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What Are Hydraulic Flow Control Valves And How To Test
Hydraulic flow valve is simply referred to as flow control valve, and the flow control is achieved by changing the flow area of the orifice. This type of hydraulic flow valve is commonly used in hydraulic circuits and is used in particular to adjust the speed of the actuator.
Hydraulic Flow Control Valve Generally Be Classified:
1) Throttle Valve, the hydraulic flow area can only be changed according to external instructions such as manual operation, mechanical or electronic control, so the actual flow through the valve is affected by the pressure difference across the valve.
2) Two-way hydraulic flow control valve, referred to as two-way flow valve, its flow area can change with the pressure difference between the two connection side of the flow valve within a certain range, to keep the flow through constant, so it is also known as pressure compensation (pressure -compensated)
Throttle control, it is often referred to as a speed control valve, in fact, it is sometimes used only to regulate the flow and is not used for speed regulation.
3) Three-way hydraulic flow control valve referred to as three-way flow control valve, the flow chamber of the priority oil chamber and the bypass oil chamber be changed as per the different pressure of each port, to achieve flow regulation, which is not the same processing like the pressure relief valve for speed control.
Hydraulic Flow Valve Function
Because the throttle valve is mostly operated by a cone shape poppet with a very small angle, it is often referred to as a needle valve. It has a simple structure and low cost, and can be completely closed.However, with the improvement of technology, there are some needle valves that are not at all like needles: with a cylindrical spool, which has a regulation indication scale for better flow adjustment precisely.
Hydraulic Flow Valve Types
From a functional classified the hydraulic flow valve can be divided into the following two types:
1) Normal throttle valve. Bidirectional throttling can be realized.
2) One-way throttle valve, which can throttle flow when the oil flows in forward. When the oil flows in the reverse direction, the throttling port is fully opened without throttling.
From the adjustment mode of view, there are fixed and not adjustable, there are manual adjustment, there are also electric proportional adjustment of screw-in cartridge flow valve.
Hydraulic Flow Valve Application
The A throttle valve is used as an inlet throttle to control the speed at which the hydraulic cylinder overcomes resistance while the cylinder rod in and out. The use of two one-way throttle valves at port B serves as an outlet throttling to prevent the cylinders from moving too fast.
Hydraulic Flow Valve Functional Principle
The two-way hydraulic flow valve is actually formed by connecting two throttle ports in series. One does not being changed with the pressure of the inlet and outlet, is called fixed throttle port, also called flow sensor, referred to as throttle valve. The flow area of the other throttle orifice changes with the pressure of the inlet and outlet, that is, the constant pressure difference valve.
The pressures p2, p3 of the throttle valve are setting at both ends of the constant pressure differential spool. The poppet moves under these two pressures and the spring force and stops at the equilibrium position, so that between these two pressures, the difference, that is, the pressure difference across the throttle valve, maintains the constant-spring pressure. In this way, the flow through the valve can be kept relatively constant, independent of the pressure of the valve.
When the oil reverses ②->①, it is like a throttle valve. Because, at this time, due to p2<p3, the constant pressure differential valve poppet will move to and stay at the left end side under the action of the spring force, and the valve port will be fully opened and the flow regulation is not being adjusted at the moment.
Type Of Hydraulic Flow Valves
Constant pressure differential valve can be pre-positioned or post-positioned installed in hydraulic system. The threaded plug-in two-way flow valve is mostly set at a constant pressure differential.
In terms of driven characteristics, the front-mounted type is more suitable for inlet throttling flow control, and the rear type installation is more suitable for outlet throttling flow control. Because of this, the constant pressure differential valve can response to the changes in the load pressure earlier and react faster.
In addition to the adjustable orifice, there is also a throttle orifice that can not be adjustable, but a constant pressure differential spring pre-tightening force is adjustable
Because, when the oil is reversed flowing, the normal two-way hydraulic flow valve operates like a general throttle valve. Therefore, in order to reduce the flow resistance, there is also a valve with a reverse flow check valve. There is also an electric proportional type flow control valve, which can be adjusted by electric signals.
Flow Valve Steady-State Characteristics And Testing The steady-state characteristics of the two-way hydraulic flow valve can be mainly reflected by its differential pressure flow characteristics.
Differential Pressure Flow Characteristics Two-way flow valve differential pressure flow characteristics can be divided into three areas:
Area I: The pressure difference between the two ends of the valve, p0-p2 is lower than the spring presetting pressure, and the constant pressure difference is fully open. The entire valve is a throttle valve. Therefore, the two-way hydraulic flow valve has a minimum working pressure difference ΔP min, which is approximately in the range of 1.2 MPa to 3 MPa.
Below this pressure difference, the flow cannot be maintained without being affected by the load pressure.
Area Il: Work area. The pressure difference across the valve is higher than the minimum operating pressure differential. The constant pressure differential valve is partially closed, which can serve to consume pressure and maintain a constant differential pressure. The bigger of the pressure difference between the two ends of the valve is, the smaller the valve of the constant pressure difference valve is closed and the corresponding spring force is increased. As a result, the pressure difference between the two throttle valves becomes bigger, and the flow rate increases. On the other hand, the bigger of the flow rate, the bigger of the hydraulic force on the spool, which in turn reduces the pressure differential.
Therefore, the slope of the characteristic curve is determined by both the spring force and the hydraulic force. Generally, it rises when the flow rate is set low, and it drops when the flow rate is set high.
Area Ill: The pressure difference is so large that the constant pressure differential valve has moved to the stroke end and it is no longer possible to maintain a constant differential.
Pressure Differential Flow Test Of Hydraulic Flow Control Valve Loop Test Of Flow Valve:
Hydraulic source. The output flow must be bigger than the test range and should be smooth. If necessary, setting an accumulator at the pump outlet
Relief valve for loading. The adjustment range must be bigger than the test range
Pressure gauges for monitoring purposes.
Pressure sensor. The conditions for using the inlet pressure to replace the difference between the inlet and the outlet: the piping to the outlet tank is short and thick, so that the display value of 3b is negligibly small relative to 3a. Otherwise, a pressure sensor should be added at 3b.
X-Y recorder, or computer data acquisition and recording display system, to record differential pressure flow characteristics.
Testing process: 1) Preparation step.
Connect the recorder: flow qy7 as the Y axis and pressure p5 as the X axis. To make the oil temperature reach the predetermined value, usually choose No. 32 hydraulic oil, at 40 °C.
2) Test procedure
Open relief valve 2 to maximum to minimize pressure. Turn on the hydraulic power source.
The tested valve 6 is adjusted to a minimum value for the flow adjustment range.
Start recording. Slowly close the pressure relief valve 2 and increase the pressure p5 to reach the maximum test pressure. Then slowly open the pressure relief valve until the pressure p5 is at its lowest point. Pause the record.
Adjusting the valve 6 to meet the maximum flow adjustment range, repeat steps
Set a few more values between the maximum and minimum values of the pressure adjustment range and repeat step c.
Keep the oil temperature relatively constant throughout the measurement process. The test curve thus obtained is the characteristic of the valve under the working conditions.