stand 128 in addition to the foregoing hydraulic drive 56 for operating the piston 40 a second hydraulic drive 130 is employed for opening and closing the door 38 while a third hydraulic drive 132 operates a locking ring 134 for locking and unlocking the door 38 from the chamber 32 the drives 130 and 132 are utilized at the completion of a fluid expression operation of the system 30 for the extraction of a cake of particulate sludge matter referring also to fig8 9 the locking ring 134 is disposed circumferentially around the periphery of the chamber 32 for engagement with the door 38 the ring 134 has a set of inwardly depending fingers 136 for engagement with a corresponding set of outwardly extending ears 138 of the plate 84 contact between a finger 136 and an ear 138 is had by a tapered wedge 139a on the finger 136 which contacts a tapered wedge 139b and the ear 138 the ring 134 is secured to the end of the sidewall 34 by a circumferential flange 140 and is rotatable about the flange 140 by two sets of rollers 142 and 144 the rollers 142 rotate about the flange 140 around axes which are parallel to the central axis of the chamber 32 the set of rollers 144 rotate about axes which are radially.