While centrifugal pump designs may vary in size, configurations, and materials, it is common to see rolling bearings incorporated in many designs. Bearing designs can be as numerous as pump designs – and one of the most used rolling element designs in pumps is angular contact ball bearings.
By Gakuo Ndonga, SKF USA Inc.
The Importance and Function of Angular Contact Ball Bearings
Bearings are vital to the shaft system and structural support of a pump, though they also reduce friction between a rotating shaft and the pump’s stationary housing. More specifically, rolling element bearings are typically used to support both static and dynamic loads generated at the impeller and driver. These loads are then transmitted through the shaft during operation.
Heavy duty centrifugal pump designs that are used in hydrocarbon processing, chemical, water and wastewater, and general service applications often use angular contact ball bearings. These bearings incorporate a contact angle between the internal components (balls and raceways) that dictates how radial and axial loads are supported. While contact angles such as 40° are common in pump applications like American Petroleum Institute (API) 610-compliant pump designs, other contact angles such as 25° and 30° (double row bearings) are also available.
As shown in Figure 2, the larger the contact angle, the higher the axial load capacity. While that may be beneficial in applications with high thrust loads, there are other cases where a smaller contact angle can improve limiting speed, stiffness, radial load capacity, and overall performance in low load conditions.
Why Choose Angular Contact Ball Bearings?
The universally matchable single-row angular contact ball bearing is one of the most common bearing designs used in pump applications. This design is often selected for its high speed and thrust load carrying capabilities in pump applications. It is common to see a duplex set of these bearings arranged in either a face-to-face or back-to-back orientation.
Single-row angular contact ball bearings are typically provided as universally matched. This matching process closely controls bearing ring widths and standout tolerances to help achieve the desired internal clearances (or preload) when mounted in pairs. The amount of internal clearance or preload will influence how well the bearings will provide axial and radial support to the shaft. The advantage of paired bearing arrangements is to share the radial load carrying ability while providing thrust load carrying ability in both directions. When thrust load is applied in one direction, only one bearing is actively supporting the main thrust load. The backup bearing functions to support radial load and any reversing thrust load.
A 25° contact angle variant for single-row angular contact ball bearings can be considered an option to improve field performance in applications that have traditionally challenged bearings with 40° contact angles. These challenges can include lightly loaded or higher speed applications. A comprehensive study was conducted by a bearing manufacturer using a 3D-simulation program to compare performance of both contact angles in a vacuum pump application.
Avoiding Bearing Damage
Avoiding Bearing Damage
There are cases where the backup bearing is insufficiently loaded as a result of the axial movement of the shaft. When this occurs, the backup bearing can be at risk of skidding damage where rolling contact between the balls and raceways changes to sliding contact. Sliding can generate increased friction, heat, and wear, which results in the reduced service life of the bearing. In conditions like these, the new 25° contact angle design can be incorporated as the backup bearing to reduce the level of unloading that occurs.
As seen in Figure 6, when a 25° (A) and 40° (B) pair is incorporated, the thrust carrying capacity in the primary thrust direction remains the same as a 40° (A) and 40° (B) pair.
While the thrust carrying capacity in the reverse direction might be reduced using a 25° (A) backup bearing, this 25° bearing can accommodate more deflection before becoming unloaded compared to a 40° (A) bearing. This 25° (A) and 40° (B) thrust bearing arrangement can be considered in pump applications where backup bearings are lightly loaded and at risk of poor field performance as a result.
Selecting the Correct Materials
Single-row angular contact ball bearings can be supplied with various cage options, however, one of the most common options in challenging pump applications is the single-piece machined brass cage.
The mechanical stability and thermal performance of this design makes machined brass cages advantageous for higher speed pump applications. Larger cross- sectional areas of machined brass cages provide a stronger alternative to pressed metal designs. Cage pocket geometries of machined brass cages aid in lubrication film formation which is critical in protecting the cage pocket contact surfaces that ride against the rolling elements.
The incorporation of seals can help simplify the housing design and eliminate the need for lubrication management for a pump. With a sealed bearing, the need to adequately seal the bearing from external contamination and manage re-lubrication intervals is eliminated, helping reduce or eliminate maintenance intervals. A non-contacting design helps keep operating temperatures down without sacrificing seal performance.
About the Author
Gakuo Ndonga is an applications engineering expert with SKF USA Inc. He has spent eight years working in the fluid industry with expertise in rolling element bearing applications for pumps. Ndonga is a graduate of Rowan University with a Bachelor’s Degree in Mechanical Engineering.