Figure 6 is an alternate method for sensing load pressure. Instead of using the 4-way spool to switch the sense line, a shuttle valve is used to sense the higher of two cylinder pressures. Some additions are needed to make this circuit practical, however this works in many cases.
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The fallacy comes in the case of over-running loads. Conventional wisdom assumes the powered end of the cylinder (be it rod end or cap end), is at the higher pressure. However, an over-running load can occur, causing the unpowered (meter-out) land pressure to be higher. In that case, the sense line to the spring cavity does not sense the pressure drop across the powered land. Instead, it has some other pressure which is a complex function of just about everything in the circuit.
The circuit’s behavior is difficult to predict and can result in sudden changes in load speed. The circuit is workable if loads behave according to conventional wisdom, except for another minor glitch. With the spool centered, if a load is on, say, the cap end of a cylinder (as would be encountered on the lift cylinder of a front end loader), then that is the pressure sent back to the unloader spool spring cavity. This becomes the pressure at which the pump is unloaded when the 4-way spool is centered.
This hardly makes for efficient operation. To combat the problem, valve designers are forced to put a special land on the spool that routes the load-sense line to tank with the 4-way spool centered, Figure 7.
The problem with the circuit of Figure 6 is best seen by considering what happens when a load is on the cylinder with the valve centered. An example would be a front end loader with the bucket raised, and the operator wants to keep the bucket raised. In this case, the shuttle valve will shift to connect the high pressure to the spring cavity of the unloader spool, and the pump will be unloaded at high pressure. This is hardly the efficiency we want.
Figure 9. This is the spool-switched load sense configuration, but it has been modified slightly so that sensing is taken off the work port side and the small V-shapes indicate a restrictive flow path. Sensing is downstream of those restrictions.Figure 9 is included to make several points. First, the shuttle valve load sense logic is not used. Instead, the spool-switched load sense that was introduced earlier is used. Second, the small V-shapes on the powered land flow arrows in the valve symbol indicate the natural restrictiveness of the land. Third, the load-sense line is taken off the work port side of the envelope, and the internal dashed lines indicate that sensing is made downstream of the powered land restriction. In other words, the load pressure is being sensed, which is exactly what should be sensed. Also, pilot-operated check valves are included but may not be needed if load motion is purely horizontal.
Additional functionality
The configuration in Figure 9, with its spool-switched sense ports and lands, is fully workable and useful as a single-spool valve. But most mobile equipment valves have several functions in the stack. Figure 6 is included to aid in understanding the features that must be added to the valve assembly when more than one 4-way section is in the stack. All the features relate to the need for getting the correct load pressure back to the spring cavity of the unloader spool while accommodating efficient unloading when all spools are centered. Also, Figure 6 shows the single-function unloader valve in conventional ISO symbols. However, the circuit is not practical unless it can accommodate more than one 4-way spool in the stack.
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.
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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.
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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