the detector 12 is comprised of a flexible resilient bar 13 preferably made of spring steel and having a round oval square or four sided cross section wherein the broadest sides of the bar are prefer ably turned to face the direction of drum rotation the detector bar 13 is connected intimately to its particular lifting device so as to be forced to follow said device and to detect occurrent variations in the position of the lifting device 7a in a diametrical and may to this end be vulcanized directly in the lifting device with the end of said detector facing towards the drum casing 4 firmly anchored in the attachment bar 10 of said lifting device or molded in a casing 14 with its end that faces towards the drum casing 4 anchored in a holder 16 which is provided with a seal 15 against the attachment bar 10 and which has the same diameter as the casing 14 as shown in the embodiment illustrated in the drawings thus in this latter case the detector 12 is intended to be mounted in a radially and inwardly extending aperture 17 which is formed in the attachment bar 10 and the lifting device 7a and the internal diameter of which should not be greater than the external diameter of the detector bar casing 14 so that intimate contact will be achieved between the detector bar and the lifting device 7a when the lifting device is in its non activated position the detector bar 13 is radially orientated i e extends radially inwards from the attachment bar 10 of the lifting device and terminates short of the.