Let-off, also known as escapement, is the mechanism of the hammer releasing from the jack and lower action when the key is struck. It is a critical part of the action mechanism, and very influential in the touch and feel of the piano. It is sometimes called ‘set-off’ in the UK.
Function:
Let-off is designed to occur in the piano action because the hammer needs to strike the tone source (strings on pianos, reeds on Wurlitzers) free from the constraints of the lower action. This so the hammer can freely rise, strike the string, and fall back ready to be struck again; all without the lower action pressing the hammer into the string, deadening the sound.
Without let-off, the hammer would simply press against the string and no tone would resonate. The combination of the lifting of the damper, and escapement of the hammer through let-off allows a note to be sustained.
In greater detail, the jack is pushed upwards by the lower action when the key is pressed. The jack then pushes the hammer knuckle (grand pianos) or the hammer butt (upright & Wurlitzer pianos). This in turn moves the hammer towards towards the string or reed. With proper let-off set, just before the hammer strikes, the jack falls away. This occurs because the tip of the L shape of the jack presses against the let-off button. This pivots the jack around its flange, causing it to free the hammer. When the hammer has struck the tone source, it returns and falls just in front of the jack. When the key is released, all the action components quickly fall back into rest position to be struck again.
Regulation process:
The way to measure let off is to press the key very gently so the hammer moves slowly. By observing the hammer at a point it will fall away or ‘let-off’ from its upwards position and fall back down.
The ideal position for let-off in theory on an acoustic piano is for the hammer to get as close to the strings as possible without physically touching them. This is to allow playing at very quiet dynamics. In practice this is not possible because other issues occur, such as double striking of the hammer against the strings. This is also known as ‘bobbling’.
Let-off on Wurlitzer electric pianos follows the same principle as grand pianos. The optimal position for let off is approximately 1/8” to 3/16” or 3-5mm. For musicians that play louder or more aggressively, then a greater let-off is advised to avoid breaking reeds. This should be 5mm (3/16”) or greater if desired. Regardless of position, it is very important that let off is even throughout the piano for even touch.
Rhodes pianos do not have a let-off mechanism as the design of the action is simpler. Instead, once the hammer strikes the tine, it falls back and the curved shape of the hammer cam is caught by the key pedestal felt. This is called checking.
Hohner Clavinets again do not have a let-off mechanism, as the hammers are located underneath the keys and the direct pressing of this key allows the hammer to strike the string.
