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How the Upright Piano Action Works
Diagram of an upright piano action
Parts' Names

1) laminated pinblock
2) tuning pin
3) agraffe
4) action mount bolt
5) string
6) damper head
7) damper lever
8) hammer butt
9) main action rail
10) sustain rod lift lever
11) damper spoon
12) key
13) capstan screw
14) wippen
15) action mount bracket
16) let-off button
17) jack
18) backcheck
19) catcher
20) hammer rail
21) hammer

When the front of key (12) is pushed down, the back end rises. As the back of the key rises it pushes up on the wippen (14) and the jack (17). The jack pushes against the hammer butt (8) causing it to rotate and move the hammer (21) toward the string (5).

When the hammer is half way to the string the damper spoon (11) pushes the bottom of the damper lever (7) towards the string. This causes the upper part of the damper lever and the damper head (6) to lift off the string. This allows the string to vibrate after it is struck by the hammer.

Once the hammer reaches a distance of about 1/8 of an inch from the string, the jack makes contact with the let-off button (16). This causes the top of the jack to slip out from under the hammer butt. Without this step the hammer would stay in contact with the string preventing it from vibrating.

The momentum of the hammer allows it to travel the remaining small distance, striking the string and then rebounding. As the hammer rebounds, the catcher (19) on the hammer butt is caught by the backcheck (18). This prevents the hammer from bouncing off of the hammer rail (20) and re-striking the string.

After the key is released the action parts return to the rest position. As the wippen returns to it's rest position the damper spoon no longer pushes on the bottom of the damper lever. This allows the damper spring to push the damper head in contact with the string causing it to stop vibrating. Also, a spring below the toe of the jack causes the top of the jack to rotate to a postion below the hammer butt. The mechanism is now re-loaded and ready for the next key press.