World’s First! Highly Accurate Prediction of Rolling Bearing Life Applying Ultrasonic Testing

After years of research and development NSK is revising-up the basic dynamic load rating of many bearings to up to double the equivalent rolling contact fatigue life (with no change to design and materials!).

You may be surprised to learn that the fundamental formulas we use today to calculate bearing life are more or less the formulas defined in a 1962 report by the International Federation of the National Standardizing Associations (ISA), the precursor to the International Organization for Standardization (ISO). Today, the formulas are codified as the ISO standard “ISO 281:2007 Rolling bearings — Dynamic load ratings and rating life.”

In 2023, over 60 years later, we have learned a thing or two more about calculating and predicting bearing life, and the materials and methods we use today have advanced by leaps and bounds so it is probably about time we reconsider how we evaluate rolling bearing life.

With the standard remaining static for so long, NSK is the first company in 60 years to make a significant breakthrough in bearing life calculation that can improve productivity and contribute to environmental protection. In this article we talk to the NSK researchers and engineers that developed an all-new highly accurate method of calculating bearing life and learn why NSK is refreshing its approach and updating the catalog life rating of many of its products.

A New, More Accurate Indicator of Bearing Life

KomataThe theory underlying the ISO standard for calculating bearing life was developed in 1962 but bearing and material technology has advanced considerably since then. About 20 years ago, NSK noticed a growing gap between the bearing life calculated in the ISO standard and the actual bearing life verified by endurance testing, and we began a more detailed study of the subject.

At that time, around when the research began, we confirmed that the life of NSK bearings was approximately 20 times longer than the life expected under ISO standards. And today, in 2023, the life of NSK bearings can be more than 50 times longer than the catalog life rating based on ISO. Erring on the side of caution is certainly fine, but longer bearing life can be used beneficially to protect the environment and improve production efficiency for example by reducing the frequency of replacement and the amount of waste.

The ISO bearing life calculation formula emphasizes the effect of lubrication conditions. Of course, lubrication greatly affects bearing life, but lubrication technology is much better today than it was in 1962. Our research has demonstrated that under well-lubricated conditions, the composition and quality (the amount of impurities) of the bearing steel is a more accurate indicator in predicting how long a bearing will last in its application.

From Theory to Real World Application

HashimotoWe knew empirically that under a favorable lubrication environment, NSK bearings last much longer than their catalog life, and we also knew that this was due to improvements in material quality and manufacturing technology. In particular, it was well known that impurities (non-metallic inclusions) in the steel material had a very significant effect. While this was very beneficial from the standpoint of developing long-life bearings, the technology for accurately predicting life had not kept pace. However, as we continued our research, we realized that an evaluation method based on fracture mechanics could provide more meaningful insight.

Although we did not have much knowledge of fracture mechanics at that point, we were able to accelerate the development of the technology through an open innovation project with Kyushu University. The result was a quantitative evaluation method that determined which factors influence the process of crack propagation to what extent in a material, which was one of the key points in establishing a technology to predict bearing life with high accuracy. As a result of this research, we were able to quantitatively reflect the effects of non-metallic inclusions on bearing life in the bearing life prediction equation.

The professor who guided our research filled in the gaps in our knowledge and thinking. This helped us clarify our research policy and landing point, which also increased the speed of our development. In addition, thanks to dramatic improvements in computer processing power, we were able to perform many high-load simulations and computer analysis in a short period of time, which accelerated the development of our quantitative evaluation method. Although we were confident in the persuasiveness of the hypotheses we had developed, we needed solid data to back it up.

Ultimately, our quantitative method and understanding of the phenomena were validated through extensive and varied endurance life tests. Although fracture mechanics was a very effective approach for quantitatively evaluating crack propagation from impurities, which is the dominant factor in determining bearing life expectancy in well-lubricated conditions, it was a great challenge to incorporate it into life prediction methods on a practical real-world level. By combining the method we developed with an ultrasonic inspection technique that scans the non-metallic inclusions in a large volume of steel, we were able to predict the life of NSK bearings with much higher accuracy.