Unloading Valves

Objectives

  • Explain how unloading valves function
  • List common uses of unloading valves

What Is It?

Unloading valves are very similar to pilot operated, balanced relief valves. The only difference is where the pilot pressure comes from.

In unloading valves when external pressure is applied to Port X that is greater than the pilot setting, the pilot poppet will crack open, allowing the main poppet to open. System flow will then unload from Port A to Port B.

Port X
Port A
Port B
Pilot Poppet
Main Poppet

Schematic Symbols

Unloading valves are drawn in a very similar way as some other types of valves.

The function of many pressure valves on a schematic can be clarified by their location within the hydraulic system.

Port X
Port A
Port B
Pilot Section
Main Section
X
P
T
Basic unloading valve symbol
X
P
T
Pilot Section
Main Section
Denison unloading valve symbol

Build It!

Drag and drop the pieces to build the unloading valve. Not all pieces will be used.

How It Works

Unloading valves are typically used in high-low circuits. The circuit is sometimes referred to as a fast approach, slow feed circuit.

The unloading valve is put to good use in a system where a high flow volume is needed at a lower pressure, and then later a low flow volume is required with a higher pressure.

Mechanical Engineering Symbol

This symbol indicates a mechanical point of mount or friction. It is used in the schematic below to indicate that the spring is mounted to a fixed point.

500
PSI
1000
PSI
Cylinder
Relief
Valve
Check
Valve
Unloading
Valve
Small,
Low Flow,
High Pressure
Pump
Large,
High Flow,
Low Pressure
Pump
Unloading valve in a high-low circuit.

Typically, two pumps, plumbed in parallel, are used in such a circuit. One fixed displacement pump provides volume for the lower pressure, high flow mode, while a small pump provides the low flow at the higher pressure.

Both pumps provide a flow volume in the low pressure, high-flow mode. The purpose of using the two pumps is to save energy draw at the prime movers during the long periods when the only function of the hydraulic system is to maintain an even maximum pressure with very little or no flow.

500
PSI
1000
PSI
Cylinder
Relief
Valve
Check
Valve
Unloading
Valve
Small,
Low Flow,
High Pressure
Pump
Large,
High Flow,
Low Pressure
Pump
Cylinder extending quickly at low pressure.

When the system cylinder is extending quickly at a low pressure (any pressure below the setting of the unloading valve), both pumps are sending full volume into the cylinder.

500
PSI
1000
PSI
Cylinder
Relief
Valve
Check
Valve
Unloading
Valve
Small,
Low Flow,
High Pressure
Pump
Large,
High Flow,
Low Pressure
Pump
Cylinder extending slowly at a higher pressure.

When the cylinders encounter enough mechanical resistance to bring the system pressure up to the setting of the unloading valve, the high flow volume pump is directed to tank through the piloted unloading valve.

The volume of the smaller pump cannot escape to tank through the unloading valve due the check valve in the system. The presence of the check valve makes the schematic symbol for the unloading valve distinct from a typical relief valve.

Unlike a relief valve, there is very little restriction on the flow passing through an unloading valve. Therefore, the unloading valve does not pose a heating problem.
500
PSI
1000
PSI
Cylinder
Relief
Valve
Check
Valve
Unloading
Valve
Small,
Low Flow,
High Pressure
Pump
Large,
High Flow,
Low Pressure
Pump
Cylinder motion stalled.

As the cylinder continues to move and the system pressure increases, the motion becomes slower as only the volume from the small pump is available.

In many cases the cylinder comes to a halt as the machine is actively clamping an assembly or machine apparatus of some sort. At this point, the main relief valve will open and limit the maximum system (or clamping) pressure.

If a relief valve is not used to limit maximum system pressure, then the small pump will be of the variable displacement or pressure compensated variety.
Small,
Low Flow,
High Pressure
Pump
Large,
High Flow,
Low Pressure
Pump

In many systems, the large pump is referred to as the volume pump.

The small pump is often referred to as the pressure pump since it is often just used to support a system pressurizing function when the cylinder is used in a clamp mode. The small pump will only supply enough volume to the cylinder to make up for any internal leakages in the system.

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Low Pressure
Medium Pressure
High Pressure
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