Machinery's Handbook, 31st Edition
Lubricants 2503 A lubricant can be in liquid (oil), solid (grease), or gaseous (oil mist) form and can be formulated from animal, vegetable, hydrocarbon, or synthetic base oil stocks. Adding to each lubricant formulation numerous chemical thickeners, solids, and chemical addi- tives, gives every single manufactured lubricant its own unique signature blend. Selec- tion of lubricant style and type is arguably the most influential factor in assuring long bearing life. In the 1970’s, Dr. Ernest Rabinowicz of MIT performed a landmark study on the effects of lubrication on the Gross National Product (GNP) of the United States. The study con cluded that at that time, US manufacturing companies spent over $600 billion US annu- ally to repair damage caused by friction-induced mechanical wear; more importantly, the study determined that over 70 percent of bearing loss of usefulness (failure) is directly attributable to surface degradation, a totally preventable condition. In his study, Rabino- wicz determined there are four major contributors to surface degradation: Corrosive Wear: All metallic-bearing surfaces will corrode if left unprotected from contact with water and corrosive acids. Water is introduced into lubricated environ- ments from outside sources penetrating the sealed reservoir or bearing (washout, prod- uct contamination), or through condensation, causing ferrous metals to rust. Corrosive acids are produced when the lubricant becomes oxidized and suffers loss or breakdown of its corrosion inhibitor additive packages. Specifying and using a lubricant with rust in- hibitors and corrosion-inhibitor additives, and replacing the lubricant in a timely manner when additives are depleted from the oil, will prevent corrosion. Mechanical Wear by Adhesion: Adhesive wear occurs when a lubricant film separating two sliding surfaces fails to completely separate the two surfaces. Metal to metal contact occurs causing metal fragment transfer from one surface to the other. This transfer is com monly referred to as seizing, galling, scuffing, or scoring of surfaces. Correct lubricant viscosity and application frequency will significantly reduce or eliminate adhesive wear. Mechanical Wear by Abrasion : Abrasive wear, sometimes referred to as cutting wear , is the result of hard particles (wear particles or introduced contaminant particles) bridging two moving surfaces, scraping and cutting either one surface (two body abrasion) or both bearing surfaces (three body abrasion). Controlling abrasive wear requires reduction of adhesive wear combined with contamination control of lubricant transfer, application, and filtration processes. Mechanical Wear by Fatigue: Fatigue wear results when bridged wear particles cause small surface stress raisers (surface rippling) that eventually expand and break away from the parent metal as a spall (flake or splinter). Repeated cyclic stress at the damaged area ac - celerates the fatigue wear process. Correct lubricant viscosity choice and contamination control are essential to retard fatigue wear. In all four types of wear, the primary solution for wear retardation lies in the correct choice of lubricant. Lubricants are categorized into two specific families—oil, and grease. The choice to use either oil or grease will depend upon temperature range, speed of rotation, environment, budget, machine design, bearing and seal design, which operating conditions. Lubricating Oil.— For the majority of industrial applications requiring the separation of moving surfaces, the lubricant of choice continues to be petroleum based oil, also widely known as mineral oil. Although any liquid will provide a degree of lubrication, hydrocarbon-based petroleum oils provide excellent surface wetting capabilities, water resistance, thermal stability, and sufficient fluid viscosity or “stiffness” to provide full film protection under load—all at an inexpensive price. By adding chemicals, metals, sol - ids, and fillers, mineral base oil stock can be formulated into an infinite number of tailored lubricating products, including grease. These modified mineral oils can be formulated for virtually any industrial application and widen the lubricant’s specification and capabili - ties even further. The fundamental defining property for all lubricating oils is viscosity , and is the starting point for choosing one specific lubricant over another.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online