Tags: hydraulics

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.

Power-saving Control
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)

Circuit Configuration
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).