The hammer butt is the wooden shank that houses the hammer tip on a Wurlitzer piano. Its function is to be pushed by the whip assembly via the jack around a fulcrum point so that it strikes a reed to generate a tone. Afterwards it returns to rest position ready to be struck again. Its role is similar to the hammer cam on a Rhodes piano, and the hammer shank on an acoustic piano.
The design of the butt has the hammer tip located on one end, with a flange screwed into the action rail on the other. A cutout section is situated in the middle lined with felt. When the hammer butt is at rest position inside the jack (also known as the fly in Wurlitzer pianos) rests against the red felt at the flange-end of the cutout section. When the key is struck the tip of the jack pushes up against the underside of the cut-out of the hammer butt and ‘throws’ the hammer into the reed. Once the hammer tip has struck the string and the butt returns back down, the jack is then caught by the hammer-side of the cut-out nearer the hammer tip. This is so that the butt will not bounce and allow the hammer to strike the reed again.
Having correct lost-motion and let-off is crucial for correct travel of the hammer butt. Both of these regulation procedures afftect the touch and feel of the key, and determine how aggressively the hammer hits the reed.
The felt in the cutout section is key to good action regulation. Over time these felts can become worn and can affect the regulation of the whip-assembly. Alongside changing the jack springs, replacing these worn felts can be an effective way to revive an action that does not respond well to conventional regulation adjustment.
