Load ratings breakthrough

BEARINGS ARE found everywhere. They’re often overlooked, but they are integral to a myriad of machinery all over the world, from the microwave in your kitchen to jets soaring over your head. As such, companies need to be able to accurately judge just how long a bearing will be able to do its job without failing. After all, it’s not good if an essential bearing in a brand-new wind turbine breaks down only a week after installation.

This is where load ratings come in. In simplest terms, the basic dynamic load rating explains what load a bearing can endure and still manage to make one million revolutions without any signs of material fatigue. Load ratings are governed by the ISO 281:2007 standard, which states that when you test a batch of identical bearings, nine out of every ten need to make it through those million revolutions without issue.

Non-destructive testing

ISO 281 does allow for an increase in the service life under certain conditions. In the past, however, any modifications to the bearing had to be followed by a further battery of tests. NSK, however, has developed a non-destructive testing method for exactly these sorts of situations which makes it possible to dispense with further testing and allows the end user to benefit from a fundamental increase in the service life of their bearings.

Generally speaking, most people use a manufacturer’s existing catalogue to calculate bearing life. Alternatively they can usually input the external loads and the bearing information into a calculation programme. The basic calculation, however, is still carried out in accordance with ISO 281.

This type of calculation has become the standard worldwide. However, since the methods of determining a bearing’s lifespan were developed and introduced (all the way back in 1962), material purity and manufacturing capabilities have both come on leaps and bounds. As a result, the actual service lifespan of a bearing tends to be quite a lot longer than that which is suggested by testing under ISO 281.

NSK’s solution has been to simplify things, bringing in computer-aided models alongside empirical tests. The basic idea is that we use these models as a ‘digital twin,’ with the ultimate aim of being able to dispense with service life tests with real, physical bearings and to determine changes to dynamic load ratings purely by means of digital evaluations.

The first step was to develop a suitable test procedure and test criteria. NSK’s own internal research has shown that the composition and quality of bearing steel, in particular the proportion of the steel which is made up of non-metallic impurities, has a huge impact on a bearing’s service life, indeed is actually a more accurate indicator of its lifespan than the above calculation methods. Our engineers realised that an evaluation method based on fracture mechanics (a field of mechanics concerned with the study of the propagation of cracks in materials) could provide more meaningful insight.

Quantitative evaluation

That was why NSK sat down with Kyushu University in Japan to develop a quantitative evaluation method that could determine which factors influenced the process of crack propagation in a material, and to what extent. Notably, by combining the new method with an ultrasonic inspection technique that scans the non-metallic inclusions in a large volume of steel, NSK discovered it could predict the life of its bearings with much higher accuracy.

As a direct result, NSK has revised the basic dynamic load rating of many bearings with no change in design or materials, while still applying the appropriate safety margin to any uprated values. With this revision, the estimated life of radial roller bearings, for example, is up to twice that which it was before.

Even better, as the basic dynamic load ratings of bearings improves, they become suitable for use in higher load conditions, meaning that company’s can opt for smaller bearings which are still able to perform the same function of their larger counterparts. Obvious benefits include machine downsizing and weight reduction. To take just one example, machines that used NSK’s HR32306J tapered roller bearing can now adopt the smaller HR33206J. This change delivers a 14% reduction in bearing outside diameter, a 13% reduction in width and approximately 38% less weight.

Those seeking carbon neutrality also benefit. Smaller bearings require fewer resources to manufacture. They also yield lower torque, resulting in less power consumption and fewer CO₂ emissions during use. With the same aforementioned change of bearing, users will see a reduction in bearing torque and power consumption of approximately 18% (around 48kWh per annum). According to NSK calculations, this converts to approximately 22kg of CO₂ emissions savings.

This might sound too good to be true, but end users can rely on the fact that the new bearing life values are determined on the basis of a carefully studied methodology supported by an extensive empirical database. All new – and fundamentally higher – values will be within the safe range. NSK’s researchers and developers used a quantitative evaluation method to determine which factors or the number and size of impurities influence the process of crack propagation in a rolling bearing steel and to what extent. Computers with extreme computing power were used to run and analyse simulations.

In short, this new testing method, and the corresponding adjustments to estimated lifespan, can see you save money, reduce downtime and maintenance, save on installation space and benefit the environment. What’s not to like?

www.nskeurope.com