Pumps & Systems, May 2008
Explore some of the issues associated with vertical pump bearings in general, and nonmetallic bearings specifically. Nonmetallic bearings may not be the best answer in every case but have been an excellent choice for many applications worldwide. In general, these product-lubricated bearings preclude additional contaminants (oils, greases) infiltrating the pumped fluid.
Nonmetallic materials include rubbers, synthetic rubbers (elastomers), plastics, graphite based materials, ceramics and lignum vitae.
XL, SXL and Composite (GM2401) are elastomeric grades that offer exceptional wear life, low friction, reduced starting torque and dry start-up capability (SXL only). The inconvenience of pre-lubricating with water and the failures that can result if the flow of liquid is interrupted are eliminated. These elastomeric grades perform particularly well in dirty water and in applications where shock loading is a factor. These grades have high resilience, readily restoring to original shape from impact or localized deflection caused by passing minor particulate. In addition, the high toughness of the material enhances the natural resistance to abrasion damage.
ThorPlas is a new, proprietary, engineered thermoplastic bearing material. While the range of high performance elastomer bearing grades clearly offers superior performance in the applications in which they can be specified, there are technical limits, such as maximum temperatures and pressures beyond which they cannot be used. To address these limitations, this new grade significantly expands the range of applications where bearings can be specified while still maintaining many recognized elastomeric performance advantages.
When selecting bearings for a given application, there are many aspects of bearing and pump design issues to consider. Bearings in a vertical pump are a necessary machine component forming the basic support structure for the power transmission system from the driver to the impeller(s). This system must be designed and selected with all pertinent mechanical aspects considered to ensure that the overall performance will be satisfactory for the pump operating life. If the pump design and operating conditions are not carefully explored, the bearings-the weak link in the chain-will indicate distress well before other components in the pump are affected. This is true whether the issue belongs specifically to the bearings or the pump.
A typical nonmetallic bearing length conforms to L/D ratios ranging from 1 to 1.5. Bearing stiffness values for the general range of pump shaft sizes will be equivalent to metallic bearings from a shaft and column dynamics standpoint.
Loading of vertical pump bearings is difficult to analyze in general. It will normally be fairly light, establishing bearing stability as a significant issue. While typical pump speeds are not high enough to develop serious observable instability effects, such activity may have detrimental outcomes on the life of a bearing.
Grooves in the bearing will tend to develop centering forces, and small side loads may result from the stack up of tolerances during the assembly stage. For this situation, minor assembly offsets may be a good thing, resulting in improved operating stability.
Bearing clearances must be adequate to allow free running of the bearings but not so large as to compromise the important shaft support mechanism provided by the bearing. Typical running clearances will be 0.0015-mm/mm of shaft diameter with a minimum of 0.08-mm. In the case of nonmetallic materials, consideration must be made for fluid absorption and thermal expansion. These allowances, although less for some materials, must be considered and may be minimized by reducing wall thickness to minimum values. In any case, the operational dynamics of the pump will be dependant on establishing correct running clearances between the shaft and bearing.
Standard pump sleeve materials such as 400 or 300 series stainless steels will function well with these bearings. For salt or brackish water applications, better corrosion resistance will be experienced with the 300 series or duplex type stainless steels. If significant abrasives are present in the pump fluid, enhanced life of the bearing system will be achieved with hardened sleeves. In such cases, superior performance has been achieved with material mated with nickel chrome boron (NiCrB) coated shaft sleeves.
In general, grooves should be provided to allow adequate flow through the bearing and to allow easy passage of any abrasive particulate debris. Some smaller bearings (under 50-mm shaft diameter) operating in clean fluids may work well without grooves. In either case, the recommended supply of clean water must flow through the bearing to ensure adequate lubrication and cooling.
Bearing spacing is the province of the pump designer, but becomes an issue for pump rebuilders if bearings of different materials are contemplated. The preferred design approach is to provide a shaft/bearing system stiffness with the first bending critical of the shafting above the operating speed by a margin of 10 to 20 percent (stiff shaft design). However, for small shafting using more flexible bearings such as rubber, designs based on the operating speed falling between the first and second bending critical speed (flexible shaft design) have been well accepted.
The above shafting criticals must be determined using the stiffness values for the actual bearings and support system in use. The stiffness of a nonmetallic bearing will be large enough compared to typical shaft bending stiffness to be considered equivalent to a metal bearing for establishing spacing requirements. Use of a less stiff rubber material may require closer spacing of the bearings or a change in philosophy to the flexible shaft design.
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.
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.
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 Bearings
There 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 Bearings
Material 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.
Nonmetallic bearings have a low dry coefficient of friction, less than cutless rubber.
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.
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.
The 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.