This invention generally relates to overhead doors and, more specifically, to an improved tension spring counterbalancing mechanism use in overhead doors, such as residential garage doors.
Overhead doors generally require a counterbalancing force which enables the door to be more easily moved between opened and closed positions either manually or by way of a powered opening device. Often, overhead door systems rely on one or more extension springs, placed in tension when the door is in a closed position, so as to provide the desired counterbalancing force. In tension spring counterbalancing systems, the tension in the spring is released as the door is lifted thereby effectively reducing the weight of the door which must be lifted either by the motor of a door opener or by hand. These extension springs must be stretched or extended during the installation of the overhead door such that they are supplied with the necessary counterbalancing tension.
Presently, the installation of extension spring systems involve labor intensive procedures on the part of the installer. That is, in order for the installer to stretch or extend the spring, the full weight of the overhead door, which may be 200-300 pounds, must be manually lifted and clamped or otherwise propped up in a fully opened position. This procedure usually requires three people, i.e., two people for lifting the door and one more for clamping it in the open position. The operating cable of the door is then attached to the free end of the spring. Since the other end of the spring is fixed, the spring is tensioned when the door is lowered to the closed position. This type of system is not only difficult to install but the door may be inadvertently released from the open position when the spring or springs have not yet been attached to the operating cable. These concerns are amplified by the fact that extension spring systems are often installed by residential homeowners who, in general, are inexperienced with overhead door installation.
Another problem associated with overhead doors utilizing extension spring counterbalancing systems concerns the prevention of damage and/or injury resulting from the spring breaking while under tension. Past solutions to this problem have generally involved the use of a rope or cable passed centrally through the spring. Such safety cables are disclosed in U.S. Pat. Nos. 3,958,367; 4,082,133; and 4,640,049. In theory, spring pieces are meant to remain suspended on the rope or cable if the spring should break under tension. However, there is no guarantee that when a spring under tension breaks, it will break in only one place at a location in which the two remaining pieces of the spring will be adequately suspended on the rope or cable. The spring may, for example be brittle and break into several small pieces which would not be retained by a rope or cable.
U.S. Pat. No. 4,757,853 discloses the combined use of a safety rope extending centrally through the spring and a pair of metal end caps placed over the two ends of the spring. The purpose of the two end caps is stated to be for containment of the extreme ends of the spring through which the safety rope does not extend. While such end caps may provide containment for the ends of the spring, this safety system still does not provide full protection for the entire length of the spring and, in addition, adds undesirable complexity to the overhead door system.
A need in the art therefore exists for improvements in the art of overhead doors employing extension springs. Specifically, an extension spring system is needed which allows easier installation of the overhead door, especially with regard to the procedures involved with supplying counterbalancing tension to the spring. Furthermore, a system is needed which provides for full containment of the extension spring to guard more completely against damage and injury in the event that the spring breaks under tension.