The real life hybrids

A new modelling approach helps engineers to understand the real-world benefits of hybrid bearings.

SKF engineers have solved one of the great mysteries of bearing performance. They are now applying their findings more than 260 times a day to help customers design machines for some of the world’s most demanding applications. The root of that mystery? Why hybrid bearings work so well.

For more than 50 years, hybrid bearings, which use ceramic silicon nitride rolling elements and steel rings, have been the preferred choice for high-speed, high-precision equipment such as machine tool spindles. Today, the hybrid bearing’s combination of low weight, good electrical resistance and good performance under demanding lubrication and contamination conditions is helping it find many new applications, from electric vehicle powertrains to industrial pumps and compressors.

Engineers know from experience that hybrid bearings can perform extremely well in these applications, often lasting many times longer than their conventional all-steel counterparts. Yet until recently, the design calculations used to estimate the operating life of bearings often gave the opposite result.

Surfacing the problem
According to Guillermo Morales-Espejel, Principal Scientist at SKF Research and Technology Development, that’s because the standard equations engineers use to calculate the rating life of a bearing don’t accurately reflect the real-world challenges faced by bearings. “The conventional bearing life model is based on sub-surface fatigue,” he explains. “As bearings rotate, their components are continually loaded and unloaded. Over millions of cycles, fatigue accumulates in the material, eventually leading to failure.”

Because fatigue behaviour is well-understood, engineers can plug information about the loads and speeds expected in their application into an equation to determine the rating life of a given bearing design. The dynamic load rating C, which can be found for any bearing in the SKF general catalogue or in the online product catalogue, is mainly used to quantify the sub-surface performance of the bearing.

This traditional model is widely used and incorporated into international standards, but Morales-Espejel explains that it doesn’t show hybrid bearings in the best light. “Because the ceramic rolling elements are stiffer than steel, they deform less under load. That means loads are concentrated over a smaller area of material, increasing stress and accelerating sub-surface fatigue.”

More significantly, however, real-world experience doesn’t always align with the traditional model. “We know from experience in the field that the majority of bearings fail due to problems at the surface, not in the body of the material,” explains Morales-Espejel. “The root cause is usually damage caused by poor lubrication or contamination.” Nobody disputes that analysis, and modern standards such as ISO 281 include correction factors in an attempt to accommodate these effects.

A new model
Those correction factors introduced in subsurface-based models didn’t attempt to represent the real behaviour of bearings in service, however, so in 2012, Morales-Espejel and colleagues at SKF set out to do better. To create a new bearing life model, he says, they needed three things. “The first was a model of sub-surface fatigue within the material, which we already had. The second was a model for failure at the surface. The third was data from endurance tests that we could use to calibrate and validate our model.”

The SKF team worked on the new model over the next two years, drawing on decades of study in materials science and tribology. The approach required a detailed understanding of the behaviour of bearing surfaces, from their friction characteristics to the way dirt particles indent them under load. Although an initial concept model was presented as a Generalized Bearing Life Model (GBLM) in 2015 at the Hannover Messe, at that time it did not cover the modelling of hybrid bearings.

“You need data to calibrate and then validate any bearing life model, and to collect enough data, there is no substitute for hard graft.” explains Morales-Espejel. “We needed to build curves describing the behaviour of bearings over a wide range of loads and surface conditions. For each point on those curves we needed to test around 30 bearings, with the expectation that several of them would fail.”

The SKF team also needed to compare bearings with steel and ceramic rolling elements, and bearings operating with poor lubrication and in contaminated environments. All this added up to hundreds of bearing tests. In total, the test programme and the adaptation of the concept model required a further four years of effort by scientists and technicians at SKF’s facilities in the Netherlands and Austria. That effort was finally completed a year ago, allowing Morale-Espejel and his team to finalise their new GBLM for hybrid bearings.

Real life insights
What does the new model mean for engineers and designers? “We already knew that hybrid bearings had advantages in many commonly experienced conditions,” explains Morales-Espejel. “When a bearing is heavily loaded, but able to run in a clean, well-lubricated environment, sub-surface fatigue is likely to be the ultimate failure mode, and a steel bearing may perform better than a hybrid. But a lot of bearings operate under lighter loads, but with a greater likelihood of poor lubrication or contamination. Our model will show if a hybrid solution would offer a longer life on those applications and will quantify the difference.”

To show just how big the difference can be, Morales-Espejel and his colleagues have run calculations for a number of representative real-world applications. In the case of a pump bearing running with oil-bath lubrication and diluted oil resulting in poor lubrication, the rating life of a hybrid bearing was eight times longer than a steel equivalent. For a screw compressor bearing running with contaminated lubricant, the hybrid offered a rating lifetime a hundred times greater than a conventional steel bearing.

Just in time
After extensive in-house testing by SKF application engineers, the GBLM for hybrid bearings has now become a standard part of the company’s customer-support toolkit. Its introduction could hardly have been timelier. Advances in manufacturing technology have increased the availability of hybrid bearings and reduced the cost gap between hybrid and conventional designs. At the same time, the range of applications where hybrids can offer advantages is growing rapidly.

“There is a significant move in industry to the use of lower viscosity lubricants and minimum lubrication,” says Morales-Espejel. “That’s being driven by the quest for energy savings and by tighter environmental regulations.” In applications from railways and car engines to industrial pumps, only hybrid bearings can provide the necessary combination of low energy consumption and high reliability under these conditions he notes.

Another hugely important growth area is e-mobility. Electric powertrains for cars, trucks and trains require bearings than can survive high speeds, accelerations and temperatures with minimal lubrication. These bearings must also resist stray electric currents, which can burn away lubricant films and damage rolling surfaces. Combined with their other benefits, the excellent electrical insulation properties of hybrid bearings make them the ideal solution for such applications.

Such is the level of interest in hybrid bearing technology that SKF’s GBLM calculation tools are currently being used 260 times a day on average by the company’s application engineers and customers, says Morales-Espejel. Hybrid bearings don’t always emerge as the winner in comparison with conventional designs, he emphasises, but that is exactly why the new modelling approach is so important. “The idea is not to replace all steel-steel bearings with hybrid designs, but to do so when it makes economic sense. Our GBLM for hybrid bearings allows customers to make those decisions based on robust, reliable data.”

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