FIGURE 6.“Bathtub” failure rate curve. APPLICATION METHODOLOGY The system concept described in the preceding section can be used as a convenient framework which may help to systematize approaches to the solution of lubrication prob- lems. 3 This will be illustrated in three examples of broad areas of lubrication technology. Comprehensive Characterization of Tribological Systems The value of all lubrication work, whether basic or applied, would be greatly increased if experimental variables would be presented in a form suitable for subsequent independent evaluation and correlation. With the help of a data sheet such as Table 4, relevant parameters can easily be compiled in a comprehensive manner. 20 For the typical application example in Table 4, characteristics of a journal bearing operating at the minimum of the Stribeck curve are compiled with data taken from the literature. 21 Since the procedure of compiling the data sheet is all that is really relevant here, only some main features of the example are pointed out. 1. The technical function of the system can be described as “guidance of motion”. 2. Operating variables are given by the type of motion, namely “continuous sliding” of a duration of 40 min together with load F N , velocity v, and bulk oil temperature T. 3. The structure of the system is described by the elements of the system, their relevant properties, and interrelations. In this case, tribo-element (1) is given by a bearing bushing consisting of a lead-tin-bronze and tribo-element (2) is a steel shaft. Lubricant (3) is a mineral oil of the type SAE 20W-20, and the atmosphere (4) is laboratory air. Together with the designation of the elements and materials, the data sheet contains all important properties of the elements. The data sheet also indicates the tribological interactions between the elements of the system which can be characterized as a “running-in” period leading to topographical changes which are within the initial roughness of the bearing element (2). 4. As tribological characteristics, emphasis is laid on the characterization of wear rates. Although the original wear rates are given in mass units, since the dimension of the bearing are given it is possible to estimate wear-time rates and wear-distance rates. Because the wear measurements have been performed by means of radiotracer tech- niques, it is possible to distinguish between the wear rates of the bearing and the shaft. In a similar manner, characteristics of other tribological systems can be described with the help of the tribological systems data sheet. Although the data sheet may be individually Volume II 655 Copyright © 1983 CRC Press LLC category are cost, availability, and physical and chemical properties such as chemical com- position, density, thermal conductivity, acidity, flash and fire point, pour point, etc. For the testing and specification of system-independent lubricant properties and charac- teristics, well-known tests have been worked out and standardized. (This has been done, for instance, in the U.S. by the American Society for Testing and Materials, (ASTM-D2), in the U.K. by the Institute of Petroleum, and in the Federal Republic of Germany by the FachausschuβMineralöl und Brennstoffnormung in Deutschen Institut fürNormung (FAM- DIN). The details of the various tests can be found in the official publications of these institutions and in other portions of this handbook. The system-dependent characteristics of lubricants depend essentially on the specifications of the whole tribological system. Thus, all of the systems characteristics described earlier must be taken into consideration, at least in principle, to make sure that no important operational aspect or influencing parameter has been overlooked. In contrast to the standardized tests for the system-independent physical and chemical properties of lubricants, the testing of system-dependent characteristics should be performed in connection with the technical function of the actual tribo-engineering system in which the lubricant is used. These tests assess predominantly the overall ability of a lubricant to permit rubbing surfaces to operate without scuffing, seizing, or other manifestation of material destruction. This can be broadly classified in three groups. 22 Simplified bench tests — These tests employ simplified test geometries leading to point, line, or flat contact. Most of these tests were devised to differentiate between EPand non- EPoils, and their accuracy is sometimes not good enough to grade different levels of EP activity. Erratic results can occur if operating variables (e.g., temperature of the lubricant) are not closely controlled. Predicting the performance of lubricants on the basis of these tests alone is almost impossible. On the other hand, they are convenient for acceptance testing, for production control, and as indicators of batch variations of lubricants. Testing with tribo-technical components — Because of the above shortcomings, a different type of lubricant testing is required to permit control of as many variables as possible while simulating actual performance requirements. Aconvenient way of doing this is to test lubricants in the laboratory, where operating conditions can be controlled, with the parts under test being those used in the complete tribo-engineering unit. Full-scale tests — There is general agreement that the only satisfactory means of evaluating the performance characteristics of lubricants is by full-scale tests of their actual use in tribo- engineering systems. Since the cost of field or proving-ground tests is considerable, this type of testing is generally used only as final proof of the decisions made while developing the design of an actual tribo-engineering system. The systematic lubricant selection procedure may follow the “flow chart” as shown in Table 5. From the technical function (A) of the mechanical system, it is often possible to make a preselection of the lubricant, i.e., to specify the “type” or “class” of the lubricant, e.g., gear oil or cutting fluid, etc. For the further specification of the lubricant, allowable ranges should be known for the operating variables (B), such as load F N (or pressure p), speed v, operating temperature T (including the friction-induced temperature rise ΔT), operating duration t, as well as the allowable limits of the tribological characteristic (C), such as friction coefficient, wear rate, and heat and vibration data. The structure of the system (D) determines the other system components which interact with the lubricant. Material and surface properties of the other system components are to be considered. A crucial factor is an estimation of the tribological processes to be expected, i.e., contact conditions, interfacial friction and wear mechanisms, and the prevailing lubri- cation mode. In addition to the system-dependent parameters from the groups (A) to (D), the system- Volume II 657 Copyright © 1983 CRC Press LLC Table 6 MACHINERY CONDITION MONITORING TECHNIQUES (a) Monitoring of operating variables • Allowable load or contact pressure → force transducers, pressure gauges • Allowable velocity → velocity transducers • Allowable temperature → thermocouples, IR-techniques (b) Monitoring of components and lubrication • Misalignments, vibration of components → proximity transducers, accelerometers • Surface conditions of components → profilometry, surface analyses • Lubricant supply → sight glasses, oil flow meters • Lubricant film thickness → distance meters, pressure gauges • Lubricant effectiveness → viscometry, chemical oil analyses (c) Monitoring of friction-induced energy losses • Friction characteristics → force transducers, torquemeters • Friction-induced noise → noise analyses, acoustic emission analyses • Friction-induced temperatures → thermocouples, IR-techniques (d) Monitoring of wear-induced material losses • Wear debris and lubricant contaminants → spectrographic oil analysis procedure (SOAP) → magnetic chip detectors → ferrography → radioactive tracer methods the use-input and use-output quantities. For the movement from diagnosis towards prognosis, signals from the loss-outputs indicating the ailments of machinery may be fed back by means of servo-control equipment or microprocessors to the input, thus influencing the operational inputs in order to establish a proper functioning of the whole system. Monitoring of Components and Lubrication Misalignment and vibration of components can be detected by proximity transducers or accelerometers. Changes in surface conditions of components or the appearance of pits or cracks are a powerful indicator of incipient failure, particularly in rolling elements and gears. However, the application of monitoring techniques is hindered by the fact that moving surfaces have to be investigated. To overcome these difficulties, attempts have been made to use lasers for in situ detection of surface roughness characteristics. Another possibility is to make a replica of the surface to be monitored. A very important aspect is the monitoring of the lubrication. If the volume of lubricant to the component is inadequate or if the physical and chemical properties of the lubricant change in service, the lubrication mode may change, leading to a deterioration of the performance of the whole system. Methods of monitoring oil supply range from visually checking the oil level in the sump or oil tank using a sight glass, to the installation of oil pressure gages and oil flow meters. These detectors are often connected to an automatic alarm system. Further, the lubricant quality or effectiveness should be monitored by taking samples of the used oils at intervals and subjecting them to laboratory tests to determine whether relevant lubricant properties have changed. Volume II 659 Copyright © 1983 CRC Press LLC Monitoring of Friction-Induced Energy Losses An increase or decrease in the friction of system components such as bearings and gears can be an indicator of decreased performance and incipient failure. Changes in the noise spectrum can also indicate incipient failure. Two types of acoustic signals may be distinguished: 25 1.Noise fields due to vibrations, impacts, or aerodynamic processes, emitted in a rela- tively low frequency range (10 to 20 kHz). 2.Impulse-like acoustic signals of low amplitude due to microstructural changes, like micro-cracking, emitted in a frequency range of about 50 kHz to 1.5 MHz (acoustic emission). Tribo-induced heat and temperature rise may lead to thermal distortion and thermal stresses and both may adversely affect mechanical strength of machine components and properties of the lubricant, thus influencing the performance and safety of the machine. Monitoring of Wear-Induced Material Losses Careful examination of wear debris or lubricant contaminants can indicate their origin and allow conclusions to be drawn about their formation, and hence the conditions of inaccessible moving parts. For example, it is not possible to examine in situ the working parts of a jet engine, but each drop of lubricant which circulates through the moving parts carries with it evidence of its experience in passage. The wear debris contained in lubricants may be monitored by the following methods: Spectrographic oil analysis procedure (SOAP) — Very small concentrations of metallic wear products (1 to 2 ppm) suspended in used lubricating oil can be identified by spectro- graphic analysis. 26 Information thus obtained often indicates which components are wearing. For example, abnormal levels of iron in the sump oil of a Diesel engine can indicate excessive cylinder bore wear. Magnetic chip detectors — Use of a magnet in a lubrication system provides a simple and effective method for monitoring contamination. Astrong magnet will attract particles in an oil stream as, for instance, metal flakes such as arise from fatigue fragmentation. The magnetic probe is replaced at regular intervals by a fresh probe while the original is retained for the assessment of the particles adhering to it. Ferrography — Petrography is a technique developed to separate wear debris from the lubricant and spread it according to size on a transparent substrate for examination in an optical or scanning electron microscope. 27 The analyzer consists of a pump to deliver a diluted oil sample at low rate, a magnet to provide a high-gradient magnetic field near its poles, and an inclined transparent substrate (Ferrogram slide) on which the particles are deposited. The quantity of wear particles and their size distribution can be determined by optical density measurement. Radioactive tracermethods — The use of radioisotopes, artificially produced by neutron irradiation, offers a convenient method for following the movement of material during deformation, transfer, or the formation of wear debris. In recent years, a great reduction has been obtained in background radiation by implanting radioactive ions instead of activating the sample. Athin-layer activation technique enables differentiation between the wear of different parts of moving machine elements. 28 From the systematic compilation given in Table 6, the most-suited condition monitoring technique may be selected for a given situation. 660 CRC Handbook of Lubrication Copyright © 1983 CRC Press LLC REFERENCES 1. Ku, P. M., Ed., Interdisciplinary Approach to Friction and Wear, NASA SP-181, National Aeronautics and Space Administration, Washington, D.C., 1968. 2. Ku, P. M., Ed., Interdisciplinary Approach to the Lubrication of Concentrated Contacts, NASA SP-237, National Aeronautics and Space Administration. Washington, D.C., 1970. 3. Czichos, H., Tribology — A Systems Approach to the Science and Technology of Friction, Lubrication, and Wear, Elsevier, Amsterdam, 1978. 4. Bertalanffy, L. von, General System Theory, Penguin. London, 1971. 5. Dixhoorn, J. J. van and Evans, F. J., Physical Structure in Systems Theory. Academic Press, London, 1974. 6. Ropohl, G., Systems Engineering — Principles and Applications, (in German), Hanser Verlag, München, 1975. 7. Faurre, P. and Depeyrot, M., Elements of System Theory, North-Holland, Amsterdam, 1977. 8. Seely, S., An Introduction to Engineering Systems, Pergamon Press, Elmsford, N. Y., 1972. 9. Thoma, J. U., Introduction to Bond Graphs and Their Application, Pergamon Press, Oxford, 1975. 10. Karnopp, D. and Rosenberg, R., Systems Dynamics: A Unified Approach, John Wiley & Sons, New York, 1975. 11. Thum, H., Reliability and wear of mechanical aggregates, (in German), Schmierungstechnik, 3, 139, 1972. 12. Fleischer, G., Problems of reliability of machines, (in German), Wiss. Z. Tech. Hochsch. Magdeburg, 16, 289, 1972. 13. Messerschmidt-Bolkow-Blohm GmbH, München, Ed., Technical Reliability, (in German), Springer- Verlag, Berlin. 1971. 14. Yoshikawa, H., Fundamentals of mechanical reliability and its application to computer aided machine design, CIRP Ann., 24, 297, 1975. 15. Bergling, G., Reliability of ball bearings, (in German), Kugellager-Z., 51, 1, 1976. 16. Eschmann, P., Safety and endurance of ball bearings (in German), Walzlagertechnik, 13, 3, 1974. 17. Bartz, W. J., Tribo-engineering as a basis for the prevention of damages of machine elements (in German), Schmiertech. Tribol., 20, 50, 1973. 18. Fleischer, G., Influence of lubrication technology on the reduction of wear intensity and on the maintenance of a high reliability (in German), Standardisierung Qualität, 21, 83, 1975. 19. Shooman, M. L., Probabilistic Reliability — An Engineering Approach, McGraw-Hill, New York, 1968. 20. Czichos, H., A systems analysis data sheet for friction and wear tests and an outline for simulative testing, Wear, 41, 45, 1977. 21. Katzenmeier, G., Wear Behavior and Load Carrying Capacity of Journal Bearings in the Transition Region from Full Fluid Lubrication to Partial Lubrication-Investigations Utilizing Radioisotopes, (in German), Kernforschungszentrum, Karlsruhe, KFK, February 1972, 1569. 22. Junemann, H., Mechanical tests for lubricants (in German), Erdöl Kohle, 29, 259, 1976. 23. Collacott, R. A., Mechanical Fault Diagnosis and Condition Monitoring, Chapman and Hall, London, 1977. 24. Woodley, B. J., Failure prediction by condition monitoring. Mater. Eng. Appl., 1, 19, 1978. 25. Ziegler, K., Condition monitoring of machines and components by means of acoustic measuring techniques (in German). Schmiertech. Tribol., 24, 5, 1977. 26. Beerbower, A., Spectrometry and other analysis tools for failure prognosis, Lubr. Eng., 32, 285, 1976. 27. Scott, D., Seifert, W. W., and Westcott, V. C., Ferrography — an advanced design aid for the 80´s, Wear, 34, 251, 1975. 28. Gerve, A., Applicability of radio nuclides for the investigation of the influences of design and lubrication on the wear of machines elements (in German), VDl-Berichte, 196, 43, 1973. Volume II 661 Copyright © 1983 CRC Press LLC . solution of lubrication prob- lems. 3 This will be illustrated in three examples of broad areas of lubrication technology. Comprehensive Characterization of Tribological Systems The value of all lubrication. wear of mechanical aggregates, (in German), Schmierungstechnik, 3, 139 , 19 72. 12. Fleischer, G., Problems of reliability of machines, (in German), Wiss. Z. Tech. Hochsch. Magdeburg, 16, 28 9, 19 72. 13. . aid for the 80´s, Wear, 34, 25 1, 1975. 28 . Gerve, A., Applicability of radio nuclides for the investigation of the influences of design and lubrication on the wear of machines elements (in German),