Low friction surface technology aids magnetic and hydrodynamic bearing reliability and failure resistance.
Magnetic bearings have the advantage of non-contacting, super-low friction performance. With no liquid lubricant needed, they are increasingly being used in applications involving vacuum operations, refrigeration, semiconductor production, biotech and pharmaceutical operations. All these are applications in which cleanliness is key and contamination from lubricants is intolerable.
A key component of magnetic bearings is the touchdown, or auxiliary bearing. These come into play if the rotating shaft delevitates for any reason—such as during spinup or spindown, power failure, intermittent contact from exceeding bearing load capacity or excess shaft radial motion due to vibration or other machine upset condition. Several options exist for auxiliary or backup bearings, with the plain bearing being the simplest.
Plain bearings are similar to hydrodynamic bearings, in that that they consist of interfacing running surfaces on a rotor and stator that may come into contact during any of the times cited above.
The main difference between auxiliary bearings in a magnetic bearing system and a hydrodynamic bearing is that the hydrodynamic bearing is supported with a liquid lubricant whereas the auxiliary magnetic bearing will run in air, inert gas or vacuum, depending on the application.
The hydrodynamic bearing needs enhanced running surfaces if the pumpage is the lubricant and of poor lubrication qualities. If these bearings lose lubricant due to system upset, they will require dry running capability until the machine is stopped and the lubricant can be restored. In this way, both auxiliary bearings in magnetic systems and hydrodynamic bearings have similar requirements for enhanced running surfaces and dry running under these conditions.
Among the auxiliary bearing alternatives of plain, rolling, planetary and zero clearance, Penfield and Rodwell1 cite plain bearings as the simplest and potentially least expensive. They cite the advantages of low cost, passivity (no moving parts), reduced potential for deterioration in standby and their wear condition can be measured via clearance between interfacing surfaces.
The only negatives cited are a higher friction coefficient and heat generation during spindown compared with the alternatives.
Another limitation is that some of the surfaces used in auxiliary bearings are impregnated with lubricant, therefore not meeting the lubrication-free requirements of a vacuum or for other clean applications. These limitations can be solved with a low-friction, self-lubricating surface that has been demonstrated in pin-on-disc testing and mechanical seal applications. Such surfaces will enhance a bearing's capability in poorly-lubricated or dry-running situations. Therefore, the reliability and wear life of plain auxiliary bearings and hydrodynamic bearings can be enhanced with this surface.
Emerging Surface Technology
Treatment of the surface of carbides to remove the carbide material (such as silicon or tungsten) can be achieved so that the carbon sintered into the carbide is transformed into a matrix of graphite and nanocrystalline diamond. This technology was described in “Surface Conversion for Dry Running Performance” (Pumps & Systems, June 2009).
This treatment is not a coating. Therefore, it avoids coating pitfalls such as dimensional growth, pitting, spalling or any other delamination mechanism. This is because no dissimilar material is added (or coated) onto the substrate.
As a result, there is no dissimilar bond line to delaminate. Also, no dimension or geometric inconsistencies are added, so the process can be performed on finished components and put into service with no post-process grinding or lapping operations.
A scanning electron microscope image of a cross-section of the resulting surface, treated to about a 10 μm depth, is shown in Figure 1. Notice that the new surface crystal structure is intimate with the virgin substrate, with no adhesive bond line to delaminate.
Figure 1. Electron microscope cross-section image of the treated surface
The performance of this surface has been demonstrated in numerous pin-on-disc tests. Those tests show the material's insensitivity of the friction coefficient to wet or dry conditions. See Figure 2.
Figure 2. CDT surface is insensitive to dry vs wet conditions. (Pumps & Systems, June 2009)
In mechanical seal testing, this surface has shown surprisingly superior performance in long duration dry running and in flashing hot water conditions. Figure 3 shows one test in which the surface ran for an hour as a mechanical seal, completely dry, and reached a temperature of 425 degrees F.