(57) Actuator for valves to be readjusted by rotational movement over a given angle, typically
being 90°, comprising an essentially cylindric housing (1) for receiving a piston
(5) arranged for axial movement in the housing under influence of a pressure fluid,
input/output-channels (22,23) in the housing for the pressure fluid, and a screw device
(9,26) for transferring the movement of the piston to rotational movement of an output
shaft (8) arranged to be connected to a valve. A second piston (6) is arranged in
the body (1) axially in line with said (first) piston (5) and arranged for an opposite
movement relative to the first piston. Both pistons (5,6) are forcibly (11,12) conducted
in the body (1) without rotational movement. An inner rotational unit (7) on the output
shaft (8) constitutes a part (26) of the screw device and portions (9) on both of
the pistons (5,6) constitutes other co-operating parts of the screw device.
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