Extension spring counterbalance system

An upwardly acting sectional door system comprising, a door having a plurality of hinged door sections movable between a closed vertical position and an open horizontal position, a drive tube mounted above said door in said closed vertical position, an operator selectively directionally rotatably driving said drive tube, cable drums mounted on said drive tube for rotation therewith by said operator, springs mounted in operative relation to said cable drums, and counterbalance cables reeved about said cable drums and interconnecting said springs and said door to counterbalance said door when moving between said closed vertical position and said open horizontal position.

In a counterbalance system for a sectional door having an operator powered drive tube mounting cable drums engaging counterbalance cables connected to the door and to springs, the cable drums comprising, a generally cylindrical exterior surface over an axial length thereof, a plurality of helical cable receiving grooves in said cylindrical exterior surface, an axially outer array of said cable receiving grooves having a first diameter, an axially inner array of said cable receiving grooves having a second diameter, and a transition array of cable receiving grooves having differing diameters intermediate said first diameter and said second diameter.