Appropriate equipment selection and best-practice maintenance procedures can make a difference in reliability, service life and production.
by Mark J. Cutler, SKF USA Inc.

Bearings in centrifugal pumps support hydraulic loads imposed on the impeller, the mass of the impeller and shaft, and loads caused by couplings and drive systems. They also keep axial and radial shaft deflections within acceptable limits for the impeller and shaft seal.

Handling all these functions—often in difficult process conditions—bearings are subjected to adverse forces that can potentially reduce their service life and reliability.

Radial shaft seals serve as critical elements within a pump’s bearing and sealing system. Seals will effectively offer a “first line of defense” for bearings in pump applications. They retain lubricants or liquids, exclude contaminants, separate fluids and confine pressure.

With pump bearings and shaft seals continually tested as an integrated system, the industry responded to the many challenges with solutions engineered to withstand even the most difficult operating conditions.

Bearing Operating Conditions

Centrifugal pump bearings are typically subjected to high axial loads, marginal lubrication and high operating temperatures and vibration—all while attempting to minimize friction. Friction, if uncontrolled, can result in power loss, excessive heat generation, increased noise and/or wear, and premature bearing failure.

Single-row angular contact ball bearingsSingle-row angular contact ball bearings are used in moderate-speed centrifugal pumps in which high thrust loads can be anticipated.

These influences can dramatically impact the service life and reliability of bearings and pumps. Bearings (types, designs and arrangements) should be evaluated in the context of their anticipated operating environment.

For example, angular contact ball bearings are usually specified as pump thrust bearings to support axial loads created by the hydraulic forces in a pump. Each variation will carry particular benefits for an application.

Angular Contact Ball Bearings

Single-row, 40-degree angular contact ball bearings represent the most popular American Petroleum Institute (API) pump thrust bearings currently in service. They are generally used in moderate-speed centrifugal pumps in which high thrust loads can be anticipated.

The bearings are normally mounted in back-to-back paired arrangements to accommodate reversing thrust loads and to provide adequate shaft support. This promotes long mechanical seal life.

Versions designed with robust, machined brass cages can run particularly well in applications in which thrust loads vary greatly during operation and periods of ball skidding are likely.

These bearings also resist destructive vibration forces when cavitation occurs. Double-row, angular contact ball bearing arrangements are used extensively as the primary thrust bearing in American National Standards Institute standard centrifugal pumps and some older API-style pumps.

The most effective types feature a Conrad-design; Annular Bearing Engineer’s (ABEC) 3 (P6) precision tolerances; 30-degree contact angle per row; one-piece, heat-treated pressed steel cages; and multiple sealing options.

Because their contact angles diverge outwardly, the bearings exhibit greater rigidity and increased resistance to misalignment.

Design variations of these types are increasing, including:

  • Steeper 40-degree contact angles to deliver increased thrust capacity
  • Machined brass cages to impart robust performance in heavy-duty and poor lubrication conditions
  • Reduced axial internal clearances to promote load sharing between the two rows of balls and a reduced possibility of skidding in the inactive ball set
  • ABEC-3 (P6) tolerances to gain better control of the bearing’s mounted condition and promote smoother bearing operation
Double-row angular contact ball bearingsDouble-row angular contact ball bearings. These bearing arrangements are used extensively as the primary thrust bearing in ANSI standard centrifugal pumps, among others.

Specialized angular contact ball bearing sets are designed for centrifugal pumps with heavy thrust loads that are not reversing or those that only reverse periodically. High-performance matched sets of 40-degree and 15-degree angular contact ball bearings can provide improved robustness in high thrust load conditions by reducing the susceptibility of ball skidding in the inactive bearing.

For pumps with minimal thrust, an alternative arrangement of paired 15-degree angular contact bearings would be appropriate.

The primary advantage of these sets is that the 15-degree bearing offers considerably less internal clearance compared with a 40-degree bearing, making it less susceptible to centrifugal and gyroscopic forces producing ball sliding and shuttling, while delivering additional radial stiffness to maintain the integrity of a pump’s shaft and seals.