What are Hydraulic Control Systems?

The role of the hydraulic system is to increase the force by changing the pressure. A complete hydraulic system consists of five parts, namely power components, actuators, control components, auxiliary components (accessories) and hydraulic oil. Hydraulic systems can be divided into two categories: hydraulic transmission systems and hydraulic control systems. The hydraulic transmission system mainly transfers power and motion. The hydraulic control system must make the output of the hydraulic system meet specific performance requirements (especially dynamic performance) ^[1] . Generally speaking, the hydraulic system mainly refers to the hydraulic transmission system.

Chinese name: Hydraulic system
Foreign name: Hydraulic systems

Principle: Change pressure to increase force
Classification: Hydraulic transmission system and hydraulic control system

Components of a hydraulic system

Scraper hydraulic system A complete hydraulic system consists of five parts, namely power components, actuators, control components, auxiliary components (accessories) and hydraulic oil.

Hydraulic system power components

The role of the power element is to convert the mechanical energy of the prime mover into the pressure energy of the liquid. It refers to the oil pump in the hydraulic system, which provides power to the entire hydraulic system. The structure of the hydraulic pump is generally gear pump, vane pump, plunger pump and screw pump.

Hydraulic system actuator

The function of the actuator (such as hydraulic cylinder and hydraulic motor) is to convert the pressure energy of the liquid into mechanical energy, and drive the load to perform linear reciprocating or rotary motion.

Hydraulic system control elements

Control elements (ie, various hydraulic valves) control and regulate the pressure, flow, and direction of the liquid in the hydraulic system. According to different control functions, hydraulic valves can be divided into pressure control valves, flow control valves and directional control valves. Pressure control valves include relief valves (relief valves), pressure reducing valves, sequence valves, pressure relays, etc .; flow control valves include throttle valves, regulating valves, diverter manifolds, etc .; directional control valves include check valves, hydraulic controls Check valve, shuttle valve, directional valve, etc. According to different control methods, hydraulic valves can be divided into on-off control valves, fixed value control valves and proportional control valves.

Hydraulic system auxiliary components

Auxiliary components include oil tank, oil filter, cooler, heater, accumulator, oil pipe and pipe joint, seal ring, quick-change joint, high pressure ball valve, hose assembly, pressure gauge joint, pressure gauge, oil level gauge, oil Thermometer and so on.

Hydraulic system hydraulic oil

Hydraulic oil is the working medium that transfers energy in the hydraulic system. There are various types of mineral oil, emulsion and synthetic hydraulic oil.

Hydraulic system system structure

The hydraulic system consists of two parts: signal control and hydraulic power. The signal control part is used to drive the control valve in the hydraulic power part.

The hydraulic power part is represented by a circuit diagram to show the interrelationship between different functional components. The hydraulic source contains a hydraulic pump, an electric motor and hydraulic auxiliary components; the hydraulic control section contains various control valves for controlling the flow, pressure and direction of the working oil; the executive section contains a hydraulic cylinder or a hydraulic motor, which can be selected according to actual requirements .

When analyzing and designing actual tasks, a block diagram is generally used to display the actual operating conditions in the equipment. Open arrows indicate signal flow, while solid arrows indicate energy flow.

Hydraulic system of crushing bed Sequence of actions in the basic hydraulic circuitreversing and spring return of control elements (two-position four-way reversing valve), extension and retraction of actuators (double acting hydraulic cylinders), and opening and closing of relief valves For actuators and control elements, the presentation is based on the corresponding circuit diagram symbols, which also prepares the introduction of the circuit diagram symbols.

Depending on how the system works, you can number all circuits in sequence. If the first actuator number is 0, the control element identifier associated with it is 1. If the component identifier corresponding to the actuator extension is an even number, the component identifier corresponding to the actuator retraction is an odd number. Not only the hydraulic circuits should be numbered, but also the actual equipment should be numbered in order to find system failures.

The DIN ISO1219-2 standard defines the numbering composition of components, which includes the following four parts: device number, circuit number, component identifier, and component number. If the entire system has only one device, the device number can be omitted.

In practice, another numbering method is to serially number all components in the hydraulic system. At this time, the component number should be consistent with the number in the component list. This method is particularly suitable for complex hydraulic control systems, where each control loop corresponds to its system number.

Advantages and disadvantages of hydraulic system

Hydraulic system advantages

1. Small size and light weight;

2. High rigidity, high precision and fast response;

3. Large driving force, suitable for heavy-duty direct drive;

4. Wide speed range and various speed control modes;

5. Self-lubricating, self-cooling and long life;

6, easy to achieve security protection. ^[1]

Disadvantages of hydraulic system

1. Poor ability to resist working fluid pollution;

2. Sensitive to temperature changes;

3. There are hidden dangers of leakage;

4. Difficult to manufacture and high cost;

5. Not suitable for long-distance transmission and requires hydraulic energy. ^[1]

Common faults of hydraulic system

Hydraulic system pressure loss

Because the liquid is viscous, friction is unavoidable when flowing in the pipeline, so the liquid must lose some energy during the flow. This part of the energy loss is mainly manifested as pressure loss.

There are two types of pressure loss: on- line loss and local loss . The loss along the path is the pressure loss due to friction when the liquid flows over a distance in a straight tube with a constant diameter. Local losses are pressure losses caused by sudden changes in the cross-sectional shape of the pipeline, changes in the direction of the flow, or other forms of flow resistance. The total pressure loss is equal to the sum of the on-line loss and the local loss. Due to the inevitable existence of pressure loss, the rated pressure of the pump should be slightly greater than the maximum working pressure required when the system is working. Generally, the maximum working pressure required for system work can be multiplied by a coefficient of 1.3 ~ 1.5 to estimate. ^[2]

Flow loss in hydraulic system

In the hydraulic system, each pressed component has a surface that moves relatively, such as the inner surface of a hydraulic cylinder and the outer surface of a piston. Because of the relative movement, there is a certain gap between them. If one side of the gap is high-pressure oil and the other side is low-pressure oil, the high-pressure oil will flow through the gap to the low-pressure area and cause leakage. At the same time, due to the imperfect seal of the hydraulic components, part of the oil will leak to the outside. The actual flow caused by this leak is reduced, which is what we call flow loss.

Flow loss affects the speed of movement, and leakage is difficult to avoid absolutely, so the rated flow of the pump in the hydraulic system is slightly greater than the maximum flow required when the system is working. It can also be estimated by multiplying the maximum flow required by the system by a coefficient of 1.1 ~ 1.3. ^[2]

Hydraulic system hydraulic shock

Reason: The reversing of the actuator and the closing of the valve cause the flowing liquid to produce instantaneous pressure peaks due to inertia and some hydraulic components are not sensitive enough to react, which is called hydraulic shock. Its peak can exceed several times the working pressure.

Hazard: Causes vibration and noise; causes pressure components such as relays and sequence valves to malfunction, and even damages certain components, sealing devices and pipelines.

Action: Find out the cause of the impact to avoid the rapid change of the flow velocity. Delay the time of speed change, estimate the pressure peak, and take corresponding measures. If the flow reversing valve and the electromagnetic reversing valve are used in combination, hydraulic shock can be effectively prevented. ^[2]

Cavitation in hydraulic systems

Phenomenon: If air is infiltrated into the hydraulic system, when the air bubbles in the liquid move to the area with higher pressure with the liquid flow, the air bubbles will burst quickly under the action of higher pressure, which will cause local hydraulic shock, causing noise and vibration. In addition, the air bubbles destroy the continuity of the liquid flow, reduce the oil flow capacity of the oil pipe, cause fluctuations in flow and pressure, and cause the hydraulic components to withstand shock loads, affecting their service life.

Reason: Hydraulic oil always contains a certain amount of air, which can usually be dissolved in oil or mixed with oil in the form of bubbles. When the pressure is lower than the air separation pressure, the air dissolved in the oil is separated out to form bubbles; when the pressure drops below the saturated vapor pressure of the oil, the oil will boil and a large number of bubbles will be generated. These bubbles are mixed in the oil to form a discontinuous state. This phenomenon is called cavitation.

Locations: Suction holes are easily generated in the suction port and the suction pipe at lower than atmospheric pressure. When the oil flows through narrow gaps such as the throttle, the pressure will drop due to the increase in speed, and cavitation will also occur.

Hazard: The bubble moves with the oil to the high-pressure area, and quickly bursts under the action of high pressure, causing the volume to suddenly decrease and the surrounding high-pressure oil to flow at high speeds to replenish, causing local instantaneous impact, the pressure and temperature rising sharply, and strong noise and vibration .

Measures: It is necessary to correctly design the structural parameters of the hydraulic pump and the suction pipe of the pump, try to avoid narrow and sharp bends of the oil passage, and prevent the occurrence of low-pressure areas. Reasonably select mechanical materials, increase mechanical strength, improve surface quality, and improve corrosion resistance. ^[2]

Cavitation in hydraulic system

Reason: Cavitation occurs with cavitation, and the oxygen in the bubbles generated in the cavity will also corrode the surface of metal components. We call this corrosion caused by cavitation as cavitation.

Location: Cavitation may occur in oil pumps, pipelines, and other places with throttling devices, especially oil pump devices. This phenomenon is most common. Cavitation is one of the causes of various failures in hydraulic systems, especially in high-speed and high-pressure hydraulic equipment.

Hazards and measures are the same as cavitation. ^[2]

Fault diagnosis of hydraulic system

Because of its unique advantages, the hydraulic transmission system has a wide range of process adaptability, excellent control performance, and lower cost, it has been increasingly used in various fields. However, objectively, the quality of components and accessories is unstable, and subjective use and maintenance are improper, and all components and working fluids in the system work in closed oil circuits, which is not as intuitive as mechanical equipment and not as electrical equipment as Use various testing instruments to conveniently measure various parameters. In hydraulic equipment, only a limited number of pressure gauges and flow meters are used to indicate the working parameters of some parts of the system. Other parameters are difficult to measure, and there are many possibilities for the root cause of general failures. This brings certain difficulties to the fault diagnosis of the hydraulic system. ^[3]

At the production site, due to the constraints of the production plan and technical conditions, fault diagnosis personnel are required to accurately, simply and efficiently diagnose the failure of hydraulic equipment; maintenance personnel are required to use existing information and site technical conditions to minimize disassembly and assembly Work load, saving maintenance man-hours and costs, using the simplest technical means, in the shortest possible time, to accurately find the fault location and cause of the failure and repair it to restore the system to normal operation, and strive to no longer The same failure occurred.

General principles of hydraulic system fault diagnosis

Correct analysis of failures is the prerequisite for troubleshooting. Most system failures do not occur suddenly. There are always warning signs before they occur. When the warning signs develop to a certain degree, a fault occurs. The causes of failure are various and there are no fixed rules to find. Statistics show that about 90% of the failures in hydraulic systems are caused by poor management. In order to diagnose faults quickly, accurately, and conveniently, it is necessary to fully understand the characteristics and rules of hydraulic faults, which is the basis of fault diagnosis.

The following principles are worth following in troubleshooting:

(1) First, determine whether the working conditions and peripheral environment of the hydraulic system are normal. First, it is necessary to find out whether the mechanical part of the equipment or the electrical control part is faulty, or whether the hydraulic system itself is faulty. Claim.

(2) Area judgment Determine the area related to the failure based on the failure phenomenon and characteristics, gradually narrow down the scope of the failure, detect the components in this area, analyze the cause of the failure, and finally find out the specific location of the failure.

(3) Grasp the types of failures for comprehensive analysis According to the final phenomenon of the failure, gradually find out a variety of direct or indirect possible causes. In order to avoid blindness, comprehensive analysis and logical judgment must be performed in accordance with the basic principles of the system to reduce the suspected Gradually approach, and finally find the fault.

(4) When verifying the possible cause of the failure, generally start from the most likely cause of the failure or the easiest place to inspect. This can reduce the workload of assembly and disassembly and increase the speed of diagnosis.

(5) Fault diagnosis is based on operating records and certain system parameters. Establish a system operation record, which is the scientific basis for preventing, discovering and handling failures. Establish a device operation failure analysis table, which is a high-level summary of experience and helps to quickly judge the failure phenomenon. It has certain detection methods and can Make accurate quantitative analysis of failures.

Fault diagnosis method of hydraulic system

1. The traditional method of routinely finding faults in hydraulic systems is to gradually approach the fault by logical analysis.

The basic idea is comprehensive analysis and condition judgment. That is, the maintenance personnel judge the cause of the failure based on experience through observation, listening, touch and simple tests and understanding of the hydraulic system. When the hydraulic system fails, there are many possible causes for the failure. The logical algebra method is used to list the possible failure causes, and then make logical judgments one by one based on the principle of easy-to-fail, and then approach each item one by one, and finally find out the cause of the failure and the specific conditions that caused the failure.

In the process of fault diagnosis, maintenance personnel are required to have basic knowledge of hydraulic system and strong analysis ability to ensure the efficiency and accuracy of diagnosis. However, the diagnosis process is cumbersome and requires a large number of inspections and verifications, and it can only be analyzed qualitatively. The cause of the diagnosis is not accurate enough. In order to reduce the blindness and experience of system fault detection and the workload of disassembly, the traditional fault diagnosis methods are far from meeting the requirements of modern hydraulic systems. With the development of hydraulic systems toward large-scale, continuous production, and automatic control, a variety of modern fault diagnosis methods have emerged. Such as the iron spectrum technique, the number, shape, size, composition and distribution of various abrasive particles that can be separated from the oil can be used to timely and accurately determine the wear position, form, and degree of components in the system. In addition, quantitative pollution analysis and evaluation of hydraulic oil can be performed to achieve online detection and fault prevention.

Expert diagnosis based on artificial intelligence, which uses computer to imitate the methods used by experienced experts in a certain field to solve problems. The failure phenomenon is input into the computer through the man-machine interface. The computer can infer the cause of the failure based on the input phenomenon and the knowledge in the knowledge base, and then output the cause through the man-machine interface, and propose a maintenance plan or preventive measures. These methods bring broad prospects to hydraulic system fault diagnosis and lay the foundation for hydraulic system fault diagnosis automation. However, most of these methods require expensive detection equipment and complex sensor control systems and computer processing systems. Some methods are difficult to study and are generally not suitable for field promotion. A simple and practical fault diagnosis method for hydraulic system is introduced below.

2. Fault diagnosis system based on parameter measurement

Whether a hydraulic system works normally depends on whether two main working parameters, namely pressure and flow, are in a normal working state, and whether the parameters such as system temperature and actuator speed are normal. The failure phenomenon of the hydraulic system is various, and the cause of the failure is also a combination of various factors. The same factor may cause different failure phenomena, and the same failure may correspond to many different causes. For example, oil pollution may cause failures in various aspects such as pressure, flow, or direction of the hydraulic system, which makes it extremely difficult to diagnose the failure of the hydraulic system.

The idea of the parameter measurement method for diagnosing faults is this. When any hydraulic system works normally, the system parameters work near the design and set values. If these parameters deviate from the predetermined values during work, the system will fail or may appear. malfunction. That is, the essence of the failure of the hydraulic system is the abnormal change of the operating parameters of the system. Therefore, when a failure occurs in the hydraulic system, it is inevitable that a certain component or some components in the system are faulty, and it can be further concluded that the parameters of a certain point or points in the circuit have deviated from the predetermined value. This means that if the working parameters of a certain point in the hydraulic circuit are abnormal, the system has malfunctioned or may have malfunctioned and needs to be handled by maintenance personnel immediately. In this way, based on parameter measurement, combined with logic analysis, you can quickly and accurately find the fault. The parameter measurement method can not only diagnose system failures, but also predict possible failures, and the prediction and diagnosis are quantitative, which greatly improves the speed and accuracy of diagnosis. This detection is direct measurement, with fast detection speed, small error, simple detection equipment, and convenient for popularization and use at the production site. Suitable for testing of any hydraulic system. During the measurement, there is no need to stop or damage the hydraulic system. It can detect almost any part of the system. Not only can it diagnose the existing fault, but it can also monitor online and predict the potential fault. ^[4]

Parameter measurement principle

As long as the working parameters at any desired point in the hydraulic system circuit are measured and compared with the normal values of the system work, it can be determined whether the system working parameters are normal, whether a fault has occurred and where the fault is located.

Working parameters in the hydraulic system, such as pressure, flow, and temperature, are all non-electrical physical quantities. When measuring with a general-purpose instrument using indirect measurement, you must first use physical effects to convert these non-electrical quantities into electricity, and then zoom in, convert, and display Processing, the measured parameters can be represented and displayed by the converted electrical signal. This can determine whether the hydraulic system is faulty. However, this indirect measurement method requires various sensors, the detection device is relatively complicated, and the measurement results have large errors and are not intuitive, which is not convenient for field promotion.

Parameter measurement method

Step 1: Measure the pressure. First, connect the hose connector of the test circuit and the three-way ball joint of the double ball valve. Open the ball valve 2, close the relief valve 3, and cut off the oil return channel. At this time, the pressure value of the measured point can be directly read from the pressure gauge (the actual working pressure of the system).

Step 2: Measure the flow and temperature-slowly release the handle of the relief valve 7, and then close the ball valve 1. Re-adjust the overflow valve 7 so that the pressure gauge 4 reads the measured pressure value, and the flow meter 5 reads the actual flow value at the measured point. At the same time, the thermometer 6 can display the oil temperature value.

Step 3: Measure the speed (speed)-whether the speed or speed of the pump, motor or cylinder depends on only two factors, namely the flow rate and its own geometric size (displacement or area), so just measure the motor or cylinder The output flow rate (input flow rate to the pump) is divided by its displacement or area to get the speed or speed value.

Examples of parameter measurement methods

The following phenomena occurred during the debugging of the system: the pump can work, but the pressure of the high-pressure pump supplied to the mold clamping cylinder and the injection cylinder does not go up (the pressure is adjusted to about 8.0Mpa, and it can no longer be increased), and the pump has a slight abnormal mechanical noise. The water cooling system works, the oil temperature and oil level are normal, and there is oil return.

There are the following possible reasons for analyzing the failure from the loop:

(1) The relief valve is malfunctioning. Possible reasons: incorrect adjustment, spring yielding, blocked damping hole, and spool valve stuck.

(2) The electro-hydraulic directional valve or the electro-hydraulic proportional valve is faulty. Possible reasons: the return spring is broken, the control pressure is not enough, the spool valve is stuck, and the proportional valve control part is faulty.

(3) The hydraulic pump is faulty. Possible reasons: the pump speed is too low, the vane pump stator is abnormally worn, the seal is damaged, a large amount of air enters the pump suction port, and the filter is severely blocked.

3.Summary

Parameter measurement method is a practical and new-type fault diagnosis method of hydraulic system. It combines with logic analysis method, which greatly improves the speed and accuracy of fault diagnosis. First of all, this measurement is quantitative, which avoids the blindness and experience of personal diagnosis, and the diagnosis results are in line with reality. Secondly, the fault diagnosis is fast, and the accurate parameters of the system can be measured after a few seconds to tens of seconds, and then the diagnosis results are obtained by simple analysis and judgment by the maintenance staff. Furthermore, this method reduces the system assembly and disassembly workload by more than half compared with the traditional fault diagnosis method.

This fault detection detection circuit has the following functions:

(1) It can directly measure and directly display the liquid flow, pressure and temperature, and can indirectly measure the speed of pumps and motors.

(2) The overflow valve can be used to simulate the loading of the measured part of the system, the pressure adjustment is convenient and accurate; in order to ensure the accuracy of the measured flow rate, the test temperature difference can be directly observed from the thermometer (it should be less than ± 3 ° C).

(3) It is suitable for any hydraulic system, and certain system parameters can realize non-stop detection.

(4) Light and simple structure, reliable work, low cost and easy operation.

This detection circuit combines a loading device with a simple detection instrument and can be made into a portable detection instrument. The measurement is fast, convenient, and accurate, and it is suitable for field promotion. It lays the foundation for the automation of detection, forecasting and fault diagnosis.

Hydraulic system conclusion

1. Apply the traditional logic analysis step-by-step approach. All the above possible causes need to be analyzed, judged and tested one by one, and finally the cause of the failure and the specific component causing the failure are found out. The diagnostic process of this method is cumbersome and requires a large amount of assembly, disassembly, and verification work. The efficiency is low, the construction period is long, and only qualitative analysis is available. The diagnosis is not accurate enough.

2. Apply a fault diagnosis system based on parameter measurement. Simply set the double ball valve tee at the three points of the pump outlet a, the front of the reversing valve b and the inlet c of the cylinder when piping the system, and use the fault diagnosis detection circuit to limit the system fault to a few seconds. The fault is diagnosed in an area based on the measured parameter values. The detection process is as follows:

(1) Connect the fault diagnosis circuit to the detection port a, open the ball valve 2 and unscrew the relief valve 7, and then close the ball valve 1. At this time, adjust the relief valve 7 to observe the working pressure of the pump from the pressure gauge 4. Change the situation to see if it can exceed 8.0Mpa and rise to the required high pressure value. If not, it means that the pump itself is faulty. If it can indicate that it is not a pump fault, it should continue testing.

(2) If the pump has no fault, use the fault diagnosis circuit to detect the pressure change at point b. If the working pressure at point b can exceed 8.0Mpa and rise to the required high pressure value, it means that the system's main relief valve is working normally and testing needs to continue.

If there is no failure of the relief valve, it can be determined whether the directional valve or the proportional valve is faulty by detecting the pressure change at point c.

The cause of the final failure was detected by the serious internal leakage of the vane pump. After disassembling the pump, it is known that the vane pump stator is abnormally worn due to poor slippage, causing internal leakage to increase, which makes the system pressure not high. It was further found that the water cooling system caused oil to emulsify due to water leakage into the oil and lost lubrication.

Maintenance of hydraulic system

The quality of a hydraulic system depends not only on the rationality of the system design and the performance of the system components, but also because of the pollution protection and treatment of the system. The pollution of the system directly affects the reliability of the hydraulic system and the service life of the components. Statistics show that about 70% of hydraulic system failures at home and abroad are caused by pollution.

Hydraulic system oil pollution

1. The harm of oil pollution to the system is as follows:

1) Contaminated wear of components

Various pollutants in the oil cause various forms of wear of the components. Solid particles enter the gap of the moving pair, which causes cutting wear or fatigue wear on the surface of the part. The impact of solid particles in the high-speed liquid flow on the surface of the component causes erosion wear. Water in oil and oxidatively deteriorated products have a corrosive effect on components. In addition, the air in the oil of the system causes cavitation, which causes the surface of the component to peel and damage.

2) Component clogging and clamping failure

Solid particles plug the gaps and orifices of hydraulic valves, causing valve plugs to become blocked and jammed, affecting work performance and even causing serious accidents.

3) Accelerate the deterioration of oil performance

Water and air in the oil are the main conditions for the oxidation of the oil with its thermal energy, and the metal particles in the oil play an important catalytic role in the oxidation of the oil. In addition, the water and suspended bubbles in the oil significantly reduce the movement The strength of the intermediate oil film reduces the lubricating performance.

2.Types of pollutants

Pollutants are substances in the hydraulic system oil that are harmful to the system. They exist in different forms in the oil. According to their physical forms, they can be divided into solid pollutants, liquid pollutants, and gaseous pollutants.

Solid pollutants can be divided into hard pollutants, including: diamond, cutting, silica sand, dust, abrasive metals and metal oxides; soft pollutants are: additives, water condensates, oil decomposition products and polymers, and maintenance Cotton silk and fibers brought in at the time.

Liquid pollutants are usually grooved oil, water, coatings, chlorine and their halides that do not meet the system requirements. It is usually difficult to remove them. Therefore, when selecting hydraulic oil, choose a hydraulic oil that meets the system standards to avoid unnecessary failure.

Gaseous pollutants are mainly air mixed into the system.

These particles are often so small that they cannot settle down and become suspended in the oil, and are finally squeezed into the gaps of various valves. For a reliable hydraulic system, these gaps have limited control, Importance and accuracy are extremely important.

3. Source of pollutants:

The sources of pollutants in the system oil mainly include the following aspects:

1) Externally invaded pollutants: Externally invaded pollutants are mainly gravel or dust in the atmosphere, which usually enter the system through the gas holes in the fuel tank, the shaft seal of the oil cylinder, pumps and motors. Mainly the impact of the use environment.

2) Internal pollutants: Residual pollutants in components during processing, assembly, commissioning, packaging, storage, transportation, and installation. Of course, these processes cannot be avoided, but can be minimized. Some special components are assembled and Commissioning needs to be performed in a clean room or clean bench environment. 3) Pollutants produced by hydraulic system: particles generated by the wear of components during the operation of the system, sand particles falling off from castings, metal particles falling off from pumps, valves and joints, rust and spalling in pipelines with their oil The particles and colloids produced by liquid oxidation and decomposition are more serious. The system pipes have a large amount of impurities that have not been washed before being put into operation. ^[5]

Hydraulic system system maintenance

A system is generally flushed before it is officially put into operation. The purpose of flushing is to remove the pollutants, metal filings, fiber compounds, iron cores, etc. remaining in the system. During the first two hours of work, even if the system is not completely damaged, it will Cause a series of failures. Therefore, the following steps should be used to clean the system oil circuit:

1) Clean the fuel tank with an easy-to-dry cleaning solvent, and then use filtered air to remove solvent residues.

2) Clean all the pipes of the system. In some cases, the pipes and joints need to be impregnated.

3) Install oil filter in the pipeline to protect the valve's oil supply pipeline and pressure pipeline.

4) Install a flushing plate on the current collector to replace precision valves, such as electro-hydraulic servo valves.

5) Check that all pipelines are of proper size and connected properly.

If an electro-hydraulic servo valve is used in the system, I might as well say a few more words. The servo valve flushing plate must allow the oil to flow from the oil supply line to the collector and return directly to the fuel tank, so that the oil can flow repeatedly. Use the flushing system to allow the oil filter to filter out solid particles. During the flushing process, check the oil filter every 1 to 2 hours to prevent the oil filter from being blocked by pollutants. At this time, do not open the bypass. If the oil filter is found to be blocked Change the oil filter immediately.

The flushing cycle is determined by the structure of the system and the degree of system pollution. If the sample of the filter medium contains no or little foreign contaminants, install a new oil filter, remove the flushing plate, and install the valve to work!

Planned maintenance: Establish a regular system maintenance system, and better maintenance recommendations for the hydraulic system are as follows:

1) Check and change the fluid at most 500 hours or three months.

2) Regularly flush the inlet oil filter of the oil pump.

3) Check whether the hydraulic oil is contaminated by acidification or other pollutants, and the smell of the hydraulic oil can roughly identify whether it has deteriorated.

4) Repair leaks in the system.

5) Ensure that no foreign particles enter the fuel tank from the vent cap of the fuel tank, the plug seat of the oil filter, the gasket of the oil return pipe, and other openings in the fuel tank. ^[6]

Hydraulic system frequently asked questions

First, the cause of hydraulic system leakage

(1) Caused by design and manufacturing defects;

(2) Loose pipe joints due to shock and vibration;

(3) The dynamic seals and mating parts wear each other (especially for hydraulic cylinders);

(4) The oil temperature is too high and the rubber seal is incompatible with the hydraulic oil and deteriorates. Let's talk about the measures to control leakage in combination with the above aspects.

Control scheme for controlling leakage of hydraulic system

Solution 1 : Design and manufacturing defect solution

1. The choice of supporting components outside the hydraulic components often plays a decisive role in the leakage of the hydraulic system. This determines that our technicians should choose cylinders, pumps, valves, seals, hydraulic accessories, etc. in the design of new products and the improvement of old products. There are comparisons made.

2. Reasonably design the mounting surface and sealing surface: When the valve group or pipeline is fixed on the mounting surface, in order to obtain a satisfactory initial seal and prevent the seal from being squeezed out of the groove and being worn, the mounting surface must be straight and the sealing surface Requires finishing, the surface roughness must reach 0.8 m, and the flatness must reach 0.01 / 100mm. The surface must not have radial scratches, and the pre-tightening force of the connecting screw must be large enough to prevent surface separation.

3. During manufacturing and transportation, prevent critical surfaces from bumping and scratching. At the same time, the assembly and debugging process must be strictly monitored to ensure the quality of the assembly.

4. Don't beware of the leakage hazards of some hydraulic systems, you must eliminate them.

Option 2 : Reduce shock and vibration

In order to reduce the leakage of the hydraulic system caused by the loose pipe joints that are subject to shock and vibration, the following measures can be taken:

Fix all pipes with shock-absorbing brackets to absorb shock and vibration;

Use low-impact valve or accumulator to reduce impact;

Properly arrange pressure control valves to protect all components of the system;

Minimize the number of pipe joints used, and connect pipe joints with welding as much as possible;

Use straight thread joints, tee joints and elbows instead of taper pipe thread joints;

Try to use oil return block instead of each piping;

According to the highest pressure used, the torque of bolts and plug torque are required during installation to prevent the joint surface and seals from being eroded;

Install the fittings correctly.

Option 3 : Reduce the wear of the moving seal

Most of the dynamic seals are precisely designed. If the dynamic seals are qualified, installed correctly, and used reasonably, they can guarantee relatively long-term work without leakage. From a design perspective, designers can use the following measures to extend the life of the moving seal:

1. Eliminate side loads on piston rod and drive shaft seals;

2.Protect the piston rod with a dust ring, protective cover and rubber sleeve to prevent impurities such as abrasives and dust from entering;

3. Design and select a suitable filtering device and an easy-to-clean fuel tank to prevent dust from accumulating in the oil;

4. Make the speed of the piston rod and shaft as low as possible.

Option four : requirements for static seals

Static seals prevent leakage of fluid between rigid mounting surfaces. Reasonably design the dimensions and tolerances of the sealing grooves, so that the seals after installation will deform to a certain extent in order to fill the microscopic depressions of the mating surface, and increase the internal stress of the seals to be higher than the pressure to be sealed. When the stiffness of the part or the pre-tightening force of the bolt is not large enough, the mating surface will be separated by the oil pressure, causing a gap or increasing the gap that may exist from the beginning because the sealing surface is not flat enough. As the mating surface moves, the static seal becomes a dynamic seal. Rough mating surfaces will wear the seals, and fluctuating gaps will eat away at the edges of the seals.

Option 5 : Control oil temperature to prevent seal deterioration

Premature seal deterioration can be caused by a number of factors, an important factor being excessive oil temperature. Every time the temperature rises by 10 ° C, the life of the seal will be halved, so a high-efficiency hydraulic system or a forced cooling device should be reasonably designed to keep the optimal oil temperature below 65 ° C; construction machinery must not exceed 80 ° C; another factor It may be the compatibility between the oil used and the sealing material. The type and material of the hydraulic oil and seal should be selected according to the instruction manual or related manuals to solve the compatibility problem and extend the service life of the seal.

Precautions for hydraulic systems

Anyone with a little mechanical knowledge knows that energy can be converted to each other, and it is best to apply this knowledge to the hydraulic system to explain the power loss of the hydraulic system. On the one hand, the power of the hydraulic system will cause energy loss and make the system On the other hand, this part of the energy lost will be converted into thermal energy, which will cause the temperature of the hydraulic oil to rise and the oil to deteriorate, resulting in failure of the hydraulic equipment. Therefore, when designing the hydraulic system, on the premise of meeting the requirements for use, full consideration should also be given to reducing the power loss of the system.

First, from the perspective of the power source-the pump, and considering the diversified working conditions of the actuator, sometimes the system needs a large flow and low pressure; sometimes it needs a small flow and high pressure. Therefore, it is advisable to choose a pressure-limiting variable pump, because the flow rate of this type of pump varies with the change of system pressure. When the system pressure decreases, the flow is relatively large, which can meet the fast stroke of the actuator. When the system pressure is increased, the flow rate is reduced accordingly, which can meet the working stroke of the actuator. This can not only meet the working requirements of the actuator, but also make the power consumption more reasonable.

Second, pressure loss and flow loss are inevitable when hydraulic oil flows through various types of hydraulic valves. This part of the energy loss accounts for a large proportion of the total energy loss. Therefore, a reasonable choice of hydraulic equipment, adjusting the pressure of the pressure valve is also an important aspect of reducing power loss. The flow valve is selected according to the flow adjustment range in the system and its minimum stable flow can meet the requirements of use. The pressure of the pressure valve should be as low as possible under the condition that the hydraulic equipment works normally. ^[7]

Third, if the actuator has speed regulation requirements, then when selecting a speed regulation circuit, it is necessary to both meet the speed regulation requirements and minimize power loss. Common speed-regulating circuits are mainly: throttling speed-regulating circuit, volume speed-regulating circuit, volume throttling speed-regulating circuit. Among them, the power loss of the throttling speed regulation loop is large, and the low-speed stability is good. The volume speed regulation circuit has neither overflow loss nor throttling loss. It has high efficiency but poor low-speed stability. If you want to meet both requirements at the same time, you can use a volumetric throttle speed regulation circuit composed of a differential pressure variable pump and a throttle valve, and make the pressure difference across the throttle valve as small as possible to reduce pressure loss.

Fourth, choose hydraulic oil reasonably. When the hydraulic oil flows in the pipeline, it will show viscosity, and when the viscosity is too high, it will generate a large internal friction, which will cause the oil to generate heat and increase the resistance when the oil flows. When the viscosity is too low, it is easy to cause leakage and reduce the volumetric efficiency of the system. Therefore, generally, an oil with a suitable viscosity and a good viscosity-temperature characteristic is selected. In addition, when the oil flows in the pipeline, there are also pressure losses along the route and local pressure losses. Therefore, when designing the pipeline, shorten the pipeline as much as possible and reduce the number of bends.

The above are some of the work proposed to avoid the power loss of the hydraulic system, but there are still many factors that affect the power loss of the hydraulic system, so if you specifically design a hydraulic system, you need to consider other aspects.

Development history of hydraulic system

In 1795, Joseph Braman (1749-1814) of the United Kingdom, using water as a working medium in London, applied it to industry in the form of a hydraulic press, and produced the world's first hydraulic press. In 1905, the working medium was changed from oil to oil, and it was further improved.

After the First World War (1914-1918), hydraulic transmission was widely used, especially after 1920, it developed more rapidly. Hydraulic components began to enter the formal industrial production stage about 20 years from the end of the 19th century to the beginning of the 20th century. In 1925, F. Vikers invented the pressure-balanced vane pump, which laid the foundation for the gradual establishment of modern hydraulic component industry or hydraulic transmission. At the beginning of the 20th century, Constantimsco's theoretical and practical research on the transmission of energy waves; his contributions to hydraulic transmission (hydraulic couplings, torque converters, etc.) in 1910, The two areas have been developed.

During World War II (1941-1945), 30% of American machine tools used hydraulic transmissions. It should be noted that the development of hydraulic transmission in Japan is nearly 20 years later than that in Europe and the United States. Around 1955, Japan rapidly developed hydraulic transmissions, and in 1956 the "Hydraulic Industry Association" was established. In the past 20 to 30 years, Japan's hydraulic transmission has developed rapidly and has taken the lead in the world.

At present, China's hydraulic technology lacks technical communication. Most of the hydraulic products are processed using foreign hydraulic technology. The hydraulic talent network reminds everyone to develop domestic hydraulic technology and revitalize domestic hydraulic system technology.

In fact, otherwise, domestic hydraulic technology has greatly improved in recent years. Companies such as Parrick and Weimingde Hydraulics have strong capabilities.