one of these unwinding rollers if the corresponding belt has a lead with respect to the adjacent belts taking into account the direction of travel or by the play of any individual drive device for each belt between the unwinding rollers and the inlet roller 6 of the calender advantageously it is possible to provide in the path of the belts devices analysing the tension of the latter such that one belt is always at the minimum longitudinal tension admissible for correct operation of the calender the construction of such a device is within the ability of a man skilled in the art the relative transverse position of the various belts is also constantly controlled and corrected in order to obtain the desired overlap at any instant by means of devices such as those shown diagrammatically by way of example in fig2 and 3 these devices control the transverse position of the borders of the various belts in the region of overlap of the latter these devices also ensure cutting of the borders 30 and 31 corresponding to the excess overlap with particular reference to the overlap region 19 located in the immediate vicinity of the latter is a so called 34 cutting 34 arrangement 32 comprising a photo electric cell 33 able to be located directly above the longitudinal edge 22 of the upper belt 14 a photo electric cell 34 able to be located directly above the longitudinal edge 35 of the central belt 13 located below the side belt 14 and a knife 36 of any known type for example mounted to rotate about a transverse axis with respect to the.