Endurance calculations for bearings

Practical experience has shown that seemingly identical rolling bearings operating under identical conditions will not necessarily exhibit the same endurance life. Standardized life calculations methods help make educated choices about a bearing size's suitability for a particular application. The industry turns to standardized life calculations for good reason. It is realistically impractical from a time and cost perspective to test the number of bearings for a specific application and its operating conditions for required acceptable confidence. A growing list of factors relevant for bearing life calculations have been integrated over the years in keeping with an increasing understanding about operating conditions and the related "system" influences of solid contamination, lubrication, operation regimes, internal stresses from mounting, residual stresses from hardening and other manufacturing processes, the material cleanness, matrix and fatigue stress limit. The evolution of standards has been to refine the life calculation methods to predict more accurately the actual life in a specific application, given certain operating parameters. In other words, a closer match of the calculated life with the experienced service life. Recent scientific advances in bearing manufacturing, tribology, materials, end-user condition monitoring, and computation abilities have led to more sophisticated and accurate bearing life calculation possibilities. In 2007, ISO (the International Organization for Standardization) published a revised ISO-281 Standard further advancing the capabilities for life calculations by including a bearing material's fatigue stress limit and a factor for the effect of solid contamination for different lubrication systems (such as grease, circulating oil, oil bath, and others) on bearing life. In defining the term "life," several types often will be referenced. A bearing's basic life or its L10 (as defined in ISO and ABMA standards) is based on the life that 90% of a sufficiently large group of apparently identical bearings can be expected to attain or exceed. The median or average life (sometimes called MTBF, or Mean Time Between Failure) is about five times the calculated basic rating life. Service life represents the actual life of a bearing in real operating conditions before it fails or is deemed necessary to be replaced for whatever reason. The specification life is generally a requisite L10 basic rating life and is grounded in a manufacturer's experience with similar applications. Getting started Bearing selection is usually based on bearing life and rolling contact fatigue models to predict the life. Therefore, selection of initial bearing size for an application normally starts by comparing its load ratings to the applied loads that generate fatigue, causing stresses with regard to the requirements for service life and reliability. Both dynamic and static bearing load conditions must be independently verified. The basic dynamic load rating will be used for life calculations involving dynamically stressed bearings, such as those bearings that will rotate under load. This rating defined in ISO 281 expresses the bearing load that will provide a basic rating life (L10) of 1 million revolutions. Dynamic loads should be checked using a representative duty cycle or spectrum of load conditions on the bearing, including any peak (heavy) loads that may possibly occur. The basic static load rating applies to calculations when the bearings will rotate at speeds less than 10 rpm, become subjected to very slow oscillating movements, or will remain stationary under load during certain periods. Static loads are not only those applied with the bearing at rest or at very low rotational speeds, but should factor in the static safety of heavy shock loads (very short duration loads). Excessive static loads can compromise the integrity of a beaMens Running Shoes & Running Trainers