Unloading Valves
By the end of this lesson, you will be able to explain how unloading valves function.
And list common uses of unloading valves.
What is It?
Unloading valves are very similar to balanced, pilot-operated 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 P to Port T.
As you can see by comparing the balanced, pilot-operated relief and unloading valve cutaways below, the only difference is the plug in the unloading valve that is blocking Port P from the pilot section. Therefore, the pilot pressure must come from an external source (Port X).
Schematic Symbols
Unloading valve symbols are also 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.
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