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Fluid Film Bearing Fundamentals and Failure Analysis

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Fluid Film Bearing Fundamentals and Failure Analysis FLUID FILM BEARING FUNDAMENTALS AND FAILURE ANALYSIS by Fouad Y. Zeidan Staff Research Engineer Amoco Corporation Naperville, Illinois and Bernard S. Herbage Vice President of Technology and Programs Imo Industries, Incorporated Houston, Texas practical aspects of fluid filmjou rnal bearings, as opposed to Fouad Zeidan is a StaffResearch Engi­ theoretical analysis. Failure modes and the mechanisms leading to neer in the rotating eq uipment group at these failures will be discussed, along with techniques used to Amoco's Research Center in Naperville, identify and cure them. The influenceof different bearing config­ Illinois. He conducts applied research on urations on the dynamics of rotor bearing systems will also be bearings and seals, rotordynamic audits of discussed. new and rerated equipment, and vibration analysis in support of the refineries, chem­ INTRODUCTION ical plants, and production fa cilities of Bearings constitute one of the most critical components in Amoco subsidiaries. Prior to joining Amo­ turbomachinery. Many problems in high performance rotating co, he worked fo r Jmo Industries, Centri­ machinery can be traced either to the rotating elements and rotor Marc Division, where he designed .fluid support system, faulty bearing designs, misapplication of a certain and bear­ film bearings, conducted rotordynamic analysis studies, bearing configuration, or faulty assembly techniques. Although ing fa ilure analysis. At Qatar Fertilizer Company he worked fo r many problems are first detected at the bearings, the source is not a Senior fi ve years as a Field Maintenance Engineer and as necessarily the bearing. Bearings are by design made more com­ ces Section, providing Mechanical Engineer in the Technical Servi pliant than the rest of the machine elements, and in cases of y. engineering support on rotating and critical machiner malfunction or distress, they tend to give first. When a bearing Dr. Zeidan received his B.S.M.E. ( 1978), M.S.M.E. ( 1979), and does fail or show signs of wear and distress, it is important for Ph.D. ( 1989) degreesfrom Texas A&M University. At Texas A&M, either the rotating equipment engineer or maintenance supervisor he conducted research on electric shaft currents, rotordynamics, to recognize the failure and pinpoint where the fault lies. An and squeeze film damper bearings. understanding of how fluid film bearings work and some of the basic principles that underline their operation is, therefore, essen­ tial for making the right decision on a critical piece of machinery. Bernard S. Herbage, Vice President of Whether the fault is in the bearings themselves or in other compo­ Technology and Programs, lmo industries, nents, modifications of the bearing design parameters has, and Incorporated, is a Registered Professional continues to be, the most direct and economical means of altering Engineer, with a B.S.M.E. degree from and correcting the fault or malfunction. This fact further adds to Bradley University, and was a member Tau their importance particularly in existing machinery. Sigma. The fi rst ten years of Mr. Herbage's HYDRODYNAMIC LUBRICATION career were spent with Allis-Chalmers in the Mechanical Design Sections of its Steam The primary requirement of hydrodynamic lubrication is that Turbine and Generator Departments spe­ sufficient oil be present at all times to flood the clearance space cializing on unit sizes through 1000 MW, between the journaland bearing. The formation of an oil wedge to and specializing on the design, application and testing of high lift the journal is dependent on the speed, load, and oil viscosity. capacity thrust and journal bearings. This is often presented by the ZN/Pcur ve. The curve is shown in For thepast 36 years, he has worked with virtually every major Figure 1, and describes three zones of operation a fluid film high speed rotating machinery manufa cturer in the design and bearing might operate in. These are: application offluidfilm thrust and journal bearings. Many of his • Full film hydrodynamic lubrication. bearing innovations are now part of numerous OEM machine • Mixed film lubrication. designs. • Boundary lubrication. Under full film lubrication conditions, no contact takes place ABSTRACT between the metallic surfaces. In mixed filmand boundary lubri­ Fluid film bearing technology is presented in this tutorial as it cation, the oil filmis too thin to separate the metallic surfaces as applies to turbomachinery equipment. The focus here is on the shown by the illustration in Figure 2. When these surfaces run 161 162 PROCEEDINGS OF THE TWENTIETH TURBOMACHINERY SYMPOSIUM I While the ZN/P curve is not totally satisfactory as an absolute I I criteria, it illustrates the characteristic coefficient of friction curve 0.150 I I ---"-- - FIIIC110II I plotted directly against speed and viscosity and inversely against the load. Other factors are involved that are not accounted for here MIXED FILM such as shaft diameter, surface finish, and cleanliness of the �----�� lubricant. BEARING LOAD The pressure over the projected area in the bearing is assumed uniform for the purpose of expressing the load capacity of a bearing. The load is, therefore, equal to the average pressure _ multiplied by the projected area. The projected area being the O.OOb _.,. FRICTION F 0 5 10 20 30 product of the bearing length multiplied by the bearing width. BEARING IWIAMETER, l!l.. In steadily loaded bearings, the load is mainly due to the weight p • VISCOSITY IN CENTIPOISE Z l.UIIRICANT [lj'( IE­ of the rotating element, in addition to the dynamic loading im­ AMBIENT TEMPERATURE • JOURNAL SPEED IN RPM parted by the unbalance in the rotating components. N • PROJECTED AREA UIIIT LOAD IN PSI P In the evolution of turbomachinery, there have been many JOURNAL BEARING PSI UIIIT � 5' WITH 100 ROTOR different types of bearing designs used to achieve acceptable LOAO OPERATING AT TEMPERATURE F OIL OF 120" WITH LIGHT TURBINE OIL. SPEED TD performance. A brief description of a number of significant de­ DETE- OPERATE AT LoWEST FRICTION. • CENTIPOISE signs follows, each with an accompanying schematic illustration. Z 19.1 • PSI P 100 SET 1!i• 10 COMMON FLUID FILM JOURNAL BEARINGS , lO P N , lll:!WOO) , RPM z -w.r- 50 Plain Sleeve Bearings EVEN AT A • N• RPM .,. 5 25 BOUNDARY N • MIICEO N • BETWEEN 211-50 _­ The plain sleeve bearing still continues to be used extensively 25 FULL FILM FLUID 50 RPM + because of simplicity and economic justification. The two major Figure 1. The Classical ZNIP Curve. classifications are (1) the pressure fed design which receives its lubricant under pressure from an external source, and (2) the self together under thin film conditions, the oil is only capable of lubricated types such as the oil ring design, the disc lube design, carrying part of the load, while the remainder is carriedby contact and the viscosity pump design. The oil ring design is the most between the two surfaces. When less oil is available, or at lower popular of these. speeds and higher loads, a point is reached where oil plays little or Pressure Fed Sleeve Bearing no part. This condition is referred to as boundary lubrication. Boundary lubrication and thin film lubrication are two modes in The most common plain journal bearing is shown in Figure 3. It which friction and wear are affected by properties of the contacting is horizontally split with axial oil distribution grooves along each surfaces, and by the properties of the lubricant. horizontal joint. Oil is fed to this groove at pressures anywhere froma few inches of Hg pressure to pressures in excess of 30 psig. BOUNDARY LUBRICATION � MOVI� � Figure 3. Plain Pressure Fed Sleeve Bearing. MIXED FILM OPERATION Oil Ring Lubricated � MOVI �G Oil ring bearings are usually utilized in low speed equipment where the load direction is constant and typically downwards. However, their range has continually been extended up to the present limiting journal surface speeds of about 3000 to 4000 ft/ � min. The rotation of the ring depends on the friction between the shaft and the ring. At low speeds, the static friction between the FULL FLUID FILM OPERATION ring and the journal is sufficient to force the ring to rotate at the Figure 2. Schematic Representation of the Three Lubrication same speed as the shaft. At higher speeds, slip occurs and the oil Regimes. filmdrag helps drive the ring. The oil liftedfrom the oil reservoir TUTORIAL ON FLUID FILM BEARING FUNDAMENTALS AND FAILURE ANALYSIS 163 as a coating is delivered to the journal largely from the inside manner which could impair their operation. The out-of-roundness surface of the ring by a squeezing action as the ring passes over the should be established based on the clearances which can be rotating journal. Oil rings are used extensively in pillow block accommodated in the particular bearing configuration. bearings, as shown in Figure 4. Lemmon and Booser [l] have Grooving. Grooving becomes important if the load is not direct­ investigated several factors that affect ring performance. The ly downwards. Side loads are caused by belt pull or gear reaction. depth of immersion, oil viscosity, guides and scrapers, ring mate­ In such instances, grooves and chamfers are important, so that the rial, and ring roundness were all found to affect the performance side loading does not block these grooves. Axial grooving and in different magnitudes. Details of the findings [ 1] are summarized proper chamfering can increase the oil flow to the loaded section below. of the bearing. Circumferential grooving is sometimes utilized to increase the oil flow. An advantage with ring lubricated bearings which is often not recognized, is their inherent capability to supply oil to the bearings during a power failure. There is no need for a coast down tank or emergency oil supply. In many installations, they are used in a pressure fed bearing configuration as a back up system during power failures.
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