The picture shows a relief valve and a hydraulic relief valve cartridge, can you tell them apart at a glance?
I. Methods of discrimination1
On the left is a hydraulic relief valve cartridge, and on the right is an unloading valve.
To be honest, the unloading and hydraulic relief valve cartridges appear very similar from the graphic symbols alone. Both are normally closed valves. This similarity makes it difficult to distinguish between them.
So we need to use another key component to assist in our judgement, and that is this check valve, which is connected in parallel with the unloading valve.
When the unloading valve is open, we only want the fluid discharged from the right hydraulic pump to flow back to the tank through the unloading valve, while the left hydraulic pump continues to supply the system.
The purpose of this check valve is to prevent the fluid discharged from the left hydraulic pump from flowing back to the tank.
Isn’t this dosing pump system in the picture above very familiar? Yes, it is our common high and low pressure circuit.
Just by using a small power motor, you can drive one large and one small hydraulic pump at the same time;
When the system has completed the rapid feed process, the right side of the low-pressure, high-flow hydraulic pump will unload the excess flow back to the tank. Meanwhile, the left side of the high-pressure, low-flow hydraulic pump will continue to provide pressure to the system. Unloading valves are often used to configure such circuits.
II. Methods of discrimination2
Another obvious difference between an unloading valve and a hydraulic relief valve cartridge is that an unloading valve requires only a very small control pressure to push the spool and open the valve port. In contrast, a relief valve needs a higher control pressure for the same action. For example, 34 bar in the diagram.
In other words, the right-hand hydraulic pump discharges the fluid and immediately returns it to the tank. The process consumes minimal motor power because the discharged fluid does not drive any load.
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