Analysing software - Calculation Tool
Predict lifespan more accurately
As a world first, NSK is introducing a new method for accurately predicting the realistic service life of rolling bearings. The method is based on the size and quantity of non-metallic inclusions in the steel from which the bearings are made. On this basis, NSK's angular contact ball bearings, cylindrical roller bearings, deep groove ball bearings and tapered roller bearings will have a significantly longer nominal service life in the future. In the best case, the service life can even be doubled due to the optimization of the dynamic load ratings.
The determination of the load ratings is regulated by the ISO 281:2007 standard. It states that when testing a batch of identical bearings, nine out of ten bearings must easily achieve the specified value (in millions of revolutions; L10) or in operating hours (L10h). The standard allows the service life to be increased under certain conditions. But in the past, every change to the bearing had to be subjected to another series of tests. NSK has now developed a non-destructive testing method for precisely these situations that makes it possible to dispense with further tests. In practice, the end user of the bearings benefits from this through a significant increase in the nominal bearing life.
Most designers use the (print or online) catalog of a rolling bearing manufacturer to calculate the bearing life. Alternatively, they can also enter the bearing characteristics and the external loads into a calculation program. However, the basic calculation is still carried out according to ISO 281, which has become the standard worldwide. However, its methodology was introduced in 1962. Since then, both material purity and manufacturing options have developed considerably. The result is that the actual service life of a bearing is usually much longer than the service life determined according to ISO 281.
NSK's approach was to use computer-aided models of the bearings in addition to empirical tests. These models are used as "digital twins" with the aim of even more dispensing with service life tests with real rolling bearings and basing increases in the dynamic load ratings on calculations.
The basis for these calculations are the findings from comprehensive studies by NSK. They have shown that the composition and quality of the bearing steel - in particular the proportion of non-metallic impurities in the steel - has a major influence on the service life of a bearing. What's more: they are actually a decisive indicator of service life.
On the basis of these findings, the NSK engineers determined that an evaluation method that deals with fracture mechanics - a branch of mechanics that is based on the investigation of the propagation of cracks in materials - allows more meaningful results.
In order to develop a quantitative evaluation method that can be used to determine which factors influence the propagation of cracks in a material and to what extent, NSK worked together with the University of Kyushu in Japan. The result: In particular, by combining the new method with an ultrasonic testing method that detects the non-metallic inclusions in a large volume of steel, the purity of the steels of rolling bearings can be determined with much greater accuracy and in a significantly larger test quantity.
As a direct result of these studies, NSK has been able to increase the dynamic load ratings of many bearings, while maintaining the same design, materials and safety margin. This revision increases the nominal life of deep groove ball bearings by up to twice as much as before.
As the dynamic load ratings of the bearings improve, the bearing can be used for higher loads while maintaining the same bearing size. Or the user can choose a smaller bearing that performs the same function as the larger bearing previously used. The obvious benefits of this optimization include more compact machine or drive train dimensions and weight reduction.
End users can be confident that the new bearing life values have been carefully determined, based on a carefully developed methodology and an extensive empirical database. All new - and generally higher - values are within the safe range. NSK researchers and developers used a quantitative assessment method to determine the exact extent to which the number and size of contaminants influence the crack propagation process in a rolling bearing steel. Computers with extremely high computing power were used to carry out and evaluate the corresponding simulations with the "digital twins".