thereof the other of the two bearing devices is a fixed bearing device 9 which is fixed to driven frame 24 so that fixed bearing device 9 can not move relative to driven frame 24 but can move therewith in the vertical direction movable bearing device 8 is movable in the axial direction of top roll shaft 7 between an engaging position a where movable bearing device 8 engages with top roll shaft 7 and a disengaged position b where movable bearing device 8 is disengaged from top roll shaft 7 movable bearing device 8 is further movable between disengaged position b and a waiting position c where movable bearing device 8 is away from an extension of top roll shaft 7 movable bearing device 8 comprises a bearing 26 and a housing 27 more particularly movable bearing device 8 is driven between engaging position a and disengaged position b by a screw shaft 10 which threadingly engages movable bearing device 8 and is rotated by an electric motor 11 via a chain 14 frame 15 further includes a movable frame 25 which is pivotally connected by a coupling 28 to driven frame 24 so as to be rotatable in a horizontal plane about the axis of coupling 28 movable bearing device 8 is adapted to be moved between driven frame 24 and movable frame 25 thus movable bearing device 8 is supported by driven frame 24 when it is disposed in engaging position a and is supported by movable frame 25 when it is disposed disengaged position b or waiting position c further.