2023 April the Second Week KYOCM Technical Knowledge: Failure Analysis of Rolling Bearings in Industrial Motors

Abstract: During the operation of industrial motors, the failure of bearings accounts for a very high proportion of the overall motor failure. As a load-bearing and rotating component, bearings have become the manifestation of many failures and are very vulnerable to damage. The failure analysis of rolling bearings in industrial motors is classified and introduced, and the corresponding failure causes and improvement measures are proposed.

Key words: rolling bearing; invalid; Industrial motors


0 Introduction

During the operation of industrial motors, faults such as bearing heating and noise often occur, and even the entire bearing may be burnt out, resulting in shutdown. In daily maintenance, if bearings are simply replaced without in-depth analysis, the key to the problem often cannot be grasped and the problem cannot be completely solved. Therefore, in order to avoid waste caused by blind replacement of bearings, when problems occur in bearings, scientific analysis should be carried out to eliminate the factors that cause bearing failure, so as to achieve the goal of fundamental maintenance.


Failure analysis of bearings is a highly demanding task. Analysts not only need to have a thorough and comprehensive understanding of bearings, but also need to understand the overall structure of the motor, the load conditions of the motor drive system, mechanical structure, and environment (including temperature, pollution, etc.). Bearing failure analysis also has some corresponding rules to follow. This article will provide a targeted introduction to the problem of bearing failure according to the classification framework of ISO 15243: 2004; At the same time, it proposes possible root causes and solutions for the processing, manufacturing, and use of the motor itself. However, the failure analysis of bearings is always driven by results, with some uncertainty. However, the failure of bearings is often mixed with multiple failures, which brings great difficulty to the analysis work.


Especially when a bearing has completely burned out, it is almost impossible for analysts to identify the root cause of the failure. Therefore, the earlier the stage of bearing failure, the more likely it is to accurately find the root cause of the failure. This is crucial for bearing failure analysis.


1.Load trace analysis

The mechanical structure of the motor itself is a system that is connected to external systems. Bearing, as one of the components, is affected by many factors. Failure analysis of bearings also requires consideration of the entire system to determine whether the bearings are operating under reasonable conditions. This is the first and crucial step in bearing failure analysis. Sometimes, simply analyzing this step can find the root cause of failure.


To determine whether the bearing is operating at a reasonable operating condition, some information can be obtained by understanding the operating conditions of the entire system, and more direct information can be obtained from the load traces inside the bearing. Generally, new bearing raceways and rolling bodies have metal machined surfaces with a certain roughness. When bearings are installed in the motor to withstand loads and operate, some areas of the raceway and rolling body surface will bear loads. After a period of operation of the bearing, the metal surface morphology of these load bearing areas will change, resulting in a difference in the morphology of the raceway and rolling body surfaces from the non load bearing parts. It can be intuitively seen that their smoothness is different. Therefore, it is possible to visually observe the raceway inside the bearing and the area where the rolling element has been subjected to load. These areas are called load traces, or load trajectories. In other words, where there is a load trajectory, there is a load bearing capacity, and vice versa.


Taking a horizontal motor using two deep groove ball bearings as an example, the two deep groove ball bearings bear radial loads from the rotor's gravity without external loads. The load situation is shown in Figure 1.



Figure 1 Load Zone of Deep Groove Ball Bearing


In Figure 1, the range of about 150 ° in the lower half of the bearing is the load area of the bearing, while the other parts are the non load areas of the bearing. In the bearings of this type of motor, the outer ring is usually fixed and the inner ring rotates, which makes the outer ring of the bearing only have a load trajectory in the load zone, while the entire inner ring has a load trajectory. From the above analysis, it can be seen that the load trajectory of a deep groove ball bearing in a normal motor is shown in Figure 2. If this type of motor is installed vertically, the deep groove ball bearing at the positioning end will withstand all axial loads. The load trajectory is shown in Figure 3.



Figure 2 Load trajectory of a normal deep groove ball bearing under radial load


Figure 3 Load trajectory of normal deep groove ball shaft under axial load


Here, both the outer and inner rings bear axial loads along the entire circumference, creating a load trajectory across the entire raceway surface.


There is also a case where axial load and radial load are borne together (commonly referred to as composite load), and the load trajectory is shown in Figure 4.


The situation shown in Figure 4 is a combination of the two situations shown in Figure 2 and Figure 3, so it will not be described again.



Figure 4 Load trajectory of normal deep groove ball bearings under combined load


The above three load trajectories are those of deep groove ball bearings under normal loads. Even if there is no problem with the operation of the bearing, if the bearing is disassembled, it is possible to see the three tracks shown in Figures 2-4.


If the load trajectory of a bearing is different from normal conditions, it should attract the attention of analysts to find the source of the load. If this load is not acceptable for this type of bearing, the bearing will soon fail.


1.1 Bearing bears eccentric load

General bearings (except self-aligning bearings) are used to carry axial and radial loads, and their ability to carry eccentric loads is very limited. If an eccentric load is applied to these bearings, it will cause unreasonable load distribution within the bearings, resulting in a dramatic reduction in bearing life and heat generation. The load trajectory under eccentric load is shown in Figure 5.


Figure 5 Load trajectory of deep groove ball bearing under eccentric load


If you find the load trajectory shown in Figure 5, you should immediately find out the concentricity of the bearing chamber and the motor end cap lip, the concentricity of the bearing chamber and the base, and the relative concentricity of the bearing chambers after the end caps at both ends are installed. If the motor is misaligned with the load it carries, this load trajectory can also occur on the bearings.


1. 2 Bearing chamber shape and position tolerance out of tolerance

The out-of-tolerance of the shape and position tolerance of the bearing chamber will affect the distribution of internal load in the bearing, resulting in abnormal load traces, as shown in Figure 6. The load trace in Figure 6 indicates that the roundness of the bearing chamber may be out of tolerance, causing the ball in the non load zone to also bear the load, which may cause heat and noise inside the bearing during operation. If the load trajectory shown in Figure 6 is found during disassembly of the bearing, it is necessary to adjust the roundness of the bearing housing. However, Figure 6 is only a case where the roundness of the geometric tolerance exceeds the tolerance. There are also cases such as out-of-tolerance cylindricity, which can also be seen from the load trajectory.



Figure 6 Bearing abnormal load trace caused by elliptical bearing chamber


As for how to check the geometric tolerance of the bearing housing, it will be introduced in subsequent articles on installation and disassembly, and will not be repeated here.


1. 3 The remaining clearance of the bearing is too small

Generally speaking, a normal deep groove ball bearing always has a certain amount of residual clearance during operation, which will be distributed in the non load area. The curve of the effect of residual clearance on the life of rolling bearings is given in document [1]. When the remaining clearance is too small, the bearing can easily get stuck, leading to early failure. The performance of these failed bearings before burning is noise and heat, and the load trajectory of the bearing raceway surface is shown in Figure 7.



Figure 7 Load trajectory of deep groove ball bearings with too small residual clearance


Factors that can cause excessive bearing residual clearance may include: tight fitting of the inner race, excessive temperature difference between the inner and outer races, etc. Timely adjustments and improvements should be made.


2 ISO Bearing Failure Classification

The failure forms of bearings are roughly classified in ISO 15243, which covers most of the failure forms of bearings and is of great guiding significance for bearing failure analysis. The specific classification is roughly shown in Figure 8.


In this classification, bearing failures are classified into six categories. There are clear definitions and atlases in each category that can be referenced. It should be noted that although this classification largely covers many cases of bearing failure, there are still some failures that are not included in these six categories, but due to their lack of typicality, they are not classified.



Figure 8 Specific Classification of Bearing Failures


In practical work, bearing failures often occur in several situations simultaneously or at close intervals. Therefore, engineers often face a situation where several failure traces are mixed together. Here, a certain amount of experience is needed to identify the initial cause of failure.


Classifying failed bearings according to ISO standards is an important step in failure analysis, but the more important task is to identify the causes of such failures. Although the standard provides some summary of the possible causes of each type of failure, engineers must conduct further inspections based on specific operating conditions to confirm. This article does not enumerate the specific descriptions and graphs of six types of bearing failures. If necessary, please refer to the corresponding standards. The following will select and analyze some typical bearing failure cases based on the application characteristics of industrial motors.


3. Typical bearing failures commonly encountered in industrial motors and its analysis

3.1 Bearing surface fatigue

Surface fatigue of bearings refers to the development of metal surface fatigue traces starting from the contact surfaces of rolling bodies and raceways. The reason for this fatigue is due to the large shear stress on the loaded metal surface. If lubrication is applied properly, this shear stress will be less than the maximum shear stress below the metal surface. When properly lubricated normal bearings reach their fatigue life after a period of operation, normal subsurface fatigue occurs and the bearings fail. However, if initial fatigue occurs on the metal surface rather than below it, the bearing may not reach its fatigue life and experience premature failure. Surface fatigue is a typical form of bearing failure that deserves attention. Figure 9 shows a shaft examples of bearing surface fatigue.



Figure 9 Deep groove ball bearing surface worn inner ring


As can be seen from Figure 9, the surface of the inner bearing ring in the load trajectory appears similar to a polished and bright appearance, while fatigue traces appear in this area from top to bottom. From this, it can be judged that this is a typical surface fatigue bearing.


As mentioned earlier, under normal lubrication, the metal surface shear stress should not cause premature surface fatigue of the bearing. Therefore, it is judged that the lubrication problem is the possible cause of surface fatigue of this bearing.


In practical work, similar bearing failures often occur in electric machines. The root cause of this failure is not the bearing, so no matter how many bearings are replaced, as long as the lubrication condition is not improved, bearing surface fatigue will still occur. If this type of bearing is found to fail prematurely, it is recommended that engineers recheck the selection of lubricant, the amount of lubricant added, the relubrication interval, and the relubrication amount. (The situation regarding bearing lubrication will be described in detail in subsequent articles.).


3.2 Bearing wear

The wear of bearings is divided into two types, one is abrasive wear, and the other is adhesive wear. The causes of these two types of wear are different, but both are common problems encountered by motor bearings.


3. 2. 1 Abrasive wear

Abrasive wear is caused by the presence of certain impurities between the rolling body and the raceway, which act as an abrasive and cause wear between the raceway and the rolling body. Its characteristic is the wear and tear of the rolling body or raceway material. Most of the reasons for this type of wear are due to poor lubrication caused by contamination and impurities entering the lubricant. Figure 10 is a typical abrasive wear bearing.


Figure 10 shows the abrasive wear on the inner ring of a spherical roller bearing. In fact, abrasive wear occurs not only on the raceway surface, but also on the cage, which is often affected by abrasive wear. In cylindrical roller bearings with inner or outer ring guide cages often used in industrial motors, if grease lubrication is used, when the rotational speed exceeds a certain range, the outer (or inner) edge of the cage will rub against the bearing ring. At this time, it is very difficult for the lubricating grease to enter and form effective lubrication, which leads to the phenomenon of so-called copper powder grinding. In this case, using a thinner grease and shortening the relubrication interval will improve the operating condition of the bearing.



Figure 10 Abrasive wear of inner ring of spherical roller bearing


Generally, when abrasive wear occurs, bearings often exhibit heat and other phenomena. At this time, it is necessary to check the cleanliness of the bearing lubricant, the integrity of the seals, and pay attention to the requirements for environmental cleanliness when installing bearings. Worn bearings cannot be reused.

3. 2. 2 Adhesion wear

The second type of wear that often occurs in motor bearings is adhesive wear. Adhesion wear is different from abrasive wear, and most of it is accompanied by the transfer of metal materials, such as from a rolling body to a raceway or vice versa. Figure 11 shows an adhesive worn bearing.



Figure 11 Adhesion Wear Bearing


There are many situations where adhesive wear occurs inside bearings in electric motors, especially in motors that are frequently started. Frequent starting and stopping (or reciprocating rotation) of the motor greatly increases the sliding friction between the rolling body and the raceway during starting, making adhesive wear very easy to occur. For adhesive wear in this situation, improvements in lubricants such as adding extreme pressure additives to the lubricating grease of reciprocating motors) to reduce the sliding friction inside the bearings during startup and shutdown.


In addition, if the load borne by the bearings in the motor is less than the minimum load required for bearing operation, then effective rolling friction cannot be formed between the rolling body and the raceway, and adhesive wear is also prone to occur at this time. The minimum load of the bearing is related to the consistency of the grease. In winter, when the temperature is very low, the consistency of the lubricating grease will increase, and the minimum load required for the bearing will also increase. If the actual load does not reach this minimum value, it will lead to adhesive wear. This situation occurs occasionally in wind power plants.


3.3 Friction corrosion

Corrosion of bearings includes wet corrosion and frictional corrosion. Wet corrosion is mostly environmental related and will not be developed here. In the use of electric motors, friction and corrosion of bearings are more subtle and often occur as a type of bearing failure.

Friction corrosion of bearings can be divided into two types: fretting corrosion and pseudo Bush indentation.


3. 3. 1 Fretting corrosion

Fretting corrosion refers to the occurrence of microscopic creep on two metal mating surfaces, resulting in metal oxidation and powdery corrosion on the fretting surface. In general industrial motors, it occurs on the mating surface of the outer ring and bearing housing, or on the mating surface of the inner ring and shaft.


The usual slight race of bearings refers to a type of fretting corrosion of the outer ring. Fretting corrosion can cause changes in the operating state of the rolling element inside the bearing, resulting in heat generation, and in severe cases, even fracture of the bearing ring. Figure 12 shows a fretting corroded bearing inner race.


Figure 12 Bearing inner ring fretting corrosion


In case of fretting corrosion of the bearing ring, the tolerance fit between the shaft and the bearing inner ring should be checked to ensure sufficient interference between the rotating ring and its mating surface. However, for general motors, the outer ring of the bearing and the bearing housing are usually loosely fitted, so if it is necessary to avoid fretting corrosion of the outer ring, measures such as adding an "O" ring are needed.


3. 3. 2 Pseudo Bush indentation

When the motor is stationary, the rolling bodies and raceways in the load zone within the bearing bear the load. At this time, if the motor is in a vibration environment, the rolling body inside the load zone will undergo slight reciprocating peristalsis (inching) with the raceway, and after a period of time, a longitudinal indentation similar to rust will form on the raceway. This is similar to the raceway when the bearing is stationary.


The Brinell indentation caused by a large radial load is somewhat similar, but the actual mechanism of its formation is a corrosion, so it is called a pseudo Bush indentation (Figure 13).



Figure 13 Pseudo Bush indentation


Some motor manufacturers have encountered situations where the noise performance of the motor during factory inspection was intact, but when the motor arrived at the customer's premises and started operating, the noise exceeded the standard. At the same time, all other parts were inspected to be free of problems. In this case, the possibility of fake Bush indentation should be considered. Because the motor may encounter bumps during transportation after leaving the factory, it is equivalent to a situation where the static motor is in vibration for the motor bearing. The bearing rolling body constantly creeps inside the raceway, which is highly likely to form a pseudo Bush indentation. At this time, when inspecting the bearing, if longitudinal traces with equal roller spacing are found (similar to Figure 13), it can be suspected that the occurrence of false Bush indentation. If there is no abnormal noise when leaving the factory, and the raceway is not. If there is installation strain, it can be confirmed as a fake Bush indentation.


The solution to this problem is to improve the packaging of the motor. The specific method has been described in document [1], and will not be repeated here.


3.4 Electrical corrosion of bearings

The mechanism and failure characteristics of electrical corrosion of bearings are described in detail in literature [2]. See.


3.5 Plastic deformation of bearings

The plastic deformation of bearings includes three types: bearing overload, indentation caused by impurities, and plastic deformation generated during installation.


3. 5. 1 Bearing overload

When a bearing is subjected to excessive static load, plastic deformation of the rolling element or raceway can occur, as shown in Figure 14.



Figure 14 Bearing Overload


From the knowledge of the load trajectory introduced earlier, it can be inferred that the bearing shown in Figure 14 bears a very large axial load, resulting in plastic deformation.


3. 5.  Indentation caused by impurities

If the lubricant inside the bearing is contaminated by impurities, the contaminated particles can cause small indentations on the surface of the rolling element raceway, thereby damaging the surface morphology of the raceway or rolling element, generating heat or noise. It was once mistakenly believed that due to the relatively high hardness of steel, some small contaminants would not have an impact. In fact, those seemingly soft small contaminants would still change the surface finish of the raceway on a microscopic level, damaging lubrication, and causing heat. Figure 15 is a good example where the contamination is a cotton fiber. Due to such contamination and indentation, the bearing is damaged due to excessive temperature.



Figure 15 Indentation caused by impurities


The situation in Figure 15 is relatively special. In fact, in motor factories, there are more impurities of solid pollution particles. Some of the damage marks of these impurities on the raceway can be directly identified with the naked eye, while others can only be detected with the aid of a microscope. To reduce the indentation damage caused by impurities, it is necessary to pay special attention to the cleanliness of the bearing during installation and use, and ensure that the bearing has reasonable sealing protection.


3. 5. Plastic deformation during installation

During bearing installation, it is not allowed to directly knock with a steel hammer or the like, and the installation force of the bearing cannot be transmitted through the rolling body, otherwise plastic deformation will occur and the bearing will be damaged. This damage is mainly manifested in the shape change of bearing components, so the failure trace varies under different working conditions, which requires specific analysis.


4 Conclusion

This article first introduces the basic knowledge of bearing failure analysis, and introduces the basic principle of load trace analysis, and lists several common forms of load trace; After that, according to the classification of ISO 15243, the failure types of rolling bearings frequently encountered by industrial motor users were selected, and a detailed analysis was conducted. The possible causes of failure and improvement measures were proposed.


It should be noted that the situation of bearing failures varies widely, and this article can only extract some of the more representative cases for analysis and classification. More analysis and judgment in daily work need to be based on experience accumulation. Having sufficient knowledge of bearing applications and understanding basic bearing failure classifications will greatly benefit the accuracy of bearing failure analysis.


More about KYOCM Cylindrical Roller Bearing:

KYOCM cylindrical roller bearings can meet the challenges of applications faced with heavy radial loads and high speeds. Accommodating axial displacement (except for bearings with flanges on both the inner and outer rings), they offer high stiffness, low friction and long service life. KYOCM cylindrical roller bearings include single row cylindrical roller bearings, double row cylindrical roller bearings and four row cylindrical roller bearings.


Cylindrical roller bearings are also available in sealed or split designs. In sealed bearings, the rollers are protected from contaminants, water and dust, while providing lubricant retention and contaminant exclusion. This provides lower friction and longer service life. Split bearings are intended primarily for bearing arrangements which are difficult to access, such as crank shafts, where they simplify maintenance and replacements.