VR4U100 pages
PILOT OPERATED UNLOADING VALVE - Series VR4U
nnnnVELJAN
nnnnVeljan Unloading Valves series VR4U are pilot operated controls and used to
nnnnjnload a circuit at low pressure when pilot pressure at portX is maintained higher
nnnnthan that of inlet pressure at port A set by the pilot spring. A typical use of an
nnnnunloader valve is to unload the low pressure side of a double pump. A Pump
nnnnconnected toan Accumulator circuit can also be unloaded by the VR4U valve.
nnnnVR4U consists of a high flow poppet type seat valve section controlled by the low
nnnnflow, adjustable pilot section mounted on top. Pressure setting is achieved by
nnnnmeans of knurled knob. For tamper proof setting, acorn nut with lead seal is
nnnnavailable as option. Optional ventvalve VW01 sandwiched between pilot section
nnnnand main body can be used forventing the VR4U valves.
nnnnExceptionally fast response is obtained due to the precise ratio between the main
nnnnpiston area and its mass. One of the unique features of VR4U is that it works as a
nnnnrelief valve at the set pressure even in the absence of external pilot signal.
nnnnnternal Drain
nnnnT O
nnnnExternal Drain
nnnnX1--
nnnnY-, (OPTIONAL)
nnnnVR4U
nnnnNormally port A is connected to pump and port B to tank. At low
nnnnpressure, flow entering port A is blocked by the main poppet.
nnnnPressure from port A passes to the pilot section through orifices in
nnnnthe main poppet and in the pilot section. No flow takes place in
nnnnthis portion until the pressure overcomes the spring setting of a
nnnnpilot ball. The pilot ball is lifted from its seat releasing a small pilot
nnnnflow to tank and resulting in a pressure drop across the main
nnnnpoppet. The main poppet opens and allows only enough flow
nnnnfrom port A to port B maintaining inlet pressure at the set value of
nnnnthe pilot spring as in thecase of a reliefvalve.
nnnnh this condition, the external pilot pressure from another part of
nnnnsystem if applied at port X causes a small override piston to act
nnnnagainst the ball pushing it further off its seat resulting in very low
nnnnpressure drop across the main poppet as long as pilot pressure is
nnnnmaintained, the main poppet opens further and unloads the inlet
nnnnflow at very low pressure drop. As soon as the pilot pressure
nnnndrops to approximately 10% below the pilot setting, the main
nnnnpoppet closes with a snap action diverting the main pump flow
nnnnback to the hydraulic system. Excessive restriction in the tank line
nnnnor the pilot pressure signal at port X should be avoided to attain a
nnnncorrect function.
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