Load sensing is achieved with a special compensator mounted on a variable displacement pressure-compensated pump. The reader should envision this special compensator mounted on the pump shown in Fig. 4.15. For this analysis, we assume that the special compensator is set to destroke the pump at 200 psi. (The pump develops 200 psi and is destroked to deliver essentially zero flow at this pressure.) As the DCV is shifted to extend the cylinder, a pilot line senses the 1300 psi required to move the load. This work port pressure is added to the “destroke” pressure, and the pump delivers the required 9 GPM at 1300 + 200 = 1500 psi. How does this actually work? The details are a bit complicated. The reader is directed to Jarboe (1983) for an explanation of how the load-sensing compensator, pump, and DCV work together to deliver less than full flow at less than full pressure with low losses. Actual power loss in this case is
A general understanding can be obtained by considering the load-sensing circuit shown in Fig. 4.19. The three-way, two-position DCV is shifted with pilot pressure. The pump is destroked when this valve is in the position shown in Fig. 4.19. There is no pilot pressure on the left side. The pump pressure applied on the right side shifts the two-position DCV to the position shown. There is no hydraulic pressure to add to the spring force (200 psi) in the compensator (spring cavity is vented through the two-position DCV to the reservoir), thus the pump builds only 200 psi pressure. It remains in this operating condition until the operator shifts.
When the operator shifts the three-position DCV, the pilot line is connected to supply pressure, which is 1300 psi in this example. This pilot pressure acts on the left side of the two-position DCV, causing it to shift to the right. Now supply pressure is added to the spring force in the compensator, and the pump builds 1300 + 200 = 1500 psi.
The means for controlling pump flow to 9 GPM is not shown in Fig. 4.19. The reader is referred to Jarboe (1983) for this detail. Figure 4.19 does have a power diagram similar to Figs. 4.17 and 4.18 to complete the visual comparison of power losses for the three circuits.
Response of a closed-center system with load sensing is slower than that of a regular closed-center system, because pressure must build from 200 psi to 1300 psi before the load begins to move. (With a regular closed-center system, the pressure is 2500 psi at the moment the four-way, three-position DCV is shifted.) Time to build pressure is reduced by designing the four-way, threeposition DCV such that it connects the pilot line to the work port a short interval before the pressure line is connected. The pump compensator is already shifted (pump is building pressure) when the pressure line is fully connected to the work port.
A cross section of a solenoid-actuated DCV with load-sensing pilot line is shown in Fig. 4.20. Several definitions are needed before discussion of the operation of this valve can proceed.
The spool and bore in the valve body are machined to a sliding fit. The spool is machined with lands (sections that have the same diameter as the bore minus clearance) and undercuts (sections where material has been removed from the spool). Spool overlap is the distance along the spool land that separates the internal passages in the valve body. When the spool is shifted, the spool undercuts move into a position where they connect adjacent passages in the valve body. For example, when the spool in Fig. 4.20 is shifted to the left, pump flow (P) is connected to Port A and Port B is connected to the return (R).
Considering the DCV shown in Fig. 4.20, when the spool (1) is in the neutral position, ports A and B are blocked at spool land (2) and metering notches (3). Pump flow enters the valve at (4) and is blocked at (5). No flow can go to ports A and B or the return for this type of closed-center valve. Overlap at (2) is less than the overlap at metering notch (6). When the spool is shifted to the left, Port A is connected to area (7) before the pressure line at (4) is connected to area (7). [Remember, the overlap at (2) is less than the overlap at metering notch (6).] There is a direct passage from area (7) to the load-sensing port (8). Pressure at area (7) is immediately communicated to the pump compensator. The pump begins to build pressure to a level 200 psi above the area (7) pressure (work pressure) before the pressure port (4) is connected to Port A via metering notch (6). The orifice formed by metering notch (6) produces a 200 psi ?P.
This ?P ensures that pump output pressure is 200 psi above the pressure required at Port A.
It should be clear that the load-sensing DCV is symmetrical. A shift of the spool to the right will connect Port B to area (7), and thus to load-sensing port (8), before the pressure port is connected to Port B.
A load-sensing circuit requires a special compensator on the variable displacement pump and a load-sensing DCV. These two components work together to minimize losses when the circuit needs to provide less than maximum flow at less than maximum pressure.