Abstract: The continuous advancement of technology has promoted the emergence and gradual application of new types of mine hoists with rolling bearings. However, due to the more stable service life and higher transmission accuracy of sliding bearings, they are still widely used in mine construction. However, the configuration of the sliding bearing hoist is quite difficult, and the maintenance requirements are relatively high. Among them, the wear and failure of the sliding bearings of the hoist is one of the key issues that should be addressed in coal mining work. This article conducts a relevant analysis on the wear and failure of sliding bearings in elevators, and proposes corresponding adjustment measures, in order to provide effective reference for improving the efficiency of using sliding bearing type elevators.
Keywords: elevator; Sliding bearings; wear failure
For coal mine hoists, sliding bearings are the key component among many parts. In order to ensure the safe operation of coal mine hoists and reduce the probability of failure, it is necessary to conduct in-depth analysis on the wear and lubrication of sliding bearings, effectively reduce maintenance costs, and ensure the smooth progress of coal mining work.
1. Overview of the Failure of Sliding Bearings in Coal Mine Elevators
When the sliding type coal mine hoist operates, the lubrication state of its sliding bearings belongs to the fluid lubrication range. The failure of the sliding bearings in this state usually includes the following reasons:
① Excessive pressure on the bearing or prolonged operation at high temperatures can lead to a decrease in the oil film bearing capacity, increasing the contact friction between the bearing neck and the bearing liner, causing serious wear and damage to the bearing system and resulting in failure; ② During the production and processing of bearings, solid metal cutting residues may fall into the bearings, causing wear and fatigue during operation; ③ Overloading of bearings leads to severe wear and failure; ④ During operation, the gap between the bearing and the shaft continuously decreases, and the meshing degree is too tight, resulting in insufficient space for both to move and causing failure; ⑤ The quality problem of bearings leaving the factory resulted in severe wear and tear during the manufacturing process.
Generally speaking, the material of the part of the bearing that comes into direct contact with the journal during operation is specifically designed to reduce wear. In order to improve the quality of bearing use and save the amount of precious metals, a layer of material with good lubrication function will be attached as a lining on the surface of the bearing in direct contact with the journal. Considering the failure behavior and causes of sliding bearings, it is required that the lining must have the following extremely good performance:
① In terms of dealing with friction, it has good fusion and anti friction effect; ② In the face of high-intensity and long-term bearing operation, the lining must have excellent fatigue resistance; ③ It can automatically hide or remove foreign solid particles, reducing the degree of wear and embedding of the shaft neck caused by it; ④ It can effectively reduce friction and thus achieve a reduction in journal work error during the running in process; ⑤ Has good corrosion resistance and resistance to friction damage; ⑥ Outstanding process performance, good economic characteristics, and controllable cost.
In addition, currently in the manufacturing of bearings, two types of materials are mostly used: tin based Babbitt alloy and lead based Babbitt alloy. These two materials have outstanding advantages in reducing friction and pressure, and are more suitable for high-speed operation of bearings.
2. Friction and Wear Mechanism of Sliding Bearings in Elevators
2.1 Formation of Friction and Wear
The friction and wear of sliding bearings are closely related to their formation forms. Relevant survey data shows that bearings made of Babbitt alloy materials are more prone to abrasion problems, but there is no occurrence of abrasive particles. However, the conclusion drawn from the study of relevant experiments using modern technological means is that Babbitt alloy sliding bearings produce a large amount of abrasive particles when friction occurs. In the actual operation process of the hoist, frequent starting is required, so when the sliding bearings experience friction and wear and form a dynamic pressure oil film, they usually go through three states: static, journal starting, and journal stable operation. Based on the author's research on relevant literature and combined with their years of work experience, the wear condition Z that occurs during the neck start-up phase in these three states is significant, and particles are produced.
2.2 Wear products of sliding bearings in elevators
The polymer formed after friction of bearings made of Babbitt alloy material is the product of bearing wear, and over time, the polymer grows from small to large. When the particle size of the abrasive particles is small, the metal content of wear can be obtained by using an atomic emission spectrometer to detect the sample using lubricating oil extracted from the elevator. It is found that the content of Sn, Pb, and Cu is usually high.
Given the understanding of the lubrication and wear status of the sliding bearings in the hoist, it is not difficult to see that the wear rate of the bearing lining is high and it is prone to operational failures. Therefore, ensuring the normal operation of the sliding bearings depends on lubrication treatment. Reducing friction and improving heat dissipation are the main functions of lubricating oil. While lubricating oil assists in lubricating various machines and components, it also generates a large amount of abrasive particles. Under the action of lubricating oil, the large number and hardness of alloy steel abrasive particles greatly reduce the frictional damage to bearings. However, sliding bearings in a mixed lubrication state will produce some small and evenly shaped metal abrasive particles, which will be embedded into the bone dynamic bearing under the combined action of high temperature, high pressure, and friction. Over time, the density of the metal abrasive particles will increase, and the frictional resistance between the journal and the bearing will greatly increase. Plastic deformation will occur during operation, forming stronger friction points, causing the gap here to gradually decrease until it bites and even bearing fracture occurs. When the external force exceeds the bonding force exerted by the node, the node undergoes shear fracture. If the shear fracture occurs at the upper end of the bonding point, it is in a zero wear state; If shear fracture occurs at low strength, material transfer problems may occur. Under the continuous friction in the future, the substance adhering to the surface of the journal gradually falls off, forming abrasive particles.
2.3 Perform lubrication work on the rolling bearings of the hoist
Through research, it has been found that due to the fixed structure and technical parameters of coal mine hoists, the focus on preventing wear and failure should be on lubrication. At present, many coal mine hoists mainly rely on centralized oil supply systems shared with reducers for lubrication of rolling shafts. In this case, in order to prevent wear and failure, we should determine the viscosity value of lubricating oil to be used according to the speed and load borne by the rolling bearings of the coal mine hoist during operation. For example, when the rolling bearing operates at a fast speed and bears a small load, due to the presence of dynamic pressure oil wedges and significant relative displacement of the oil layer, the viscosity value of the lubricating oil we choose should be relatively low. Meanwhile, if the coal mine hoist frequently experiences reciprocating, variable speed, and vibration during operation, in order to prevent the wear and failure of rolling bearings, we should choose lubricating oil with a higher viscosity value. In addition, for some coal mine hoist rolling bearings that have been subjected to relatively stable loads, the lubricating oil should be selected with a lower viscosity value.
3. Determination of clearance and ground adjustment of sliding bearings
Radial clearance and axial clearance are the two main types of clearances in sliding bearings, with the former being further divided into top clearance and side clearance. The existence of radial clearance is an important guarantee for accurate operation between the journal and the bearing, while axial clearance is to ensure that the journal temperature can rise while the length can freely expand and contract.
3.1 Gap determination
The determination of the radial clearance of sliding bearings is usually based on the nature of the design and assembly. Generally speaking, the top clearance is twice the side clearance. The determination of the axial clearance with the sliding bearing must be based on its specific structural form, ensuring that it is greater than the Z-length expansion and contraction of the journal during operation. Usually, the structural form needs to be determined according to the different structures. Generally, it should be below 2 millimeters, but it should also be ensured to be effective at 2 millimeters.
3.2 Measurement and adjustment of clearance between sliding bearings
Before determining the clearance of the sliding bearing, the journal must be arranged at the extreme position. By using a dial gauge, feeler gauge, etc. for relevant measurement operations, and then superimposing the data from these two extreme positions, the axial clearance size can be obtained. The feeler gauge is the main measuring tool used to measure the side clearance of sliding bearings, and the measurement process should be carried out with a normal length of a 30 degree arc. For measuring the top gap, the method of using a feeler gauge to draw a pressure lead wire can be used, and usually the latter is more accurate. It should be noted that choosing a softer lead wire with a radius of 2-3 times the top gap value is advisable, and the length should be controlled between 15mm-20mm.
3.3 Gap adjustment
If the clearance between the top of the sliding bearing is greater than the limit value, the adjustment force of the tile mouth should be weakened to ensure that the force is appropriate. At the same time, careful inspection and adjustment should be carried out on the position of the shaft neck and the lower end of the bearing, usually to ensure that the contact angle is within 120 °.
4. Conclusion
The mine hoist is extremely important for coal mining activities, and the sliding bearing type hoist still occupies a very core position and is widely used. Therefore, maintenance and repair of sliding bearings are also extremely important. Thoroughly analyzing the wear and failure of the sliding bearings of the hoist is necessary to effectively ensure the normal operation of the hoist during maintenance, while effectively reducing maintenance and repair costs and improving economic efficiency.
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