Bearing cleaning, overhaul and judgment

1, the cleaning of the bearing

    When disassembling the lower bearing for inspection, first record the appearance of the bearing, confirm the residual amount of the lubricant, and then wash the bearing after sampling the lubricant for inspection. As a cleaning agent, gasoline and kerosene are commonly used.

    The cleaning of the removed bearing is divided into coarse cleaning and fine fine washing. In the container, the metal mesh bottom is placed first, so that the bearing does not directly contact the container. When rough cleaning, if the bearing is rotated with dirt, it will damage the rolling surface of the bearing and should be noted. In the rough cleaning oil, use a brush to remove the grease and the adhesive. After it is almost clean, it is transferred to the fine wash.

    Fine washing is to carefully clean the bearing while rotating it in the cleaning oil. In addition, the cleaning oil should always be kept clean.

    2. Inspection and judgment of bearings

    In order to judge whether the removed bearing can be reused, check the bearing after cleaning, carefully check the raceway surface, the rolling surface, the state of the mating surface, the wear of the cage, the increase of the bearing clearance and whether there is dimensional accuracy. Falling damage, abnormality. For non-separable ball bearings, use one hand to support the inner ring and rotate the outer ring to check if it is smooth.
 
    Separate bearings such as tapered roller bearings can be inspected separately for the rolling surfaces of the rolling elements and the outer ring.

    Because large bearings cannot be rotated by hand, pay attention to the appearance of rolling elements, raceways, cages, ribs, etc. The higher the importance of bearings, the more careful inspection is required.

    The judgment of whether or not to reuse is determined in consideration of the degree of bearing damage, mechanical properties, importance, operating conditions, and the period until the next inspection. However, if there are the following defects, they can no longer be used, and new bearings must be replaced:

    (a) cracks or notches on any of the inner ring, the outer ring, the rolling elements, and the cage;
    (b) rupture on any of the ferrules and rolling elements;
    (c) Significant jams on the raceway surface, ribs and rolling elements;
    (d) the cage is worn significantly or the rivet is significantly slack;
    (e) There is rust and injury on the raceway surface and the rolling elements;
    (f) There are severe indentations and marks on the raceway surface and rolling elements;
    (g) The inner diameter of the inner ring or the outer diameter of the outer ring has obvious creep;
    (h) discoloration due to heat;
    (i) Bearings sealed with grease, the seal or dust cover is damaged.

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Bearing knowledge

1, bearing maintenance
In order to maintain the original performance of the bearing in a good condition for as long as possible, it is necessary to maintain, repair, and prevent accidents, ensure the reliability of operation, and improve productivity and economy.
Maintenance is best carried out on a regular basis in accordance with the operating standards for the mechanical operating conditions. The content includes monitoring of operating conditions, replenishment or replacement of lubricants, and periodic disassembly inspections.
As a maintenance item during operation, there are bearing rotation sound, vibration, temperature, lubricant state, and the like.
2, bearing overhaul
Cleaning of the bearing: When disassembling the lower bearing for inspection, first record the appearance of the bearing, confirm the residual amount of the lubricant, and then wash the bearing after sampling the lubricant for inspection. As a cleaning agent, a cleaning agent or kerosene is generally used.
The cleaning of the removed bearings is divided into coarse cleaning and fine cleaning. They are placed in the container respectively, and the metal mesh bottom is placed first so that the bearing does not directly contact the container. In rough cleaning, if the bearing is rotated with dirt, it will damage the rolling surface of the bearing and should be noted. In the rough cleaning oil, use a brush to remove the grease and the adhesive. After it is almost clean, it is transferred to the fine wash.

Fine washing is to carefully clean the bearing while rotating it in the cleaning oil. In addition, the cleaning oil should always be kept clean.

Bearing inspection and judgment: In order to judge whether the removed bearing can be used, it should be checked after the bearing is cleaned. Check the raceway surface, the rolling surface, the state of the mating surface, the wear of the cage, the increase of the bearing clearance, and the damage and abnormality that are irrelevant in dimensional accuracy. For non-separable miniature ball bearings, use one hand to support the inner ring horizontally and rotate the outer ring to check if it is smooth.
Separate bearings such as tapered roller bearings can be inspected separately for the rolling surfaces of the rolling elements and the outer ring.
Large bearings cannot be rotated by hand. Pay attention to the appearance of rolling elements, raceways, cages, and ribs. The higher the importance of bearings, the more careful inspection is required.
3, the choice of bearing
Summary of bearing selection:
The market requirements for various mechanical devices and instruments using rolling bearings are becoming increasingly stringent, and the conditions and performance required for bearings are becoming increasingly diverse. In order to be able to select the most suitable bearing from among a large number of structures and sizes, it is necessary to study from various angles. When selecting a bearing, generally considering the bearing arrangement as a shaft system, the ease of mounting and dismounting, the space allowed for the bearing, the size, and the marketability of the bearing, the bearing structure is roughly determined. Secondly, the bearing dimensions are determined while comparing the design life of various machines using bearings and the various endurance limits of the bearings. When selecting a bearing, it is often preferred to consider only the fatigue life of the bearing. The grease life, wear, noise, etc., which occur due to aging of the grease, also need to be fully studied. Furthermore, depending on the application, it is necessary to select a bearing that is specially designed for accuracy, clearance, cage structure, grease, and the like. However, there is no order or rule for selecting bearings. It is especially practical to consider the conditions, performance, and most relevant requirements for the bearings.

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Type of damage in plain bearings

1. Scratch (two-body abrasive wear)

The hard particles moving together with the shaft diameter are in contact with the friction surface, which is the lower contact stress between the particles and the metal surface, and they are drawn on the surface of the bearing pad; the hard particles semi-inlaid on the surface of the bearing pad are also on the surface of the shaft diameter. Lines are scarred and are called scratches. Scratch is a two-body abrasive wear, and the direction of the linear flaw is consistent with the direction of the shaft diameter.

The lubricating oil film is broken, and the hub peak on the shaft diameter surface will also scratch the bearing bush, and many linear scratches appear, which is also the two-body abrasive wear.

The hard particles are embedded in the surface of the bearing shell and fall off, causing scratches of the point-like scar.

Most of the above particles are iron and sand.

The scratching causes the surface of the friction pair to be roughened, thereby reducing the load bearing capacity of the lubricating oil film, and forming new hard particles and hub peaks that can scratch the friction surface, causing a vicious cycle.

2. (three-body) abrasive wear

The smaller hard particles entering the bearing gap migrate between the two friction surfaces, creating extremely high contact stress on the friction surface, forming a three-body abrasive wear, similar to the grinding action, causing the bearing pad and the shaft diameter surface to wear. The high contact stress between the hard particles and the friction surface causes plastic deformation or fatigue damage to the friction surface of the ductile metal, causing brittle fracture or peeling of the friction surface of the brittle metal.

The scratches of the abrasive wear are also linear, and the direction is also consistent with the direction of the shaft diameter.

When there is edge contact, lack of lubricating oil or cracking of the oil film, severe abrasive wear will occur. Abrasive wear will result in shaft diameter and/or bearing geometry and shape changes, loss of precision, and increased bearing clearance, causing the sliding bearing performance to deteriorate dramatically before the expected life.

3. Bite (gluing)

In the state where the lubricating oil film is broken or lacks oil, a large friction factor causes a large amount of frictional heat to be generated, and the bearing temperature rises. At high temperatures, the low melting point metal of one friction surface adheres to the other friction surface due to softening, and the adhered metal is detached from the original surface and transferred to the other friction surface due to the shearing action caused by the rotational movement of the shaft diameter. Causes obvious pits and raised scratches on the friction surface. This damage is adhesive wear.

When bite sticking occurs, the friction sharply increases and the bearing temperature further rises, forming a vicious circle. When the adhesion is serious, the power of the shaft diameter rotation can no longer cut the bonding point, the shaft diameter movement will be terminated, commonly known as the "holding shaft", and the bearing is completely damaged.

4. Fatigue wear

Fatigue wear is also called fatigue damage. Under the repeated action of the cyclic load, in the direction perpendicular to the sliding direction, the friction surface appears fatigue crack, and the crack develops perpendicularly to the surface of the bearing pad to the joint surface of the lining and the backing, and rotates to extend parallel to the friction surface. Eventually the material is peeled off from the friction surface, causing crater damage.

5. Stripping

When manufacturing the bearing bush, if the bonding force between the lining layer and the backing is insufficient or the bonding is poor, the material of the partial lining will be peeled off from the bearing bush under the action of the load during the operation of the bearing. Peeling is somewhat similar to fatigue spalling, but the periphery of the fatigue peeling pit is irregular, and the periphery of the peeling pit caused by poor bonding is relatively smooth. 6. Corrosion

The lubricating oil is continuously oxidized during use, and a weak organic acid is often generated during oxidation. It is corrosive to the bearing material, especially the lead of the cast copper-lead alloy, and is characterized in that the lead is spotted off and the surface is roughened.

Strong inorganic acids are more susceptible to corrosion on steel journal surfaces.

After the tin in the tin-based bearing alloy is oxidized, a black hard coating layer of SnO2 and SnO is formed on the surface of the bearing pad, and the hardness is in the range of 200 to 600HS. This cover is extremely harmful to the bearing, it is very hard, it can scratch the surface of the journal and make the bearing clearance smaller.

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Bearing storage

The bearing should firstly take into account the size and weight of the bearings to be stored, and secondly consider the necessary handling space and equipment to ensure that the bearings are not caught when stored and taken. The bearings should not be placed directly on the ground. Therefore, it is necessary to configure the appropriate Shelves, and the bottom of the shelf should be 30cm above the ground.

    In the general packaging state, depending on the storage environment, the rust prevention effect is 13 years. When it is necessary to store for nearly 10 years in special cases, it can be stored in oil such as turbine oil (as a bearing lubricant).

    (1) Operation of installing bearings

    There are many ways to install the bearing. After the same operation, the bearing is installed. After the bearing is installed, the outer ring must be inclined to apply the lubricant for the bearing. {TodayHot}

    1) Application of lubricant

    1 grease lubrication

    Coating lubrication covers all roller surfaces and restores the position of the outer ring after application.

    2 When lubricating oil

    Apply oil to all roller surfaces and restore the position of the outer ring after application.

    2) After coating, cover the dust with a plastic film or the like.

    (2) Precautions for disassembling the bearing

    The disassembly of the bearing is carried out during regular maintenance or when the bearing needs to be replaced due to abnormal operation. When it is necessary to replace the bearing with frequent bearing maintenance, it is necessary to understand the cause of the abnormality when disassembling the bearing to prevent the occurrence of abnormality. At least keep the following items.

    1 collect and retain used lubricant

    2 retaining damaged bearings

    3 Keep records of abnormal events during operation

    4 Keep track of bearing conditions when abnormal operation

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