At the Hannover Messe a few years ago, SKF announced the launch of the SKF Universal Bearing Life Model, an innovative model that helps engineers calculate bearing rating life in a more realistic way.
The model is a major breakthrough for the bearing industry, allowing supporting customers and end users to play an important role in better matching bearing products with applications, thereby improving equipment life and reducing operating expenses.
At the Hannover Messe, SKF showcased the EnCompass field performance program and launched the SKF Universal Bearing Life Model
As part of the SKF EnCompass field performance program, the development of this life theory successfully separated the surface fatigue model from the sub-surface fatigue model, taking advantage of the existing bearing life theory (developed by SKF and widely used for more than 30 years) and introducing more parameters to provide new insights for calculating bearing rating life.
The promotion of the Universal Life Calculation concept in Hannover included two days of live interviews with experts, demonstration software to demonstrate the calculation method, and one-on-one interactions with customers and journalists. The conceptual model was well received by the audience and customers, and has revived interest in bearing rating life calculations.
Next, we will introduce the principles of the new model in two parts.
The power of tribology
Until now, the life calculation of rolling bearings has been based on an equivalent stress-based engineering model, which considers equivalent stresses originating below the contact surface and applied to the stress volume of the rolling contact.
For many years, surface-originated fatigue was considered to be caused by poor lubrication or contamination, and the impact of this failure mode on life has been realized by adding a correction factor to the total equivalent stress of the rolling contact and incorporating it into the bearing life calculation formula.
In this paper, we address this problem of surface fatigue failure by developing a universal rolling contact life method. In this method, surface-originated damage is explicitly stated in the basic fatigue formula for rolling contact. This new formula better reflects the tribological characteristics of rolling bearings in rating life calculations.
In addition, it provides a better understanding of surface fatigue life, which plays a major role in the field performance of rolling bearings. The role of this universal method is to explain the friction of the bearing and discuss the conflicting fatigue mechanisms occurring in the surface and subsurface of the rolling bearing.
Under the premise of correct use and good lubrication, rolling bearings are becoming more and more reliable. This is due to the correct practice methods and the successful understanding and application of the traditional rolling contact fatigue mechanism.
At the same time, the improvement of steel purity and processing quality, combined with reliable life calculation methods, have also become an important part of the improvement of bearing reliability.
However, the trend of miniaturization of industrial equipment and the higher requirements for field performance efficiency have brought more demanding application conditions to rolling bearings, especially on the contact surface, which is why most bearing failures are related to surface fatigue [Reference 1].
In order for bearings not to become a bottleneck for further improving the performance of modern equipment, better evaluation of surface friction performance is needed in terms of bearing performance. In the past decade, SKF has made substantial progress in the field of surface life models [References 2-8].
Finally, by introducing the SKF universal bearing life model, surface fatigue was separated from the life theory of subsurface fatigue and this knowledge was integrated into the calculation of rolling bearing rating life [Reference 9].
In this approach, different physical models are used for the two areas. Subsurface rolling contact fatigue can be calculated using the classic Lundberg and Palmgren dynamic load theory [10], but when dealing with surface fatigue, more advanced friction models are needed that take into account more complex physical effects such as lubrication, friction, wear, fatigue or running-in due to stress concentrations occurring at the Hertzian contact surface.
This allows SKF to include more customized designs with special features in the bearing life calculation that can affect the performance of the bearing in the field application. For example: special heat treatment of the bearing, advanced micro-geometry, unique design or higher quality.
Customers can take advantage of different unique features of the bearings in the SKF product catalog and use them in the rating life calculation. Ultimately, customers no longer have to simply use the basic dynamic load rating (C) that only represents "subsurface fatigue" as is currently the case, but can better take advantage of the unique features of SKF products and higher product quality.
The new model can specifically deal with bearing aging mechanisms, and during the development of bearing products, friction on the raceway surface will be more widely used in this more advanced version of the general bearing life model.
SKF engineers will use the universal bearing life model to develop improved bearing designs for special applications or specific field performance requirements. In summary, the universal bearing life model represents a more modern and flexible bearing performance assessment tool that can incorporate new knowledge and technologies as it continues to evolve.
Universal modeling approach
The current model will continue to retain the standardized probabilistic rolling bearing life calculation method that is still in use today, which is based on the two-parameter Weibull distribution theory, as discussed in [12].
Varodi Weibull [13] introduced the concept of randomness in determining the strength and fracture of structural elements in his weakest link theory of the chain model.
If a structure consists of n elements subjected to different stress states, under different life probabilities S1, S2, ..., Sn, according to the reliability product law, the overall structure life probability can be obtained by formula (1).
In their early classical basic dynamic load rating formula for rolling bearings [Reference 10], Lundberg and Palmgren substituted the Weibull reliability product law into formula (1) and derived the life function formula (2) for a structure composed of n independent physical elements, which includes the aging process from 0 to N load cycles.
Considering that G represents the function of material aging caused by the cumulative effect of load cycles (fatigue), the volume parameter V can be divided into two or more independent structural damage origin factors.
Therefore, different regions can be characterized by different material aging functions, which describe different (or single) material aging processes Gv.1, Gv.2, ..., Gv.n, and their combined effect on the life of the entire structure can be expressed by formula (2). However, considering that there are only two regions, one is the subsurface (v zone) and the other is the surface (s zone), formula (3) can be derived.
According to [Reference 14], the fatigue damage volume integral formula (4) can be obtained using the stress amplitude σv generated in the Hertz stress zone.
Where c and h are exponents, e represents the Weibull slope of the subsurface, N is the contact life under the number of load cycles, z represents the subsurface depth to be analyzed, Vv is the volume integral, σu.v is the fatigue limit on this volume, and Ā is a setting constant.
In a similar way, the surface damage function can be rewritten. If the constant ĥ is substituted into the surface damage probability constant B ̅, formula (5) can be obtained.
Where m is the surface Weibull slope, A is the surface integral, σu.s is the fatigue limit of the surface, and B ̅ is a setting constant.
In the surface damage formula (5), the surface stress σs must be obtained from the actual surface shape and friction stress of the contact surface.
Now, by combining formulas (4) and (5) with formula (3), it is possible to obtain the contact life formula for the surface and subsurface separate terms. Note that the life in revolutions can be related to the number of load cycles by L = N/u, where u is the number of load cycles per revolution, and taking into account the very similarity of the two Weibull slopes e = m, this is the relevant surface fatigue model in the bearing, and finally obtains Equation (6).
This is the basis for a bearing life model that explicitly separates the surface terms from the subsurface terms. The subsurface terms, represented by the volume integral, can be calculated according to the traditional Hertz rolling contact fatigue method of [14].
The surface terms, represented by the area integral, include many friction phenomena that describe the life of the raceway surface in a more compatible way in the new life theory.
Of course, more advanced data models are needed in the development of the new model. In fact, it is necessary to describe the interaction between two more complex and contradictory aging mechanisms.
For example: i) surface fatigue combined with slight wear, ii) the evolution of indentation damage, iii) tribochemical interactions, and many other factors.
