Yuken Poppet Type Solenoid Controlled Pilot Operated Directional Control Valves
These valves are solenoid controlled pilot operated directional valves, which are made multi-functionable by granting individual poppet functions, such as directional control, flow control, and pressure control. They consist of a main valve with four poppets, a solenoid operated directional valve for the pilot line, and a pilot selector valve. These valves are used in large-scale hydraulic systems including press and compressing machines.
They have mounting faces of the same size as those of the conventional spool valves; however, they provide the following features.
(1) They have three functions in one body: directional control, flow control, and check valve for pilot operation (counterbalance valve). They reduce the number of valves in applications and installation space.
(2) Changeover response time is short as the valves are the poppet type. This characteristic eliminates valve overlap. Neither hydraulic leakage from the seat parts nor hydraulic locking is expected.
(3) By selecting proper orifice diameters for the pilot, open/close timing of the flow passage can be set freely. Therefore, together with the shockless type poppets, a smooth start/stop of the actuators can be achieved. Both the noise of the actuator operation switch and line vibration in hydraulic systems can also be minimized.
(4) Control of the pilot line offers independent open/ close operation of four poppets; the four-position four-way valve type is also available to create a differential circuit. By adopting a differential circuit, gaining high-speed feed by using a smaller volume pump is possible, and the electric power for the systems can be saved.
Figure 4.6 shows the poppet type solenoid controlled pilot operated directional valve (DSLHG-06), and Table 4.8 shows examples of the valve functions.
Yuken Solenoid Controlled Pilot Operated Directional Valves
These valves are a combination of a small solenoid operated directional valve and a large pilot operated directional valve. The small four-way solenoid valve is used for directional control of the pilot line. The main valve (main spool) provides directional control of the main line. The pilot operated directional valve includes the spring offset, spring-centered and no-spring types. The main valve (main spool) combinations are as shown in Table 4.6. A pilot choke can be installed between the pilot valve and the main spool to adjust the pilot flow for
decreasing the changeover speed of the main spool. Thus, shock upon actuator start or stop can be reduced. Delay of the spool response to a solenoid changeover varies according to the fluid viscosity and the pilot pressure and flow.
The pilot pressure must be 0.5 to 1 MPa (72.5 to 145 psi) or higher.
Table 4.6 shows combinations of pilot operated directional valves and main spools.
Figure 4.4 shows a solenoid controlled pilot operated directional valve of the 3/4 size (DSHG-06).Table 4.7 and Figure 4.5 show the specifications and changeover time of the valve.
Yuken Solenoid Operated Directional Valves
These valves control the flow direction in hydraulic circuits, electrically operated with manual switches, limit switches, or pressure switches. They are the most popular for use in practical hydraulic systems. Three types of solenoids are available: for direct current (DC), for alternating current (AC), and with a rectifier. The solenoids can be grouped into wet and dry types in respect to the structure. Also, the explosion-proof type is available for special purposes. The solenoid with a rectifier consists of a DC solenoid, a rectifier, and a surge absorber; therefore, it can be used for systems operated with AC power supply. This type of the solenoid is characterized by its low noise and low hydraulic shock during the operation. In addition, it assures the non-burnout feature of the coil when the spool is stuck at the half way point of its changeover.
The solenoid operated directional valves are commercially available in the following sizes: less than 1/8, 1/8, 3/8, 1/2, 3/4, and 1 1/4. Among them, the 1/8 size is most often selected for practical hydraulic systems. Figure 4.1 shows the 1/8 solenoid operated directional valve (DSG- 01), and Table 4.4 lists the major specifications of the standard and shockless types.
Aside from the maximum working pressure, flow, tank-line back pressure, and changeover frequency, performance characteristics of the solenoid operated directional valves include power consumption. The maximum flow refers to the highest flow rate free from any abnormality caused in the valve operation (changeover), and it is determined by the spool profile and service condition (connecting port, voltage, frequency, etc.). To select appropriate valves for a hydraulic system, it is best to consult with valve specifications offered by the manufacturers.
Other performance characteristics to be considered are pressure drop and changeover time. Figures 4.2 and 4.3 show the pressure drop characteristics of the standard and shockless types, respectively. The pressure drop depends on the spool profile in the valves. Table 4.5 indicates changeover time, which varies according to the solenoid type.
Directional Control Valves Classification by Operation Method and Spring Arrangement
These valves are classified according to the function, operation method, and spring arrangement. The operation method is classified into: manual, mechanical, hydraulic, solenoid-operated, electro-hydraulic, and pneumatic. The spring arrangement is classified into: spring offset for the two-position type, spring centered for the three-position type, and no spring for both two- and three-position types. The no-spring type includes the detent type, which holds the spool position at a certain position. Table 4.3 shows classification of the valves according to the operation method and spring arrangement.
Directional Control Valves Classification by Spool Types
The directional control valves in hydraulic systems must work such that when the spools are in a neutral position, the fluid flow patterns meet the purpose of the systems, in addition to causing reversible motion of the hydraulic cylinders and motors.
For example, take the “three-position” valve in Table 4-1. This closed-center valve (all ports are closed when the spool is in the neutral position) locks the cylinder at its
position. The pressure in the pump line is maintained at a preset level for the relief valve or the variable pumps, and other systems can be operated as desired. On the other hand, the center-bypassed valve (ports P and T are open when the spool is in the neutral position) unloads the pump line while locking the cylinder, which is desirable for energy saving: lower heat generation and reduced pump load.
However, the overall performance of the hydraulic system should be taken into consideration to choose the best valves for it. Table 4.2 lists common spool types with their functions and usage. Note that the numbers in the double quotation marks “**” may vary according to valve manufacturers.
Directional Control Valves Classification by Port/Position Count
The port count indicates the number of connectable lines, and the position count indicates the number of change- overs in the directional control valves. Table 4.1 lists the classifications. The valves with four ports and three
positions are very popular. The four ports include: pump port (P), tank port (T), and cylinder ports (A and B). The symbols are often appended with graphic symbols of the directional control valves.
Yuken Pressure Switches
Pressure switches are used in hydraulic systems to make or break an electrical circuit at a preset hydraulic pressure. A sensing component made of semi-conducting materials detects the pressure. The switches are used with relays to operate solenoid valves to unload pumps or start/stop motors.
Yuken Balancing Valves (Pressure Reducing and Relieving Valves)
These valves are combination valves that have pressure reducing and counterbalancing functions developed for applications such as a hydraulic balance circuit in a vertical machining center. When the pressure reducing function is employed, the outlet pressure is maintained at the preset level for pressure reduction, regardless of the inlet pressure. If the counterbalancing function is employed, the outlet pressure is maintained at the preset level for pressure relief, which is higher than the pressure reducing.
Yuken Brake Valves
These valves smoothly stop actuators that have a large inertia force. When a directional control valve is closed, a relief valve at the outlet side releases the accumulating pressure while maintaining the circuit pressure at the preset level. At the inlet side, a check valve feeds the flow, supply of which has been blocked by the directional control valve, to reduce a risk of cavitation.
Yuken Pressure Reducing Valves/Pressure Reducing and Check Valves
These valves set hydraulic circuit pressure equal to or below a pressure in the main circuit. When the outlet pressure reaches a preset level, the valve opens, and the balanced piston moves to throttle a passage to keep the outlet pressure constant; thus, the pressure is reduced. The outlet pressure is maintained constant, regardless of the inlet pressure. If flow resistance at the inlet side is equal to or lower than the preset pressure for the pressure reducing valve, the outlet pressure is given at the same level as the inlet pressures. If flow resistance at the outlet side is equal to or lower than the preset pressure for the valve, the valve opens fully; therefore, both the inlet and outlet pressures are the same.
