body the dry bearings each comprise plastics sleeves 105 supported by and secured to a metal cylinder which can be press fit into the bore 102 the plastics sleeve 105 generally of ptfe i e the polymer polytetrafluoroethylene acts to form a bearing between the metal surfaces of the body and the member and additionally acts to prevent improper travel of air such as by leakage along the surface of the valve member the parts 103 and 104 each include a flange 108 for axially locating the parts when firmly connected together by a bolt not shown which passes through a bore 109 in the part 104 and engages a screw threaded bore 110 in the part 103 so as to draw a shoulder 111 of the part 103 against the end of the part 104 by slackening and retighting the bolt the relative angular positions of the parts 103 and 104 can be adjusted as explained in more detail hereinafter the part 103 includes a stub shaft 112 for connection to a drive wheel 113 shown in fig1 connected to the main drive motor 20 of the machine via a variable gear box not shown in this way the valve member 103 104 can be rotated in synchronism with machine speed and at a preselectable rate relative thereto the valve body 101 includes a first pair of ports 114 115 fig1 and 9 which are connected to a source of high pressure air shown schematically at 30 via intervening valves not shown for controlling the air pressure at the ports 114 115 each of the ports 114 115 is connected via a short bore to.