Yuken Multiple Vane Pumps
These pumps are categorized into double and triple types. The double pump has two sets of cartridge kits on one shaft, and each of the kit works independently. Generally, this type contains a combination of low-pressure large- volume and high-pressure low-volume pump elements. The triple pump has three sets of cartridge kits and forms a circuit to sum up the output pressures for applications requiring a larger displacement. Figure 2.14 shows a PV2R series double pump. Figure 2.15 shows circuit examples of low-pressure/large-volume and high-pressure/small- volume combinations for press machines.
Yuken Single High Pressure Vane Pump
These pumps operate at 31.5 to 40 MPa (4 569 to 5 802 psi). The PV11R series pumps are available in two sizes, ranging from 2 to 22 cm3/rev (.122 to 1.34 cu.in./rev). Some of them have the pressure loading mechanism, while the others are provided with a unique valve structure that decreases and introduces the pressure to the vane base to reduce the sliding friction between the vane and the cam ring. Those with the unique valve structure are used in high-pressure press machines, etc.
Yuken Single Middle Pressure Vane Pump
These pumps operate at 16 to 21 MPa (2 321 to 3 046 psi). The PV2R series, specially designed to reduce the noise level, are available in four sizes, ranging from 6 to 237 cm3/rev (.366 to 14.46 cu.in./rev). The pumps have a plate with a pressure loading, which changes the clearance between the rotor and the vane side according to the pressure. Figures 2.9 and 2.10 show a PV2R series pump and its typical characteristics.
The simplest vane lift mechanism is the straight type; it introduces the discharge pressure to the base to counterbalance the discharge pressure applied to the bottom of the vane. Other mechanisms, such as the intra vane type where small vanes are embedded into the vanes and the pin type where vanes are pushed with pins, as shown in Fig. 2.11, are available.
These two types have better mechanical efficiency than the straight type because they need less force to lift the vanes from the base. The pin type pumps operating at 25 to 28 MPa (3 626 to 4 061 psi) have been commercially supplied.
Yuken Single Low Pressure Vane Pump
These low pressure vane pumps, operated at 5 to 7 MPa (725 to 1 015 psi), are supplied as the PVL1, 50T, 150T, 250F, and 500F series. The pumps are available in the displacement of 1.5 to 498 cm3/rev (.092 to 30.4 cu.in./rev). They have a simple structure: clearances between the rotor and the vane side are maintained with a fixed plate. The inner space of the cam ring is structured with an oval circle and complete round circle. The curved and connected space between the circles is where the vanes make the elevation movement. The pumps have two pairs of suction and discharge areas and balance the internal radial force with the discharge pressure; thus, these pumps are called the pressure balanced type (Table 2.1).
Yuken Vane Pumps
These pumps intake and discharge fluid according to the change of space enclosed by the vanes and the cam ring that rotates by means of the rotor. Vane pumps in a low/middle pressure range from approximately 7 to 25 MPa (1 015 to 3 626 psi) and with middle displacement; for example, the single middle-pressure type has a displacement of approximately 300 cm3/rev (18.3 cu.in./rev). These pumps provide the following advantages: (1) minimized discharge pressure pulsation; (2) compactness and light weight for high output; (3) less efficiency degradation due to vane wear, and; (4) reliability and ease of maintenance.
The pumps are quieter because of the structure and are less susceptible to working fluid contamination than piston pumps. Therefore, they are conveniently used in a wide range of applications. The pumps typically have a structure where the vane is pressed against the cam ring by inducing pressurized flow to the bottom of the vane. With the improved structure, pumps capable of operating at a high pressure of up to 42 MPa (6 092 psi) are also commercially available.
Vane pumps are categorized into fixed and variable displacement types. Each type is further subcategorized into single and multiple pumps. With vane pumps, it is easy to construct double and triple pumps by mounting pump elements (components such as rotors, vanes, and cam rings) in tandem to the pump shaft. Such multiple pumps with displacements of 300 to 500 cm3/rev (18.3 to 30.5 cu.in./rev) have been commercialized.
Variable displacement type vane pumps, with changing ring eccentricity, are also available. These pumps, with displacements of 30 cm3/rev (1.83 cu.in./rev) or less, are widely used as hydraulic pressure sources for small machine tools.
Yuken Radial Piston Pumps
These pumps have pistons installed in a pattern radial to the pump rotating shaft. The pumps are more suitable for high pressure operation than the axial type.
Figure 2.7 shows the structure of a typical radial piston pump. Piston stroke is achieved with an eccentricity of the piston-sliding ring to the pump rotating shaft. These pumps switch suction and discharge per piston stroke in the ring. With a mechanism for ring eccentricity change added, the pumps allow their displacement to be adjusted.
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.
Piston Pumps
These pumps assure high performance in high pressure operation, compared to the other types, and are easy to convert to the variable displacement type. Thus, they can operate with various control types. The piston pumps provide advantages including: (1) high efficiency; (2) ease of operation at high pressure; (3) ease of conversion to the variable displacement type, and; (4) various applicable control types.
The pumps are categorized into axial, radial, and reciprocal piston types. This section explains the axial piston type, which is most widely applied in industrial machinery, from low-/middle-pressure general industrial machines to high- pressure press machines and construction machines.
Flow Resistance in Pipelines
The friction between the flowing layers of liquid and the adhesion of the liquid to the pipe wall form a resistance which can be measured or calculated as a drop in pressure.
Since the flow velocity has an influence on the resistance to the power of two, the standard values should not be exceeded.
Friction, Heat, Pressure drop
Friction occurs in all devices and lines in a hydraulic system through which liquid passes.
This friction is mainly at the line walls (external friction). There is also friction between the layers of liquid (internal friction).
The friction causes the hydraulic fluid, and consequently also the components, to be heated. As a result of this heat generation, the pressure in the system drops and, thus, reduces the actual pressure at the drive section.
The size of the pressure drop is based on the internal resistances in a hydraulic system. These are dependent on:
• Flow velocity (cross-sectional area, flow rate),
• Type of flow (laminar, turbulent),
• Type and number of cross-sectional reductions in the system of lines (throttles, orifices),
• Viscosity of the oil (temperature, pressure),
• Line length and flow diversion,
• Surface finish,
• Line arrangement.
The flow velocity has the greatest effect on the internal resistances since the resistance rises in proportion to the square of the velocity.
