lateral guides not shown in the figure the lap being retained resting on a stop arm 21 which is able to assume two alternate positions namely a raised position to retain the lap 1 39 as indicated in fig1 or a lowered position which releases the lap 1 39 as indicated in fig3 the replacement laps can be advantageously transported by automated means for example by the transportation system described in ep a 312 503 in the name of the present applicant which is able to operate both with complete laps and with empty tubes the initial edge of the reserve lap 1 39 must be prepared before the replacement it is prepared by inserting it both into the gripper 22 and into the pair of stretching rollers 23 which are both closed after this insertion after which said stretching rollers are briefly operated the edge is prepared by stretch breaking within the length between said two members downstream of the pair of rollers 23 there is a suction duct 24 which draws off the portion removed from the lap the edge 25 of the new lap 1 39 is thus prepared the unwinding of the lap 1 resting on its rollers 2 proceeds to the end it can be seen.