trends in liquid lubrication

MECHANICAL ENGINEERING

TRENDS IN LIQUID LUBRICATION

ACKNOWLEDGMENT

This article, by Prof. E. E . Klaus of The Pennsylvania State Univer- s i t ! / , apperrred in the October 1963 issue of “Mechanical Engineenng.”

THE development of liquid lubncants has followed a significant evolutionary pattern over the past several decades. Primary emphasis in the 1930’s was placed on improved mineral oil-processing to p r ~ r duce a quality lubricant without additives. The 1940’s were marked by the development of an effective and extensive additive package applied to conventionally refined mineral oils. The development and commercialization of a wide variety of synthetics were the major contributions of the 1950’s. The present decade is marked by the integration of the addtive package with the synthetic or superrefined base stock to provide tailor-made lubricants for specific applications. Demands of aerospace hardware frequently necessitate collaboration be- t ween the design and the lubrication engineer.

FLUID PROPERTIES

Aerospace hardware demands have resulted in hetter definition of the pertinent properties of lubricants. These fundamental studies, in turn, result in a wider spectrum of lubricants available for more conventional uses. An example is the development

of multigrade crankcase oils in widespread use today from hydraulic fluids developed for the Air Force and Navy during World War 11.

A list of some of the more important classes of synthetics and some common applications of these materials is shown in Table 1.

A list of fluid properties whch are considered to be important in various hydraulic fluid and lubricant applications is shown in Table 2. Thus far, no single synthetic lubricant studied shows optimum properties in all of the areas shown. There have been a number of important fundamental studies and correlations concerned with synthetic lubricants which may be of considerable help to the design engineer.

VISCOSITY-VOLATILITY PROFTRTIES

The relationship between viscosity and b o h g point, or volatility, of a lubricant is an important factor in determining its application. Viscosity-volatility and viscmity-temperature properties of fluids and lubricants are closely interrelated. ”€us genera1 correlation can be applied to all classes of synthetics

Naval Enqimers Journal, October. IT64 701

LUBRICANT TRENDS MECHANICAL ENGINEERING

TABLE 1. Classes of Fluids and Lubricants Chemlcal Clam Primary Use Common Name

Hydrocarbon

Phosphate esters

Polyglycol ethers

slikonca

Halocarbons

Crankcay l u b e s Hydraulic flulds

~ e e Inflpmmable hydraullc fluIda

Automotive hydraulic brake fluld

spcclal flulds. lubes. and InNument 0118

Nonlnflammable flulds and lubes

Spec Mll-I.-7808 spcc. MI-L-9236 Conventlonal Mlneral Super refined rnlneral S thetlc hydrocarbon Srdml Pydraul

Ucon

Dc-200. Dc-500 Versl luk Fluorolube Aroclor

Cefiulude

011 011

Polyphenyl ethers Experlmcntal hl- tcrnpraturr flulds and lube8

discussed here. Frequently, poor viscosity-volat~lity properties are associated with a more limited liquid range.

It is apparent from the data that substantial differ- ences in viscosity can be achieved for the same boiling point. Or for a given viscosity level, lubricants of varying boiling points or volatility can be chosen. The major consequences of viscosity-volatility relationships are reflected in oil consumption. evaporation, inflammability, and capacity for utiliz- ing polymeric materials. The effect o f lubricant volatility on oil consumption has been demonstrated in the case of automotive crankcase oils [ l ] . Evapo- ration has b e n shown to be of major imprtance in several hydraulic and instrument oil applications

The relationship between volatility and inflammability has also been established for the hydrocarbons and synthetics, other than those containing halogens and or phosphorus. Thus the inflammability of the lubricant is determined primarily by the most vola- tile constituent present in the lubricant in appre- ciable quantity. There are substantial benefits regarding volatility and inflammability to be de- rived from the use of single compounds, or at least narrow boiling-range mixtures, over conventional mineral-oil fractions which may have a boiling range of 200 to 300 F or more.

Organic liquids containing substantial quantities

[21.

TABLE 2. F l i t d Properties of Lubricants 1. Fiydrodynrmlc Lobdcstlon

( a ) Viscosity-temperaturn relationship (b) Viscosity-pressure relationship ( C ) Viscosity-shear relationship

2 Boundary hbrlcatlon

(a) Friction (b) Wear ( c ) Seizure

3 . Fluld Sbbl l l ty

( a ) Oxidation stability (b) Thermal stability ( c ) Metal corrosion (d) Storage &ability (e) Inflammability

of phosphorus and or halogen do not follow this general volatility-dammability correlation. The phosphate and phosphonate esters are materials which show fireretardant properties, and have found application as hydraulic fluids and lubricants primarily because of thu property [3]. These materials are not nonrnflammable but are less inflammable than the usual organic lubricants. "hLS property has been demonstrated in a variety of comparative tests. A noninflammable lubricant can be achieved in the halocarbon-type synthetic lubricant. In general, halocarbons containing 60 weight percent or more halogen in the molecule are noninflammable.

VISCOSITY PROPERTIES

The viscosity properties of lubricants are of particular concern to the designer considering hydrodynamic lubrication. In essentially all of the classes of synthetics discussed, synthesis techniques have provided a wide range in viscosity grades of lubricants. One exception is the polyphenyl ethers which are currently available in four, five, and six-ring products. Of particular interest is the relative effect of temperature and pressure on the viscosity of characteristic examples of these synthetic classes. Viscosity-temperature properties of several synthetics of the same viscosity level at 100 F (20 centi- stokes) and atmospheric pressure are shown in Table 3.

The viscosity-temperature properties of the lubricants are expressed in three ways in Table 3. The viscosity index scale is used primarily for mineral oils in the range of 0 to 120. Above and below these values, the scale has little meaning [4]. The ASTM slope is an indication of the slope of the line rep- resented by the measured viscosities on an ASTM viscosity-temperature chart [4] . This slope provides a good relative measure for all fluids. The use of a temperature range equivalent to the useful viscosity range of the lubricant is helpful in many cases to interpret the more abstract measures of viscosity- temperature coefficients in terms of operational range.

TABLE 3. Viscosity-Tenlpernture Charactenstics of Luhricnllts

Vl.- w e . F. for V l s o d t y Centldoke at ASTM cwity 10 to 158 CL I.ubrlcant

Type 21OF IOOF 4 0 F Slope Index V k . Range

Halocarbon

Hydrocarbon Hydrocarbon Ester Polyglycol ether Phosphate- base

formulation Ester-base

formulation Silicone

( fluorolube) 2 9 20 3.4 20 3.9 20 4.4 20 4.6 20

4 6 20

6 3 20 9.5 20

0.973 0.861 0.771 0.696 0.670

0.670

0.496 0.297

-132 0

loo 151 164

164

197 195

84 90

101 118 115

115

160 240

702 Naval Enginears Journal. October. I9M

MECHANICAL ENGINEERING LUBRICANT TRENDS

Viscosity-pressure effects show a general correlation with viscosity-temperature properties for these fluids with the exception of the silicones [5]. The silicones are relatively compressible liquids and, as a result, show a relatively high rate of viscosity increase with pressure. The role of viscosity-pressure effects has not been as clearly demonstrated as that of viscosity-temperature effects in lubricant applications.

The viscosity-temperature properties and the ASTM viscosity-temperature chart can be used to determine the low-temperature properties of a lubricant [S]. Studies have shown a general correlation between a viscosity of 0.25 to 0.5 million centistoke and the viscous ASTM pour point. This correlation can be used to determine whether the pour point of a lubricant is due to viscosity or phase change (solids' separation) and solubility problems. This method can also be used to determine whether synthetic lubricants will have the desired liquid range, providing a viscous pour point can be achieved.

POLYMERIC MATERIALS

Polymers have been used widely to obtam viscosity-temperature characteristics not possible with the base stock alone. These chemicals have made the viscosity-temperature characteristics of the fluid relatively independent of the base stock properties.

Polymer-containing formulations, in general, dis- play non-Newtonian viscosity properties. Viscosity of these lubricants is a function of shear rate. The viscosity change with shear rate is a reversible change, providing only streamline flow is involved [ 71. Viscosity loss at a high shear rate is completely recovered at low shear rates under these conditions. Some advantages of non-Newtonian viscosity properties have been shown in fundamental studies of the hydrodynamic lubrication of journal bearings. It has been shown that a non-Newtonian fluid may show a 43 percent reduction in friction over a New- tonian fluid of the same low shear viscosity while both fluids exhibit the same film thickness in the bearing [8].

Polymeric materials usually introduce mechanical stability problems. These polymers tend to be per- manently degraded to less viscous materials when subjected to turbulent flow under high shear, wire drawing, and attrition. This process results in a permanent loss in viscosity.

Overall stability of the fluid dictates fluid life and, therefore, the maintainance and overhaul schedules and reliability of the mechanism in whch the lubricant is used. Fluid stability plays an important role in the cost of operation and rates special attention.

Most lubricants are used in an air atmosphere and oxidative degradation is the most severe h i t a - tion to lubricant life. The synthetics, in general, have provided a substantial margin of oxidative

stability over the conventional mineral oils whch were previously used. With respect to oxidation stability, the synthetics fall into two general classes. The silicones, halocarbons, and phenyl ethers do not readily react with air or oxygen a t operating temperatures of 400 F or less. The remainder of the synthetics including the esters, phosphate esters, polyglycol ethers, and hydrocarbons provide good oxidation stability through their response to oxidation inhibitors. This latter group without such additives and inhibitors is frequently less stable than the conventional mineral oil.

The effect of oxidation inhibitors (antioxidants) on conventional mineral oils as well as esters and hydrocarbons is shown in Figure 1. These data were obtained at 347 F and atmospheric pressure, and are expressed as stable life. Stable life is defined as the period during which little or no oxidation occurs, or the period during which the oxidation inhibitor is effective. The stable life of these fluid types has also been studied as a function of temperature and these data show that stable life for ex- tended periods cannot be provided by oxidation inhibitors at temperatures of 500 F and above.

At high temperatures, deterioration of the fluid is a function of i ts oxidation rate and its oxygen tolerance. Oxygen tolerance is the amount of oxygen that can be assimilated by the fluid or lubricant without exceeding the property changes considered tolerable by the system. Oxidation rate has been shown to be a function of the temperature, the intimacy of contact between the oxygen and the lubricant, the time of contact, and the partial pressure of the oxygen in the atmosphere [9]. One way to increase lubricant life a t high temperatures where stable life is not available from an additive, is to

YC,. r w . *Dv"'

Figure 1. W k c t of super rsfiniru on oxidation stability of mineral oils.

Test procedure and techniques in accordance with spec. Mil-L-7808. Test conditions include: Test temperature-347 +3"F; test time as indicated; air rate=5*0.5 liter per hr.; test fluid cha.rged=lOO mls.; and catalysts=Al-in. sq. each of copper, steel, aluminum, and magnesium.

0 Ester containing antioxidant 0 Hydrocarbon containing antioxidant

Note: Shaded area indicates values for conventionally refined mineral oils with synthetic inhibitor and esters or hydracarbons without an antioxidant.

Naval Enqirwors Journal. 0ctob.r. IW 703

LUBRICANT TRENDS MECHANICAL ENGINEERING

reduce the amount of oxygen in contact with the lubricant. Almost complete inerting of the atmosphere in the lubricant system with nitrogen or helium will provide essentially intinite lubricant life up to the thermal stability limit, or loss via evaporation.

THERMAL STABILITY

Thermal stability refers to the fluid’s stability a t a given temperature under an inert-nitrogen atmosphere. It should be noted that the thermal stability values are higher than the temperatures at which these materials cease to show good oxidation resistance properties. There are no addtives available for raising the thermal stability of fluids. The use of inert atmospheres may raise the temperature at which a fluid can be used to the limit imposed by the thermal stability or evaporation of the fluid. Sustained liquid lubricant life is, therefore, limited by the thermal stability of the lubricant. Again, there are design possibilities which will allow lubricant use above oxidation or thermal-stability limits, such as a single-pass or throw-away system.

BOUNDARY LUBRICATION

Boundary lubrication refers to the regon of lubrication in which metal--metal contact occurs relatively persistently. The chemical action of the lubricant and or the atmosphere with the metal surface is involved in providing adequate lubrication under these conditions. The group of synthetics which respond to oxidation inhihtors are also responsive to lubricity additives. In general, the silicones, halocarbons, and polyphenyl ethers are less responsive to lubricity additives.

The silicones have received special attention in the field of boundary lubrication because of their excellent physical properties coupled with poor steel-on-steel boundary lubrication [lo]. The silicones also show a marked decrease in boundary lubricating ability with increasing temperatures. Frequently, the least sophistication in boundary lubricity additives provides the best life for a given class of lubricant. The mineral oils o r hydrocarbons represent some of the best base stocks for the effective lubricity additives because of the lack of re- activity of such hydrocarbons with the additive package under storage and most use conditions.

LUBRICANT COMPOUNDING

The additive package is a function of the type of base stock and the intended use. Additives in common use in high-performance lutmcants comprise oxidation inhibitors, viscosity-index improvers, de- tergents and dispersants, antiwear and extreme- pressure additives, corrosion idubitors, antirust additives, pour point depressants, metal deactivators, and antifoam agents. In general, the most desirable

iormulation is the minimum additive to provide the properties for the particular application.

It should be emphasized that most of the addtives enumerated are polar and surface active materials. In many cases, these additives compete with the antiwear additive and reduce its effectiveness, or they may reduce the oxidation s tabhty by providing a catalyst for oxidation. The additive package may also add needlessly to the cost of the final pro- duct. There is evidence to show that in some cases the additive package causes poor storage stability in a synthetic lubricant. For example, an ester-base lubricant containing only an oxidation inhibitor and the same formulation with 1 percent of an antiwear additive shows more than 10 years of storage stability in metal cans or bottles a t -20 to i 1 2 0 F storage temperature. The addition of 5 instead of 1 weight percent of the antiwear additive, or the addition of more effective antiwear and EP additives, results in a storage life of less than one year.

h e n t l y superclean fluids and lubricants have become available. These materials have been subjected to carefully controlled filtration. Such materials provide the designer with lubricants for precision, close-tolerance equipment. This additional cleanliness has had some influence on the additive package. The antifoam additives w h c h are dis- persed rather than dissolved in many lubricant formulations can be removed effectively by this type of mechanical filtration.

SUPERREFINED MINERAL OILS

The widespread interest in synthetics as fluids and lubricants has been responsible for a recent resurgence of work in the area of mineral oils and hydrocarbons. This work has been designed to show the optimum properties that can be achieved by additional refining of mineral oils or by the de- liberate synthesis of hydrocarbons [ll]. The major steps involved in superrefining are: (a) distillation to produce a narrow boiling fraction; ( b ) hydro- genation and ‘or acid extraction to remove polar impurities and unsaturated hydrocarbons; and ( c , deepdewaxing or low-temperature filtration to pro- 1:ide the wide liquid range desired. The hydrocarbons resulting from this sequence of operations exhibit a liquid range of -75 to above 700 F; good thermal stability to 700F; excellent lubricity properties; and additive response of the same order of magnitude as that shown by high-quality organic esters for controlling foaming, oxidation, corrosion, wear, and lubrication. These stocks can also be USHI to advantage as base stocks for polymeric materials.

These superrefined hydrocarbons are beginning to be produced commercially and are used in missile hydraulic fluid and lubricant applications. This group of lubricants should provide some interesting and useful lubricants for the design engineer in the near future.

MECHANICAL ENGINEERING LUBRICANT TRENDS

REFERENCES

[ I J C. W. Georgi, Motor Oils a d Engine Lubrication Reinhold Publlshing Corp.. New York, N. Y., 1950

12) E E. Klaus, “A Study of the Critical Properties of Gyro-Bearing Lubricants.” Appendix 11, WADD TR 60898, part III, January, 1963.

[ 31 R. C. Gunderson and A. W. Hart, Synthetic Lubricants. Remhold Publishing Corp., New York, N. Y., 1962.

[ 4 ] E. E. Klaus, R. E. Hersh, and M. J. Pohorilla, “Slope Index-An Expression for Viscosity-Temperature Characteristics,” Lubrication Engineenng, Vol 14. 1958, p. 439.

IS] R. H. Johnson, MS thesis, “Design and Use of a Preci- sion Pressure Viscometer,” The Pennsylvania State University, 1962.

IS] E. E Klaus and M. R. Fenske. “The Use of ASTM Slope for Predicting Viscosities.” ASTM Bidleton. No.

[ 7 I F. M. Angeloni. MS Thesis. “Development of a Capillary 215, J u l y , 1956, pp. 87-94.

High Shear Viscometer,” The Pennsylvania State Uni- velsity, 1959.

j8J G. B. Dubois. F. W. Ocvirk, and R. L. Wehe, National Advisory Committee for Aeronautics, Contract No. NAw6197, Progress Report 9, August, 1953.

(91 E. E. Klaus and M. R. Fenske, “Fluids. Lubricants. Fuels and Related Materials,” W A X TR 55-30, part 4, 1957.

[ 101 E. E. Klaus, E. J. Tewksbury, and M. R Fenske, “Criti- cal Comparison of Several Fluids as High Temperature Lubricants,” Journal of Chemical and Engineering

111 ] E. E. Klaus, E. J. Tewksbury, and M. R. Fenske. “Preparation Properties. and Some Applications of Super-Refined Mineral Oils.” ASLE Trans.. Vol. 5, No.

[ 121 L. T. Eby, Tables for Determination of ASTM Slope and Predwtion of Vlscositics. Esso Research and Engi- neering Company. Linden, N. J., 1946.

Data. Vol. 6, NO. 1. 1961. pp. 99-106.

1, 1962, p ~ . 115-125.

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Naval Engimer i Journal, October . IW 705

trends in liquid lubrication

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