Bearing Design Considerations for Pumps and Compressors – Part 1

Turbomachinery is an indispensable component found across many industries. Rolling bearings play a crucial role in ensuring smooth operation, minimal downtime, and longevity of this equipment. In this technical article, AST^® explores several important bearing design elements for consideration to ensure optimal performance of all types of turbomachinery. We discuss bearing load ratings in Part I.

Pumps and compressors are ubiquitous in industrial settings. These machines can be found anywhere there is a need to transport liquids or circulate or compress gasses. Manufacturing, chemical processing, food and beverage, construction, mining, oil and gas, agriculture, and power generation industries all employ the use of turbomachinery. Reliability and minimizing downtime are critical in these applications. Rolling bearings are integral to the operation of pumps and compressors and must be able to withstand moderate to heavy loading and harsh operating environments. A wide variety of ball, roller, and needle bearings can be found in these machines; however, this article will focus on the most common type of bearing used, the deep groove ball bearing. Benefits of using deep groove ball bearings include the ability to handle high speeds and carry loads in the radial - and to a lesser extent - axial, directions. To ensure reliable and efficient operation, correct bearing selection is an essential step in the design process of pumps and compressors. It is important to perform an analysis of the components and operating conditions from both a static and dynamic perspective to gain a complete understanding of the loads (both magnitude and direction) the bearings will encounter in the application. Deep groove ball bearings are designed to primarily carry radial loads, but they can also support moderate axial loads as well. In most turbomachinery applications, bearings are subjected to both radial and axial, or combined, loading. Once the loading is understood, the basic bearing life is calculated and then adjusted if necessary. This adjusted life is then compared to the desired design life to determine if the bearing is suitable for the application. Lastly, specifications including shaft and housing fits, bearing material, and lubrication need to be determined.

Bearing Load Ratings

Manufacturers of ball bearings typically publish load ratings - both static (Cor) and dynamic (Cr) - for each bearing part number they produce. The static load rating, or static capacity, is the amount of load a non-rotating bearing can support without causing permanent deformation to its rings and/or balls. Industry standards specify that a total deformation of 0.0001 of the rolling element diameter (at the center of the most heavily loaded element) is acceptable for most bearing applications. The dynamic load rating, or dynamic capacity, is the amount of constant radial load a rotating bearing can withstand for one million revolutions. While the methods used to calculate load ratings vary from one manufacturer to the next, both the American Bearing Manufacturers Association (ABMA) and the International Organization for Standardization (ISO) have published equivalent standards related to load ratings and bearing life:

• ABMA Standard 9 – Load Ratings and Fatigue Life for Ball Bearings
• ISO 76 – Rolling Bearings - Static Load Ratings
• ISO 281 – Rolling Bearings - Dynamic Load Ratings and Rating Life

An important point to remember about load ratings: the static and dynamic load ratings are derived differently and have no direct relationship. While both load ratings depend on bearing component geometry, the static load rating also depends on the applicable contact stress level, whereas the dynamic capacity depends on the accuracy to which the components are manufactured, and the quality of the material used.

In the next installment of this series, AST addresses bearing life, including calculations and the use of life adjustment factors. Look for Part 2 of Bearing Design Considerations for Pumps and Compressors, coming April 24, 2024.