Types and causes of bearing damage of natural gas collector and some measures to improve the service life of coal mine machinery bearings.

Abstract: according to the field investigation, this paper discusses the common damage conditions and main causes of coal mine machinery bearings.

Key words: coal mine machinery bearing life

The bearing is an important basic component of the coal mine machinery. Its function is to support the transmission shaft and the parts on the shaft, keep the shaft at the determined working position, ensure the rotation accuracy of the shaft and reduce the friction and wear between the shaft and the bearing. The reliability and service life of the bearing have a great influence on the normal operation of the coal mine machinery. In recent years, during the use of the coal mine machinery, the mechanical transmission system failures caused by the bearing failure have increased, and even the early damage of the bearing has caused the gear teeth in the reduction box, and the shaft is stuck, forcing the coal mine machinery to disassemble and go to the shaft for maintenance, which has brought serious losses to the production. Therefore, how to improve the service life of the bearing has become an urgent problem to be solved for the working reliability of the coal mine machinery and the realization of high yield and high efficiency in the fully mechanized face.

1. Types and causes of coal mine machinery bearing damage

Among the transmission shaft supports of the coal mine machinery, the parts with weak strength and easy to be damaged include:

(1) The first shaft of the cutting section (small spiral bevel gear shaft) has a high speed. The temperature rise and heat make the radial clearance smaller, and burns in case of oil shortage, resulting in abnormal noise and vibration.

(2) The planetary axle of the cutting planetary mechanism bears a large force (twice the tangential force of gear meshing). However, due to the limitation of space size and rim wall thickness, the bearing diameter cannot be increased, the contact stress between the rolling element and the raceway surface is high, and early pitting corrosion and serious wear often occur.

(3) The rotation speed of rocker arm rotary shaft sleeve and drum shaft is low, but the load is high and there is serious impact force and large axial force. The bearing ring is often deformed and the retaining edge is broken.

(4) The traction traveling axle bears the heavy load of impact and alternation, and the sealing and lubrication conditions are poor. The coal dust is easy to enter the raceway to crush the cage, and the rollers are scattered off the raceway.

2. Some measures to improve the service life of coal mine machinery bearings

2.1 Improve the design life by using improved bearings

The general design life of coal mine machinery bearings is less than 5000h (gear design life), and some are only 1000~2000h, while in foreign countries it is as high as 10000~20000h. It can be seen that there is a big gap. In the design of coal mine machinerys, imported products can be used for important bearings. Generally, the improved reinforced bearings should be preferred. For example, for cylindrical roller bearings, the diameter, length and number of rollers are increased by E-type compared with the original ones, and logarithmic generatrix variable curvature crown rollers are used to realize the reasonable distribution of axial contact stress along the rollers. These measures have increased the rated dynamic load of the bearing by an average of 40%-50% compared with the old bearing, and the calculated life of the bearing has been increased by 2.1-2.9 times (for example, the 42610 has increased the rated dynamic load by about 37% compared with the 42610, and the calculated life has been increased by 1.8 times). For another example, the self-aligning roller bearing replaces the asymmetric type with the symmetrical type, and changes the middle flange of the inner ring to be movable, so as to improve the roller guide and stress distribution and the coincidence rate between the roller and the inner and outer raceways, so that the rated dynamic load capacity is increased by 25%~40% on average, and the calculation life is increased by 1.1~2.1 times (e.g. 53610 vs. 3610. The rated dynamic load is increased by about 30%, and the calculation life is increased by 1.4 times).

2.2 Special sliding bearing is used for rocker arm support

When the coal mine machinery is raised, the rotation speed of the rocker arm is very slow, but it has to bear a large radial and axial impact load. Generally, self-aligning roller bearings and short cylindrical roller bearings are used for coal mine machinerys. It is found that the bearings are damaged early in use, resulting in the deformation of the casing. The rollers are scattered into the casing, and the gears are crushed. At present, the solution is to use radial joint ball bearings and sliding bearings made of three-layer composite materials.

2.2.1 Spherical plain bearings can bear large radial and axial loads, as well as large impact. They have simple structure and compact size. It is especially suitable for swing bearing with low sliding speed and heavy load. The split outer ring and joint ball bearing with lubricating oil groove (Fig. 1) have been used in the rocker arm support of coal mine machinery since the early 1980s. For example, this bearing is used for Japanese 6565 coal mine machinery to continuously produce 2.4 million tons of coal without any sagging.

Fig.1

1 - Snap ring, 2 - Outer ring, 3 - Inner ring

Fig.2

1 - Outer ring, 2 - Inner ring fault

At present, structure is adopted in domestic high-power coal mine machinerys, such as MG300 and AM500 (imitation). Medium and small power coal mine machinerys, such as MG20 andBM150, also use the integral spherical joint ball bearing with notch on the inner spherical surface of the outer ring. Although the impact resistance of this type is slightly poor, it is convenient for processing and installation. The service conditions of spherical plain bearings (Fig. 2) are:

Shaft diameter: 6~3oomm

Material: steel to steel (GCr15SiMn)

Temperature: 60 ℃ <t<200 ℃

Specific pressure: P_max≤100N/mm²

Speed: v ≤ 0.1m/s

PV value: ≤0.4N/mm² · m/s

2.2.2 SF three-layer composite material is a self-lubricating bearing material with steel plate as the matrix, copper mesh as the middle layer and plastic as the surface layer. It effectively gives full play to the advantages of metal and plastic, and has reliable interlayer bonding force, thus achieving good mechanical and physical properties and wear resistance. For example, SF-2 is coated with modified POM and engraved with oil storage pit with a diameter of 2.5mm. It can be used without oiling for a long time. It is suitable for low speed and high load (v≤2.5m/s, p≤140N/ mm², PV≤1.8-2.0N/ mm² · m/s) and continuous working temperature from - 40 ℃ to 100 ℃. For example, the sliding bearing of SF-2 three-layer composite material is used in the rocker arm support of dy150 coal mine machinery. The coal mining capacity is 270000 tons, and the unilateral wear is only 0.03~0.07mm, which is more than twice the service life of the original 9-4 aluminum bronze material.

2.3 Development of mining heavy-duty bearings

The bearing is made of high carbon chromium manganese bearing steel. After overall quenching treatment, the surface hardness is high. The requirement is HRC60~64. However, it is sensitive to notch, and fatigue crack is easy to occur at the stress concentration point under impact load. If low-carbon alloy steel (such as 17CrNiMo6, 20Cr2Ni4A, etc.) is used for carburizing and tempering, the surface hardness can reach hrc58~62, and the core hardness can reach about HRC46. The bearing can resist wear and has sufficient surface strength. It also has high heart strength and toughness. Therefore, it can resist impact and is suitable for manufacturing large roller bearings. This new material and technology can greatly improve the impact resistance and prolong the service life of the drum shaft and rocker shaft bearings of the coal mine machinery. In order to further improve the bearing capacity of the existing bearings, vacuum degassing steel and electroslag remelting steel can be used to replace the commonly used electric furnace bearing steel, so that the segregation, slag inclusion, sulfide, hydrogen and oxygen content in the steel are reduced, the microstructure and hardness are uniform, the plasticity and toughness are improved, and the fatigue strength is improved.

2.4 Improve the lubrication and sealing of bearings

When the bearing works, there is friction between the rolling element and the raceway, the cage, the inner and outer rings and the rolling element. The lubricant can reduce the wear. In particular, the oil film formed between the rolling element and the raceway can reduce the contact stress, reduce the temperature and prolong the service life of the bearing. The lubricating oil for coal mine machinery bearings is generally N220 and N320 extreme pressure industrial gear oil, and the oil pool splash or splash plus circulating combined lubrication mode is usually adopted. The main problem is that the seal is unreliable, resulting in a large amount of oil leakage, continuous immersion of external pulverized coal and dust, increased bearing wear, poor bearing lubrication, and even overheating and burning of the surface due to lack of oil. At present, the measures to solve the sealing of the main parts of the coal mine machinery include:

(1) The high-speed shaft oil seal improves its service life by improving the sealing material and structure,

(2) The swing bearing of the rocker arm is lubricated with grease (2* lithium grease) and separated from the oil pool of the fixed tank with an oil seal,

(3) For low-speed shaft (such as roller shaft, traveling axle, etc.), use end floating oil seal (Fig. 3). The end surface specific pressure is generated through the elastic deformation of the 0-ring, which makes the two floating rings close and transmit torque to compensate for wear. The oil seal is insensitive to vibration, impact and axial and radial deflection. It is especially suitable for bearing sealing at low speed (below 2m/s) and with pulverized coal slurry.

Fig. 3

1-sealing chamber    2 -O-ring,   3 - floating ring

2.5 Strict order acceptance to ensure bearing manufacturing quality

The quality of bearing itself is an important factor affecting its installation performance and service life. For example, the internal and external diameters of some bearings are out of tolerance and cannot be correctly matched with the shaft and shell holes. Some bearings have too large clearance and large radial and axial displacement after installation. It is also found that the bearing material has segregation, slag inclusion, low hardness, unqualified structure, micro cracks and other defects, resulting in premature damage of the bearing. At present, there are many domestic bearing manufacturers, and the quality difference is large and unstable. Therefore, the manufacturer shall be selected when ordering, and the quality tracking and testing shall be carried out through strict acceptance to ensure the quality requirements of the bearing.

2.6 Improve the manufacturing and installation quality of bearing components

Even though the bearing itself has high precision and excellent quality, due to the manufacturing and installation errors of its components and improper clearance adjustment, the shaft will still run unsteadily, generate additional load, and affect the normal use of the bearing.

(1) The diameter of the shaft and shell hole for installing the bearing is out of tolerance, which changes the correct matching requirements of the bearing. The large interference makes the radial clearance smaller, and the inner ring generates tensile stress. Negative interference (clearance) increases the radial clearance, reduces the rigidity of components and causes the ferrule to slide.

(2) Shell hole oval or conical error, so that the ring raceway deformation. When the rolling element passes through, the pressure in the small diameter area of the raceway increases significantly. This causes premature wear and fatigue failure of the raceway surface in this area.

(3) The shaft and shell hole retaining shoulder are not perpendicular to the mating surface, and the different shaft errors at the two sides of the mating part cause the axis of the inner and outer rings of the bearing to skew, and also increase the local surface stress. For self-aligning roller bearings, different axial degree errors can be compensated through self-aligning performance (adjustment angle 0.5 - 2). For ring column roller bearings, the allowable deflection angle of roller bearings is small (2 '~4'), and the requirements for coaxiality of shaft and shell hole are high.

(4) During bearing installation, the axial clearance must be adjusted to meet the design requirements. Especially for cylindrical roller bearings, if the axial clearance is small and the inner ring movement is blocked, it is easy to crack the retaining edge when subjected to impact load. Inadequate lubrication can also cause bearing burns.

After each transmission box of the coal mine machinery is installed, it is necessary to carry out empty running in and loading test run to check the temperature rise, operating noise and no-load power, so as to assess the installation quality of gears and bearings.

2.7 Strengthen the maintenance of bearings in use

Good maintenance can make the bearing in the best working condition, which is conducive to the improvement of bearing life. The bearings for coal mine machinery shall be well packed during storage and transportation before installation, free from collision and rust due to water immersion. During use, pay special attention to the quantity and quality of lubricating oil. Specific requirements:

(1) Check the oil level frequently and add enough oil.

(2) Avoid mixing different types of oil.

(3) During cover cleaning and oiling, impurities such as coal dust and water shall be prevented from entering, so as to prevent oil damage and intensify abrasive wear and corrosion on the friction surface. If the oil is found to be dirty, it shall be drained and cleaned in time before adding new oil.

When the machine operates for a certain period of time, there is abnormal noise and vibration. This is often due to the increase of bearing clearance caused by the looseness of bolts (nuts) or the wear of the bearing itself, which causes the axial movement or impact of the shaft and gear. It is necessary to tighten the bolts and readjust the clearance in time.

More about ITS Slewing Bearings：

ITS slewing bearings top quality brand has guaranteed production and highly active management. Slewing bearings comprise an inner ring and an outer ring, one of which usually incorporates a gear. Together with attachment holes in both rings, they enable an optimized power transmission with a simple and quick connection between adjacent machine components. The bearing raceways, in conjunction with the rolling elements and cages or spacers, are designed to accommodate loads acting singly or in combination, and in any direction.

ITS manufactures standard and customized slewing bearings in various designs, with outside diameters from 0.1 to 18 m to meet the demands of particular applications.