A sleeve assembly consisting of a cylindrical tube, formed from one of: an engineering plastics material, a metal or an alloy thereof, having a density less than 3.5g/cm 3 and having exterior and interior cylindrical surfaces extending between a pair of annular end surfaces, said tube having machined or otherwise provided therein a plurality of recesses occupied by magnetic means which are firmly secured therein, said recesses being of substantially uniform depth along their length, and being uniformly spaced apart circumferentially of the tube and extending axially from one end of the tube to the other over substantially the entire axial length of the tube, wherein said sleeve assembly further comprises a tubular radially compressible liner component which is shorter in axial length than the cylindrical tube and which is bonded inside said tube to the interior cylindrical surface thereof axially centrally thereof thus leaving interior cylindrical end regions of the tube exposed beyond the annular end surfaces of said liner component, said interior cylindrical end regions being rebated radially outwardly from the circular interface between liner and said tube so as to define annular shoulders within the interior of the tube at either end thereof, said interior cylindrical end regions being provided with screw threads so as to be capable of receiving a pair of correspondingly threaded end rings constituted of a material having a modulus of elasticity which is at least twice that of the material of which the cylindrical tube is constituted, and having an inner diameter which is greater than the inner diameter of the compressible liner thus allowing the interior cylindrical surface of said liner to be radially expanded towards a position in which it lies flush with an inner cylindrical surface of said end rings, and wherein said annular shoulders are provided internally of the tube at a shallower axial depth from the respective annular end surfaces of said tube than the axial distance between the terminal end of at least one of the recesses and the annular end surfaces of the tube most proximate thereto, the arrangement being that when said end rings are screwed fully into said rebates and into abutting relationship with the annular shoulders and tightened thereagainst, not only do said end rings exert axial compression on the cylindrical tube, but their existence provides underlying structural support for the end regions of the sleeve assembly underneath said at least one recess. wherein said sleeve assembly further comprises a tubular radially compressible liner component which is shorter in axial length than the cylindrical tube and which is bonded inside said tube to the interior cylindrical surface thereof axially centrally thereof thus leaving interior cylindrical end regions of the tube exposed beyond the annular end surfaces of said liner component, said interior cylindrical end regions being rebated radially outwardly from the circular interface between liner and said tube so as to define annular shoulders within the interior of the tube at either end thereof, said interior cylindrical end regions being provided with screw threads so as to be capable of receiving a pair of correspondingly threaded end rings constituted of a material having a modulus of elasticity which is at least twice that of the material of which the cylindrical tube is constituted, and having an inner diameter which is greater than the inner diameter of the compressible liner thus allowing the interior cylindrical surface of said liner to be radially expanded towards a position in which it lies flush with an inner cylindrical surface of said end rings, and wherein said annular shoulders are provided internally of the tube at a shallower axial depth from the respective annular end surfaces of said tube than the axial distance between the terminal end of at least one of the recesses and the annular end surfaces of the tube most proximate thereto, the arrangement being that when said end rings are screwed fully into said rebates and into abutting relationship with the annular shoulders and tightened thereagainst, not only do said end rings exert axial compression on the cylindrical tube, but their existence provides underlying structural support for the end regions of the sleeve assembly underneath said at least one recess.