Understanding Journal Bearings

Understanding Journal Bearings

Malcolm E. Leader, P.E. Applied Machinery Dynamics Co.

Durango, Colorado

ABSTRACT

This paper covers the basic aspects of journal bearings including lubrication, design and application. Descriptions of various types of journal bearings are presented. Guidance is given for choosing the proper bearing type and keeping your bearings healthy. A section on do’s and don’ts gives practical information.

INTRODUCTION

Bearings are used to prevent friction between parts during relative movement. In machinery they fall into two primary categories: anti-friction or rolling element bearings and hydrodynamic journal bearings. The primary function of a bearing is to carry load between a rotor and the case with as little wear as possible. This bearing function exists in almost every occurrence of daily life from the watch on your wrist to the automobile you drive to the disk drive in your computer. In industry, the use of journal bearings is specialized for rotating machinery both low and high speed. This paper will present an introduction to journal bearings and lubrication. Lubrication technology goes hand-in-hand with understanding journal bearings and is integral to bearing design and application.

Since they have significant damping fluid film journal bearings have a strong impact on the vibration characteristics of machinery. The types of machinery we are concerned with range from small high speed spindles to motors, blowers, compressors, fans, and pumps to large turbines and generators to some paper mill rolls and other large slow speed rotors.

Not covered here is the topic of bearings for reciprocating machinery. While some of the same principals apply, engine bearings have special needs and design considerations and deserve a more complete study. Reciprocating machinery bearings tend to be simpler in geometry and much more complicated in application than turbomachinery bearings. For example, the typical turbomachinery journal bearing consists of a thin layer of babbitt on steel while a connecting rod bearing may have numerous different layers of copper, steel, nickel, or other metals with a thin layer of babbitt on top. This layering is done for fatigue resistance to the pounding loads encountered in such machinery. Engine bearings are often required to withstand peak specific loads in excess of 3,000 PSI or about ten times a typical motor or turbine bearing. Reciprocating machines rely primarily on the squeezing of the oil film for load support.

WHEN TO USE FLUID FILM BEARINGS

There are applications where anti-friction bearings are the best choice. Commonly, smaller motors, pumps and blowers use rolling element bearings. Paper mill rolls often use large specialized spherical roller bearings. Clearly, anti-friction bearings are best for these applications. However, once the size of a pump (or fan or motor, etc.) gets large enough and fast enough, a gray area is entered. Here you will still find rolling element bearings used successfully but as speeds increase and temperatures rise, rotor dynamics often become a concern and critical speeds are encountered. This is when damping is required and fluid film bearings become increasingly necessary. My experience is that turbomachinery designers (and users) should consider using fluid film bearings if running above 3,000 RPM or the machine exceeds 500 HP. In my opinion, at 1,000 HP and up, all machines except very special cases should be on journal bearings specifically designed for that service. There are exceptions of course, and the decision where to apply what type of bearing is ultimately done for every machine individually based on good engineering practice and experience. Unfortunately, this decision is sometimes based on economics which keeps maintenance engineers and consultants employed.

ADVANTAGES OF FLUID FILM BEARINGS

The primary advantage of a fluid film bearing is often thought of as the lack of contact between rotating parts and thus, infinite life. In a pure sense, this is true, but other complications make this a secondary reason for using these bearings. During startup there is momentary metal-to-metal contact and foreign material in the lubricant or excessive vibration can limit the life of a fluid film bearing. For these reasons, special care must be taken when selecting and implementing a lubrication system and special vibration monitoring techniques must be applied. The most important aspects of the health and longevity of a fluid film bearing are proper selection, proper installation, proper lubrication, and the alternating hydrodynamic loads imposed on the bearing surface by relative shaft-to-bearing vibration.

FLUID FILM BEARING DO’S and DON’TS

DO:

1.   Always use the proper lubricant as determined by the manufacturer or engineering.

2.   Understand the additive package in your lubricants to avoid potential conflicts with process fluids and/or component materials.

3.      Provide proper cooling to the bearing and the lubricant.

4.       If applicable, provide proper filtration to the lubricant.

5.    Implement a regular oil analysis program for all critical machinery. Monitor lubricants for viscosity changes, wear metals and contamination, especially water.

6.        Stay within design guidelines on clearances. General rule is 1.5 mils (0.0015 inches) per inch of shaft journal diameter. 3.0 mils/inch diameter is excessive clearance in most cases.

7.  Handle bearings carefully. Babbitt surfaces are very soft and thin liners are easily distorted.

8.     Inspect bearings under magnification or have a professional evaluation before reuse. Early fatigue damage is usually invisible and other damage like electrostatic discharge may not be readily apparent. Replace bearings if any doubt exists as to the serviceability of the used bearing. Tilting pad bearings may have back-of-pad and pivot wear and brinelling concerns.

9.  Lubricate bearings during replacement with a heavy prelube oil such as an ISO 460 especially if a lot of shaft rotation will occur as during alignment.

10.   Monitor bearing temperatures, preferably metal temperature with embedded thermocouples or resistance temperature detectors (RTD). Calibrate transducers before installation in the bearing. Install dual sensors in case one fails - leave the spare unconnected. Drain temperatures, while useful, will not give an early enough indication of heat problems.

11.      Carefully protect the temperature device leads during handling and installation.

12.   If a bearing is spherically mounted, ensure line-to- line contact or a slight crush. These bearings do not self align! The user must manually align these bearings.

13.   Inspect all bearings and inspect the shops making or repairing your bearings. Many rebabbitting shops are not qualified to repair all types of bearings. Ask where they get their babbitt and what quality controls are used. Is babbitt welding and babbitt casting done?

14.  Use common sense. Treat the bearing with respect and you will get good service. Follow the rules specified by API and the guidelines proposed in this

article.

15.     Fix damaged shaft journals with submerged arc welding or plasma spray - NOT chrome plating.

16.      If you find any lead babbitt bearing material replace it with type 2 high tin babbitt which is much stronger and environmentally friendly.

DON’T:

1.   Never use unapproved substitute lubricants.

2.      Never mix different lubricants unless the combination has been evaluated and approved - different additive packages may react. Never mix mineral oils and synthetic lubricants. Using synthetic oil in a system previously using mineral oil may loosen old deposits.

3.   Never bypass oil filters or coolers.

4.   Installing bearings too tight or too loose is a recipe for disaster. Never set clearances in a tilting pad bearing in the field - this must be done in a qualified shop.

5.     Do not use automotive viscosity enhancers (e.g. STP) when fitting bearings.

6.  Never hand scrape bearings for proper fit. Take the time to have bearing properly machined by a qualified shop. No pocket knives touching babbitt!

7.  Never pry or hammer a bearing liner out or back into place.

8.    Throwing a babbitt bearing into the parts bucket usually means you ruined it.

9.     Never install temperature sensors in the babbitt, rather 0.030" behind in the steel backing. Those wires sticking out of the bearing are not used to carry the part!

10.    Don’t use the low bid to buy or repair bearings. Reusing babbitt from old jobs or overflow is forbidden. Tiny contaminations can lead to early bearing failure.

11.    If there is a choice, don’t use spherically seated bearings, use cylindrical fits.

12.   Do not roll shafts in Teflon? strips or “V” blocks due to micro embedment. This could result in the shaft being unable to properly “wet” and causes failure.

13.  Never let oil reservoir or sump temperature exceed 200°F.

14.  Don’t expect thick babbitt (e.g. 0.060") to be better than “thin” babbitt (<0.015") The fatigue resistance of thin babbitt can be more than 10 times greater than thick babbitt.

15.   Don’t use copper backed pads in the initial design unless there is no other option. This method of increasing capacity should be “held-back” in case additional capacity is needed in the future. It is easier than increasing bearing size.

16.         Don’t ever disassemble a machine without measuring the bearing clearances “as-found”. This is the only chance you get to obtain this data which is an invaluable diagnostic tool.

17.     NEVER coat a fluid film bearing journal with chrome. Use almost any other coating. Chrome is porous and water may get trapped behind the chrome and pop off the chrome layer causing catastrophic failure.

REFERENCES

1.  Machine Design, Part III by International Textbook Company, 1907

2.    Bearings and Their Lubrication by L. P. Alford, McGraw Hill, The American Machinist, 1911

3.  Lubrication of Industrial and Marine Machinery by

C. L. Pope and W. T. Everitt, John Wiley and Sons, 1954, LC number 53-9023

4.      Lubrication Fundamentals by J. George Wills, Marcel Dekker, Inc., 1980, ISBN 0-8247-6976-7

5.  Bearings and Lubrication by Joseph E. Shigley and Charles R. Mischke, a Mechanical Designer’s Workbook by McGraw Hill, 1990, ISBN 0-07-056928- 2

6.     Basic Lubrication Theory by Alistair Cameron. John Wiley & Sons, 1976, ISBN 85312-057-9

7.      Machinery’s Handbook, 25th Edition, Industrial Press, Inc., 1997, ISBN 0-8311-2424-5

8.   Design of Film Bearings by Paul Robert Trumpler. The Macmillan Co., 1966, LC number 66-11212

9.      Bearing Design and Application by Donald F. Wilcock and E. Richard Booser, McGraw Hill, 1957, 195, LC number 56-9641

10.          Applied Tribology - Bearing Design and Lubrication by Michael M. Khonsari and E. Richard Booser, John Wiley and Sons, Inc., 2001, ISBN 0-471- 28302-9

11.   Nicholas, J. C.,"Hydrodynamic Journal Bearings - Types, Characteristics and Applications," Mini Course Notes, 20th Annual Meeting, 1996, The Vibration Institute, Willowbrook, Illinois, pp. 79-100.

12.   Nicholas, J. C. and Allaire, P. E., 1980, "Analysis of Step Journal Bearings - Finite Length, Stability," ASLE Transactions, 23 (2), pp. 197-207.

13.  Nicholas, J. C., 1986, "Stabilizing Turbomachinery with Pressure Dam Bearings," Encyclopedia of Fluid Mechanics, 2, Gulf Publishing Co.

14.       Mehta, N. P. and Singh, A., 1986, "Stability Analysis of Finite Offset-Halves Pressure Dam Bearings", ASME Journal of Tribology, 108 (2), pp. 270-274.

15.  Lanes, R. F. and Flack, R. D., 1982, "Effects of Three-Lobe Bearing Geometries on Flexible Rotor Stability", ASLE Transactions, 25 (3), pp. 377-385.

16.    Nicholas, J. C., 1985, "Stability, Load Capacity, Stiffness and Damping Advantages of the Double Pocket Journal Bearing," ASME Journal of Tribology, 107 (1), pp. 53-58.

17.   Lund, J. W., 1974, "Stability and Damped Critical Speeds of a Flexible Rotor in Fluid Film Bearings," ASME Journal of Engineering for Industry, 96 (2), pp. 509-517.

18.    Newkirk, B. L. and Taylor, H. D., 1925, "Shaft Whipping Due to Oil Action in Journal Bearing," General Electric Review, 28, pp. 559-568.

19.   Gunter, E. J., 1966, "Dynamic Stability of Rotor- Bearing Systems," NASA SP-113, pp. 153-157.

20.     Nicholas, J. C., Gunter, E. J. and Barrett, L. E., 1978, "The Influence of Tilting Pad Bearing Characteristics on the Stability of High-Speed Rotor B e a r i n g S y s t e m s , " R e p o r t N o . UVA/643092/MAE81/141, School of Engineering and Applied Science, University of Virginia, Charlottesville, Virginia, pp. 30-32. Also in Topics in Fluid Film Bearing and Rotor Bearing System Design and Optimization, an ASME publication, April 1978. pp. 55-78.

21.   Theory and Practice of Lubrication for Engineers by Dudley D. Fuller, Wiley and Sons, 1984, ISBN 0- 471-04703-1

22.        API Standards 612 (Special Purpose Steam Turbines), 617 (Centrifugal Compressors), 670 (Vibration and Temperature Monitoring) and RP684 (Rotordynamics) are available from the American Petroleum Institute in Washington, D.C. or from WWW.API.ORG

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