also referred to as a tensioning unit the roller 33 here is assigned a pressure measuring apparatus 51 by way of which the pressure exerted on the roller 33 by the felt belt 1 is detected the output signal of the pressure measuring apparatus 51 is transmitted via a measuring line 52 to a control unit 53 by way of which the tension of the felt belt 1 is detected the output of the control unit 53 is transmitted via a control line 54 to a correcting unit 55 for the roller 32 by way of which the roller 32 is displaceable in the direction of movement of the felt belt 1 the roller 32 is wrapped to 90 u00b0 by the first felt belt 1 the wrapping is at least 10 u00b0 and may also be 180 u00b0 on account of this wrapping the tension of the felt belt 1 can be controlled to be a predetermined value by way of an adjustment of the roller 32 two end positions of the displaceable roller 32 are illustrated in fig1 furthermore a unit 6 for detecting deviations of the first felt belt 1 from the correct movement path and for correcting the direction of movement of the felt belt 6 is provided which unit 6 includes a sensor 61 which is assigned to a periphery of the felt belt 1 and.