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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.

Materials

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.

Design Issues

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.

Bearing Issues

L/D

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

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 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.

Mating materials

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.

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 (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. 

Pump Issues

Bearing Spacing

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.

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 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.