Flow Control Valves/Flow Control and Check Valves
These valves consist of a pressure compensator (pressure reducing valve that keeps the pressure difference constant) and a restrictor. They maintain a constant flow rate, independent of the inlet-outlet differential pressure. Provided with a sharp-edge orifice, they can also work regardless of fluid temperature or viscosity.
In a circuit where the flow rate is regulated to a low level, the control flow may be momentarily exceeded, leading to jumping of the actuator. This phenomenon is related to a time lag until the pressure compensating piston is properly positioned for flow control. To prevent the phenomenon, the piston stroke should be adjusted according to the inlet- outlet differential pressure.
Flow control valves are basically used as follows.
(1) Meter-In Control
The control valve is connected in series with the cylinder inlet to directly control the input flow. Prior to the control valve, a relief valve is applied to excess flow, which escapes through a relief valve. In a circuit where load is applied in the direction of piston travel, the control valve may lose cylinder speed control.
(2) Meter-Out Control
The control valve is connected in series with the cylinder outlet to directly control the output flow. Prior to the control valve, excess flow escapes through a relief valve to a tank. This circuit design is used for applications where the piston could move down faster than a control speed, as in the case of vertical drilling machines, or where there should always be a back pressure in the cylinder. Careful attention should be paid to the fact that the cylinder outlet pressure may rise above the relief pressure produced in the circuit.
(3) Bleed-Off Control
The control valve is installed on a by-pass line to regulate flow to the tank and control the actuator speed. Compared to the other control circuits, this circuit works with small power consumption because the pump’s discharge pressure is fully delivered to the load resistance. Given that the bleed flow is constant, the fluctuation of pump flow determines the actuator speed. In other words, the pump discharge flow directly influences the load and the pump’svolumetric efficiency.
This circuit does not allow for control of multiple actuators.
Figure 5.4 shows flow control circuits provided with the control types (1) to (3). PG* indicates the pressure at each point observed when the cylinder is operated at the load pressure of 4 MPa (580 psi). The mechanisms of the meter-in and -out control can be understood by comparing the positions and inlet/outlet pressures of the flow control valves and the inlet/outlet pressures of the cylinders in the meter-in and -out control circuits. The higher cylinder outlet pressure in the meter-out control circuit suggests that the cylinder area difference results in pressure intensification. Also, differences between the bleed-off control and the other control types are clear in the relief valve pressures and the cylinder inlet pressures. The pressure values in the figure are based on the assumption that there are no pipe resistance and pressure loss through the valves.
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