TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Section 2 - Lubricants

SECTION II: LUBRICANTS

8. General.-a. A lubricant may be defined as a substance having greasy properties and used almost exclusively for reducing friction between bearing surfaces; however, it may also be used as a rust preventive on metallic parts subject to corrosion.

b. Common lubricants may be classified as animal, vegetable, and mineral, according to the source from which they are derived.

(1) Animal lubricants, such as tallow, sperm oil, lard oil, etc., are excellent lubricants provided they are not subjected to high temperatures. They are not suitable for lubricating internal-combustion engines, since above certain temperatures they form fatty acids.

Porpoise-jaw oil, used for lubricating expensive watches, is an excellent high-quality animal oil.

(2) Certain vegetable lubricants, such as castor oil, olive oil, and cottonseed oil, have satisfactory lubricating qualities but are chemically unstable under conditions prevailing in internal-combustion engines. Straight mineral oils of high quality have almost completely replaced the blends of castor oil formerly used in high-output engines.

(3) Mineral lubricants have many desirable properties for use in the lubrication of internal-combustion engines and are, therefore, described in detail in subsequent paragraphs.

9. Description and classification of mineral lubricants.a. Lubricants may be readily classified according to their physical properties as solids, semisolids, and fluids. Solid lubricants, such as mica, soapstone, and graphite in finely powdered form, serve to fill the low spots in a bearing surface to form a perfectly smooth surface and at the same time to provide a slippery film to reduce friction. A finely divided solid lubricant also acts as a mild abrasive, smoothing the surface previously roughened by machining or excessive wear. Solids are fairly satisfactory on slow speed machines but lack the ability to dissipate heat, which is often an essential requirement. Certain solid lubricants have the ability to carry heavy loads and for this reason are often added to fluids to reduce wear between surfaces subjected to high unit pressures.

FIGURE 5-Shaft and bearing with fluid lubricant (cross-section view).

b. The semisolid lubricants include such substances as extremely heavy oils and greases. Modern industrial operations require a great number of such lubricants for special applications. Greases give good service when applied periodically, but because of their consistency they are not suitable for continuous or circulating lubrication systems. A continued discussion of the properties of these lubricants will appear later in this manual under a separate heading. From this point, fluid lubrication will be emphasized, especially the requirements of internal combustion engines.

c. (1) Fluids (oils) are universally used in internal combustion engines for many reasons. They may be readily pumped or sprayed, they provide a good cushioning effect, and are effective, in absorbing and distributing heat. In theory, fluid lubrication is based on the actual separation of surfaces so that no metallic contact occurs (fig. 5). In this way, as long as the oil film is unbroken, metallic friction is replaced by the internal friction (fluid friction) of the lubricant itself, and obviously under such an ideal condition no wear can occur. Many vital engine parts are given adequate protection by supplying oil under direct pressure, but where this method is impractical a mist or spray will generally be satisfactory. Parts carrying heavy loads at high rubbing velocities are, where possible, lubricated by direct pressure. In the process of circulating through the engine, oil absorbs heat from different parts and will later dissipate most of the heat through suitable coolers or heat exchangers. In this way engine parts are protected from both wear and excessive temperatures.

(2) An ideal fluid lubricant would be capable of providing a strong oil film to prevent metallic friction, and at the same time create a minimum amount of oil drag or viscous friction. Unfortunately, however, the body or viscosity of oils is affected by temperature changes to such an extent that ideal conditions are difficult to attain. Variations in climatic temperatures alone will often create an astounding change in oil viscosity. It is not at all uncommon for some grades of oil to become completely solid in cold weather with consequent high oil drag and impaired circulation. Conversely, at high operating temperatures oil may thin out to such extent that the oil film is broken, which permits rapid wear of the moving parts. The major problem in lubrication is to obtain a satisfactory compromise between the above conditions.

10. Engine lubricating oils.-a. Crude petroleum, which furnishes the fuel for internal-combustion engines, also supplies the most satisfactory oils for their lubrication. The numerous individual compounds contained in crude oil are arranged in groups, such as the paraffin series (saturated hydrocarbons), naphthalene series, olefin series, aromatic series, and others of lesser importance. A single crude sample may contain all of the above groups of the hydrocarbons in varying proportions. As a group, the paraffins are most satisfactory for engine lubricants since they are very stable, have good lubricating qualities, and are affected least by temperature variations. Almost any crude contains a certain percentage of paraffin compounds, but the crudes taken from eastern United States oil fields contain a higher proportion than midcontinent or western crudes.

b. Since the paraffin hydrocarbons are most desirable in a finished oil, the refining process should endeavor to eliminate all of the undesirable constituents without damage to the paraffin compounds. Two principal methods of refining are in common use: acid extraction and solvent extraction, the latter being of more recent development.

(1) The acid extraction process involves the treatment of the lubricating stock with sulphuric acid which reacts chemically with many of the undesirable compounds, thus removing them from the mixture. The acid concentration and the duration of the treatment must be carefully controlled, since insufficient exposure will fail to remove the undesired compounds, and an overtreatment will destroy a part of the valuable constituents. In most cases, the type of crude has a direct bearing on the quality of the finished product when acid extraction is employed. Thus, it would be very difficult, if not impossible, to produce a superior lubricating oil by this process, except from crudes containing a high percentage of paraffin hydrocarbons.

(2) Solvent extraction, on the other hand, does not require a chemical reaction but, as the name implies, separates the various compounds by dissolving them with certain selected solvents. Advantage is taken of the fact that the desirable compounds are soluble in particular fluids such as propane, whereas the undesirable ingredients are soluble in other liquids. By the use of one or more solvents the lubricating stock may be accurately divided into the desired fractions. By this process a good lubricating oil may be produced from practically any crude.

(3) Oils extracted by either of the above methods require many treatments other than the basic extraction process. In order to remove heavy wax which will cause cold weather difficulties, the oil is chilled as low as -30° F., and the solid wax is removed by special filters or centrifugal separators. Filtration through fuller's earth or other decolorizing earth is an essential and valuable part of most refining methods. After being properly purified, the various "Cuts" taken by low pressure distillation are accurately blended to give the various grades of oil required by modern industrial operations.

11. Determining properties of lubricating oils. -a. Since mineral oils are produced in many grades, it is highly important to examine particular specifications when selecting oils for use in internal combustion engines. Of special significance are the factors of viscosity, flash point, and pour point, each of which will be discussed in detail.

b. Viscosity is generally considered as the resistance the oil offers to flow. Thus, if an oil flows slowly, it is described as a viscous oil or an oil of high viscosity. An oil which flows readily is said to possess a low viscosity. As previously stated, the body or viscosity of an oil determines the amount of fluid friction. In general, it is desirable to select an oil of the lowest viscosity which will provide an unbroken film, so that friction may be held to a minimum. However, when consideration is given to the fact that the oil must often lubricate through a temperature range of 0° F. to 300° F., the problem becomes a very complex one and worthy of extensive study.

(1) In order to measure oil viscosity, the Saybolt viscosimeter (fig. 6) is employed. By a suitable arrangement, a certain amount of oil is heated to a standard testing temperature, generally 130° F. or 210° F. The oil tube is provided with an outlet orifice through which the oil is permitted to flow into a 60 cubic centimeter flask. The time (in seconds) required for the delivery of 60 cubic centimeters of oil gives the Saybolt universal viscosity at that temperature. For example, if the time required for a, particular sample is 120 seconds at 210° F., the oil is said to have a Saybolt viscosity of 120.

(2) If actual Saybolt numbers were used to designate the viscosities of various oils on the market, there would probably be several hundreds of grades listed and the purchaser would be faced with a rather complex problem. In order to simplify the selection of oils, they are often classified under an SAE rating system which divides all oils into seven groups (SAE 10 to 70, incl.) according to viscosities at either 130° F. or 210° F. These SAE ratings are purely arbitrary and bear no direct relationship to the Saybolt or other ratings. They are defined, however, in terms of the Saybolt universal viscosity. By reference to the chart (fig. 7), the relation

FIGURE 7-SAE conversion chart.

between Saybolt seconds and SAE ratings can readily be determined.

It will be noted that grades 10 to 40 are tested at 130° F. and the heavier grades at 210° F., a procedure which is quite normal since the heavier oils are intended for use at higher engine temperatures. To determine the SAE rating of an oil sample, it is first tested at the correct temperature in the Saybolt viscosimeter. The reading thus obtained is compared with the ranges listed on the chart. For example, if an oil tests 82 Saybolt seconds at 210°, F. it is evidently an SAE 50 oil, because the 50 classification covers all oils between 75 and 105 seconds. By the same method, an oil testing 130 seconds at 130° F. is found to be an SAE 20 grade. Occasionally the letter "W" will be included in the SAE number giving a designation such as SAE 20W. This letter indicates that in addition to meeting the viscosity requirements at the testing temperature the oil also meets additional low temperature specifications, showing that it is a satisfactory oil for winter use.

(3) Although the SAE scale as explained above has eliminated some confusion in the selection of lubricating oils, it must not be assumed that this specification covers all of the important viscosity

FIGURE 8.-Viscosity temperature chart of two SAE oils.

requirements. An SAE number indicates grade only; it does not indicate quality or any other essential characteristics. It is well known that there are good oils and inferior oils having the same viscosities at a certain temperature and are therefore subject to classification in the same grade. The SAE letters on an oil container are not an endorsement or recommendation of the oil by the Society of Automotive Engineers.

(4) In order to appreciate the difference between apparently similar oils, viscosity must be known at more. than one temperature. Figure 8 shows two oils of the same SAE rating but having widely different characteristics, as their temperatures are changed through the range required by engine operation.

(5) This chart clearly indicates a need for a more complete, study of the viscosity temperature characteristics of lubricating oils. Sample A is obviously superior to sample B in its ability to withstand temperature changes with less change in body or viscosity. Sample B congeals readily at low temperatures thus causing excessive drag and cold starting difficulty, and at 300° F. is actually lower in viscosity than sample A.

(6) A convenient method of recording viscosity temperature curves is by the use of a special graph prepared by the American Society for Testing Materials (A. S. T. M.), a copy of which is shown in figure 9. By intricate spacing of the temperature and viscosity readings on the graph, resultant curves will appear as straight lines. To examine an oil, it is tested in a Saybolt viscosimeter at two temperatures, such as 130° F. and 210° F., and the viscosities are properly located as points on the chart. A straight line is then drawn through the two points so as to include all temperature readings. It is then a simple matter to determine the viscosity at any temperature

FIGURE 10-Alinenient chart for the estimation of viscosity index.

between - 30ture F. and 450ture F., although the temperature values below 0° F. and those above 300° F. are of little significance, because most oils reach their chill points near zero temperatures and are seldom subjected to engine temperatures above 300° F.

(7) Upon examination of the oils plotted on the graph in figure 9 certain significant points are observed. Sample, A is a good grade of SAE 50 oil; sample B is an inferior grade of SAE 50 oil; sample C is a superior grade of SAE 50 oil, and sample D is a good grade of SAE 60 oil. Sample B line on the chart indicates that at low temperatures this oil is actually more viscous than the SAE 60 oil represented by the sample D line on the chart. Sample C line shows less tendency to congeal at low temperatures and to thin out at high temperatures than other samples tested, thus indicating its superior characteristics. Many other comparisons may be readily made by use of the A. S. T. M. graph. It is generally agreed that an oil of 60 to 100 Saybolt seconds at all temperatures represents an ideal lubricant for internal -combustion engines, but it is highly improbable that such a lubricant will ever be available.

(8) For somewhat greater convenience in indicating the viscosity temperature characteristics of oils, a viscosity index chart (fig. 10) may be used. This is an arbitrary chart on which superior oils of any grade will give a reading at the higher end of the scale, and inferior oils will show much lower readings. Indexes are determined by performing two Saybolt tests at different temperatures and locating the readings thus obtained on the proper lines on the left side of the chart. A straight line is then drawn through these two points and projected so as to cross the index scale at the right. The point of intersection of the viscosity line and index curve gives the index rating of the particular oil. The two oils listed in figure 8 furnish an excellent sample of oils having similar SAE ratings but widely different index values. When compared on the index chart, sample A oil is found to have a viscosity index of 95, and sample B (also an SAE 50 oil) has a rating of only 26. This further emphasizes the fact that an SAE rating alone cannot be interpreted as an indication of quality in lubricating oils. In general, high index ratings are very desirable in all grades of oil, a quality which is found particularly in compounds of the paraffin series. Some oils containing a high percentage of paraffin hydrocarbons have indexes well above the 100 figure.

c. The lubrication of cylinder walls and pistons of internal-combustion engines requires consideration of factors other than oil viscosity. Because of the large amount of heat generated in cylinders during the power stroke, the flash test of an oil assumes considerable importance. The test is performed by heating an oil sample in an open cup (fig. 11), and as the temperature rises a flame is periodically passed slowly over the surface of the oil. The first test wihich momentarily ignites the oil vapor above the cup is recorded as the flash point. Other factors being equal, an oil of high flash test is preferred because of the greater degree of protection afforded cylinders and pistons and the probable lower oil consumption during continuous engine operation. A fire point test, which indicates the temperature of continuous burning, is often performed along with the flash test, but this specification is considered of little importance in aircraft engine oils.

d. In addition to the above properties, a good oil must show a high degree of chemical stability in order to resist the action of high temperature, moisture, and acids, all of which are often present in engine crankcases. Here again, the paraffin hydrocarbons (saturated series) show a marked superiority over the other groups of petroleum products.

FIGURE 11-Open cup flash test apparatus.

e. Cold weather starting and operation also require special consideration. It is not at all uncommon for internal combustion engines to be started cold at temperatures of 0°' F. or below. A study of the A. S. T. M. chart (fig. 9) reveals that at these low temperatures oil viscosities reach surprisingly high figures, which, of course, necessitates a very high cranking torque in order to rotate the crankshaft. Furthermore, at certain low temperatures oil will congeal or chill completely so that circulation is impossible even if starting is accomplished by special means.

(1) In regard to the question of whether a low pour point or a good viscosity curve (high viscosity index) is best for low temperature starting of internal combustion engines, experience indicates clearly that both are essential. The viscosity curve must be good (viscosity index about 95 or above is desired), and the pour point should be within 5° F. of the average starting temperature. The, pour point of an oil is the lowest temperature at which it will pour or flow when it is chilled without disturbances. The oil to be tested is placed in a. glass test jar and cooled in a cooling bath until it ceases to flow, when the test jar is removed from the bath and held in a horizontal position for exactly 5 seconds. A sectional view of a pour-test apparatus is shown in figure 12.

FIGURE 12.-Pour test apparatus.

(2) Many ingenious methods have been employed for starting aircraft engines in cold weather, the most satisfactory of which appears to be the dilution of the lubricating oil with gasoline prior to stopping the engine whenever a cold start is anticipated. By adding aircraft gasoline directly to the lubricating oil, the oil viscosity is greatly reduced, and cranking is comparatively easy even at subzero temperatures. Upon starting an engine so prepared, the cold diluted oil circulates quite readily, providing adequate lubrication, and as the engine is brought to normal operating temperature the gasoline evaporates, returning the oil to its original condition. Increased corrosion of some engine parts has been found, caused by the introduction of the corrosive ethyl fluid into the oil, but the better fluidity of the oil at starting temperature and the lower drain on electric or manpower for starting are believed greatly to outweigh any disadvantages. This system of oil dilution, with proper modifications in the oiling system, provides the first real solution to the problem of maintaining comparatively low oil viscosities in cold weather to facilitate starting, and at the same time meeting the high temperature requirements of continuous operation.

12. Requirements of aircraft-engine oils.-a. The chief lubrication problems presented in an aircraft engine are the high temperatures encountered during operation, the high bearing pressures on working parts, and oil drag at starting temperatures. All of these conditions require an oil of high initial viscosity and preferably one of flat viscosity temperature characteristics. Only such oils will afford the proper cushioning effect, especially between heavily loaded parts to protect them from friction and wear. A high degree of chemical stability is also an essential quality for aircraft lubricants.

b. In order to fulfill the requirements of aircraft-engine operation at atmospheric temperatures ranging from -20° F. to 110° F., many grades of oil are sometimes prescribed. Much attention has been given to this problem in recent years, with the result that it now seems possible to meet all requirements with only two grades of oil of intermediate viscosity. Oils of especially high viscosities are apparently not necessary when suitable oil temperature regulators are installed in the lubrication system. By means of such regulators, oil temperatures are reduced in warm weather, with a consequent increase in viscosity which insures satisfactory lubrication. On the other hand, the light grades formerly required in cold weather are also unnecessary, since the system of oil dilution (par. 11) has removed most of the cold weather difficulties.

c. In summary, the desirable properties of an aircraft oil are as follows:

(1) High viscosity (90-130 Saybolt seconds at 210° F.).

(2) Flat viscosity temperature curve (high viscosity index).

(3) High flash point.

(4) Chemical stability.

(5) Low pour point.

d. Only oils refined from crude petroleum will meet all of the above requirements. Oils containing compounds other than petroleum hydrocarbons are in most cases unsatisfactory for continuous use.

13. Oil reclamation.-a. In service, oil is constantly exposed to many harmful substances which will in time seriously reduce its ability to protect moving parts. The chief agents of contamination are heavy ends of gasoline, acids, moisture, dirt, carbon, and metallic particles. Because of the accumulation of these injurious substances, it is common practice to drain the entire lubrication system at regular intervals and refill with new oil. This practice is open to the objection that it is not generally possible to determine exactly when the oil should be drained. If changed too frequently, needless waste and expense will result, and on the other hand, if an oil is used too long, excessive wear and mechanical trouble are probable. Although there is no exact relation between the length of service of an oil and its condition when drained, regular periodic draining as generally practiced is a fairly safe procedure.

b. Oil filters of many types have been designed to remove the solid impurities from oil during circulation. Really efficient filters of the full flow type which may be readily cleaned are very satisfactory for removing solids, but they cannot remove the liquids such as acids and water. Part flow filters of the treated cotton waste type are efficient when new, but they usually require replacement after a moderate amount of use.

FIGURE 13-Oil reclaimer.

c. A very novel oil supply arrangement has been developed for use with aircraft lubrication systems (dry sump with external supply). A small tank or hopper is incorporated within the main oil tank in the direct path of oil circulation. By this arrangement only a small part (10 percent or less) of the total oil supply passes through the engine. The new oil in the tank merely serves to replenish the supply in the hopper at a rate equal to the oil consumption of the engine. The advantages of the hopper tank are as follows:

(1) More rapid consumption of that portion of the oil in use in any given time, permitting less oxidation and other deteriorating effects.

(2) More rapid warming up of the oil in use after starting.

(3) Less frequent oil change.

d. It will be noted that oil requires periodic change because of contamination and not because of any chemical change in the lubricant itself. Realizing this fact many concerns manufacture oil reclaimers similar to that shown in figure 13 to remove the impurities so as to return the oil to its original condition. These machines generally operate on the principle of distillation to remove foreign liquids and of filtration to remove the solid matter. Experience indicates that the most economical and practicable oil reclamation is to make no attempt at chemical refining, but simply to remove water, acid, fuel, solids, dirt, carbon, metal particles, etc. This can be done by any simple and reasonable efficient vacuum heating chamber, followed by filtration through tight felts or similarly effective filters, or by centrifugal separation of solids. The resulting oil may be slightly inferior in some respects to the original new oil; probably equal to the new oil after 2 or 3 hours of use. Chemical treatment of oils is practicable on a large scale, and if accomplished by experienced personnel with adequate technical supervision results in quite acceptable lubricants. Various stocks may be reclaimed, but if the charging stock is not controlled there will be a corresponding lack of control in the finished product.

14. Miscellaneous lubricants.-a. In certain cases because of special stresses or peculiar design it is impossible or unsatisfactory to lubricate units by means of engine oil. In these cases special lubricants (generally greases) are required. A great variety of such lubricants are available, each of which has certain special properties. For example, a grease for one purpose may require a resistance to the action of water, whereas in another unit a high melting point may be the most important. Other qualities often desired are low shearing stress, chemical stability, and the ability to support unusually heavy loads. To meet this condition a series of EP (extreme pressure) lubricants have been developed. They contain various active chemical compounds which protect parts from wear when temporary high unit pressures break through the normal film of lubricant. At normal loads the EP characteristic is not essential.

b. Since a large number of lubricants must be available in order correctly to service aircraft units and accessories, it is necessary to emphasize the fact that detailed instructions must be carefully followed. A certain lubricant is prescribed for an item of equipment only after extensive tests have been conducted. The substitution of another lubricant will normally be unsatisfactory and often dangerous. For example, identical parts, such as bearings, used in different airplane accessories may require different lubricants. In one case engine oil may be specified, in another case high melting point grease is required, and in still another case graphite grease is correct, even though the bearings are identical. When lubricating airplane parts, the detailed instructions available in technical publications must be closely followed in regard to the type and grade of lubricant and to the actual lubricating procedure.