Flat rate spring particularly adapted for typewriter cartridges

Claim 1

A ribbon cartridge for a typewriter, said cartridge including a case and a ribbon extending externally of said case; a take up member for said ribbon in said case, said take up member comprising a ribbon take up spool mounted for rotation and translation in said case and for effecting a winding of layers of ribbon thereon, and a drive member for engagement with the periphery of said ribbon take up spool; and biasing means for biasing said take up member against said drive member, said biasing means comprising a spring under tension for operation beyond the elastic limit of the material of which said spring is constructed, said spring comprising a length of resilient material having terminal end portions and a central body portion, one of said terminal end portions including means for connection to one of said case and members; the other of said terminal end portions comprising a convoluted portion of said resilient material having a central axis substantially perpendicular to said body portion and for connection to one of the other of said case and members, at least said convoluted portion being dimensioned so that upon separation of said drive and take up members due to an increase in the diameter of said take up member caused by an increase in the layers of ribbon on said spool, said convoluted portion uncoils and is stressed beyond the yield point of said material whereby a substantially constant and limited load placed on said convoluted portion by separation of said drive and said take up members effects further uncoiling of said convoluted portion.