The deflection bar (01) is provided with holes (08) over at least part of its peripheral mantle (02), enclosing a pressure space supplied with compressed air, with at least one blocking element, e.g. a piston, for selectively blocking the holes upon its pivot movement through 90 degrees about a pivot axis perpendicular to its longitudinal axis. The movement of the blocking element is coupled to the pivot movement of the deflection bar about a pivot axis perpendicular to its longitudinal axis relative to a frame.
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