Counterbalancing mechanism for rolling doors

Claim 1

A door assembly comprising a standard rolling door comprising a multiplicity of articulated sections mounted for movement vertically along the plane of the doorway and into a stored overhead position above the top of the doorway when the door is opened, the weight of the vertical portion of the door producing variable forces urging the door toward its closed position as the door is moved between its open and closed positions, and a counterbalancing mechanism comprising the combination of: a. a horizontal shaft rotatably mounted above the doorway and operably connected to said door, and a drum fixed to said shaft for rotation therewith in response to movement of the door between its open and closed positions, said drum having a spiral groove formed in the outer surface thereof for guiding a cable along the drum when the drum is rotated, b. a cable connected to said drum and wrapped around said drum in said spiral groove in response to rotation of the drum in one direction and unwrapped from the drum in response to rotation in the opposite direction, the lengths of said cable and said spiral groove being such that the cable is completely unwrapped from the spiral groove when the door is in the open position, c. and a constant counterbalance weight suspended from said cable directly adjacent the doorway for applying a torque to said drum, and thus applying a force to said door, urging the door toward its open position, the radius of curvature of said spiral groove about the axis of said drum gradually decreasing along the length of the drum so that the torque arm between the axis of the drum and said cable gradually decreases as the door moves from its closed position to its open position for counterbalancing the variable forces urging the door toward its closed position, said radius of curvature decreasing at an increasing rate at the smaller end of the spiral drum so that the force applied to the door diminishes at an increasing rate as the door approaches the fully open position, said spiral groove terminating at about the outside diameter of the shaft in a radius of curvature that produces substantially zero torque on the drum, and thus the door, when the door is in its open position so that the door smoothly decelerates as it approaches said open position and is readily withdrawn from said open position for movement toward said closed position, said drum and weight providing the sole counterbalance for said door.