Hydraulic pumps - part 3

Fig. 15. Typical performance of a single- and two-stage pressure compensation.

Fig. 16. Schematic of proportional pump compensator that provides load-sensing capability.

Fig. 17. Pressure-flow curve of pump with load-sensing control.

Fig. 18. Schematic of pump control that provides load sensing and pressure limiting.

Fig. 19. Load-sensing gear pumps with two different types of hydrostats installed. The spring adjustment allows tuning pressure drop for different manufacturers' valves or line lengths.

Fig. 20. Unloader control has been added to the load-sensing gear pump. The control uses a poppet or a plunger to allow maximum flow at the minimum pressure drop across the unloader with minimal control movement.

Fig. 21. Combined control is achieved by incorporating a pilot relief, which causes the hydrostat to act as the main stage of a pilot-operated relief valve.

Fig. 22. This cutaway shows combined control, which has an adjustable hydrostat contained within the unloader control. Locating the hydrostat within the low-unload control allows all piston areas to operate from a single load-response signal. It is intended for applications using large pumps where secondary flow bypasses to tank.

Superior dynamically

With respect to its matching function, a two-stage compensator is identical to the proportional compensator control shown in Figure 12. The dynamic performance of the two-stage control is superior, however. This becomes obvious when one analyzes a transient which involves a sudden decrease in load flow demand, starting from full stroke at low pressure.

The single-stage control spool ports pressure fluid to the stroke piston only when pump discharge pressure reach the compensator setting. The main-stage spool of the two-stage control starts moving as soon as pump discharge pressure minus spring chamber pressure exceeds the 300-psi spring setting. Because pilot fluid flows through the orifice and because of the flow needed to compress the fluid in the spring chamber, the spring chamber pressure lags pump discharge pressure. This causes the spool to become unbalanced and shift to the right.

Pump destroking starts before pump discharge pressure reaches the compensator setting, Figure 15. Note that in system equipped with an accumulator, a two-stage compensator control provides little advantage. In excavator hydraulic systems, however, superiority of the two-stage compensator is evident: it provides system components much greater protection against pressure transients.

Load sensing: the next step

A similar control, which has recently become popular, is the load sensing control, sometimes called a power matching control, Figure 16. The single-stage valve is almost identical to the single-stage compensator control, Figure 12, except that the spring chamber is connected downstream of a variable orifice rather than directly to tank. The load-sensing compensator spool achieves equilibrium when the pressure drop across the variable orifice matches the 300-psi spring setting.

Any of three basic load-sensing signals control a load-sensing pump: unloaded, working, and relieving. In the unloaded mode, the lack of load pressure causes the pump to produce zero discharge flow at bias or unload pressure. When working, load pressure causes the pump to generate discharge flow in relation to a set pressure drop, or bias pressure. When the system reaches maximum pressure, the pump maintains this pressure by adjusting its discharge flow.

Like the pressure-compensated pump, a load-sensing pump has a pressure-compensation control, but the control is modified to receive two pressure signals, not just one. As with pressure compensation, the load-sensing control receives a signal representing discharge pressure, but it also receives a second signal representing load pressure. This signal originates from a second orifice downstream from the first. This second orifice may be a flow-control valve immediately beyond the pump outlet, the spool opening of a directional control valve, or it may be a restriction in a fluid conductor.

Comparison of these two pressure signals in the modified compensator section allows the pump to sense both load and flow. This reduces power losses even further, Figure 17. Output flow of the pump varies in relation to the differential pressure of the two orifices. Just as the pressure-compensated pump increased its discharge pressure by the amount required to run the pressure compensator, the load- and flow-sensing pump's discharge pressure typically is between 200 and 250 psi higher than actual load pressure.

Furthermore, a load-sensing pump can follow the load and flow requirements of a single circuit function or multiple simultaneous functions, relating horsepower to maximum load pressure. This consumes the lowest possible horsepower and generates the least heat.

Operator control

If the variable orifice is a manually operated flow control valve, the system can operate in a load-matched mode at the direction of an operator. As he opens the flow control valve, flow increases proportionally (constant pressure drop across an increasing-diameter orifice), at a pressure slightly above load pressure.

As suggested in Figure 17, wasted power is very low with a load-sensing variable volume pump compensator. Since the control senses pressure drop and not absolute pressure, a relief valve or other means of limiting pressure must be provided.

This problem is solved by a load-sensing/pressure-limiting control, Figure 18. This control functions as the load-sensing control previously described, until load pressure reaches the pressure limiter setting. At that point, the limiter portion of the compensator overrides the load-sensing control to destroke the pump. Again, the prime mover must have corner horsepower capability.

Load-sensing gear pumps

Piston and vane pumps rely on their variable-displacement capability to accomplish load sensing. How, then, can a gear pump accomplish load sensing if its displacement is fixed? Like standard gear pumps, load-sensing gear pumps have low initial cost when compared to other designs with equivalent flow and pressure capabilities. However, load-sensing gear pumps offer the versatility of variable-displacement axial-piston and vane pumps but without the high complexity and high cost of variable-displacement mechanisms.

A load-sensing gear pump is a versatile pump of patented design that can:

• provide the high efficiency of load sensing without the high cost associated with piston or vane pumps
• produce zero to full output flow in less than 40 milliseconds with little or no pressure spiking and without pump inlet supercharging
• drive circuits with low (approaching atmospheric) unload relief pressures
• provide priority flow and secondary flow with a low unload pressure to reduce standby and secondary loaded power draw, and
• interchange with load-sensing vane or piston pumps without having to change line or component sizes.

Load-sensing piston pumps use a pressure compensator and a hydrostat to vary volumetric output to a system in reference to load pressure and flow requirements. A hydrostat is a spring-loaded device that meters flow according to the spring force across its equal but opposing effective areas. It may be restrictive, as in a series circuit, or it may bypass primary load pressure to a secondary or tank pressure. In simple terms, a hydrostat separates the total flow into two flows: one represents the required flow and the other represents the required pressure of the primary circuit. A load-sensing piston pump uses its hydrostat to regulate output flow relative to load pressure and bypasses the excess pump flow to a secondary route, which may be ported to tank or to a secondary circuit.

A load-sensing gear pump, on the other hand, uses a hydrostat in combination with an unloader to vary its volumetric output in response to load and flow requirements. Because load-sensing piston and gear pumps both use a single load-sensing signal to control pump discharge pressure and flow, they are interchangeable in load-sensing circuits. Both types have much in common and offer substantial power savings over systems using fixed-displacement pumps. Both offer reduced power consumption in the running mode - when flow and pressure are required to operate a function. They also conserve power in the standby mode - when the system is idling or in a non-operational mode. Furthermore, they can reduce the required size - and, therefore, cost- of valves, conductors, and filters needed for the circuit.

The load-sensing gear pump illustrated in Figure 19 minimizes power consumption in the running mode by separating total discharge flow according to a remote primary function pressure and a primary flow. This is accomplished through a single load-sensing signal originating from the priority circuit and routed as close as possible to the discharge side of the pump's gears.

Adding an unloader control to the pump circuit, Figure 20, allows the system to conserve power in the standby mode of operation as well as in the running mode. This control must be installed in parallel with the inlet port of the hydrostat and as close as possible to the discharge side of the gears. It must be piloted by the same load-sensing signal as in Figure 19. This signal causes the pump to dump all flow from the outlet to the secondary circuit and at a pressure well below the hydrostat's pressure-drop setting in the standby mode.

The unloader control must operate off the same remote load-sensing signal that controls the hydrostat. Unlike the hydrostat, the unloader poppet of the unloader control is designed with opposing areas having a ratio of at least 2:1. Any line pressure sensed that exceeds 50% of pump discharge pressure will close the unloader control. The ability of the unloader control to unload the pump to near atmospheric discharge pressure is controlled by the poppet or plunger spring force. The unloader control is set to the lowest value to maintain the internal pressure loading of the gear pump. When compared to a standard fixed-displacement gear pump circuit, this control can reduce standby power consumption by 90%.

Dual and combined controls

The load-sensing signal can be conditioned by limiting pressure in the remote sensing line or taking it to 0 psig. Doing so causes the hydrostat and the unloader control of the load-sensing gear pump to respond to the conditioned signal according to the discharge pressure. This is accomplished by providing a pilot relief, Figure 21, which causes the hydrostat to act as the main stage of a pilot-operated relief valve. The ability to condition the load-sensing line is patented and makes the load-sensing gear pump useful for functions other than just load sensing.

The combined-control load-sensing gear pump, Figure 22, is intended for large-displacement pumps and bypasses secondary flow to tank. It also is patented, and can be used in the same applications as the dual-control pump. However, because secondary flow must be routed to tank, it cannot be used when the secondary circuit drives a load.