Last of Two Parts
by Heinz P. Bloch, P.E.

In modern industry, hundreds of millions of horizontal process pumps move fluids. The larger of these, typically pumps in ranges above 500 kilowatts (kW), incorporate sliding bearings. Except for a relatively small number of pumps with product lubrication or with exotic and expensive bearings, horizontal process pump bearings need oil lubrication.

Part 1 of this series dealt extensively with lubrication. Part 2 discusses lubrication matters but focuses on bearings and surprising findings on allowable bearing temperatures.

Figure 1 illustrates the extended temperature ranges available from modern synthetic lubricants. Properly formulated synthetic lubricants allow continuous operation at 400 degrees F (205 C). Although no rolling element bearings in process pumps are ever exposed to this temperature, it would be correct to point to the information conveyed by Figure 1. It supports the findings that premium synthetic formulations can be used with every bearing style or configuration found in process pumps.

Figure 1. Temperature ranges of modern synthetics (Courtesy of the author)Figure 1. Temperature ranges of modern synthetics (Courtesy of the author)

Bearing Styles & Configurations

Process pumps incorporate a radial bearing and a thrust bearing. A radial bearing is usually located near the mechanical seal, and the thrust bearing is fitted close to the drive end. Fluid pressures acting on impellers cause a net axial force on the shaft system, or axial thrust. In operation, thrust acts in one direction, which would make the two tandem-mounted bearings in Figure 2(a) suitable for thrust acting from right to left. If the bearings are precision-ground, they will share the load and each will carry 50 percent. If they are not precision ground, the configuration will create issues.

Figure 2. Sets of thrust bearings with different orientations: (a) tandem, for load sharing of a pump shaft thrusting from right-to-left; (b) back-to-back, the customary API-610 recommended orientation with shafts possibly exerting axial load in each direction; (c) face-to-face, rarely desirable in centrifugal process pumps.Figure 2. Sets of thrust bearings with different orientations: (a) tandem, for load sharing of a pump shaft thrusting from right-to-left; (b) back-to-back, the customary API-610 recommended orientation with shafts possibly exerting axial load in each direction; (c) face-to-face, rarely desirable in centrifugal process pumps.

Expect the thrust action to briefly shift from the normally active to the opposite, or inactive, direction when process pumps are started and accelerating to operating speed. This concern favors using two angular contact thrust bearings mounted in back-to-back fashion, as depicted in Figure 2(b).

The back of an angular contact bearing is the wider outer ring land. The more narrow outer ring land is the face. Figure 2(c) is a face-to-face mounted set. If the inner ring became hotter than the outer ring, the resulting thermal growth would load up the bearing. This is one reason face-to-face bearings are rarely used in pumps.

Care must be taken at all times to prevent interference at the bearing and shoulder radii ra and rb. Also, precision-ground bearings are more expensive than bearings with more liberal manufacturing tolerances. However, after thinking through how the thrust bearing set in Figure 2(a) works, the end user will realize that with one bearing heavily loaded, the other one might be unloaded. When that happens, the unloaded bearing may skid and wipe the lubricant off the raceways. Metal-to-metal contact on the skidding bearing creates heat and accelerated bearing failure.

American Petroleum Institute (API) 610 asks for the contact angles in each bearing making up a set to be equal, which is why they are shown as equal in Figure 2(b). The API recommendation is influenced by the desire for standardization, by the economics of initial cost and by the desire to simplify training of technicians. Economics aside, two angular contact thrust bearings with equal load-carrying capacities are not necessarily best for pumps that briefly experience thrust reversal at startup. An unloaded bearing may accidentally skid while the rolling elements in the loaded bearing are rolling as intended. Skidding bearing elements tend to wipe off the oil film, which causes metal-to-metal contact. This contact can initiate failures and often manifests itself as high bearing temperature.

Special sets of back-to-back mounted thrust bearings may be a better choice in certain applications. These sets would use dissimilar angles. As an example, instead of the customary 40/40 degrees, a 40/15 or 29/15 degree design might be used to avoid skidding. Even an occasional set of triple-row bearings can be found in multi-stage pumps. Like Figure 2(b) and related sets, they must be carefully matched and precision-ground at all contacting faces and surfaces.

Finally, there are thrust bearing sets with matched/mated split inner ring (separable ring) and angular contact bearing geometry (see Figure 3). Because they accommodate the maximum number of rolling elements, split inner ring bearings can impart favorable load capabilities but are more expensive.

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