The Calculation of Bearing Life
In modern industrial applications, bearings are used to support rotating and static shafts and are subject to a variety of loading conditions that affect their life and reliability. Bearing life, defined as the total number of revolutions or a given time to failure, can be calculated and predicted with reasonable accuracy. In general, bearing life is calculated and expressed as the number of revolutions to several different periods of failure such as SKF fatigue life, L10 life, catalog life and catalog dynamic load ratings. Understanding the calculation of bearing life is important for industrial machine design and operation.
The most common calculation method for bearing life is the SKF rating life system. This system relies on bearing ratings to calculate the life of a bearing under a given load and speed. The SKF rating life calculation takes into account all of the possible sources of failure, such as material fatigue, lubricant shear, and thermal stress, which a bearing may encounter under normal operating conditions.
To begin calculating bearing life we must first determine the SKF rating (C), which is based on maximum radial and axial loading of a bearing. This value consists of a dynamic load rating (C0) and an on-load factor (C). C0 is the maximum pressure that a bearing can withstand without failure and is used to calculate the complete static and dynamic load ratings for both radial and axial loading. The on-load factor (C) is a multiplier which accounts for the tolerance of bearings to the combined effects of preload, lubrication, and misalignment. The resultant SKF rating (C) is used to calculate the bearing life as defined by the ISO 281 formula:
L10 = (C/P)2 x 106/N
where P is the applied total load on the bearing, N is the speed in RPM, and L10 is the number of revolutions to failure.
The SKF rating life system is commonly used to calculate the life of bearings in industrial applications and the ISO 281 formula used to calculate its life is universal. However, due to the different materials and tolerances between bearings, industry standards suggest using more conservative bearing life predictions for industrial applications.
In general, the catalog summary dynamic load ratings of a bearing are used as a life index for industrial applications and are generally recommended as the most reliable type of bearing life prediction. Summarized dynamic load ratings contain a significant safety factor to account for possible factors of change such as velocity, operation with oil, installation, side loading, misalignment, and housing design. Therefore, it is recommended that the catalog summary dynamic load ratings are used as an alternative method to calculate bearing life.
Understanding the calculation of bearing life is essential to assessing the overall durability and performance expectations of a bearing in an industrial application. With the proper calculation of bearing life, engineers can accurately predict the effectiveness, reliability, and life of a bearing in any given application. Moreover, calculating the bearing life of a given machine component can provide vital data for machine design, optimization, and operation.
