where such a belt shift takes place is downstream of the sprocket 13 a as viewed in the direction c of belt travel thus in case the flat brush roller 17 is stationarily that is non shiftably mounted the toothed belt 23 together with the flat bars 14 may be lifted for a desired period of time toward the flat brush roller 17 in such a manner that in the region of the flat brush roller 17 the clothing 17 a of the flat brush roller 17 is in engagement with the dirt strip not shown in the flat bar clothing 14 a in fig6 a between the points of the clothings 17 a of the stationary flat brush roller 17 and the points of the clothings 14 a of the flat bars 14 a clearance b is present that is the clothings 17 a and 14 a are out of engagement with one another in accordance with fig6 b the toothed belt 23 with the flat bars 14 u2014 as compared to fig6 a u2014 are shifted in the direction i locally to such an extent that the clothings 14 a and 17 a are in engagement with one another in operation the flat brush roller 17 rotates in the direction f and the upper flight return flight 23 a of the toothed belt 23 travels in the direction c while the lower flight 23.