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Wear particle analysis, using tools such as ferrous density and ferrographic analysis, can play a valuable role in determining the root cause of active machine wear. However, unlike many common oil analysis tests that are quantitative in nature, successfully interpreting information on wear particles requires a fundamental understanding of tribology.
Tribology is defined by ASTM as “the science and technology concerned with interacting surfaces in relative motion including friction, lubrication, wear and erosion.”
Developing a fundamental understanding of tribology and the role it plays in diagnosing lubrication- related problems can be a daunting task for those new to the area of oil analysis. However, help is on the way!
In a new book written by Dr. Jian Ding, one of the world’s foremost experts in the field of tribology and wear debris analysis, the fundamentals of this fascinating field are explained, complete with wear debris photographs and case studies to illustrate the point.
The following is a highly- abbreviated and condensed excerpt from Dr. Ding’s book on one of the most common causes of rolling element bearing failure: contact fatigue.
Surface fatigue wear, also called rolling contact fatigue, predominantly occurs in rolling element bearings. Fatigue wear in rolling bearings generally starts with micropitting - small areas on the bearings’ surface where material has been removed due to repetitive stress.
At its terminal point, surface fatigue causes significant surface spalling - large craters often several hundreds of microns across, which are easily visible to the naked eye. While the effects of fatigue on bearings are well-documented, wear debris analysis offers a unique insight into fatigue failure.
Because the particles that are removed from the bearing surface are deposited in the oil and become the mirror image of the surface distress, the onset and progress of rolling contact fatigue can be detected.
Surface fatigue begins with microcracking on the surface or subsurface of a rolling contact bearing. The subsurface cracking typically nucleates at material defects or inclusions in bearing steels. With high stress on the rolling contact surfaces, subsurface microcracking propagates parallel to the surface, causing the material to eventually dislocate or spall and form fatigue wear particles.
Recently, particulate-indentation induced surface fatigue has attracted greater attention among tribologists. The risk for particle-induced surface fatigue is greatest when solid particles are roughly the same size as bearing dynamic clearances (clearance size particles) and are harder than bearing surfaces and not too friable. This enables them to enter bearing interfaces and dent bearing surfaces, as illustrated in Figure 1.1
Figure 1. Clearance-Size Particle Denting Bearing Surface1
Bearing surfaces that have suffered from this type of surface fatigue show massive indentations. These indentations have irregular circumferences that correspond to the sizes of the solid particles that form them (Figure 2, top).
As the particle concentration rises, the surface indentation density increases (Figure 2, center). Then, several individual, adjacent indentations appear, connecting them by coalescence to form larger micropits and eventually macropits (Figure 2, bottom).
Figure 2. Surface Cracking
that Started with Solid
There are four typical types of wear particles that have been recognized as being representative of rolling contact fatigue: microspall particles, laminar particles, chunky particles and spherical particles.
Particles that are the result of micro-spalling at an early stage of rolling contact fatigue are called microspall fatigue wear particles. Removal of these particles usually causes a slight surface frosting without visible pitting or spalling.
The material detached from these surfaces can be over-rolled into small plate-like particles (platelets) by passing through a region of rolling contact. The microspall particles that are generated this way are relatively small, ranging between 10 micron and 30 micron in their major dimension; however, their major dimension is sometimes as large as 50 micron.
After over-rolling, the microspall particles have a smooth surface with a thickness typically on the order of one-tenth or less of the major axis dimension. Figure 3 shows micro-spall fatigue wear particles on a ferrogram; and Figure 4 shows these particles on a filtergram.
Figure 3. Opt, M 1000X, Ferrogram
Figure 4. Opt, M 1000X, Filtergram
Microspall fatigue wear particles are similar to rubbing wear particles in appearance. However, the ratio of large particles (greater than 10 micron) to small particles (less than 10 micron) is much higher for fatigue particles than for rubbing wear (Figures 3 and 4). In addition, the concentrations of microspall wear particles in both oil and grease samples are usually low compared to those of rubbing wear (Figure 5).
Figure 5. Opt, M 1000X, Ferrogram
Microspall particles are often present at the same time as small spherical particles because both are the product of early rolling contact fatigue (Figure 6). Figure 7 shows microspall particles mixed with massive solid contaminant particles.
Figure 6. Opt, M 1000X, Ferrogram
Figure 7. Opt, M 1000X, Filtergram
Laminar particles are the most characteristic fatigue wear particles generated in rolling element bearings. Laminar particles are the result of microspalling that has further deteriorated into visible surface pitting and spalling.
Figure 8 shows large laminar particles on the surface of a bearing raceway before spalling and the laminar particles are dislodged. These large two-dimensional particles can be broken into several smaller platelets by reworking (over-rolling) further in the rolling contact zone.
Figure 8. Opt, Surface Pitting
of a Bearing Raceway
After being repeatedly over-rolled before and after spalling and before dislodging from the bearing surface, laminar particles have several distinct features:
Major Dimension Measurements
(L=50 m to 100 m) and Thickness Measurements
(H=1 m to 4 m) of Individual Laminar Particles
Major Dimension Measurements (L=100m to 200 m) an
d Thickness Measurements
(H=2 m to 8 m) of Individual Laminar Particles
Chunky fatigue wear particles are the result of further deterioration of surface pitting and spalling. At this stage, the fatigue cracks have penetrated and propagated deeper into the subsurface, at an angle of approximately 45 degrees to the rolling direction, as illustrated in Figure 12.2
Figure 12. Formation of Chunky Fatigue Wear Particles2
Thus, the propagation of fatigue cracks has changed direction. This means there is a higher risk of bearing fracture. A bearing with deep spalling is usually recognized as a fatigue failure. Accordingly, the presence of chunky fatigue wear particles should be used as an important indicator of a bearing fatigue failure event.
Deep-spalling chunky particles have two possible physical features generated by their passage through rolling contacts. A majority of chunky particles are thick platelets resulting from over-rolling. Their widths generally range between 5 micron and 20 micron, and sometimes even thicker. They have a low aspect ratio of approximately 10-to-1, as shown in Figures 13 and 14.
Figure 13. Opt, M 500X, Filtergram
Figure 14. Opt, M 500X, Filtergram
When fatigue wear has deteriorated into a deep-spall, the surface integrity is destroyed. Metal-to-metal boundary contact can occur, leading to oxidized or overheated chunky particles (Figure 15), and surface sliding (striations) or scratch marking on the chunky particles, as shown in Figure 16.
Figure 15. Opt, M 200X, Filtergram
Figure 16. Opt, M 500X, Filtergram
Due to fewer over-rolling effects and their considerable thickness, no holes or folds are found in these thick laminar particles, compared to surface-spalling laminar particles. These features clearly demonstrate that these chunky particles are a result of deep-spalling, which indicates a more severe bearing fatigue condition than that of surface spalling.
Another type of deep-spalling produces chunky pebble-like particles without experiencing over-rolling. The pebble-like particles, which can also be seen as chunky particles, are easily identified by their prominent three-dimensional features. These particles range between 10 micron and several hundreds of microns in their major dimension. Their aspect ratio ranges between 5-to-1 and 1-to-1.
Figures 17 and 18 show the pebble-like chunky particles from oil-lubricated rolling bearings that range from 50 micron to 200 micron on filtergrams.
Figure 17. Opt, M 500X, Filtergram
Figure 18. Opt, M 500X, Filtergram
Compared to other types of fatigue particles, the diameters and quantity of spherical particles generally better reveal the severity of rolling-contact fatigue wear. It has been extensively recognized that spherical particles are generated not only in early fatigue crack propagation, but also from other wear modes and at different wear stages.
In general, the small spherical particles of less than 5 micron are associated with rolling bearing fatigue; whereas spheres larger than 5 micron are the products of other wear modes or ingression sources, such as sliding, ploughing or cavitation.3
Small spherical particles are predominantly generated in the fatigue cracks of a rolling element bearing, as shown in Figure 19, and can be seen in the ferrogram as long strings of small spheres (Figure 20).3 The presence of these small spherical particles indicates an onset of early surface pitting. Most likely, these small spheres occurred with microspalling particles, which are small plate-like particles of less than 30 micron to 50 micron (Figure 21).
Figure 19. Small Spherical Particles
in a Fatigue Crack
Figure 20. Small Spherical
Particles on a Ferrogram3
It is estimated that during the failure of rolling element bearings by surface fatigue, several million spherical particles are generated. Figure 22 shows numerous small spheres in a rolling element bearing oil sample. However, bearings that have been tested at higher-than-normal operating loads in clean lubrication systems experience surface fatigue without generating significant quantities of spherical particles.3
Some grease-lubricated bearings, which include small rolling bearings of less than 2 micron in outer race diameter and large slew bearings of up to 5 meters in diameter, have also suffered significant surface fatigue without producing small spherical particles.
Figure 21. Opt, M 1000X, Ferrogram
Figure 22. Opt, M 1000X, Ferrogram
Small spherical particles of less than 5 micron are frequently found in other lubrication systems - typically in diesel engines associated with normal rubbing wear and sliding wear.
Large spherical ferrous particles (greater than 10 micron) in rolling element bearings are believed not to be generated in the bearing fatigue cracks. It is more likely that they come from surface sliding, welding, ploughing, etc.
If they are mixed with large laminar particles and/or chunky particles (Figure 23), they were probably generated at the deep-spalling stage. These large spherical particles can range from 50 micron to 100 micron and often show signs of overheating and melting (Figure 24).
Because large spherical particles are the product of localized metal-to-metal contact and high frictional temperatures between rolling contact surfaces, their presence is often considered a supplementary or supporting symptom for assessing wear severity levels.
For instance, sliding wear associated with large spherical particles is probably more severe than similar sliding wear situations that have no spherical particles. This is because spherical particles are an indication of higher surface temperature.
In addition to discussing fatigue wear particle formation and the various types of fatigue wear particles, the fatigue wear particle analysis chapter in Dr. Ding’s book also talks about the techniques used to identify surface sliding striations of fatigue wear particles, striated fatigue wear particles from grease and oil-lubricated rolling element bearings and the thermal effect of fatigue wear particles, which is a useful supplementary feature in assessing the severity of surface fatigue wear. Each of the technique’s explanation includes numerous photographs of particles generated from the various causes mentioned.
Because of the correlation between fatigue wear deterioration and wear particle characteristics, several Rolling Fatigue Wear Severity Atlas Charts (Chart 1) are included in this chapter.
These charts rate the severity level of bearing fatigue into five levels, from initial microspalling fatigue wear (Level 1) to significant deep-spalling fatigue wear (Level 5). At each severity level, photos of typical fatigue wear particles, correspondent fatigue wear surfaces and representative applications are displayed with the Rolling Element Bearing Fatigue Wear Severity Atlas.
This chapter, as well as many others in the book, also contains a case study that demonstrates how fatigue wear can damage equipment, in this particular case how it caused damage to bearings in various mining equipment. The case study covers a long condition-monitoring period, from 1992 to 2000.
It includes several photographs of damaged components, as well as photographs of wear particles before and after the bearing failure. The case study demonstrates that solid particulate concentration measurements combined with wear particle analysis can aid in identifying solid contaminant particles and help predict how bearing life and reliability will be impacted.