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Basic hydraulic circuit design: energy conservation, Part 3.

A special class of hydraulic systems has evolved to improve the efficiency of transmitting power from the prime mover to the load. They are load-sensing systems.

There are two basic methods of sensing load: with pump controls and valve controls. Most commercial load-sensing systems use pump controls. The control signal sometimes consists of only a load-pressure signal, but often the load-pressure signal is combing with a flow signal. Load-sensing pump control systems resemble those used in pressure-compensated pumps with one significant difference--control pressure for pressure-compensated pumps is sensed inside the pump and reflects all system pressure variations.

Control pressures for load-sensing systems are sensed close to the load, reflecting only variations in that specific load. These load pressures are usually sensed at the motor port of the directional control valve or at a motor or actuator inlet port. Differential pressure reflecting flow is usually measured in the pump or in the control valve (Fig. 1 and Fig. 2).


In load-sensing valve control systems, sensed pressure is used to adjust the setting of a control signal to adjust the relief valve setting, but it would be possible to apply the technique to flow control, as is done in pressure-compensated directional control valves (Fig. 3).


Valve Differences

There is a fundamental difference between load-sensing and pressure-compensated directional control valves. Load-sensing valves are used for external pilot control, so piloting is external and had no direct effect on the valves themselves. Instead, the external pilot control sends load signals to the pump or valve control.

On the other hand, pressure compensated valves are self-contained; piloting is internal, helping to reduce flow forces acting on the spool, as demonstrated in Fig. 4.


Shifting the spool of a load-sensing valve opens the correct sensing passage, simultaneously blocking the other, as system flow is directed to the proper actuator port, (Fig. 5). Load-sensing pilot lines can be connected directly to a compensator, but most commonly are connected to other controls within a packaged control unit.


One common packaged unit, shown in Fig. 6, combines a load-sensing directional control valve with a brake valve and load valve. The brake valve accommodates overrunning loads, while the load valve provides improved metering characteristics and isolation for simultaneous operation of multiple branched circuits.


In a pressure-compensated directional control valve, the spool-land orifice is used as a control orifice in series with the pressure-compensated variable orifice. This confines the major portion or the pressure differential across the valve to the compensator and keeps fluid pressure differential across the spool low, resulting in low shifting forces. Depending on the application, the metering orifice can be formed with tapered throttling areas on the spool, as shown in Fig. 6, tapered grooves in the spool, or semi-circular orifices in combination with tapered lands in the valve body.

Some functional combinations for pressure-compensated load-sensing valves are: inlet-compensated, load-sensing (with or without return flow) regeneration capability; outlet compensation (brake valve) with or without flow regeneration; and variations on inlet or outlet compensators to allow straight load-pressure sensing, differential-pressure sensing, etc.

The second major component used in load-sensing systems is the pressure-compensated, variable displacement pump. Fig. 7 summarizes other types of pressure-compensated pump controls which offer versatility in matching pumps to energy conservation systems.


More Complex

Here are some illustrations that show increasing complexities of control hardware as designers require more sophisticated controls, and the correlation of power relationships for pumps equipped with these controls.

Fig. 8 illustrates proportional, pressure-compensated pump controls. Fig. 8(b) shows the reduction in power loss over a fixed displacement pump. Note, however, that the prime mover must still be able to provide required corner horsepower.


If the pump supplies fluid to more than one load, a throttling device, such as a flow control valve or a pressure-reducing valve, is typically installed between the pump and one or more of the loads. The power loss occurs at these throttling devices.

Performance characteristics of the proportional pressure compensator control can be improved with a two-stage, or pilot-operated compensator, Fig. 9(a).


Here, pilot fluid at load pressure is admitted to one end of the main stage spool. Fluid flows through a small orifice in the spool, creating a relatively small pressure drop. The resulting force is opposed by a spring. Pilot flow then returns to reservoir though a small relief valve.

The relief valve setting determines maximum pump pressure. In this design, Fig. 9(b), the main spool starts opening earlier in response to a sudden decrease in flow demand. This anticipation results in less pressure overshoot than typically produced by a single-stage compensator control.


To obtain better dynamic-performance characteristics, the control piston of the single-stage proportional compensator is supplemented with a bias-control piston.

Load sensing, or power matching, control can be provided by a relatively simple variation of the single-stage proportional pressure compensator, Figure 10(a). Here, the spring chamber is connected to the downstream side of a variable orifice. When the pressure drop across the orifice matches the spring setting, the spool achieves equilibrium. If the circuit is arranged so the variable orifice is a manually operated valve, the result is a load-matched flow-control arrangement.


When the valve opens, flow increases proportionately because the pressure drop is constant across the orifice, which keeps increasing in size. The flow is at a pressure only slightly above load pressure--wasted power is very low, Figure 10(b).

However, if load flow drops to zero, as when an actuator reaches the end of its stroke, for example, the pressure drop across the orifice also drops to zero. Then, the spring drives the spool to vent the control piston to reservoir, and the pump is stroked to full displacement. Thus, a relief valve must be included to protect the pump. In this design, full pump output returns to reservoir over the relief valve and the prime mover is loaded to corner horsepower.

The undesirable waste of power over a relief valve in a load-sensing system can be avoided by adding a pressure-limiting control. With this combination, Fig. 11(a), the load-sensing portion controls the pump until load pressure reaches that of the compensator setting. Then the pressure-limiting portion overrides the load-sensing portion and destrokes the pump. Even with this arrangement, the prime mover must be able to provide corner horsepower, Figure 11(b).


Some information and illustrations for this article are from "Fluid Power Systems & Circuits," by Russell W. Henke, published by Penton.

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Author:Henke, Russ
Publication:Diesel Progress North American Edition
Geographic Code:100NA
Date:Sep 1, 2006
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