When a system is at rest, unless the bearing has hydrostatic lubrication applied, the sliding surfaces will be in contact. As the components begin to rotate, the surfaces remain in contact until the lubricant film is established. During this time, the highest incident of failure persists. As a film is formed and surface active additives begin to protect, a lubrication regime is established. Prior to this occurring, surface asperities and particulate will contribute to surface wear and possible part failure. Wear is defined as simply the loss of any material from one or both faces of two interacting surfaces. These losses of material occur over three phases: break-in wear, normal wear, abnormal wear. Of these phases there is an occurrence that persists. Early failures occur due to new component defects or inadequate tolerances resulting in compromised load. In a normal situation, new components go through a short “break-in period” until a smooth low wearing surface is produced. There is a shear mixed layer of approximately 1mm, highly ductile surface that exists. The major asperities on the surface are flattened at the peaks which eventually break off to form long flat particles. In a “stable” shear mixed layer, platelets continue to be exfoliated from the metal surface with a particle size less than 15um long and 0.15um to 1umm thick. This is considered normal and acceptable wear. When there is an imbalance or non-uniform tolerances between part surfaces or the wrong materials or components are used, the situation arises for premature wear. Poor installation, poor maintenance practices and poor workmanship all will result in an early failure event. There are various particles that are generated that indicate these scenarios.
Another failure class is random, event dependent. Random, event dependent failures can occur at any point during the systems life cycle. These types of failures are the result of the over speed, or over loading a component. As speed and load increase, there is an opportunity for the sliding or rolling surfaces to come in contact resulting in increased heat as well as the opportunity for particulate to contact the surfaces. In both situations, the surfaces and particles produce wear.
In a third case, condition based failures are the result of several sources primarily due to contamination – both internally generated from wear debris (normal or accelerated) or outside sources such as dirt or processed product. Contamination can also be liquid or gas. Gas such as hydrogen sulfide or pure oxygen can produce severe corrosion which produce wear particles. Liquids such as fuel and coolants will contribute to system failures in engines. These fluids compromise the viscosity of the oil resulting in surface contact or oxidation product development resulting in an increase in failure opportunity. Water is the most prevalent contaminant resulting in metal oxidation as well as lubricant degradation. Water contamination can be found in oil lubricated systems and the result is either a reduction in oil viscosity or as a catalyst for oil oxidation and the formation or sludge, varnish, and lacquer.
Michael D. Holloway, MLA I, MLA II, OMA1, MLT I, MLT II, CLS, LLA I
Principle Consultant, Certified Reliability Leader