Common types of bearing faults and root causes of bearing damage
Bearings are the core components of most equipment. Once the NSK bearings fail, the equipment will stop. It is particularly important to find the root cause of the bearing failure to avoid accidents. Each type of failure has its own characteristics, and it is possible to determine the root cause of the failure by observing the damage condition. Now we will analyze them one by one.
1. Wear
Abrasion is one of the most common failures of rolling bearings. It is usually caused by foreign matter to cause grinding damage, scratches, groove marks, circumferential line marks or debris contamination. It is divided into two categories: abrasion wear and viscous wear. Abrasion and wear are caused by small foreign bodies inside the bearing. They may be contaminated particles from the outside or inside. These particles will wear or grind the raceways and rolling elements. This wear will cause the axial clearance or internal clearance to expand, thereby reducing the bearing Life, resulting in bearing eccentricity. In addition, wear will also affect other parts of the machine where the bearing is located. Lubricant analysis may reveal the source of contaminants, which helps to find solutions to problems.
The viscous wear mainly occurs on the contact surface with light load, poor lubrication, and large rolling element speed difference, resulting in sliding contact surface. In the early stage, the appearance was a luminous surface, but soon it became a matte surface, with more or less glued material.
2. Fatigue
The manifestation of fatigue is peeling, that is, pitting or shedding of the bearing material, and the peeling initially occurs on the raceways or rolling elements. Fatigue from below the raceway surface is the deterioration of the material, which is caused by the alternating stress below the raceway surface, which ultimately leads to material decay. At first, cracks appeared, and the cracks slowly developed with the work of the bearing. When the cracks reached the surface, spalling occurred. Fatigue from the surface is usually caused by insufficient lubrication. When the lubrication is poor, metal-to-metal contact occurs. The uneven surfaces (protruding parts) shear each other, causing shear stress on the surface. Due to material fatigue, small cracks appear, followed by micro-flaking. Initially, the surface roughness is reduced, so the surface may be bright and smooth, but if it continues to develop, the surface will become dull, the surface will be more and more broken, and pitting will appear.
3. Mechanical corrosion
Erosion is usually caused by temperature changes in the bearing housing, internal air condensation, and continuous accumulation of moisture. However, moisture or water often enters the bearing from a damaged or improper sealing ring. Compared with other damage processes, it occurs faster and can develop deep into the material and cause serious damage to the bearing. High air humidity or touching the raceway with your fingers can cause this type of corrosion. Therefore, good protection is very important. Moisture corrosion usually occurs in a static state, and deep rust can cause early damage to the bearing.
The root cause of friction corrosion is the micro-movement between the two bearing surfaces. The micro-movement is mainly caused by the cyclic load generated when the rolling elements pass by. Poor fit, bent shafts, or defective contact surfaces can cause or accelerate the occurrence of friction corrosion. Friction corrosion can easily cause cracks in the bearing ring. False hardening is also a kind of friction corrosion, which occurs in the contact area between the rolling element and the raceway, and is caused by the micro-movement and springback of the plastic contact surface caused by cyclic vibration.
4. Electric corrosion
When the current passes through the bearing, it will be conducted between the inner and outer rings of the rolling elements, and damage caused by excessive voltage may occur. Similarly, current leakage can also cause damage. Current leakage comes from stray currents passing through the bearing, and stray currents are usually caused by frequency changes. The main visible damage is the groove, shaped like a washboard pattern. Compared with excessive voltage damage, in current leakage, the area through which the current passes is larger, resulting in a smaller current density and a lower damage temperature. Therefore, the main visible damage is the tempering effect, that is, the softening of the bearing surface. When observing the damaged part with a high-power magnifying glass, the pit can usually be seen.
5. Plastic deformation
Static load or shock load can cause overload, which can lead to plastic deformation and form dents on the raceway. Usually improper installation is the source of this problem, that is, improper force on the bearing ring, resulting in an impact load on the rolling elements. The dent caused by debris is because foreign particles enter the bearing and are pressed into the raceway by the rolling elements. Concavity caused by handling is due to the bearing surface being damaged by hard sharp objects. Due to the extremely high accuracy of the bearing surface, when a local overload, such as the bearing falling on the ground, dents may be formed on the surface, making the bearing unable to work.
6. Rupture and cracking
Compression rupture is caused by local overload or excessive stress, which causes stress concentration to exceed the tensile strength of the material. It is common to disassemble the bearing or conical seat or bushing too tightly. Fatigue rupture begins when the stress exceeds the fatigue strength in the bending state. At first, a crack appeared, and then it continued to increase, and finally the entire bearing ring or cage cracked. Fatigue cracking may occur when using a tight fit. Thermal cracking occurs when two surfaces rub against each other severely. The generated frictional heat causes cracks, usually at right angles to the sliding direction.
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 cost of failure can be reduced.