Abstract: A rotary, variable-displacement hydraulic pump (1) in use driven by a prime mover (4), the pump (1) comprising control means (5) for varying the displacement of the pump and means (17) responsive to a signal related to the rotational speed of the pump and operable to override the control means so as, in use, to prevent stalling of the prime mover (4) within a predetermined operational range of the prime mover.

Abstract: A flow sensor comprises a body (4) formed with a through bore (5), at least one fluid inlet (6, 7) and one fluid outlet (7,6) for permitting ingress and egress of fluid to and from the sensor, and a spool (8) slidably mounted within the bore and having at least one shaped end (38) past which fluid flows from the inlet to the outlet, characterized in that the spool (8) also has resilient means (26) mounted internally thereof and acting against and between two spaced abutments (27, 28) which are connected to the interior of the spool in such a way that they move with the spool in one direction of movement thereof and slide relative to the spool in the opposite direction of movement thereof, the spool being arranged to close the outlet in the null position of the spool when no fluid takes place, and being movable against the action of the resilient means when fluid flow is present, to open the outlet, the extent of movement being representative of the rate of flow of the fluid and being sensed by sensor means (3

Abstract: A hydraulic system includes a pilot controlled, pressure operated, spool valve (2) which acts substantially as a by-pass valve when regulating supply of fluid to an actuator (8), and as an in-line valve when regulating the return flow of fluid from the actuator (8) to tank. Both these regulating functions are provided by means of a single main valve spool (31). A flow sensor (20) provides negative feedback to the pilot valve in both by-pass and in-line modes of operation of the main valve, which makes the operation of the system substantially independent of load, temperature, and pump flow. If it is essential that the actuator (8), once it has reached a desired level, remains at that level, the system may additionally incorporate a releasable non-return valve (7) in the service line (71) from the main valve (2) to the actuator (8).

Abstract: In a hydraulic valve with flow feedback there is provided a position sensor (13) to sense the position of the flow-controlling valve member and provide a position feedback signal. The flow and position feedback signals are compared and a switching circuit (143) selects whichever of the signals indicates the larger flow.

Abstract: A pilot operated relief valve has a main relief valve (10) and a pilot relief valve (31) connected between a control chamber (23) and drain. A restriction (16) connects the control chamber (23) to the inlet chamber (22) which is separated from the control chamber by a piston (24) integral with the main relief valve closure member (12). Thus opening of the pilot relief valve against the adjustable force of a spring (14) determines the maximum pressure in the inlet chamber (22). The control chamber (23) can also be connected to drain (70) by a pilot valve (19) which is operated by a force motor (52) against pressure in a feedback chamber (56) connected to the inlet passage (22). The energization of the force motor (52) can be adjusted to adjust the relief pressure to a value below that at which the pilot relief valve (31) opens. Alternatively the pressure in the feedback chamber (56) can depend on the pressure at at least one side of a load fed via a directional valve from the inlet passage (22).

Abstract: A hydraulic system in which a pilot operated hydraulic four-way main valve (11) controls the direction and rate of fluid flow to a hydraulic load (13), includes a flow sensor (15) which is capable of operating bidirectionally and may therefore be located in one of the service lines (25, 26) from the main valve (11) to the load (13). The main components of the flow sensor are a variable or fixed aperture flow sensing orifice (52, 53), and a pressure transducer (18) to which the pressure differential developed across the orifice is applied and which converts the pressure differential into an electric negative feedback signal. This electric feedback signal is compared in an electronic circuit (17) with a demand signal, and a corresponding command signal sets the pilot valve (12) which controls the main valve (11).

Abstract: In a flow sensing device in which flow is measured by the linear displacement of a moveable bobbin within a throat, the bobbin contains a cylinder and piston arrangement, with the piston linked to the same linear displacement sensing device as the bobbin. Fluid pressures upstream and downstream of a calibrated orifice parallel to the throat, is applied to opposite ends of the piston. Low flow rates are measured by the displacement of the piston, with movement of the bobbin together with the piston taking over the flow sensing function at higher flow rates.

Abstract: A pilot actuated hydraulic control valve has a flow setting member in the form of a valve piston an end of which co-operates with radial outlet ports of the valve to form a variable metering orifice. A flow sensor provides a feedback differential pressure proportional to the flow through the valve and that feedback differential is used to control a pilot stage which actuates the flow setting member.

Abstract: A pilot actuated hydraulic control valve has a flow setting member in the form of a valve piston an end of which co-operates with radial outlet ports of the valve to form a variable metering orifice. A flow sensor provides a feedback differential pressure proportional to the flow through the valve and that feedback differential is used to control a pilot stage which actuates the flow setting member.

Abstract: The flow of fluid to a hydraulic load (18) is controlled by controlling a bleed-off path (31, 32) by means of a main valve (20) responsively to the difference between a first input force produced by a force motor (52) and a second (hydraulic feedback) force produced by a flow sensor (74) in the passage (33) leading to the load (18). The pressure difference across the flow sensor is applied by lines (75, 76 and 71, 70) to opposed feedback chambers (56, 55) to act on the pilot spool (40) upon which the force motor (52) acts.

Abstract: A fluid flow sensor comprises a hollow spool (58) displaceable on a spindle (66) against a spring (68). A sharp peripheral edge of a peripheral bead (80) on the spool (58) co-operates with the wall (61) of a throat (60) to form an annular flow path between chambers (92 and 90) in a housing (28) as the spool is displaced by the fluid pressure difference between the chambers (92 and 90). In the illustrated resting position of the spool (58), the peripheral bead (80) seats against a valve seat (57) and a residual flow path between the chambers (90 and 92) is provided by a small bleed orifice (89) in the wall of the spool. The pressure difference between the chambers (92 and 90) provides a measure of the fluid flow rate over a very wide range of flow rates.In a modification, the bleed orifice is pre-adjustable.