Print hammer mechanism having dual pole pieces

摘要

A print hammer mechanism which forms a magnetic path between the opposite fixed and free ends of an elongated resilient hammer element using a permanent magnet, a first pole piece terminating in a pole tip adjacent the free end of the hammer element and electromagnetic means for cancelling the effects of the permanent magnet employs a second pole piece having a pole tip disposed adjacent the free end of the hammer element between the first pole piece and the fixed end of the hammer element. The relatively short magnetic path of low reluctance along the hammer element between the pole pieces combines with the effect of flux flowing in two gaps perpendicular to the flat surface of the hammer element to significantly improve the magnetic properties of the print hammer mechanism including the ability to quickly release and then retract the hammer element and the ability to increase the resonant frequency and/or the amount of energy stored in the hammer element for a given amount of magnetic flux available. The second pole piece forms a gap with the hammer element when the hammer element is in a spring-loaded retract position against the first pole piece, providing better retraction of the hammer after release and reducing the amount of magnetic energy required to overcome the effects of the permanent magnet and release the hammer element. The electromagnetic means for cancelling the effects of the permanent magnet consists of a coil wound about the first pole piece in direct contact therewith, enabling the first pole piece and adjacent parts of the magnetic circuit to provide adequate heat dissipating without the need for fins or other heat dissipating elements on the coil.