VR4U100 pages

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PILOT OPERATED UNLOADING VALVE - Series VR4U

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VELJAN

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Veljan Unloading Valves series VR4U are pilot operated controls and used to

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jnload a circuit at low pressure when pilot pressure at portX is maintained higher

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than that of inlet pressure at port A set by the pilot spring. A typical use of an

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unloader valve is to unload the low pressure side of a double pump. A Pump

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connected toan Accumulator circuit can also be unloaded by the VR4U valve.

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VR4U consists of a high flow poppet type seat valve section controlled by the low

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flow, adjustable pilot section mounted on top. Pressure setting is achieved by

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means of knurled knob. For tamper proof setting, acorn nut with lead seal is

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available as option. Optional ventvalve VW01 sandwiched between pilot section

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and main body can be used forventing the VR4U valves.

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Exceptionally fast response is obtained due to the precise ratio between the main

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piston area and its mass. One of the unique features of VR4U is that it works as a

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relief valve at the set pressure even in the absence of external pilot signal.

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nternal Drain

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T O

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External Drain

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X1--

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Y-, (OPTIONAL)

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VR4U

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Normally port A is connected to pump and port B to tank. At low

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pressure, flow entering port A is blocked by the main poppet.

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Pressure from port A passes to the pilot section through orifices in

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the main poppet and in the pilot section. No flow takes place in

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this portion until the pressure overcomes the spring setting of a

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pilot ball. The pilot ball is lifted from its seat releasing a small pilot

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flow to tank and resulting in a pressure drop across the main

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poppet. The main poppet opens and allows only enough flow

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from port A to port B maintaining inlet pressure at the set value of

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the pilot spring as in thecase of a reliefvalve.

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h this condition, the external pilot pressure from another part of

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system if applied at port X causes a small override piston to act

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against the ball pushing it further off its seat resulting in very low

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pressure drop across the main poppet as long as pilot pressure is

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maintained, the main poppet opens further and unloads the inlet

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flow at very low pressure drop. As soon as the pilot pressure

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drops to approximately 10% below the pilot setting, the main

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poppet closes with a snap action diverting the main pump flow

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back to the hydraulic system. Excessive restriction in the tank line

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or the pilot pressure signal at port X should be avoided to attain a

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correct function.

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