How and upright piano action functions.
When the key is pressed, it rocks on the center rail, moving upwards at the back. In doing so, it raised the wippen. The wippen operates the jack, which in turn pushes the hammer butt. The butt pivots on its flange and moves the hammer towards the string. When the key is half way down, the damper spoon engages with the damper level, lifting the damper off the strings. When the hammer is almost hit to the string, the jack heel meets the set-off button and as wippen keeps moving up, the jack pivots and moves out from under the butt. the hammer then continues under its own inertia to the string, instantly rebounding. the catcher is caught by the backcheck and is held in the position as long as the key remains depressed.
When the key is released, the wippen drops, the backcheck releases the catcher, the bridle tape exerts a tug on the hammer butt and with the help of the butt spring, the hammer returns to the hammer rail . The damper spring returns the damper to the strings and the jack spring returns the jack under the butt, ready for the next repetition. This entire sequence is momentary, allowing rapid repetition of the notes.
Summary
Upright piano action: upright piano internal operation.
Hammer Top Felt: cloth surrounding the hammer.
Hammer: piece that hits the string to make it vibrate.
Hammer Rail: hammer support when it is not operated.
Catcher: piece that catches the hammer tail when it falls.
Backcheck: piece that catches the hammer tail when it falls.
Key: part of the keyboard that is pressed to produce a note.
Wippen: piece to which the strings are attached and that transmits the sound to the soundboard.
Jack: piece that sends the hammer head towards the string.
Hammer Butt: part of the hammer that is pushed by the jack.
Spring Rail: support for the damper when it is not operated.
Damper: piece that prevents the string from vibrating.
String: part of the piano that produces the sound by vibration when hit.
The animation in the illustration is in slow motion. The actual time from when a finger begins to depress a key to the point where the hammer strikes the string can be less than tenth of a second.
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