2023 May the Fourth Week XBRZ Technical Knowledge: Hybrid ceramic bearings in the real world

Time : Май 26, 2023

SKF engineers have solved a major puzzle in bearing performance. Nowadays, they use this discovery more than 260 times a day to help customers design devices for use under the most stringent application conditions in the world. What is the root of the puzzle? Why do hybrid ceramic bearings have high working performance?

For over 50 years, hybrid ceramic bearings using ceramic silicon nitride rolling elements and steel rings have been the preferred bearing in industries such as machine tool spindles that require high speed and precision applications. Nowadays, hybrid ceramic bearings have the advantages of light weight, good insulation performance, and can maintain good performance even under harsh lubrication and pollution conditions, making them suitable for applications in many new fields, from electric vehicle power transmission systems to industrial pumps, compressors, and so on.

Engineers have learned from experience that hybrid ceramic bearings have excellent performance in these applications, and their service life is usually many times longer than traditional all steel bearings. However, until recently, the calculation models used to estimate the working life of bearings still gave opposite results.

Problems that arise

In the opinion of Guillermo Morales Espejel, Chief Scientist of the SKF Research and Technology Development Center, this is because the standard formula used by engineers to calculate the rated life of bearings cannot accurately reflect the challenges faced by bearings in real work. He explained, “The traditional bearing life model is based on subsurface fatigue. When the bearing rotates, the component will continue to be under load and not under load for millions of cycles, reaching the material fatigue limit and ultimately leading to failure.

The fatigue performance of materials is well-known, and engineers can substitute the expected load and speed data in applications into formulas to determine the rated life of a given bearing design model. The rated dynamic load C of bearings is mainly used to quantify the subsurface performance of bearings. The rated dynamic load C of various types of bearings can be found in SKF’s overall product catalog or online product catalog.

The traditional models mentioned above are widely used and have been incorporated into international standards. But Morales Espejel explained that this model is not entirely applicable to hybrid bearings. Because ceramic rolling elements have greater stiffness than steel, they deform less under load. This means that the load will be concentrated in smaller material areas, leading to increased stress and accelerating subsurface fatigue.

However, more importantly, real-life experiences do not always match traditional models. Morales Espejel explained, “Based on our industry experience, most bearing failures are due to surface issues rather than the problems of the components themselves. The root cause is usually damage caused by poor lubrication or contamination.” The above analysis is not controversial, and modern standards such as ISO281 have also added correction factors to consider these effects.

A brand new model

However, introducing correction coefficients into subsurface based models still cannot represent the true performance of bearings in operation. Therefore, Morales Espejel and colleagues from SKF began trying better solutions in 2012. He stated that to establish a brand new bearing life model, three conditions need to be met. Firstly, we need a subsurface fatigue model of the material, which we already have on hand. Secondly, we need a surface fatigue model. Finally, we need durability testing data that we can use for model calibration and validation.

The SKF team devoted themselves to the research of new models in the next two years, absorbing decades of research achievements in materials science and tribology. When using this method, it is necessary to have a detailed understanding of the bearing surface performance, including its friction characteristics and the way dust particles form dents on the bearing surface under load. Although the initial conceptual model as the General Bearing Life Model (GBLM) was already presented at the Hanover Industrial Expo in 2015, modeling of hybrid ceramic bearings was not involved at that time.

Morales Espejel explained, “You need data to calibrate and validate any bearing life model, and in order to collect enough data, it can only rely on hard work without any shortcuts. We need to draw curve graphs to describe the performance of bearings under various loads and surface conditions. For each point on the curve, approximately 30 bearings need to be tested, and it is expected that several of them will fail.

The SKF team also needs to compare bearings using steel and ceramic rolling elements, as well as bearings operating in poorly lubricated and polluted environments. All of these tasks add up to hundreds of bearing tests to be completed. In summary, scientists and technicians at the SKF factories in the Netherlands and Austria need to spend another four years developing testing programs and modifying conceptual models. This work was finally completed a year ago, and Morale Espejel and his team finalized the new hybrid bearing GBLM.

A deep understanding of the real environment

What does the new model mean for engineers and designers? Morales Espejel explained that, We already know that hybrid bearings have advantages in many common operating conditions. When the bearing load is large but can operate in a clean and well lubricated environment, subsurface fatigue may be the ultimate failure mode, and steel bearings may perform better than hybrid ceramic bearings. However, many bearings operate under smaller loads and are likely to have poor lubrication or contamination. Our model will be able to display hybrid shafts Can the bearing scheme have a longer service life in those applications and quantify these differences.

To display differences? Morales Espejel and his colleagues conducted calculations for some representative real-world applications. If the bearing used in the water pump works under the condition of oil bath lubrication and thin oil that causes poor lubrication, the rated life of the hybrid bearing is 8 times longer than that of the same steel bearing. For screw compressor bearings operating under contaminated lubricating oil, the rated life of hybrid ceramic bearings is 100 times longer than that of traditional steel bearings.

Just in time

After extensive internal testing by SKF application engineers, the GBLM calculation model for hybrid ceramic bearings has become a standard component of the company’s customer support kit. Its emergence can be said to be at the right time. The advancement of manufacturing technology has improved the utilization rate of hybrid bearings and narrowed the cost gap between hybrid bearings and traditional bearing designs. Meanwhile, hybrid ceramic bearings can bring performance advantages in an increasing number of applications.

Morales Espejel stated that “the use of lubricants with lower viscosity and minimal lubrication is a significant shift in the current industry. The pursuit of energy conservation and stricter environmental regulations have jointly driven this shift.” He also pointed out that in applications ranging from railway locomotives to automotive engines to industrial pumps, Only hybrid ceramic bearings can simultaneously achieve low energy consumption performance and high reliability in applications such as railway and automotive engines and industrial pumps.

Another important growth area is electric transportation. The electric transmission systems of cars, trucks, and trains require bearings to operate reliably at high speeds, high acceleration, high temperatures, and poor lubrication. These bearings must also withstand the influence of shaft current, as it can damage the lubricating oil film and the rolling surface. Hybrid ceramic bearings have excellent electrical insulation performance and many other advantages, making them the most ideal solution for such applications.

Morales Espejel stated that the level of attention paid to hybrid ceramic bearing technology is so high that the company’s application engineers and customers are currently using SKF’s GBLM calculation tool an average of 260 times a day. However, he emphasized that hybrid ceramic bearings do not always stand out in comparison to traditional bearings, but this is precisely why the new modeling method is so important. Our view is not to replace all steel steel bearings with hybrid bearing designs, but only when it is cost-effective. “Our hybrid bearing GBLM ensures that customers make these decisions based on stable and reliable data.”

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