Lubricating oils wear metals

Antiwear Compounds. Additives are used in many lubricating oils to reduce friction, wear, and scuffing and scoring under boundary lubrication conditions, ie, when fuU lubricating films cannot be maintained. Two general classes of materials are used to prevent metallic contact. 

Lubricating oil analysis, as the name implies, is an analysis technique that determines the condition of lubricating oils used in mechanical and electrical equipment. It is not a tool for determining the operating condition of machinery. Some forms of lubricating oil analysis will provide an accurate quantitative breakdown of individual chemical elements, both oil additive and contaminates, contained in the oil. A comparison of the amount of trace metals in successive oil samples can indicate wear patterns of oil wetted parts in plant equipment and will provide an indication of impending machine failure. 

Spectrographic analysis allows accurate, rapid measurements of many of the elements present in lubricating oil. These elements are generally classified as wear metals, contaminates, or additives. Some elements can be listed in more than one of these classifications. Standard lubricating oil analysis does not attempt to determine the specific failure modes of developing machine-train problems. Therefore, additional techniques must be used as part of a comprehensive predictive maintenance program. 

The main function of most lubricants is to reduce friction and wear between moving surfaces and to abstract heat. They also have to remove debris from the contact area, e.g. combustion products in an engine cylinder, swarf in metal-cutting operations. Sometimes they have to protect the lubricated or adjacent parts against corrosion, but this is not a prime function of most lubricants. On the other hand, many lubricants do contain corrosion inhibitors and some lubricating oils, greases, mineral fluids and compounds are specially formulated to prevent the corrosion of machinery or machine parts, particularly when these components are in storage or transit. These temporary protectives are described in Section 17.3. 

Barcroft, F. T. and Park, D., Interactions on Heated Metal Surfaces Between Zinc D. P. and Other Lubricating Oil Additives , Wear, 108, 213 (1986)... 

Oils used to lubricate machinery, motors, hydraulic systems and other mechanical devices can sometimes contaminate fuel systems. These oils often carry with them low levels of the metals which wear from the lubricated surfaces of the mechanical components. Some common wear metals and possible sources of origin are listed in TABLE 4-12. 

Cylinder and piston ring wear have been observed when using alcohol fuels. Metal loss could be due to wear caused by removal of the lubricating oil film by liquid alcohol during cold starting or by the corrosive action of formic acid or other acids formed during the combustion process. Use of more corrosion- and wear-resistant metal alloys in engine construction have helped resolve this problem. 

Aside from the primary function of friction and wear control, lubricating oils are often called on to serve other purposes, such as corrosion prevention, electrical insulation, power transmission and cooling. This last is particularly important in metal cutting and grinding. 

In industry analysis for metals in lubricating oils, greases or fuels is important for checking characteristics on delivery or for testing the quality of used lubricants for the purpose of preventing maintenance. In the first case one talks of the determination of metal ions in oils among others, in the second of wear metals, such as Cd, Cr, Cu, Fe, Mn, Ni, Sb, Sn and Ti. Metals which may be present in lubricants or fuels, are Al, Ba, Ca, K, Li, Mg, Mo, Na, P, Pb, S, Si, V and Zn [74]. This analytical area is very important because considerable financial considerations are related to the use of the correct lubricants, to the possibility of harmful substances developing in fuel oils (S or V) and also to maintenance. 

In used lubricating oils the wear metals are normally present in an extremely finely divided form and thus flame chemistry, similar to that for metals in true solution, is normally found. However, in some oils, such as those from heavy duty equipment or where excessive dust has been accumulated, larger particles may be present. In these cases more accurate results may be obtained by employing an ashing procedure to ensure proper dissolution of the metals (see Section II.D). 

The determination of wear metals in used lubricating oils... 

Dithiophosphoric-0,0-diester with long alkoxy-groups (> 5), optionally with aryloxy-groups, are neutralized in large quantities with zinc oxide to oil-soluble zinc salts and utilized in this form as lubricating oil additives. The zinc dialkyldithiophosphates form a protective layer on metallic materials, which reduces the wear and corrosion of these materials and protects the lubricating oil from oxidation. 96 10 t of zinc dialkyldithiophosphate and other lubricating oil additives on the basis of diphosphorus(V) sulfide were utilized in the USA in 1994. 

The composition of crude oils differs in different parts of the world as does the type of metal present and concentration. The different metal contents can be useful for identification of the source of oil and also help in determining those responsible as part of forensic and environment pollution investigations. Wear metals in lubricating oil are also monitored to provide early detection and correction of major problems in the various machine parts. 

Wear Metals and Metal Contaminants in Lubricating Oils [3]... 

Fassel, V.A., Peterson, C.A., Kniseley, R.N. and Abercombie, F.N. (1976) Simultaneous determination of wear metals in lubricating oils by ICP-OES, Analytical Chemistry, 48, pp516-519. 

Mag plugs—magnetic drain plugs and other magnetic inspection devices enable wear metals to be examined and removed from lubricating oil. 

Friction and wear of metal oxide filled PTFE can be improved by liquid paraffin lubrication, and the friction coefficient decreases by one order of magnitude. On the other hand, the interaction between liquid paraffin oil and metal oxide filled PTFE reduces load-carrying capacity because of absorption on the surface layers of composite. ... 

Zinc dithiophosphates The dominating position of ZnDTPs as additives for lubricating oils is due to their multifunctional performance. Not only do they act as antioxidants, but they also improve the wear inhibition of the lubricant and protect metals against corrosion. ZnDTPs are mainly used to formulate anti-wear hydraulic fluids and engine oils. 

Metals Metals should not be present in re-refined base oils either as wear metals, or from previous use of metal lubricant additives, or from the clay treatment or hydrofinishing catalyst fines. Metals are measured either by determining the total sulphated ash content or as individual metals by ICP analysis. 

Reducing friction. Putting lubricating oil between moving metal surfaces decreases friction. This results in energy conservation and significantly less wear. 

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