Faults and Maintenance of Excavator Hydraulic Systems

Abstract: The hydraulic system is one of the most important systems of an excavator and also one of the systems most prone to failure. Taking the hydraulic system of the PC200-5 excavator as an example, this paper describes the working principle of the hydraulic system and the methods for diagnosing and eliminating faults, in order to provide certain guidance for the maintenance of excavators and other equipment containing hydraulic transmission devices.

Key words: Hydraulic fault diagnosis and elimination of excavators

The hydraulic transmission systems applied on excavators mainly include pilot control hydraulic systems, rotary hydraulic systems, traveling hydraulic systems, and working device hydraulic systems, etc. They have the advantages of compact structure, flexible movement, smooth operation, and convenient operation. The hydraulic system uses hydraulic oil as the medium for power transmission. Due to the wear of internal components, leakage may occur, accompanied by overheating. Faults such as lack of energy at work. The occurrence of hydraulic transmission faults is sudden and concealed, and involves a large number of components, which brings certain difficulties to fault diagnosis and elimination. Therefore, when maintaining a hydraulic system, it is necessary to understand its working principle and correctly analyze the cause of the fault to ensure the quality of maintenance. This article takes the common faults of the PC200-5 excavator as an example to introduce the working principle of hydraulic transmission and analyze the diagnosis and elimination methods of its common fault phenomena.

The working principle of the hydraulic system of the PC200-5 excavator

The hydraulic system of the PC200-5 excavator is composed of some basic circuits and auxiliary circuits, including the working circuit, pressure-limiting circuit, unloading circuit, buffer circuit, throttling speed regulation and throttling speed limit circuit, travel speed limit circuit and pilot valve control circuit, etc. Its components mainly consist of working pumps, make-up oil pumps, pilot control valves, distribution valves, safety valves, large arm cylinders, small arm cylinders, bucket cylinders, oil tanks and related pipelines, etc.

During the operation of the PC200-5 excavator hydraulic system, the hydraulic oil is drawn in by the working pump through the oil filter from the bottom of the oil tank, and the hydraulic oil with a certain pressure is output from the oil pump and enters a set of parallel distribution valves. The corresponding actions are achieved through the handle → pilot valve → working valve group. The system limits the total pressure of the entire system through the main safety valve on the main oil circuit. The safety valves of each working oil circuit respectively provide overload protection and oil replenishment for the corresponding oil circuits.

Fault Diagnosis and Elimination of the Hydraulic System of the PC200-5 Excavator

For the PC200-5 excavator, the set pressure of the hydraulic system is 30MPa. If the pressure is lower than this, it indicates that the system pressure is too low. The faults of the hydraulic system are mainly manifested in two aspects: slow and weak lifting of the large arm; The rotation is slow and weak. The main cause of the two faults is the low working oil pressure, while the main reasons for the low pressure are blockage and leakage. Unobstructed and well-sealed oil circuits are the guarantees for the normal operation of the system. Blockage and leakage are the most common faults in hydraulic transmission. Therefore, when checking for hydraulic transmission faults, the inspection usually starts from the hydraulic oil circuit. The following are the diagnostic and troubleshooting methods for different fault phenomena in the hydraulic system.

2.1 The upper arm lift is slow and weak, while other movements are normal

2.1.1 Fault Diagnosis

In a hydraulic system, if only the boom lifts slowly and weakly, while the bucket flips normally. It is not difficult to see from the working principle diagram that other operations are normal. This indicates that the working pump and the main safety valve are functioning properly, and the pressure they provide to the entire system is sufficient. At the same time, it also shows that the oil quantity and quality in the pipeline at the pump's oil inlet end, the oil filter, and the oil tank are all fine. At this point, it is only necessary to inspect the large arm slide valve, the large arm cylinder, the oil pipes of the large arm section, and their sealing parts.

(1) Check the blockage of the oil circuit

First, carry out routine treatment. Remove the oil pipe, take off the valve body, valve stem and related components of the large arm slide valve for cleaning. Clean the oil passage thoroughly and blow it through and dry with compressed air.

(2) Check for oil circuit leakage

Leakage in hydraulic systems usually occurs after they have been in use for a period of time. From the surface appearance, it is mostly caused by the failure, damage, extrusion of the sealing parts or the scratching of the sealing surface. The main reasons are: oil contamination, improper roughness of the sealing surface, unqualified sealing grooves, loose pipe joints, increased clearance of mating parts, excessively high oil temperature, deteriorated sealing rings or poor assembly, etc. Leakage is divided into internal leakage and external leakage. Usually, faults are mainly caused by internal leakage.

(3) Handling of internal leakage faults

The internal leakage in this part mainly occurs in the leakage of the boom spool valve and the cylinder. Internal leakage mainly occurs inside the valve body and the cylinder and is not easy to check. But we can rely on some auxiliary methods to determine the leakage situation.

(4) Inspection of the large arm cylinder

When the piston of the large arm cylinder is retracted to the bottom, remove the oil pipe without a rod cavity to allow the rod cavity of the large arm cylinder to continue to be filled with oil. If a large amount of working oil leaks out of the oil pipe opening in the rodless cavity, it indicates that the hydraulic cylinder has internal leakage. It is also possible to fill the bucket with load, lift it to the limit position, place the boom control lever in the middle position, and turn off the engine to observe the sinking speed of the boom. Then, place the large arm control lever in the ascending position. If the sinking speed of the large arm significantly increases at this time, it also indicates that the internal leakage occurs in the hydraulic cylinder. If the change in the sinking speed is not obvious, the cause of the internal leakage lies in the large arm slide valve.

(5) Inspection of the large arm slide valve

The leakage of the large-arm slide valve is mainly due to excessive fit clearance between the valve stem and the valve body, damage to the pressure regulating spring, and damage to the sealing parts inside the valve, etc. Check the fit clearance between the valve stem and the valve body, inspect the pressure spring, and see if the sealing parts inside the valve body are damaged.

2.1.2 Exclusion Methods

If the leakage test result in the oil cylinder exceeds the specified value, it should be disassembled for further inspection. If the sealing ring is damaged, it should be replaced; if the cylinder wall is severely scratched, it should also be replaced. If the boom valve is severely worn, replace it.

2.2 The large arm is working properly; The forearm (bucket) works slowly and weakly

2.2.1 Fault Diagnosis

In the hydraulic system, if the large arm is working properly; The forearm (bucket) works slowly and weakly. It is not difficult to see from the working principle diagram that the lifting of the large arm is normal, which indicates that the working pump and the main safety valve are working properly. At the same time, it also shows that the oil quantity and quality of the pipeline at the oil inlet end of the pump, the oil filter and the oil tank are fine. At this point, it is only necessary to pay attention to checking the working slide valve, safety valve and their sealing parts of the small arm (bucket).

2.2.2 Exclusion Methods

The methods for diagnosing and eliminating faults in hydraulic transmission are largely similar. Similarly, the fault handling methods for the small arm (bucket) part and the large arm part are also basically the same. Therefore, for dealing with faults in the working part of the small arm (bucket), the operation can be carried out by referring to the fault phenomenon handling methods in 2.1.

2.3 Slow and weak lifting of the upper arm; Slow and weak rotation

2.3.1 Fault Diagnosis

In the hydraulic system of the working device, both the large arm and the rotary operation are abnormal. There are many reasons for this. Besides the previous fault phenomena, it may also be related to the faults of the main safety valve, hydraulic pump, oil filter, hydraulic oil, and the inlet and return oil circuits of the distribution valve. Since there are many points involved, we can start checking from the easy to the difficult and from the key points. First, let's check the most intuitive points: check if the oil level in the tank is sufficient, and examine the cleanliness, color, viscosity, consistency and odor of the hydraulic oil. When hydraulic oil flows from the high-pressure side to the low-pressure side without performing mechanical work, heat will be generated within the hydraulic system. If the temperature of the hydraulic oil is too high, it will cause the sealing parts to deteriorate and the oil to oxidize and fail. It will also lead to corrosion and the formation of deposits, which may clog the damping holes and accelerate the wear of the valves. Excessively high temperatures will cause the valves and pumps to get stuck. If there are any problems, eliminate them first, and then check several key parts. In terms of probability, the possibility of both the large arm slide valve and the rotary slide valve being damaged simultaneously, as well as the simultaneous internal leakage of the large arm cylinder and the rotary cylinder, is relatively low. Therefore, we first conduct the detection of system pressure. System pressure detection: Install a pressure gauge with a range of 40MPa at the pressure measurement point. When the engine is running at its rated speed, lift the boom to the highest position. At this time, the gauge shows the highest pressure, and the reading should be 27 to 28MPa.

2.3.2 Exclusion Methods

If the system pressure is low, the following aspects should be mainly analyzed and excluded:

(1) Internal leakage in the distribution valve: The main reasons for internal leakage in the distribution valve are: the main valve core of the main safety valve is stuck, the fit clearance between the valve stem and the valve body is too large, the pressure regulating spring is damaged, the sealing parts inside the valve are damaged or there are sand holes in the valve body, etc. Disassemble and inspect whether the cone valve of the main safety valve is stuck. Check the fit clearance between the valve stem and the valve body. The normal fit clearance should be between 0.005 and 0.025mm, and the repair limit is 0.04mm. Check the fit clearance between the main valve core and the main valve sleeve. The normal fit standard clearance is 0.010 to 0.018mm, and the repair limit is 0.03mm. Check whether the pressure spring and the sealing parts inside the valve are damaged.

(2) Internal leakage of the working pump: Internal leakage of the pump is manifested as: high noise during operation, and the higher the engine speed, the greater the noise; A large amount of copper shavings can be seen in the oil filter. The pump should be disassembled and inspected. If any damage is found, it should be repaired or replaced.

(3) After checking and resolving the overall pressure issue of the system, if it still does not work properly, the boom part and the bucket part can be eliminated respectively in accordance with items 2.1 and 2.2.

(4) If all the above checks are normal, then measure the pressure of the corresponding working oil circuit. According to the pressure gauge, when in the two working states of "rotation" and "boom lifting", the pressure of the working oil circuit is only 10MPa, and the oil pressure of the pilot control system is only 0.5MPa (the normal value is 2.8MPa). However, the working oil pressure and the oil pressure of the pilot control system in other working states were in line with the normal values. After a careful analysis of the working principle diagram of the hydraulic system, it was found that the pilot oil circuit with priority for rotary control and the pilot oil circuit with priority for boom control could be connected with the oil circuits of the left travel control valve and the linear travel valve. Therefore, it is initially judged that it might be due to the high-pressure oil in the "left-walking control valve" acting on the "linear walking shuttle valve", causing the valve core of the "linear walking shuttle valve" to be out of the center position. After the pilot control oil circuits for "rotation" and "boom lifting and lowering" pass through the "shuttle valve" and the "linear walking valve", they are deflated. When the "left-walking" operation handle is slightly operated, The movements of "rotation" and "boom lifting" have accelerated. Therefore, it was determined that the "spring and oil seal" at the end of the "left travel control valve" were damaged. After disassembling the "left travel control valve", it was found that the oil seal and spring were obviously damaged. After replacing the new spring and oil seal, the fault was eliminated.

Precautions for Maintaining the hydraulic system of excavators

This article only introduces the typical faults of the PC200-5 excavator's hydraulic system and their maintenance methods. In fact, there are other faults as well, such as slow movement and frequent breakage of pipe joints, etc. However, regardless of the cause, all hydraulic transmission problems can be summarized into three major issues: pressure, flow, and direction. The causes of the three major problems are generally leakage, blockage, incorrect connection of oil pipes and incorrect pressure regulation. Therefore, when repairing faults in the hydraulic system, we must pay attention to:

(1) Hydraulic components must be thoroughly cleaned and the oil circuits must be unobstructed before assembly.

(2) Do not use unclean hydraulic oil and do not use inferior seals.

(3) Components must be assembled correctly. For instance, the opening of the "Y" ring must not be installed in reverse and the oil pipe must not be connected incorrectly.

(4) Do not make any random adjustments to the displacement of the working pump and the safety valve before you have a clear understanding. Otherwise, if you increase the displacement, it may damage the hydraulic components; if you decrease it, it may cause slow, weak or no operation.

In conclusion, when a hydraulic system malfunctions, do not handle it blindly. Analyze and solve the problem in the order of "easy first, then difficult; external first, then internal; key points first, then general". Generally, first check for external leakage, oil quantity and quality, and blockage conditions. For faults caused by internal wear of components, first understand the principle before conducting targeted inspections Only by having a deeper understanding and mastery of the hydraulic system and constantly improving technical and working capabilities can we better solve the main problems faced by users of hydraulic equipment and manage the hydraulic system well. When problems occur in the system, the true cause of the system failure can be identified. More work starts from daily inspections, paying attention to the details of equipment inspection and maintenance work. Various factors causing the failure can be eliminated at an early stage. Through continuous improvement and enhancement of the work cycle, predictive maintenance work can be further advanced in the constantly changing working environment. Ensure that the equipment operates at greater efficiency and achieve the goal of zero equipment accidents.