579,734. Combustion-product plant, pumps, compressors &c. MUNTZ & CO., Ltd., A., and BEALE, E. S. L. June 25, 1942, No. 8764. [Classes 7 (i) and 7 (vi)] [Also in Group XXVIII] A dead point follower for a free-piston pump, compressor or motive-gas generator includes an actuating element 24 oscillated in relation to the free piston assembly, a master follower 25 which is moved in one sense, in response to the movement of the actuating element into a dead-point, to a position depending on the dead point, and then remains locked in that position, and means serving periodically to move the master follower through a limited distance in the other sense when the actuating element 24 is not in or near its dead-point. The master follower 25 determines the position of a slave follower 48, which is locked in position during the part of the cycle in which the master is displaced in the other sense, the master being locked in position during the part of the cycle in which it corrects the position of the slave. The followers may control the free-piston machine by the means described in Specification 579,733. The mechanism is shown as applied to a free-piston gas generator, Fig. 1, comprising opposed'power pistons 10 integral with pistons 14 moving in cushioning cylinders 15, one face of each piston 14 compressing air into a receiver 18 communicating with scavenging air ports 12 in the power cylinder 11. The pistons are synchronized by racks meshing with pinions on a shaft 23, Fig. 3, which carries cams 90, 91. The cam 90 engages the spring- loaded actuating member 24 co-operating with the master follower 25, and the cam 91 actuates a pump 36 delivering oil to the cylinder 26 above the follower piston 25. The stem 27 of the member 24 engages a recess 28 in the piston 25, tapering ports 29 being provided for the escape of oil from the recess. After the piston 25 has been given a downward movement by the delivery of the pump, it is brought to rest by the closing of the delivery port 35; afterwards it is driven upwards by the member 24, 27 and retarded towards the dead-point by the resistance to discharge of oil through a braking valve 31. The rod 40 of the piston 25 passes in a fluid-tight manner through the cover of the cylinder 26 and is integral with the piston valve 41 of an oil relay which incorporates the slave follower 48. The cylinder 42 of the relay has flanges 43, 44, between which enters an oil inlet port 45 controlled by a valve groove 47 in the pump plunger 36. The slave follower 48 is a tube sliding in the flanges 43, 44 and terminating in flanges 49, 50 fitting the bore of the cylinder 42. The piston valve 41 works in the tube 48 and has flanges 51, 52 which normally register with ports 53, 54 opening into annular spaces 55, 56. Oil admission ports 57 opening between the flanges 43, 44 and the open ends of the tube 48 form discharge ports. When oil under pressure is supplied through the groove 47 and inlets 45, 57, the tube moves until its ports 53, 54 are in register with the flanges 51, 52 of the valve 41, and is held in that position by trapped oil, when the groove 47 is shut off, while the piston 25 is moved down by the pump and up again by the member 27. The movement of the slave follower 48 may be controlled by an oil brake controlled by a valve actuated by the shaft 23. Means for controlling the relative pressures in the cushioning cylinders 15 and the receiver 18, Fig. 1, comprises a balance pipe 67 connected to the receiver through a piston valve 72 and non-return valves 68, 69 opening in opposite directions. The valve 72 is connected to a pilot valve 76, Fig. 3, supplied at 77 with oil under pressure and controlling ports 74, 75 leading to the top and bottom of the master cylinder 26. A starting-position valve 66 also controlling the ports 74, 75 prevents change in cushioning during the starting period and is actuated by air pressure from a cock 63, Fig. 1, which also admits air behind a piston 60 on a rod 64 for moving the piston assemblies into starting position. The valve 66 closes a connection between the bottom of the cylinder 26 and a duct 59 leading to the oil sump. In a modified form of dead-point follower, Fig. 4, the actuating element is a cylinder 78 sliding in a fixed casing 80, the master follower including a piston 79; the slave follower may be similar to Fig. 3. A metering port 82 in the cylinder 78 is connected to an inlet 86 for oil under pressure by a grooved piston valve 87 slidable in a lug 88 on the cylinder and engaged by a friction plunger 89. As the port 82 passes the port 86, oil is admitted to the cylinder and displaces the master 79 against a spring 83, uncovering an exhaust port 84 registering with a channel 85 in the casing and allowing the master 79 to follow the cylinder 78, under the influence of the spring 83, to the inner dead-point; it then remains locked by the oil trapped in it, until the port 82 again admits a charge of oil. During the outward movement of the cylinder, the valve 87 is moved by the plunger to a closed position, a collar 87 abutting against the casing, so that the master remains locked. In an alternative form, the cylinder oscillates and the master follower includes a vane co-operating with the cylinder.