Patent ID: 6918211

Claim:
An apparatus for urging a bidirectionally moving door ( 13 ) in a first direction, comprising: a cable ( 23 ), having a first end ( 23 b ) adapted to be attached to said door and oriented so as to deliver a tensile force thereto in said first direction; a spring apparatus ( 16 ), engaged with said cable, for imparting the tensile force thereto, said spring apparatus further comprising; a winding drum ( 22 ) having an outer peripheral surface ( 27 ) upon which said cable is wound; and, a torsional spring ( 24 ), engaged with said winding drum, wherein said torsional spring imparts a least magnitude torque (T 1 ) to the winding drum when the moving door is at a first ultimate displacement nearest the drum in said first direction, and imparts a maximum magnitude torque (T 2 ) to the winding drum when the moving door is at a second ultimate displacement furthest from the drum in a second direction opposite said first direction, wherein said outer peripheral surface is shaped to impart a substantially constant force to said cable as said cable is wound onto said winding drum.