There are usually warning signs before a hydraulic system fails. Such as pressure imbalance, excessive noise, excessive vibration, excessive temperature rise, excessive leakage and so on. If these phenomena can be found in time and properly controlled or eliminated, system failures can be reduced or avoided.

1. The working pressure of the hydraulic system is out of balance

       Pressure imbalance is often manifested as unstable pressure, inability to adjust pressure up or down, lag in pressure conversion, and high unloading pressure. The main reasons for stress imbalance are as follows:

1. Pressure imbalance caused by hydraulic pump

1) The axial and radial clearances of the hydraulic pump increase due to wear;

2) The "trapped oil" of the pump has not been satisfactorily resolved;

3) Poor machining and assembly accuracy of parts in the pump;

4) Individual parts in the pump are damaged, etc.

2. Pressure imbalance caused by hydraulic control valve

1) In the pressure control valve:

① The poppet valve of the pilot valve and the valve seat are poorly matched;

②The pressure regulating spring is too soft or damaged;

③ The damping hole of the main spool is blocked, and the spool valve loses control;

④The main spool is stuck in the open or closed position by dirt;

⑤ When the relief valve is used for remote control, its remote connection channel is too small or leaks;

⑥ When the relief valve is used as an unloading valve, the reversing valve that controls the unloading fails, etc.

2) In the directional control valve:

①The oil circuit is switched too fast and the hydraulic shock is generated;

②The reversing push rod of the electromagnetic reversing valve is too long or too short.

3. Pressure imbalance caused by auxiliary elements

1) The oil filter is blocked;

2) The liquid flow channel is too small and the oil return is not smooth;

3) The viscosity of the oil is too thick or too thin, etc.

4. other

1) The mechanical part is not adjusted well, and the frictional resistance is too large;

2) Air enters the system;

3) Oil pollution;

4) The motor power is insufficient or the speed is too low;

5) The pressure indicating device is faulty, etc.

2. Vibration and noise

      Vibration and noise are two aspects of the same physical phenomenon. When the hydraulic system vibrates, in addition to having a certain amplitude and frequency, it is also accompanied by noise. It not only causes failure of the hydraulic system, but also is harmful to human health. Therefore, in hydraulic transmission, the noise level is usually limited to below 80dB. The main reasons for the noise of the hydraulic system are as follows:

1. Noise caused by mechanical system vibration

1) The unbalance of pulley, coupling, gear, rotating body and the rolling body of rolling bearing vibrate;

2) The hydraulic pump shaft and the motor shaft are not concentric or the coupling is loose;

3) Poor pipe support, etc.

2. Noise caused by poor quality of hydraulic pumps and hydraulic motors

1) The precision of parts processing and assembly is not high;

2) The "trapped oil" of the pump has not been satisfactorily resolved, and the flow and pressure pulsation are large;

3) The bearing quality is not good, etc.

3. Noise caused by poor system sealing

1) The air entering the system is the main reason for the noise of the hydraulic system.

4. Noise caused by hydraulic control valve failure.

1) The pressure regulating spring of the relief valve is fatigued or the verticality between the end face and the axis line is poor;

2) The small hole on the valve core is blocked;

3) The valve core does not move flexibly in the valve body;

4) The impact of the flowing liquid in the valve body on the valve wall;

5) Eddy current or fluid shear of flowing liquid, etc.

3. Cavitation and cavitation

       Cavitation and cavitation are one of the reasons for various failures of hydraulic components and hydraulic systems, especially in hydraulic systems with high pressure and high-speed flow. Now, this problem has been fully paid attention to in the design of hydraulic components and systems. For operation and maintenance managers, the most important thing is how to effectively prevent air from entering the hydraulic system. Special attention should be paid to the following aspects:

1. Keep the connection of each joint surface of the hydraulic pump and the tightness of the connection of the pump suction pipe joint;

2. Note that the oil level in the fuel tank should not be too low, and the oil return pipe should not be exposed to the liquid level;

3. The oil filter at the pump suction pipe end should neither be close to the oil surface nor close to the bottom surface of the oil tank;

4. Clean the oil suction oil filter regularly to prevent the oil filter from being blocked by dirt and cause insufficient oil suction of the pump.

4. The temperature rise of the hydraulic system is too high

      The working temperature of the hydraulic system is generally 30 to 55 °C. Exceeding this temperature will adversely affect the system, resulting in failure of the hydraulic system. The specific effects of excessive oil temperature are: the viscosity of the oil decreases, the leakage increases, the volumetric efficiency of the pump and the system efficiency decrease significantly; the gap between the motion pairs with different expansion coefficients changes, or the motion of the moving parts is ineffective or even stuck. It will die, or cause the leakage to increase, which will reduce the working performance; make the oil oxidation aggravate and reduce the service life; make the rubber seals accelerate aging and failure. The main reasons for the high oil temperature of the hydraulic system are as follows:

1. Excessive oil temperature due to mechanical friction losses

1) The parts processing and assembly quality of hydraulic components are poor;

2) Poor lubrication conditions between relatively moving parts;

3) The quality of the seals is not good and the adjustment is too tight.

2. Oil temperature is too high due to pressure loss

1) The specifications of various valves in the system are too small, the pipe diameter is small and many bends, there is no unloading circuit, and the throttling speed control mode is improperly selected;

2) Improper adjustment of the working pressure of the hydraulic system;

3) The quality of the working oil is too poor or the viscosity is too high.

3. Oil temperature too high due to volume loss

1) The rated flow of the selected hydraulic pump is too large, and most of the pressure oil flows back to the oil tank from the relief valve during operation;

2) The matching gap between the relative moving parts is too large, and the internal and external leakage is large;

3) The seal is damaged or the joint surfaces are poorly contacted, resulting in leakage;

4) The volume of the fuel tank is too small, the heat dissipation conditions are poor, the cooler is faulty, etc.

Five, hydraulic system leakage

       Leakage is not only a harbinger of failure of the hydraulic system, but also a failure of the hydraulic system, which includes external leakage and internal leakage. At present, oil leakage is still a common problem of hydraulic equipment in my country, and the solution of this problem is directly related to the development and popularization of hydraulic technology. Therefore, measures must be taken to reduce leakage. The main reasons for hydraulic system leakage are as follows:

1. Leakage caused by poor processing at the joint surface

1) The sealing groove is too deep or too shallow;

2) The depth of the fixed screw holes of the plane seal is not suitable, and the positioning error of the plane seal part is too large;

3) The combined gasket and the seat hole are not concentric and skewed;

4) The sealing surface is too rough, etc.

2. Leaks due to poor installation

1) In the installation of pipe joints, for example, the tightening nut of the pipe joint and the joint thread are not properly matched;

2) It is not easy to align when installing the pipe joint, and there is a phenomenon of different necks;

3) When tightening pipe joints, the amount of sealing tape and sealant used is inappropriate;

4) When welding pipe joints, leakage will be caused by skewing the connecting parts after welding.

5) Another example is that the pressing force is too large when installing V and Y-shaped sealing rings;

6) When installing the oil seal at the journal of the oil pump, the oil motor and the outer end of the spool valve stem, the lip is scratched by the key or pushed off by the spring, etc., which will cause leakage.

3. Leakage caused by improper maintenance and selection of materials

1) The surface of the sealing ring is damaged;

2) The material of the sealing ring is soft, the sealing gap is large, and it is bitten by being squeezed into the gap;

3) Damage to hydraulic components and seals after oil pollution.

       From the above analysis, it can be seen that the occurrence of any fault omens or faults in the hydraulic system is not caused by a single factor, but is a very complex problem. It is not only related to component structure, processing and assembly quality, but also closely related to system design, installation and maintenance. Therefore, people are required to deal with the failure signs of the hydraulic system seriously on the basis of mastering the fault diagnosis technology of hydraulic equipment.