The Progression of Surface Rolling Contact Fatigue Damage of Rolling Bearings

By
Mfame Team
-
3832

fatigue.jpg

The mechanism of surface rolling contact fatigue in rolling bearings is investigated by means of dedicated experiments and numerical simulations of the damage progression.  Rolling contact fatigue (RCF) is a typical failure mode in rolling bearings and similar types of machine components.  The fundamental work in RCF is due to Lundberg and Palmgren.

Fig. 2: Ball bearing experiments: microphotographs showing the onset and typical evolution of the dent spalling. The typical V-shaped crack can be seen in the stage (A). The arrows indicate the overrolling direction.

In this article, the progression of SRCF is investigated by modelling the contact and the interaction with deviations of the surface micro-geometry that generate stress concentrations.  Comparison of the numerical simulations with a set of experimental results indicates good correlation, allowing the formulation of a hypothesis about the underlying mechanisms of SRCF, as well as its inception and growth in rolling bearings.  This new knowledge fits very well with the basic idea behind the SKF Generalized Bearing Life Model (GBLM) that separates surface from subsurface fatigue damage.

Fig. 3: Roller bearing experiments. The spalled area of a roller bearing grows across the raceway (A) before progressing along the rolling path (B). Arrows indicate the overrolling direction.

Often, rolling contact fatigue damage originated around surface micro geometry features develops into a spall. Spall propagation, in its advanced form, is strongly influenced by macro geometry aspects – for example, the evolution of the raceway contact geometry and resulting overall stress field in a rolling bearing.  A recent investigation carried out by the present authors has studied SRCF propagation of pre-dented rolling bearings, both with a model and experiments, concluding that the mechanisms involved in ball bearings require the consideration of lubrication conditions and the interaction of stresses between the surface and subsurface to understand the development of the typical V-shaped cracks along the raceway, differently from the initial transverse damage growth observed in roller bearings that can be explained sufficiently with only dry contact assumptions.  Experimental observations in damage progression.

Fig. 4: Experimental results of spall propagation from an artificial indentation in the centre of the inner ring raceway of a tapered roller bearing. The ellipse shows the approximate size of the Hertzian contact.

From the theoretical and experimental investigations found in the literature, at least two distinctive spall propagation phases from surface defects are clearly recognized.

Fig. 5: Progression of the spalled area originated from an initial dent versus number of bearing revolutions: Measurements are related to six dents for which damage progression was monitored. Thick solid line: progression of average and maximum damage values measured from the experiments. Dashed lines: results from the numerical simulation of the progression of the damage area.

  1. The 1st one is when the spall grows across the raceway at a more or less slow rate, and the second is when it grows along the rolling path in a more accelerated fashion.  The reason for the across-raceway propagation of the spall, in its initial phase, is understood as a consequence of the higher stresses present at the diametric edges of the spall.  Experiments were conducted on standard tapered roller bearings; see table 1.Tapered roller bearings were artificially indented using an indentation load of 1,250 N and a 1 mm diameter tungsten carbide ball indenter.  However, in this article, only the progression of damage of the dents located at the centre of the raceway will be discussed in detail.
  2. The initial damage progression phase, which extends for 30 to 40 million revolutions, as expected, displays an exponential growth of the damaged area.
  3. The accelerated growth. This extends for 20 to 25 million revolutions, during which the growth rate substantially increases (more than twice compared to the previous period).  Damage propagation model simulations.

Table 1. Bearing data, operating conditions and dent geometry used in the experiments.

In the simulated results the mechanism of the damage progression is also interesting.  Because the indentation is a bit larger than the Hertzian width in the rolling direction, the most loaded zone in the raceway is the lateral area of the indentation, where the damage indeed will initiate and progress.

Fig. 6. Model predictions for spall, pressure and subsurface stress evolution with number of revolutions from the initial indentation. (Test conditions: table 1.)

The project was partially financed by the European Commission Marie Curie Industry-Academia Partnerships and Pathways (IAPP) – iBETTER Project.

Fig. 7. Damage area versus the number of revolutions for test conditions given in table 1. Average values of the initial damage observed in the experiments compared with the results of the numerical simulations using hmin. Full line: curve fitting using the exponential growth rate.Source: Evolution