The Piano Action consists primarily of four main sets of parts: (1) the KEY & BACKCHECK (2) the WIPPEN & JACK, (3) the HAMMER, and (4) the DAMPER.
1. The KEY, when pressed down, activates its corresponding action parts to subsequently cause the HAMMER to strike the string. The BACKCHECK, on the opposite end of the KEY, is a wooden block covered with felt and leather on which the HAMMER is caught after striking the string.
2. The WIPPEN & JACK is a complex mechanical unit of parts between the KEY and the HAMMER designed to transmit energy from the KEY to the HAMMER for striking the string, then to release the HAMMER so it can rebound and be caught and stabilized by the BACKCHECK, and then reset and repeat the cycle.
3. The HAMMER is a felt mallet on a shank, that when activated by the KEY swings toward and strikes its respective set of strings.
4. The DAMPER is a felt-covered wooden block that presses against a string. It is lifted by the back of the KEY to allow the string to vibrate when struck by the HAMMER, and then lowered to extinguish its sound.
A set of events happen simultaneously in the piano's action:
1. With the KEY up and the HAMMER in position about 1 7/8 inches from the string, the front of the KEY is moved downward. This causes the back of the KEY to pivot upward. The back of the KEY raises the WIPPEN & JACK, which begins to raise the HAMMER. At the same time the BACKCHECK is raised to prepare it to catch the HAMMER on the rebound. When the front of the KEY is about half way down, the back of the KEY also begins lifting the DAMPER, freeing the string to eventually vibrate when struck.
When the HAMMER has been raised to about 1/8 inch from the string, the WIPPEN & JACK disengage from the HAMMER. The HAMMER continues by its own inertia to strike the string. With the WIPPEN & JACK now out of the way, the HAMMER rebounds unimpeded and is caught by the BACKCHECK. When the KEY is released the HAMMER is then freed to be reset and repeat, and the DAMPER is lowered on to the strings to stop the sound.
by Robert Callaghan, RPT
Piano Tuning and Allied Arts, William Braid White
Piano Parts and Their Functions, Merle H. Mason
Piano Tuning, Servicing, & Rebuilding, Arthur A. Reblitz
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