Basic Hydraulics - Flow Control Valves
CHAPTER 6 - Flow Control Valves
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| Figure 6.1 - the internal workings of a flow control valve |
Introduction
Flow control valves are used to regulate the volume of fluid supplied to different areas of hydraulic
systems. In this section you will be given an overview of the two types of flow control valves, as well as their application and location in a hydraulic system. Because a proper placement of these devices is critical to optimum system performance, a section has been provided to help you learn why and where flow control devices should be used.
Overview
The function of the flow control valve is to reduce the rate of flow in its leg of the circuit. Flow reduction will result in speed reduction at the actuator. A flow control valve builds added resistance to the circuit, thus increasing pressure, resulting in a partial bypassing of fluid over the relief valve or a de-stroking pressure of a compensated pump. This reduces flow downstream of the flow control valve.
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| Figure 6.2 - a hydraulic system with a needle valve |
With a fixed volume pump, to reduce flow to the actuator we must bypass a portion of the fluid over the relief valve. As the needle valve is closed, pressure increases upstream. As the pressure approaches 1500 psi, the relief valve begins to open, bypassing a portion of fluid to the reservoir.
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| Figure 6.3 - a flow control valve with a pressure compensated pump |
When flow control is used in a pressure compensated pump, fluid is not pushed over the relief valve. As pressure approaches the compensator setting of 1500 psi, the pump will begin to de-stroke, reducing outward flow.
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| Figure 6.4 - schematic drawings for the different types of flow control valves |
Flow control valves may be fixed or non-adjustable or adjustable. In addition, they may also be classified as throttling only or pressure compensated.
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| Figure 6.5 - pressure gauges illustrating the change in pressure as flow goes through a flow control valve |
The amount of flow through an orifice will remain constant as long as the pressure differential across the orifice does not change. When the pressure differential changes, the flow changes. Changing load or upstream pressure will change the pressure drop across the valve.
Throttling vs. Pressure Compensating (Needle Valves)
Needle valves may be designated as non-compensated flow control or throttling valves. They are good metering devices as long as the pressure differential across the valve remains constant.
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| Figure 6.6 - a hydraulic system with a needle valve |
A pressure compensated flow control valve is designed to make allowances for pressure changes ahead or after the orifice. The pressure compensated flow control valve symbol adds a pressure arrow to the orifice. With a pressure compensated flow control valve, the speed of the cylinder does not change with the change in load.
Meter-In/Meter-Out
Meter-in is the method of placing a flow control valve in such a way that fluid is restricted to the actuator. In Figure 6.7, the cylinder extends and retracts at an unrestricted rate when a flow control valve is not present. When a flow control valve is placed into the circuit, flow will be restricted to the cylinder, slowing the extend rate of the cylinder. The check valve allows return flow to bypass the flow control when direction of flow is reversed.
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| Figure 6.7 - a check valve used for a meter in application |
When the flow control is moved to the other line, as in Figure 6.8, the cylinder extends at an unrestricted rate. Flow can be restricted to the cylinder so that it will retract at a reduced rate.
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| Figure 6.8 - a check valve used for a meter out application |
The advantage to meter-in is that it is very accurate with a positive load. However, when the load goes over center, the load becomes negative or overrunning. The load is no longer being controlled by the cylinder. As the load overruns, it causes the cylinder to cavitate as in Figure 6.9.
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| Figure 6.9 - the meter in application is used for controlling the speed of the actuator |
Although meter-in is usually the best placement for controlling a constant speed, because it also dampens flow and pressure transients, it may be required in some applications to meter-out. To meter out, simply change the direction that the flow is allowed to pass through the reverse check. This will cause the fluid to be metered as it leaves the actuator, which is opposite of meter-in.
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| Figure 6.10 - pressure intensification due to metering out |
An advantage of meter-out is that it will prevent a cylinder from over-running and consequently cavitating. A disadvantage of meter-out can be pressure intensification. This can occur with a substantial differential area ratio between the rod and piston. When metering-out on the rod side of the cylinder without a load, the pressure is intensified on the rod side. This may damage the rod seals. Meter-in or meter-out has advantages and disadvantages. The application must determine the type of flow control valve placement.
SUMMARY
-The function of the flow control valve is to reduce the rate of flow in its leg of the hydraulic circuit.
-Flow reduction will result in speed reduction at the actuator.
-Flow control valves may be fixed or non-adjustable or adjustable.
-Metering-out will prevent a cylinder from overrunning and consequently cavitating.
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