Wear evolution process

The bearing lifetime is described based on five stages. The wear evolution model assumes that, at a certain time interval of the steady-state stage, a transition into the defect initiation stage will take a place. Later, the evolution model describes the wear progress with the help of two assumptions: the existence of multiple stages that have specific transition events and existence of multiple wear and stress concentration mechanisms that are acting in each progress stage. There- fore, the wear evolution model that has been described in details with the help of literature (i.e. experimental findings) can be summarized as follows:

Some topographical changes might occur when the stresses in rolling contact increase due to the increased operating loads, additional loads due to faults i.e. imbalance, misalignment, bent shaft, looseness, and/or distributed defects i.e. high degrees of surface roughness and waviness, contaminations, inclusions. These topographical changes in the contact area generate stress concentration points and lubrication film disturbances.

At early stage, the concentrated stresses are not strong enough to produce a defect. It is mainly located in the loading zone and in the normal running track i.e. pure rolling points. Later, some sliding events, lubrication film transfers, false brinelling events due to stand-still events, might occur and introduce some degree of surface interaction. These surface interactions might appear as a reduction in the lubrication film, gapping, miss-matching between the rolling element profile and race profile, etc. Therefore, such surface conditions allow some abrasive wear events, some contaminations to enter the contact zone and minor vibration im- pacts. As a result of these actions, some surface dents might be generated. Therefore, the model describes how dents and in particular their asperities has the main role in the defect initiation and propagation process. The main assumption is that, as long as the asperity is large and sharp, the contact force between the rolling element and the asperity is large. The contact force and other loading forces contribute to the applied tangential force. The tangential force and the fric-

tion force are the main forces that generate a sufficient stress intensity factor (SIF) for crack opening and later for the crack propagation. However, since the rolling elements are rolling over the asperity and the asperity can be abraded, the original shape of the dent might change over the time. Therefore, the impact force and the crack opening and propagation progress are influenced. However, although the asperities are plastically deformed during the over-rolling cycles and degraded by abrasive and adhesive wear actions, they remain sufficiently high to produce ten- sile surface stresses. In the end, the propagated crack needs a secondary crack to reach the surface or it can attach to the rolling element once a sufficient adhesive bonding exists. During this process, the lubrication film is distorted and transfers into surface crack where another mechanical and chemical actions might acceler- ate the defect creation. When the defect is completed and the material is detached from the surface, new asperity is generated, new debris is generated, severe disturbances of lubrication film are generated, and the less hardening material (i.e. the material that was below the removed defect material) became the new sur- face.

This new descriptive model highlights the features of the generated defect and its asperity. The length of the generated defect depends on how long the crack could propagate in parallel to the surface before the detachment process occurred. The depth of the generated defect depends on how deep the crack was opened before it matches with a secondary or sub-surface inclusion. It also depends on the fric- tion force and its depth of stress concentration. The width of the generated defect depends how far the force trajectories were propagating. The defect’s length, depth and width are the basic elements of the new impact area. Therefore, a new impact force will be generated when the rolling element passes over the new de- fect i.e. especially at the trailing edge of new defect. The new defect will generate a number of defect serials, and the generated debris will generate several dents in different locations. First, the tangential force will be larger, since the asperity is larger and rougher than the initial dent asperities. Second, the new debris will generate a more severe dent, since it is larger and sharper than contamination particles. Debris might act as moving and distributed asperity. Moving asperities have a more random and non-linear way of action compare to the fixed ones, i.e. dent asperities. Large portion of debris will be pressed into the surface and gener- ate more dents. Moreover, debris can act as an asperity and minor indenter and generate abrasive wear.

The five stages are schematically illustrated in Figure 2. The wear evolution pro- gress produces several surface topographical changes. These topographical changes have a significant effect on the physical measurements, in particular, for condition monitoring purposes. At the end of the running-in stage, the roughness of the surface becomes smooth. Therefore, the steady-state stage is characterized by uniform lubricant film and contact mechanics, under normal operating condi- tions. When the surface is dented, the surface looks like peelings. Later, the shapes of the dents change due to the over-rolling and wear actions. However, the trailing edge acts as a stress raiser and the micro-cracking is initiated and opened

on and under the surface. The micro-cracks propagate from the surface downward with inclination, which depends on the rolling direction. The crack propagates later in parallel with rolling direction until it meets a secondary crack and connects to the surface or detaching process occur. Therefore, a relatively large material will be detached from the surface as debris particles. After the first pit, a number of pits and spalls are expected to occur in a serial pattern and extend in a wider and deeper manner, where the defect area becomes larger and rougher.

Figure 2. Evolution of dynamic behaviour and surface topography due to wear evolution.

It is worth explaining that several bearing manufacturers indicate that their lifetime estimation formula (e.g. the SKF bearing rating life formula) is based on the load and capacity assuming that the lubrication, oil contamination and operating condi- tions are ideal. Based on this assumption, such lifetime prediction shows quite a long lifetime and they fit very well with applications where the influences of the lubrication, oil contamination and operating conditions are controlled and ne- glected. The damage in the bearings of such applications are related to sub-surface defect which appear after a long period of operation i.e. after high repeated stress cycles, inclusions in the sub-surfaces and the degradation of the material properties. However, several studies in the literature (Xu & Sadeghi 1996), (Mota & Ferreira 2009), (Morales-Espejel & Brizmer 2011), etc. show that under the normal loading conditions, the damage might appear much earlier once the lubrication, oil con- tamination and operating conditions are disturbed. Moreover, the rating life for- mula expresses the capacity to load ratio with an exponential factor that is related to contact type i.e. ball contact, rolling contact. Therefore, the studies show that lubrication disturbances might generate boundary lubricated contact, abrasive actions, pressing particles between surfaces, etc. and these actions in fact initiate

Lifetime Running-in _Steady-state Defect initiation Defect propagation Damage growth 2 3 5 1 4 D y n a m ic i m p a c t o f w e a r s e v e ri ty

the surface dents. Therefore, the “service life factors” have been introduced to the life rating formula including lubrication, the degree of contamination, misalignment, proper installation and environmental conditions. These service life factors can easily influence the bearing lifetime at an early stage and initiate surface dents much earlier than sub-surface defects, in particular, for bearings that are operated under normal loading conditions (with some degree of variation) and assuming high material quality (which have been used in other applications and showed a long lifetime). In fact, several studies (Dwyer-Jones 1999), (Maru et al. 2007), (Morales-Espejel & Brizmer 2011) highlighted the fact that the probability of getting disturbances in lubrication, contamination degree and operating conditions are higher than having sub-surface inclusions at an early stage.