Load detection is a term used to describe a type of pump control used in open circuits. This is called because the pressure induced by the load downstream of an orifice is detected and the flow of the pump adjusted to maintain a constant pressure drop (and therefore flow) through the orifice. The "orifice" is usually a directional control valve with proportional flow characteristics, but a needle valve or even a fixed orifice may be used, depending on the application.
A load sensing circuit typically comprises a variable displacement pump, usually an axial piston design, equipped with a load sensing controller and a directional control valve with an integral loading signal gallery (Figure 1). The load signal gallery (LS, shown in red) is connected to the load signal port (X) of the pump controller. The loading signal gallery on the directional control valve connects ports A and B of each of the control valve sections through a series of shuttle valves. This ensures that the actuator with the highest load pressure is detected and returned to the pump.
To understand how the load sensing pump and directional control valve work together in operation, consider that a winch is driven through a manually operated valve. The operator summons the winch by moving the spool on the directional valve 20% of its stroke. The lathe drum rotates at five rpm. For clarity, imagine that the directional valve is now a fixed orifice. The flow through an orifice decreases as the pressure drop through the orifice decreases. As the load on the winch increases, the pressure induced by the load increases downstream of the orifice (directional valve). This decreases the pressure drop through the orifice, which means that the flow through the orifice decreases and the vice decreases.
In a load sensing circuit, the pressure induced by the load downstream of the orifice (directional valve) is returned to the pump through the loading signal gallery on the directional control valve. The charge sensing controller responds to the increase in charge pressure by slightly increasing the displacement of the pump (flow) so that the pressure upstream of the orifice increases by a corresponding amount. This keeps the pressure drop constant through the orifice (directional valve), which keeps the flow constant and, in this case, the speed of the constant winch. The value of the pressure drop or delta P maintained through the orifice (directional valve) is typically 10 to 30 bar (145 to 435 PSI). When all reels are in the center position, the charge signal port is vented to the tank and the pump maintains the "hold" pressure equal to or slightly greater than the delta P configuration of the load sensing controller.
Because the pump always receives the load signal from the function operating at the highest pressure, the high-end load sensing directional control valves have a pressure compensator (not shown) at the pressure inlet of each section. The section pressure compensator works with the opening of the selected hole for the spool to maintain a constant flow, regardless of the pressure variations caused by the operation of multiple functions at the same time. This is sometimes called load sensitive sensitivity.
A load sensing pump only produces the flow required by the actuators, which makes it efficient in energy (less heat losses) and as demonstrated in the previous example, provides more precise control. The load sensing control also provides a constant flow independent of the pump shaft speed variations. If the pump drive speed decreases, the load sensing controller will increase the displacement (flow) to keep the delta P adjusted through the directional control valve (hole) until the maximum displacement is reached.