Chemical Processing - February 2008 - (Page 25)

and arrangements) in the context of their anticipated operating environment. Suitable bearings are available to satisfy even the most dif cult conditions faced by centrifugal pumps. As an example, thrust-type bearings (to support axial loads created by hydraulic forces in the pump) include various versions of angular contact ball bearings that suit many applications: Single-row 40° angular contact ball bearings. These are the most popular API pump thrust bearings in service today and generally are used in moderate-speed centrifugal pumps where high thrust loads can be expected. Variations designed with robust machined brass cages suit applications where thrust loads vary greatly during operation and periods of ball skidding are likely. These bearings also will resist destructive vibration forces should cavitation occur. They normally are mounted in back-to-back paired arrangements to accommodate reversing thrust loads and to provide adequate shaft support to promote long mechanical-seal life. Pay particular attention to internal clearance. When mounted and at operating temperature, the bearings should have enough residual internal clearance to operate cool but not so much as to promote skidding of the inactive bearing. Double-row angular contact ball bearings. These (Figure 1) are used extensively as the primary thrust bearing in ANSI standard centrifugal pumps and some older APIstyle pumps. The most effective types feature a Conraddesign, ABEC-3 precision tolerances, a 30° 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. As with paired single-row angular contact ball bearings, operating conditions dictate the need for normal or greater-than-normal (C3) internal radial clearance. Design variations are becoming more popular. Examples include steeper (40°) contact angles to deliver increased thrust capacity, machined brass cages to offer robust performance under 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, and ABEC-3 (P6) tolerances to contribute better control of the bearing’s mounted condition and smoother bearing operation. Specialized angular contact ball bearing sets. For pumps with minimal thrust, paired 15° angular contact bearings generally suf ce. However, for high-thrust-load conditions, use high-performance matched sets of 40° and 15° angular contact ball bearings to provide improved robustness by reducing the susceptibility of ball skidding in the inactive bearing. These designs are intended for centrifugal pumps that don’t reverse or only periodically reverse. The primary bene t of these sets is that the 15° bearing www.chemicalprocessing.com is designed with considerably less internal clearance than the 40° bearing, making it less susceptible to centrifugal and gyroscopic forces producing ball sliding and shuttling, while providing additional radial stiffness to maintain integrity of the shaft and seals. Split-inner-ring angular contact ball bearing sets. These, by matching a single-row 40° angular contact ball bearing with a split-inner-ring ball bearing or four-point contact ball bearing, are designed to accommodate thrust loads in either direction. This arrangement commonly is used in vertical pumps to handle the primary thrust load but also can be utilized in horizontal arrangements, provided the loading is such that the split-inner-ring bearing doesn’t support radial load on its own. Because two bearings acting in tandem share the thrust load, this arrangement offers an extremely high thrust-carrying capacity. Reversing thrust load can be accommodated on the backside of the split-inner-ring bearing. These two-bearing sets behave like “triplex” sets with the added advantage of saving space and costs. Supplying the lubrication Proper lubrication for pump bearings is essential for reliable service — improper lubrication accounts for more than 30% of bearing failures, according to some studies. Good lubricants primarily provide a separating lm between a bearing’s rolling elements, raceways and cages to prevent metal-to-metal contact and undesired friction that otherwise would generate excessive heat that could cause wear, metal fatigue and potential fusing of the bearing contact surfaces. Adequate lubrication for bearings also acts to inhibit wear and corrosion and help guard against contamination damage. The common methods for the effective lubrication of pump bearings include: >> Double-row angular contact ball bearings Figure 1. ANSI standard pumps typically rely on such bearings to handle thrust loads. February 2008 • 25 http://www.chemicalprocessing.com

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Chemical Processing - February 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Nanoparticle Safety Raises Questions Take the Pressure Off Vacuum Systems Achieve Optimum Centrifugal Pump Performance Rethink Batch-Manufacturing Alarm Systems Dr. Gooddata Orlando Plant Pioneers HMI Migration Strategy Process Puzzler Plant InSites Equipment & Services Ad Index Product Spotlight/Classifieds End Point

Chemical Processing - February 2008

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