| Nonmetallic Pump Bearings |
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| Written by Keith Laskey, PhD, Thordon Bearings, Inc. | |
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Pump Dynamics Most of the installed vertical pumps worldwide do not have any lateral structural support below the pump mounting floor. This means that the casing holding the bearings, supporting the shafting, is itself quite flexible and subject to possible resonance in the operating speed range. If this issue is not carefully investigated in the machine design, the bearings may suffer odd wear patterns which may not be easily interpreted. Suction Conditions Many vertical pumps are located in sumps without proper attention to approach velocities, or clearance guidelines provided in the literature for bottom, back wall, sidewall or neighboring pumps. This may result in cavitation and/or separation, producing excessive turbulence in operation. In addition, if minimum submergence recommendations by pump manufacturers are contravened, vortexing may be generated, allowing air to be entrained in the suction flow and causing associated, undesired machine vibrations. Operating Conditions Many pumps are operated across the performance curve without appropriate consideration for the best efficiency point (BEP). If a pump is highly throttled, or allowed to run out well beyond BEP, excessive vibration, possible overheating and damage of product lubricated bearings could result. It is common practice to dynamically balance impellers of vertical pumps to ensure smooth operation without vibration. However, if an impeller core shifts in the casting process, it will not only result in mechanical unbalance, but hydraulic unbalance as well. No amount of dynamic balancing can correct for the latter condition which may lead to excessive vibration and shorter bearing life. Advantages of Nonmetallic BearingsThere are several advantages offered by nonmetallic bearings such as impact capability, low friction, self-lubricating qualities, edge loading capability. Most nonmetals offer significant electrical resistance. Consequently, stray currents will not be a factor in bearing erosion, and a connection point is not provided for galvanic activity. Specific Features for Nonmetallic BearingsMaterial Grades and Configurations Nonmetallic pump bearings can be found in three grades and two configurations to allow selection of the optimal bearing for each application. These can be supplied in standard tube or bonded into a metal housing. In addition, the new thermoplastic material may also be used for pump bearings handling relatively clean fluids. It offers a broader chemical compatibility and higher operating temperatures compared with other nonmetallic grades. Low friction Nonmetallic bearings have a low dry coefficient of friction, less than cutless rubber. Dry Start-Up If dry start is a requirement for the application, specific materials may be selected for initial dry start periods of up to two minutes. Easily Machined to Size Nonmetallic bearings can be easily machined to exact finished dimensions as determined by sizing calculation programs. Costly sleeve or shaft replacement can often be avoided by machining the bearing to compensate for existing wear or damage. Load Bearing Capability The bearing length in a pump designed for some elastomeric bearings can be reduced by up to 50 percent compared with some nonmetallic bearings. The higher load bearing capability allows a reduction in length without sacrificing performance. High Resilience and Stiffness Elastomer bearings exhibit an ideal blend of resilience and stiffness. With a modulus of resilience many times that of bronze, it absorbs impact or shock loads without permanent deformation and yet exhibits an elastic modulus nearly five times that of rubber. A nonmetallic bearing can generally be assumed to be stiffer than the bearing column support and need not be considered as a flexibility point when performing shaft whirling analysis. Damping (loss factor) of elastomeric materials, as a proportion of material stiffness, will be similar to rubber. Consequently, damping will also be nearly five times that of rubber for similar geometries. Long Wear Life/High Abrasion Resistance Wear of the bearing and journal sleeve due to third particle abrasion is minimal due to the elastomeric nature. Abrasive particles will depress the bearing, roll with the journal to the next lubrication groove and be flushed out without becoming embedded. Easily Installed After machining to the proper dimensions for an interference fit, the bearing can be installed quickly and easily by freezing or press fitting into place. As an alternative, the bearings can also be bonded in place using an adhesive approved by the bearing manufacturer. Conversion of Existing Bearings Worn metallic bearings or other nonmetal materials can usually be replaced with these elastomeric materials and with a minimal amount of machining and preparation of the existing housing. ConclusionThe significant advantages of using nonmetallic pump bearings will ensure many years of excellent service life, provided the various design issues imposed upon the pump and system are carefully investigated and considered in the pump design or rebuild stage prior to putting the machine into service. Dr. Keith Laskey, PhD, is chief design engineer at Thordon Bearings Inc., 3225 Mainway, Burlington, ON, Canada L7M 1A6, 905-335-1440, Fax: 905-335-4033, www.thordonbearings.com. Comments (0)
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