Rolling bearing lubrication

Citation

(1998), "Rolling bearing lubrication", Industrial Lubrication and Tribology, Vol. 50 No. 3. https://doi.org/10.1108/ilt.1998.01850cae.001

Rolling bearing lubrication

Rolling bearing lubrication

Keywords: Germany, Tribology, Worksheets

The German Society for Tribology (GfT), best known for the journal Tribologie und Schmierungstechnik it publishes under the direction of Professor Wilfred Bartz, has produced a number of worksheets. These publications give guidance to managers, designers and users on specific aspects of the application of lubricants. The GfT has issued five updated worksheets (GfT-Arbeitsblaetter) since 1991 with two more, on Oil Containing Waste and its Disposal and Methods of Lubrication and Lubrication Equipment in Preparation.

The worksheets cover the following topics:

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  the organisation of tribotechnology ­ tasks in the company (10 pp.);

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  lubrication in metalforming (7 pp.);

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  rolling bearing lubrication (47 pp.);

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  plant oils as lubricants (20 pp.);

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  gear lubrication.

So far only the worksheet on rolling bearing lubrication, issued in 1993 to replace the 1984 edition, has been translated into English. Its 43 pages explain the current theory and practice of bearing lubrication for the benefit of designers and users. The five authors come from FAG, SKF, INA, Fuchs and Klueber, among the most respected companies in the world in this field.

The theme of the publication is that the lubricant is a vital machine element. Rolling bearings require less lubricant than sliding bearings but a suitable lubricant must be available in sufficient amounts at all times for rolling bearings to give a long life.

Contents

The effect of lubrication on bearing life, rolling bearing friction and the selection of the lubrication system are the main subjects of the worksheet. It also touches on ceramics as bearing elements, gives practical tips and tackles the subjects of lubricant contamination (both solid and liquid) and of filtration. The final section lists methods of monitoring the condition of bearings. There are 56 references, mainly in German, and a list of 15 relevant German DIN standards.

Bearing life

Bearing fatigue life calculations assume the presence of a lubricant film and "a few tenths of a micron will be sufficient" to give elastohydrodynamic lubrication.

Minimum loads required to prevent skidding and the effect of viscosity are mentioned. The increase in fluid viscosity caused by the pressure generated at the lines or points of contact points in the bearings with both mineral oil and non-mineral oil fluids is treated in detail. There is graph plotting pressure/viscosity coefficients for various fluids against viscosity over the range 4 to 200 cSt (mm2/s) showing the coefficients increasing with increase in viscosity. At 60 cSt the values of ×10^-8m²/N, for the pressure range 0-2000 bar together with range values from a later table (p. 25), are:

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  Naphthenic mineral oil 2.5 (range 1.1 to 3.5);

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  Paraffinic mineral oil 2.2;

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  Polyalphaolefin (range 1.5 to 2.2);

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  Diester 1.5 at 20 cSt;

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  Ester (range 1.5 to 4.5);

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  Triaryl phosphate ester 3.1;

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  Fluorohydrocarbon 4.2 (range 2.5 to 4.4);

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  Polyglycol 1.4 (range 1.2 to 3.2);

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  Silicone 1.9 (range 1.0 to 3.0).

The units shown for pressure/viscosity coefficients are usually x10^-8m²/N not 10⁸ as appears in figure 3 nor mm²/N as shown in figure 80. The curves were based on the VDI-guideline 2202 updated with more recent information. However the worksheet warned that the published alpha values had been obtained under unrealistic, nearly static, low temperature conditions and "it is doubtful whether the high viscosity increase can be expected at all".

The ISO 281 adjusted bearing life equation is used. In it the life adjustment factor a₃ takes into account the viscosity and amount of the lubricant. The graph of a₃ plotted against kappa, the ratio of the viscosity at the operating temperature and the rated viscosity, widely used by the bearing industry and proposed as an appendix to DIN ISO 281, is shown and its use fully described.

Rolling bearing friction

The section on friction from rolling, sliding and the lubricant also follows the general approach adopted by rolling bearing manufacturers' catalogues but with fuller explanations. Some of the many friction coefficient values and exponent factors given differ slightly from those provided in some catalogues. The worksheet warns that starting friction coefficients may be two to three times the given values. There is a useful graph of friction versus externally acting load angle for a range of bearings.

The familiar equation for heat generated in a bearing

Watts = newton metes per second is given in the form

Heat flow in watts = 1.047 x 10⁴ x n x M where M is the friction torque (Nmm) and n the speed in revolutions per minute (min^-1). A graph is included of heat transition coefficient versus bearing seated area as an aid to calculating heat dissipation using a simple equation. Much of this section comes almost verbatim from the 1978 FAG book Ball and Roller Bearings published as an English translation in 1985. Helpful guidance on operating temperatures to be assumed for a range of bearing applications is provided.

Speed

In addition to the well-established limiting speed, the concepts of reference speed n_b and thermally admissible speed n_adm are introduced and illustrated. Reference speed allows the comparison of different bearing types and sizes for use in high speed applications. It is used when calculating admissible speed based on the heat balance for the bearing in its location. n_adm is proportional to n_b . A table of the constants of proportionality called "speed ratios" derived from load and lubrication parameters is provided.

Lubrication systems

The worksheet advises that lubrication should be one of the first aspects of a bearing design to be considered and not left as an afterthought. About 90 per cent of all rolling bearing are grease lubricated. For speeds with values of d_mn (mean diameter in mm × rpm) above half a million (or 1.3 million in special circumstances) oil lubrication is preferred. The various lubrication methods and where they should be used, together with the equipment needed and an indication of cost are detailed and illustrated by line diagrams.

Guidance on the selection of the correct grade of oil depending on the speed, load and temperature is provided, aided by charts and tables. The properties and characteristics of mineral oil and the five synthetic fluid alternatives shown above are detailed in two tables. Other aspects of lubrication treated are oil maintenance, deterioration in service and the oil circulation rates necessary for various types of machine.

There are eight pages of information and guidance on grease lubrication including four tables and three graphs. Aspects covered are grease types and their selection, the relevance of consistency (penetration hardness), base oil type and viscosity, miscibility, how much grease to use, grease service life and lubrication intervals. For high-temperature applications a cheap grease supplied at the rate of 1 to 2 parts per thousand (ppt) of the free space in the bearing is suggested.

Alternatively an expensive, thermally stable, special grease in amounts as small as 0.2 ppt "will do".

Biodegradability

For biodegradability a pass at 80 per cent in the CEC-L-33-A-93 test is required together with acceptable toxicity and water pollution classifications. A table of the degradability characteristics of a range of oils and thickeners is given.

Solid lubricants

Solid lubricants as additives to oils and greases or as dry coatings are mentioned and two tables of solid lubricant types and their applications provided. The use of tin (melting point 232^oC) and lead (melting point 327^oC) as soft metal film lubricants at temperatures above their melting points and up to 450^oC as shown in a table seems most unlikely. Solid lubricants are required where liquid lubricants prove unsatisfactory or where high sliding friction or extremely high loads are present. They can also be used to improve the run-in behaviour of roller bearings. Where solid matter is present speeds should be limited. For example, a reduction of 40 to 70 per cent is suggested when the solid content in a grease is 3 per cent.

Contaminants

The Worksheets give charts showing the influence of indentation size and filter mesh size on bearing life in the lubricant contamination section. There is a drawing illustrating how filtration ratio beta values are calculated. Beta values greater than 75, corresponding to the removal of 98.7 per cent of particles above a certain size, usually 10 micrometres, are recommended. The damage caused to bearings by liquid contamination is outlined.

Conclusion

Overall this GfT Worksheet provides a valuable and authoritative guide to effective lubrication of rolling bearings. It goes into greater detail than manufacturers' catalogues enabling users to make better informed decisions. The main criticisms are the poor standard of some of the English translation from the German original and many inaccuracies which should have been picked up during editing.

The GfT is to be congratulated on producing its range of excellent Worksheets for industry. They are available at prices between 5 and 60 DM each, depending on the subject, the language and the number ordered.

Contact: Gesellschaft für Tribologie e.V. Ernststr. 12, D-476443 Moers, Germany. Tel: + 2841 54213; fax: + 2841 59478.