Damage classification of rolling bearings

1. Fatigue spalling Fatigue spalling is the flake like spalling on the working surface of a bearing caused by rolling fatigue.

When the rolling bearing works, the load on the rolling element and inner ring (or outer ring) is not constant, but changes repeatedly with the operation of the bearing. This load is an alternating load; Moreover, because the contact area between the rolling element and the inner ring (or outer ring) is very small, the contact stress is very large. At a higher speed, the metal will fatigue under the repeated action of the huge alternating contact stress. At the beginning, tiny cracks are visible at the parts where the stress is concentrated. Due to the action of lubricating oil and metal corrosion, the cracks gradually expand. When they expand to a certain extent, the surface metal will peel off, forming small pits, similar to pits. When we remove and inspect the bearings, we often find pits and pieces of metal peeling on the surface of the inner or outer rings and rolling bodies, which is the phenomenon of metal fatigue peeling.

If the fatigue peeling of rolling bearing occurs after the bearing has been running for a long time, it should be considered as natural damage. If it occurs in the middle or early period of the bearing's use, it should be considered as unnatural peeling. At this time, the reason for peeling should be found out.

2. Non natural spalling Non natural spalling is caused by human factors and occurs in the middle or early period of bearing use.

The main reason for unnatural peeling is due to improper selection of bearing type or improper installation.

(1) Spalling caused by improper selection of bearing type. Early peeling may occur if deep groove ball bearing is selected under large axial load. The spalling mainly occurs in the raceway, which makes the rolling track of the rolling element move towards the ball loading port, even roll over the ball loading port, and finally causes serious spalling damage to the raceway. The measure to prevent such damage is to select bearings reasonably according to their working conditions.

(2) Spalling due to improper installation

1) During installation, the shoulder of the shaft and housing is not perpendicular to the mating surface, or the inner and outer rings are skewed due to the bending of the shaft, which will cause excessive contact stress on the rolling surface, resulting in peeling.

2) The outer ring is installed in an oval or tapered seating hole. When the rolling element passes through the irregular raceway, it will increase the contact stress, resulting in the early peeling of the raceway and rolling element.

3) During the installation of cylindrical roller bearing, the inner or outer ring will be injured due to the installation deflection and excessive force when knocking.

4) When the deep groove ball bearing is installed with interference on the shaft, if the applied pressure is transmitted through the outer ring and the steel ball, it will inevitably cause pits where the raceway contacts the steel ball. These pits are the source of peeling, and the rapid development will soon make the bearing scrapped. Therefore, when installing the inner or outer ring with interference, force can only be applied to the end face of the inner or outer ring to be installed.

5) In addition, if the bearing is installed too tight, too loose or adjusted improperly, it will also cause early peeling of the bearing.

3. When the worn rolling bearing works, the main movement between the rolling element and the raceway is rolling, with little sliding. Therefore, under normal conditions, the wear of bearing parts is not serious. However, when impurities invade or the lubrication is poor, the wear will accelerate rapidly.

(1) Impurities intrusion Impurities particles (sand, dirt, etc.) mixed in the lubricating oil invade the bearing, and abrasive wear will occur on the raceway and rolling body. In this case, no metal peeling can be seen, but uneven scratches can be seen on the working surface. Due to abrasive wear, the clearance between the inner or outer ring and the rolling element increases significantly, the raceway loses luster, and uneven scratches appear.

(2) Insufficient or deteriorated lubricating oil When the lubricating oil is insufficient or deteriorated, the bearing lubrication cannot be guaranteed, which will lead to serious consequences. As the heat generated in the bearing operation cannot be taken away by the lubricating oil, the bearing will lose its original mechanical properties due to overheating, and its hardness will be greatly reduced, so the wear is extremely rapid, and the normal operation of the bearing will be damaged.

(3) It should be pointed out that the severe wear of bearings is mainly caused by improper use. Therefore, it is necessary to ensure the reliability of the installation of the bearing sealing device during the use of the bearing, systematically monitor the working condition and lubrication condition of the bearing, and ensure that the bearing works in a good lubrication condition.

4. Cracking There are two cases of rolling bearing cracking, one is caused by fatigue, the other is caused by improper installation.

(1) The rolling bearing parts bear great alternating load due to fatigue. Due to material defects or other reasons of the bearing, stress concentration occurs at a certain part of the bearing. When the stress is greater than the strength limit, small cracks will occur there. The continuous action of alternating load and the oil wedge action of lubricating oil cause the crack to expand continuously. When the crack expands to a certain extent, it will break.

(2) The more common cause of bearing fracture caused by human factors is human factors. For example, hard knocks during disassembly and assembly, as well as the intrusion of hard particles and impurities into the bearing interior, may cause fractures of inner and outer rings and cages. The clearance of tapered roller bearing is adjusted too tightly, which usually breaks at the big end of tapered roller; If the clearance is adjusted too much, it usually breaks at the small end of the tapered roller.

5. Corrosion is the phenomenon that dark black spots and canker are formed on the surface of bearing parts. The main causes of corrosion are:

(1) Moisture or moisture is immersed in the bearing.

(2) The use of lubricants with poor properties can also cause corrosion.

(3) Another kind of corrosion is called contact corrosion, which mainly occurs on the contact surface of the non rotating inner and outer rings and rolling bodies.

6. In addition to the above-mentioned damages, there are other damages.

(1) Spark melting If the current passes through the bearing, at the contact point between the raceway and the rolling element, the current passes through a very thin oil film and causes sparks, resulting in local surface melting and crater on the raceway. These pits gradually expand due to wear until the entire bearing is damaged.

(2) Sliding, oxidation and grinding When the bearing does not rotate and is subject to vibration from nearby punches and air compressors or the protective measures are not good during transportation, relatively repeated small sliding occurs between the rolling body and the inner or outer ring, causing some small particles to fall off from the surface. Once oxidized, these small particles can become abrasives to accelerate the wear of contact surfaces. It will increase the bearing noise and affect the normal operation of the equipment.

(3) Unreasonable design and unreasonable design structure cause bearing damage; If the outer ring is fastened with set screws on the bearing seat or the local stress concentration is caused by grinding grooves on the bearing, the raceway will be broken.

(4) Cage damage Cage can not be used due to wear, breakage and breakage.

(5) Residual magnetic adsorption The residual magnetic causes the adsorption of metal debris, which causes damage to the shaft.