Vickers, Inc.

John A Hammond

Ronald B Walters

Andrew C Morley

A solenoid-operated pilot valve for a two stage hydraulic device comprises a spool (4) slidably mounted in a housing for movement in opposite directions from a normal null position in order to deliver fluid under pressure to the second stage (2) of the device to actuate the latter in one direction or an opposite direction from a normal null position, the spool (4) being formed with two passageways (27,28) each extending internally of the spool (4) between one end face (15) of the latter and the periphery of the spool, and the pilot valve further comprising two spool control devices (13), at least one of which is a solenoid, associated with respective ends of the spool and in normal operation seating against the end faces (15) of the spool so as to close the passageways against fluid flow therethrough, but becoming unseated in the event of an interruption in the electrical input to the solenoid, whereby both ends of the second stage are at a common pressure so as quickly to relieve the pressure in the second stage of the device, thereby permitting fast return of the latter to the null position.

Pilot valves for two-stage hydraulic devices

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). The pressure transducer (18) in the described embodiment is a cantilever beam pressure transducer (36), acted on by the inner ends of a pair of diameter-matched sliding pins (34, 35), across the outer ends of which the pressure differential is applied.

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Frinton-on-Sea, United Kingdom

John A Hammond