FUNCTIONS OF LUBRICANTS
Lubricants are used for three purposes-to reduce friction, to prevent wear, and as a protective cover against corrosion. As a protective cover against corrosion, the use is obvious. As a preventive against wear, the use is equally obvious when you consider the matter of friction. Lubricants form a layer or film between the metal surfaces that actually keeps the metals from touching. The moving parts literally ride on the lubricant. In the instance of two metal surfaces sliding across each other where space cannot be provided for ball bearings, the lubricants themselves serve as liquid bearings. In all mechanical devices, lubrication is necessary to counteract fiction as much as possible. The presence of only a thin film of lubricant separating metal surfaces keeps machinery running. If the film disappears, you have hotboxes, burned-out and frozen bearings, scored cylinder walls, leaky packings, and a host of other troubles-the least of them being excessive wear. All of these troubles are the result of direct metal-to-metal contact without adequate lubricant.
Because proper lubrication is an absolute necessity, selection of high-quality lubricants having the right viscosity and other properties for each job is of vital importance. When you drive your car into the local falling station and tell the attendant, "One quart of 10W30 oil," you are specifying the kind of oil you want.
Most lubricants, hydraulic fluids, cleaners, coolants, and related materials used by the Navy are purchased under federal or military (MIL) specifications. These specifications supersede all specifications formerly issued by the Navy. Table 2-3 in NAVSEA OD 3000, Lubrication of Ordnance Equipment, cross-indexes superseded Navy specifications and designations with the current federal or MIL specifications.
The specifications are identified by names, numbers, and letters. Lubricants, preservatives, hydraulic fluids, and many other items are often known and referred to by these specification numbers and symbols alone. An example of an item you have probably already used is MIL-F-17111-a power transmission fluid.
Many oils used by the Navy in ordnance equipment are identified by four-digit symbols preceded by the letters MS (military symbols). You need to be able to read and translate these symbols.
The following classes of MS oils are approved for use in naval ordnance:
Class 1 - Aircraft engine oils
Class 2 - General-purpose lubricating oils (130°F)
Class 3 - General-purpose lubricating oils (210°F)
Class 5 - Mineral cylinder oils
The viscosity for oils of classes 1, 3, and 5 is determined at a temperature of 210°F and for oil of class 2 at 130°F. These classes are straight petroleum-based oils without corrosion inhibitors.
Now, assume the MS on a certain oil is 2135. What does this tell you about this oil?
The first digit tells you that the oil is class 2, a general-purpose lubricating oil. The last three digits indicate the viscosity as 135 at a temperature of 130°F. That is how you read military symbols for oils.
On lubrication charts, note that the number of the lubricant to be used at each place is not repeated, but a "target" symbol is used instead. This avoids confusion with the number used to identify the part to be lubricated, which may be used several times in the chart. The meaning of the target symbol is explained in the notes on the chart, as well as in OD 3000.
Lubricating greases are a mixture of soaps-commonly, calcium or sodium soap-and lubricating oil. The oil may be a mineral oil (petroleum-based) or a synthetic oil.
The purpose of the soap is to make the oil stay put at the point of application. The soap traps the oil within its mass, but the actual lubrication is done largely by the oil in the grease. You might think of it as an oil-soaked
sponge. The heat of friction squeezes the sponge, melting the grease and releasing the oil to perform the lubrication.
Greases are classified according to the kind of soap used in making them. Each kind of soap has specific properties.
Calcium (lime) soap grease will not absorb moisture or emulsify (separate into its original ingredients). Consequently, it is specified for general lubricating purposes where bearings are exposed. However, calcium soap grease has a low-melting point and is not suitable for hot-running bearings.
Sodium-soap grease emulsifies in the presence of moisture but has a higher melting point. It should be protected from moisture. It is used for ball and roller bearings.
Other kinds of soap bases used in greases are aluminum soap and lithium soap, with others added to the list through experimentation and test at laboratories.
Graphite grease, as the name implies, contains graphite. The graphite acts as a mild abrasive to smooth roughened wearing surfaces, as a filler to smooth over any pits in the surfaces, and as a friction reducer. However, because of its abrasive action, graphite grease should not be used in bearings that are in first-class condition, except under high temperatures when ordinary greases would be destroyed. Technically, since graphite grease contains no soap, it is classed as a lubricant oil, but it looks and is applied like other
Gear lubricants are a mixture of high-viscosity oils and just enough sodium soap to cause jellying. Gear lubricants are suitable for high gear tooth pressure and moderate speeds where the design of the case is such that ordinary oil cannot be retained.
As with oils, the viscosity of greases varies with temperature. If temperature changes make it necessary to change oil, check your lubrication chart to find out whether you have to change the grease too.