Published: Jan 2009
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THIS CHAPTER DEALS WITH THE FUNDAMENTALS of lubrication. It covers the lubricant functions, nature and composition of the lubricants market, concepts of friction, lubrication, viscosity, and wear, lubricant types, and lubricant selection, performance specifications, and composition. A brief description of the lubricant classes and additives is also provided. The concepts addressed in this chapter are invaluable in understanding the modern lubricant technology and the material covered in the subsequent chapters. Almost all modern machines require the use of a lubricant. Power generation in such equipment is achieved by the use of engines that mostly comprise metal parts that move against one another. In many cases, there is metal-to-metal contact that leads to the generation of friction and heat, which results in wear. The extent of wear in equipment depends upon the degree of the metal-to-metal contact, either due to the equipment design or the nature of the operation. For example, the equipment that is designed to experience minimal metal-to-metal contact, as is the case in most parts of an internal combustion engine, there is little friction and wear. However; the parts that are designed to have intimate metal-to-metal contact, such as gears and bearings, wear due to friction is extensive. With respect to the effect of equipment operation on wear, high-speed, low-load operation leads to lower wear than slow-speed, high-load operation. This is because in the former case there is minimal metal-to-metal contact. A lubricant can be a solid, liquid, or gas, and lubrication is its primary function. The usual objective of the lubrication is to lubricate surfaces to minimize direct metal-to-metal contact and, hence, reduce friction and wear. The term lubricant is also loosely applied to many other fluids that do not specifically perform this function. Examples include power and heat transmission fluids, hydraulic fluids, dielectric fluids, process oils, and the others. Incidentally, in this book the term “lubricant” pertains to a finished lubricant, that is, it comprises base fluid and additives. A lubricant performs many diverse functions, which help protect and prolong the life of the equipment . These include the following: 1. Lubrication (reduce friction and wear)—Lubricant helps reduce friction and wear by introducing a lubricating film between mechanical moving parts, such as gears and bearings. Essentially the presence of a lubricating film minimizes the metal-to-metal contact and reduces the force necessary to move one surface against the other, thereby reducing wear and saving energy. 2. Cooling (heat transfer)—Lubricant acts as a heat sink and dissipates the heat away from the critical moving parts of the equipment, thereby decreasing the possibility of the machine component deformation and wear. The heat is either frictional heat that results from the metal surfaces rubbing against one another, such as in gears, or is conducted and radiated heat, which is due to the close proximity of the parts to a combustion source, such as the combustion chamber in an automobile engine. 3. Cleaning and Suspending—Lubricant facilitates smooth operation of the equipment by removing and suspending potentially harmful products, such as carbon, sludge, and varnish, and the other materials, such as dirt and wear debris. This lubricant function is important in operations that involve high operating temperatures, as in the case of an internal combustion engine or a transmission. This is because in these applications the lubricant gets oxidized to form deposit precursors that can separate on hot surfaces and get converted into deposits. 4. Protection—Lubricant prevents metal damage due to oxidation products, corrosion, and wear. It achieves this by forming a physical film on metal surfaces that is impervious to oxygen, water, and acids, or by forming physical and chemical films by additives, such as rust and corrosion inhibitors, extreme-pressure (EP) additives, and anti-wear agents, that are present in the lubricant. 5. Transfer Power—Lubricant is used as a power transfer medium in some applications, for example, in hydraulic systems. The lubricant performs this function in addition to its normal function of lubrication. Examples of equipment that use hydraulics technology include transmissions, circulating systems, lifts used in automotive service stations, log splitters, fork lifts, dump trucks, and underground continuous mining equipment such as drills, loaders, and miners.