greater than equ1 where d m is the diameter of the mandrel in a multipurpose installation as preferably envisaged according to the invention the counterrollers must be able to exert a sufficient compression for all types of shells to be shaped by the installation including shells with an inside diameter on the order of 12 mm which means that the main counterrollers cannot have a diameter greater than 77 6 mm the maximum diameter of the auxiliary counterrollers is of course also limited by the diameter of the shell however the calculations show that if according to an embodiment of the invention the auxiliary counterrollers are put in contact with the shell only when it reaches 200 mm in diameter with main counterrollers of 77 6 mm in diameter the theoretical maximum diameter of the auxiliary counterrollers is greater than 700 mm for practical reasons and although this theoretically does not correspond to the most favorable conditions for a good shaping of the shells auxiliary counterrollers of much smaller dimensions are used for example with a diameter equal to 80 mm now we come to the difficulties posed by the winding itself around a heated revolving mandrel of a section of mineral fibers whose length can amount to about twenty meters for the purpose of shaping an insulating shell with an outside diameter that can reach up to 500 mm as already mentioned to operate such a winding according to an increasing feeding speed of a felt of mineral fibers with a heated mandrel revolving at a constant speed leads to very great winding times as soon as the outside diameter of the shaped shell exceeds 200 mm for example also according to the invention operating with a constant feeding speed of.