Engineers know from experience that hybrid bearings can perform extremely well in applications such as machine tool spindles, 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.
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. SKF’s dynamic load rating C is mainly used to quantify the sub-surface performance of the bearing.
Although this traditional model is widely used, it doesn’t always align with real-world experience. “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
In 2012, Morales-Espejel and colleagues at SKF set out to create a new bearing life model to better reflect reality. To do so, 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. An initial concept model was presented at the Hannover Messe in 2015.
Then came phase three. “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 in 2019, allowing Morale-Espejel and his team to finalise the new generalised bearing life model for hybrid bearings.
APPLICATIONS OF INTEREST
The new model reveals the extent to which hybrid bearings are appropriate for a particular type of duty. “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.”
For example, 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.
And the range of applications where hybrid bearings can offer advantages is growing rapidly, the company asserts. “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 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.
The GBLM model has now become a standard part of SKF’s customer support toolkit. Hybrid bearings don’t always emerge as the winner in comparison with conventional designs, emphasises Morales-Espejel, but that is exactly why the new modelling approach is so important.
He concludes: “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.”
BOX: Why bearings fail
SKF manufactures some ten billion bearings each year and, given the harsh conditions to which they are often subjected, they are reliable. Approximately 90% of these bearings outlive the equipment to which they are fitted. Only 0.5% of bearings fail in service, but this still means that some 50 million are replaced due to damage or failure every year, and each of these failures will likely have financial implications for their operators in terms of lost production, damage to adjacent parts and the cost of repairs, the manufacturer says.
There are numerous reasons why bearings can damage or fail. Generally speaking, around one-third fail due to fatigue, while another third fail due to issues with lubrication. Contamination causes a sixth of bearing failures, while the balance is accounted for by other factors, such as improper handling and mounting, heavier or different loading than was anticipated, and poor fitting.
When attempting to predict how, and perhaps more importantly when, a bearing will fail, a huge number of variables must be taken into account, including the application for which it will be used, the environment in which it will operate, the lubricants used and the loads to which it will be subjected, to name but a few.