Unloading valves are very similar to balanced, pilot-operated relief valves. The only difference is where the pilot pressure comes from.
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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.
Figure 3. Cutaway diagram of the unloader spool and 4-way valve as a variable orifice, showing how the unloader spool senses the differential pressure across the powered land.
Figure 3 is a simplified diagram and schematic consisting of the unloader spool in cutaway with the 4-way valve characterized only as a simple variable orifice, KVPL. A sense line connects the load to the spring cavity on one side of the unloader spool. Meanwhile, another sense line connects to the opposite side of the unloader spool. Other spools in the stack and other lands in the valve are omitted for simplicity in illustrating the operating principle behind unloading valve operation.
Note that the unloader spool&#;s bias spring, labeled KS, biases the unloader spool in the closed position. That is, the unloader function is normally closed. The unloader spool will not open until the pressure through the pump sense line exceeds the load sense pressure by the amount of bias spring plus flow forces acting on the unloader spool.
Valve designers select the spring so that the differential pressure required for cracking the unloader spool (just starting to open) requires from about 50 psi to as much as 250 psi. The higher pressure provides for better control, but it also results in higher unloading pressure and, thus, lower efficiency of the valve system.
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Unloader valve issues
A few issues with unloader-type valves are worth discussing. The first is that the unloader spool processes two pressure signals in a way that attempts to maintain a constant pressure across the powered land, KVPL. In so doing, the unloader spool regulates the flow to the load, at least in this simplified scenario.
This raises the second point: As the load pressure changes, the powered land&#;s pressure drop is maintained, and flow tends to remain at a value determined by the degree of shift of the 4-way spool &#; not by the load or even pump pressure variation. This kind of control is easy for even the novice operator to use because of the nearly independent nature of the flow control. A human operator need not compensate for load pressure changes; the unloader does it automatically, somewhat.
However, this is a somewhat idealized description. Practical aspects bring us to a third point: Although many designers of these valves insist that the pressure drop across the powered land remains constant, indeed, it does not. When tested at a variety of loads and KVPL settings (differing amounts of 4-way spool shift), the differential pressure will vary between the cracking value up to as much as twice that amount. Nonetheless, the differential pressure drop will not, in this simple, single function scenario, vary by any large amount from the design cracking value.
This brings up a fourth point: Because the differential pressure drop is maintained at a relatively low level at all operating conditions, the flow forces acting on the 4-way spool are small, making it easier to shift and maintain than in other designs of previous discussions. This results in less operator fatigue, and when electrically activated with proportional solenoids, it also means that the solenoid current determines the flow, not the load and supply pressure. This contributes to operator friendliness of these valves.
A fifth point is that the flow regulation is imperfect. It can vary depending on supply pressure and load pressure, but not nearly as much as without the unloader function. Also, flow forces act to close the unloader valve and, in essence, make the bias spring seem stiffer than it is. The result is overcompensation; that is, as load pressure approaches the pressure rating of the valve, the load flow decreases.
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