US Patent 7343958 Overhead door lift system

A garage door cable drum is disclosed for a sectional overhead door of a type having a substantially non-linear lift-weight to lift-height characteristic. The drum includes a generally spiral cable groove having a variable minor radius. The groove minor radius at any intermediate point along the groove is sized to provide a lift-cable moment arm that yields a corresponding cable lift force that is slightly less than an instantaneous lift weight of the garage door at any intermediate door elevation.