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Variable-volume, pressure-compensated bent-axis pumps
The cutaway view in Figure 8-25 shows a bent-axis pump that is capable of variable volume as well as pressure compensation. This means the pump output can be varied by a manual control or it can automatically change as pressure increases to a predetermined setting. (This cutaway represents only one way such a pump might be built.) The operates in the same way as the pump in Figure 8-24, but the angle of the cylinder block can vary to reduce flow on a pressure demand. Also, the maximum-volume screw can limit the maximum angle of the cylinder block to establish maximum flow. This is an option on many manufacturers’ designs.
As pressure at the outlet builds to the setting of the pressure adjustment, the compensator spool is pushed back. The spool forces the compensator piston to push the cylinder block to a lesser angle. When pressure reaches the preset level, the cylinder block stays in any position required to maintain the flow needed at the preset pressure.
Two symbols can be used to show pressure-compensated pumps schematically. The complete symbol on the left shows all the functions, while the simplified symbol on the right omits the case drain and shows the compensating arrow inside the pump circle. Because most schematic drawings are done on CAD systems now, the simplified symbol is seldom used.
Figure 8-26 shows the symbol and a cutaway view of a bi-directional, bent-axis pump for closed-loop circuits. This pump operates in the same manner as the previously described bent-axis pumps, but is capable of drawing in and discharging fluid from either port while turning the same direction. This design needs an external pilot supply because it has no integral pilot pump.
The cutaway shows optional features such as: maximum-volume screws in both flow directions and a proportional-control valve for infinitely variable flow from either port. Manual, mechanical, and solenoid controls also are available. A control piston that is offset by a resisting piston with a smaller diameter moves the cylinder block. The 3-way servovalve ports fluid to or exhausts fluid from the larger piston to position the cylinder block. This pump design is not readily available currently, but there still are many of them operating in the field.
All pumps that can be pressure compensated can also be made load sensing. Load sensing is a control technique that keeps the pump compensator from holding full pressure until an actuator stalls. Normally a pressure-compensated pump circuit operates at full compensator pressure setting unless an actuator is using all the pump flow. While an actuator is using all pump flow, pressure is whatever it takes to move the load. This is an ideal setup because all energy -- except for component inefficiencies -- is being used to do work. There is no wasted energy except for inefficiencies and very little heat is generated. A load-sensing circuit uses a feedback signal from the actuator that keeps pump pressure at 100 to 300 psi above the load. Some load-sensing pumps have a fixed differential while others are adjustable. When no actuator is moving, system pressure is at the load-sensing setting of 100 to 300 psi instead of the compensator setting. Energy savings is the main advantage of a load-sensing function, but it also makes a non-compensated flow control perform like it is pressure compensated.
The schematic drawing in Figure 8-27 is a typical load-sensing circuit with two actuators. Notice that the sensing lines from the actuator flow lines to the pump compensator. A load-sensing pump must be able to read any load it is powering so that ample pressure can be maintained. Also notice that the load-sensing lines go through check valves to isolate the flow lines from each other.
All flow controls in a load-sensing circuit must be meter-in type so pressure at the actuator is always high enough to move the load. In the case of the vertical cylinder, a counterbalance valve keeps it from running away while extending. Notice that the load-sensing line from the rod end of the vertical cylinder is connected between the counterbalance valve and the directional control valve so it does not see a load when the circuit is at rest.
Because the pump is pressure compensated, the directional control valve’s pump port is blocked in center position. This circuit uses a bar manifold with modular meter-in flow controls and a modular “B” port counterbalance valve sandwiched under float-center directional control valves for piping convenience and leak prevention. The lines connected to the “A” and “B” ports below the meter-in flow controls go to isolation check valves, then on to the load-sensing connection.
With the circuit at rest as shown, the load-sensing connection sees little or no pressure because the actuator ports are connected to tank. At this point, circuit pressure is equal to the load-sensing bias spring, regardless of the setting of the adjustable compensator spring. At this low pressure, the circuit consumes very little horsepower and generates little heat. The pump’s internal parts are subject to low stress, which makes them last longer and maintain high efficiency
When a cylinder cycles, the load-sensing connection sees whatever pressure it takes to move it. Pump outlet pressure rises to load pressure plus load-sensing bias-spring force. When both cylinders operate simultaneously, the load-sensing connection receives pressure from the highest load through the isolation check valves. Pump pressure is always that needed to move the highest load plus a value added by the load-sensing bias spring. (Some load-sensing bias springs are adjustable within a narrow range.)
When load-sensing valves have low or no bypass flow, use shuttle valves in place of the isolation check valves. Shuttle valves will not trap backflow when a directional control valve shifts to center position. Check the chosen pump to see if this feature is standard, must be specified, or is not available.