A frame is provided with at least two axially parallel cylindrical rollers on which a drum can be supported on its axially spaced wheels. The drum has axially projecting central pins, and a servicing arrangement is adjacent the frame so that the drum may be moved into the servicing arrangement and back to the frame, for servicing and operating purposes. The servicing arrangement has a guide surface extending from the region of the cylindrical rollers outwardly of the frame and having a free edge remote from the rollers. Two or more support elements are provided having edge faces provided with notches into which the axially projecting pins of the drum can engage when the drum rolls on its wheels over the guide surface and off the free edge.