Introduction Te concepts of rolling bearing rating life and basic load rating (load carry- ing capacity) were introduced by A. Palmgren in 1937 (Ref. 1). At that time and until the 1950s, most bearing man- ufacturers listed in their catalogues the load admissible on the bearing for thousands hours of operation at fve diferent speeds. In those days the se- lection of a bearing size for a given ap- plication was rather a simple matter. Te concept of a single rating factor to characterize the dynamic capacity of the bearing was new and it was initially used only within the bearing company that developed this new technology. Tis rating method was backed by the theory of Lundberg and Palmgren (L- P) (Ref. 2) and by the Weibull statis- tics (Ref. 3). It was found that it could provide a correct interpretation of the many series of endurance tests avail- able at the time, (Refs. 2, 4 and 5). Tis calculation method prevailed on all the others methods used at the time and it was adopted by ISO in 1962. Before ISO acceptance the L-P model for life ratings was independently vali- dated by Lieblein and Zelen in 1956 (Ref. 4) of the U.S. National Bureau of Standard, using endurance test data provided from diferent bearing manu- facturers. In total, 213 test series were analyzed amounting to a total of 4,948 endurance-tested bearings. Further- more, the statistical setting of the bear- ing life dispersion was also assessed by Tallian of the Philadelphia testing labo- ratories in 1962 (Ref. 5). In the Tallian investigation, a composite sample to- taling more than 2,500 endurance-test- ed bearings was analyzed. Te original L-P model constituted the foundation and it is remains today the nucleus of all national and international standards for fatigue life rating of rolling bear- ings — including subsequent theories and developments. Basically, the L-P theory (Ref. 2) developed the basis for the calculation of the dynamic load rating and equivalent dynamic load of rolling bearings as it is applied today in the ISO 281 (Ref. 8) basic rating life equation: (1) L10 = ( C ) p P Where L 10 is rated fatigue life, at 90% reliability, in million revolutions C is basic dynamic load rating of the bearing for a rated fatigue life of one million revolutions P is standardized dynamic equivalent load of the bearing p is life equation exponent Te availability of a standard method for the dynamic rating of rolling bear- ings is useful to the mechanical indus- try, as it allows streamlining product specifcations for large-scale manu- facturing and worldwide compatibility and exchangeability of rolling bearings. Te dynamic load rating allows bear- ing users to compare similar bearing types made by diferent manufacturers. Manufacturers, on the other hand, can proft from the ISO standards to rate their products, of any size and type, us- ing just the internal nominal geometry of the bearing. Apparently the ISO stan- dard for bearing load ratings provides a win-win situation for all parties, and this explains the widespread use of this standard in the mechanical industry. Mechanical designers, however, need to be well informed in order to take full advantage of the opportunities ofered by standardized bearing load ratings. In particular, they must be aware of the many aspects and changes that have taken place in this feld through the years and how these changes have im- pacted gearbox performance and de- sign practices. In this paper we will frst examine the evolution of standardized bearing life rating that has taken place after ISO 281 was frst instituted in 1962. Te techni- cal justifcations behind each diferent change will be explained, showing also the impact that variation of bearing Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Fifth Floor, Alexandria, VA 22314-1587. Statements presented in this paper are those of the author(s) and may not represent the position or opinion of the American Gear Manufacturers Association. The Modifed Life Rating of Rolling Bearings: A Criterion for Gearbox Design and Reliability Optimization Antonio Gabelli, Armel Doyer and Guillermo Morales-Espejel Engineers typically learn that the bearing L10 life can be estimated using the so called “C/P method” — or the “basic rating life” of the bearing, a method rooted in the 1940s. Major developments have since led to the “modified rating life,” released in ISO 281:2007, which includes the a iso life modification factor. In this paper a succession of equations used for bearing life ratings are reviewed, and current bearing life rating practices are discussed in detail. It is shown that — despite the introduction more than 30 years ago of the adjustment factor of the basic rating life, and the standardization in 2007 of the a iso modification factor — use of these improved calculation methods are not practiced by all engineers. Indeed — many continue referring to the old model as a way of seeking compliance with existing, established practices. The result is the potential for many gearbox manufacturers to continue making design decisions based on the old ISO 281:1977 “basic rating life” standard. This paper addresses these issues in the specific context of industrial gearbox bearing design, using as an example the design analysis of a helical gearbox application. The implication of not adopting modern rating life as described in ISO 281:2007 is equivalent to disregarding 30 years of bearing technology development. 46 Power Transmission Engineering ]———— WWW.POWERTRANSMISSION.COM MARCH 2015 TECHNICAL