A new life model of SKF for hybrid ceramic bearings

Hybrid ceramic bearings composed of ceramic silicon nitride rolling elements and steel rings have always been the first choice bearings in industries that require high speed and high precision applications such as machine tool spindles, because hybrid ceramic bearings are light in weight and have good insulation properties. Moreover, it can maintain the advantages of good performance under harsh lubrication and pollution conditions, and is suitable for applications in many new applications ranging from electric vehicle power transmission systems to industrial pumps and compressors.

Engineers know from experience that hybrid ceramic bearings perform extremely well in these applications, and their service life is usually many times longer than traditional all-steel bearings. However, until recently, the calculation model used by SKF engineers to estimate the working life of a bearing still gave the opposite result. This is because engineers’ standard formulas for calculating bearing rating life cannot accurately reflect the real working challenges faced by bearings. Guillermo Morales-Espejel, the Chief Expert of SKF Research and Technology Development Center, explained: “The traditional bearing life model is based on subsurface fatigue. When the bearing rotates, the components will continue to be loaded and unloaded up to millions. After two cycles, the material fatigue limit is reached, which will eventually lead to failure.

The fatigue properties of materials are well known, and the expected bearing load and speed data in the application can be substituted into the formula to calculate the rated life of a given bearing design model. Engineers mainly use the dynamic load rating of the bearing to quantify the subsurface performance of the bearing. To determine the dynamic load rating of various types of bearings of a certain manufacturer, please refer to its general product catalog or online product catalogue.

The above-mentioned traditional models are widely used and are included in international standards. But Morales-Espejel explained that this model is not entirely suitable for hybrid ceramic bearings. "Because ceramic rolling elements are more rigid than steel, they deform less under load. This means that the load will be concentrated in a smaller area of the material, resulting in increased stress and accelerated subsurface fatigue." 

However, more importantly, the experience in the real environment does not always match the traditional model. Morales-Espejel explained, “According to our industry experience, most bearing failures are due to surface problems rather than the problems of the components themselves, that is, the root cause of bearing damage is usually caused by poor lubrication or contamination.

The introduction of correction coefficients into the subsurface-based model still cannot represent the true performance of the bearing in operation. Therefore, Morales-Espejel and SKF colleagues began to try a better way in 2012. He said that to establish a new bearing life model, three conditions need to be met. "First, we need a subsurface fatigue model of the material. We already have this model on hand. Second, we need a surface fatigue model. Finally, we need durability test data. We can use these data for model calibration and verification."

When this method is used, it is necessary to understand the bearing surface performance in detail, including its friction characteristics and the way dust particles form dents on the bearing surface under load. Although the initial conceptual model as the Universal Bearing Life Model (GBLM) was shown at Hannover Messe in 2015, it did not involve modeling hybrid ceramic bearings at that time.

Morales-Espejel explained, “We already know that hybrid bearings have advantages in many common operating conditions. But when the bearing is loaded with heavy loads and works in a clean, well-lubricated environment, subsurface fatigue may be the ultimate failure mode. At this time, the performance of the steel bearing may be better than the performance of the hybrid ceramic bearing. And many bearings are working under a small load, and there is a possibility of poor lubrication or pollution. Our model will be able to show whether the hybrid bearing solution can in those applications it has a long life and can quantify these differences."

Morales-Espejel and his colleagues performed calculations for some representative real applications. If the bearings used in the water pump are operated under oil bath lubrication and thin oil conditions that cause poor lubrication, the rated life of the hybrid bearing is 8 times longer than that of the equivalent steel bearing. For screw compressor bearings that operate with contaminated lubricating oil, the rated life of hybrid ceramic bearings is 100 times longer than that of traditional steel bearings.

After extensive internal testing by SKF application engineers, the GBLM calculation model for hybrid ceramic bearings has now become a standard part of the toolkit supported by the company's customers. With the advancement of manufacturing technology, the utilization rate of hybrid bearings has been improved, and the cost gap between hybrid bearings and traditional bearing designs has been narrowed. At the same time, hybrid ceramic bearings can bring performance advantages in more and more applications.

Morales-Espejel said, “Using lower viscosity lubricants or as little lubrication as possible is a major change in the current bearing industry, and people’s pursuit of energy saving and stricter environmental regulations have jointly promoted this shift.” He also It is pointed out that in applications ranging from railway locomotives to automobile engines to industrial pumps, only hybrid ceramic bearings can simultaneously achieve low energy consumption and high reliability in applications such as railway and automobile engines and industrial pumps.

Another important growth area for hybrid bearings is electric transportation. The electric transmission systems of cars, trucks and trains require bearings to operate reliably at high speeds, high accelerations, high temperatures and poor lubrication. At the same time, these bearings must withstand the influence of shaft currents, because shaft currents will destroy the lubricating oil film and damage the rolling surface. Hybrid ceramic bearings have excellent electrical insulation properties and many other advantages, making them the most ideal solution for such applications.

At present, SKF's GBLM calculation tool is used on average up to 260 times a day. Hybrid ceramic bearings are not always the best in comparison with traditional bearings, but this is precisely why the new modeling method is so important. The actual application is not to replace all steel-steel bearings with hybrid bearing designs, but to use hybrid bearings only when economical. , GBLM can ensure that customers make decisions based on stable and reliable data.