GQZ Technology Knowledge: Failure Analysis of Rolling Bearings

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Basic failure modes of rolling bearings, factors affecting bearing failure and prediction and prevention of bearing failure.




Abstract: the basic concept and significance of failure analysis of rolling bearings are summarized; the basic failure mode, the factors affecting bearing failure, and the working ideas and methods of bearing failure analysis are emphatically discussed; the prospect of bearing failure prediction and prevention is also discussed.


Key words: bearing; failure analysis; analysis method; influencing factors; Predictive prevention


In Electromechanical industry, bearing is one of the most widely used basic parts, especially rolling bearing. No matter ordinary mechanical equipment, means of transportation, or aviation, navigation, aerospace, where there is rotation, there are bearings working. Obviously, it is very important to ensure that the bearing can work normally under various environmental conditions.


The phenomenon that the bearing loses its specified function during operation, resulting in failure or failure to work normally is called failure. Bearing failure can be divided into normal failure and early failure according to its service life. The analysis work is mainly aimed at the early failure of bearings, find out the causes of their failure, and put forward improvement measures to improve the service life and reliability of bearing operation.


Bearing failure analysis is an important part of system engineering to improve bearing reliability. It is an interdisciplinary technical field. It is both comprehensive and applicable. The so-called comprehensiveness is reflected in its wide range of disciplines and technical categories, including product structure design, mechanical manufacturing process, material selection and metallurgical technology, as well as tribology, corrosion, engineering mechanics, fracture mechanics, metal physics, surface physics and so on. The so-called practicality is that the failure analysis of bearings must start from the actual production and closely serve the production, which has a strong application background. Its positive significance lies in:  it can analyze the main causes of bearing failure, summarize experience and lessons, put forward improvement measures, and constantly improve the quality of bearing products;  We can judge whether the design is reasonable and correct some unreasonable aspects to improve the reliability of bearing products;  The problems existing in the cold and hot processing of bearing parts can be found, and the unreasonable processing technology can be corrected;  We can judge the rationality of material selection and the problems of raw material quality. Therefore, bearing failure analysis is an important work closely related to bearing product quality and production development. Its working characteristics require that the technicians engaged in this work have a wide range of knowledge, rich experience and advanced technology.


1. Basic failure modes of rolling bearings

Bearing failure can generally be divided into two types: rotation failure and sperm loss failure. Stalling failure means that the bearing stops rotating due to loss of working capacity. For example, jamming, fracture, etc. Loss of precision failure is that the fit clearance of the bearing changes due to the geometric dimension, and the design accuracy required by the original design is lost. Although it can still continue to rotate, it is abnormal operation. Such as wear, corrosion, etc. The influencing factors of bearing failure are very complex, and due to the differences of working conditions and failure factors of various bearings, the failure forms and morphological characteristics are also different. According to its damage mechanism, it can be roughly divided into: contact fatigue failure; Friction and wear failure; Fracture failure; Deformation failure; Corrosion failure, and clearance change failure.


1.1 Contact fatigue failure

Contact fatigue failure is one of the most common failure modes of various bearing surfaces, which is caused by the alternating stress on the bearing surface. Contact fatigue spalling also has the process of fatigue crack initiation, propagation and fracture on the bearing surface. The initial contact fatigue crack first occurs at the maximum orthogonal shear stress below the contact surface, and then extends to the surface to form spalling, such as pitting, which is called pitting or pitting spalling; Flaking into small flakes is called shallow flaking. The initial crack occurs at the junction of the hardened layer and the heart, and the early peeling of the hardened layer is called the peeling of the hardened layer.


1.2 Wear failure

The phenomenon of continuous loss of surface metal caused by relative sliding friction between bearing parts is called wear. Continuous wear changes the size and shape of parts, increases the fitting clearance of bearings, deteriorates the morphology of working surfaces, and thus loses the rotating accuracy, making the bearings unable to work normally, which is called bearing wear failure. Wear failure is also one of the most common modes of various bearing surfaces. According to its wear forms, it can be divided into abrasive wear, adhesive wear, corrosion wear, fretting wear and fatigue wear. The most common ones are abrasive wear and adhesive wear. The friction surfaces of bearing parts are crowded with foreign hard particles or wear debris on the metal surface, which causes the wear of the friction surface, which is called abrasive wear. It often causes gouging or furrow abrasion on the bearing surface. Foreign hard particles often come from dust in the air or impurities in lubricants. Adhesive wear is mainly due to the uneven stress on the friction surface caused by the micro protrusions or friction foreign bodies on the friction surface. Local friction heat may cause the friction surface to form micro welding. The temperature rise of the friction surface will cause the rupture of the lubricating oil film. In serious cases, the surface layer metal will melt locally, and the contact point will produce a cycle of adhesion, tear off and re adhesion, forming adhesive wear. Serious adhesive wear will cause welding and jamming of the friction surface.


1.3 Fracture failure

The fracture of bearing parts will cause sudden failure accidents. The main causes of bearing fracture are overload and defect. Because the applied load exceeds the strength limit of the bearing part material, the bearing part fracture is called overload fracture. The cause of overload may be the failure of the host machine, or the unreasonable structure or installation of the bearing. In addition, bearing parts have defects such as microcracks, shrinkage cavities, bubbles and large foreign inclusions, which will also cause fracture at the defect under normal load conditions, which is called defect fracture. Overheating, overburning, local burns and surface cracks caused by forging, stamping, hot rolling, heat treatment and grinding of bearing rings and rolling elements may cause bearing fracture failure. In particular, grinding burns are not easy to find during inspection. Once the ferrule with grinding burns is impacted or vibrated, it may break.


1.4 Plastic deformation failure

Under the action of external force and ambient temperature, the partial plastic flow or overall deformation on the surface of bearing parts makes the whole set of bearings unable to work normally, and the failure is called deformation failure. For example, cage warping, skewing, elongated pocket or deformation of frame cage, backup sleeve, etc. will cause early failure of bearing. In addition, the surface plastic scratch caused by bearing friction will also cause the increase of vibration, noise and temperature, thus accelerating the early failure of the bearing.


1.5 Corrosion failure

The surface damage and bearing failure caused by the chemical or electrochemical reaction between the metal surface of bearing parts and environmental media are called corrosion failure. The environmental media that can play a chemical role on the surface of bearing parts include atmosphere, moisture, oxidation products of fuel and lubricating oil (acids, ketones, ethanol, etc.) and vapors of oxidation products, etc.

Generally, bearing surface corrosion can be divided into dielectric corrosion, organic acid corrosion, other medium corrosion (such as sulfide in lubricating oil) and current corrosion.


Corrosion causes oxide film or corrosion holes on the metal surface of bearing parts, causing local or total discoloration of the surface. The hard, brittle and loose oxide film and corrosion reactants peel off under the action of load, forming corrosion pits on the bearing surface or roughening the working surface, and then forming corrosion wear or corrosion fatigue failure. Electric shock scars on the surface of bearing parts due to electrostatic charge or other discharge phenomena are called current corrosion, which is also a kind of corrosion failure.


1.6 Clearance change failure

In the working process, the bearing changes the original fit clearance under the influence of external or internal factors, which reduces the accuracy and even causes the phenomenon of seizure, which is called clearance change failure.


If the structure (such as retained austenite) and stress of bearing parts are in an unstable state, their size will change with the extension of time, making the bearing lose operating accuracy. Because the size and shape of bearing parts are different, and the expansion coefficient or expansion amount is different, working under super normal temperature will cause the change of bearing working clearance, and the bearing will also cause early failure due to the loss of operating accuracy.


2. Factors affecting bearing failure

The causes of bearing failure are often multifactorial, and all influencing factors in the design and manufacturing process are related to bearing failure, which is difficult to judge by analysis. In general, it can be considered and analyzed from two aspects: external factors and internal factors.


2.1 External factors

External factors mainly refer to whether the installation and adjustment, use and maintenance, maintenance and repair meet the technical requirements. Therefore, it is also called use factor.


The installation condition is one of the primary factors in the use factors. The improper installation of the bearing often leads to the change of the stress state between the parts of the whole bearing, and the bearing will operate in an abnormal state and fail early. According to the technical requirements of bearing installation, use, maintenance and repair, monitor and inspect the load, speed, working temperature, vibration noise and lubrication conditions of the running bearing. In case of any abnormality, find the cause immediately and adjust it to make it return to normal. It is also important to analyze and test the quality of lubricant and the surrounding medium and atmosphere. In particular, the correct use of lubricants is crucial to prolong the service life of bearings.


2.2 Internal factors

Internal factors mainly refer to the three factors that determine the quality of bearings, such as design, manufacturing process and material quality. It can also be called manufacturing quality factor.


In order to improve the service life and reliability of bearings, people have done a lot of research work around the above three factors. First of all, if the structural design is unreasonable, it is certainly impossible to have a reasonable bearing life; Only the rationality of structural design without considering progressiveness will not have a long bearing life; Only when the structural design is reasonable and progressiveness at the same time, can there be a long bearing life.


The manufacturing of bearings should go through various processing procedures such as steel smelting, forging, stamping, heat treatment, turning, grinding and assembly. The rationality, progressiveness and stability of various processing technologies will also affect the life and failure analysis of bearings. In particular, the heat treatment and grinding processes that directly affect the quality of finished bearings are often more directly related to the failure of bearings.


The metallurgical quality of bearing materials was once the main factor affecting the early failure of rolling bearings. With the improvement of Metallurgical Technology (such as vacuum degassing of bearing steel, etc.), the quality of raw materials has been greatly improved, and its proportion in bearing failure analysis has decreased significantly, but it is still one of the main influencing factors of bearing failure. In addition, proper material selection is still a factor that must be considered in bearing failure analysis. The main task of bearing failure analysis is to comprehensively analyze and find out the main influencing factors of bearing failure according to a large number of background materials, analysis data and failure forms, so as to put forward targeted improvement measures and improve the service life of bearings. Avoid sudden early failure of bearings.


3. Bearing failure analysis method

In the bearing failure analysis method, we often encounter many complex phenomena, and various test results may be contradictory, or the primary and secondary are not easy to distinguish, which requires repeated tests and demonstrations to obtain sufficient evidence or counter evidence. In the whole analysis process, only by using the correct analysis methods, procedures and steps, can we find the real failure reason and get the correct conclusion. The extensive basic knowledge, rich practical experience and advanced analysis methods of failure analysis workers are very important. A well-trained bearing failure analysis worker must make a comprehensive analysis from the external conditions that affect the bearing life (including the environmental conditions of the host machine, installation and commissioning, maintenance and lubrication conditions, etc.) to the internal quality factors (including structural design, material quality, cold and hot processing quality, etc.) to confirm the main procedures of the analysis as soon as possible.


Generally, bearing failure analysis can be divided into three steps: the collection of failure material and background data, and the macro inspection and micro analysis of failure material.


3.1 Collection of invalid objects and background materials

All parts and fragments of the failed physical object should be collected as far as possible. Learn as much as possible about the actual working conditions, use process and manufacturing quality of the failed bearing. This is essential for correct failure analysis. It specifically includes the following contents:


(1) The working condition, load and running speed of the mechanical equipment used by the bearing, and the design working conditions of the bearing on the equipment.


(2) In the case of bearing failure, only the bearing fails or other parts also fail. What type of bearing failure does it roughly belong to.


(3) The installation and operation records of bearings, and whether there is abnormal operation during operation.


(4) What is the real load borne by the bearing during operation, and whether it conforms to the original design.


(5) The actual speed of bearing operation and the frequency of different speeds.


(6) Whether there is a sharp increase in temperature or a sudden increase in smoke, noise and vibration during failure.


(7) Whether there is corrosive medium in the working environment, and whether there is special surface oxidation or other contamination color at the bearing and its contact journal.


(8) The installation record of the bearing (including the re inspection of the dimensional accuracy of the bearing before installation), the clearance, assembly and alignment of the shaft and bearing, the rigidity of the bearing seat and frame, and whether the installation is abnormal.


(9) Whether there is thermal expansion and power transmission change during bearing operation.


(10) Bearing lubrication, including lubricant brand, composition, color, viscosity, hybrid content, filtration, replacement and supply, and collect its sediment for analysis.


(11) Whether the material selection of bearings is correct and whether the quality of materials meets the requirements of standards or drawings.


(12) Whether the manufacturing process of the bearing is normal, whether there are plastic changes on the surface, and whether there are surface grinding burns.


(13) Repair and maintenance records of invalid bearings.


(14) Failure of the same batch or similar bearings. In the actual failure analysis background material collection work, it is difficult to meet all the above requirements. However, the more data collected will undoubtedly be more conducive to the correct analysis conclusion.


3.2 Macro inspection

It is the most important link of failure analysis to carry out macroscopic inspection on the failed bearing (including dimensional accuracy measurement and surface condition inspection and analysis). Through the overall appearance inspection, we can understand the general situation of bearing failure and the characteristics of damaged parts, estimate the cause of failure, observe the size, shape, position, quantity and characteristics of defects, and determine the intercepted parts for further microscopic inspection and analysis. The contents of macro inspection shall include:


(1) Changes in shape and dimensional accuracy (including vibration measurement analysis, dynamic function analysis and raceway out of Roundness Analysis).


(2) Change of clearance.


(3) Whether there is corrosion, where it is, what type of corrosion, and whether it is directly related to failure.


(4) Whether there is crack. What is the shape of the crack and the nature of the fracture.


(5) What kind of wear is it and how much effect it has on failure.


(6) The condition and position of discoloration on the working surface of each bearing component to determine its lubrication and surface temperature effect.


(7) The failure characteristic area is mainly observed for abnormal wear, foreign particles embedded, cracks, scratches and other defects.


(8) Check the original surface of bearing parts for soft spots, decarburized layers and burns, especially surface grinding burns, by cold acid washing or hot acid washing.


(9) Use X-ray stress tester to measure the stress changes of the bearing before and after operation. Sometimes, the mode and cause of failure can be basically judged by the results of macro inspection, but micro analysis is also necessary to determine the nature of failure and obtain more evidence.


3.3 Microanalysis

The microscopic analysis of the failed bearing includes optical metallographic analysis, electronic microscopic analysis, probe and electronic energy spectrum analysis. It is important to provide more sufficient criteria or counter evidence for failure analysis mainly based on the microstructure changes in the failure characteristic area and the analysis of fatigue sources and crack sources. The most commonly used and popular methods in microanalysis are fracture analysis, optical metallographic analysis and hardness testing. The contents of the analysis shall include:


(1) Whether the raw materials meet the standards and design requirements.


(2) Whether the matrix structure and heat treatment quality of bearing parts meet the standard requirements.


(3) Whether there is decarburization layer, troostite and other surface processing metamorphic layers in the surface structure.


(4) Measure the depth of the surface strengthening layer such as carburized layer and the structure of each layer of multi-layer metal, the shape and depth of corrosion pits or cracks, and determine the cause and nature of cracks according to the shape of defects and the structural characteristics of both sides.


(5) Judge the deformation degree, temperature rise, material type and process according to grain size, structural deformation, local phase transformation, recrystallization, phase aggregation, etc.


(6) Measure the surface hardness, matrix hardness, hardness uniformity and hardness changes in the failure characteristic area.


(7) Fracture observation and analysis. Scanning electron microscope has the advantages of large depth of field, high magnification and clear image, so it has more advantages in the observation, qualitative analysis and measurement of fracture.


(8) In the analysis of fatigue source and crack source, the composition of foreign matters in the fracture can be measured by electron microscope, probe and electron spectroscopy, and the nature of the fracture and the cause of fracture can be found.


The three steps of the general method of bearing failure analysis introduced here is a step-by-step analysis process from the outside to the inside. The specific analysis should be based on the type and characteristics of bearing failure, and not every problem and method in the three steps should be used correspondingly. It depends on the specific situation, but every step of the whole process of analysis is indispensable. Moreover, in the whole analysis process, the analysis results should always be connected with many internal and external factors that affect the bearing failure for comprehensive thinking and judgment.


4. Prediction and prevention of bearing failure

4.1 Predict the main ways to prevent bearing failure

(1) Adopt all possible and effective means to maximize the service life and reliability of bearings. It includes optimization of structural design, reform of processing technology, selection and refining of raw materials, lubrication with high efficiency and cleanliness, fine assembly and installation, etc. This is also the most fundamental and proactive way to prevent early bearing failure.


(2) Strengthen the quality inspection and supervision of bearing products to ensure that the quality indicators of bearing products meet the design requirements. Before the bearing is put into operation, strict quality inspection and supervision are also positive measures to prevent early failure of the bearing.


(3) Strengthen the diagnosis and prediction of the working state of the bearing, find the abnormality as soon as possible, and take preventive measures to prevent the possible heavy losses caused by sudden failure accidents.


4.2 Fault diagnosis and detection system

Modern bearing diagnosis and prediction technology is associated with precise testing system and means. At present, the more mature detection systems in practical application include the following:


(1) The bearing pulse vibration measuring device generates vibration after bearing wear (or fatigue peeling), and the receiver converts the mechanical pulse signal into an electrical signal and amplifies it. When the number of pulses exceeds the normal range and reaches a sudden change, an alarm will be given immediately and the bearing will stop using.


(2) Bearing temperature alarm device: poor bearing lubrication, surface wear or fatigue will make the surface hot. When the heat reaches a certain limit temperature, it will alarm and the bearing will stop running.


(3) Regularly detect the current state of the bearing in operation, and find or monitor the existing defects and their development trend.


(4) The iron powder recording diagnosis method regularly takes lubricant samples of working bearings, and uses Ferrograph to detect the number, size and shape characteristics of wear particles, which can judge the degree of fatigue and wear, and find signs of early bearing failure.


In recent years, the newly developed bearing diagnosis device abroad not only makes this kind of measurement technology continue to deepen, accurate and sensitive on the original basis, but also has new development in form and principle. In short, the prediction and prevention technology of bearings is developing rapidly. No matter which method, principle or instrument is used, the most important thing is to find the limit state and estimate the remaining life as soon as possible. Diagnosis and prediction are only methods and means, and the purpose is to prevent the loss caused by early bearing failure. In this regard, we must also do a lot of work to quickly improve the technical level of bearing diagnosis and prediction, and catch up with the advanced technical level of foreign countries.


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