Analysis of Typical Lubrication Problems for Shield Tunnel Machine

1. Necessity of shield machine oil detection
Shield tunneling machines are important mechanical equipment in tunnel construction such as subway projects and water conservancy projects. They are expensive in cost and difficult to maintain if they fail, so they require good equipment integrity. Due to the presence of a large amount of dust, moisture, and other pollutants in the subway tunnel construction environment, the shield machine lubrication failure rate is high, resulting in significant economic losses and delays in the construction period. Oil detection, as a "lubrication physical examination technology" for mechanical equipment, can ensure long-term stability and optimal operation of the equipment and prevent and avoid major accidents by regularly sampling and analyzing representative lubricating oils in use to determine the operating wear and potential lubrication risks of the equipment.

2. Analysis of typical lubrication problems

The Oil Detection Center of China Railway First Bureau Group Urban Rail Transit Engineering Co., Ltd. was established in 2015, specializing in the detection of lubricating oil in use for shield tunneling machines, and the diagnosis and analysis of lubricating faults. In a certain year, the center provided in-service lubricating oil testing services for 51 shield machines, and inspected 1481 samples of lubricant used by shield machines. The distribution of the detected oil sample is shown in Figure 1. Among the oil samples used by all shield machines, there are 279 oil samples with lubrication problems, accounting for 18.8% of the total oil samples used by shield machines. The high rate of lubrication problems indicates that the lubrication condition of shield machines is not optimistic, and the equipment is often in a poor state of lubrication.

Typical lubrication problems include: oil degradation, water pollution, abnormal kinematic viscosity, hydraulic oil solid particle pollution, abnormal wear, grease pollution, and light fuel oil pollution. The distribution is shown in Figure 2. Among them, oil degradation, water pollution, abnormal kinematic viscosity, solid particle pollution of hydraulic oil, and abnormal wear accounted for 95.7% of the total problem oil samples, which require special attention and belong to the main lubrication problems. Grease pollution and light fuel oil pollution account for 4.3%, which belong to a few special lubrication failures. Typical lubrication problems are analyzed as follows.

(1) Oil degradation, which accounts for 35.5% of the total problem oil samples, is the most prominent lubrication problem. It is manifested as deterioration of the lubricating oil, usually accompanied by the deepening of the color of the lubricating oil compared to the new oil, even turning into black ink, unstable oil properties, delamination, insoluble particles in the upper or lower layers, and deterioration products such as black sludge. After oil degradation, the oil is insufficient to maintain normal lubrication, which can lead to a continuous increase in oil temperature and severe abnormal wear, which requires special attention. The reasons for the deterioration of oil products are: ① The long-term use of lubricating oil and the gradual deposition of pollutants in the equipment lead to the deterioration of the oil products; ② The high operating temperature of the equipment exceeds the normal operating temperature of the lubricating oil, resulting in oxidation and degradation of the oil product Inadequate and incorrect daily lubrication and maintenance of the shield machine can cause unknown pollutants to enter the oil tank through refueling, refueling, and other procedures, resulting in oil degradation.

(2) Water pollution, which accounts for 22.2% of the total problem oil sample rate, is a secondary prominent lubrication problem characterized by a large amount of external water entering the lubricating oil, causing turbidity, emulsification, and even oil-water separation. The presence of water can hydrolyze lubricating oil additives and cause the lubricating oil to lose its special anti wear properties, causing oxidation and deterioration of the lubricating oil Base Oil, leading to accidents such as poor lubrication. Due to the high humidity in the shield tunnel environment, oil and water pollution is extremely easy to occur, which requires special attention. The common causes of water pollution are: ① The sealing of the main bearing of the cutter head, the reduction gearbox of the screw machine, and other components is damaged, causing cooling water and environmental water to enter the oil tank; ② Improper storage of new lubricating oil may result in water pollution caused by new lubricating oil, which may contaminate lubricating components; ③ The refueling or refueling work is not clean and the process is not standardized, resulting in external moisture entering the tank.

(3) The kinematic viscosity is abnormal, accounting for 19.0% of the total problematic oil sample rate. According to the national general oil change standard, the kinematic viscosity of general lubricating oil is normal compared to that of new oil, with a change rate of hydraulic oil within ± 10% and a change rate of Gear Oil within ± 15%. Exceeding this range is abnormal. For each lubricating component of the shield machine, the manufacturer will recommend the type of lubricating oil and provide the optimal kinematic viscosity value, which should be observed as much as possible. Abnormal kinematic viscosity can be divided into two types: excessive and excessive kinematic viscosity. Too low kinematic viscosity can cause the oil film produced by the lubricating oil to be insufficient to separate the lubrication contact surface of the equipment, resulting in abnormal wear in the direction of scratches and scratches. Excessive oil viscosity can lead to increased transmission energy of the equipment, increased lubricating oil temperature, oil degradation, abnormal mechanical energy consumption, and other situations. Kinematic viscosity is the core indicator for determining whether an oil product can be used. The main reasons for abnormal kinematic viscosity of oil samples are as follows: ① Deterioration of the oil product or significant changes in the molecular structure of the oil product due to mechanical shear action, resulting in a decrease in the kinematic viscosity of the oil sample; ② The consumption of viscosity increasing additives in oil products greatly changes the kinematic viscosity of lubricating oil Equipment users or maintenance personnel do not work in a standardized manner and do not fill the lubricating oil required by the equipment as required, resulting in incorrect filling or refueling; ④ Unknown contaminants enter the tank, causing a significant change in the kinematic viscosity of the oil.

(4) Solid particle pollution in hydraulic oil, which accounts for 12.5% of the total problem oil samples, is the most common lubrication problem in hydraulic oil samples. Hydraulic systems have high requirements for hydraulic oil, and if there are too many solid particles in the oil sample, and the size and quantity exceed the tolerance of the hydraulic system, significant lubrication problems can occur. Hydraulic oil solid particle pollution is characterized by particle size detection, which is a key indicator of whether hydraulic oil is qualified. The main reasons for solid particle pollution in hydraulic oil are: ① The solid particles in the oil tank are not cleaned up during oil change and maintenance of the equipment; ② The hydraulic system filter element fails, resulting in the presence of a large amount of solid particles; ③ Failure to operate as required during refueling and oil change resulted in a large amount of solid particles and abnormal wear, resulting in equipment failure; ④ Not paying attention to the role of respirators, some shield machines have dirty and poor hydraulic oil respirators, which can contaminate the hydraulic oil; ⑤ During repair or maintenance, inappropriate seals or rubber pads are used, which react and deteriorate in hydraulic oil, resulting in material degradation and the appearance of large amounts of solid particles.

(5) Abnormal wear, accounting for 6.5% of the total problem oil sample rate, is characterized by high metal particle concentration and large wear particle size in the oil sample. Through continuous sampling and testing, the concentration of metal particles in the oil sample of the same lubricating component continues to increase, with a significant increase, indicating that the equipment has experienced abnormal wear. If abnormal wear is found on the equipment, it is necessary to disassemble the machine for inspection in a timely manner, find out the cause of abnormal wear, and repair and restore it in a timely manner. There is also a false wear anomaly, which typically occurs when shield machine maintenance personnel do not completely remove residual wear particles from the oil tank during maintenance of the lubricating components of the shield machine. During subsequent equipment operation, historical residual wear particles will run with the lubricating oil, resulting in a high concentration of oil sample particles.

(6) Grease contamination, which accounts for 2.5% of the total problematic oil sample rate, represents a significant increase in the kinematic viscosity of the oil sample. Due to its special reasons and great harm, it shall be listed separately for explanation. Grease contamination is caused by damage to the sealing of the main bearing of the shield cutter head or the gearbox of the screw machine, resulting in external grease entering the gear oil tank through pressure. A large amount of lubricating grease continues to enter the gearbox, resulting in an increase in the kinematic viscosity of the oil product, which transforms the oil from hydraulic pressure to semi solid or even solid, posing a significant safety hazard to the operation of the equipment.

(7) Other lubrication problems mainly include light fuel oil pollution and oil mixing, accounting for 1.8% of the total problem oil sample rate. Light fuel contamination is manifested as light fuel entering the lubricating oil. Usually, during shield machine maintenance, light fuel is used to clean lubricating components, and the cleaned light fuel is not discharged completely and remains in the oil tank. In subsequent equipment use, the remaining light fuel is mixed with newly added lubricating oil, which can lead to a significant decrease in the kinematic viscosity of the lubricating oil and generate a large amount of abnormal wear. Although light fuel oil pollution is an individual case, it is highly harmful and requires special attention. Equipment maintenance personnel and users must pay attention to this situation.

Oil mixing refers to the mixing of different grades and types of oil, usually manifested as the mixing of hydraulic oil and gear oil. Due to the different properties of hydraulic oil and gear oil, the kinematic viscosity varies several times, resulting in sharp changes in the kinematic viscosity of oil products. The main reasons for this situation are: ① The equipment lubrication personnel incorrectly filled and replenished oil, and misused hydraulic oil and gear oil; ② The mixing of hydraulic oil and gear oil refueling tools causes residual oil in the refueling tool to enter the tank by mistake.

3. Lubrication management recommendations

From the above analysis, we can conclude that:

(1) The unit using the shield machine should strengthen the lubrication management of the shield machine, train professional lubrication management personnel, and avoid lubrication problems caused by poor management such as refueling and refueling errors;

(2) The oil testing report should be qualified for use, and the daily oil replacement should be guided by the testing and analysis conclusions to ensure that the oil is replaced according to the quality;

(3) It is necessary to strengthen the storage management of the oil products used by the shield machine user, institutionalize the refueling, refueling, and oil change systems, and fundamentally avoid the situation of incorrect or ineffective implementation of the system, resulting in new lubricating oil pollution; Avoid process pollution caused by process errors such as refueling, oil replenishment, and oil change;

(4) For typical lubrication problems of shield machines, equipment maintenance units and user units should develop special plans to gradually eliminate hidden dangers item by item, and conduct special treatment to eliminate the problems in the bud.

(5) The lubrication management personnel shall establish and improve the feedback system for rectification of problem oil samples, escalate the lubrication problems to the leadership of the equipment management department, urge the equipment maintenance personnel, investigate the causes of lubrication problems, and rectify the problems.

(6) For hydraulic oil samples, direct oil change due to solid particle contamination can cause significant resource waste. Currently, hydraulic oil purification technology is complete, and purification equipment can be used to remove a large amount of particulate pollutants and a small amount of water, which can revitalize hydraulic oil and extend its service life;

(7) For newly purchased shield machines, the manufacturer will use debugging lubricating oil for debugging when leaving the factory. The goal is to test the function of the equipment, and a small amount of wear particles generated during production, manufacturing, and installation will also be brought into the debugging lubricating oil. The equipment user shall timely replace the commissioning lubricating oil according to the manufacturer's requirements to remove the wear particles generated during operation;

(8) Currently, construction companies often rent shield machines for construction, and gradually normalize them. However, shield machine leasing can also have adverse effects on lubrication management. Leasing companies are unable to understand the lubrication and maintenance status of shield machines in the previous construction project. It is recommended to conduct oil sample testing for key lubrication parts at the early stage of the lease to timely understand the lubrication status of the equipment and prevent any potential problems.

4. Conclusion
Typical lubrication problems are mostly caused by inadequate emphasis on basic lubrication management or inadequate systems. It is recommended that enterprises attach importance to lubrication management, standardize the maintenance and repair of shield machines by means of oil detection technology, reduce the occurrence of typical lubrication problems, and ensure that shield machines are in a healthy state.