- 16 August 2006 -

The non-metallic bearing - what you need to know

Specifying the bearings for your vertical pump is one of the most important steps in a pump design or rebuild. Non-metallic materials offer some enticing benefits, including price point. Dr. Keith Laskey, chief design engineer for Thordon Bearings, reviews some of the most common considerations for incorporating non-metallic bearings into your application.

This article discusses some of the issues associated with vertical pump bearings in general, and non-metallic bearings specifically. Non-metallic bearings may not be the best answer in every case but have been an excellent choice for a great many applications worldwide. Most non-metallic bearings are product lubricated, preventing outside contaminants such as oils and greases from infiltrating the pumped fluid. Non-metallic materials include rubbers, synthetic rubbers (elastomers), plastics, graphite-based materials, ceramics and lignum vitae.

Design carefully

When selecting bearings for a given application, there are many design issues to be considered involving aspects of both bearing and pump design. Bearings in a vertical pump are a necessary machine component. They form the basic support structure for the transmission of power from the driver to the impeller/s. This system must be designed and selected with all the pertinent mechanical aspects taken into account to ensure that the overall performance will be satisfactory over the pump's entire operating life. If the pump's design and operating conditions are not carefully explored, the bearings, as the weak link in the chain, will indicate distress well before other components in the pump are affected. This is true whether the problem is caused by the bearings or the pump.

Bearing considerations

The typical length of non-metallic 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 of vertical pump bearings is difficult to analyze. It will normally be fairly light, and bearing stability can be a significant issue. While typical pump speeds are not high enough to develop serious observable instability, such activity may shorten 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 big enough to enable free running of the bearings but not so large that they compromise the important shaft support mechanism provided by the bearing. Typical running clearances are 0.0015 mm/mm of shaft diameter with a minimum clearance of 0.08 mm. In the case of non-metallic bearings, consideration must be made for fluid absorption and thermal expansion. These allowances must be considered and may be minimized by reducing wall thickness to minimum values. In any case, the operational dynamics of the pump depend on establishing correct running clearances between the shaft and bearings.

Standard pump sleeve materials such as 400- or 300-series stainless steel will function well with non-metallic materials. For salt or brackish water applications, better corrosion resistance will be experienced with 300-series or duplex stainless steel. If the pumped fluid contains significant abrasives bearing life can be enhanced with hardened sleeves.

In general, bearings should be grooved. These grooves enable adequate flow through the bearing, allowing 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.

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