Damage growth fault

One important physical feature in the damage growth is the defect width. A wider defect means a wider impact area at the trailing edge. A wider impact area means higher peak amplitudes at the bearing fault frequency zone. Moreover, the length and the depth of the defect are expected to propagate over time until a failure occurs. In Figure 16, the wear fault features are illustrated. It is worth mentioning that Figure 16 represents an early-stage of damage growth. As the damage grows, it becomes more complicated to visualise the features of the spectrum due to the complex distortions associated with damage growth process.

Figure 16. Time response and spectrum of the bearing under damage growth state.

In summary, the wear evolution stages can be illustrated in the following descrip- tions, with physical justification related to the topographical and tribological evolu- tion of the defected bearing.

High frequency zone: The experimental findings that are discussed in (El-Thalji & Jantunen 2013) have shown that surface roughness is the main reason behind the amplitude peaks at the high frequency zone of the spectrum. The surface rough- ness and waviness generate the surface peaks and valleys which increase the probability of race surface peaks contact with rolling element surface. These con- tact events present amplitude peaks in the high frequency zone. Over time, the contact events between the surface peaks of the race and rolling element surface smooth the surface. Moreover, the film thickness will be stabilised and become uniform. However, the bearing geometrical and tribological characteristics might change due to the loading and operating conditions. For example, there is all the

time some degree of error in the contact profile between the race and the rolling element which disturbs the pure rolling contact (Morales-Espejel & Brizmer 2011). Another example is the lubrication transfer into surface valleys (Kotzalas & Doll 2010). Such contact disturbances produce high stresses when the rolling elements pass over the surface asperities. These stresses will appear as amplitude peaks in the high frequency zone in the spectrum. These contact events might involve minor abrasive wear which make the surface smoother, and higher stresses will be generated from those events. Later, the distributed wear debris will be the main issue in generating high stresses. That means higher amplitude peaks in the high frequency zone are expected at a later stage.

Bearing natural and bearing fault frequency zones: The impact events when the rolling element passes over the defective area (i.e. with the help of a small degree of loose fit between the raceway and the housing might) excite the natural fre- quency of the raceway. Epps (1991) observed that the defect signal is of two parts: the first part originates from the entry of the rolling element into the defect, which generates an amplitude peak in the bearing fault frequency zone. The sec- ond part originates due to the impact event between the rolling element and the trailing edge of the defect, which generates an amplitude peak at bearing natural frequency zone. However, the defect signal might change as the wear (defected area) is evolved over time.

The abrasive wear generates some internal debris, which might be transferred with the oil lubrication into the valleys of either the surface waviness or the contact deflection. At the moment when the rolling element passes over the valleys that contain debris, the rolling element might press the debris into the surface and generate a localised dent. This localised dent generates impact events that might excite the bearing natural frequency. Therefore, the peak amplitudes at the bear- ing natural frequency might be seen. In fact, the dent acts as a stress riser in par- ticular at the trailing edge of the dent (Alfredsson et al. 2008). However, the impact event which is generated when the rolling element passes over the new dent is very small. Moreover, it becomes even smaller due to the over-rolling and mild abrasive wear of the asperity at the trailing edge of the new dent. However, the high stresses at the trailing edge are still enough to initiate a crack. The crack will propagate and end eventually become a defect. The defect will have leading and trailing edges as well.

However, the impact events at the trailing edge of the defect are much higher compared to that of the dent. Therefore, it is expected to see higher peak ampli- tudes at the bearing natural frequency in the spectrum. In fact, the impact event (i.e. generated when the rolling element passes over the trailing edges) generates impulsive impact and distorts the rolling element motion. The distortion phenome- non is responsible for producing peak amplitudes at the harmonics of the bearing defect frequencies. It depends on whether the impact area (the trailing edge area in contact) is large enough to distort the impact signals.

The over-rolling and mild abrasive wear will act again to smooth the trailing edge of the new defect. Therefore, a clear reduction in peak amplitudes at both the bearing defect and bearing natural frequency zones is expected. However, the high stresses at the trailing edge are enough to initiate a crack for the next defect. The width of the defect is the key parameter in the impact severity. In fact, the impact area of a dent is smaller than the impact area of the defect. Therefore, the crack trajectories of the defect will be further from each other when compared to the crack trajectories of the dent. The crack trajectories are the main issue that determines the width of the new defect. A wider impact area means higher peak amplitudes at both bearing defect and bearing natural frequency zones.

A significantly important issue of this simulation model is the consideration of the slippage phenomenon and its effect on frequency domain features. The time be- tween impact events is known as the epicyclical frequency, which we try to extract from the spectrum. For example, the topographical change due to the wear evolu- tion might most probably change the drag and driving tangential forces which make the cage and rolling element travel more slowly than its epicyclical value. In fact, as the wear become more severe, the topographical and tribological features of the surface generate and influence stronger the drag and driving tangential forces, which means more slip and disturbances. Therefore, the amplitude peaks at the defect frequency might not be clear, and several harmonics and sideband peaks will appear.

Rotating frequency zone: The machine faults e.g. imbalance, bent shaft, mis- alignment, looseness have specific characteristic features at the rotating speeds and their orders. In fact, those machine faults have in-direct influence on the wear initiation and evolution, as they introduce some changes in the contact character- istics and load distribution; for example, the misalignment introduces higher con- tact stresses at specific part of the raceway, which is one of the main reasons for wear initiation.