cycles which are estimated to last between 3 to 7 years. One cycle is defined as a single opening and closing sequence. Many, if not all manufacturers offer a 30,000 cycle spring. However, it is important to remember that if the weight of the garage door is increased by adding glass, additional insulation, or even several coats of paint, the life of the torsion spring may be greatly reduced.
Other factors like poor garage door maintenance, loose tracks, or components will shorten the life of torsion springs. Owners are advised to avoid applying grease to the garage door tracks, this will prevent the wheels on the rollers from doing their job. Only the bearings, hinges, and spring wire require lubricant.
Extension spring lift mechanism 
An extension spring counterbalance system consists of a pair of stretched springs running parallel to the horizontal tracks. The springs lift the door through a system of pulleys and counterbalance cables running from the bottom corner brackets through the pulleys. When the door is raised, the springs contract, thus lifting the door as the tension is released.
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).
A garage door with a broken spring, or the wrong strength spring, can fall. Because the effective weight of the door increases as the garage door sections transfer from the horizontal to vertical door tracks, a falling garage door accelerates rapidly. A free falling garage door can cause serious injury or death.
The sections and rollers on garage doors represent a major pinch hazard. Children should never be allowed near a moving garage door for this reason. On manually operated garage doors, handles should be installed vertically, to promote “vertical orientation of the hand”.
Mechanical garage door openers can pull or push a garage door with enough force to injure or kill people and pets if they become trapped. All modern openers are equipped with “force settings” that will cause the door to reverse if it encounters too much resistance while closing, and to stop if too much resistance is encountered while opening. Any garage door opener sold in the United States after 1992 requires “safety eyes”, which are sensors that will prevent a garage door from closing if there is an obstruction under the door. Force settings should cause a door to stop or reverse upon encountering more than approximately 20 lbs of resistance. Safety eyes should be installed a maximum of six inches above the ground. Many garage door injuries, and nearly all garage door related property damage, can be avoided by following these precautions.
Certain parts, especially springs, cables, bottom brackets, and spring anchor plates, are under extreme tension. Injuries can occur if parts under tension are removed. 
Extension spring systems should always be restrained by a “safety cable” which runs through the middle of the spring, tying off to a solid point at the rear and front of the horizontal door track. Extension springs represent a hazard to bystanders when a spring, pulley, or cable breaks under tension. Metal parts from extension spring systems can suddenly be launched.
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