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.
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.
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.
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.
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.
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.
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.
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.
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.
Become a member to get immediate access to the rest of this lesson, and all the other great content on LunchBox Sessions.