Abstract: In the field of fluid power control, there is a need for a control scheme that supplies working fluid under pressure to a load in the event of failure of power supplies to control apparatus associated with the circuit. The disclosure relates to a control circuit 10 in which output pressure from a pump 11 is fed to a control piston 13 therefor via a pressure compensator 17. The pressure compensator 17 is biased to disconnect the supply of fluid at outlet pressure when this pressure is below a threshold value, and permit such supply when the outlet pressure exceeds the threshold value determined by the biassing force on the pressure compensator. A proportional control valve 16 may be employed further to counteract the biassing of the pressure compensator 17, whereby to control the threshold pressure at which the compensator 17 operates to feed fluid at control pressure to the piston 13. The invention is of particular use in control circuits eg. for cooling pumps and cooling fans.

Abstract: A fluid power control system for use in a mini-excavator, in which a first control section (S2) includes first and second control valves respectively connectable to first (2) and second (4) implement functions; and in tandem with one another. A first gear pump (P2) supplies the actuator connected principally to the first implement (2), and a second gear pump (P3) supplies working fluid to the interconnection between the first and second control valves. The advantage of this arrangement is that, as the first control valve switches from its neutral position to a position selecting its associated implement (2), progressively more of the fluid passing through the second control valve is supplied by the second pump (P3) until, when the implement (2) to which the first control valve is principally connected is fully selected, the two implements (2, 4) are supplied separately by the respective gear pumps (P2, P3).

Abstract: A hydraulic system including a re-circulation flow loop including a hydraulic pump and an actuator in fluid communication via a high pressure feed line and a low pressure return line, the system further including an air separator in fluid communication with the low pressure line for removing air from hydraulic fluid flowing along said re-circulation loop.

Abstract: An hydraulic circuit controlling an actuator including a main signal branch which includes a pump (11), a valve (12) and an actuator (18) coupled in series, the valve being responsive to first and second applied biasing pressures (19,23), the first biasing pressure being operable to bias the valve (12) into a closed position and the second biasing pressure being operable to bias the valve into an open position; an auxiliary branch (20,21,22) connected in parallel with the actuator (18); the first biasing pressure (19) being derived from the main signal branch downstream of the valve (12) and the second biasing pressure being derived from the auxiliary branch.

Abstract: A method and apparatus for controlling a two-stage valve using a closed-loop control system, the system including a variable gain element, the method involving continually performing the steps of estimating the flow gain of the pilot stage of the valve; comparing the estimated flow gain with a reference flow gain; and in response to said comparison step, adjusting the gain of the variable gain element to maintain a substantially constant closed-loop gain.

Abstract: An electrically operated, two-stage hydraulic spool valve comprising a maid stage (2) having a main spool (4), centering springs (9) acting on the main spool, a pilot stage (1) having a pilot spool (12), and two solenoids (39) acting on respective ends of the pilot spool to control the operating position of the latter the solenoid being opposed by return springs (18) also acting on respective ends of the pilot spool, the pilot stage comprising tank port means (23,24), supply port means (26), and two service ports (27,28), and the pilot spool comprising two portions (13,14), a spring (15) interposed between the two portions and operable to urge the two portions apart in a direction axially of the pilot spool so that on de-energization of the solenoids, the portions of the pilot spool separate under the action of the spring to provide an interconnection between the pilot service ports and the tank port means.