LUBRICANTS AND CORROSION CONTROL
LEARNING OBJECTIVES: Discuss the characteristic properties and functions of lubricants. Identify lubrication symbols used on charts and drawings. Explain why corrosion control is an important part of any maintenance program.
If you grew up in a large city, perhaps the only connection you had with lubrication was taking the family car to the garage or the gas station for greasing and an oil change. But, if you grew up on a farm or had a car that you had to keep in running condition yourself, you are well aware of the need for regular lubrication of all moving parts. If your car had a burned out bearing, you learned a lesson that you are likely to remember. Since you have been in the Navy, you have heard a lot about the importance of lubrication. We place additional emphasis on the subject in this section as we describe the different lubricants and lubrication tools that you will encounter. We will also discuss some of the fundamentals and practices involved in corrosion control.
QUALITIES OF LUBRICANTS
Lubricants are of two general classes-oils and greases. Oils are fluids; greases are semisolids at ordinary temperatures. Both have several qualities that determine their suitability for a particular lubrication job. One of the most important is viscosity.
Viscosity is the measure of the internal resistance of a fluid that tends to prevent it from flowing. It varies with the temperature as well as with the nature of the substance. Petroleum jelly can hardly be said to flow at room temperature, but it can be melted to a rather thin liquid. On the other hand, many kinds of oils flow readily at ordinary temperatures but become much thicker when they are cold. A fluid that flows easily has a low viscosity, while a fluid that flows slowly has a high viscosity.
Figure 12-2.-Sample IEM schedule for ROH.
Viscosity is expressed in terms of S.S.U. units. (S.S.U. means Seconds Saybolt Universal and represents the number of seconds it takes a given quantity of the lubricant at a specified temperature to pass through the Saybolt Universal Vicosimeter or Viscometer.) The higher the S.S.U. number of a lubricant at a given temperature, the more viscous the liquid. The Navy uses the S.S.U. measurement, rather than the S.A.E. (Society of Automotive Engineers), grades to designate lubricants.
The viscosity index (VI) is an indication of the variation of viscosity of the lubricant with variation in temperature. The higher the index, the less the viscosity varies with the temperature. Thus a high index is a desirable quality. You want a lubricant that will not solidify and gum up in cold weather nor liquefy and leak away in hot weather.
A viscosity index can be improved up to a point by putting chemical additives into the oil. (Additives are put in by the manufacturer. Do not try to brew up your own special oil by adding anything to it.)
The flash point of a lubricant is the temperature at which it gives off flammable vapors. The fire point (always higher than the flash point) is the temperature at which it will catch fire if ignited and continue to burn. The pour point (of an oil) is the lowest temperature at which it will pour or flow.
Oiliness is the characteristic of an oil that prevents scuffing and wear. You might think this depends on viscosity, but a complicated relationship of many factors is involved. Certain substances have been found that increase the oiliness of a lubricant.
Chemical stability is the ability of the lubricant to "take it." Certain oils and greases tend to deteriorate under the influence of high temperatures, exposure to air or water, or introduction of impurities. A lubricant with good chemical stability will resist such deterioration. You can often detect deterioration by change in color, by formation of varnish or gum deposits, by formation of sludge, by change in viscosity (of oil) or consistency (of grease), by hardening (of grease), or by other telltale signs. Change in viscosity can be more accurately measured by a viscometer, but serious change is easy for the expert to detect.
These signs of deterioration mean that the lubricating and corrosion-preventing qualities of the substances are impaired. You will find it useful to know the signs of deterioration in oils and greases well enough to recognize them should they appear.
Lubricants, preservatives, and hydraulic fluids all protect metal against corrosion, at least to a certain extent. Corrosion prevention is, of course, the main function of a preservative. The corrosion-resisting qualities of lubricants and hydraulic fluids can be improved by adding chemicals, called inhibitors. In general, inhibitors are added to the substance by the manufacturer before delivery to the Navy.
Other qualities or properties of lubricants are the dropping point, the penetration, the neutralization number, the work factor, the viscosity change, and the aniline point. Chemists working with lubrications need to understand the meaning of these terms; we list them only to impress upon you that not just any oil will do.