May, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
493
Lubricating Value as Related to Certain Physical and Chemical Properties of Oils' By L. W. Parsons and G. R. Taylor T I D EWATEROIL Co., BAYOWE,PIT. J.
The selection of the best lubricant for a particular service cannot always be made on the basis of the common physical and chemical tests. Many specifications covering various types of lubricating oils are based on insufficient data as to the effect of variations in these tests, and either impose unnecessary restrictions or omit essential requirements. In this paper there is given a brief review of the theory of lubrication and a discussion of the application of this theory to a few special cases, with particular reference to the value of certain tests.
FFICIENT lubrication is dependent upon two main factors: (1) proper design and construction of the bearing and bearing surfaces, and (2) an adequate supply of the proper lubricant. This paper deals only with the second factor. It has been shown that the main variables influencing the lubrication of a given journal bearing of normal design and construction are (1) the ratio z N / p , where z is the viscosity of the lubricant, N is the speed of rotation of the journal, and p is the pressure on the bearing; and (2) a property of the oil variously described as oiliness, film-forming tendency, etc., which is the capacity of the oil to form, on the bearing surfaces, an adsorbed film which will resist rupture a t low speeds and high loads and thus prevent metal-to-metal contact and abrasion. If we furnish to a given journal bearing an adequate supply of any given lubricant, vary the speed and the load on the bearing, determine the coefficient of friction, and plot this against z W / p , we obtain a curve of the t m e shown on Plate I. w k e r e t h e coefficient of friction to the right of the critical point increases slowly and alm o s t l i n e a r l y with z N / p , whereas to the left of the critical point the coefficient rises very sharply. The region to the right of the critical point has been termed the region of fluid film or stable lubrication; the region to the left, the region of partial l u b r i c a t i o n . I n the region of fluid film lubric a t i o n the friction is all in the fluid medium and the only variable of importance i s t h e ratio z N l m When we approach the critical point, however, under conditions of low speed, or high load, or a combination of both, the main bulk of the oil is forced out from between the bearing surfaces and the only protection against abrasion is a tenaciously adsorbed film of lubricant on the bearing surfaces. Therefore, the film-forming properties of the lubricant be-
E
1 Meeting title "Relation between Certain Physical and Chemical Properties of Oils and Their Lubricating Value."
come of prime importance. Wilson and Barnard* state that this iilm of lubricant adsorbed by metal surfaces is thicker than a monomolecular film and probably has the nature of a plastic solid of a high yield point since it is forced out only very slowly from between the metal surfaces under high pressures. It will be observed that in the region of stable lubrication lowest friction coefficients are obtained a t values of zN/p as near the critical point as possible, but that the closer this point is approached the greater is the danger of entering the region of partial lubrication. Inasmuch as the pressure and speed are fixed by the bearing design and operating conditions, the only real variable is the viscosity of the lubricant. T o "c"'4;& :C;-) A U C E
PLATE
II
REWCING VALVE-
-
j
MODIFIED BINGHAM PLASTOMETER
obtain lowest friction coefficients, therefore, it is essential to use a lubricant of the lowest viscosity which will insure stable lubrication. For protection under conditions of an inadequate supply of lubricant due to overloading the bearing a t low speeds, or to any other cause, the lubricant should have the maximum film-forming tendency. Many methods have been tried for measuring the property of oiliness, but the two most practicable methods proposed to date are the static friction test used by Wilson and Barnard and the Deeley friction test. Considerable data have been obtained and are being obtained a t the Tide Water Laboratories by the use of the static friction test, but in so short a paper it will only be possible to point out some of the conclusions which have been drawn from the results. The general laws as applied above to journal bearings can be applied also to such problems as the lubrication of cylinder walls, thrust bearings, cutting tools, etc., and should be borne in mind in the following discussion of special cases. Furthermore, in testing lubricants for their lubricating value it is always necessary to consider the changes which will take place in the lubricant under service conditions and to consider the effect of those changes on the value of the partly used oil. Steam Turbine Lubrication
The large steam turbines of a modern power station are equipped with a storage reservoir, from which the oil is 2
THISJOURNAL, 14, 682 (1922).
494
INDUSTRIAL A N D ENGINEERING CHEMISTRY
pumped to the bearings under pressure. The system also includes an oil cooler and an oil purifier of some type. Under the conditions of operation of such a unit, oxidation products form in the oil which are partly acidic and partly oil-insoluble compounds or sludge. The first sludge formed in the fresh oil is soluble in hot oil but separates on cooling. On further oxidation of this soluble sludge there is formed an insoluble type. These oxidation products cause much trouble in lubricating steam turbines, as they tend to form sludges with water which deposit on the oil cooler tubes, decreasing the coefficient of heat transfer and allowing the average temperature of the oil to increase so that i t oxidizes more rapidly. These sludges also clog the oil lines and screen, and may be the cause of an inadequate supply of oil to the bearings. The acidic compounds formed also tend to corrode various parts of the system. The work of Funk, of the Philadelphia Electric Company, and the cooperative tests of the National Electric Light Association have shown the desirability of testing turbine oils for their relative resistance to oxidation and it is concluded that the oils which give best results on the laboratory oxidation tests can be expected to give best results on a laboratory sludging test and longer life in actual service. All the oils which were most satisfactory on the N. E. L. A. tests had very good nonemulsifying properties before use,
Vol. 18, No. 5
investigated. The emulsion tests are also being investigated and standardized by the A. S. T. M., but no decision has yet been reached as to the best test for turbine oils. It is always necessary, in applying special tests to oils, that enough check results be obtained to determine the variations due to the method of testing. It is also necessary to use great care in interpreting the results, since frequently oils of different characteristics, especially viscosity, give variations due to the effect of these variables on the test. Cutting Oils
For the lubrication of cutting tools, where a heavy cut is taken, a high degree of oiliness is essential as we have a condition of operation in the region of partial lubrication. Straight fatty oils, such as 1ard.or sperm oil, have been found to be the best lubricants for such work. Other lubricants, such as blends of small amounts of fatty acids or fatty oils with mineral oil, may show on the static friction test as low results for the static coefficient of friction as the animal oils but cannot be substituted for them as cutting lubricants. This is probably because the temperature of the surfaces being lubricated a t the tool tip is high and the animal oils must possess a higher oiliness a t these temperatures than the compounded mineral oils, although both may be similar on tests a t room temperature. A friction test at high temperatures would be very difficult to run, so i t is more feasible to test the lubricant under actual operating conditions. Ford Lubrication
The problem of lubricating the engine and transmission of a Ford car is complicated by the fact that the engine lubricant must also lubricate the bands and drums of the planetary transmission. A fabric-lined band is used as a brake on a rotating metal drum. The pressure developed between the two surfaces is very high and the speeds low, so that the oiliness or film-forming properties of the lubricant are important. The lubricating value of the oil also deteriorates in use, owing to crank-case dilution and to the presence of abrasive particles in the oil from road dust and engine wear-all of which increases the necessity for an oil of a high degree of oiliness. Internal Combustion Engine Lubrication
I n selecting the best lubricant for an automobile engine many factors must be considered, since the oil changes radically in its properties during use. Insoluble sludge and organic acids form in the oil owing to oxidation. Carbonaceous matter forms in the combustion chamber and on the lower surface of the piston owing t o the cracking and oxidation of the oil, and some of this material finds its way into the crank case. The oil is diluted with the higher boiling fraction of the gasoline to varying degrees depending on the design of the car, operating conditions, etc. Road dust is carried in oil down past the piston rings into the crank case. Abrasive Darticles of metal wQrn from the engine are held in suspension 4 In the oil. Water fiom the exhaust gases, which leak past the pistons, condenses in the crank case. Moreover, in cold P R L S S U R E - P O U N O S P C R SQUARE INCH weather the oil becomes very viscous, or even congeals, when as shown by the Herschel demulsibility test or the A. S. T. M. the car is allowed to stand in a cold garage or outdoors, and in steam emulsion test. The film-forming properties of a tur- most cars this lubricant must be handled by an oil pump on bine oil are of secondary importance, as the high speeds of starting and splashed onto the cylinder walls in order to luoperation insure fluid film lubrication even when the vis- bricate them. The formation of excessive amounts of oxidized insoluble cosity is low. The American Society for Testing Materials is a t present sludge in a crank-case oil may be the cause of an inadequate working on the development of a reproducible oxidation test supply of oil to the bearings, since this material tends to clog which can be used in writing specifications for lubricating oils. the oil screen and feed pipes. Oxidation tests are therefore The tests proposed by T. S. Sligh of the Bureau of Standards of great value and, although the best type of oxidation test and H. G. Smith of the Gulf Refining Company are now being has not been selected, such tests have aided materially in
May, 1926
INDUSTRIAL AND ENGINEZRING CHEMISTRY
developing many of the best grades of oils now used in the trade. A definite emulsion test is sometimes required in specifications for motor oils. This test is of little value except that it indicates the degree of refining of a straight mineral oil. However, addition agents which increase the oiliness of the lubricant frequently lower the emulsion test rating. A consideration of the above-mentioned factors which affect the
495
by very thorough cleaning. After starting the engine, all samples showed very poor emulsion tests a t the end of one hour’s running. The &hour samples showed practically no tendency to separate from water. The foregoing considerations demonstrate the advisability of depending on an oxidation test rather than an emulsion test in selecting a motor oil of the proper degree of refining. Table I-Emulsion Tests on Crank-Case Oils A. S. T. M. Steam Emulsion -Demulsibility-R. E. Number (Min.) 130’F. 180’F. 180’F. 300, 380 300 380
-
Viscosity Fresh oil After 15 minutes’ motoring Engine started (run After 5 minutes Atter 15 minutes After30minutes After 1 hour After 2 hours After 3 hours After 4 hours After 8 hours
1.5
6.0 at 1500 r. 6.0 7.0 12.0 20f
Viscosity (1) (2) 4.0 13.5
8.0 p. m.) 8.0
9.5 11.0 20f 20+
Viscosity
Viscosity
$“h,
1320
(1) 480
15.0
120
300
84
15.0 20+ 20+ 20+ 20+
120
300 240
72 72 69 67 66
120
110 76 90
192 128 120
120 110
20f
20+
20f
57
20 f
%$
%$
60 0
48
66
64 0
I n the past much reliance has been placed upon the present A. S. T. M. pour point test as an indication of the suitability of an oil for use as a winter motor oil. Recent work a t the Bureau of Standards on airplane engine oils shows that the use of this test as a criterion often leads to erroneous conclusions as it does not show the relative tendency of oils to flow under pressure at low temperatures.
properties of the lubricant, and of the facts that pistons, piston rings, and cylinder walls operate largely in the region of partial lubrication, indicates that the oiliness of the lubricant is important, and the use of the lubricant, with a high degree of oiliness would undoubtedly reduce engine wear. If an oil with good emulsion tests is introduced into a crank case, i t is immediately contaminated by the small amount of used oil in the crank case containing oxidized compounds which are excellent emulsifying agents. During the first 50 to 100 miles of use of the oil, sufficient more oxidized material is formed in the oil to cause it to show but very little tendency to separate from water. I n order to determine the effect of using oils in the crank case of an engine upon the emulsification properties, the following tests were made using a 6-cylinder Continental motor connected to a Sprague dynamometer on the testing block. The crank case of the engine was opened and thoroughly cleaned before starting the test. The engine was then driven for 15 minutes a t 1000 r. p. m., using the dynamometer as a motor, and with no fuel being fed to it. The engine was then started and run a t 1500 r. p. m. and samples were removed frequently. The results are shown in Table I. It will be observed that all the oils showed a decided change in emulsion tests after motoring the engine for 15 minutes. The oil is not heated under these conditions, so the poorer emulsion tests are due entirely to the mixing, with the fresh oil, of the small amount of used oil which could not be removed
A modified form of the test used a t the Bureau of Standards has been used at the Tide Water Laboratories. The apparatus shown on Plate I1 was constructed in order to measure the rate of flow of oils under pressure a t low temperatures. This apparatus is a modified form of the Bingham plastometer, consisting of a brass Saybolt viscometer tube immersed
496
INDUSTRIAL A V D E-VGI-VEERING CHEMISTRY
in a cooling bath and connected to a tank of compressed air. The temperature of the oil is measured by immersing in the bath another brass tube containing the same oil, into which a thermometer is placed. When the system has come to equilibrium at the desired temperature, the stopper is removed from the bottom of the tube, the capillary is closed by means of a rubber plug held by the operator, and the desired pressure is applied. A small amount of oil is then forced out into the tared dish, the operation being carefully timed with a stop watch. When sufficient oil has been obtained, the pressure is released and the capillary again closed. The viscosity tube has a replaceable capillary, a smaller capillary being used for the lower viscosity oils than for the higher. The results are shown in Plates I11 and IV. Plate I11 shows that paraffin-base oils a t 0” F. are not viscous but plastic, giving rather low rates of flow a t low pressures; however, a t higher pressures these oils show a much greater rate of increase in flow with pressure than the asphaltic base oils, which indicates that for two oils of similar viscosity a t 100” F., the paraffin-base can be handled more easily by a properly designed oil pump and oil lines. Plate IV is for oils of lower viscosity using a longer and smaller capillary. I n this set of curves the 300-viscosity asphaltic oil shows much higher rates of flow under pressure than 295-viscosity paraffin-base oil. If oils of similar viscosity a t 210’ F. are compared, it will be seen that the paraffin-base curves cross those of the asphaltic base at the higher pressures. The relative tendency of these oils to splash may be indicated by the lower portions of the curves, but further work is needed along these lines, It is possible that plasticity curves on samples of oil which have been stirred after reaching the low temperature may show different results. The temperature chosen for these tests is that of rather a n extreme case and further work must be done on oils at slightly higher temperatures. Since the nature of the crank-case oil also changes markedly during operation, further work must be done on oils containing oxidation products and average amounts of crank-case dilution. Further tests along these lines are under way. Gear Lubrication
For the lubrication of gears an oil or grease of high viscosity and a high degree of oiliness should be used, since the surfaces of gears operate largely in the region of partial lubrication.
Vol. 18, No. 3
I n testing gear lubricants for their suitability for use a t low temperatures, especially in transmission and rear-axle housings of automobiles, the plastometer described above using a slightly larger capillary was found of value. This is a much more accurate test than has previously been available for this purpose. Plate V gives the results obtained on a series of automobile gear lubricants. The upper three curves are for lubricants that have been found satisfactory for use in cold weather, while the lower two cause trouble in gear shifting and give a high power absorption until they warm up due to the internal friction of the lubricant. Conclusions
These special cases have brought out the following points: (1) It is desirable t o apply a n oxidation test and a n emulsion test to turbine oils as well as the usual physical tests in order t o prevent the use of oils which will deteriorate in service or become unsatisfactory lubricants. (2) No test, except use in actual service, has been found satisfactory for determining the relative lubricating value of cutting oils. (3) An oil for the lubrication of the Ford transmission should possess a high degree of “oiliness” due to the condition under which it must operate. (4) I n developing the best grades of motor oils an oxidation test is of value and an emulsion test is only of secondary importance. ( 5 ) Motor oils for use a t low temperatures should be tested for their relative rates of flow under pressure a t low temperatures, since the A. S. T.M. pour point test may lead t o erroneous conclusions as t o the amounts of oil which will be supplied to the bearings and cylinder walls when the oil is cooled t o low temperatures. (6) Automobile gear oils and greases can also be tested by the use of a plastometer for their value as lubricants for use in cold weather.
These typical cases are only a few which might be cited to show the necessity of a careful inspection of the physical and chemical properties of oils in order to determine their relative lubricating value under service conditions. The type of tests applied must take into consideration modern theories of lubrication and the conditions under which the oils are to be used, as well as the probable changes in the 1ubricant . Acknowledgment
The authors wish to acknowledge the valuable assistance of Mortimer C. Bloom in the preparation of this manuscript.
Prize Essay Contest of 1926-1927 public or private, listed as such in the Directory issued by the The generosity of Mr. and Mrs. Francis P. Garvan enables CHEMICAL SOCIETY again to conduct a Prize U. S. Department of the Interior, Bureau of Education, will the AMERICAN Essay Contest, t h a t for the academic year 1926-1927 differing be eligible for this contest, and no others. In this contest the first prize will probably be $500, the second $300, and the third in some respects from previous contests. In the Proceedings will be found the letter of gift, in which it is stated that, as in $200 for the best essays in each of the six subjects. The comprevious years, contests will be conducted among high school mittee is now engaged in preparing detailed announcements, students on the same subjects and with the same prizes as before. but it is hoped by disseminating this information as early a s The college contest for the past two years has been open t o stu- possible several who desire t o compete will take advantage of dents of all grades, but in the next contest will be available only the summer months for reading and preparation of outlines. t o bona fide members of the freshman class. The SOCIETY’SThe office of the committee continues t o be a t 85 Beaver St., New York, N. Y. committee believes t h a t such limitation will be beneficial in The merits of the contest have been recognized t o a greater encouraging those who have just entered college t o take a new degree during the past year than in any previous contest. Many interest in chemistry a t a time when they are choosing majors for study and perhaps deciding upon the choice of a career; high schools have adopted the contest as a piece of required work also t h a t as the contest is among freshmen there will be no for which the student is given some degree of credit as a n English composition and also as an alternative for one of the quizzes inclination t o refrain from entering because of the presence in in chemistry. Many teachers look upon the stimulus t o the the contest ’bf higher classes. An entirely new contest is announced on the same subjects for use of the library as adequate return for the efforts of the student those preparing to teach. While the committee has not made in the contest and see an increased interest in the general class work resulting from this new conception of the relations of a final announcement, it is expected that students preparing t o chemistry which the contestant soon acquires. be teachers in the normal schools and teachers’ colleges, whether
