5 Things to Know Before Buying unloading valve in hydraulic system

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Figure 11. A shuttle valve is used to select the higher load pressure when two functions are simultaneously shifted, giving the system load pressure priority.

Figure 11 shows the circuitry that can change the load-sense logic. A shuttle valve connects the two sense ports of the two 4-way valves. The unloader spool is now motivated, not by a 4-way&#;s position in the stack but by the higher load pressure. If more than two 4-way spools are in the stack, the highest pressure priority can be maintained by connecting shuttles in binary fashion. That is, each pair of 4-way spools is connected to its own shuttle. Their outputs are paired with one other shuttle, and that output is then paired with one other pair, until they are &#;Christmas treed&#; together until a single output feeds back to the unloader spool. In this manner, two spools require one shuttle, four spools require three shuttle valves, eight spools require seven shuttle valves, and so on.

Simultaneous operations
As is the case with more conventional open-center valves, simultaneous operation of several functions can become complex. System designers are faced with complex mathematical configurations, many of which can be solved only with simulation methods and computers. Operators can have difficulty in training because the behavior of the controls can depend on the cylinder loading and the relative loading between cylinders.

None of these problems is solved when going from conventional open-center designs to the unloader/compensator design. In fact, simultaneous operation of multiple functions can be even more difficult with unloader designs, analytically speaking, and operators can be surprised by sudden changes in load speeds. This is especially true when there is disparity between loads, and the 4-way valves are shifted one after the other.

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All of the unloader/compensator designs shown until now provide for predictable output flow or actuator speed when only one function is shifted. To help make the point, here is a brief recap of single-function operation: First, the unloader compensator adjusts in an attempt to keep the load flow or speed more dependent on the amount of shift in the 4-way spool than on the load. Second, pump pressure rises only enough to support that operation, assuming the pump is not undersized. Third, the action of the compensator function eliminates the apparent, load-dependent dead zone of the conventional open-center valves. Fourth, all 4-way stacks are configured for parallel operation, not series operation. To my knowledge, this the only configuration that has been commercially viable.

However, when more than one 4-way spool is shifted, predictability can be problematic. For example, what happens when a heavily loaded spool is shifted, and after its load is set into motion, a second spool is shifted, but it has a very low load? In fact, consider that the second load is so small that it steals the flow from the high-pressure load. Flow stealing calls for a reduction in pressure at the pump. Will it drop until the high pressure load stalls, which would argue in favor of a rising pump pressure? Or will the system reach some new condition of equilibrium where both loads are moving at some &#;compromised&#; speed? Or maybe the system will break into oscillation and chatter as pressure rises and falls. Will the high pressure load drop?

These questions can be answered for specific cases but are difficult to answer in a general sense. The answers are best predicted by having good, detailed mathematical models of the valves that can be analyzed in a simulation program.

Unfortunately, no standard test methods exist to evaluate all the parameters needed for detailed mathematical modeling of dynamic responses. The critical item to evaluate is the dynamic impedance of the sense lines, which have earned total silence on all valve testing standards. Sense lines are always small and often long, meaning that they have modes of operation with laminar flow. But they are also subject to large differential pressures, which means they can at times have turbulent flow.

In the next issue we&#;ll discuss a logical extension of unloader/compensator valves, namely, valves that have a compensator dedicated to each 4-way spool as well as an unloader. Such systems ease the challenges in trying to predict the consequences of simultaneously shifting more than one 4-way spool without the need for detailed math models and computer simulations. But they, too, have limitations.

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