Rheology and tribology of engineering oils

Rheology and tribology of engineering oils

and Iskandar show in their contribution for alumina sliding against a range of counterface materials. The purposes of a tribological test may include ...

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and Iskandar show in their contribution for alumina sliding against a range of counterface materials. The purposes of a tribological test may include conducting basic scientific research, measuring the relative wear resistance of a set of materials, developing new wearresistant materials or lubricants or evaluating the wear resistance of candidate materials for a specific application. Several papers in this volume well illustrate the last of these points, for applications including the evaluation of carbon seals of various geometries (Au), plain thrust-bearing materials (including engineering ceramics, hard steels, cemented carbides and polycrystalline diamond) for use under very severe conditions in mud-driven motors for oil and gas drilling ( H o r m a n and Eyre), rolling-element and plain bearing materials (including advanced polymer matrix and metal matrix composites) for aerospace applications (Au and Yung), the fretting wear of continuous fibre polymer matrix composites (Jacobs et al.) and the wear of materials for slurry pump casings (Pagalthivarthi and Helmly). In each case, the paper provides insights into the methodology of wear test selection and interpretation which will be useful to any reader with similar, even if not identical, problems. Maps and diagrams which represent regimes and rates of

wear are now becoming more common. Such diagrams can provide valuable illustrations of the influence of time, contact pressure and sliding velocity on transitions in wear mechanisms and thereby be of great value to the designer. Several authors in this book use these types of graphical representation, notably Lim et al. to show the results of sliding wear tests on silicon nitrides, and Au, and Blau and Yust, who use the now wellestablished IRG transition diagram to describe the behaviour of lubricated concentrated contacts. Of course, any diagram or map is only as accurate as the data on which it is based, and the remaining papers present considerable evidence of the difficulty of generating meaningful data from wear tests. T u r e n n e and Fiset demonstrate the complexities of particle motion in slurry erosion testing by the jet-impingement method, applied to the study of metal matrix composites. Gee shows in an excellent contribution how the m e t h o d of applying the load to the specimen in the ubiquitous pin-on-disc test (and, by inference, in most other sliding wear tests as well) can have a profound effect on the results. In tests on alumina, under nominally identical contact conditions, he found that wear

rates could differ by nearly two orders of magnitude between samples tested in apparatus with different mechanical characteristics. In a second paper, he demonstrates how uncontrolled variables, particularly atmospheric humidity, can lead to substantial variation between the results of pin-on-disc tests carried out under apparently the same conditions on identical samples of alumina in different laboratories. The paper also includes a useful simple discussion of the statistical methods used to describe repeatability (within each laboratory) and reproducibility (between laboratories). This short collection of papers confirms that the subject of wear testing is moving out of the Dark Ages. T h e r e is a growing appreciation of the many factors which can influence the results of a wear test, and of the importance of closely defining the data which are required before attempting to obtain them. It can be warmly r e c o m m e n d e d as useful background reading for all those involved in the design, selection or interpretation of wear tests; its value is by no means restricted to those concerned with the tribology of "advanced' materials. lan Hutchings

Rheology and Tribology of Engineering Oils Eds: M. J. Covitch and S. C. Tung This book comprises a collection of 21 papers prepared by experts and specialists in the twin areas of rheology and tribology. The focus is on the performance of engine oils in relation to research and development allied to engine design, particularly Original Engine Manufacture ( O E M ) . As such, the papers represent an Society of Automotive Engineers, 1992, ISBN 1-56091-300-2, £53.00 (European price)


international sampling of current research work being performed in industrial and academic institutions. As automotive design has evolved, durability and maintenance of low wear over the useful life of a vehicle are seen to be important aspects which have an impact on the performance of several critical components piston rings and cylinder liners, bearings and journals, cams and valve lifters, and gears. It is in these critical areas of performance

that the two interdisciplinary fields of study are seen to converge. Rheology, representing the study of fluid flow, and tribology are seen to be closely interrelated especially in relation to the role of viscosity. The first half of the book is devoted to a series of papers covering the rheological aspects with some attention being given to the relationship between viscosity and wear. Later, the 1993 VOLUME 26 NUMBER 4

focus is directed more to the tribology of engine components and bench wear-testing devices, including techniques developed for the measurement of oil film thickness and a lubrication system for uncooled diesel engines. Interspersed among these two major topics are some innovative contributions such as the study of perfluorinated fluids as a basis for characterizing mineral and common synthetic base oils. Likewise, the possibilities are examined for using electrorheological fluids for shock absorbers and engine mounts. Other emerging developments include microscopic diagnosis of lubricant surface interactions and the application of threedimensional surface topography, together with SIMS and EDAX, to correlate mechanisms of surface interaction with macroscopic performance. A key aspect in attempts to achieve better performance is the scope for introducing improved lubricant formulations which will adequately satisfy the demands imposed by modern engines. In terms of rheological properties, the viscosity characteristics and how they are determined under realistic conditions have received attention. The viscometric properties of detergent/dispersant olefin copolymer (DOCP) interaction products are reported in which their reaction with over-based calcium sulfonate detergents can lead to a large increase in relative viscosity in hydrocarbon solution ~. Four-ball test results revealed increased weld loads which appear to be related to increased dispersant alkyl groups on the metal surfaces 2. Rheological characterization of lubricant methanol-water emulsions showed that viscosity increases with water concentration in an emulsion and that they are shear stable 3 up to 105 s- ~. To meet demands for improved fuel economy standards, the use of low-viscosity, low-volatility base stocks has been investigated in relation to the formulation of high-performance motor oils 4. A TRIBOLOGY INTERNATIONAL

study of vapour phase lubrication for lubricating the piston ring/ liner interface of low heatrejection engines revealed that reduced wear was obtained in an uncooled single-cylinder engine test when using tricresyl phosphate 5. In connection with the methods currently used to measure viscosity, a comparison between rotational and high-temperature, high-shear rate capillary viscometers revealed that viscous heating and viscosity-temperature dependence are the largest sources of error in viscometry 6. Shearing thinning effects are also investigated, including the effect on cavitation 7. The behaviour of thin films is reported in relation to pistoncylinder lubrication 8,9, engine main bearings 1° and between cam and rocker followers H. The wear mechanisms and characteristics of coating films have been investigated using a Falex blockon-ring tester involving SEM and EDAX analysis ~2, while the relationship between the regime of lubrication (ehl) and the lubricant properties of perfluoropolyalkylether (PEPAE) fluids is evaluated in terms of their pressure-temperature characteristics 13. The maximum film temperatures generated in journal-bearing operation are compared for synthetic, mineral and viscosity index enhanced engine oils 14.

Among novel developments, one paper describes a newly developed prototype engine mount which is electronically controlled for variable spring and damping force by using electrorheological fluids whose apparent viscosity can be varied by the applied electric field strength 19. As this book comprises a sampling of specialist topics, it is not surprising to find a lack of overall cohesion. Rather, it is a pot-pourri of knowledge and results gleaned from research being performed in industrial and academic institutions. Nevertheless, within the confines of the subject matter, there is much for the rheologist and tribologist to draw upon. It may also provide new stimulus for those who have the task of applying such knowledge in pursuit of improved durability and the maintenance of low wear in automotive components.

References 1. Papke B.L. and Rubin I.D. Detergent-polymer interactions in hydrocarbon solvents: a viscometric study. 2. Kulp M.J., Gundic D.T., Hanna M.E. and Fabian L.E. Zinc dialkyldithiophosphate-dispersant interactions: effects on solution behavior and wear. 3. Sorab J. and Chui G.K. Rheological characterization of lubricant- methanolwater emulsions. 4. Kiovsky T.E., Yates N.C. and Bales J.R. Use of low-viscosity, low volatility basestocks in formulation of high performance motor oils.

In the field of applications, the problems of predicting engine oil performance at low temperature are highlighted in relation to the present SAE J300 Engine Oil Viscosity Classification Systems tS.

5. Groeneweg M., Barber G.C. and Freiheit T. Vapor delivered lubrication of diesel engines - uncooled single cylinder engine test.

Field test results are analysed for different oils used in taxi cabs in New York City to establish the effects of viscosity and other characteristics 16. Theoretical analysis has been carried out to determine load-bearing capacity for journal-bearing bench tests 17. To assist in determining wear characteristics, a 3-D surface topography measurement application is described 8.

7. Rastogi A. and Gupta R.K. Effect of shear thinning on cavitation in lubricant films.

6. Girshick, F. Non-Newtonian fluid dynamics in high temperature high shear capillary viscometers.

8. Millard W. Jr, Johnson and Mangkoesoebroto, S.P. Thin film theory for the power law fluid with application to piston ring lubrication. 9. Richardson D.E. and Borman G. L. Theoretical and experimental investigations of oil films for application to piston ring lubrication. 10. Choi J.K., Lee J.-H. and Dong-Chuihan. Oil film thickness in engine main bear-