On-site diagnosis of hydraulic system faults in small and medium-sized excavators

At present, small and medium-sized excavators with a bucket capacity of 0.6 to 1.6 cubic meters account for over 80% of the total number of excavators. If the hydraulic system of these excavators malfunctions, accurately and quickly diagnosing the location and cause of the fault on site and promptly eliminating it will be of great significance for accelerating the project progress and reducing economic losses.

I. Basic Requirements for On-site Fault Diagnosis

On-site diagnosis requires maintenance personnel to have certain knowledge and practical experience in hydraulic transmission. Before conducting a fault diagnosis on a new model, it is necessary to carefully read the random operation and maintenance manual to have a basic understanding of the hydraulic system of this machine. At present, the hydraulic systems of small and medium-sized excavators are almost all composed of two parts: the construction oil circuit and the pilot control oil circuit. The main oil circuit adopts a dual-pump and dual-loop variable open system, while the pilot control oil circuits are all quantitative systems. By reading the technical materials, master the main parameters of its system, such as the opening pressure of the main safety valve, the pilot control pressure and flow rate, etc. Be familiar with the schematic diagram of the system, master the functions and interrelationships of each component symbol in the system, and analyze the function of each branch circuit. One should also have an understanding of the structure and working principle of each hydraulic component. Analyze the possible causes leading to a certain malfunction; Compare the machine to understand the location of each hydraulic component and the connection methods between them. When diagnosing faults specifically, the sequence of "from the outside to the inside, from the easy to the difficult" should be followed, and the possible causes leading to a certain fault should be investigated one by one.

Ii. Methods for On-site Fault Diagnosis

The main method for on-site diagnosis of hydraulic system faults is still the empirical diagnosis method. The empirical diagnosis method refers to a technique where maintenance personnel, by leveraging their theoretical knowledge and accumulated experience, and in combination with the actual situation of the machine, employ the means of "asking, observing, listening, touching, and testing" to quickly identify the location and cause of the fault. Specifically:

1. Ask

"Ask" means to inquire about the basic situation of the faulty machine from the operator. Mainly understand what abnormal phenomena the machine has; Was the malfunction sudden or gradual? Whether there are any violations of operation and maintenance conditions during use; Whether the grade of hydraulic oil is correct and the situation of replacement; The timing of the malfunction, that is, whether it occurs at the beginning of the work or after a period of operation, etc. After obtaining this information, the characteristics of the faults that occurred in the hydraulic system can be basically determined. Generally speaking, sudden malfunctions are mostly caused by overly dirty hydraulic oil or broken springs, resulting in poor valve sealing. Gradual faults are mostly caused by severe wear of components or aging of rubber seals and pipe fittings.

If the excavator starts working normally but after working for a period of time, its movement slows down, accompanied by noise and an increase in oil temperature (the oil temperature gauge reading is greater than 75°C), apart from excluding reasons such as insufficient oil volume, long-term heavy-load operation in a high-temperature environment, excessive dirt on the radiator fins of the oil cooler, and slippage of the fan belt, it may be caused by internal leakage in the pump or valve. For instance, for an excavator, the pilot control pressure was normal at first, but soon its value did not drop. The inspection result was that the rubber oil inlet pipe of the pilot pump was deflated due to heat, causing the oil inlet to be blocked.

2. Look

"Looking" means inspecting the working condition of the hydraulic system through the eyes. Whether the oil level in the fuel tank meets the requirements, and whether there are any bubbles or discoloration phenomena (the noise, vibration and crawling of the machine are often related to a large number of bubbles in the oil); Oil leakage at sealed parts and pipe joints, etc. The changes in the indicated values of pressure gauges and oil temperature gauges during operation; Check if there is any damage to the faulty part, gradual detachment of the connection or loosening of the fixing parts. When the fault of hydraulic oil leakage occurs, after eliminating the possibility of insufficient or uneven torque of the locking bolts, before replacing the oil seal that may have been severely worn or damaged, it is also necessary to check whether the pressure exceeds the limit. When installing the oil seal, the model and quality of the oil seal should be inspected, and accurate assembly should be ensured.

3. Listen

"Listening" means using your ears to check if there are any abnormal sounds in the hydraulic system. The normal operation sound of a machine has a certain rhythm and pitch and remains stable. Therefore, be familiar with and master these rules and maintain stability. Therefore, by being familiar with and mastering these rules, one can accurately diagnose whether the hydraulic system is working properly. At the same time, based on the changes in rhythm and pitch, as well as the components that produce abnormal sounds, the component where the fault occurs can be determined, and thus the location and degree of damage of the fault can be identified. A shrill, high-pitched whistling sound is usually due to the inhalation of air. The "clattering" or "giggling" sound of a hydraulic pump is often due to damage to the pump shaft or bearings. The "sizzling" sound from the directional control valve is due to insufficient opening of the valve stem. The heavy "tap-tap" sound might be the sound of the overload valve being overloaded. If there is a cavitation sound, it might be due to the oil filter being clogged with dirt, the suction pipe of the hydraulic pump being loose, or the oil level in the oil tank being too low, etc.

4. Touch

"Touching" involves using sensitive fingers to sense the touch and check whether the pipelines or components of the pressure system have faults such as vibration, shock, and abnormal temperature rise of the oil. If you touch the pump casing or hydraulic components with your hand, you can determine whether there is an abnormal temperature rise in the hydraulic system based on the degree of cold or heat, and identify the cause and location of the temperature rise. If the pump casing is overheated, it indicates that there is severe leakage inside the pump or air has been sucked in. If abnormal vibration is felt, it may be due to faults such as poor installation balance of rotating parts, loose fastening screws or gas in the system. The conditions of temperature and hand feel sensing are shown in the attached table.

Temperature and hand feel conditions

At around 40°C, it feels like touching a patient with a high fever

At around 50°C, it feels quite hot. After touching it for a long time, there will be sweat on the palm

At around 60°C, it feels very hot to the touch, and most customers can tolerate it for about 10 seconds

At around 70°C, fingers can tolerate it for about 3 seconds

At around 80°C, fingers can only make instantaneous contact, and the pain intensifies, which may cause burns

5. Test

"Test" means operating the actuating components of the machine's hydraulic system to determine the location and cause of the fault based on their working conditions.

(1) Comprehensive test. According to the design function of the hydraulic system, conduct experiments one by one to determine whether the fault is in a local area or the entire area. If the full motor operation fails or is weak, first check whether the pilot control pressure is normal, whether the clutch (coupling) is slipping (loose), whether the engine power is sufficient, whether the hydraulic oil quantity is adequate and the sealing condition of the hydraulic pump inlet. If the fault symptoms of an excavator are only manifested as the automatic descent of the boom, the cause of the fault may lie in the reversing valve, overload valve or the oil circuit of the hydraulic cylinder, and has nothing to do with the hydraulic pump and the main safety valve.

(2) Exchange test. When only one circuit or a certain function is lost in a hydraulic system, it can be exchanged with an oil circuit of the same (or related) function to further determine the faulty part. If an excavator has two independent working circuits, each with its own components, when a fault occurs in one circuit, the other pump can be connected to this circuit by exchanging the high-pressure oil pipe. If the fault is still on one side, it indicates that the fault does not lie with the pump, and the other components of this circuit should be inspected. Otherwise, it indicates that the fault lies with the pump. For instance, in the case of the traveling device of an excavator, when one side can move while the other cannot or deviates automatically, the oil pipes of the two newly purchased motors can be swapped to determine whether the fault lies in the motor or inside the reversing valve.

(3) Replacement test. Replace the suspected faulty components with those in good technical condition. By comparing the phenomena reflected before and after the replacement of the components, confirm whether the components are faulty.

(4) Adjustment test. Adjust the relief valve or directional control valve of the system and diagnose faults by comparing the changes in the machine's operating conditions before and after the adjustment. When adjusting the pressure of the hydraulic system, if the pressure (indicated by the pressure gauge) fails to reach the specified value or rises and then drops, it indicates that there is severe internal leakage in the system.

(5) Open-circuit test. Remove a certain oil pipe of the system (or loosen the joint), observe the oil output situation, and check exactly on which section of the oil circuit the fault occurs.

Finally, it is recommended that maintenance personnel keep good records of fault diagnosis, collect the phenomena, causes and troubleshooting methods of the faults, and continuously accumulate and improve them in actual work.