A guiding roller, that is used to turn a web of material, has a jacket surface provided with holes over at least a portion of its periphery. An inner surface of the guiding roller is provided with compressed air. A piston is axially displaceable in the roller inner surface and can occlude selected ones of the holes. A motor is provided for displacing the piston in response to a control signal supplied by a control circuit. The guiding roller is pivotally supported by a frame and can pivot through 90° about a pivot axis which is perpendicular to the longitudinal axis of the roller. A gear mechanism couples a rotational movement of the piston about the axis to the pivotal movement of the guiding roller.