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Lubricant contamination is with about 25% one of the most frequent causes of premature failure of roller bearings [Tec00, p. 11]. In addition, the detection of contamination is very complex and often not possible, especially in the case of grease lubrication. In order to prevent follow-up costs due to machine downtime, either the bearing must be replaced long before it actually reaches the end of its service life or the lubricant must be changed as a preventive measure. Both approaches are often associated with high economic and ecological costs (consumption of resources, energy consumption for bearing production). However, these measures can at most reduce the risk of bearing failure. In the event of exceptional contamination, bearing damage can still occur, which then leads to machine downtime and additional costs.
What are common contaminations?
Contamination often originates from the bearing’s environment and gets past the seal into the bearing. However, abrasion within the machine or directly in the bearing can also be a source of contamination. In the case of oil lubrication in particular, such contaminants can spread and enter the bearings involved.
The influence of solid contaminants or particles depends on their size, hardness and concentration. Particles with a higher hardness than the bearing shorten the service life drastically. They lead to pitting and chipping of the bearing surface. Softer particles can also damage the bearing. They can cause the hydrodynamic lubricating film in the contact zone to break off, resulting in increased wear. They can also adhere to the raceway surface and then lead to failure due to the increased surface pressure [Kou14].
Hard particles that are larger than the lubricating film thickness are ground up, softer particles are flattened. Such particles whose diameter is within the range of the lubricating film (10^-6m) also lead to a lubricant deficiency by disturbing the hydrodynamic lubricating film [Dwy9].
The speed of bearing damage increases linearly as the concentration of contaminants increases. At the same time, the damage, measured in terms of abrasion, is most severe in the first 10 hours after the contamination occurs [Cel77].
Another very common contamination in bearings is water. It can be found in the environment of most applications, either in liquid form or due to air humidity. Water can also significantly impair the effectiveness of lubricants and is very difficult to detect.
Undissolved water within the bearing can also be split by the high pressure in the contact zone of the rolling elements. The atomic hydrogen then can lead to so-called hydrogen embrittlement of the bearing steel and consequently to pitting. At the same time, the oxygen produced leads to accelerated ageing of the lubricant due to oxidation [Car10, p. 2].
The detection of water contamination is often even more difficult than that of particles. Conventional vibration monitoring would only react to damage, the cause of which cannot be assigned. In bearings with circulating oil lubrication, for example, the water content in the oil can be detected using H20 spectroscopy, but this requires additional effort and does not allow live monitoring [Lei18].
Is it possible to identify contamination using HCP Sense sensor technology?
The HCP Sense technology is based on the electrical behavior of the bearing when an AC voltage is applied. The measured impedance can be used to draw conclusions about the lubricating film thickness via the electrical resistance and the capacitance. The behavior is similar to that of a plate capacitor, for which the following relationship between capacitance, capacitor area and distance between the two electrodes applies:
The decisive factor for the capacitance in the roller bearing is the rolling contact that is subjected to the greatest load at this point in time. Due to the flattening of the rolling elements (increasing surface area A) and the decreasing lubricant film thickness d, the capacitance C increases.
This measurement provides a direct insight into the lubrication condition of the rolling elements under the highest load. In other words, exactly at the point where any bearing damage would occur. As the measurements are carried out at high sampling rates, the exact behavior during a rotation can be analyzed.
If a particle enters the contact zone between the rolling elements, this is shown, for example, by a break in the lubricating film and consequently an increase in capacitance with a simultaneous drop in resistance. This can be observed in the section of raw measurement data shown here. These events occur randomly and can be distinguished from damage that has already occurred or irregularities in the bearing surface. In addition, a distinction can be made based on different electrical properties, such as mineral contaminants like sand and metallic contaminants.
In this way, individual particles that have entered the bearing can be recognized and identified and the concentration of these can be deduced from the number of occurrences. This is important, as the ingress of small quantities or particularly soft materials does not automatically justify or necessitate the replacement of the lubricant or even the bearing. It may then be possible to take measures at an early stage to counteract further contamination or to find and eliminate the cause. This not only prevents damage, but also efficiently minimizes maintenance intervals and the associated costs. HCP Sense provides you with comprehensive support in this regard, with many years of experience in roller bearing technology in the fields of research and industry.
Benefits with HCP Sense
- Prevention of damage due to lubricant contamination
- Identification of the smallest amounts of contamination down to individual particles
- Gaining knowledge about the source of contamination as a basis for root cause analysis instead of symptom treatment
- Recommendations for countermeasures and lubricant changes, tailored to the needs of your specific use case
- Support with experience and expertise gained from research and industry
References
[Car10] Carlos Goncalves und L.R. Padovese. „Vibration and oil analysis for monitoring problems related to water contamination in rolling“. In: International Brazilian Conference on Tribology (2010).
[Cel77] B. CeleFitzsimmons und H. D. Clevenger. „Contaminated Lubricants and Tapered Roller Bearing Wear“. In: A S L E Transactions 20.2 (1977), S. 97–107.
[Dwy99] R.S Dwyer-Joyce et al. Surface damage effects caused by debris in rolling bearing lubricants, with an emphasis on friable materials. 1999.
[Kou14] D. Koulocheris et al. „Experimental study of the impact of grease particle contaminants on wear and fatigue life of ball bearings“. In: Engineering Failure Analysis 39 (2014), S. 164–180.
[Lei18] Dirk Leimann, Gerhard Gajewski und Dirk Arnold. „Wassergehalt in Ölen: Einfluss unterschiedlicher Wassergehalte in Ölen auf die Ermüdungslebensdauer von Wälzlagern und die Grübchentragfähigkeit einsatzgehärteter Stirnräder“. In: FVA 1299 (2018).
[Tec00] Schaeffler Technologies. Wälzlagerschäden, Schadenserkennung und Begutachtung gelaufener Wälzlager. 2000. url: https://www.schaeffler.com/remotemedien/media/_sharedmedia/ 08_media_library/01_publications/schaeffler_2/tpi/downloads_8/tpi_176_de_de.pdf (23. Juli 2023)
