About Hydraulic Solenoid Valves, How To Test

//What Are Hydraulic Solenoid Valves And How To Test

What Are Hydraulic Solenoid Valves And How To Test

Hydraulic solenoid valve can open and close one or more flow channels by energizing and de-energizing the solenoid, usually there are modular hydraulic solenoid valves and cartridge solenoid valves installed in the hydraulic system or equipment.

The 2/2-way hydraulic solenoid valve type has a much more connected form in the de-energized condition than the modular solenoid valve.

Hydraulic solenoid valve, from the point of view of the structure of the main valve spool, only two position, two-way and two position and three-way seat valve are spool with seat valve, the other are all spool valve.

The valve spool and spool seat are generally made of steel and hardened to achieve long life. However, there are also individual varieties that use softer seats in order to meet the strictly requirements for internal leakage in certain applications.

From the internal structure, the hydraulic solenoid valve can be divided into direct-acting and pilot differential. In general, only the two-way type and the three-way valve in the hydraulic solenoid valve have a pilot type that is electrohydraulic control, and the other types are direct-acting spool valves, that is, electric control. For simplicity and ease of comparison, the graphical symbols do not distinguish between electrically and electro-hydraulic valves.

The solenoid valve is generally composed of three parts: solenoid coil, solenoid piston assembly and solenoid valve parts assembly.

The solenoid coil converts the input current into a magnetic field. The armature sleeve assembly converts the magnetic force into a pulling force or thrust force in a magnetic field. The hydraulic solenoid valve parts assembly utilizes this force to overcome the spring force and the liquid force, to opens or closes the corresponding flow channel. The solenoid coils are fixed with nuts for easy replacement.

The steady state characteristics of the hydraulic solenoid valve are mainly examined from the differential pressure flow characteristics and working range.

The steady state characteristics of the hydraulic solenoid valve are mainly examined from the differential pressure flow characteristics and switching limit.

Differential Pressure Flow Characteristics And Test Of Hydraulic Solenoid Valve:
Differential pressure flow characteristics:

From the differential pressure flow characteristics of the hydraulic solenoid valve, it can be realized that there will be much pressure loss when a certain flow rate is passed.

Because the direct-acting solenoid valve is an on-off valve, in normal operation, there are only two states that are off and on. Unlike a continuous regulating valve, there is an intermediate state. Therefore, the differential pressure characteristic curve of a certain channel is usually a parabola.Differential-Pressure-Flow-Curve For Hydraulic-Solenoid-Valve
Pilot solenoid valves are different. Its main port is gradually opened at a relatively small flow rate. Therefore, this is not completely parabolic.

Many hydraulic solenoid valves have multiple different channels when de-energized or energized, and the flow resistance of these channels varies. Therefore, to fully express the pressure differential flow characteristics of hydraulic solenoid valve often requires multiple curves.

Testing Of Differential Pressure Flow Characteristics Of Hydraulic Solenoid Valve:

(1) In the test circuit diagram:

  1. A hydraulic power source. Its output flow rate should be adjustable. The maximum flow rate should exceed the estimated nominal flow rate. The minimum flow rate is not necessarily small, generally as long as the corresponding pressure difference is less than 0.1 MPa. Because, the differential pressure flow characteristics of solenoid valves at very small flow rates are generally not the focus of attention. Variable hydraulic pump can be used, in order to reduce the flow fluctuations, an accumulator can be added if necessary.
  1. A pressure relief valve. For safety protection only, the set value should not exceed the allowable pressure of the testing valve.
  2. A flow sensor. Generally, the maximum and minimum flow ratios are 10 or more.
  3. The testing valve
  4. A thermometer.
  5. A pressure sensor.
    6a. Measures the inlet pressure. 6b, 6c. Measure the pressure at ports A and B separately.
    If the pressure at the outlet T cannot be ignored, a pressure sensor should also be provided.
    Because of the measurement range of the differential pressure flow curve, 1 to 2MPa is sufficient. Therefore, the pressure sensor should Choose a small range for higher measurement accuracy.
  1. X-Y recorder, or digital oscilloscope, or computer-aided test system, used to record the steady state characteristics.

(2) Test process

1) Preparation stage

Connect the X-Y recorder: The output qv3 of the flow sensor 3 acts as the X axis.

Allow the oil temperature to reach the predetermined value, and use the VG32 hydraulic oil, and maintain the temperature at 40°C.

The flow of hydraulic power source 1 is minimized.

3) Test procedure

  1. The testing valve is switched to an open position. The difference in the output of the corresponding pressure sensor, for example p6a-p6b, or p6a-p6c, is the Y-axis of the X-Y recorder.
  2. Start recording.
  3. Slowly increase the flow of the hydraulic source until the pressure difference exceeds, say, 1 MPa
  4. Slowly reduce the flow of the hydraulic source to a minimum.
  5. Stop recording.

The recorded is pressure difference flow characteristic curve of the corresponding channel.

  1. According to the need, change the pressure sensor output, or change the valve connection, repeat the process b.

Hydraulic Solenoid Valve Switching Limit And Testing
Allowable pressure

The allowable pressure of common hydraulic solenoid valves in the market is mainly two levels: 21MPa (20.7MPa) and 35MPa (or 34.5MPa). But there are also 24MPa, 25MPa and 28 MPa, etc.Differential-Pressure-Flow-curve-Hydraulic-Solenoid-Valve

Hydraulic solenoid valves with different allowable pressures use different materials and characteristics for their components, since the manufacturing precision and manufacturing process are different, the price will naturally be different accordingly. Therefore, the high allowable pressure cannot be purchased in no target.

The permissible pressures at all outlets are generally the same except that individual T outlets are lower. However, whether or not it can work and switch reliably under this pressure depends on the operating range curve.

Switching Limit Curve

The spool switching limit of the hydraulic solenoid valve is that the solenoid valve can be reliably held in a certain working position within this range and reliably switched to another working position. If the actual operating parameters exceed this range, the switching speed may slow down, may not even switch at all, or may not be maintained in the normal working position.

The spool switching limit curves given on the general product samples are made under ideal laboratory conditions: clean mineral oil, oil temperature 40°C, viscosity 32mm, input voltage is 90% of the rated voltage. If the actual working conditions fluctuate greatly, they should be conservatively selected.

Factors That Affect The Switching Limit Of Solenoid Valve

The factors affecting the switching limit of the solenoid valve are different in the direct acting type, the pilot differential type, the spool valve and the poppet seat valve.

Direct-Acting Spool Valve: The factors affecting the spool switching range of the direct-acting spool solenoid valve is mainly the solenoid force of the coil, the spring force, the static pressure of the pressure medium to the valve spool, the hydraulic force and the friction force.
It is the electromagnetic force that causes the spool to switch to or remain in the energized position. The electromagnetic power of the hydraulic solenoid valve is generally between 14-30W, and the electromagnetic force is very limited, about 70-120N.Retaining the spool or returning to the de-energized position is the return spring force. The spring force must be sufficient to overcome the maximum value of the hydrodynamic force.

The side port oil fluid balances the static pressure of the valve spool. The liquid pressure on the slide valve spool at the end face port is balanced with each other through the hole in the valve spool, or it can only be connected to the T port.
Impeding the movement of the spool from one working position to another, or deviating the spool from its working position, is the spring force, the combined force of the static pressures of the chambers, and the hydraulic force that is approximately proportional to the flow rate and flow speed.

The hydraulic power reaches a maximum at small openings, that is, in a transitional state.
The valve spool and valve hole manufactured by Finotek have normal fit size and shape position deviation, and when immersed in clean hydraulic oil, the friction force is generally small relative to solenoid force and spring force and can be ignored.

Pilot Differential Valve: The pilot type and the pilot differential type valve is usually very small, the flow rate is very small as well, the fluid power is also very small. They are generally poppet valves with static pressure imbalance. As long as the solenoid force overcomes the spring force and static pressure, the pilot valve core can be removed.
The main factors affecting the working range of the main valve are: spring force, static pressure of the hydraulic oil on the valve spool, and hydraulic power.
The difference between the static pressures at both ends of the main spool overcomes the spring force and the hydraulic force, pushing the main spool and opening the related chamber. Since the difference in static pressure and the area of ​​action can be much larger than the electromagnetic force, the working flow rate of the pilot differential valve can be much larger than the direct-acting type.

Testing Of Hydraulic Solenoid Valve Switching Limit

For the determination of the switching limit of the solenoid valve: ISO 6403:1988 or refer to the GB/T 8106-1987 standard version

Test loops

  1. Hydraulic source. Its output flow is adjustable. Variable pumps can be used. In order to reduce the flow fluctuations, an accumulator can be added if necessary
  2. Relief valve. 2a as a safety valve, its set value should be the allowable pressure of the valve under test. 2b, 2c simulated load, the set value should be lower than the allowable pressure of the valve under test
  3. Flow sensor
  4. The Tested Valve
  5. Check valve
  6. Temperature sensor.
  7. Pressure sensor. The inlet pressure is measured at 7a, and the pressures at ports A and B are measured at 7b and 7c, respectively.

Test Procedure

The solenoid coils are pre-powered until the balance is reached. Input voltage: 90% of rated voltage.
The spool of directional valve can move in at least 6 full strokes in both directions.
If the hydraulic directional valve cannot be normally switched, reduce the pressure or flow. On the coordinate paper, with the horizontal axis of the flow and the vertical axis of the pressure, the normal working points are marked.
Finally, connecting the boundary points results in the valve’s working range.


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