Abstract: An actuating device (10) actuates jaws (12) of a steady rest (11). The jaws can be used for clamping or supporting a workpiece (13). The actuating device includes a carrier (18) with a mounting side (20) to which the steady rest (11) can be mounted. An outer end (27b) of an actuating part (27) projects away from the mounting side (20) of the carrier (18) and includes a connecting arrangement (29) as the interface for the connection with an input part (14) of the steady rest (11). An inner end (27 c) of the actuating part (27) is movably coupled with a piston (35) of a pneumatic cylinder (36) via a coupling arrangement (34). The coupling arrangement 34 includes a slide (40) that is immovably connected to the piston (35), said slide having on it at least one slotted gate (44). The slotted gate (44) defines a gate path oriented obliquely to longitudinal direction L and to height direction H, along which gate path the at least one gate element (46) can be guided in a movable manner.