is obtained which are located within the cleaning ring 1 and which are open towards the outside only through the first connection 6 and the nozzles 10 as a result an application of pressure onto the first connection 6 and thus onto the second groove 16 will lead to an increase in pressure in the second groove 16 the bore 12 and the first groove 15 the fluid introduced there under pressure flows through the nozzles 10 radially inwards into the gap between the insert 17 and the cladding tube 18 the corresponding increase in pressure in the first groove 15 generates a force which pushes the scraper ring 7 radially inwards i e towards the cladding tube 18 fig4 shows a device wherein a total of nine cleaning rings 1 are arranged in a common holder 25 the holder 25 comprises a connection tube 26 holding elements 27 and connection elements 28 the holding elements 27 are of essentially equal design and each hold three cleaning rings 1 and they are arranged at such a distance from one another that one cleaning ring 1 of each holding element 27 is respectively aligned with a second and a third cleaning rings 1 in relation to the longitudinal axis 11 the connection elements 28 keep the holding elements 27 spaced apart and parallel to one another inside of the connection tube 26 a channel is disposed which may be fed with.