Unloading Valves

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Unloading valves are used in hydraulic circuits to offer high speeds up to a pre-determined pressure level. At valve set ting, a remote pressure signal unloads part of the original flow source to tank. A prime example of an unloading application is a two pump “hi-low” unloading circuit.

In many applications, where high speed and high force are not needed simultaneously, you can achieve a considerable savings of installed horsepower by selecting a two pump system. The first pump supplies high flow at lower operating pressures, and the second pump offers higher pressures, but at a lower flow. Remember that:

(Flow x Pressure) / 1714 = HP

You can see that, at low pressure and high flow, the calculated horsepower can be the same as it would be if the system were operating at high pressure and low flow. Let us now take a look at a circuit which accomplishes this task.

Above: (See Color code legend for above image)

As shown in the example, a multifunction valve with external pilot and internal drain can be used to unload the high volume pump. As long as the system is operating at a pressure lower than that set on the unloading valve, the valve remains closed. Under these conditions, the output of the high volume pump supplements the output of the small pump, and the system operates at full speed. As the system offers an increased resistance to flow, pressure builds on the outlet of both pumps, and on the pilot spool in the unloading valve. When the pressure is high enough to move the pilot spool against the spring setting, the output of the high volume pump is connected to tank. Whenever the main system is operating at a pressure of approximately 100 PSI over the setting of the unloading valve, the output of the high volume pump is circulated back to tank at minimum pressure. If, for some reason, the system pressure drops below the setting of the unloading valve, the circuit automatically shifts itself into high speed operation.

(See Color code legend for above image)


Since the outlet of the unloading valve is connected to tank satisfactory opera can normally be achieved by internally draining the valve However, if it becomes desirable to circulate the output of the high volume pump through filters and heat exchangers, the resistance to flow added by these components can adversely affect the efficiency of the system. Let us now take a closer look at a pilot operated unloading valve, and how it is affected by back pressure at its outlet.

In unloading applications, the efficiency of the system de pends on the minimum pressure at which the high volume pump circulates oil. If, for instance, we were to unload a 100 GPM pump at 50 PSI, our horsepower to drive the pump would be a little less than 3 HP. The same pump, with a 200 PSI resistance to flow in the unloaded condition, would require almost 12 HP to circulate the oil. This not only draws unnecessary power from the prime mover, but, in turn, it generates an equal amount of heat. A slight modification to the pilot spool of a standard multifunction pressure control optimizes the minimum pressure at which the valve unloads its pump.

(See Color code legend for above image)

Hydraulic line color-coded legend. Used in schemaics.

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Thursday, November 27, 2008 17:52

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. High pressure low flow pump 5. so the full input horsepower can then be used… 4. Very little horsepower is required here,... High volume low pressure pump 3. the high volume pump is unloaded to tank at very low pressure. High Pressure Relief Valve To system Isolating check valve 6. to develop a high pressure with a lower flow potential. High-Low Unloading Circuit 2. exceeds this setting... 1. When pressure here… To Tank From High Volume Pump High Pressure Remote Pilot 6. which increases the pressure at which the high volume pump circulates its output to tank. 5. is exposed on the top area of the main poppet... 4. However, back pressure here... 3. pressure here is relatively high and can hold the spool in the wide open position. 2. a nominal back pressure in this chamber has little influence on proper operation, because… 1. When this spool is fully shifted against this spring… External Drain To Tank From High Volume Pump High Pressure Remote Pilot 5. Back pressure here has no influence on the pressure at which the valve opens. 4. which is the minimum pressure required to move the main poppet against this spring. 3. the high volume pump unloads at 45 PSI... 1. By plugging this drain passage... 2. and draining the top of the main poppet externally through these cross drillings...