HYDRAULIC CONTROL VALVES

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HYDRAULIC CONTROL VALVES

THESE ELEMENTS OF A HYDRAULIC CONTROL SYSTEM FUNCTION TO REGULATE THE FLOW OF HYDRAULIC FLUID FROM THE HIGH PRESSURE SIDE TO THE ACTUATOR, I.E. THE HYDRAULIC MOTOR. THERE ARE THREE MAIN TYPES OF VALVES.
·         THE PISTON OR SPOOL TYPE
·         THE FLAPPER AND NOZZLE TYPE
·         THE JET PIPE VALVE

IN ALL CASES, THE MECHANICAL INPUT MOTION CAN BE CONTROLLED BY MANUAL OPERATION, OR BY A LIMITED MOTION CAN BE CONTROLLED BY MANUAL OPERATION, OR BY A LIMITED MOTION ELECTRIC MOTOR, OR BY HYDRAULIC PILOT METHOD. THE OUTPUT RESULTS IN A CHANGE OF HYDRAULIC PRESSURE.

PISTON OR SPOOL VALVE: THE COMMONLY USED SPOOL VALVE IS CONSTRUCTED IN EITHER A THREE-WAY OR A FOUR-WAY VALVE ARRANGEMENT

WHEN THE SPOOL IS IN THE NEUTRAL POSITION, THE OIL FLOW TO THE ACTUATOR IS COMPLETELY BLOCKED. DISPLACEMENT OF THE SPOOL TO RIGHT AND LEFT CAUSES ALTERNATELY PRESSURE IN ONE PORT TO BE HIGHER THAN THAT IN THE OTHER PORT. THIS IS BECAUSE WHEN ONE OF PIPE LINE COMMUNICATES WITH THE DRAIN. THE DIFFERENTIAL PRESSURE CAUSES THE HYDRAULIC MOTOR TO ROTATE IN A PARTICULAR DIRECTION. THE FLOW AND THEREFORE THE MOTOR SPEED ARE FUNCTION OF THE SPOOL-VALVE OPENING, AFFECTED SOMEWHAT BY THE LOAD PRESSURE.

     FLAPPER NOZZLE VALVE: THE OPERATION OF THE UNIT BASED ON AVAILABLE LEAKAGE ARRANGEMENT, WHICH HAS THE GREAT VIRTUES OF SIMPLICITY AND RELIABILITY. THE UNIT INCORPORATES TWO ORIFICES IN SERIES, ONE OF WHICH IS A FIXED RESTRICTION AND THE OTHER VARIABLE ORIFICE CONSISTING OF A FLAPPER AND NOZZLE ARRANGEMENT. THE NOZZLE RESTRICTION IS CHANGED AS THE FLAPPER IS POSITIONED CLOSER TO OR FARTHER FROM THE NOZZLE. THE FLUID AT CONSTANT PRESSURE PASSES THROUGH THE RESTRICTION AND A BRANCH IS LED TO THE NOZZLE. WHEN THE FLAPPER MOVES INTO A POSITION THAT COMPLETELY BLOCKS THE NOZZLE OPENING, THERE IS VERY LITTLE LEAKAGE AND THE OUTPUT PRESSURE APPROACHES THAT OF THE SUPPLY. WITH THE NOZZLE OPENING, THERE OCCURS AN INCREASING IN PRESSURE DROP ACROSS THE RESTRICTION AND CONSEQUENTLY THE OUTPUT PRESSURE DIMINISHES. THUS THE DEVICE PRODUCES A VARIABLE OUTPUT PRESSURE WITH FLAPPER POSITION.

JET - PIPE VALVE: THE DEVICE COMPRISE A PIVOTED NOZZLE AND TWO ADJACENT ORIFICES. THE NOZZLE CONVERTS THE STATIC PRESSURE OF THE SYSTEM INTO KINETIC ENERGY AND THEN DIRECTS THE HIGH VELOCITY JET OF HYDRAULIC FLUID TOWARDS THE ORIFICES. DURING FLOW THROUGH ORIFICES, THE KINETIC ENERGY IS RECONVERTED TO PRESSURES; THE CONVERSION BEING APPROXIMATELY 90% EFFICIENT AT MODERATE SUPPLY PRESSURE.  THIS RESULT FROM THE FACT THAT FRICTION CAN BE REDUCED TO MINIMUM BY MAKING THE SPACE BETWEEN THE NOZZLE AND ORIFICES RELATIVELY LARGE.

THE FLAPPER NOZZLE AND JET PIPE VALVE ARRANGEMENT S ARE FREQUENTLY USED AS PREAMPLIFIER TO A PISTON VALVE.



5 comments:

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