A jack is the L shaped component that is found in piano actions, and the whip assembly in Wurlitzer pianos. In Wurlitzer pianos, it is commonly known as the fly – but the two terms are interchangable.
Function:
The jack plays a crucial roll in let-off, a major part of the action process when the key is struck. The role of the jack in the piano action is to transfer the energy of the key into the movement of the hammer so it strikes the string or reed. It is designed in such a way that when the lower action is raised by the key, the jack raises and pushes the hammer shank towards the string. The jack is adjusted so that the hammer strikes the string without choking it. This is achieved by adjusting the let-off.
Design:
The reason the jack is L shaped is this allows the jack to disengage from under the hammer shank. It achieves this by the smaller tip of the L shape touching the let-off button, pivoting the jack around its flange, freeing the hammer, and thus allowing the it to strike the string. Let-off is controlling where we want this to happen, usually 2-3mm (1/8”) on acoustic pianos. This let-off adjustment is essential for even touch and feel of the piano, particularly with quiet dynamics; as the closer to the string the hammer ‘lets-off’, the more softly a musician can play. Adjusting the let-off is essentially adjusting at which point the jack will escape from the hammer shank, and therefore how much free movement the shank has before the hammer strikes the string.
Wurlitzer:
In a Wurlitzer piano, the procedure of the jack and let-off is the same as acoustic pianos. However the jack pushes up into the hammer butt which houses the hammer. The motions acts in a similar way to that of grand pianos, except that the hammer butt has a recess underneath in which the jack is housed. This is done to make the action more compact. Let-off is usually adjusted to 3mm or 1/8” on Wurlitzer actions, though it should be increased for musicians who play harder to avoid broken reeds. In this case 4-5mm or 3/16” is recommended.
Other Electric Pianos:
Rhodes and Clavinets do not have a jack as their action design is different. A rhodes bypasses the lower action entirely and the raising of the key pedestal directly throws the hammer into the tine. On a Hohner Clavinet there is no conventional piano action at all: The key itself houses the hammer which when pressed strikes the string.
