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The Issues and Advantages of Non-Metallic Pump Bearing Design

Mon, 10/01/2007 - 11:20am

By Keith Laskey, P.E.


This article discusses some of the issues associated with vertical pump bearings — specifically non-metallic ones — and their advantages including impact capability, low friction, self lubrication, edge loading capability, and electrical resistance. Non-metallic bearings may not be the best answer in every case, but they have been an excellent choice for a great many applications worldwide. In general, they are product-lubricated bearings that prevent additional contaminants, such as oils and greases, from infiltrating the pumped fluid.
Material Review
Non-metallic materials include rubbers, synthetic rubbers (elastomers), plastics, graphite-based materials, ceramics, and lignum vitae. A low-friction elastomeric synthetic polymer alloy — one that is a hard, tough, cross-linked polymer alloy — can have performance characteristics superior to those of most other bearing materials. It performs well in dirty water and in applications where shock loading is a factor. It has high resilience, readily restoring to its 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.
Focusing on Design

ThorPlas pump bearings can be used for relatively clean fluids.
When selecting bearings for a given application, there are many issues to be considered involving aspects from both bearing and pump design. 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 or impellers. This system must be designed and selected with all the 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 — being 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.
Bearing Issues
L/D: The length of certain bearings 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: 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.

Stability: 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.

Clearances: 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 non-metallic materials, consideration must be made for fluid absorption and thermal expansion. These allowances, although less for certain designs than for most other 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.

Mating Materials: Standard pump sleeve materials such as 400 or 300 series stainless steels will function well with certain designs. 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. For example, superior performance has been achieved with Thordon GM2401 material mated with nickel chrome boron (NiCrB) coated shaft sleeves.

Grooves: 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 (less than 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.

Pump Issues

Vertical pump design
(Click image for larger version)
Bearing Spacing: Bearing spacing is the province of the pump designer, but it is often an issue for pump re-builders if bearings of different materials are contemplated. The preferred design approach is to provide a shaft/bearing system stiffness having 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, it has been well accepted to design based on the operating speed falling between the first and second bending critical speed (flexible shaft design).

Bearing Stiffness: The above shafting "criticals" must be determined using the stiffness values for the actual bearings and support system in use. The stiffness of certain bearings will be large enough — in comparison with 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.

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, which are supporting the shafting, is itself quite flexible and subject to the possibility of resonance in the operating speed range. If this issue is not carefully investigated in the design of the machine, 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 to clearance guidelines provided in the literature for bottom, back wall, side wall, 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 with associated undesirable machine vibrations.

Operating Conditions: Many pumps are operated across the performance curve without appropriate consideration for the best efficiency point. If a pump is highly throttled or allowed to run out well beyond the best efficiency point, excessive vibration can again result with possible overheating and damage to product-lubricated bearings. It is a common practice to balance impellers of vertical pumps dynamically 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 the latter condition, which may lead to excessive vibration and shorter bearing life.

Advantages Outlined
There are several advantages offered by non-metallic bearings such as impact capability, low friction, self-lubricating qualities, and edge loading capability. In addition, most non-metallics offer significant electrical resistance. As a consequence, stray currents will not be a factor in bearing erosion, and a connection point is not provided for galvanic activity.

Pump bearings are available in various grades and configurations to allow for the selection of the optimal bearing for each application. These can be supplied in standard tube or bonded into a metal housing. Furthermore, there is a new thermoplastic material that can be used for pump bearings handling relatively clean fluids. It offers a broader chemical compatibility and higher operating temperatures compared with other grades.

Some bearings have a lower dry coefficient of friction that other non-metallics and definitely less than cutless rubber. If a dry start is a requirement for the application, specific materials can be selected for initial dry start periods of up to two minutes. In addition, some bearings can be easily machined to exact finished dimensions, and costly sleeve or shaft replacement can often be avoided by machining the bearing to compensate for existing wear or damage.

With a modulus of resilience many times that of bronze, some bearings absorb impact or shock loads without permanent deformation and exhibit an elastic modulus that is nearly five times that of rubber. These bearings can be assumed generally 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 the materials, as a proportion of material stiffness, will be similar to rubber.

After machining to the proper dimensions for an interference fit, some bearings can be installed easily by freezing or press-fitting into place. Or, they can be bonded in place using an approved adhesive.

In conclusion, the significant advantages of using non-metallic pump bearings ensures many years of service life as long as the various design issues imposed upon the pump and system are carefully investigated and considered in the pump design or re-build stage prior to putting the machine into service.

Keith Laskey, P.E., is the chief design engineer with Thordon Bearings Inc. in Burlington, Ontario, Canada, a supplier of non-metallic pump bearings. He is intimately involved with new product development and testing and received his doctorate from the University of Waterloo in Ontario. More information is available at www.thordonbearings.com.

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