Contaminant Particles are considered the single most significant cause of abnormal component wear. The wear initiated by contaminants generally induces the formation of larger particles, with the formation rate being dependent on the filtration efficiency of the system. In fact, once a particle is generated and moves with the lubricant, it is technically a contaminant.
Rubbing Wear Particles: Normal-rubbing wear particles are generated as the results of normal sliding wear in a machine and result from exfoliation of parts of the shear mixed layer. Rubbing wear particles consist of flat platelets, generally 5 microns or smaller, although they may range up to 15 microns, depending on the equipment application. There should be little or no visible texturing of the surface and the thickness should be 1 micron or less.
Abrasive Wear Particles: Abrasive wear particles are generated as a result of one surface penetrating another. Because abrasive wear particles are very abnormal, their presence and quantity should always be carefully monitored. There are two ways of generating this effect.
- A relatively hard component can become misaligned or fractured, resulting in a hard sharp edge penetrating a softer surface. Particles generated this way are generally coarse and large, averaging 2-5 microns in width and 25-100 microns in length.
- Hard abrasive particles in the lubrication system, either as contaminants such as sand or wear debris from another part of the system may become embedded in a soft wear surface, such as a lead/tin alloy bearing. The abrasive particles protrude from the soft surface and penetrate the opposing wear surface. The maximum size of cutting wear particles generated in this way is proportional to the size of the abrasive particles in the lubricant. Very fine wire-like particles can be generated with thickness as low as .25 microns. Occasionally small particles, about 5 microns long and 25 microns thick, may be generated due to the presence of hard inclusions in one of the wearing surfaces.
Spherical Particles: These particles are generated in the bearing cracks. If generated, their presence gives an earlier warning of impending trouble, as they are detectable before any actual spalling occurs. Rolling bearing fatigue is not the only source of spherical metallic particles. They are known to be generated also by cavitation erosion and, more importantly, by welding or grinding processes. Spheres produced in fatigue cracks may be differentiated from those produced by other mechanisms through their size distribution. Rolling fatigue generates few spheres over 5 microns in diameter, while the spheres generated by welding, grinding and erosion are frequently of 10 microns in diameter.
Severe Sliding Wear Particles: Severe sliding wear particles are identified by parallel striations on their surface. They are generally larger than 15 microns, with the length-to-with thickness ratio falling between 5-30 microns. Severe sliding wear particles sometimes show evidence of temper colors, which may change the appearance of the particle after heart treatment.
Bearing Wear Particles: These distinct particle types have been associated with rolling bearing fatigue:
1. Fatigue Spall Particles constitute actual removal from the metal surface when a pit or crack is propagated. These particles reach maximum size of 100 microns during the micro-spalling process. Fatigue spalls are generally flat with a major dimension to thickness ratio of 10:1. They have a smooth surface and a random, irregularly shaped circumference.
2. Laminar Particles are very thin free metal particles with frequent occurrence of holes. They range between 20 and 50 microns in major dimension, with a thickness ratio of 30:1. These particles are formed by the passage of a wear particle through a rolling contact. Laminar particles may be generated throughout the life of a bearing, but at the onset of fatigue spalling. An increasing quantity of laminar particles in addition to spherical wear is indicative of rolling bearing fatigue micro-cracks.
Chunk Particles (Gear Wear): There are two types of wear that are usually associated with Chunks:
- Pitch Line Fatigue Particles from a gear pitch line have much in common with rolling-element bearing fatigue particles. They generally have a smooth surface and are frequently irregularly shaped. Depending on the gear design, the particles usually have a major dimension-to-thickness ratio between 4:1 and 10:1. The chunkier particles result from tensile stresses on the gear surface causing the fatigue cracks to propagate deeper into the gear tooth prior to spalling.
- Scuffing or Scoring Particles are caused by too high a load and/or speed. The particles tend to have a rough surface and jagged circumference. Even small particles may be discerned from rubbing wear by these characteristics. Some of the large particles have striations on their surface; this indicates a sliding contact has occurred. Because of the thermal nature of scuffing, quantities of oxide are usually present, and some of the particles may show evidence of partial oxidation (that is blue or tan temper colors).