The Navy identifies lubricating oils by number symbols. Each identifying symbol consists of four digits and, in some cases, appended letters. The first digit shows the series of oil according to type and use; the last three digits show the viscosity of the oil. The viscosity digits indicate the number of seconds required for a 60-milliliter (ml) sample of oil to flow through a standard orifice at a certain temperature. Symbol 9250, for example, shows that the oil is a series 9 oil which is specified for use in diesel engines. It also shows that a 60-milliliter sample should flow through a standard orifice in 250 seconds when the temperature of the oil is 210F. Another example is symbol 2135 TH. This symbol shows that the oil is a series 2 oil, which is suitable for use as a force-feed lubricant or as a hydraulic fluid. It also shows that a 60-milliliter sample should flow through a standard orifice in 135 seconds when the oil is at a certain temperature (130F, in this case). The letters H, T, TH, or TEP added to a basic number indicate a primary specific usage within the general category.

PROPERTIES OF LUBRICATING OILS

Lubricating oils used by the Navy are tested for a number of properties. These include (1) viscosity, (2) pour point, (3) flash point, (4) fire point, (5) auto-ignition point, (6) demulsibility, (7) neutralization number, and (8) precipitation number. Standard test methods are used for each test.

The properties of lube oil are briefly explained in the following paragraphs.

1. VISCOSITY-The viscosity of an oil is its tendency to resist flow. A liquid of high viscosity flows very slowly. In variable climates, for example, automobile owners change oil according to prevailing seasons. Oil changes are necessary because heavy oil becomes too thick or sluggish in cold weather, and light oil becomes too thin in hot weather. The higher the tem-perature of an oil, the lower its viscosity becomes; lowering the temperature increases the viscosity. On a cold morning, it is the high viscosity or stiff-ness of the lube oil that makes an automobile engine difficult to start. The viscosity must always be high enough to keep a good oil film between the moving parts. Otherwise, friction will increase, resulting in power loss and rapid wear on the parts.

Oils are graded by their viscosities at a certain temperature. Grading is set up by noting the number of seconds required for a given quantity (60 ml) of the oil at the given temperature to flow through a standard orifice. The right grade of oil, therefore, means oil of the proper viscosity.

Every oil has a viscosity index based on the slope of the temperature-viscosity curve. The viscosity index depends on the rate of change in viscosity of a given oil with a change in temperature. A low index figure means a steep slope of the curve, or a great variation of viscosity with a change in temperature; a high index figure means a flatter slope, or lesser variation of viscosity with the same changes in temperatures. If you are using an oil with a high viscosity index, its viscosity or body will change less when the temperature of the engine increases.

2. POUR POINT-The pour point of an oil is the lowest temperature at which the oil will barely flow from a container. At a temperature below the pour point, oil congeals or solidifies. Lube oils used in cold weather operations must have a low pour point. (NOTE: The pour point is closely related to the viscosity of the oil. In general, an oil of high viscosity will have a higher pour point than an oil of low viscosity.)

3. FLASH POINT-The flash point of an oil is the temperature at which enough vapor is given off to flash when a flame or spark is present. The minimum flash points allowed for Navy lube oils are all above 300F. However, the temperatures of the oils are always far below 300F under normal operating conditions.

4. FIRE POINT-The fire point of an oil is the temperature at which the oil will continue to burn when it is ignited.

5. AUTOIGNITION POINT-The auto-ignition point of an oil is the temperature at which the flammable vapors given off from the oil will burn. This kind of burning will occur without the application of a spark or flame. For most lubricating oils, this temperature is in the range of 465 to 815F.

6. DEMULSIBILITY-The demulsibility, or emulsion characteristic, of an oil is its ability to separate cleanly from any water present- an important factor in forced-feed systems. You should keep water (fresh or salt) out of oils.

7. NEUTRALIZATION NUMBER-The neutralization number of an oil indicates its acid content and is defined as the number of milligrams of potassium hydroxide (KOH) required to neutralize 1 gram of the oil. All petroleum products deteriorate (oxidize) in air and heat. Oxidation produces organic acids which, if present in sufficient concentrations, will cause deterioration of (1) alloy bearings at elevated temperatures, (2) galvanized surfaces, and (3) demulsibility of the oil with respect to fresh water and salt water. The increase in acidity with use is an index of deterioration and is measured as a part of the work factor test. This test is not applicable to 9250 oil.

8. PRECIPITATION NUMBER-The pre-cipitation number of an oil is a measure of the amount of solids classified as asphalts or carbon residue contained in the oil. The number is reached when a known amount of oil is diluted with naphtha and the precipitate is separated by centrifuging-the volume of separated solids equals the precipitation number. This test detects the presence of foreign materials in used oils. An oil with a high precipitation number may cause trouble in an engine. It could leave deposits or plug up valves and pumps.