Conclusion: Hydraulic Pressure-Control Valves

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This section has been devoted to an in-depth study of how to manipulate forces by controlling pressure. With a thorough understanding of the five basic pressure valve functions, the design engineer’s ability to control the interaction of forces is limited only by his imagination. Hydraulic systems, when properly designed, put tons of force under precise, finger-tip control.

In reviewing your knowledge of pressure control, you should be able to differentiate between the five basic control functions, namely: relieving, reducing, sequencing, counterbalancing, and unloading. Likewise, you should understand that, no matter how complex the function, all pressure controls operate by balancing a hydraulic force with a spring. This spring balancing is readily apparent in direct operated valve designs, but it is also the key operating principle in pilot operated versions.


In reviewing the different pilot operated functions, you will discover that no matter what the design or desired function, pilot operation always works on the principle of creating either balanced or unbalanced pressure conditions across the main control element.

More specifically, in relation to pilot operated pressure reliefs, you should know what is meant by the terms remote piloting, venting, and high vent option. You should also understand the three uses of the external pilot drain: for more stable pressure adjustments, for remote pressure control, and for load sensing.

The important points covered in our discussion of pressure reducing valves were: first, the relieving ability of a pressure reducing valve, and, second, the differences between pilot operated versions with either primary or secondary control. We also mentioned the inherent ability of the reducing valves to generate heat in the hydraulic system.

In addition to your knowledge of relief and reducing functions, you should also have a good understanding of the versatility offered by the multi-function family of direct and pilot operated valves. You should know the assembly variations and application requirements in using the valve in sequence, counterbalance, overcenter counterbalance, or unloading functions. In addition, you should realize the special requirements of accumulator circuits.

The hydraulic specialist, who understands pressure and how to control it, knows just about half of everything there is to know in his field. When he combines this knowledge with the principles of flow, the designer has the world of hydraulics at his fingertips. section 3 will deal with flow and its relation to speed in the hydraulic system.

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Wednesday, September 19, 2007 1:34

Make-Up Check Port Relief Valve Directional Control From Pump From Pump Directional Control Fly wheel Hydraulic Motor Crossport Reliefs X port Y drain P1 P2 Tank 1. When the pilot relief opens... 2. a constant pilot flow is established... 3. by this pressure compensated flow control. 4. The pressure in the pilot circuit determines... 5. the outlet pressure on this area needed to close the main spool. 6. The main spool orifices flow to the secondary system, which keeps outlet pressure in equilibrium with pilot pressure. Main System Flow (GPM) Inlet to outlet pressure: 3000 PSI delta P, 1500 PSI delta P Pilot Flow in^3/Min 3. This feeds back an inaccurate downstream pressure... 4. and lessens the pilot oil flow. 2. creates turbulence downstream of the main control spool. 1. A high flow rate… Pressure Compensated Flow Control Metered Pilot Flow Pilot Relief Over Pressure Relief Protection Drain Port High Pressure Inlet Reduced Pressure Outlet Gauge Port 1. When this pilot relief is closed... 2. the pressure above the control spool is the same... 5. Consequently, the main control spool is held in the WIDE OPEN POSITION, since there is a larger opening force than closing force. 4. The pressure at the outlet is only slightly lower than that at the inlet because of pressure drop due to flow. 3. as the pressure at the inlet, since the pilot system is static. 1. When the secondary system become static... 2. leakage by the main control spool… 3. could cause downstream pressure to equalize... 5. However, this relief opens when pressure builds to 45 PSI above that set on the pilot... 4. with inlet pressure... 6. which limits the pressure build-up downstream of the valve.