Calculation of a Safety Margin for Hydrogenerator Thrust Bearings

During the design or refurbishment of a thrust bearing a question that occurs frequently is, “How safe is this bearing?” Factors of safety are widely used in engineering design, and some applications require compliance with safety factors that are specifically stated in codes of practice. Most manufacturers have limits on the maximum allowable bearing temperature. These can be useful as a guide but offer no graded answer as to how much safer (or less safe) a bearing is made by applying a particular design decision. A numerical factor of safety would give a quantitative measure of the safety of a bearing and would be useful in judging whether a proposed design change is worthwhile in terms of improved safety. Experience has shown that failure of a babbitt bearing often develops from a local yielding (and subsequent flow) at a discrete point on the surface. This develops into rippling and folding of the material and escalates rapidly to a bearing “wipe.” This article describes the application of the Tresca and von Mises criteria of yielding to give a numerical factor of safety against plastic flow of the babbitt. The method uses data for the yield point of the babbitt as a function of temperature and assumes that the material on the bearing surface is constrained in both lateral coordinate directions. In practice, any data on the failure of hydrothrust bearings are invariably regarded as proprietary by manufacturers and there is apparently no published work available that can be used to correlate the model. However, the method developed in this paper is quite general and applicable to any fully hydrodynamic bearing loaded to the yield point of the surfaces. Results for large babbitt-faced journal bearings that were deliberately taken to failure are used for a correlation. Reasonable agreement is shown between predicted and actual failure. The calculation of a factor of safety can be a useful tool for evaluating initial designs or modifications during refurbishment. Some factors concerning the assessment of an acceptable minimum film thickness are also discussed.

Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2004 Manuscript approved June 1, 2005

Review led by Itzhak Green

KEY WORDS:

• Hydroelectric Generators
• Load Carrying Capacity
• Surface Roughness Effects
• Tilting–Pad Bearings

ACKNOWLEDGMENTS

The authors are grateful to Dr. Booser for loaning the original photograph in Fig. 1 for reproduction. This figure is copyrighted by the Society of Tribologists and Lubrication engineers. Permission from the Society to reproduce Fig. 1 is gratefully acknowledged. The authors thank R. Knox (Michell Bearings, Ltd.) for data on the yield point of babbitt.

Notes

Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2004 Manuscript approved June 1, 2005

Review led by Itzhak Green