Why is the bearing damaged?

1. The main reasons for bearing damage are:

① Material fatigue; ② Poor lubrication; ③ Pollution; ④ Installation problems; ⑤ Improper handling.

Generally speaking, one-third of bearing damage is caused by material fatigue; one-third is due to poor lubrication; and the other third is due to pollutants entering the bearing or improper installation and handling.

2. The beginning of bearing damage:

The length of time from the beginning of the bearing to the appearance of the first material fatigue point is related to the number of revolutions, load size, lubrication and cleanliness of the bearing during this period. Fatigue is the result of the periodic appearance of shear stress under the loaded surface. After a period of time, tiny cracks will be initiated and then gradually extend to the surface. When the rolling element passes through the small area formed by these cracks, some cracks begin to fall off, forming the so-called peeling phenomenon. As the peeling continues to expand, the bearing is damaged and cannot be used. Initially occurs under the surface. Although the initial peeling is usually very slight, as the stress increases and cracks increase, the peeling area spreads. This process usually lasts for a long time with obvious vibration and noise. Therefore, there should be enough time to replace it before it is broken.

3. The bearing force traces

Generally, after a bearing has been running for a period of time, there will be obvious stress marks on the working surface. Not all marks indicate that the bearing is broken. The bearing will also leave stress marks when used under normal conditions.

The bearing runs under load, and the contact surface of its raceway is dull and black in appearance. But this does not mean wear and tear, and it has nothing to do with life. The trace structure of this black hair

It becomes a force trace of the bearing. This trace varies with its operation and load conditions, and its appearance is also different. Careful inspection can help us determine whether the bearing is operating under normal conditions.

Radial bearings, ball bearings, spheres, and double-row balls only bear radial force and can bear a small amount of axial load. Often the damage is caused by excessive axial force.

(1) Unidirectional radial load: two situations, one is the inner ring rotation; the other is the inner ring fixed.

(2) One-way axial load: inner ring or outer ring rotates.

(3) Combination of unidirectional radial load and axial load.

(4) The radial load rotates synchronously with the inner ring, and the outer ring is fixed.

(5) The radial load rotates synchronously with the outer ring, and the inner ring is fixed.

4. Common forms and causes of bearing damage:

1) The raceway surface and the roller surface are full of dents, the particles on the cage and the raceway surface are worn, and the grease (agent) is discolored. Usually this situation is caused by the unclean installation process.

Keep it clean during installation, use new grease, and check whether the seal is intact.

2) Wear caused by improper lubrication

The abrasion of the surface is mirror-like, and the color is blue or brown. This situation is caused by insufficient lubrication. The lubrication condition should be improved and the lubrication period and oil seal should be re-determined.

3) Dent caused by improper installation

The working surfaces of the inner and outer rings have dents with a pitch equal to the distance between the rollers. The reason is that it is not knocked on the correct ring during installation, or is pushed excessively on the conical shaft, or is overloaded in a static state.

4) Dents caused by foreign objects

There are dents on the work surface and roller surface, which may be caused by foreign matter or lubricant foreign matter and the surrounding environment during installation. The bearing is installed clean, use clean lubricant and check the oil seal.

5) Scratches on the roller end

The roller end surface rubs against the guide ribs, causing nicks and discoloration. This situation is caused by sliding under excessive axial load or insufficient lubrication. For this type of damage, a lubricant with a higher viscosity can be selected.

6) Abrasion of roller and raceway

At the beginning of the load zone of the raceway, there is abrasion and partial discoloration with the roller, which is caused by the sudden acceleration of the roller entering the load zone. There are two possible ways: one is to choose a higher viscosity lubricant; the other is to reduce the bearing clearance.

7) Abrasion on the outer surface

The inner hole of the inner ring and the outer surface of the outer ring have nicks and partial discoloration. This situation is caused by the relative movement of the ring and the shaft or the bearing housing.

The only solution is to increase the interference between the ring and the shaft or bearing box to prevent mutual rotation. Axial braking or clamping cannot solve such problems.

8) Surface pit marks

Small and shallow pit marks on the surface of raceway, rolling element or large end face are crystalline and strong. This is due to poor lubrication. For example; less oil or viscosity change caused by temperature rise, so that the oil film cannot separate the contact surface, and the surface has instant contact. Method: Improve lubrication.

9) Fretting corrosion

The phenomenon occurs when there is relative movement between the bearing ring and the shaft or the bearing box. This is due to too loose fit or deformation of the bearing seat.

10) Current corrosion

There are dark brown or gray-black straight streaks or pits on the surface of the raceway or rolling element. When current flows through the bearing, the surface of the bearing parts will be welded. Solution: Prevent current from flowing through the bearing.

11) Spalling at symmetrical positions on the raceway surface

In the two rings, there are obvious stress marks in radially symmetrical positions of the rings, and the skin peels off. This is due to the deformation and elliptical compression of the bearing box. The bearing box is remanufactured.

12) Spalling caused by axial load

The force traces are obvious, and the skin of one side of the ring or a raceway of the double-row bearing is peeled off.

the reason:

a. Axial load caused by incorrect installation;

b. Excessive pressure;

c. The non-fixed bearing is stuck or the reserved axial displacement is insufficient.

13) Peeling caused by imprints

The surface of the raceway is peeled off, and there are marks equal to the distance between the rollers. This kind of bearing is excessively loaded at rest caused by incorrect installation. Other minor impressions may be foreign matter brought in during installation or foreign matter in the lubricant.

14) Cracks caused by brutal installation

This crack is a cracked gap, usually only on one side. This is due to the brutal beating that transmits force to the end face of the ring through the rolling elements to form an impact force. Do not strike the bearing ring directly.

15) Cracks caused by excessive extrusion

The crack passes through the full section. This is caused by too large interference fit of the bearing inner ring or excessive advancement on the conical shaft.

16) Cracks caused by fretting corrosion

It is the transverse direction on the inner ring and the circumferential direction on the outer ring. It is caused by too loose fit or poor shape of the bearing housing.

17) Peeling caused by fretting corrosion

The raceway surface of the bearing ring is peeled off. There is corrosion on the outer surface peeled off here. It is caused by too loose fit or incorrect shape of the bearing housing.

5. Damage to the cage

Damage to the cage is usually not easy to judge. If the cage is broken, other parts of the bearing are also broken, which makes it difficult to analyze the cause. The main reasons for frame damage are as follows: vibration, high speed and wear, jamming and skew.

1) Vibration: When the bearing is under vibration, the force inside the bearing may cause fatigue cracks in the cage, which will gradually break the cage.

2) Too high speed: If the bearing runs at a speed beyond the capacity of the cage material, the inertial force will break the cage.

3) Wear: Improper lubrication or hard foreign matter can cause the cage to wear. Because the frame is made of soft materials, there is sliding friction between the frame and the rolling elements during operation. Once the lubrication is poor, the frame wears quickly. Gradually, the pockets of the frame will continue to expand due to wear and tear, so impact force is generated to quickly break the frame.

4) Jamming: Broken objects or hard particles of parts may be stuck between the frame and the rolling elements, hindering the rolling of the rolling elements, and causing the frame to break.

Bearing temperature rise

The friction loss of the bearing almost becomes heat inside the bearing, which causes the temperature of the bearing to rise. The heat generated by friction torque can be expressed by the following formula:

If the heat generation and the heat rejection are balanced, the bearing temperature is stable. In normal operation, the temperature rises sharply at the beginning, but it is basically stable when it reaches the normal state. The time until it reaches a stable state, and the temperature varies depending on the amount of heat generated, the heat capacity of the bearing housing, the cooling area, the amount of lubricating oil, and the surrounding temperature. If it is always unstable and cannot reach a stable state, it can only be judged as some kind of abnormality.

The reasons for the abnormal temperature rise are: bearing torsion (torque load); too small clearance, too large preload, too much or insufficient lubricant; mixing of foreign matter and heating of the sealing device, etc.

Friction: The friction coefficient of rolling bearings is based on the bearing inner diameter, which can be expressed as:

Where:

μ: coefficient of friction

M: Friction torque N.mm

P: Load N

d: bearing inner diameter mm

Elimination of common bearing problems

If the bearing is not running normally, it usually shows many symptoms, which can be roughly divided into seven categories:

A: The bearing is overheated;

B: Loud noise;

C: vibration;

D: The mechanical properties are not satisfactory;

E: The bearing is loose on the shaft;

F: High frequency of replacement;

G: The shaft is difficult to rotate.

With different degrees of bearing damage, it is often the result of secondary damage. To effectively eliminate these bearing problems, you must first observe the application of these bearings, and then analyze the causes of these damages.

1. Bearing overheating:

the reason:

1. Lubricating grease or oil is invalid or incorrectly selected. The corresponding method is: select the correct grease or oil, and check the compatibility of the grease or oil.

2. The oil level is too low, the lubricant is lost from the oil seal, and the grease in the bearing box is insufficient. Countermeasure: The oil level should be slightly lower than the center of the bottom rolling element, and the grease in the bearing box should fill about 1/3 to 1/2 of the space.

3. The oil level is too high or the grease in the bearing box is completely filled, which will cause the lubricant to be fully stirred and cause high temperature or oil leakage. Countermeasures: add grease to the box to 1/2; if the oil is lubricated, the oil level is slightly lower than the center of the lowest rolling element.

4. Inappropriate bearing clearance, when there is heat flow through the shaft, the inner ring will expand excessively. Countermeasure: Check whether the gap of the overheated bearing is within the original design range. If it is, please use a larger gap and change it to C3 or C3 to C4.

5. The contact type (friction) oil seal is too dry or the spring is too tight. Countermeasures: Replace the contact type oil seal and lubricate its oil seal surface.

6. The inner hole of the bearing box is not round, the bearing box is deformed, the supporting surface is not flat, and the inner diameter of the box hole is too small. Countermeasures: Check the bearing box and inner hole, and adjust the base plate to be evenly distributed.

7. The rotating oil seal rubs against the gland, or the shaft shoulder rubs against the bearing seal cover. Countermeasures: Check the operating clearance of the rotating oil seal to avoid friction and prevent misalignment.

8. Cross positioning or there are two positioning bearings on the shaft, the internal clearance of the bearing is insufficient due to excessive axial expansion.

Countermeasures:

1) Insert an adjusting piece between the bearing box and the flange of the end cover to release the axial preload of the bearing.

2) Move the end cover of any bearing box to the outside, use the adjusting piece to obtain the gap between the bearing box and the outer ring, and if possible, apply axial spring force on the outer ring to reduce the axial float of the shaft .

9. The adapter sleeve is too tightly locked: loosen the fixed nut and sleeve and re-lock to ensure that the bearing can rotate freely.

10. When the shafts with two or more bearings are coupled, incorrect linear deviation or angular skew occurs. Countermeasure: Adjust the correct alignment by the adjusting piece to ensure that the shaft is coupled in a straight line, especially when there are three or more bearings running on the shaft at the same time, pay more attention.

11. The shaft diameter is too large, resulting in excessive expansion of the inner ring and reducing the bearing clearance.

Countermeasures:

1) Grind the shaft diameter to obtain a proper fit between the shaft and the inner ring of the bearing.

2) Switch to bearings with large radial clearance.

12. Because the material of the bearing box hole is too soft, the hole diameter becomes larger after being stressed, which causes the outer ring to slip in the box hole. Countermeasure: Improve the box material or process a steel bushing to squeeze into the box hole, and then process the inner hole of the bushing to the correct size.

2. Noise

1. Lubricating grease or engine oil is invalid, and the lubricant model is not selected properly. Countermeasures: Choosing appropriate lubricants should confirm their compatibility.

2. The oil level is too low or the bearing box grease is insufficient. Countermeasures: The oil level should be slightly lower than the center of the lowest rolling element, and the bearing box should be filled with grease 1/3 to 1/2 of the space.

3. Inappropriate internal clearance of the bearing, excessive locking of the tightening sleeve, too large shaft diameter and too large interference in the bearing inner hole, etc. can all cause the bearing clearance to decrease. When heat flows through the shaft surface, the inner ring will expand excessively. .

Countermeasures:

1) Check whether the overheated bearing clearance is consistent with the original design. If it is still, you can use a larger clearance. For example, change from "0" group to "C3" from "C3" to "C4"

.

2) Improve the locking amount of the tightening sleeve, detect the radial clearance, and determine the proper position.

3) Adjust the matching relationship between the shaft and the inner hole of the bearing.

4. Dirt, sand dust or other pollutants enter the bearing box. Countermeasures: Clean the bearing box, replace the new oil seal or improve the oil seal design.

5. Water, acid, paint or other corrosive substances enter the bearing box. Countermeasures: Install a certain protective anti-pressure cover or throw-off ring to prevent foreign objects from entering the bearing.

6. The inner hole of the bearing box is not round, twisted and deformed, and the inner hole is too small. Countermeasures: Check whether the bearing box squeezes the bearing and whether the hole size is correct.

7. The debris and foreign matter in the bearing box before installing the bearing have not been cleaned up. Countermeasure: Carefully clean the bearing housing and the bearing itself.

8. (Cross positioning) There are two positioning bearings on the same shaft, causing misalignment or insufficient internal clearance of the bearing due to shaft thermal expansion. Countermeasures: Adjust the adjusting shim between the bearing box and the end cover, so that there is a certain gap between the bearing box and the outer ring.

9. The shaft shoulder rubs against the bearing seal cover, and the shaft shoulder diameter is incorrect and rubs with the cage. Countermeasures: Rework the shaft shoulder, check the diameter and fillet of the shoulder.

10. The outer ring and the bearing housing are twisted, and the shaft and inner ring are twisted. The reason is that the rounded corners of the box hole are too large; there is not enough support. The shoulder fillet is too large, there is not enough support, and the two ends are not reliable. Countermeasures: Rework the fillet of the box hole and the shoulder.

11. Incorrect installation method, directly hitting the bearing with a hammer, resulting in a knock on the working surface of the bearing. Countermeasures: Choose the correct installation method: sleeve method, heating method, oil pressure method, etc.

12. The sun sheet (locking washer) of the fixed washer rubs against the bearing. Countermeasure: knock down the sun sheet or replace it with a new one.

13. The rotating parts in the equipment interfere with the stationary parts. Countermeasures: Check carefully to avoid interference.

14. Excessive bearing clearance causes vibration.

15. Equipment vibration, check the balance of the rotating parts of the equipment and correct it.

Three, vibration

1. Dirt, foreign matter, sand or other pollutants enter the box. Countermeasure: Clean the bearing box and replace the new oil seal.

2. Water, acid, paint or other corrosive substances enter the bearing box. Countermeasure: Install a protective dust cover and improve the oil seal.

3. The inner hole of the bearing box is not round, twisted and deformed, and the supporting surface is not flat. Countermeasures: Check the bearing box, adjust the supporting surface, and adjust the gasket.

4. The shaft diameter is small or the adapter sleeve is not locked. Countermeasure: Check the shaft diameter, select the appropriate amount of fit, and re-tighten the adapter sleeve.

5. Unbalanced load, large box hole clearance, and outer ring slips in the box hole. Countermeasure: Replace the bearing housing with the bore diameter that meets the design requirements.

6. Two or more bearings are coupled to produce axis linear deviation and angular deviation. Countermeasure: Readjust the gasket so that the shaft is coupled on the same straight line.

7. Incorrect installation, directly hit the bearing with a hammer.

8. The bearing clearance is too large.

9. Equipment vibration.

Fourth, the mechanical cannot meet the design requirements

1. The internal clearance of the bearing is not appropriate, and the bearing preload is not enough: select the appropriate bearing.

2. Debris in the bearing box,

Dirt is not cleaned up, or dirt, water, or acid pollutants enter the cabinet during operation.

3. The adapter sleeve is too tightly locked or not locked.

4. The accuracy of the bore diameter of the bearing box is not high, and the box hole is deformed and twisted.

5. The shaft diameter is too small and the shaft shoulder size does not match, which interferes with the bearing.

6. The internal clearance of the bearing is too small due to excessive shaft diameter or box hole.

7. Multiple bearings are coupled, resulting in misalignment.

5. Difficulty in axis rotation

1. The choice of lubrication failure, oil level and bearing clearance.

2. Cleanliness, debris.

3. Friction: oil seal, shaft shoulder and seal, anti-loosening sheet (solar sheet).

4. The concentricity of the shaft is not right, and the axial jam caused by cross positioning.

5. The bearing box has low accuracy and distortion.

Six, the bearing is loose on the shaft

1. The shaft diameter is small and the bearing bore is large.

2. Insufficient interference.

3. The adapter sleeve is not locked enough.