Hydraulic systems are widely used in various industrial sectors such as machine tool industry, engineering machinery, mining machinery, agricultural machinery, and automobile industry. During the use of the hydraulic system, various failures may occur due to mechanical wear, improper use and maintenance, or accidental damage. To determine the fault location as soon as possible, analyze the fault cause, and take effective measures to troubleshoot the fault in a timely manner, you must understand the methods of fault diagnosis.
The picture above is the simplest working principle diagram of a hydraulic system (or hydraulic pump station). The hydraulic pump, electric motor, etc. form a power source to transport oil to the hydraulic cylinder, and the solenoid valve plays a direction change function, making the piston rod of the hydraulic cylinder extend or retract.
Here is a simple analysis of three common types of faults in hydraulic systems: rise in hydraulic oil temperature, hydraulic system vibration and noise, and hydraulic system leakage.
1. Excessive Oil Temperature
1.1 Effects of High Oil Temperature in Hydraulic Systems
If the oil temperature of the hydraulic system is too high, it will lead to a series of serious consequences:
1. Because the oil temperature is too high, the oil viscosity decreases, resulting in increased leakage and even affecting the normal movement of the working mechanism.
2. Excessive oil temperature will also cause the oil to deteriorate and produce oxidative substances, which will block the small holes or gaps of hydraulic components and prevent the components from working properly.
3. Excessive oil temperature may cause thermal deformation of the machine tool or machinery, destroying its original accuracy.
1.2 Causes of High Oil Temperature in Hydraulic Systems
1. Poor cooling. The selected oil tank is too small during design, causing the oil to circulate too fast, or due to the high ambient temperature, although a cooler is used, its cooling effect is poor.
2. Impact during direction change and speed change.
3. Air or moisture enters the oil. When the hydraulic pump converts the oil into pressurized oil, if there is air and moisture in the oil, it will increase heat and cause the oil to overheat.
4. Use the hydraulic oil with too high viscosity or the viscosity of the hydraulic oil increases. Oxidation of hydraulic oil or low ambient temperature will increase the viscosity. The hydraulic oil with too high viscosity will increase friction and cause heat generation.
1.3 Solutions to High Oil Temperature in Hydraulic Systems
1. Design an oil tank of sufficient size and install a cooler if necessary. If the ambient temperature is too high, isolate the system from the outside world. Strictly control the system temperature within the range of 20-60℃, and the maximum temperature shall not exceed 70℃.
2. The pipeline should be as short as possible and should not be too slender or curved to ensure smooth oil flow.
3. When the setting value of the pressure regulating valve is high, it is best to reduce the working pressure to reduce unnecessary energy loss.
4. Select the hydraulic oil with appropriate viscosity.
2. Vibration and Noise
First, let’s give an overview of the sources of vibration, so that we can understand the causes of vibration discussed below.
Overview: Hydraulic impact, unbalanced force during rotation, changes in frictional resistance, etc. are all sources of vibration. In hydraulic equipment, vibration is often followed by noise. In actual use, vibration and noise in hydraulic pumps and valves often occur, mainly for 5 reasons.
2.1 Causes of Vibration and Noise in Hydraulic Systems
1. Air Invasion
It is the main cause of vibration and noise in hydraulic systems. Because air entering the hydraulic system will cause cavitation, which is the sudden explosion of air bubbles.
2. Hydraulic Pump Unable to Suck Oil
The oil level in the oil tank is too low, the oil suction port is too high, the oil filter port is too small, the viscosity of the hydraulic oil is too high, and the oil tank is airtight, etc., which will cause the hydraulic pump to be unable to suck oil, thus producing noise.
3. Hydraulic Pump Damaged during Use
During use, noise will also be generated due to wear of hydraulic pump parts (vanes, oil distribution plates), excessive clearance, insufficient flow, excessive speed, and pressure fluctuations.
4. Relief Valve Failing to Operate
The relief valve is a pressure regulating valve, whose instability can cause pressure fluctuations, resulting in system vibration and noise. Contaminants in the hydraulic oil clog the orifice, causing the oil to pass through for a while and then be blocked for a while. In addition, the deformed spring of the relief valve and stuck valve core will cause pressure fluctuations in the relief valve.
5. Mechanical Vibration
The most common mechanical vibrations in hydraulics are caused by pipe jitter and unbalanced rotating parts of the motor and hydraulic pump.
2.2 Solutions to Vibration and Noise in Hydraulic Systems
1. In view of the cavitation phenomenon, an air exhaust device is often installed on the hydraulic cylinder. In addition, after starting the actuator, allow it to reciprocate several times in a rapid full stroke to discharge air.
2. In view of the phenomenon that the hydraulic pump cannot suck oil, the oil inlet sealing must be strengthened, the height from the oil suction pipe inlet to the pump oil suction port must be less than 500MM, and select the hydraulic oil with appropriate viscosity.
3. If the hydraulic pump is damaged during use, repair and maintain it.
4. Pay attention to the cleaning and inspection of the overflow valve, and repair or replace it in time if it is damaged.
5. In view of the phenomenon of mechanical vibration, when the oil pipe is slender and has many elbows, fixed pipe clamps should be added, and the installation accuracy of the electric motor and hydraulic pump should be adjusted to not less than 0.1mm.
3. Leakage
Hydraulic system leakage is divided into external leakage and internal leakage. The causes of leakage are complicated and mainly related to vibration, temperature rise, pressure difference, gaps, improper design, manufacturing, installation and maintenance. Compared with other types of faults, it is more visible and can be seen through visual inspection. Leakage can cause oil loss, environmental pollution, and equipment wear.
The main reasons for leakage are: damaged and aged seals, excessive oil filling resulting in excessively high oil level, excessive oil temperature, damaged components, excessive fit clearance, etc.
3.1 Causes of Hydraulic System Leaks
1. Due to poor lubrication between the kinematic pairs, improper material selection, and poor processing, assembly, and installation accuracy, early wear will occur, resulting in increased gaps and leakage;
2. Seal failure. Seals are widely used in hydraulic components. If the seal material is inferior, physically and chemically unstable, has low mechanical strength, elasticity and wear resistance, it will easily lead to leakage;
3. It can also cause leakage that the designed size of the groove for installing the seal is unreasonable;
4. It is caused by oil leakage from components and poor oil return pipelines;
5. The oil temperature is too high, the oil viscosity drops, or the selected oil viscosity is too small, causing leakage;
6. Hydraulic shock and mechanical vibration directly or indirectly cause system pipeline connectors to loosen and cause leakage.
3.2 Solutions to Hydraulic System Leaks
1. Strictly control the machining accuracy and fit clearance of kinematic pairs that use clearance seals.
2. Replace the seals with appropriate ones.
3. Minimize the number of oil pipe joints and flanges. Installing the hydraulic valve kit closer to the actuator can greatly reduce the total length of the hydraulic pipeline and the number of pipe joints.
4. The amount of oil leakage is inversely proportional to the viscosity of the oil. Small viscosity means a large amount of leakage. Therefore, the hydraulic oil should be replaced in time according to the temperature to reduce leakage.
5. Control temperature rise.
4. Conclusion
For someone who is familiar with the principles, structure and various components of the hydraulic system and understands common faults of the hydraulic system in advance, he will know which aspects to analyze and eliminate, instead of blindly disassembling components when an emergency occurs during work.
Not only technicians, equipment managers and operators should also understand the knowledge of hydraulic systems, so that they can achieve reasonable maintenance and correct use, effectively prevent and reduce the occurrence of hydraulic faults, and even solve these faults as soon as possible.
