Moment 1
The key at rest
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Here we see the key at rest. The vertical position of the key tail is determined by the thickness of the lower key cloth, usually two or three layers of heavy woolen cloth. The air space between the beak and the underside of the pawl is called the Schnabelluft, or literally "beak air". This space is determined by a combination of factors, including hammer rest pad thickness (again usually two or three layers of woolen cloth), the depth to which the kapsel has been screwed into the key, and the thickness of the beak leather. For all practical purposes, however, Schnabelluft is initially adjusted by installing the kapsel to the proper depth, and fine-tuned by either planing or gluing paper or card shims to the underside of the keytail. After the action has been playing for some time, further regulation of this space by turning the kapsel is NOT recommended, since this also changes the forward/back position of the hammer, which can adversely affect other aspects of how the action functions.
Lost Motion is the name given to the first portion of key travel, and it is defined by the amount of key tail rise required to consume all of the Schnabelluft, that is, rise until the beak leather bumps up against the underside of the pawl. "Lost" Motion is a misnomer, implying "wasted" motion or something that should be eliminated. With the Prellzungenmechanik, however, Schnabelluft has two very important functions.
The primary function of Schnabelluft is to assure good repetition. The action will not reset itself for another stroke until the beak drops back to a position low enough for the pawl to snap back above it. If there is no Schnabelluft, the key must return all the way before this happens. Free space above the beak also gives the action a safety margin, making sure that the pawl can come back above the beak under all kinds of weather conditions. An action that is set up with almost no Schnabelluft will sometimes not repeat, because the sizes and positions of the action components have changed just enough to cause the pawl to hang on the edge of the beak leather.
The second advantage to Schnabelluft and the resultant "lost" motion is that it keeps the action from acquiring an unpleasant heaviness. When a key is played, the finger of the player must do three separate bits of work: (1) the entire key/hammer system must be set in motion; (2) when the beak contacts the underside of the pawl, the actual work of lifting the hammer through the leverage system begins; (3) shortly thereafter, the key begins to lift the damper. Each one of these jobs creates a moment when energy must be put into the system to overcome the inertial resistance of the stationary object(s); after each of these moments, the entire system essentially "coasts" on its own inertia. By staggering the point at which each of these three jobs must be done, the work is distributed over time. If all three moments of work happen at the same instant, the touch feels heavy. If the same action is adjusted so as to have no Schnabelluft by putting shims under the lower key cloth to raise the rest elevation of the key tail, it will feel noticeably heavier than when the rest position is low enough to provide a good amount of lost motion, even though nothing affecting the actual weight of any component or leverage of the system has been changed. In this sense, Schnabelluft and "lost" motion are directly analogous to the staggering of choirs in harpsichords.
How much Schnabelluft should there be? Most builders/restorers agree that about 1 mm is good for 5 octave instruments, and up to 1 1/2 mm for big late instruments. In general, the heavier the hammer, the more Schnabelluft you will want. However much there is, it is of the utmost importance that there be no drastic differences from key to key. It should be more or less equal over the entire action, though some prefer to give the bass more than the treble. Some builders/restorers/players recommend no lost motion whatsoever, but these people are almost inevitably coming from a modern piano background.
Some actions, such as Stein's second PZM, have such steeply upwardly sloping beak angles that a setup with no Schnabelluft is impossible; when the pawl springs forward over the beak, the highly oblique angle causes space to open up merely through the motion of the pawl. If we take normal sizes for the pawl, the key, and the hammer head as reference, the drawing shows about 1 1/2 mm space above the beak:
However, even with this healthy amount, the action could not function. If the drawing of the pawl is rotated to the position it would have after a complete key stroke, when the key had fallen back to its rest position, we see that the pawl cannot return, because the beak has not dropped low enough. The action would need about another full millimeter to allow the pawl to reset itself, or in other words, a good 2 1/2 mm just to work at all. If a safety margin were desired for all weather conditions, Schnabelluft would increase to almost 3 mm.
