Patent ID: 7886803

Claim:
A covering for an architectural opening, comprising: an extendable material movable from an extended position for covering the opening to a retracted position for uncovering the opening; a drive spool mounted for rotation, wherein rotation of said drive spool drives a first load, which includes moving said extendable material; a capstan; a drive cord having first and second ends, said first end of said drive cord being attached to said drive spool, said drive cord extending from said drive spool, wrapping around said capstan, and extending to said second end; and a second load acting on said second end of said drive cord; wherein, when said second load is relieved, the drive cord surges the capstan and the drive spool is driven by said first load, wrapping the drive cord onto the drive spool, wherein said drive spool rotates about a fixed axis of rotation; and further comprising a frame, defining a cord emanation point that is axially fixed relative to said fixed axis of rotation; a guide surface between said cord emanation point and said drive spool, said guide surface being fixed relative to said emanation point; wherein said drive cord extends from said drive spool, over said guide surface and through said cord emanation point; and wherein said guide surface is a compound arcuate surface which projects radially outwardly away from said spool a greater distance where it is axially aligned with said cord emanation point and tapers to be a smaller radial distance away from said spool as the axial distance from the emanation point increases, so that the distance of travel of said cord from said cord emanation point across said guide surface to said spool is approximately equal for all axial positions of said cord along said spool.