Ch 3 - Pressure Control Chapter 3 Material taken from Fluid...

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1 Pressure Control Chapter 3 Material taken from Fluid Power Circuits and Controls, John S. Cundiff, 2001 Introduction Pressure control is a key element in the design of any circuit. Used correctly, it can achieve a given functional objective, as well as safe operation. In circuit design the pressure must be limited to a level below the working pressure of the lowest-rated component in the circuit.
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2 Introduction Six pressure-control valves are discussed: ± Relief valves ± Unloading valves ± Sequence valves ± Pressure-reducing valves ± Counterbalance valves ± Brake valves Introduction Each of these valves works on the same principle; a spring force balances a hydraulic force. The hydraulic force is produced by fluid pressure acting on a given area. When hydraulic force exceeds the spring force, the valve spool moves.
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3 Primary Flow Control Valves Introduced Review of Needed Symbols An open-center (float) valve allows flow between all four ports when the valve is in the center (nonactuated) position. The actuator (downstream from the valve) is not held in position but is free to float.
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4 Review of Needed Symbols The open-center valve also allows free flow from the inlet port to the return (or tank) port, but it blocks the actuator ports. The actuator can not move (neglecting leakage) when the open- center valve is in the center position. Review of Needed Symbols The closed- center valve has all four ports blocked when it is in the center position. There is no pathway through the valve between any of the four ports.
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5 Relief Valve The relief valve is used to limit pressure in an entire circuit. It is generally the first component downstream from the pump. Relief valves can be direct acting or pilot operated. Direct-Acting Relief Valve Pressure acts on the annular area of the valve spool. The hydraulic force is given by F h = Pa a ± Where F h = hydraulic force (lb f ) ± P = pressure (psi) ± A a = annular area (in 2 )
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6 Direct-Acting Relief Valve The notation F s will be used for the spring force. When F h equals F s , the valve cracks open, meaning that the spool lifts off its seat and allows fluid to flow to the reservoir. As pressure increases, the spool lifts higher, allowing more flow to bypass to the reservoir. At some pressure level, the total flow bypasses to the reservoir. Direct-Acting Relief Valve A typical flow vs. pressure curve for a direct-acting relief valve is shown. The valve is set to open at 1500 psi. This pressure is known as the cracking pressure.
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7 Direct-Acting Relief Valve At 2000 psi, the valve is fully open, and all flow is bypassed to the reservoir. The 500 psi differential between cracking and full bypass is needed in direct-acting valve when it is used for flow control, as well as pressure limiting. Direct-Acting Relief Valve
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This note was uploaded on 11/22/2011 for the course ABE 5152 taught by Professor Burks during the Fall '08 term at University of Florida.

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Ch 3 - Pressure Control Chapter 3 Material taken from Fluid...

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