Pianoshop :: Info :: Figures :: Hammer Height
The keyframe and action are set up on the workbench. For each section of strings in the grand piano a measurement has been taken of the string height. A "Letoff Rack" is set up so that the wooden target is the same height as the string height above the hammers. See picture below:
The "Hammer Stroke" is set, usually to 47mm. This is the distance between the top of the hammers and the strings.
"Letoff" is set next. When the key is pressed slowly, the jack is released when the hammer is 1.5mm from the strings, this is adjusted by turning set off buttons, visible under the rail that the hammers are screwed to.
Once the hammer is let off it drops another 1.5mm down to the "Drop Height". This is set by a small screw on the hammer flange.
"Backchecking" is set last. This is the distance between the hammer's final resting place and the strings when a reasonable blow is struck on the key. It is usually 16mm. The repetition spring is set so that when the key is released, the whippen just holds the hammer up.