In selecting a lubricant for a particular job, the service conditions requirements are to be related to the properties of the lubricant. The properties of a properly selected lubricant should not change under service conditions. If the property of chosen lubricant alters during services, it may cause certain problems. For example if a lubricant used at a high temperature undergoes volatilization of a portion of it, leaving being a residual oil, which will have different lubricating properties like higher viscosity. Such a mistake must be avoided. So a careful study of various properties and their correct interpretation is necessary for the selection of lubricants for a particular job.
Selections of a lubricant for a few typical jobs are illustrated as follows:
Lubricants for cutting tools: cutting fluids are lubricants used in machining, cutting, turning and grinding of metals. In these operations a metal is continuously removed from the surface and a fresh metal surface, free from any oxides, is continually exposed to the tool used. This consequently causes high friction which results in excessive power consumption and liberation of large amount of heat. Heat liberated may overheat the tool and may lower its temper. Therefore the main functions of cutting fluids are
• To cool the tool so as to reduce wear and prevent metal losing its harness
• To cool the metal work piece so as to prevent distortions and dimensional inaccuracies
• To reduce power consumption by lubricating action
• To improve surface finish
Two situations can be observed in cutting tool operation;
a) For heavy cutting, the most effective lubricants are cutting oils. The cutting oils are essentially mineral oils of low viscosities containing additives like fatty oils, sulphurised fatty oils and chlorinated compounds which by virtue of their polar groups attach themselves to the surface of continuously exposed fresh metal. As the shear strength of such an oil film is much less than that of the metal a considerable reduction in friction results, thereby decreasing both the power consumption and the extent of heat generation.
b) In light cuttings: the most effective lubricants are oil-emulsions. Oil-emulsions have somewhat smaller lubricating effects that cutting oils but they are more effective as cooling media due to high heat capacity of water which is present in them as an external phase.