The modern grand piano has over 10,000 moving parts. To say that it's one of the world's engineering marvels would not be an exaggeration. Have you ever noticed how luxury car manufacturers always compare their vehicles to Steinway grand pianos? There's a reason for this: the fine tuning and precision required for a piano to run as smoothly as a fine automobile.
A piano's "action" is the mechanism that connects the 88 white and black keys to the hammers, and ultimately, the strings, to produce sound. A properly adjusted piano action is capable of producing a wide variety of volumes, tone colors, and articulations based on the input of the player. With over 100 parts in each key, the piano has almost 10,000 moving parts in the action, alone.
The Grand Piano Action in Motion
The key is pressed down, the back end rises and pushes up on the escapement mechanism, consisting of the wippen, the jack, and the repetition lever. Notice that the jack sticks up through a slot in the repetition lever (window). The repetition lever and the jack jointly push up on the knuckle attached to the hammer shank, sending the hammer toward the string.
When the hammer is a third to half of the way toward the string, the back end of the key contacts the damper underlever, causing the damper to rise off the string. This allows the string to vibrate freely when struck. The timing of this action is critical. Too soon and the action will not feel right; too late and the string will not vibrate when struck.
When the hammer is about 1/8 inch from the string, the toe of the jack contacts the let-off button, causing the top of the jack to pivot out from under the knuckle. Simultaneously, the end of the repetition lever contacts the drop screw, preventing the repetition lever from rising any further. These two actions constitute an event called escapement or let-off. Let-off is critical in grands for the same reason as in verticals.
With both the jack and the repetition lever disengaged, the hammer goes the remaining small distance to the string on its own momentum, strikes the string, and rebounds. On the rebound, the hammer knuckle lands on the repetition lever, causing it to pivot and compress the repetition spring. At the same time, the tail of the hammer head is caught by the back check, which prevents the compressed repetition spring from pushing the hammer away (bottom).
When the key is released even a little bit, the back check releases the hammer tail just enough so that the compressed repetition spring can now push the hammer (via the repetition lever and knuckle) toward the string again. The hammer doesn’t actually strike the string a second time, however, because the drop screw is still limiting the upward movement of the repetition lever. But the upward movement of the hammer gives the jack, pulled by its spring, enough room to pivot back underneath the knuckle, ready for another stroke, even though the key has not yet returned to its rest position.
When the key is fully released, everything falls back to its rest position. The back end of the key releases the damper underlever, allowing the damper to drop back onto the string and stop its vibration. The hammer shank falls back to its rest position, just a little above the hammer rail or rest cushion.
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