concave grooves 29 30 31 and 32 extending in circumferential directions along the inner peripheral surface of the pipe joint 21 concave grooves 29 to 32 prevent displacement of the positions of the o rings 25 to 28 owing to the o rings 25 to 28 mounted to bridge the vent holes 22 and 23 in the manner described above it is possible to prevent air leakage through the clearance 24 between the pipe joint 21 and the rotary shaft 16a moreover at opposite axial ends 33 and 34 of the pipe joint 21 there are provided bearings 35 and 36 which rotatably support the rotary shaft 16a extending through the pipe joint 21 the clearance 24 between the rotary shaft 16a and the pipe 21 leading to the hollow portion 19 of the rotary shaft 16a is communicated with a hollow portion or passage 38 of an air feed pipe 37 extending from the pipe joint 21 with the end portion of the air feed pipe 37 being connected to an air compressor pump not shown the compressed air fed from the air compressor pump passes through the air feed pipe 37 and is fed from clearance 24 through vent holes 22 and 23 into the hollow portion 19 of the rotary shaft 16a the compressed air is further fed from the hollow portion 19 of the rotary shaft 16a through the hollow portion 14 of the outer loop taker 2a and through the air blast holes 12 thus the compressed air is jetted from the air blast holes 12 into a clearance 4a between the track groove 6a and the projection 5a as a result an air film is formed in the clearance 4a and thus the sliding resistance or friction between the rotating.