Single piece hammer module

Abstract

A hammer module (11) for a dot matrix line printer including a cantilever mounted multi-arm hammer (23) formed from a single piece of resilient ferromagnetic material is disclosed. The multi-arm hammer (23) comprises a plurality of hammer arms (25) each including a thin spring region (51) and a thick head region (53) that lie in a plane that forms a slight angle with the plane of the base (27) of the multi-arm hammer (23). Each module also includes magnetic circuits for each hammer arm (25) formed by a common permanent magnet (13), a post (19), an arm (29) of a flux plate (15) and an arm (33) of a return plate (17). The post (19) is mounted on the tip of the flux plate arm (29). The flux plate and return plate arms (29 and 33) lie in parallel planes located on opposite sides (poles) of the permanent magnet (13). The posts (19), flux plate arms ( 29) and return plate arms (33) are sized and positioned such that the tip of the posts (19) lie coplanar with the outer surface of the return plate arms (33) and such that a gap exists between each post tip and an associated return plate arms (33). The head regions (53) of the print hammer arms (25) are positioned to be attracted to the post tip and the adjacent area of the return plate arms by the magnetic flux created by the permanent magnet (13) when coils (21) wrapped around the posts (19) are not energized. This attraction pulls the head regions 53 across the small gap created by the slight angle bend of the hammer arms (25). Thus, the thin spring region (51) of the hammer arms (25) is stressed, cocking the print hammer arms. When a coil (21) is energized, it produces a magnetic field that counteracts the permanent magnet attraction force, releasing the associated, cocked hammer arm (25). Releasing of the cocked hammer arm (25) impacts a ball (49), welded to the side of the hammer arm remote from the pole tip, against a ribbon to create a dot on a print receiving medium.