Understanding Bearing Damage – Causes, Failure Mode and Prevention

Rolling bearings are robust and reliable components in mechanical engineering and custom machine construction. Under certain conditions, however, bearing damage to these widely used machine elements can occur, usually resulting from unusual behavior during operation. In the worst case, bearing damage can result in a complete failure of the rolling bearings, which can result in unwanted downtime. Therefore, it is important to detect the first signs of rolling bearing damage early on and to remedy their causes. In this article, we discuss the symptoms of bearing damage, its causes and typical failure modes.

How does bearing damage occur?

Bearing damage can be caused by a variety of causes, some of which are due to wear and tear and incorrect assembly. However, bearing damage often occurs due to improper design during the design phase, which can lead to overloading of the bearing. In addition to faulty installation and overload, contamination and insufficient or even missing lubrication are typical reasons why bearings fail.

In any case, an in-depth review of the causes is necessary in case of bearing damage in order to ensure the longevity and safety of the application. The damaged bearings should not be inspected in complete isolated. Surrounding components can be negatively affected by the disturbed operating behavior of the bearing or can even be considered a potential cause of the bearing damage.

What symptoms can be used to identify bearing damage?

Bearing damage can usually be recognized by the fact that the operating behavior is gradually changing. Depending on the operating environment and application, bearing damage can be noticed in the form of different symptoms.

Very often, bearing damage can be detected by unusual operating noise. Wistling, wining, or uneven noises are a clear indication of damage. Bearing noise is often caused by inadequate or unsuitable lubrication. But excessive loads and bearing installation errors can also be a reason for bearing noise.

If the bearings are accessible or visible from the outside, damage can also be detected by uneven bearing operation. Uneven or inaccurate operation can be caused by shocks, vibrations, but also by dirt. Another symptom of bearing damage is excessive temperatures. Excessive temperatures in the bearing can generally be attributed to increased friction. However, an excessively low or high amount of lubricant, overloads or installation errors can also increase the bearing temperature.

In principle, bearings should be inspected for these symptoms during routine inspections in order to avoid rolling bearing damage and to preemptively counteract any resulting downtime of the application.

What are the typical failure modes?

Each type of damage has its own characteristic features and failure modes. Based on the failure modes, conclusions can be drawn about the causes of the defect and appropriate countermeasures can be initiated. Typical bearing damage for example includes wear, corrosion, indentations, burnishing, and current flow. Damaged or failed bearings often have a combination of different damage.

In the following, some of the most common failure modes, their characteristics and possible causes are examined in more detail and highlighted with examples.

Bearing damage due to wear

Wear includes surface damage caused by sliding friction on the roller bodies, journal surfaces or end faces, among other things. In the case of rolling bearings, there is hardly any noticeable wear under normal circumstances. However, wear can occur if the bearing comes into contact with foreign particles or the lubrication is insufficient. Vibrations on stationary bearings can also cause wear.

Small particles such as metal abrasion can cause wear on the raceways, rolling bodies and the cage of the bearing, which in turn causes the surfaces to appear matte. The granulation and the type of particles determine the degree of matting. For example, a greenish discoloration of the bearing grease can indicate abrasion on the brass cage. Additional signs of wear can be small indentations on the raceways. Among other things, this is due to a lack of cleanliness during installation, ineffective seals or contaminated lubricants.

If too little lubricant is present or the latter loses its lubricity, a sufficiently viable lubricant film cannot form. This leads to metal-on-metal contact between the rolling bodies and raceways. When the lubricant is completely consumed, the temperature rises significantly. As a result, the hardened rolling bearing steel loses its hardness and the surfaces discolor. In extreme cases, the temperature can rise so high that the bearing seizes. If the lubricating film is missing and this leads to metal-on-metal contact between rolling bodies and tracks, vibrations can cause small movements between rolling bodies and raceway rings. This in turn can lead to small particles breaking off the surfaces. Over time, this creates depressions in which rust often develops due to the oxidation of the detached particles.

Bearing damage due to indentations in raceways

Indentations or pitting in the bearing surfaces occur when high loads are applied to a stationary bearing or when foreign objects enter the bearing. However, overloading due to improper installation can also cause indentations.

If foreign particles penetrate the bearing and are rolled into the raceways by the rolling bodies, indentations can occur. Foreign objects and other dirt particles can remain in the bearing due to insufficient initial cleaning or lack of cleaning after work processes. If bearings are under load and vibration for a longer period of time, so-called idle markings can form. Typical for idle markings are the uniform impressions distributed over the raceway at the distance of the rolling bodies. Peak loads due to shocks or static overload can also lead to depressions in raceways that are also manifested as indentations at the distance of the rolling bodies. The failure modes often result from excessive force on one of the raceway rings during assembly and disassembly.

Since the contours of the indentations are subjected to a great deal of stress each time the rollover occurs after foreign bodies are pressed in, so-called breakouts can occur. These can propagate over the entire raceway in a V-shaped manner in rolling direction. The stronger the definition of the impression contour (e.g. for hard minerals) and the higher the load, the higher the probability of further breakouts and bearing failure.

Bearing damage due to corrosion

Corrosion is one of the most severe bearing failure modes. It occurs in operating conditions where moisture or aggressive media come into contact with the bearing and the lubricant can no longer protect the bearing surfaces.

Corrosion due to moisture (rust) forms unevenly distributed across the bearing in the form of brown-colored rust scars. Extended downtime also frequently results in rust defects at the distance of the rolling elements. Corrosion damage due to moisture occurs due to improper storage, condensation due to temperature fluctuations, but also due to inadequate sealing and defective cover discs.

In addition to moisture, aggressive media can also cause corrosion damage to bearings. Therefore, it is important to take into account the operating materials in contact with the bearing during the design and to only perform maintenance with the approved cleaning agents. The color of the defects is usually different and are caused by sealing errors or contact with aggressive chemicals or unsuitable lubricants.

Other types of bearing damage

On rolling bearings in current-carrying environments (e.g. in electric motors or during welding), current flow through the bearing can lead to thermal material damage. Such failure modes are usually manifested as craters or riffling on the bearing and are formed at the contact points of the rolling partners. This form of bearing damage can extend over the entire bearing surface.

Bearing damage due to improper lubrication is a common problem that can significantly impair bearing performance and service life. Too much lubricant can be as harmful as too little. Overfilling leads to increased resistance in the bearing, which in turn leads to excessive heat generation. This heat can impair the lubricity of the lubricant and cause deterioration of the bearing components. In contrast, a lack of lubricant leads to increased friction between the moving parts of the bearing. This results in excessive wear, increased heat generation, and ultimately in premature bearing failure. Water, dust, and other contaminants can contaminate lubricants and reduce their lubricity. In addition, lubricants lose their effectiveness over time. Old lubricants can lose their lubrication properties, which can lead to increased wear and bearing failure.

Stresses in the mounting orientation affect the raceway insofar as non-uniform wear becomes apparent. The type of stress that is present is decisive here. Radial stress on a ball bearing, such as a bearing gap that is too narrowly dimensioned, can manifest itself by showing circumferential traces on both rings of the bearing. Failure modes on ball bearings due to oval stresses, on the other hand, are discernible from a higher wear of opposing running surfaces, while the parts in between have a significantly lower wear. An example of this would be an outer ring stressed by radial compression loads. The causes of stress can often be found in the deformation of bearings or surrounding components. An unsuitable selection of a bearing with regard to the acting forces of the application can also be the cause of the stress.

How can you prevent bearing damage?

In general, carefully selected rolling bearings reach their expected service life. Nevertheless, often avoidable errors lead to premature wear of these bearings. These premature defects usually do not result from material fatigue, but are due to a number of defects:

• Assembly error
• Faulty handling
• Insufficient lubrication
• Ingress of foreign bodies
• Excessive heat development.

In order to take effective measures against such failures, a thorough examination of the failure mode, the type of failure, and the potential causes is required. This requires accurate knowledge of all relevant pre-failure and post-failure conditions, including specific application, operating conditions, and environment. Implementing successful countermeasures can largely prevent similar problems in the future.

Therefore, potential risks to bearing damage and failure should already be taken into account when selecting bearings and determining specifications. For example, in case of doubt, non-serviceable (permanently greased) bearings should be preferred over re-lubricated bearings. In addition, the bearing material and lubricant should correspond to the bearing capacity, static and dynamic loads, expected temperatures, and ambient media in contact with the bearing.

MISUMI offers a comprehensive selection of high-quality, individually configurable rolling bearings. In addition, the specifications of the bearing manufacturer and the lubricant supplier should always be observed. Proper assembly and regular inspection and maintenance helps you maintain rolling bearings for a long service life and a high level of safety in your application.

In modern production management, the monitoring of bearing locations can also be done by noise monitoring (microphone), frequency analysis or vibration analysis. The function principle is based on the determination of excessive sound emissions or on the examination of ball bearing-specific damage frequencies that indicate damage to the bearings concerned. These systems are available for practitioners and are directly integrated into production control. However, it is also possible to use appropriate tests independently of production control as part of maintenance or preventive maintenance. MISUMI offers the following measuring instruments for monitoring: Vibration meter and industrial strobe (monitors vibration while running).