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When things go awry with a piece of hydraulic equipment, the maintenance technician is usually the first on the scene. For the technician’s troubleshooting efforts to be effective, he or she must understand how the equipment operates.
One type of hydraulic control system in widespread use, but not well understood, is load-sensing control.
Load-sensing describes a type of variable pump control used in open circuits. It is also termed this because the load-induced pressure downstream of an orifice is sensed and pump flow is adjusted to maintain a constant pressure drop (and therefore flow) across the orifice.
The orifice is typically a directional control valve with proportional flow characteristics, but a needle valve or even a fixed orifice can be employed, depending on the application.
In hydraulic systems subject to wide fluctuations in flow and pressure, load-sensing circuits can save substantial amounts of input power (Figure 1). In systems where all available flow (Q) is continuously converted to useful work, the amount of input power lost to heat is limited to inherent inefficiencies.
In systems fitted with fixed displacement pumps where 100 percent of available flow is required only intermittently, the remaining flow not required passes over the system relief valve and is converted to heat.
This situation is compounded if the load-induced pressure (p) is less than the set relief pressure - resulting in additional power loss due to pressure drop across the metering orifice (control valve).
A similar situation occurs in systems fitted with pressure-controlled (pressure-compensated) variable pumps, where only a portion of available flow is required at less than maximum system pressure. Because this type of control regulates pump flow at the maximum pressure setting, power is lost to heat due to the large pressure drop across the metering orifice.
A load-sensing controlled variable pump largely eliminates these inefficiencies. The power lost to heat is limited to the relatively small pressure drop across the metering orifice, which is held constant across the system’s operating pressure range (see bottom of Figure 1).
Figure 1. Flow-pressure-power Diagrams for Fixed,
Variable and Load-sensing Controlled Pumps (Peter Rohner)
A load-sensing circuit typically has a variable displacement pump, usually axial-piston design, fitted with a load-sensing controller, and a directional control valve with an integral load-signal gallery (Figure 2).
Figure 2. Typical Load-sensing Circuit Schematic
The load-signal gallery (LS, shown in red) is connected to the load-signal port (X) on the pump controller. The load-signal gallery in the directional control valve connects the A and B ports of the control valve sections through a series of shuttle valves. This ensures the actuator with the highest load pressure is sensed and fed back to the pump control.
To understand how the load-sensing pump and directional control valve operate together, consider a winch being driven through a manually actuated valve. The operator summons the winch by moving the spool in the directional valve 20 percent of its stroke. The winch drum turns at five rpm.
For clarity, imagine that the directional valve is now a fixed orifice. Flow across an orifice decreases as the pressure drop decreases. As load on the winch increases, the load-induced pressure downstream of the orifice (directional valve) increases. This decreases the pressure drop across the orifice, which means flow across the orifice decreases and the winch slows down.
In a load-sensing circuit, the load-induced pressure downstream of the orifice (directional valve) is fed back to the pump control via the load-signal gallery in the directional control valve.
The load-sensing controller responds to the increase in load pressure by slightly increasing pump displacement (flow) so that pressure upstream of the orifice increases by a corresponding amount. This keeps the pressure drop across the orifice (directional valve) constant, which keeps flow constant and in this case, winch speed constant.
The value of the pressure drop or delta p maintained across the orifice (directional valve) is typically 10 to 30 bar (145 to 435 PSI). When all spools are in the center or neutral position, the load-signal port is vented to tank and the pump maintains standby pressure equal to or slightly higher than the load-sensing control’s delta p setting.
High-end load-sensing directional control valves feature a pressure compensator at the inlet to each valve section. The section pressure compensator works with the spool-selected orifice opening to maintain a constant flow rate, independent of the pressure variations caused by the operation of multiple functions at the same time. This is sometimes referred to as “sensitive load-sensing”.
Because the variable pump produces the flow demanded only by the actuators, load-sensing control is energy efficient (fewer losses to heat) - which may result in lower oil oxidation rates and longer fluid life, and improves actuator control.
Load-sensing control also provides constant flow independent of pump shaft speed variations. If pump drive speed decreases, the load-sensing controller will increase displacement (flow) to maintain the set delta p across the directional control valve (orifice) until displacement is at maximum.
Load-sensing pump controls typically incorporate a pressure limiting control, also referred to as a pressure cut-off or pressure compensator. The pressure compensator limits maximum operating pressure by reducing pump displacement to zero when the set pressure is reached.