The Physics Behind Garage Door Springs
You may never have thought much about what makes your garage door open/close smoothly, reliably and what part a garage door spring plays in that. In fact, this mechanism works thanks to some simple laws of physics. Torque, or rotational force, acts on these springs, also known as torsion springs. This bending motion causes the movement of the door.
The density of these springs allows more energy storage than would be the case if they were conventional springs. Each time the garage door comes down, the spring is rewound. Opening the door releases that energy. The torque created by the movement of the spring means that despite the weight of a garage door, it is relatively easy to open.
This torque moves along a rotating shaft to the cable drums on either side of the door. This converts the rotating force to a linear force. The resulting tension means that most people are easily able to lift the garage door, which may weigh upwards of 150 pounds, above head height without any problems.
The garage door spring, shaft and cable drums are called the counterbalance. The “counter” in this word refers to the way this mechanism counters another force, that of gravity, that is pulling the door down. Because the garage door is rolled up into a horizontal position and supported there by tracks, it gets lighter as it is lifted so that at its highest, very little force is required to move it.
The stationary end of the torsion spring is attached to the wall with brackets on one side with the shaft able to rotate in the spring. A winding cone fastens the spring to the shaft at the other end and also allows the spring itself to be wound. Although the spring is then held in place, force is pushing it toward unwinding. The amount of force exerted by the spring or springs must be equal to the weight of the garage door. When the spring is in its fully wound position, the force is transferred along the shaft to the cables.
The spring unwinds and loses tension as the door is lifted although it should still contain a little bit of tension even when lifted all the way to keep the cables taut. As the door is closed and the spring is rewound, that tension returns.
Rust, cold weather and repeated use will eventually break down a garage door spring, but in regular use, its lifespan is generally about five to seven years.