Yuken Axial Piston Pumps
These pumps have pistons installed in parallel, or axially, with the pump shaft. The pumps are subcategorized into the swash plate type and the bent axis type according to the piston stroke mechanism, as shown in Figs. 2.1 and 2.2.
For both the types, pump displacement depends on the stroke of the pistons in the cylinder block. The displacement
at the maximum angle of the swash plate or the bent axis represents the pump size. The pumps operate with the cylinder block rotating on the shaft. The cylinder block contacts with and rotates on the valve plate (or port plate), which is fixed opposite to the piston, to provide alternate suction and discharge strokes. Some of the swash plate type axial piston pumps have a fixed cylinder block and a rotating swash plate, which rotates so that the piston moves. This type uses a check valve in each cylinder to switch suction and discharge.
The displacement of the piston pumps can be changed by adjusting the angle of the swash or bent axis. The swash plate type allows easier adjustment of the angle; thus, it is generally used as a variable displacement piston pump. Figure 2.3 shows the appearances of the swash plate type variable displacement pumps (A and A3H series) and a graphic symbol of the variable displacement piston pump.
The variable displacement piston pumps are based on pressure compensator control (Table 2.4). When the discharge
pressure rises to a preset level, the compensator valve is actuated to reduce the discharge pressure and feed it to the control piston. The control piston reduces the angle of the swash plate to decrease the output flow and keep the pressure constant. This control type eliminates the necessity for a relief valve to be installed for the maximum pressure in fixed displacement pump circuits. Figure 2.4 shows the typical characteristics of “A-series variable displacement pumps with pressure compensator control type (A16-01).” The performance characteristic curve indicates that the pumps maintain high efficiency of 96 % at a high pressure of 20 MPa (2 901 psi). When the pressure exceeds 20 MPa (2 901 psi), the flow rate starts to decline (cut-off point). At a pressure of 21 MPa (3 046 psi), the pump produces the flow rate required to maintain the pressure (full cut-off). As shown in the graph of full cut-off power, the pressure is controlled with low input power. In the full cut-off state, the drain increases to more than that during the maximum output flow; drain piping should be carefully selected. The A series pumps are considerably quiet and energy-saving, and they are suitable for operation patterns where pressure holding is frequently required.
Tables 2.4 and 2.5 explain features, characteristics, and graphic symbols of various control types, including those that provide multistage flow and pressure control and those that maintain the output at a constant level. Electrohydraulic pressure and flow control type with proportional solenoid control valves and pressure/swash plate angle sensors, and also electro-hydraulic load sensing type are available. This type offers high efficiency, ease in adjusting the output, and quick response through the use of mechatronic pumps, which are a harmonization of mechanics and electronics. Such pumps are energy saving and suited for complicated hydraulic systems involving continuous flow and pressure control in multiple processes. These pumps are supplied as the “A series” and the “A3H series.” Generally, output of the variable displacement piston pumps can be manually adjusted with an adjustment screw.
In a system using the variable displacement pump, a pressure surge is caused within the output line when a rapid shutoff of the output line with a solenoid operated valve or the stroke end of a cylinder changes the operation to the full cut-off from the maximum flow. In contrast, a pressure undershoot is caused by abruptly opening the line. These pressure variations and response times depend on the piping conditions (material (steel pipe or rubber hose), capacity, etc.). Figure 2.5 shows the response characteristics of the A series pumps.
Figure 2.6 compares electric power consumption by hydraulic injection-molding machines using a fixed displacement pump and a variable displacement pump, respectively. The fixed displacement pump PV2R is used with proportional pressure and flow control valves (proportional electro-hydraulic flow control and relief valves EFBG and ELFBG). The variable displacement pump is used with an A-series proportional electro-hydraulic load sensing type, which can keep the flow rate and pressure at the optimum level according to load. EFBG and ELFBG are energy-saving; they bleed excess flow from the pump according to load and maintain the pressure slightly higher than the load pressure. The comparison result, however, indicates that the variable displacement pump is still more energy-saving than the other.
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