Abstract: This article analyzes the failure modes and materials used in the sliding bearings of mine hoists, and proposes two measures to control the wear of sliding bearings. The first is to control the clearance of sliding bearings well, and the second is to do a good job in lubrication management of sliding bearings.

【 Keywords 】 Mine hoist; Sliding bearings; wear and tear

Among the various components of the coal mine hoist, the sliding bearing is a crucial component. Analyzing the characteristics of the sliding bearing in terms of friction, wear, and lubrication can ensure the safe operation capability of the hoist, reduce the probability of faults, increase the start-up rate, and reduce maintenance costs, thereby achieving the goal of improving the management level of large-scale operating equipment in coal mines.

1. Failure modes and materials of sliding bearings in elevators

According to the lubrication state during the operation of the hoist, sliding bearings belong to fluid lubricated bearings. The main failure modes of sliding bearings under this lubrication state are: (1) due to overload or high temperature, the oil film bearing capacity is insufficient, and the contact between the journal and the bearing liner causes wear; (2) Bearing damage caused by abrasive wear due to cutting and residual sand particles generated during processing; (3) Bearing wear and fatigue caused by prolonged work; (4) Due to extreme working conditions causing temperature rise and resulting in bearing and shaft biting (or sticking); (5) Local wear, corrosion, erosion or cavitation of bearings caused by manufacturing or assembly errors, corrosion of lubricating oil, impact of external loads, etc.

The part of the sliding bearing that comes into direct contact with the journal during operation, namely the bearing shell or sleeve, is generally made of anti friction materials. In order to improve the performance of bearings and save precious metals, a layer of material with good anti friction performance is often attached to the inner surface of the bearing shell, called a bearing liner.

Corresponding to the failure mode of sliding bearings, it is generally required that the material of the sliding bearing liner should have the following properties: (1) good friction compatibility, that is, the ability to prevent adhesion when the journal comes into contact with the bearing material; (2) Sufficient anti fatigue performance to have the ability to resist fatigue failure under cyclic loading; (3) Better embeddedness of foreign particles to reduce journal wear; (4) Better wear resistance during the running in process by reducing machining errors of the journal or bearing, same column errors, surface roughness, and making contact uniform, thereby reducing friction and wear cracks; (5) Sufficient ability to resist corrosion and cavitation; (6) High wear resistance and abrasion resistance.

The materials for bearings are generally two types: tin based Babbitt alloy and lead based Babbitt alloy. Therefore, they have strong ability to reduce friction and are suitable for high-speed and high gravity environments.

2. Measures to prevent wear of sliding bearings

2.1 Control the clearance of sliding bearings well

There are two types of clearances for sliding bearings, namely radial clearance and axial clearance, and radial clearance can be divided into two types, namely top clearance and side clearance. In order to ensure accurate operation between the journal and the bearing, there must be radial clearance in the sliding bearing. To ensure that the journal can freely expand and contract in length while increasing temperature, sliding bearings must have axial clearance.

(1) Determination of Gap

The radial clearance of sliding bearings is generally determined based on the nature of the design assembly, and the lateral clearance is usually half of the top clearance. The axial clearance of sliding bearings should be determined according to their structural form. Generally, it should be less than 2 millimeters, but it should also be ensured to be greater than the Z-large expansion and contraction during the expansion process of the shaft neck.

(2) Measurement and adjustment of clearance between sliding bearings

When determining the clearance of sliding bearings, it is necessary to arrange the shaft neck at the extreme position and use a feeler gauge and dial gauge for measurement. Then, by superimposing the data from the two extreme positions, the size of the axial clearance can be determined.

When measuring the side clearance of sliding bearings, a feeler gauge is used for measurement, and a circular arc with a length of 30 degrees is generally recommended for insertion.

In the measurement of top clearance, it is possible to use a feeler gauge or lead wire pressing method for related application operations, and the lead wire pressing method has higher accuracy. The radius of the lead wire should be 3 to 2 times the value of the top clearance, and the length Z should be 15 to 20 millimeters. The lead wire should not be too hard.

(3) Adjustment of Gap

When the clearance between the top of the sliding bearing is greater than the limit value, the force of adjusting the tile mouth should be reduced. At the same time, the contact angle between the journal and the lower end of the bearing should be checked. Generally, the contact angle should be within 120 degrees.

2.2 Proper lubrication management of sliding bearings

The experimental study on the lubrication and wear state of the sliding bearings of the hoist found that the growth rate or wear rate of the friction polymer produced by the bearing lining is relatively high, and the risk of failure is also relatively high. Considering that the structure and technical parameters of the hoist, such as lifting capacity, drum speed, motor power, bearing material and structure, are certain, lubrication is the main factor restricting the normal operation of sliding bearings. The sliding bearings of coal mine hoists mostly use centralized oil supply lubrication stations shared with reducers. It is generally believed that the specific viscosity value of lubricating oil used in sliding bearings should be determined based on their speed and load. For example, bearings with high unit load and low speed cannot form favorable dynamic pressure oil wedges. To ensure their semi fluid lubrication or boundary lubrication state, the viscosity of the oil should be higher; Bearings with low unit load and high speed have a greater ability to form oil wedges, which can increase the relative displacement of the oil layer and the heat generated by internal friction of oil molecules. Therefore, the viscosity of the oil should be selected to be lower; For sliding bearings with reciprocating motion, variable speed motion, vibration and impact loads, the viscosity of the oil should be selected to be higher; For bearings with stable loads, the viscosity of the oil can be chosen to be lower. The viscosity of the lubricating oil for the elevator reducer lubricated with sliding bearings is also selected based on the load and speed. The faster the speed, the shorter the instantaneous meshing, the lighter the load, and the lower the viscosity requirement; On the contrary, the viscosity is higher. A small load and high viscosity will inevitably increase the additional resistance of the transmission and affect the dissipation of frictional heat. Obviously, in the same lubrication system, although the main factors for selecting lubricating oil for sliding bearings and reducers are basically the same, due to their respective working characteristics, it is difficult to balance the selection of lubricating oil. Therefore, if conditions permit, the sliding bearings of the hoist should be lubricated separately and magnetic filters should be installed in the oil tank.

Some people believe that the hoist operates at high speeds and heavy loads, and it is safer to use lubricating oil with high viscosity and good extreme pressure performance. Based on this, many users choose N150, N220, N320 medium load (extreme pressure) industrial gear oil as the lubricating oil for the hoist, and supply oil to the sliding bearings and reducer at the same time. But in reality, it cannot achieve good lubrication effect. The following is an example of using N320 medium load industrial gear oil as the lubricating oil for the hoist.

According to on-site measurements, when using N320 medium load industrial gear oil lubrication for the hoist, the surface temperature of the sliding bearing part reaches 95 ℃, and the kinematic viscosity at this temperature is 16.20mm2/s. The actual temperature between the sliding bearing lining and the coupling friction pair will definitely be 95 ℃ higher than the temperature on the outer surface of the bearing seat. Figure 1 shows the viscosity temperature characteristic curve of N320 industrial gear oil under load. It can be seen from the figure that as the temperature increases, the viscosity of the oil reaches a turning point at 90 ℃, and then sharply decreases. At 125 ℃, the corresponding kinematic viscosity is 6.9mm2/s. If the working temperature of the sliding bearing is 125 ℃, the lubricating oil with a kinematic viscosity of 6.49mm2/s cannot form a strong extreme pressure oil film, and the instantaneous high temperature generated by friction and wear cannot be effectively carried away, resulting in deterioration of lubrication conditions. In addition, when lubricating oil is pumped into sliding bearings, the clearance between the bearing liner and the shaft neck is small, and the viscosity of the oil is high and the fluidity is poor. Especially during the start-up and shutdown stages, the bearing coupling pair cannot receive effective lubrication and is in a mixed lubrication or dry friction state, resulting in increased friction and wear. Therefore, choosing lubricating oil with good extreme pressure performance and high viscosity did not achieve the expected lubrication effect due to its incompatibility with the actual working conditions of the operating pair. The actual monitoring results indicate that the appropriate kinematic viscosity for lubricating oil used in sliding bearings is 30-100mm2/s.

Figure 1 Viscosity temperature characteristics of N320 medium load industrial gear oil

3. Conclusion

Heat dissipation is crucial in the lubrication of coal mine hoists, which is also an important condition for selecting lubricating oil for sliding bearings. The actual monitoring results have shown that the appropriate viscosity for lubricating oil used in sliding bearings is 30-100mm2/s.

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