Composite dielectric coatings for electrical insulation of bearings

Abstract

A bearing including an inner ring (104), an outer ring (108) surrounding the inner ring, a plurality of rolling elements (111) supported between the inner ring and the outer ring, and a non-stick coating (140) provided on an exterior surface of at least one of the inner ring or the outer ring. The non-stick coating further provides electrical insulation to the bearing.

Description

Attorney Docket No. 023644-0007-US01COMPOSITE DIELECTRIC COATINGS FOR ELECTRICAL INSULATION OF BEARINGSRELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63,679,750 filed August 6, 2024, the entire content of which is hereby incorporated by reference herein.BACKGROUND

[0002] Bearings in rotating electrical machines, such as electric motors, turbines, and generators, operate in environments that increase the risk of damage by electrical currents from the drivetrain. Static electric discharges are well-known to cause frosting and fluting damage, during which electrical arcs locally melt and erode the material on the raceways and introduce debris into the rolling contacts. This damage can lead to increased vibration noise, heat generation, and wear that ultimately reduce the bearing performance. More recently, stray currents in wind turbine main shafts have been found to alter the microstructure of steel bearing elements locally and have been associated with premature cracking and failure. Hence, methods to prevent the passage of electrical current into the bearings within the same drivetrain are important to ensure long bearing life and to reduce costly downtime.

[0003] Today several mitigation strategies are used: installation of shaft grounding brushes or bearing protection rings on the shafts, use of ceramic rolling elements, applying electrically insulating ceramic coatings on bearing outer surfaces. Shaft grounding brushes and bearing protection rings are mounted on the shafts to direct current away safely to the ground, but they are subject to wear and must be replaced to remain effective. Alternatively, using ceramic rolling elements prevents the flow of currents through the bearing, but this option becomes commercially unfeasible to use on larger bearings due to the high costs of larger size rolling elements. Ceramic coatings applied to the exterior surfaces of the bearing achieve the same effect by adding a thick electrically insulating layer on the outer surfaces of the bearing. However, the thermal spray processes required for ceramic coatings are expensive and create porous coatings. Therefore, the ceramic coatings are sealed with polymer to ensure the effectiveAttorney Docket No. 023644-0007-US01 electrical insulation under humid conditions and must be finish-machined to meet dimensional tolerances and surface finishes, all of which greatly add to the manufacturing costs.

[0004] There is a need to provide a durable, inexpensive, and scalable coating solution for insulating bearing elements from stray currents and arcing. Polymers and polymer composites have a demonstrated track record of being cost-effective electrical insulators, given their widespread use in isolating electronic components in electronic devices and exterior insulation on electrical wiring. Numerous polymer systems based on epoxy, polypropylene, polyester, polycarbonate, and fluropolymers are characterized by attributes that make them useful in this regard, including high dielectric strength, low dielectric constants, low permeability of water vapor, and low processing temperatures. Among these, fluoropolymers are distinguished in being low-friction or lubricious materials that may have ancillary benefits of fretting wear resistance and non-stick properties.

[0005] Prior work has demonstrated that these properties of polymers can be harnessed as coatings that form electrically insulating layers on the exterior surfaces of bearings. In one known method described in U.S. Patent No. 11,708,859, polymer powders are electrostatically sprayed on the exterior surfaces of bearing rings and heated to melt and cure the powders into continuous, electrically insulating layers. These relatively thick coatings (100-500 micrometers) are generally only applied to the outer surfaces of the bearing and cannot withstand the heavy stresses applied on certain bearing surfaces during installation and operation. Consequently, they cannot be applied to the heavily stressed bearing bore surfaces, which can be the entry point for stray currents emerging from the drivetrains. In another method described in U.S. Patent Application Publication No. 2021 / 0140479A1, thermoplastic resin coatings are proposed to be applied to the outer surfaces of a bearing, and in conjunction with an insulating washer, prevent electrolytic corrosion of the bearing by preventing the passage of electric current. Thus, the polymer coatings in the prior work fail to simultaneously provide electrical insulation and fretting wear protection to the bearings.Attorney Docket No. 023644-0007-US01SUMMARY

[0006] Considering the aforementioned state of the art, the following disclosure provides a method for effectively providing thin polymer composite coating layers capable of providing electrical insulation resistance for bearings and mitigating fretting wear on exterior contact surfaces.

[0007] The proposed disclosure utilizes spray coating processes to selectively coat the exterior surfaces of roller bearing rings with low-porosity polymer composite layers containing at least the following components: (a) thermosetting polymer resin binder, (b) lubricious thermoplastic fluoropolymer, and (c) pigmentation compounds. The as-deposited fdms are hardened by subsequent heating steps that evaporate the carrier solvent in the coating solution and cure or melt the remaining polymeric components at temperatures that do not compromise the prior heat-treated microstructure of the hardened and tempered bearing steel. Careful selection of the amounts and type of filler compounds will electrically insulate the bearing, aid in press fitting the bearing on the shaft, and provide additional fretting wear protection during operation at the bore / shaft interface. The coated bearing element provides protection against electrical discharge at DC voltages of at least 1000 V, under both dry and wet conditions, and AC voltages of at least 1000 V at frequencies of up to 6 kHz.

[0008] The present disclosure provides, in one aspect, a bearing including an inner ring, an outer ring surrounding the inner ring, a plurality of rolling elements supported between the inner ring and outer ring, and a non-stick coating provided on an exterior surface of at least one of the inner ring and the outer ring. The non-stick coating further provides electrical insulation to the bearing.

[0009] The present disclosure provides, in another aspect, a method of applying a non-stick, electrically insulating coating to a bearing component, the method including priming, using phosphating chemical surface conversion, an exterior surface of the bearing component; degreasing, using a solvent, the bearing component; applying masking to rolling / sliding contact surfaces of the bearing component; spraying, using a wet spray gun apparatus, a polymer coating on the exterior surface of the bearing component; drying the bearing component; curing theAttorney Docket No. 023644-0007-US01 polymer coating; and removing the masking from the rolling / sli ding contact surfaces of the bearing component.

[0010] Other features and aspects of the disclosure will become apparent by consideration of the following detailed description and accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. l is a perspective view of a bearing.

[0012] FIG. 2A is a schematic view of a portion of the bearing of FIG. 1 supporting a shaft and mounted in a housing, with only the outer ring of the bearing coated.

[0013] FIG. 2B is a schematic view similar to FIG. 2A except that only the inner ring of the bearing is coated.

[0014] FIG. 2C is a schematic view similar to FIG 2A with both the outer ring and the inner ring of the bearing coated.

[0015] FIG. 3 is a schematic view of a coating apparatus for applying a coating to the bearing of FIG. 1.

[0016] FIG. 4 is a flow-chart of a method for applying a coating to the bearing of FIG. 1.

[0017] Before any embodiments of the disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.DETAILED DESCRIPTIONAttorney Docket No. 023644-0007-US01

[0018] FIGS. 1 and 2A-C illustrate an assembled roller bearing 100 for use within a wind turbine 10. It should be noted that while the bearing 100 of the present embodiment is contemplated to be within a wind turbine, the bearing 100 may be used in any of a plurality of different machines or applications (e.g., electric motors, generators, etc.). Additionally, while the disclosure describes a rolling element bearing, the coating and methodology disclosed herein can also be applied to plain bearings (e.g., bearings with no rolling elements). In the present embodiment, the bearing 100 is disposed within a bearing housing 102. The bearing 100 supports a shaft 106 within the wind turbine 10. The bearing 100 includes an inner ring 104, an outer ring 108, together defining a bearing raceway 110 that supports rolling elements 111. The inner ring 104 includes a bearing inner surface 112 which defines an inner diameter or bore of the bearing 100, an inner ring outer surface 113 which defines an outer diameter of the inner ring 104, oppositely-facing inner ring axial faces 116, and an inner ring shoulder 120 configured as a chamfer between the bearing inner surface 112 and the respective inner ring axial faces 116. The outer ring 108 includes a bearing outer surface 124 which defines an outer diameter of the bearing 100, an outer ring inner surface 125 which defines an inner diameter of the outer ring 108, oppositely-facing outer ring axial faces 128, and an outer ring shoulder 132 configured as a chamfer between the bearing outer surface 124 and the respective outer ring axial faces 128. A bearing cage 136 is disposed between the inner ring 104 and the outer ring 108 and maintains the proper positioning of the rolling elements 111.

[0019] Selected exterior surfaces of the bearing 100 are covered with a thin, polymer, nonstick coating 140 configured as a protection barrier for the bearing 100. More specifically, the coating 140 is configured to provide electrical insulation for the bearing 100 and to mitigate fretting wear on exterior contact surfaces, such as the bearing outer surface 124 and / or the bearing inner surface 112. In the present embodiment, the illustrated coating 140 is 954G-304 One Coat non-stick finish available from The Chemours Company of Wilmington, Delaware. The -304 portion of the coating numbering scheme relates only to the color of the coating. Other colors of the base 954G coating are also contemplated for use. The coating 140 is a mixture of thermosetting binder resins (e.g., epoxy, phenolics), thermoplastic fluoropolymer components (e.g., fluorinated ethylene propylene, polytetrafluoroethylene), functional fillers (e.g., pigmenting compounds, ceramic particles or fibers), and curing catalysts which are dispersed in a carrier solvent and spray-coated in liquid form on select surfaces of the bearing 100 (e.g., the bearingAttorney Docket No. 023644-0007-US01 inner surface 1 12, the inner ring axial faces 116, the inner ring shoulders 120, the inner ring outer surface 113 excluding the raceway 110, the bearing outer surface 124, the outer ring axial faces 128, the outer ring shoulders 132, and the outer ring inner surface 125 excluding the raceway 110). It is helpful to avoid fillers and pigmenting compounds that are electrically conductive, such as carbon black particles, which are commonly used to pigment black-colored organic coatings. Conductive fillers in amounts as low as 0.5 % are sufficient to make the polymer composite electrically conductive and compromise the insulating properties. In alternate embodiments, the coating may be other thin, polymer, non-stick coatings that can be applied and cured in a similar manner to the coating 140 discussed herein.

[0020] In a first application or embodiment shown in FIG. 2A, the coating 140 is applied only to the bearing outer surface 124, the outer ring inner surfaces 125 excluding the raceway 110, the outer ring axial faces 128, and the outer ring shoulders 132. In a second application or embodiment shown in FIG. 2B, the coating 140 is applied only to the bearing inner surface 112, the inner ring outer surfaces 113 excluding the raceway 110, the inner ring axial faces 116, and the inner ring shoulders 120. In a third application or embodiment shown in FIG. 2C, the coating 140 is applied to the bearing inner surface 112, the inner ring outer surfaces 113 excluding the raceway 110, the inner ring axial faces 116, the inner ring shoulders 120, the bearing outer surface 124, the outer ring inner surfaces 125 excluding the raceway 110, the outer ring axial faces 128, and the outer ring shoulders 132. Each embodiment allows for the bearing 100 to be pressed into or interference fitted into the bearing housing 102 and onto the shaft 106, respectively, due to the presence of dry lubricating fluoropolymer resins in the coating 140. In the second and third applications, the coating 140 is configured to withstand high hoop stresses of at least 80 MPa applied on the bearing element during bearing installation onto the shaft. The coating 140 is further configured to withstand fretting wear and fretting corrosion on the shaft 106 as a result of bearing vibration during operation of the wind turbine 10.

[0021] FIG. 3 is a schematic illustration of a portion of the bearing 100 and a coating apparatus 144 configured for applying the coating 140 to the bearing 100. The coating apparatus 144 is known to practitioners and is commercially available as a wet spray gun apparatus. The coating apparatus 144 includes a coating solution reservoir 148 for housing a liquid form of the coating 140 prior to being applied to the bearing 100, a pressurized air supply 152, and a sprayAttorney Docket No. 023644-0007-US01 gun 156 having a spray gun nozzle 160. In operation, pressurized air enters the spray gun 156 to force the coating solution through the spray gun nozzle 160, thereby aerosolizing the coating 140 and allowing a user to apply the coating 140 to the bearing 100. An appropriate separation between the nozzle 160 and the bearing 100 and a nozzle tip size must be selected and maintained to obtain a uniform application of the polymer coating 140. Mechanical or adhesive masking 142 may be used to prevent coating of any rolling / sliding contact surfaces of the bearing 100 (e.g., the raceway 110, including the raceway surface itself and the flanges at either axial end of the raceway surface, etc.).

[0022] FIG. 4 illustrates a method 200 for applying the coating 140 to the bearing 100. At step 201, the bearing 100 is degreased and chemically pre-treated with a phosphate conversion layer (e g., zinc phosphate, manganese phosphate) to prime the surface. Step 201 further includes immersing via any method, such as immersion in water, in displacing fluid and drying to remove excess water from the phosphated surface. Step 201 may also include masking of the rolling / sliding contact surfaces by any method known, which masking may be removed after the phosphate pre-treatment is completed. The phosphate conversion is configured to be added as a continuous intermediate layer containing inorganic metal phosphates, applied according to guidelines in known industry standards such as ISO 9717, between the exterior surface of the bearing 100 and the polymer coating 140.

[0023] At step 202, the bearing is further degreased in an organic solvent (e.g. methyl ethyl ketone, naptha, etc.) and pre-baked in an oven at low temperatures (e.g., 150 °C) to ensure clean dry surfaces free of debris and surface contaminations. At step 203, mechanical or adhesive masking is applied to surfaces of the bearing 100 that are not desired to be coated in the polymer coating 140 (e.g. the bearing raceway 110). At step 204, the polymer coating 140 mixture is forced through a spray nozzle 160, driven by pressurized air, to form a film, or coating 140, on select surfaces of the bearing 100. At step 205, any residual solvent is evaporated from the as- coated film by drying in an oven at low temperatures (e.g., 50-60°C) for a prescribed time. At step 206, the coated part is subsequently oven-baked at less than 180°C for a prescribed time to cure the thermosetting resin components and melt the thermoplastic components. Alternately, laser curing at step 206 may be used in lieu of oven baking to cure the thermosetting resin without heating the steel bearing component when heating the steel above 160°C risks overAttorney Docket No. 023644-0007-US01 tempering and compromising the mechanical properties of the bearing component. The spraying and heating sequences (steps 204, 205, and 206) may be repeated to progressively build up the coating 140 to a desired thickness while minimizing coating defects (porosity, blistering, orange peel, etc.). At step 207, the masking is removed from the surfaces of the bearing 100 which are not desired to be coated with the polymer coating 140.

[0024] After the final heating step (step 206) of method 200, the coating 140 has a thickness of between 35-55 micrometers and acts as a barrier against electrical current and provides protection against discharge at various voltages. Accordingly, the coating 140 achieves dielectric breakdown voltages under DC voltage excitation of at least 1000 V DC in ambient air conditions or after water immersion and removal of visible water. In ambient air conditions under AC voltage excitation at up to 6 kHz, the coating 140 provides protection against at least 1000 V AC.

[0025] Various features of the disclosure are set forth in the following claims.

Claims

Attorney Docket No. 023644-0007-US01CLAIMSWhat is claimed is:

1. A bearing comprising: an inner ring; an outer ring surrounding the inner ring; a plurality of rolling elements supported between the inner ring and the outer ring; and a non-stick coating provided on an exterior surface of at least one of the inner ring or the outer ring; wherein the non-stick coating further provides electrical insulation to the bearing.

2. The bearing of claim 1, wherein the non-stick coating is a 954G One Coat nonstick finish available from The Chemours Company of Wilmington, Delaware.

3. The bearing of claim 1, wherein the non-stick coating has a thickness of between 35 and 55 micrometers.

4. The bearing of claim 1, wherein the inner ring includes a bearing inner surface which defines an inner bore of the bearing, and wherein the non-stick coating is present on the bearing inner surface and prevents fretting wear at an inner bore-shaft interface.

5. The bearing of claim 4, wherein the outer ring includes a bearing outer surface which defines an outer diameter of the bearing, and wherein the non-stick coating is provided on the bearing outer surface.Attorney Docket No. 023644-0007-US016. The bearing of claim 1 , wherein the outer ring includes a bearing outer surface which defines an outer diameter of the bearing, and wherein the non-stick coating is provided on the bearing outer surface.

7. The bearing of claim 1, wherein the inner ring includes oppositely facing inner ring axial faces, and wherein the non-stick coating is provided on the inner ring axial faces.

8. The bearing of claim 1, wherein the outer ring includes oppositely facing outer ring axial faces, and wherein the non-stick coating is provided on the outer ring axial faces.

9. The bearing of claim 1, wherein the inner ring includes an inner ring shoulder and an inner ring outer surface, and wherein the non-stick coating is provided on the inner ring shoulder and the inner ring outer surface.

10. The bearing of claim 1, wherein the outer ring includes an outer ring shoulder and an outer ring inner surface, and wherein the non-stick coating is provided on the outer ring shoulder and the outer ring inner surface.

11. The bearing of claim 1, wherein the non-stick coating is configured to be applied using a wet spray gun.

12. The bearing of claim 1, wherein the non-stick coating has a dielectric breakdown voltage of at least 1000 volts AC in dry conditions at frequencies to at least 6 kHz, at least 1000 volts DC in dry conditions, and at least 1000 volts DC in wet conditions.

13. The bearing of claim 1, wherein the non-stick coating is a mixture of thermosetting binder resins, thermoplastic fluoropolymer components, functional fillers, and curing catalysts which are dispersed in a carrier solvent.Attorney Docket No. 023644-0007-US0114. A method of applying a non-stick, electrically insulating coating to a bearing component, the method comprising: priming, using a phosphate surface conversion, an exterior surface of the bearing component; degreasing, using a solvent, the bearing component; applying masking to rolling / sliding contact surfaces of the bearing component; spraying, using a wet spray gun apparatus, a polymer coating on the exterior surface of the bearing component; drying the bearing component; curing the polymer coating; and removing the masking from the rolling / sliding contact surfaces of the bearing component.

15. The method of claim 14, wherein curing the polymer coating includes baking the bearing component in an oven.

16. The method of claim 14, wherein the phosphate surface conversion provides a coating to the bearing component, wherein the coating provided by the phosphate surface conversion is an intermediate layer between the exterior surface of the bearing and the polymer coating.

17. The method of claim 14, further comprising repeating the spraying, drying, and baking steps of the method to achieve a coating thickness of between 35 and 55 micrometers.

18. The method of claim 14, wherein the bearing component is a bearing inner ring including an inner bearing surface which defines an inner bore of the bearing, and wherein the method further includes spraying the coating on the inner bearing surface.

19. The method of claim 14, wherein the bearing component is a bearing outer ring including a bearing outer surface which defines an outer diameter of the bearing, and wherein the method further includes spraying the coating on the bearing outer surface.

20. The method of claim 14, wherein the solvent is an organic solvent.

21. The method of claim 14, wherein the masking is a mechanical or adhesive masking.Attorney Docket No. 023644-0007-US0122. The method of claim 14, wherein the coating is configured to withstand hoop stresses of at least 80 MPa applied on the bearing component during bearing installation onto a shaft.

23. The method of claim 14, wherein curing the polymer coating includes heating the coating with a laser.

24. The method of claim 14, wherein the polymer coating is a 954G One Coat nonstick finish available from The Chemours Company of Wilmington, Delaware.

25. The method of claim 14, wherein the polymer coating is a mixture of thermosetting binder resins, thermoplastic fluoropolymer components, functional fillers, and curing catalysts which are dispersed in a carrier solvent.

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