The selection, installation and lubrication of sliding bearings should be strictly controlled to prolong their service life and ensure the normal operation of the equipment especially in farm machinery.
Abstract: The bearing mainly uses for bearing and on the axis the components, and the maintenance axis revolves between the precision and the reduced revolution axis and the supporting friction and the attrition. Therefore, has discussed the guard measure which the sliding bearing principle of work, the common breakdown type and adopts, carries on the routine maintenance in view of the breakdown reason to the sliding bearing, for impels the bearing provides the reference in farm machinery reasonable use and the maintenance .
Key words: machine design; sliding bearing; analysis; farm machinery; use; maintenance
0. Introduction
Bearing is mainly used to support the shaft and its parts, maintain the rotation accuracy of the shaft and reduce the friction and wear between the shaft and the support. It is widely used in agricultural machinery. Because agricultural machinery often works in the harsh environment of heavy load, vibration and impact, the rational use and maintenance of bearings is very important. The sliding bearing has the advantages of large bearing capacity, reliable operation, impact resistance, vibration absorption and splitting. It is very suitable for agricultural machinery operation with high speed, heavy load and huge impact and vibration.
Due to the wide application of sliding bearing, its failure has also become a common mechanical failure in agricultural machinery operation. Mastering the working principle of sliding bearing, understanding the common fault types and fault causes, taking specific measures to eliminate the fault causes, and even carrying out daily maintenance of sliding bearing according to the working principle of sliding bearing to prevent faults are the technical objectives that mechanical maintenance personnel must achieve.
1. Structure and working principle of sliding bearing
The sliding bearing is composed of bearing cover, bearing seat, bearing bush, connecting bolt, etc. Bearing bush is an important part of sliding bearing, which is used to directly support the journal. On the inner surface of the bearing bush, an oil groove is opened longitudinally, obliquely or transversely with the oil inlet as the center, so as to facilitate the uniform distribution of lubricating oil on the whole journal.
The sliding bearing works through the movement between the inner surfaces of the bearing bush, and the sliding friction generated by the movement will hinder the movement of the sliding bearing. If a lubricating film can be formed on the surface of the sliding bearing to separate the surfaces of the moving pair, the sliding friction can be greatly reduced. Since the surfaces of the moving pair are not in direct contact, wear is also avoided. Therefore, the formation of lubricating film is the basic condition for the normal operation of sliding bearing. The factors affecting the formation of lubricating film include lubrication mode, relative motion speed of moving pair, physical properties of lubricant and surface roughness of moving pair. In the application of sliding bearing, maintaining good lubrication can not only reduce friction power consumption and wear, but also play the role of cooling, vibration absorption, rust prevention and so on.
2. Failure mode of sliding bearing
In the process of normal use, the natural damage of sliding bearing is inevitable because of gradual wear until it finally loses its working capacity and ends its service life. However, if the bearing is prematurely worn or has various faults due to improper assembly and adjustment, poor lubricating oil quality or poor service conditions, it is an early damage caused by man-made. Early damage not only greatly reduces the service life of bearings, but also affects the normal operation of agricultural machinery. According to experience, the common forms of early damage of sliding bearing include mechanical damage, bearing pitting corrosion, fatigue pitting corrosion, bearing alloy corrosion, bearing melting, bearing walking on the outer circle, etc.
2.1 Mechanical damage
Mechanical damage of sliding bearing refers to different degrees of groove marks on the alloy surface of bearing bush. In serious cases, metal peeling and large-area random scratches occur on the contact surface. Generally, the contact surface damage and ablation exist at the same time. The main reason for the damage is that it is difficult to form oil film on the bearing surface or the oil film is seriously damaged.
2.2 Bearing cavitation
Under the repeated action of impact pressure, the surface layer of sliding bearing will undergo plastic deformation and cold work hardening, lose the deformation ability locally, gradually form cracks and expand continuously, and then form pitting corrosion in the loaded surface layer with the falling off of wear debris. In general, when the bearing bush is pitted, the pit first appears, and then the pit gradually expands and causes the cracking of the alloy layer interface. The crack extends along the parallel direction of the interface until it peels off. The main reason for the cavitation of sliding bearing is that the sudden change of the cross section of structural elements such as oil groove and oil hole causes the strong disorder of oil flow, and bubbles are formed in the vacuum area with disordered oil flow. Then cavitation occurs due to the collapse of bubbles due to the increase of pressure. Cavitation usually occurs in the high load area of the bearing, that is, the lower bearing bush of the bearing. Bearing cavitation often occurs in sliding bearings with heavy load, high speed and large changes in load and speed.
2.3 Fatigue pitting
Bearing fatigue pitting is caused by overload work, overheating of bearing and excessive bearing clearance, resulting in fatigue damage, fatigue pitting or fatigue falling off in the middle of bearing. This damage is mostly caused by overload, excessive bearing clearance, or unclean lubricating oil mixed with foreign matters. Therefore, during use, attention should be paid to avoid bearing overload, do not operate at too low or too high speed, and ensure normal bearing clearance.
2.4 Bearing alloy corrosion
The corrosion of bearing alloy is generally due to the impure lubricating oil. The chemical impurities contained in the lubricating oil oxidize the bearing alloy to form acidic substances, causing part of the bearing alloy to fall off and form irregular small cracks or pits. The main causes of bearing alloy corrosion are improper selection of lubricating oil and poor corrosion resistance of bearing materials.
2.5 Bearing melting
There are tiny convex metal surfaces between the journal and the bearing friction pair, which form local high temperature. In the case of insufficient lubrication and poor cooling, the bearing alloy will be blackened or locally burned. This fault is often caused by too tight fit between journal and bearing; Insufficient lubricating oil pressure is also easy to burn the bearing.
2.6 Bearing outer circle
The outer circle of the bearing means that the bearing has relative rotation in the seat hole. After the bearing goes to the outer circle, it will not only affect the heat dissipation of the bearing, which is easy to ablate the alloy on the inner surface of the bearing, but also damage the back of the bearing and burn the bearing in serious cases. The main reason is that the bearing is too short and the processing or installation does not meet the specifications.
3. Reasonable use and maintenance of sliding bearing
The early damage of sliding bearing is much more common than the burning of bearing, so it is very important to prevent the early damage of sliding bearing. The correct maintenance of sliding bearing is not only an effective way to reduce the early damage of bearing, but also a reliable guarantee to prolong the service life of bearing. Therefore, in the daily maintenance and repair of agricultural machinery, we must start with the reasonable selection of bearing alloy materials, improving the maintenance and assembly quality of bearings, reasonably selecting and filling lubricating oil and improving the working conditions of bearings. Once the sliding bearing has abnormal phenomena or signs of premature wear, it is necessary to carefully find out the causes, take corresponding measures, and pay attention to the prevention of early damage of sliding bearing.
3.1 Reasonable selection of sliding bearing
When selecting sliding bearing, the heat balance of bearing should be considered to control the temperature rise. The oil inlet hole shall be opened from the top of the upper bearing bush of the bearing to introduce the lubricating oil from the non-bearing area; An oil groove is opened longitudinally or transversely on the inner surface of the bearing bush with the oil inlet hole as the center, which is conducive to the uniform distribution of lubricating oil on the journal to control the temperature rise. The oil groove is generally opened by experience, but the main principles are as follows: first, the lubricating oil should be input into the bearing from the place with the minimum oil film pressure; Second, the oil groove is opened in the non-bearing area, otherwise the bearing capacity of the oil film will be reduced; Third, the oil groove cannot be opened axially to avoid a large amount of oil loss from the end of the oil groove; Fourth, the oil groove of the horizontally installed bearing shall be opened for half a circle and shall not extend to the bearing area. The full circle oil groove shall be opened near the end of the bearing.
In addition, an oil inlet hole is opened at the top of the bearing bush. The oil groove can be centered around the oil inlet hole, and an oil groove with the same width as the hole diameter can be opened. The shape can take “一” “ ×”, “王”, etc.
In addition to ensuring the reasonable structure of the oil groove of the sliding bearing, the material of the bearing should also be selected reasonably according to the working conditions of the sliding bearing. Bearing materials must have low friction coefficient, good thermal conductivity, low-coefficient of thermal expansion, strong wear resistance, corrosion resistance and gluing resistance, and sufficient mechanical strength and plasticity. Therefore, Babbitt alloy is often selected as the bearing material.
3.2 Improve the maintenance and assembly quality of bearings
The maintenance and assembly quality of sliding bearing plays a vital role in giving full play to the performance of bearing. During the maintenance and assembly of the bearing, ensure that the back of the bearing is smooth without spots and the positioning convex points are intact; The spring opening is generally controlled within the range of 0.5 ~ 1.5mm, which can ensure that the bearing bush fits closely with the bearing seat hole with the help of its own elastic force after assembly. Each end of the upper and lower bearing shells installed in the bearing seat shall be 30 ~ 50 µm higher than the plane of the bearing seat. The higher amount can ensure that the bearing fits closely with the bearing seat after tightening the bearing cover bolts according to the specified torque, so as to produce sufficient friction self-locking force, prevent the bearing from loosening, have good heat dissipation effect and prevent bearing ablation and wear. The working surface of the bearing cannot be measured by 75% ~ 85% contact marks by scraping and matching method. The matching clearance between the bearing and journal shall meet the requirements without scraping.
In addition, the maintenance and assembly shall be carried out in strict accordance with the process specifications to prevent improper installation and uneven or non-conforming torque of bearing bolts due to improper installation methods, resulting in bending deformation and stress concentration, resulting in early damage to the bearing.
3.3 Reasonably select and fill lubricating oil
The lubrication quality of sliding bearing is another important factor affecting its performance. When selecting and using lubricating oil, the lubricating oil with low surface tension of oil film should be used. When the formed bubbles collapse, the impact of oil flow will be reduced accordingly, which can effectively prevent bearing cavitation. The viscosity grade of lubricating oil shall not be increased at will to avoid increasing the coking tendency of bearing. The quantity of lubricating oil must be within the standard range, the lubricating oil and refueling tools must be clean, and the place where lubricating oil is filled should be free of pollution and sand, so as to prevent the invasion of all pollutants. At the same time, take sealing measures and pay attention to regular inspection and replacement of lubricating oil. It is forbidden to mix lubricating oils of different qualities, viscosity grades and types of use. The precipitation time before lubricating oil filling shall generally not be less than 48h.
There are many lubrication methods for sliding bearings, which should be selected according to the specific structure and working conditions of the machine. The specific lubrication methods include:
1) Manual lubrication. When it is found that the lubricating oil of the bearing is insufficient, the oiler is usually used to inject oil. This method is difficult to maintain a certain amount of oil, and is usually only used for bearings with light load, low speed or intermittent movement. In order to prevent dirt from mixing in, it is best to set a dust cover or ball valve on the oil filling hole, and use felt, cotton, wool, etc. as the filtering device.
2) Drip oil for lubrication. Roughly quantitative lubricating oil is supplied from the container through holes, needles, valves, etc. the most classic is the oil drip cup. The amount of oil dripping varies significantly with the viscosity of lubricating oil, bearing clearance and the position of oil supply hole. This method is commonly used for light load and medium load bearings with circumferential speed less than 4 ~ 5m / s.
3) Oil ring lubrication. It can only be used for the lubrication method of horizontal shaft. The lubricating oil in the oil pool is brought into the bearing by the ring hanging on the shaft and rotating. Suitable for medium and high speed bearings with shaft diameter greater than 50mm. The oil ring is preferably seamless. When the width diameter ratio of the bearing is less than 2, only one oil ring can be used, otherwise two oil rings are required.
4) Lubricate the oil rope. The lubricating oil in the oil cup is led into the bearing by the capillary action and siphon action of the oil rope. It is used for light load and medium load bearings with circumferential speed less than 4 ~ 5m / s. The oil rope also has the function of filtration.
5) Oil pad lubrication. Use the capillary action of the oil pad to apply the lubricating oil in the oil pool to the surface of the shaft diameter. This method can keep the friction surface clean, but the dust will also block the pores and cause insufficient oil supply. The oil supply of oil pad lubrication is usually only 1 / 20 of that of oil lubrication.
6) Oil bath lubrication. The lubrication method of immersing a part of the bearing in lubricating oil. This method is often used in vertical shaft bearings rather than horizontal shaft bearings.
7) Splash bearing. The lubricating oil splashed by the tapping of the rotating parts in the oil tank is supplied to the bearing, which is suitable for bearings with higher speed.
8) Spray lubrication. The lubrication method of atomizing and spraying lubricating oil on the friction surface is suitable for high-speed bearings.
9) Pressure supply lubrication. Supplying oil to the bearing by the pressure of the lubrication pump and recovering the lubricating oil from the bearing to the oil pool for recycling is the most stable lubrication method with the largest amount of oil supply. It is suitable for high-speed, heavy-duty and important sliding bearings.
4. Conclusion
Sliding bearing is an important part of agricultural machinery. Whether its use and maintenance are reasonable or not will directly affect its performance and play a vital role in the normal operation of agricultural machinery. Therefore, the selection, installation and lubrication of sliding bearings should be strictly controlled to prolong their service life and ensure the normal operation of the equipment.
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