The mechanical system includes an interface for moving parts, with the mechanical system including a driving assembly and a driven assembly, wherein the driving assembly mates with the driven assembly at a mechanical joint (41). The driving assembly includes a rotating disc (16) and a drive arm (18) rotatably connected thereto. The driven assembly includes a pivot arm (40) which is driven by the drive arm and also a grounded member (44) fixed to the handle about which the pivot arm moves. A preload assembly, which includes a spring or springs (54, 56) is connected to the drive arm and the pivot arm so as to place a preload at the axis of rotation of the mechanical joint between the drive arm and the pivot arm.