Hybrid bearings have numerous advantages over conventional steel components, especially under difficult operating conditions. But are they always the best choice? With SKF's Generalised Bearing Life Model (GBLM) for hybrid bearings, customers can now make this decision based on robust, reliable data.
Hybrid bearings with ceramic (silicon nitride) rolling elements and steel rings have been proven for more than 50 years and are the preferred choice for high-speed precision applications. The components have low mass forces, high electrical resistance and good running performance under difficult lubrication and contamination conditions. That is why they are still used in new applications today - from drive trains in electric vehicles to industrial pumps and compressors. Even though hybrid bearings perform excellently and often last many times longer than steel versions, service life predictions have so far usually led to the opposite result.
Bringing the problem to light
According to Guillermo Morales-Espejel, Principal Scientist at SKF Research and Technology Development, this is because the current standard models for lifetime calculation do not represent the properties of bearings accurately enough. "The conventional bearing life model is based on fatigue below the surface," he explains. "During rotary motion, the individual components of a bearing are permanently loaded and unloaded. Over millions of cycles, the material gradually fatigues, eventually leading to failure."
This fatigue behaviour is well researched. Therefore, engineers can determine the nominal life of a particular bearing design based on the anticipated loads and speeds of an application. The basic dynamic load rating C, which can be found for each bearing in the SKF or online product catalogue, quantifies the long-term deep fatigue of the component. "This model is widely used and integrated into internationally valid standards, but it does not do justice to the performance of hybrid bearings," explains Morales-Espejel. "Ceramic rolling elements are stiffer than those made of steel and deform less under load. Here, the load is concentrated on a smaller area of the material, which leads to higher stress and faster deep fatigue."
Experience shows, however, that bearings rarely fail because of this. "We know from practical experience that the majority of failures are due to problems on the surface," says the SKF expert. "The failures are usually based on insufficient lubrication or contamination." Analyses and standards such as ISO 281 take this into account with the introduction of correction factors, but this did not help in depth fatigue-based models to accurately represent the actual behaviour of bearings in service. Therefore, in 2012, Morales-Espejel and his colleagues started looking for a more accurate solution.
A new model emerges
"Creating a new bearing life model required three things," he says. "First, a model for deep fatigue within the material. That was already available. Second, a model for surface failure, and third, data from endurance testing that we could use to calibrate and validate our model." The SKF team worked on the new model for the next two years. The basis was decades of studies in materials science and tribology. This is because the new concept required detailed knowledge of bearing surfaces - from their frictional behaviour to the way they are scored by dirt particles under load.
Although an initial concept was presented at Hannover Messe 2015 in the form of the Generalised Bearing Life Model (GBLM), this did not yet cover the modelling of hybrid bearings. "A lot of data is needed to calibrate and validate a bearing life model, and collecting a sufficient amount of information is tedious and not easy," explains Morales-Espejel. "We had to create curves that describe the behaviour of bearings under a long series of loads and surface conditions. For every single point on these curves, we tested around 30 bearings - always hoping that several of them would fail."
The SKF team also had to compare bearings with rolling elements made of ceramic with those made of steel, as well as those used with poor lubrication and in contaminated environments. So, over the next four years, the experts at SKF's facilities in the Netherlands and Austria tested a total of several hundred components and adapted the existing concept until Morales-Espejel and his team were finally able to present their new GBLM for hybrid bearings in 2019.
Findings from practice
What are the concrete advantages of the new model for engineers and designers? "We already knew that hybrid bearings have their advantages under many conditions," explains Morales-Espejel. "If a bearing is running under high load but is used in a clean, well-lubricated environment, deep fatigue is likely to be the reason for failure. Then a steel bearing may be the better choice. However, many bearings operate under lighter loads, but often with poor lubrication or in contaminated environments. Our model shows whether a hybrid solution offers longer life in such a setting and can quantify this."
Morales-Espejel and his colleagues performed calculations for a number of representative real-world applications. They found that in the case of a pump bearing operating with oil bath lubrication, for example, whose lubrication deteriorates due to diluted oil, the life of a hybrid bearing is eight times longer than that of a comparable steel bearing. In a screw compressor bearing with contaminated lubricant, a hybrid bearing even offers a hundred times longer service life in comparison.
GBLM enables decisions based on solid data
After extensive testing, the GBLM for hybrid bearings has become an integral part of SKF's customer support toolkit. And not without reason. Advances in manufacturing technology have led to higher availability of hybrid bearings and reduced the cost difference between hybrid and steel bearings. At the same time, the number of applications where hybrid bearings can bring their well-known advantages to bear is growing very fast. "There is a clear trend in the industry towards lower viscosity lubricants and minimal lubrication," says Morales-Espejel. "The fact that companies are also trying to save energy and the introduction of stricter environmental regulations are further reinforcing this trend. In applications ranging from rail vehicles and car engines to industrial pumps, only hybrid bearings can provide the needed combination of low energy consumption and high reliability under the conditions mentioned."
Another important growth area is e-mobility. Electric drive trains for cars, trucks and trains require bearings that can withstand high speeds, accelerations and temperatures with minimal lubrication. These must also withstand stray currents that can burn away lubricating films and damage running surfaces. Hybrid bearings are the ideal solution for such applications thanks to their very good electrical insulation properties. "The interest in hybrid bearing technology is so great that our GBLM calculation programs are used by our engineers and customers an average of 260 times a day," Morales-Espejel is pleased to say.
He stresses that hybrid bearings are not always the better choice compared to conventional designs, but that is exactly why the new modelling concept is so important. "It is not a question of replacing all steel bearings with hybrid designs, but exactly when it makes economic sense. Thanks to our GBLM for hybrid bearings, customers can now make that decision based on robust, reliable data."