Typewriter ribbon conveying mechanism

A ribbon conveying mechanism for a typewriter including a printing head carriage, said mechanism comprising a mounting plate, a support mounted on said plate, a cassette mounted on said support, said cassette including a ribbon and two hubs, means for alternately driving each of said hubs to wind the ribbon on the driven hub, said cassette support pivotally mounted for tipping between first and second positions, spring means for biasing the support into either of said first and second positions, said driving means comprising two toothed wheels mounted on said pivotally mounted support, driving pawls cooperating with each of said toothed wheels, connecting means securing each cassette hub to each toothed wheel for rotation therewith, a spring blade for each toothed wheel and cooperating with its related toothed wheel to prevent return movement thereof, control means responsive to the position of said support and engageable with said pawls to render them alternately inoperative, said control means comprising a stirrup member having two arms, said stirrup member being mounted on the carriage for translational movement between a first position in which one arm disengages one driving pawl and the corresponding spring blade from one toothed wheel and a second position in which the other arm disengages the other driving pawl and the corresponding spring blade from the other toothed wheel, wherein the direction of the action of the pawls on said wheels is such that the pawl action on a related wheel rocks said support into a position corresponding to the inoperative position of that particular pawl, and wherein the biasing action of said spring means is greater than the rocking action exerted by the pawl, except when the driven wheel is held immobilized by the ribbon being fully unwound from the other hub.