Bearing is widely used in equipment and its service life is very important. According to statistics, the failure of rotating machinery in the bearing damage fault accounted for about 30%, and these causes are very simple, such as the derailment of a freight train, according to the investigation found that a bearing in the axle box overheated), resulting in axle fracture, causing the carriage overturned, traffic disruption for several hours, the accident loss is huge.

Analysis on the root cause of bearing fault damage:

Bearing is the core part of most equipment. Once the bearing fails, the equipment will stop. It is very important to find the root cause to avoid the accident. In fact, once the bearing is damaged, it gradually deteriorates until it becomes inoperative, at which point all evidence of the fault may be destroyed.

The whole process of damage from small to large is revealed, in which external particles (pollutants) enter the bearing and create dents in the movement (pollutants are pressed into the racetrack). The edge of the dent will rise, so the dent will look like a pit. The local geometry of the raceway is changed, so that this area cannot form a good oil film to separate the contact surface, resulting in material fatigue. It starts with surface cracks, ends with the damaged area peeling off, and as it continues to operate, the peeling gets worse. Later, the damage area becomes so large that the initial damage point, or dent, disappears completely. At this point, the inspection of the damaged bearing, it has been difficult to find the root cause of the problem: that may be due to poor sealing of pollutants into the bearing.

Each fault has its own characteristics, and it is possible to determine the source of the fault by observing the damage condition.

1, wear and tear

Abrasion is one of the most common faults of rolling bearings. It is usually caused by grinding damage, scratch, groove mark, circumferential line mark or debris pollution caused by foreign matter. There are two types of abrasion and viscous wear.

Wear and tear is due to small foreign matter inside the bearing, which may be pollution particles from outside or inside, such as metal particles or carbides from the gear. These particles will wear or grind the raceway and rolling body. In tapered roller bearings, the roller end face and inner ring flank are far more severely worn than the raceway. Such wear will cause the axial clearance or internal clearance to expand, thus reducing the bearing life and leading to bearing eccentricity. In addition, wear can affect other parts of the machine where the bearing is located. Contaminants can enter the bearing interior, usually due to severe wear or failure (or absence) of the seal ring. Lubricant analysis may reveal the source of the contaminant and help to find a solution to the problem.

Viscous wear mainly occurs in light load, poor lubrication, rolling body speed difference is large, resulting in sliding contact surface. For example, the process by which a rolling object enters the loading zone from the no-load zone. The rolling object stalls in the no-load zone and gains acceleration when re-entering the loading zone. This will cause the lubrication film to tear, slide, and heat, which may cause material to be transferred from the rolling body to the raceway or from the raceway to the rolling body. In the early stages, the appearance was a luminous surface, but soon it became a matte surface, more or less with a sticky material.

2, fatigue

The manifestation of fatigue is spalling, that is, the bearing material appears pitting or falling off, spalling initially occurs on the raceway or rolling body.

Fatigue from beneath the raceway surface is material deterioration caused by alternating stresses below the raceway surface, which ultimately leads to material decay. At first, there are cracks, which develop slowly with bearing work. When the cracks develop to the surface, they flake off.

Fatigue from the surface is usually caused by insufficient lubrication. The role of the lubricant is to create an oil film separating the raceway from the rolling body. Metal-to-metal contact occurs when lubrication is poor. Surface bumps and dents (protrusions) shear each other, resulting in shear stress on the surface. Due to material fatigue, there are small cracks, followed by microspalling. Initially, the surface may be bright and smooth due to reduced surface roughness, but if it continues to develop, the surface will become dull and more and more surface breakage and pitting will occur.

3. Mechanical corrosion

Erosion is usually caused by the temperature change in the bearing housing, condensation of the internal air, and the accumulation of water. And moisture or water often from damaged or inappropriate sealing ring into the bearing, due to the bearing raceway and rolling surface precision is extremely high, vulnerable to moisture and water corrosion. Compared with other damage processes, it occurs faster and can develop deep into the material, resulting in severe damage to the bearing. High humidity in the air or touching the raceway with your fingers can cause this type of corrosion. So it's very important to protect. Damp corrosion usually occurs at rest, and deep rust can cause early damage to the bearing.

The cause of frictional corrosion is the micro-movement between two bearing surfaces. In most cases, this frictional corrosion occurs between the bearing outer diameter and the bearing housing, and between the bearing hole and the shaft, and the micro-movement is mainly caused by the cyclic load generated when the rolling body passes by. Poor alignment, shaft bending or defective contact surfaces can cause or accelerate frictional corrosion. Air can enter unprotected surfaces, accelerating the development of corrosion. The resulting iron oxide, which is much larger in volume than pure steel, causes the material to become larger and more stressed, even for bearing raceways, leading to premature subsurface fatigue. Friction corrosion is easy to cause bearing ring cracking.

Pseudohardening is also a kind of frictional corrosion, which occurs in the contact area between the rolling body and the racetrack. It is caused by micro-movement and springback of plastic contact surface caused by cyclic vibration. Because it occurs when the bearing is at rest and under load, the damage occurs at the pitch circle of the rolling body. Depending on the strength of the vibration, lubrication conditions and load, corrosion and wear may occur simultaneously, forming shallow depressions in the raceway. In general, vibration leads to local loss of lubricant, metal-to-metal contact, and wear and tear. Depressions are usually dull in appearance, often faded, and sometimes reddish because of damp corrosion. Occasionally, the dimples show a shiny surface, probably mainly because lubricant is still present and no wear and tear has yet occurred. False hardening damage can cause holes in the spherical surface of ball bearings and lines in roller bearings.

4. Electrical corrosion

When the current passes through the bearing, it will be conducted between the inner and outer rings of the rolling body, which may cause damage caused by excessive voltage. The process that occurs on the contact surface is similar to arc welding (high density current on small contact surfaces), in which the material is heated to varying degrees of tempering to melting. Where the material has been tempered, re-hardened or melted, discolored areas of varying sizes appear in appearance. Where the material melted, a pit of about 0.1mm to 0.5mm was formed.

Damage can also be caused by current leakage, which is caused by stray current passing through the bearing, which is usually caused by a change in frequency. The main visible damage is the groove, shaped like a washboard pattern. These grooves are shaped as contact ellipses in ball bearings and contact lines in roller bearings, and the rolling body usually appears uniformly faded. Compared with excessive voltage damage, the current passes through a larger area in current leakage, resulting in a smaller current density and a lower damage temperature. Therefore, the main visible damage is the tempering effect, that is, the bearing surface becomes soft. When a high magnifying glass is used to look at the site of the injury, pits are usually also visible.

5. Plastic deformation

Static load or impact load can generate overload, resulting in plastic deformation and indentation on the raceway. Usually improper installation is the root cause of this problem, that is, improper force on the bearing ring, resulting in impact load on the rolling body. The dents caused by debris are caused by external particles (contaminants) entering the bearing and being pressed into the raceway by the rolling body. The size and shape of the indentation depends on the nature of the particle. The geometry of the raceway at the notch is broken and lubrication is affected. Stress occurs on the surface and fatigue leads to premature peeling of the surface.

The dent caused by handling is due to the surface of the bearing being damaged by a hard sharp object. Due to the high precision of the bearing surface, local overloads, such as the bearing falling to the ground, may cause dents in the surface, making the bearing unable to work.

6. Cracking and cracking

Compression fracture is caused by local overload or excessive stress, resulting in stress concentration that exceeds the tensile strength of the material. It is common for the bearing (impact) or taper seat or bushing to be unscrewed too tightly. When the bearing ring is struck directly with a hammer and chisel, tiny cracks may form. Small pieces of bearing ring may fall off when the bearing is put into use. When the taper seat or bushing is screwed too tight during installation, as a result, the bearing ring appears tensile stress (circumferential stress) and cracks occur when the bearing is put into use.

Fatigue fracture begins when the stress in the bending state exceeds the fatigue strength. First a crack appears, then more and more, and eventually the entire bearing ring or cage cracks. Fatigue failure may occur when tight fits are used because tight fits may produce high circumferential stress. Then alternating stress and circumferential stress act together to cause premature fatigue of the bearing ring and the whole ring cracks.

Thermal cracking occurs when two surfaces rub against each other severely. The resulting heat of friction causes a crack, usually at right angles to the direction of sliding.

Many bearing failures can be avoided. In-depth analysis of the root cause of bearing damage can find the root cause of the problem. By taking appropriate measures, the failure can be avoided in advance and the failure cost can be reduced.