Lubrication Guide
Revision 3 (Formerly NP-4916-R2)
Technical Report
L I C EN SED M ATE R IA LEquipment
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WARNING:Please read the License Agreement on the back cover before removing the Wrapping Material.
EPRI Project Manager M. Pugh
Revision 3 (Formerly NP-4916-R2)
1003085INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:
(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR
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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Bolt & Associates
This report was prepared by
Nuclear Maintenance Applications Center (NMAC) EPRI
1300 W.T. Harris Boulevard Charlotte, NC 28262
This report describes research sponsored by EPRI.
The report is a corporate document that should be cited in the literature in the following manner:
A large number of lubricants are used in power plants for various purposes. Maintenance personnel need concise guidelines for selecting the correct lubricant for a specific application. Also, specific knowledge is required regarding a lubricant’s characteristics to determine its applicability.
Background
This lubrication guide has traditionally provided useful information to power plant personnel involved in this area of plant operation and maintenance. This revision of the Lubrication Guide incorporates changes within the lubrication industry including consolidation and discontinuation of product lines and features. As in Revision 2, it also includes topics that were covered under EPRI report, Radiation Effects on Lubricants, NP-4735.
Objectives
• To provide general guidance to plant personnel involved with lubricants
• To provide information on current oils and greases and their operating limitations for different plant applications
Results
This guide addresses lubricants, lubrication, testing, and friction and wear. It includes sections on basic lubrication, application problems, tests and analysis. Tables are provided that profile each use category, listed lubricants for specific applications, and temperature and radiation tolerances of these lubricants. A glossary of technical terms is also included. Guidance on selecting the correct lubricant for a specific application is also provided. Information on determining the remaining life of a lubricant is addressed, which can help reduce unnecessary and costly lubricant change-outs.
EPRI Perspective
Knowledge of lubrication is important to maintenance personnel in their day-to-day work. This guide provides, in a concise form, a substantial amount of information on properties of
commonly used lubricants. Selection of correct and compatible lubricants can help prevent unscheduled maintenance or shutdown. Information contained in this guide can be useful to a training instructor and to persons being initiated in the technology of lubrication. This revision to the NMAC Lubrication Guide attempts to incorporate recent changes within the lubrication industry including consolidation and discontinuation of product lines and features.
Plant operations Lubricants Lubrication
ACKNOWLEDGMENTS
This publication was developed by the Nuclear Maintenance Application Center (NMAC). The first versions of the Guide were prepared by Dr. Bob Bolt and the late Jim Carroll. This third version, built on the prior work, was prepared largely by Dr. Bolt with the major assistance of Dr. Howard Adams. Additionally, Dr. Bolt would like to acknowledge the valuable
contributions from the following:
Chesley Brown TXU
Jim Fitch Noria
Doug Godfrey Wear Analysis; Bolt & Associates Bill Herguth Herguth Laboratories
ABSTRACT
This Guide gives information on lubricants from many manufacturers, suitable for various nuclear power plant applications. Lubricant operating limits with respect to temperature and radiation dose are listed. The Guide also addresses the basics of how lubricants work, how radiation affects them, and how this relates to their composition. Friction and wear is another basic topic presented, along with lubricant stress effects, shelf life, compatibility, troubleshooting and testing, all important in maintenance. The testing section has received particular attention with the addition of several new test methods. A summary of the lubricants study in the
EPRI/Utilities Motor-Operated Valve Performance Prediction Program is also included, as it was in Revision 2. The Guide is intended for use by power plant maintenance and engineering
CONTENTS
1 LUBRICANTS: WHAT THEY ARE AND HOW THEY WORK ... 1-1
1.1 Base Oils... 1-1 1.2 Key Measurements ... 1-2 1.3 Additives ... 1-3 1.3.1 Vl Improvers ... 1-4 1.3.2 Detergent/Dispersants ... 1-4 1.3.3 Basic Metal Compounds... 1-4 1.3.4 Antiwear and Antiscuff (EP) Additives... 1-4 1.3.5 Antioxidants... 1-5 1.3.6 Rust Inhibitors and Antifoamants ... 1-6 1.3.7 Gelling Agents ... 1-6 1.4 Synthetic Lubricants... 1-6
2 RADIATION EFFECTS ON LUBRICANTS... 2-1
2.1 Effect on Elastomers ... 2-8
3 LUBRICATION, FRICTION, AND WEAR ... 3-1
3.1 Hydrodynamic Lubrication (HDL)... 3-1 3.2 Elastohydrodynamic Lubrication (EHL) ... 3-2 3.3 Boundary Lubrication (BL)... 3-3 3.3.1 Physically Adsorbed Film... 3-3 3.3.2 Chemisorbed Film ... 3-4 3.3.3 Chemical Reaction Film ... 3-5 3.4 Solid Lubricants... 3-5 3.5 Nature of Machined Surfaces... 3-6 3.6 Wear ... 3-6
4 APPLICATION PROBLEMS... 4-1
4.1 Compatibility of Mixed Products ... 4-1 4.1.1 Oils ... 4-1 4.1.2 Greases... 4-1 4.2 Shelf Life ... 4-4 4.3 Time/Temperature/Radiation Considerations ... 4-5 4.4 Continuous Versus Intermittent Use and Lube Performance... 4-7
5 TESTS AND ANALYSES ... 5-1
5.1 Sampling ... 5-1 5.2 Troubleshooting ... 5-1 5.3 Lubricant Testing... 5-2 5.3.1 Sensory Tests ... 5-2 5.3.2 Other Simple Tests ... 5-4 5.3.3 Diagnostic Laboratory Tests ... 5-5 5.3.4 Standard Laboratory Tests ... 5-12 5.3.5 Analytical Test Methods... 5-14 5.4 Using Test Results ... 5-19 5.5 Trending... 5-19 5.6 Warning Limits ... 5-20 5.7 Cleanup Considerations ... 5-22
6 LUBRICATING MOTORIZED VALVE ACTUATORS ... 6-1
6.1 Stem Nut Friction and Wear – Off-the-Shelf Products ... 6-2 6.2 Stem Nut Friction & Wear – Solid Lubricants and Improved Nut Cutting
Procedure... 6-4 6.3 Search for Improved Actuator Lubricants ... 6-6 6.4 Long-Term Thermal Effects On Greases... 6-9 6.5 Conclusions ... 6-11
LIST OF FIGURES
Figure 1-1 Effect of Antiwear and Antiscuff Additives ... 1-5 Figure 1-2 Hydrocarbon Oxidation Process... 1-6 Figure 2-1 Dose Levels for Radiation Effects ... 2-1 Figure 2-2 Interaction of a Gamma Photon with Organic Matter... 2-2 Figure 2-3 Upper Limits of Radiation Doses Resulting in Failure of Various Base Fluids ... 2-3 Figure 2-4 Radiolysis Effects on a Lithium Complex-Gelled, Mineral Oil-Based Grease... 2-4 Figure 2-5 Relative Oxidation Stability of Irradiated Mineral Oil-Based Steam Turbine
Oils in Turbine Oil Stability Tests (TOST) (ASTM D 943) ... 2-5 Figure 2-6 Effect of Temperature and Irradiation on Bearing Life of a Sodium
Salt-Thickened, Mineral Oil-Based Grease ... 2-6 Figure 2-7 Relative Sensitivity of Common Lubricants and Elastomers to Irradiation ... 2-8 Figure 2-8 Resistance of Elastomers to Irradiation... 2-9 Figure 3-1 Hydrodynamic Lubrication... 3-2 Figure 3-2 Elastohydrodynamic Lubrication ... 3-2 Figure 3-3 Boundary Lubrication (Fragmented Roughness) ... 3-3 Figure 3-4 Representation of Physically Adsorbed Film—Non-Polar Molecules ... 3-4 Figure 3-5 Physically Adsorbed Film—Polar Molecules ... 3-4 Figure 3-6 Chemisorbed Film ... 3-4 Figure 3-7 Effects of Various Parameters on Friction Coefficient... 3-5 Figure 3-8 Machined Surface ... 3-6 Figure 4-1 Compatibility of Mixtures of Greases With Different Gelling Agents ... 4-3 Figure 4-2 Time/Temperature/Irradiation Interplay Continuous Operation in Air of High
Quality Lubricant Under Stress ... 4-6 Figure 5-1 Observing the Appearance... 5-3 Figure 5-2 Detecting the Odor... 5-3 Figure 5-3 Viscosity Gage for Measuring the Viscosity of Oils... 5-4 Figure 5-4 Sample Blotter Spot Test ... 5-5 Figure 5-5 Wear Particle Size/Concentration and Machine Condition ... 5-8 Figure 5-6 Detection of Wear and Other Particles ... 5-9 Figure 5-7 Schematic of TGA Setup... 5-15 Figure 5-8 Schematic of DSC Apparatus... 5-15 Figure 5-9 Ruler™ (Remaining Useful Life Evaluation Routine) Instrument ... 5-16
Figure 5-10 Example of Three Additives and Voltammeter Response... 5-17 Figure 5-11 Chromatographs of Fresh and Used Gear Oils ... 5-18 Figure 5-12 Sample Plot of Lubricant Properties... 5-20 Figure 6-1 Composite of Friction Coefficient (@10,000 lbs) Versus Number of Strokes ... 6-4 Figure 6-2 Cross-Section of Macrograph of New SMB-O Stem Nut Thread – Standard
Machining... 6-5 Figure 6-3 Pin-On-Disk Machine Schematic (Tribometer) ... 6-7
LIST OF TABLES
Table 1-1 Oil and Grease Requirements ... 1-1 Table 1-2 Common Additives in Various Lubricants ... 1-3 Table 1-3 Synthetic Base Oils and Their Application ... 1-7 Table 1-4 Comparative Properties of PAO Synthetic Base Oil and Various Mineral Base
Oils... 1-8 Table 2-1 Effects of Irradiation on Common Oils ... 2-7 Table 2-2 Effects of Irradiation on Common Greases... 2-7 Table 2-3 Resistance of Elastomers to Effects of Common Oils and Greases... 2-8 Table 4-1 Compatibility of Greases ... 4-2 Table 4-2 Grease Compatibility Tests ... 4-4 Table 5-1 Sequence of Lubricant Testing... 5-2 Table 5-2 IR Peak Regions of Interest... 5-6 Table 5-3 Sources of Metals in Lubricants ... 5-8 Table 5-4 Wear and Its Causes... 5-9 Table 5-5 Range Number Determination... 5-11 Table 5-6 Key Tests for Lubricants... 5-13 Table 5-7 Typical Warning Limits for Certain Lubricant Services... 5-21 Table 6-1 Friction and Wear Performance Summary (500 Stroke Stem/Stem Nut
Lubricant Tests with SMB-0)... 6-3 Table 6-2 Bleeding Tests on Grade 1 Greases (including effects of gelling agents) ... 6-6 Table 6-3 Pin-on-Disk Tribometer Data for Some Grease Types... 6-8 Table 6-4 Grease Consistency Changes in Long-Term Thermal Tests ... 6-10 Table A-1 Turbine Oils ISO Viscosity Grades 32, 46, 68 ... A-1 Table A-2 Engine Oils for Large Diesels... A-2 Table A-3 Low-Pressure Hydraulic Oil ISO Viscosity Grades 32, 46, 68, 100... A-3 Table A-4High-Pressure Hydraulic Oil ISO Viscosity Grades 32, 46, 68, 100 ... A-4 Table A-5 Compressor Oils ... A-5 Table A-6 High Load Extreme Pressure (EP) Gear Lubricants ... A-6 Table A-7 Open Gear Lubricants... A-7 Table A-8 Antiseizure Compounds ... A-8 Table A-9 Limitorque Valve Actuator Lubricants... A-9 Table A-10 Fire Resistant Hydraulic Fluids ... A-10
Table A-11 General Purpose Greases—Grades 00, 0, 1, 2, 3... A-11 Table A-12 Coupling Greases ... A-12 Table A-13 Grease Types and Performance ... A-13 Table B-1 Viscosity Equivalents ... B-4
1
LUBRICANTS: WHAT THEY ARE AND HOW THEY
WORK
Oils and greases have to meet the several requirements shown in Table 1-1.
Table 1-1
Oil and Grease Requirements
Properties Oils Greases
Prevent metal/metal contact x x Act as a hydraulic medium x
Act as a coolant x Carry away contaminants x
Protect against wear x x Protect against corrosion x x Protect against deposits x x Resist foaming x
Remain in place x
Note that the only function exclusive to greases is the ability to stay in place. This results from the semi-solid nature of greases. On the other hand, there are several functions exclusive to oils that are derived from their fluid nature.
1.1 Base Oils
To perform the indicated tasks, commercial lubricating oils consist of about 85 to 99+ % base oil. The remainder consists of additives. Additives are used to enhance the properties of the base oil or to create a necessary property in it. Base oils are classified as:
•
Mineral oils•
Synthetic oilsThe principal advantage of synthetic oils is their relatively low viscosity at low temperatures. They also can have somewhat better high temperature performance. However, the cost of synthetic-based lubricants is 3-8 times the cost of mineral oil-based products. (For additional discussion on synthetic lubricants, see Section 1.4.)
The term “mineral oil,” as opposed to “synthetic oil,” implies that little processing is involved in the manufacture of mineral base oils. This is not true. The fraction distilled from selected
petroleum crude oils for subsequent base oil manufacture contains many organic molecular species. Several of these must be removed to yield a high quality final base oil. Aromatic and wax compounds are two classes that are removed. Aromatics (alternating carbon-to-carbon double bonds in six membered rings) show a particularly high rate of viscosity change with temperature. This is not a good property in a lubricant. Waxes are solids at room temperature and are, therefore, unsuitable in base oils. Removing these requires considerable processing. Physical treatment, for example solvent refining, is still used as a method of removal, but catalytic
hydrogenation under pressure and temperature is now the preferred method of removal. The product of solvent refining of a base oil feed is called a Group I base oil. Relatively mild catalytic hydrogenation yields a Group II base oil, while more rigorous hydrogenation produces a Group III base material. Some properties of these and of a common synthetic hydrocarbon base oil (Group IV) are listed in Table 1-4.
1.2 Key Measurements
Viscosity is a measure of a fluid's resistance to flow, in other words, its fluidity. It is measured in
centistokes (cSt.). The viscosity at 40°C is used in industrial oil grading. For example, a 32 grade has a viscosity at 40°C of around 32 cSt. Other grading methods exist but they are used primarily for engine oils. Some of these, including their interrelationships, are shown in the Glossary (Appendix B).
Viscosity Index (VI) is a measure of viscosity change with temperature. VI has its origins in
petroleum antiquity. An oil derived from a Gulf Coast crude oil showed a high rate of change of viscosity with temperature and was arbitrarily given a VI value of 0. A Pennsylvania crude-derived oil, with a low rate of change of viscosity with temperature, was given a VI of 100. All oils since then have been compared on this scale. The best of the normal mineral base oils (Group I and some Group IIs) have VIs in the 90's. Synthetic oils and some very highly refined mineral oils (Group III, some Group IIs) can have VIs in the 105 to 160 range, reflecting their superior viscosity/temperature properties.
Lubricants: What They Are and How They Work
cone sinks into a standard cup of grease at 77°F (25°C). Because consistency can change with shear or “working,” greases are often worked in a standard worker before penetrations are measured. The worked penetrations corresponding to the various grease grades are shown in the Glossary (Appendix B). P60
refers to the penetration after 60 double strokes in the worker; P 10,000
refers to 10,000 double strokes, and so on. Grease grades are determined by P 60 values (see Appendix B for grade determinations).
Dropping Point is another ASTM grease measurement. It is the temperature at which a grease
just begins to melt or separate. The use temperature of a product is related to its dropping point.
1.3 Additives
Up to about 15% of a finished lubricant consists of materials added to the starting base oil to create properties or enhance those that already exist. Table 1-2 shows finished lubricants and the additives they might contain.
Table 1-2
Common Additives in Various Lubricants
Common Lubricants Engine Oils Turbine Oils Hydr. Oils Gear Oils Compr. Oils Greases Gasoline Diesel Additives VI Improvers x x x x Detergent/Dispersants x x x Basic Metal Compounds x x x Antiwear Agents x x x x x x
Antiscuff (EP) Agents x x*
Antioxidants x x x x x x x
Rust Inhibitors x x x x x x x
Antifoamants x x x x x x
Gelling Agents x
1.3.1 Vl Improvers
Viscosity Index (VI) improvers are listed first because they are used in the largest amounts to perform their function. They thicken lower viscosity base oils and, in the process, flatten the mixture's viscosity/temperature slope. This improves VI. These additives are widely used to make mineral oil-based multigrade engine oils. VI improvers are not required to make
multigrade products from synthetic base oils or some Group III mineral base oils. This is because of the superior viscosity/temperature properties of such base oils (see Table 1-4).
1.3.2 Detergent/Dispersants
Detergent/dispersants keep any deposit precursors in suspension instead of agglomerating to plug piston rings, key oil passages, etc. or collecting as sludge. Detergent/dispersants were among the first additives used and continue to be of high importance in engine oils where deposits can come from combustion products. They are sometimes used in compressor oils, as well.
1.3.3 Basic Metal Compounds
Basic metal compounds have some detergency and good rust preventing properties but their main function is to neutralize acids in diesel engine oils. The acids come from the combustion of sulfur in fuel and the fixation of nitrogen in combustion air. Reaction with water converts the sulfur and nitrogen oxides to corresponding acids. If not neutralized, they cause corrosive wear of engine parts. The need for basic metal compounds (base reserve, high base number) in part depends on the sulfur content of the fuel – the lower the sulfur the less need for base. The
national trend toward low sulfur diesel fuel to control emissions will eventually reduce the use of basic metal compounds.
1.3.4 Antiwear and Antiscuff (EP1) Additives
Antiwear additives are very widely used in engine and industrial lubricants, but not universally so. Antioxidants, on the other hand, are universally used. Antiscuff additives are less widely used, as indicated in Table 1-2. Antiscuff materials can be viewed as more surface-invasive and, therefore, stronger in action than antiwear additives. Both antiwear and antiscuff additives function by interposing a relatively shear-resistant chemical film between load bearing metal surfaces. The general mechanism by which these additives work is shown in Figure l-l2.
Lubricants: What They Are and How They Work
antiscuff agents. All of these additives act similarly in both oils and greases and they can be temperature-sensitive. Mild antiwear can also be provided in greases from the gelling agents.
Figure 1-13
Effect of Antiwear and Antiscuff Additives
1.3.5 Antioxidants
The principal enemy of any lubricant is oxidation. The onset of oxidation cannot be prevented but only delayed. The delay is called the induction period. Antioxidants extend the induction period very effectively. Once this period is exceeded, however, oxidation can occur
exponentially, as shown in Figure 1-2. This results in physical and chemical property changes, for example, fluidity change and acid formation. In common with all chemical reactions,
oxidation increases with temperature – the rate doubles with each increase of about 18°F (10°C). However, doubling a very low rate still yields a low rate and the rate is low during the induction period.
3
Figure 1-2
Hydrocarbon Oxidation Process
1.3.6 Rust Inhibitors and Antifoamants
Rust or corrosion inhibitors are also widely used. They perform by forming a weakly adsorbed film on the surfaces to be protected. An antifoamant is also used in most oils. They are polymers and silicone fluids in low concentration, which affect surface tension to reduce the foaming tendency. They also help provide good deaeration properties. Recently, there is a move away from silicone antifoam materials for oils, for example, turbine oils. This is because there can be tight silicon content specifications to control dirt contamination.
1.3.7 Gelling Agents
A gelling agent is used to convert an oil into a grease, thus providing the lubricant with its unique stay-in-place function. The oil that is gelled also contains the other additives required to provide the necessary properties shown in Table 1-2. In addition, the gelling agent
identity defines many of the grease's other performance characteristics. These are detailed in Appendix A, Table A-13.
Lubricants: What They Are and How They Work
Table 1-3
Synthetic Base Oils1 and Their Application
Engine Oils Industrial Oils Greases Fire Resistant Oils Relative Cost2 Jet Other Synthetic Oils Poly(alpha-olefins) (PAOs) x x3 x3 4-8 Diesters x x4 5-7 Polyolesters x 10-14 Phosphate Esters x5 10 Polyethers (Polyglycols) x 6-8 Silicones (Siloxanes)7 x6 x6 30-100 Perfluoropolyethers x x 80-800 Polyphenylethers x x 100+ Chlorofluorocarbons x 100+ 1
In the field of metalworking/cutting fluids, water-based fluids are sometimes called “synthetic.”
2
Approximate cost multiplier relative to most common mineral oil.
3
Mobil SHC series, Mobilgrease 28.
4
Beacon 325 (Exxon).
5
Fyrquel (Akzonobel), etc.
6
Dow Corning; GE.
7
Including halogenated species.
The poly(alpha-olefins) (PAOs - Group IV) are the most widely used synthetic base oils in industrial and automotive lubricants. However, the differences between them and the new highly refined (hydrocracked4) mineral oil base stocks (Group III) are becoming blurred as shown in Table 1-4. Because of this, the marketplace is likely to see fewer PAO-based products in the future. The hydrocracked base oils cost half as much as the PAOs and their properties are often similar.
4
This process involves hydrogenation of normal mineral oil feed material with special catalysts. These catalysts direct the process to rearrange the undesirable molecular constituents of the feed into species that resemble those in the polymerization of the alpha-olefins (PAOs). The severity of the process dictates the properties of the final product as in Table 1-4.
Table 1-4
Comparative Properties of PAO Synthetic Base Oil and Various Mineral Base Oils
Mineral Oils Group I* Mineral Oils Group II* Mineral Oils Group III* PAO
API Group IV*
Viscosity, 40°C, cSt 32 44 39 32 Viscosity, 100°C, cSt 5.3 6.6 7.0 6.0 Viscosity Index 95 102 135 136 Pour Point, °C -15 -15 -20 -66 Flash Point, °C 210 230 240 246 Fire Point, °C 240 — — 272 Evaporation Loss, Wt% (6.5 Hr. at 204°C) 16 — — 4 Aniline Point, °C (ASTM D 611) 108 115 127 127
* American Petroleum Institute (API) base stock classification
The good low temperature properties of the PAOs are reflected in the viscosities, viscosity index, and pour point. They are matched, except for the last, by the Group III mineral base oil. The lower volatility for a given viscosity shows up in higher fire point and lower evaporation loss. The aniline point is a measure of solvency – the lower the number, the higher the solvency. Here the PAO and Group II and III oils are inferior to the normal, or Group I, mineral oil. That is, if sludge is formed, it will precipitate out later with a Group I-based product. However, the sludge, which is oxidized material, might not form so readily with the synthetic oil- or Group II- or III-based product. This is because the Group II, III, and IV oils generally give a higher degree of oxidation resistance with a given amount of antioxidant.
Improved performance with synthetic oil-based lubricants comes with an increased price tag. This is shown in Table 1-3. Such costs make it hard to justify the use of synthetic-based products unless the application demands their superior properties. For example, if equipment needing lubrication is used in subzero weather, it is worth the added cost reliably to start or operate the
2
RADIATION EFFECTS ON LUBRICANTS
5In normal operation, lubricants must withstand the stresses of temperature, shear, pressure (load), and exposure to oxygen in the air. In nuclear power plants, exposure to nuclear radiation is an added stress. Overall effects of thermal and radiation exposures are similar. For example, both show thresholds below which changes in bulk properties of exposed materials are not significant. Both also accelerate oxidation, the main foe of lubricants in service.
With radiolysis, as well as pyrolysis, color change occurs first, signaling beginning oxidation and other structural changes. Gas evolution also takes place early, followed by changes in fluidity as secondary reactions take over. The final product of very high thermal or radiation exposure is an intractable solid, no longer a lubricant.
Radiation effects are directly related to the radiation energy input. This input is expressed in terms of the rad (100 ergs/gram of absorber = 4.3 X 10-6
Btu/lb). The radiation sensitivity of lubricants versus other things is shown in Figure 2-1. The more complex the irradiated object the less tolerant it is of irradiation. Note the effect on the ultimate in complexity – homo sapiens!
Figure 2-1
Dose Levels for Radiation Effects
5
Bolt, Carroll, “Radiation Effects on Organic Materials,” chapter 9, Academic Press (1963); Bolt chapter in Boozer, “Handbook of Lubrication,” Volume 1, CRC Press (1983).
Mechanistically, incident gamma radiation affects organic matter through initial collisions with electrons of individual atoms of molecules. This is shown in Figure 2-2. About half an incoming ray's energy is given up to a scattered electron and the weakened gamma ray goes on to repeat the process. The charged electron, knocked from its position by the incoming gamma ray, goes on to lose its added energy by creating increasingly intense ionizations and excitations in neighboring molecules.
Figure 2-2
Interaction of a Gamma Photon with Organic Matter
Incident high energy neutrons interact initially with atomic nuclei of irradiated material instead of with the electrons in gamma ray interactions. This knocks out protons and these charged particles go on to act in the same fashion as described for incident gamma rays.
Primary interactions in radiolysis take place in some 10-14 seconds. Secondary reactions that result in new molecular products occur in the next 10-2 seconds. To minimize change, excitation without decomposition needs to be fostered. Use of additives, for example selected compounds containing sulphur that neutralize excitation without C-C bond fissure, is a means of doing this. Another means is to employ base oil molecules that dissipate the input energy largely through the generation of heat (resonance), that is, aromatic compounds. Thus, the effect on lubricants depends on the chemical makeup of both the base oil and additives. Figure 2-3 shows this for base oils.
Radiation Effects on Lubricants
Figure 2-3
Upper Limits of Radiation Doses Resulting in Failure of Various Base Fluids
Note the effect of aromatic content – the polyphenyls, poly(phenyl ethers), and alkylaromatics head the list in radiation resistance. Phenyl groups are basic units of aromaticity. Aromatics, because of their poor viscosity/temperature properties, are deliberately removed from mineral base oils. However, aromatic compounds can be designed through synthesis to have good properties. Such materials (alkylaromatics) are employed in making lubricants designed for maximum radiation resistance. The introduction of phenyl groups even into poor performing molecules will improve their radiation resistance. For example, phenyl silicones are a notch better than methyl silicones in radiation resistance.
The physical effect of radiolysis on greases is that they mostly soften with initial exposure, reflecting degradation of their sensitive gel structure. Eventually, this is followed by hardening as the effect on the oil component takes over. Figure 2-4 shows the typical softening effect. Although this grease exhibits stability in the 106
-108
rad region, other greases can either harden or soften in this region. This is before the major softening indicated in Figure 2-4 and before effects on the oil component set in.
Figure 2-4
Radiolysis Effects on a Lithium Complex-Gelled, Mineral Oil-Based Grease
The effect of radiation exposure on oxidation stability, a key property of turbine oils, is shown in Figure 2-5. Other effects on oils include gas evolution, evidenced by a decrease in flash point and increase in vapor pressure. The gas is hydrogen and low molecular weight hydrocarbons that come from C-H and C-C bond fissure. The C-C bond breakage can also yield compounds that eventually “double” or similarly polymerize to cause viscosity increase.
Radiation Effects on Lubricants
Figure 2-5
Relative Oxidation Stability of Irradiated Mineral Oil-Based Steam Turbine Oils in Turbine Oil Stability Tests (TOST) (ASTM D 943)
The effect of radiation dose rate is also highlighted in Figure 2-5. The doses shown were delivered to the test samples at widely different rates – differing by a factor of about one
thousand. Yet the variation in the test results falls within the reproducibility limits of the ASTM D 9436
test. Thus, there appears to be no appreciable dose rate effect. All the exposures were made in air for the indicated doses and then the oils were tested. Note that the dose below which no significant oxidation takes place is about 5 X 106 rads.
This dose rate concern comes up primarily in applying radiation effects studies to plant
situations. Most radiation effects studies are accelerated, that is, at higher dose rates than those in the plant, to allow results in a reasonable time. The answer is complicated by oxidation effects – more oxidation would be expected over the longer term, simply due to heating in air under irradiation. Oxidation is mitigated by oxidation inhibitors. All high quality lubricants have such antioxidants. Without them oxidation could be interpreted as a dose rate effect.
Even with good inhibitors, the acceleration of oxidation in the presence of radiation is an
important consideration from a maintenance point of view. Lubricant life will be reduced if there is excessive exposure to oxygen in the air, for example, where there are unrepaired air leaks on the inlet side of a pump in a radioactive area. In the example, a rich supply of oxygen and irradiation at high temperature can take its toll on the lubricant.
6
ASTM D 943-81 (91), “Test Method for Oxidation Characteristics of Inhibited Mineral Oils” [Turbine Oil Stability Test (TOST)].
Generally, radiolysis of lubricants is not a problem in nuclear power plants. It takes radiation doses above those prevailing in normal nuclear plant operations to make appreciable changes in bulk properties of lubricants. An accident scenario (a DBA) may produce high enough radiation exposure to cause significant property changes. In such a case, the equipment being lubricated doesn't have to operate very long or be maintained. The equipment itself is very tolerant of fluidity changes in lubricants. For example, antifriction bearings in motors can go just fine, at least in the short run, with grease worked penetrations from about 200 to over 400. This is equivalent to a change in consistency from a 4- to a 00-grade – a wide variation.
This tolerance exists even under stress. Figure 2-6 shows test data for a grease in a 10,000 rpm bearing at various temperatures. An Arrhenius plot (log bearing life versus inverse of absolute temperature) is shown. Note the change in life of irradiated grease versus that of unirradiated product. It took over 108 rads to make much of a difference in the grease's performance.
Radiation Effects on Lubricants
The effects of irradiation on oils and greases are summarized in Tables 2-1 and 2-2.
Table 2-1
Effects of Irradiation on Common Oils
Radiation Dose Effect
< 106 Rads No unusual problems.
106 - 107 Rads Things begin to happen; some turbine oils borderline.
107 - 108 Rads Most oils usable; some marginal.
108 - 109 Rads The best oils usable; most become unusable.
109 - 1010 Only special products will work.
> 1010 No oil usable.
Table 2-2
Effects of Irradiation on Common Greases
Radiation Dose Effect
< 106 Rads No unusual problems.
106 - 107 Rads Things begin to happen; some greases borderline. 107 - 108 Rads Most high quality products usable; others not. 108 - 109 Rads Most greases unusable.
109 - 5 x 109 Rads Special products required. > 5 x 109 Rads No grease usable.
Values for temperature and radiation operating ranges are given for individual products in Appendix A, Tables A1-A12. In these tables, the first number listed in each category is the value below which little, if any, property change will occur and long use life can be expected. The second number is the point where appreciable change is expected and surveillance of the equipment is required. The need for lubricant changeout should be anticipated at this point.
2.1 Effect on Elastomers
Elastomers are used frequently as seal materials in nuclear power plants. If one is concerned with radiation-resistance, elastomers are the weak link. Figure 2-7 shows the resistance to irradiation of elastomers versus lubricants. The elastomers are about ten times more sensitive to radiation than lubricants.
Figure 2-7
Relative Sensitivity of Common Lubricants and Elastomers to Irradiation
Table 2-3 shows the effect of common lubricants on various elastomers. Neoprene and Nitrile rubber and the epichlorohydrins are the principal oil and grease resistant products.
Table 2-3
Resistance of Elastomers to Effects of Common Oils and Greases
Elastomer Resistance
Radiation Effects on Lubricants
The picture changes somewhat as the elastomers are exposed to radiation. Figure 2-8 illustrates this performance. The natural rubbers and urethanes are most resistant to radiation, with the nitriles ranked a close second.
Figure 2-8
3
LUBRICATION, FRICTION, AND WEAR
Three lubrication mechanisms have been established in tribology – the study of surfaces in relative motion. These are:
• Hydrodynamic lubrication (HDL)
• Elastohydrodynamic lubrication (EHL)
• Boundary lubrication (BL)
A single mechanism might not prevail in any one application but a combination might exist depending on geometry and/or operating conditions. For example, the balls in ball bearings involve EHL in their relationship to the bearing races and BL in their relationship to the cages or retainers. It is important to understand the three types of lubrication in order to be clear about lubricants and how they function.
Friction is the resistance to the relative motion of surfaces and is an indicator of the efficiency of this motion. It is important because poor efficiency relates to high energy consumption. Wear, or the undesirable removal of material from contacting surfaces due to relative motion, shortens equipment life and decreases its reliability.
3.1 Hydrodynamic Lubrication (HDL)
HDL conditions exist when a fluid film completely separates moving surfaces and there is no surface-to-surface contact. This is the most desirable regime of lubrication because friction and wear are low under these conditions. HDL is the most common mode of lubrication for
components of industrial machines. Examples include simple journal bearings and bushings, and turbine shaft bearings. Factors affecting HDL are the viscosity of the lubricating fluid, its
adhesion to the surfaces, the sliding or rolling velocity of the components, the shape of the surfaces, and pressure (load) between them.
Film thicknesses for effective HDL range from 0.0001 to 0.005 inches (40-200 microns). The creation of such films is fostered when the shape of the surfaces allows a wedge of lubricant to form between them (see Figure 3-1). The failure of HDL usually results from too thin a film, due to high temperatures, that reduces the viscosity of fluids, low speed that discourages wedge formation, and shock loads. Another very common cause of film failure is damage by contaminants, such as dirt, in the oil.
Figure 3-1
Hydrodynamic Lubrication
3.2 Elastohydrodynamic Lubrication (EHL)
The name, elastohydrodynamic, implies that a full oil film exists between moving surfaces that are elastically deformed. EHL occurs only in situations where loads are concentrated over small areas, for example between balls/rollers and races in rolling element bearings and between gear teeth. In EHL the load is sufficient to deform the surfaces elastically at the point or line of near contact (Figure 3-2). The oil is trapped between the deformed surfaces and the resulting high pressure increases the oil's viscosity by several orders of magnitude. The surface deformation also increases the load bearing area. The combination of extremely high oil viscosity and increased area over which the load is applied keeps the surfaces from touching.
Lubrication, Friction, and Wear
Because of the cyclic elastic deformation, fatigue cracks and pits are formed. This contact fatigue determines the catalog life of a rolling element bearing.
3.3 Boundary Lubrication (BL)
BL conditions prevail when HDL and EHL fail and surface-to-surface contact occurs (see Figure 3-3). The word, boundary, suggests surface involvement. BL occurs with high loads and
temperatures, low sliding velocities, and rough surfaces. Examples of BL are bearings during start up and shut down, oscillating bearings, piston rings at top-dead-center, worm gears, and metal cutting operations. Friction and wear in BL are dependent upon the shape and composition of the surfaces and the properties of the lubricant. Friction results from the shear of the
interfacial material, which includes adhesion between the surfaces and the shear of other solids or liquids in the contact. For example, if the additives in an oil form a soap film of low shear strength on the surface, friction will be low. If the film formed is a shear resistant inorganic salt, for example iron sulfide, friction will be higher. Three types of films might form in BL,
physically adsorbed, chemisorbed, and chemical reaction films.
Figure 3-3
Boundary Lubrication (Fragmented Roughness)
3.3.1 Physically Adsorbed Film
Physically adsorbed film involves the adsorption of the non-polar molecules of the base oil at random on the surfaces (see Figure 3-4). The adsorption is reversible so, as temperature
increases, the film desorbs and fails to keep the asperities in the surfaces apart (for asperities, see Section 3.5). Mineral oils or PAO synthetic base oils are in this category. If the oil molecules are polar, for example a polyester synthetic, their adsorption is stronger because of their close packed nature (see Figure 3-5). Higher temperatures are required to desorb them.
Figure 3-4
Representation of Physically Adsorbed Film—Non-Polar Molecules
Figure 3-5
Physically Adsorbed Film—Polar Molecules
3.3.2 Chemisorbed Film
Chemisorbed films (see Figure 3-6) are chemical reaction products between long chain polar compounds in the oil (or compounds that are added to it) and compounds in the metal surfaces. An example is the reaction between a fatty acid in the oil and a metal oxide film from the surface to form a soap. The reaction is irreversible so an increase in temperature increases its rate. The melting point of the soap film is the temperature limitation. The additives in an oil that
chemisorb are termed lubricity additives because they reduce friction as compared to that of the base oil alone.
Lubrication, Friction, and Wear
3.3.3 Chemical Reaction Film
Chemical reaction films are also formed through irreversible reactions but the products are inorganic salts. Additives such as sulfur compounds react with surfaces containing iron to form iron sulfide. Such high melting point compounds inhibit scuffing by preventing bare metal-to-metal contact. They are called antiscuff (formerly known as EP) additives. Oxygen, which is in oils from the air, can also act as an antiscuff agent by reacting with metals to form thicker oxide films and prevent metal-to-metal contact.
The relationship between HDL and boundary lubrication (BL) for various operating conditions is shown in Figure 3-7. Note the effects of the various parameters on the friction coefficient. With a given speed and load, a low viscosity oil will allow boundary lubrication and a very high
viscosity oil will increase fluid friction.
Figure 3-7
Effects of Various Parameters on Friction Coefficient
3.4 Solid Lubricants
The presence of a film or a coating of other solids between surfaces reduces surface-to-surface contact. It might also reduce friction and wear. Solid lubricants are classified as follows:
• The metal oxides that form in air, for example iron oxide, Fe3O4, on steel (which reduces
friction), or aluminum oxide (which increases friction).
• Preformed coatings such as soft lead or Babbitt on aluminum in a journal bearing, the laminar graphite or molybdenum disulfide on steel, or poly(tetrafluoroethylene) (Teflon) on steel.
• Boundary lubricant films such as soap from a fatty acid in the oil, or iron phosphate from tricresyl phosphate additive, iron borate from boron additive compound, or iron sulfide from a sulfur additive compound in the oil.
3.5 Nature of Machined Surfaces8
Machined metallic surfaces are rough on a microscopic scale (see Figure 3-8) and covered with a thin film of oxide. The microscopic bumps contained on these surfaces are called asperities. When two machined surfaces are placed together, the area of real contact (where a few asperities touch) is much less than the apparent area of contact. This real contact area increases with load because more asperities are crushed, thus increasing the contact surface.
Figure 3-8
Machined Surface
3.6 Wear
Wear is the undesirable removal of solids from a sliding or rolling component. There are many kinds of wear. In analyzing a wear problem in a machine, it is necessary to determine the kind of wear that occurred. Analysis requires microscopic examination of the worn area and a close look at the used lubricant. Wear is generally proportional to the applied load and the amount of sliding. The major kinds of wear are:
• Adhesive Wear — the removal of material due to adhesion between surfaces.
– Mild adhesion — is the removal of surface films, such as oxides, at a low rate. This is the minimum wear expected under BL conditions.
– Severe adhesion — the removal of metal due to tearing, breaking, and melting of metallic junctions. This leads to scuffing or galling of the surfaces and even seizure.
• Abrasive Wear — the cutting of furrows on a surface by hard particles, (for example, sand particles between contact surfaces, or hard asperities on an opposing surface). Hard coatings
Lubrication, Friction, and Wear
• Contact Fatigue — the cracking, pitting, and spalling of a surface in sequence due to cyclic stresses in a contact. Contact fatigue is most common in rolling element bearings, gear teeth, and cams.
• Corrosive Wear — the removal of corrosion products from a surface by motion, such as the rubbing off of rust.
• Fretting Corrosion — the removal of metal oxides from a surface due to a reciprocating sliding motion of extremely low amplitude generated by vibration.
• Electro-Corrosive Wear — the removal of metal by dissolution in a corrosive liquid with the aid of electric currents. One source of currents is streaming potential from high velocity fluids. The oil serves as the electrolyte.
• Fretting Wear — localized wear of lubricated surfaces due to reciprocating sliding of extremely low amplitude because of vibration.
• Electrical Discharge Wear — the removal of molten metal from surfaces due to electrical sparks between them. High static voltages are sometimes generated by large rotating machinery and these are relieved by sparking to regions of lower potential.
• Cavitation Damage — the removal of material due to cracking and pitting caused by high-energy implosions of vacuous cavities in a cavitating liquid. Liquids cavitate when suddenly subjected to low pressures.
• False Brinelling — localized wear in lubricated rolling element bearings due to slight rocking motion of rollers against raceways. Wear depressions match the position of the rolling
4
APPLICATION PROBLEMS
4.1 Compatibility of Mixed Products
Lubricants can be incompatible with one another on mixing and can potentially cause degradation of properties and performance. Solid formation with oil mixtures can take place because of additive interaction or solubility difficulties. With greases, the usual result of incompatibility is breakdown of the grease gel structure to produce softness. Both of these effects can be undesirable in lubricant applications.
Incompatibility can be avoided by not mixing products. Procedures should be set up to eliminate unwanted mixing. When a change to a new product is dictated, careful cleanup should be
employed to keep less than about 5% of the old material in the new. Remember, don't mix! If you inadvertently do, you face incompatibility risks.
4.1.1 Oils
Lube oils are mostly compatible and miscible with one another in all proportions. A notable exception is mixing a product that contains a chemically acidic additive, for example a turbine oil, with a product that contains a basic additive, for example an engine oil. One will neutralize the other in the presence of moisture and frequently cause a precipitate to form. Precipitates can plug filters and/or other oil passages and cause oil starvation and equipment failure. If you don't know the chemical makeup of the particular products you have, your lubricant supplier can give guidance on this point so you can avoid the acid/base concern. (Anyhow, mixing of lubricants
should be avoided.)
4.1.2 Greases
These products present a different case. With inadvertent mixing, possible additive interactions (other than those involving gelling agents) pose only some loss of those functions provided by the reactants. Precipitates are generally no problem (grease is already semi-solid). Gelling agent interaction is a concern, depending on the application. Table 4-1 gives compatibility information (Meyers, E. W., NLGI Spokesman 47, (1), 24,1983; Meade, F.S., “Compatibility of Greases,” Rock Island Arsenal Report 61-2132, 1961). Examples of data on which the table is based are in Figure 4-1. (See also Note No. 5, NMAC Lube Notes, July 1993.)
A consistency change of 30 points or less in worked penetration in more than one mixture in a given set denotes compatibility (“C”) in the table. This change is measured by deviation from the straight line between the two 100% points. Softening is the most likely result of incompatibility,
although hardening can take place (< 10% of the cases). Softening is of little concern in a contained system, such as a Limitorque gearbox (unless leakage is rampant). It is only the stay-in-place function that is affected – the lubrication function is largely handled by the oil
component and its soluble additives. A problem does occur if the grease flows away from the part being lubricated. Rolling element bearings are vulnerable here although they have quite a tolerance for changes in grease consistency. This tolerance runs from about 200 to about 400 in worked penetration. However, the departure from around the 280 norm might cause some increase in required maintenance.
Table 4-1 Compatibility of Greases A lum inu m C o m p le x Ba ri u m S o a p C a lc iu m So a p C a lc iu m 1 2 -H y d ro x y s tear at e C a lc iu m C o mp le x Inor g a ni c ( C la y ) Lit h iu m So a p Lit h iu m 1 2 -H y d ro x y s tear at e L it h iu m C o m p le x Poly ur e a Sodi um So a p C a lc iu m Su lf o n a te C o mp le x ( C al ci u m C a rb o n at e/ S u lf o n at e – C C S ) Aluminum Complex I I C I I I I C I NA I Barium Soap I I C I I I I I I NA B Calcium Soap I I C I C C B C I C NA Calcium 12-Hydroxystearate C C C B C C C C I NA NA Calcium Complex I I I B I I I C C NA C Inorganic (Clay) I I C C I I I I I B I Lithium Soap I I C C I I C C I C C Lithium 12-Hydroxystearate I I B C I I C C I NA C Lithium Complex C I C C C I C C I NA C Polyurea I I I I C I I I I C I Sodium Soap NA NA C NA NA B C NA NA C I Calcium Sulfonate I B NA NA C I C C C I I
Application Problems
As with oils, different greases should not be mixed. The data cited in the table should be
considered generic in nature. A “C” in Table 4-1 is not an endorsement to allow mixing because different grease formulations might give different data. With inadvertent mixing, compatibility risks are generally less if products with at least the same gelling agent are involved. However, reversals do occur. To be sure of compatibility or incompatibility, tests on specific greases must be run.
Figure 4-1
Compatibility of Mixtures of Greases With Different Gelling Agents
Compatibility test results will sometimes vary with the method used. Table 4-2 lists some of these methods. High temperatures in the storage (aging) phase are employed to provide test acceleration and assure that any incompatibility will be picked up. A consideration here is not to exceed the heat stability of the individual mixture components. The more severe mix procedures are undertaken to assure thorough mixing.
The method we prefer involves 25/75, 50/50, and 75/25 mixtures (10/90 and 9/10 are sometimes also used) of two components stirred with a hand-held electric mixer before aging at 250°F (121°C) for 72 hours. The starting materials get the same treatment. Then, after cooling to room temperature, the 60-stroke worked penetrations are run on all samples. Compatibility/
incompatibility is determined as in Figure 4-1. Dropping points can also be run on the treated samples. ASTM has now developed the compatibility test listed in Table 4-2. It is more complex and, therefore, three times as expensive to run as the method just cited. Its interpretation is also much more restrictive.
Table 4-2
Grease Compatibility Tests
Group Mix Storage (Aging) Time Temp Difference in P60 1 to Fail Rock Island Arsenal Hand Mix + P10,000 1 0 70°F (21°C) ±10
Meyers Hand Mix 72 hr. 250°F (121°C) ±30 for > one mixture Mobil RIV Tester 2 hr. 200°F (93°C) 0 - 30 (Compatible)
31- 60 (Borderline) 61+ (Incompatible) Bolt &
Associates
Motor Stirrer 72 hr. 250°F (121°C) ±30 for > one mixture
ASTM D 6185 P100,000 1 248°F (120°C) 167°F (75°C) 70 hr. 1400 hr. 2
> about 11 above the value for the thickest component or 11 below that of the thinnest component
1
ASTM 60-stroke or 10,000- or 100,000- stroke worked penetration
2
Applies to low dropping point greases.
4.2 Shelf Life
In general, lubricants are very stable when exposed to the mild conditions encountered in storage or “on the shelf.” Storage life of many years should result. This assumes, of course, no exposure to rain, sunlight, or sources of heat such as adjacent steam lines. Why then do suppliers often limit recommended shelf life to some two to three years? For several reasons:
• Formulations change from time to time for supply and performance reasons – base oil changes, additive changes, and so on. Incompatibility between old and new versions sometimes is a problem. Storage life restrictions limit the supplier's responsibility for old formulations.
• Conditions of storage can vary widely and some deterioration can take place under situations over which the supplier has no control. For example:
Application Problems
– Surface color change.
– Surface cracking from shrinking on cooling after manufacture or on heating and cooling in storage.
– Bleeding, or oil separation. The separated oil can be decanted or stirred back in; it is only a small portion of the total. This occurs mostly with soft greases made with low viscosity oils. A small amount of bleeding is acceptable. (See ASTM D 1742 for perspective.) Suppliers' reluctance to sanction extended shelf life is understandable. Although lubricant changes in storage are mostly cosmetic, they can be sources of many complaints. However, attention to storage conditions (including those for drums), for example, avoidance of temperature and other environmental extremes, will eliminate virtually all the potential problems. A few simple tests, for example, sensory tests and infrared (see Section 5.3, “Lubricant Testing”) on the questionable lubricants versus an authentic sample will give confidence that stored material is still acceptable. Storage of the drums should be indoors if possible. If outdoors, drums should be out of the sun and stored with a plastic lid or on their side (bung on the upside) to avoid standing water and its leakage into the drum contents.
4.3 Time/Temperature/Radiation Considerations
Figure 4-2 shows how time, temperature, and irradiation relate to lubricant life (point at which change-out is necessary). The vertical scale is logarithmic and gives lubricant life in hours. The horizontal scale is the inverse of absolute temperature.
The slope of the band represents an approximate doubling of life for every 10°C (18°F)
temperature decrease. One expects this for chemical reactions. The band is used to illustrate that the change might be more or less, depending on the chemical make-up of the lubricant. Also, the best performing lubricants will be on the right side of the band and the poorest performing lubricants on the left. Note that the whole band moves to the right in a parallel fashion as less stress is involved. The band moves to the left if there is more stress.
Figure 4-2
Time/Temperature/Irradiation Interplay
Continuous Operation in Air of High Quality Lubricant Under Stress
As an illustration, suppose a piece of equipment must be relubricated every 36 months in an application at 93°C (200°F) (A). Then at 104°C (220°F), the relubrication interval would decrease to 18 months (B). At 121°C (250°F), the required interval would be 9 months (C). It would be somewhat more than this (C') or less (C"), as the temperature effect is smaller or greater within the band, depending on the lubricant. Note that at 66°C (150°F) lubricant life would be extended and off the chart at 300 months! Of course, lubricant life cannot be extended indefinitely – contamination from dirt, wear debris, etc., might dictate a shorter interval.
Application Problems
effects are relatively minor but, above that threshold, the thermal component of total stress can become increasingly large. This is tied in, of course, to the approximate doubling of chemical reaction rate by each increase of 10°C (18°F) in temperature. If the rate is very low, a doubling doesn't do much. When the reaction rate is appreciable, doubling has a discernible effect. The threshold is where this rate becomes apparent. Note that temperature and radiation dose thresholds are shown for various lubricants in Appendix A.
Oxidation is not addressed specifically in the figure except as an increased stress that would shift the band to the left. However, the lubricant life shown is for products exposed in the presence of air. This is a normal condition and only abnormal exposure conditions, for example bubbling air through the lubricant, would be considered an increase in stress.
4.4 Continuous Versus Intermittent Use and Lube Performance
In any plant, much lubricated equipment operates continuously under relatively stable
conditions, as when a grease lubricates a motor bearing. The life of that grease, or of the greased bearing, can be estimated from prior experience or, more generally, from a knowledge of
lubrication practice. Often such bearings can run continuously for years. Sometimes, the
lubricant must be replenished at prescribed intervals. Now and then, the bearing must be replaced when it becomes noisy or shows other distress.
In other situations, a piece of equipment might be on stand-by status until a specified event occurs. Then, on signal, the equipment must quickly come up to speed and perform its function. This intermittent duty is not always benign. Start-stop operation of bearings (especially under load) can create wear debris from unusual slippage, even with proper lubrication. A spinning bearing also tends to deflect dirt, dust, and debris more readily than does a stationary unit. Further, as a heated bearing cools after running, it tends to attract rust-producing moisture. Also, a grease in a stationary bearing can slowly separate oil from the gel, causing the lubricant to dry out. Then, too, stationary bearings are vulnerable to vibrations that can shorten bearing life due to fretting or false brinelling (see Section 3.6). Thus, extended periods of inactivity are not good for long-term performance. Care must be taken to “exercise” the lubricated equipment
occasionally.
When radiation is involved during lubrication, one would expect frequent operation to be more damaging to the lubricant than intermittent operation. This is because more exposure to oxygen in the air is involved during agitation and oxidation is accelerated by irradiation. However, this does not hold for greases. Their key gel structure generally benefits from shearing action (agitation) and this offsets the effect of increased oxidation.
In any event, good maintenance practices dictate that the lubricated equipment should undergo:
• Periodic inspections for signs of leakage of oil, accumulation of dirt, oil thickening, grease drying, or wear fragments in the lubricant.
•
Periodic “exercise” to assure that it functions properly without distress. This also maintains adequate distribution of grease to lubricated parts.•
Periodic lubricant changes based on experience. Lacking experience, change should be based on intervals established in similar applications. In some instances, lubricant changeout periods are specified by the equipment supplier.5
TESTS AND ANALYSES
Lubricant testing is recommended for a host of reasons. These include:
• To check an incoming lubricant to verify its authenticity.
• To determine if a lubricant in storage is still of acceptable quality.
• To study the condition (wear, etc.) of the machine being lubricated. If there is a problem with the lubricant, there is a strong possibility that the machine will need maintenance.
• To determine if preventive maintenance is being performed properly and effectively.
• To know when it is time to relubricate the machine.
Lubricant testing is both an art and a science. The art is in determining how much science to use in addressing a concern. The full complement of lubricant tests is very broad in its scope and complexity but seldom is this full set of tests required. Part of the process is:
• Selecting adequate and appropriate tests.
• Not overkilling with the unnecessary – do the minimum that will resolve the concern.
5.1 Sampling
The first and most crucial step in lubricant testing is to get a representative sample. Samples should be taken as follows and handled carefully:
• When the system is stabilized, neither just before nor just after makeup lubricant has been added.
• Ahead of filters or centrifuges so as not to miss the contaminants that they remove.
• In suitable, clean, well-labeled containers. Be consistent in sampling method. Take the sample from the same location and under the same operating conditions. In addition, be aware that sampling from the bottom of sumps, where dense materials (for example, water and metals) settle, can give valuable information on the history of the lubrication.
5.2 Troubleshooting
Operating equipment has a great tolerance for lubricant property changes. Greases or oils can change by a consistency grade or two and the machinery being lubricated will continue to
operate smoothly. However, an off-grade or contaminated product can hasten equipment distress, which might be manifested by:
• Temperature increase (at the lubricated part)
• Output decrease
• Noise
• Change in vibration pattern
• Visual indicators, for example leakage
• Wear and corrosion
Often the equipment distress can be anticipated by trending the data from lubricant analyses. (More details are provided on trending in Section 5.5.) Whenever any of these symptoms occur, corrective action must be taken. The action required might sometimes be evident from the information derived from the lubricant analysis program itself.
5.3 Lubricant Testing
The first line of surveillance in lubricant testing, or the first step in isolating a problem, is simple on-site sensory examination. A lot can be learned from looking at, feeling, and smelling the used lubricant. These sensory tests can signal the need for more complex laboratory tests. A hierarchy, or sequence of tests from the simple to the complex is shown in Table 5-1. Remember, do the simple ones first!
Table 5-1
Sequence of Lubricant Testing
Test Type Description
Sensory Tests Simple tests on-site; compare to known product.
Other Simple Tests Easily done on-site; again back-to-back with known product.
Diagnostic Tests Laboratory; relative test - compare to known product. Skill of technician is vital.
Standard Tests Laboratory; well developed, ASTM methods formulated from round-robin testing. Can be compared on the basis of determined repeatability and reproducibility.
Analytical Tests Laboratory; Not always standard - compare to known product. Skill of technician is vital. Often a judgment call is involved.
Tests and Analyses
• Appearance: Look at the sample, as shown in Figure 5-1. Is the oil clear and bright? Or is it
hazy and cloudy, indicating the presence of water? Is it foamy? Or does it show suspended matter? When examining grease, smear a small amount on a piece of white paper with a knife or spatula. Examine the sample for lumps and other particles, and don't forget the comparison with the fresh, unused sample.
• Color: Compare with that of the original product. This observation is sometimes useful with
light-colored materials. Darkening can indicate oxidation and/or exposure to high
temperatures. Remember that color can change by just adding the new lubricant to the system being lubricated!
Figure 5-1
Observing the Appearance
• Odor: (Figure 5-2) Again, compare with that of the original product. Oxidized oils and
greases eventually acquire an acidic, pungent, or “burned” smell. This occurs also at a radiation dose of about 100 megarads. The strong odor of some additives might for a time mask the developing pungent smell.
• Feel: Oils should feel slippery; greases should feel buttery, not stringy or lumpy. Neither
should feel gritty, as from wear debris.
Figure 5-2
5.3.2 Other Simple Tests
• Viscosity: This is a measure of the resistance to flow of an oil and is its single most
important property in hydrodynamic lubrication (see Section 3.1). The various grading systems for oils are given in Appendix B. Oil viscosity is generally specified by the
equipment builder for operating machinery. If the viscosity is too high (thick), performance can be sluggish because of increased drag. This also can cause increased temperature, which has an adverse effect on lubricants and sometimes machine life. If viscosity is too low, the oil film might not be able to keep the moving parts separated. In the absence of an antiwear or antiscuff additive, this can result in metal-to-metal contact, contamination with wear debris, and shorter life for both the lubricant and the machine. It is important to remember that rotating machinery has a tolerance for everything but major changes in viscosity in service. The simplest means of determining viscosity is to compare an unknown to a known material through sensory-like tests – sight and feel. If this is not accurate enough for the required purpose, a viscosity gage, shown in Figure 5-3, can be used. This works on the principle that the rate a ball falls in a column of oil depends on the viscosity of the oil.
Figure 5-3
Viscosity Gage for Measuring the Viscosity of Oils (courtesy of Visgage by Louis C. Eitzen Co.)
With this device, the unknown is drawn into a tube containing a ball. A parallel tube containing a known oil and a like sphere is used for the comparison. After the two oils are allowed to reach equal temperatures and each ball the same starting point, the instrument is inclined at a slight angle. This starts the spheres rolling. The inclination is stopped when either oil's sphere reaches a calibration point. Then the position of the lagging ball in either tube shows directly the viscosity of the unknown. Both high and low viscosity oils can be used in this equipment. Accuracy of 95% or so is achievable with little effort.
