663,723. Hydraulic machines. RUSTON-BUCYRUS, Ltd. Sept. 16, 1947 [July 22, 1946 ; July 22, 1946 ; Feb. 21, 1947 ; May 23, 1947 ; June 19, 1947], Nos. 25263/47, 25264/47, 25265/47, 25266/47 and 25267/47. Class 69 (ii) [Also in Group XXVIII] A double acting differential cylinder piston assembly comprises a main piston having opposite fluid pressure faces, an auxiliary piston of less effective area and also having opposite fluid pressure faces, corresponding main and auxiliary cylinder spaces, and a piston rod acted upon by each of said pistons at each single stroke, ports in continuous open communication with the cylinder spaces and serving as supply and exhaust or vice versa according to the direction of stroke, and means to neutralise the effective fluid pressure on at least a portion of one of said pistons during a predetermined part of a stroke in each direction whereby the force speed ratio is automatically reduced during such portion of a stroke. To effect downward movement of a cylinder 18 Fig. 1 having trunnions 20 to which a workpiece may be secured, pneumatic or hydraulic pressure fluid is supplied through a port 16 to a passage in a piston rod 12 and acts against a main piston formed by a piston 11 and the end of an inner cylinder 19 which makes a fluid-tight sliding fit with the piston 11 and the cylinder 18. The latter continues its downward stroke at a relatively high force speed ratio until its upper end abuts the upper end of the cylinder 19, whereupon the pressure on the second mentioned part of the main piston is neutralised. Cylinders 18, 19 complete the downward stroke together at a lower force speed ratio, i.e. at a higher speed but lower force under the action of fluid pressure acting only on the piston 11 which forms the auxiliary piston of the assembly. Fluid is continuously exhausted from the cylinder spaces of the opposite faces of the main and auxiliary pistons through a second duct in the rod 12 leading to a port 17. The initial stage of the reverse stroke which is effected by interchanging the ports 16, 17 is similarly maintained at a higher force speed ratio than the final stage, and the ratio may be further varied during either stroke by varying the diameters of the rod 12 and that of a lower rod 13. In a modification the workpiece is secured to the end 126 Fig. 5 of a hollow piston rod 116 of a movable hollow piston 115. The ends of the piston 115 comprise the main piston working in main cylinder spaces formed by the interior surface of a cylinder 114 and a rod 113, 112 integral with the base of the cylinder, and the auxiliary piston comprises the cross-section of the wall of the piston 115 and operates in the space bounded by the said inner surface of cylinder 114 and the peripheral wall of a constriction 111. On supplying pressure fluid to a port 117, the piston 115 and rod 116 are displaced upwardly at a relatively high force speed ratio until by-pass grooves 123 along the rod 113 become uncovered resulting in pressure being neutralised on part of the face of the main piston and being maintained only on the face of the auxiliary piston. Upward displacement continues at a lower force speed ratio until the ends of grooves 123 are reached when the high ratio is restored. Fluid is exhausted from the space above the hollow piston through an outlet 118, and the space between the constriction 111 and the upper end of the piston 115 is supplied through grooves 121, 122. The reverse stroke is effected by interchanging the ports 117, 118. In a still further modification the main piston comprises the outer faces of the opposite ends 217, 216 Fig. 8 of a piston 215 having an intermediate portion 218 which forms the auxiliary piston working in a space formed by the inner wall of a constriction 212. Upward displacement of the piston 215 and its rod 219 at a relatively high force speed ratio is effected by supplying fluid through a duct 221 to the underface of the end 217 ; fluid being exhausted from a chamber 214 through grooves 225 to a chamber 213. On uncovering grooves 224 pressure on the radially projecting parts of the piston end 217 is neutralised and upward displacement continues at a reduced ratio until the piston end 217 passes beyond the upper ends of the grooves 224, whereupon the high force speed ratio is restored. Reversal is again effected by interchanging the ports 220, 221. In a further modification the main piston 820 Fig. 26 works in an annular chamber 813 connected by ports 828 to a coaxial auxiliary chamber 814 in which is mounted the auxiliary piston 821. Aligned rods 822, 823 are connected to a single external rod 824 for connection to the workpiece. In operation, fluid entering at a port 818 acts against the lower force of the auxiliary piston 821 and displaces it and the main piston connected to it at a relatively low force speed ratio until the ports 828 are reached and by-pass grooves 826 passed, whereupon fluid acts on the main piston of greater area to give a higher force speed ratio. During the downward stroke the fluid first acts on the main piston and hence the initial force speed ratio exceeds the final ratio. In further modifications the auxiliary piston is guided by a fixed rod running along its axis, or by vanes projecting from the interior face of the auxiliary cylinder, or by a sleeve slidingly mounted within the auxiliary piston and within a chamber mounted above the auxiliary cylinder. The Specification as open to inspection under Sect. 91 comprises also a system wherein the two faces of the main piston comprise upper and lower faces of radially inward and outward projections 417, 416 Fig. 15 (Cancelled) of the end 433 of a hollow piston 415 having a rod 419 and operating in a chamber formed by the inner wall of a cylinder 411 and a central rod 431 carrying a constriction 412. In operation, upward displacement of the piston 415 due to pressure over the full area of end 433 continues at maximum force speed ratio until the lower ends of the grooves 424 become uncovered, thus neutralising the pressure on the face of projection 417 and reducing the effective area to that of projection 416. Displacement at the reduced force speed ratio continues until the upper ends of grooves 424 are passed when the full force speed ratio is restored. Exhaust of the hollow piston space above the constriction 412 occurs through grooves 425, a passage 435 and a duct 420. During the downward stroke the effective pressure area of the main piston is less than for the upward stroke by the external area of the piston 415, with a corresponding decrease in force speed ratio. This subjectmatter does not appear in the Specification as accepted.