A Detailed Description
The key (1) is depressed by the pianist's finger. As the back of the key begins to rise, the capstan (2) pushes up on the wippen heel (3) which in turn raises the wippen body (4). The wippen is hinged on the wippen flange (5), which is attached to the wippen flange rail (6). As the wippen body is lifted by the key, it raises in turn the jack (7) and the repetition lever (8), which work together to begin to push the hammer towards the string. The jack and repetition lever push up on the hammer knuckle (9), which raises the hammer shank (10) and hammer head (11). The hammer rotates on the hammer flange (12), which is attached to the hammer flange rail (13). When the key is halfway down, the damper lever lift felt (14) on the back of the key begins to lift the damper lever (15), which in turn raises the damper assembly (16). The damper assembly lifts the damper head off the string, allowing it to vibrate and sound freely (click here for more information on the damper). When the hammer is one-sixteenth of an inch away from the string, the top of the repetition lever comes into contact with the drop screw (17), which stops the upward motion of the repetition lever. At this exact same moment the jack toe (18) comes into contact with the let-off button (19), which is attached to the let-off rail (20). The let-off button causes the jack to rotate out from beneath the hammer knuckle - at this point, the hammer continues to move towards the string on its own inertia - it is no longer in contact with any other part of the action. This process is called single escapement, and is what allows the hammer to rebound immediately from the string while the key is still depressed. Without escapement, the hammer would be held against the string (stopping its vibrations) for as long as the key was depressed.
After the initial escapement, the key is completely depressed and the hammer head strikes the string - only to immediately rebound and have the hammer tail (21) be caught by the back check (22), which is attached to the back of the key by the back check wire (23). As the hammer rebounds to its original position, the knuckle forces the repetition lever to rotate downward on the repetition lever flange (24), causing the repetition spring (25) which is attached to the bottom of the repetition lever to compress. As the key is being released, the hammer is released by the back check, and the repetition spring pushes up on the repetition lever and raises the hammer. As the hammer is pushed upward, the jack to returns to its original position beneath the hammer knuckle. This resetting of the action is known as double escapement. The adavantage of the double escapement action is that the action is reset without the key having to be completely released, which allows for much faster note repetition. If the key is completely released, the action parts resume their original at-rest positions. The Hammer falls back onto the hammer shank cushion (26), which is attached to the wippen. The repetition lever regulating button (27) stops against the wippen body and returns the repetition lever to its rest position, and the jack regulating button (28) is stopped by the spoon (29), which is also attached to the wippen body. The entire action assembly is supported by rails which are attached to the action bracket (30).
Follow the link to watch an animation of the action in motion...