Abstract:
A non-conventional, low temperature, process for applying a thin electrically insulating coating arrangement with high density, high purity, minimal porosity, and improved adhesion strength to a steel bearing component is provided. The bearing component is formed from steel and machined or otherwise formed to a near net shape. A high purity aluminum is electro-chemically deposited on the steel bearing component using a non-aqueous electrolyte in an inert environment to form a high purity aluminum coating at least over a portion of the steel bearing component. A surface of the high purity aluminum coating is then converted by an acid-bath into aluminum oxide to form an insulating layer. A bearing component and a bearing having such components is also provided.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to devices for insulating shaft electrical currents in electrical machines such as AC and DC electrical motors, generators, alternators, and other rotating shaft machines. More specifically, the invention relates ceramic electrically insulated rolling element bearings. 
       BACKGROUND 
       [0002]    The advent, development, and increasing use of electrical machines coupled to Pulse Width Modulated (PWM) variable speed inverter drives utilizing rapid switching devices such as Insulated Gate Bi-Polar Transistors (IGBTs) has resulted in an increase in rolling element bearing failures in electrical machines. The reduced bearing life and increased rate of bearing failures is due to bearing currents flowing from the rotating shaft to the machine frame through the bearings. Such bearing currents manifest as damage due to Electrical Discharge Machining (EDM) between the bearing raceways and rolling elements thus causing physical mechanical damage of the bearing raceways and rolling elements which may ultimately cause premature failure of the bearing assembly. 
         [0003]    Shaft induced electrical currents are inherent to electric motors and generators due to mechanical asymmetries in the electrical machine. Additionally, due to the very high rate of switching frequencies of PWM inverter drives, the Common Mode Voltage (CMV) is increased over non-PWM driven machines. This increased CMV causes a difference in electrical potential between the rotor shaft and the stator frame of the machine. When the CMV potential exceeds the impedance of the bearings, a current is induced in the shaft, and the current passes through the bearings seeking ground, potentially causing EDM damage to the bearings. 
         [0004]    During static (stationary) conditions the lubricant (e.g. oil or grease) is displaced between the rolling element and raceway mating surfaces in contact and under load thus causing a low resistance for electrical currents to pass between the rolling elements and raceway surfaces. 
         [0005]    During transient (startup condition greater than ˜15% of rated rotational speed) and normal steady-state operating conditions, a lubricant film thickness is generated and maintained between the rolling element and raceway surfaces. This Elasto-Hydrodynamic Lubricant (EHL) film thickness causes the mating surfaces to separate from contact and “ride” on a film of lubricant. 
         [0006]    Many methods have been employed to prevent shaft induced currents from causing EDM damage to the bearings. Such technologies include dielectric ceramic coatings such as aluminum oxide or silicon oxide on the bearing external surfaces or bearing housing, use of Faraday shields to prevent the charge build-up on shafts, electrically conductive bearing grease, and shaft-contacting ground brushes made of electrically conductive materials such as copper, brass, or carbon. 
         [0007]    Di-electric ceramic bearing coatings have been utilized with some success but have several drawbacks. The coating is very hard and brittle and can fracture during installation or after installation under mechanical load. These di-electric ceramic coatings have limited di-electric strength due to porosity. 
         [0008]    Typically, these ceramic insulating coatings are applied to the metal substrate through the High Velocity Oxygen Flame (HVOF), Low Pressure Plasma, or other high temperature deposition processes. The resulting coating is inherently porous due to an imperfect deposition process. Additionally, the ceramic material must be heated to very high temperatures during the deposition process which can negatively affect the metal substrate material (e.g. material microstructure, hardness, heat treatment). The metal deposition process such as HVOF or Low Pressure Plasma also produces a relatively impure ceramic coating. The resulting coating will have limited di-electric strength due to the porous and impure structure of the coating, which includes numerous voids and impurities. At high voltage potential, current can pass through these voids and imperfections inherent within the coating structure, causing an arc that compromises the coating, passing through the coating and potentially causing EDM damage to the bearing. 
         [0009]    The coating porosity can also promote crack formation when mechanical load is applied to the coating. The resulting cracks can allow current to pass through the coating resulting in damage to the bearing. 
         [0010]    Ceramic insulating coatings applied via HVOF, or Low Pressure Plasma deposition processes also have limited adhesion strength to the substrate material. 
         [0011]    Faraday shields have been employed with success but are very expensive to implement in most applications in industry. 
         [0012]    Electrically conductive grease allows the current to continuously pass through the bearing surfaces but may contain elements such as copper, carbon, or phosphorus which can cause excessive wear of the bearing surfaces leading to premature failure of the bearing. 
         [0013]    Spring loaded shaft grounding brushes are also prone to issues such as excessive wear, material transfer, and oxide formation on the mating surface. Such oxides can become a di-electric and may prevent effective current transfer from the shaft to ground. This can cause electrical arcing and lead to damage of the shaft. Additionally, once the impedance of the shaft grounding brush exceeds the impedance of the bearings, the current can seek ground through the bearings once again potentially causing EDM damage to the bearings. Shaft grounding brushes may also vibrate due to imperfections in the shaft surface such as an out of round condition. When the brush momentarily loses contact with the shaft during vibration, the current can arc from the brush to the shaft thus causing EDM damage to the shaft. Alternatively, the current can flow through the bearings if the voltage potential is sufficiently high while the brush loses contact with the shaft during vibration. Traditional spring loaded shaft grounding brushes are prone to rapid and excessive wear and can ultimately lose contact with the shaft causing the current to seek ground through the bearing potentially causing EDM damage in the bearing. Finally, the shaft grounding brushes are not usually installed close to the bearing which is usually the component of most concern. Close proximity of grounding near the bearing is essential to protecting the integrity of the bearing surfaces. 
         [0014]    What is needed in the art is an electrically insulating coating with high density, high purity, minimal porosity, and improved adhesion strength to the substrate material. 
         [0015]    Historically, the HVOF, Low Pressure Plasma, and other high temperature material deposition processes have been utilized to deposit an aluminum oxide ceramic coating, or variant thereof, to the metal bearing substrates (steel bearing inner or outer rings). This is due to conventional acid bath electrochemical depositing (plating) processes not being feasible for depositing aluminum to steel substrates. 
       SUMMARY 
       [0016]    A non-conventional, low temperature, process for applying a thin electrically insulating coating arrangement with high density, high purity, minimal porosity, and improved adhesion strength to a steel bearing component is provided. The bearing component is formed from steel and machined or otherwise formed to a near net shape. A high purity aluminum is first electro-chemically deposited on the steel bearing component using a non-aqueous electrolyte, such as Toluene, in an inert environment to form a high purity aluminum coating at least over a portion of the steel bearing component. A surface of the high purity aluminum coating is then converted by an acid-bath into aluminum oxide. This is typically referred to as acid anodization, and forms an insulating layer. 
         [0017]    Optionally, the insulating layer is sealed. 
         [0018]    Preferably, the high purity aluminum has a final thickness of 25-200 μm, and more preferably 50-100 μm. 
         [0019]    Preferably, the insulating layer has a final thickness of 25-200 μm, and more preferably 50-100 μm. 
         [0020]    Preferably the high purity aluminum and the aluminum oxide insulating layer have less than 2% voids or inclusions, and more preferably less than 0.2% voids or inclusions. In accordance with the preferred method, there are virtually zero voids or inclusions as indicated by a test sample examined at a 2500:1 scale enlargement. 
         [0021]    In one embodiment, a surface of the steel bearing component to be coated with the high purity aluminum has a surface roughness Ra of greater than 4 μm for enhanced adhesion of the high purity aluminum. 
         [0022]    In one embodiment, a raceway surface of the bearing component is masked prior to the electro-chemical coating with pure aluminum and/or the acid bath conversion of the surface of the high purity aluminum coating into aluminum oxide. 
         [0023]    Preferably, the insulating layer or the sealed insulating layer has a resistance of at least 10 MΩ at 50 VDC. More preferably the insulating layer or the sealed insulating layer has a resistance of at least 50 MΩ at 1000 VDC. 
         [0024]    Optionally, the raceway surface of the steel bearing component can be finished after forming the insulating layer. 
         [0025]    A steel bearing component as well as a rolling bearing having a steel bearing component with the insulating layer formed by the above method is also provided. The rolling bearing preferably includes an inner ring, outer ring, and a plurality of rolling elements, such as balls or rollers. The inner ring and/or the outer ring include the insulating coating applied to, at a minimum, either the outer mounting surface of the outer ring or the inner mounting surface of the inner ring, or both inner and outer ring mounting surfaces. This provides an EDM resistant rolling bearing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The foregoing Summary as well as the following Detailed Description will be best understood when read in conjunction with the appended drawings. In the drawings: 
           [0027]      FIG. 1  is an elevational view of a rolling bearing including one or more bearing components with an insulating coating. 
           [0028]      FIG. 2  is a greatly enlarged cross-sectional view through either of the bearing rings as indicated by lines  2 - 2  in  FIG. 1  showing the insulating coating applied to one or both of the bearing rings. 
           [0029]      FIG. 3  is a greatly enlarged cross-sectional view showing the insulating coating on the bearing component of  FIG. 2 . 
           [0030]      FIG. 4  is a flow chart showing the process for applying and insulating coating on a bearing component according to one embodiment. 
           [0031]      FIG. 5  is a 500× magnification photo of an example of the insulating coating on the bearing component. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Certain terminology is used in the following description for convenience only and is not limiting. The words “inner,” “outer,” “inwardly,” and “outwardly” refer to directions towards and away from the parts referenced in the drawings. A reference to a list of items that are cited as “at least one of a, b, or c” (where a, b, and c represent the items being listed) means any single one of the items a, b, c or combinations thereof. The terminology includes the words specifically noted above, derivatives thereof, and words of similar import. 
         [0033]    Referring to  FIG. 1 , a rolling bearing  10  including an outer ring  12  and an inner ring  14  is shown. The outer ring  12  and the inner ring  14  are bearing components formed of bearing grade steel. Rolling elements  16  are located between the outer ring  12  and the inner ring  14 . The rolling elements are preferably spaced apart via a cage  18 . 
         [0034]    As shown in detail in  FIG. 2 , at least one of the inner ring  14  or the outer ring  12  include, at least over a portion thereof, an insulating coating arrangement  30  having a layer of high purity aluminum  32  located on the steel used to form the inner ring  14  or the outer ring  12 . The high purity aluminum coating  32  includes an aluminum oxide insulating layer  34  on an outer surface thereof that is formed from the high purity aluminum layer  32 . The layers in  FIG. 2  are not shown to scale. 
         [0035]    The high purity aluminum layer  32  and the aluminum oxide insulating layer  34  preferably have less than 2% voids or inclusions, and more preferably less than 0.2% voids or inclusions. In accordance with one embodiment, there are virtually zero voids or inclusions as indicated by a test sample examined at a 500:1 scale enlargement. This is shown in  FIG. 5 . 
         [0036]    As shown in detail in  FIG. 3 , the aluminum oxide insulating layer  34  can be sealed, as indicated by seal layer  36 . This seal layer can be formed via a hot water seal in which the aluminum oxide layer or at least an outer surface thereof is converted to a hydrated form which reduces the pore size. A further coating such as Teflon or any other insulating sealant can optionally be used with or in place of the hot water seal. 
         [0037]    As shown in  FIG. 2 , preferably the insulating coating arrangement  30  is located on the outer mounting surface  26  of the outer ring  12  and the raceway  22  does not include the insulating layer. Alternatively, the inner mounting surface  28  of the inner ring  14  includes the insulating coating arrangement  30  and the raceway  24  of the inner ring  14  does not include the insulating coating arrangement  30 . It is also possible for both the outer ring  12  and the inner ring  14  to include the insulating coating arrangement  30 . The aluminum oxide insulating layer  34  preferably has an electrical resistance of at least 10 MΩ at 50 VDC. More preferably, the aluminum oxide insulating layer  34  or aluminum oxide insulating layer  34  with the seal layer  36  has a resistance of at least 50 MΩ at 1000 VDC. 
         [0038]    Preferably, the high purity aluminum layer  32  has a final thickness of 25-200 μm. Additionally, the aluminum oxide insulating layer  34  has a final thickness of 25-200 μm. More preferably, the high purity aluminum layer  32  and the aluminum oxide insulating layer  34  each have a thickness of 40-75 μm. Preferably, the total coating thickness is 50-400 μm and more preferably in the range of 75-125 μm. 
         [0039]    Referring to  FIG. 4 , a method for providing an electrically insulating coating on a bearing component, which can be the outer ring  12  and/or the inner ring  14  is provided in the flowchart  40 . The method includes providing the bearing component formed from steel and machined or otherwise form to a near net shape. As show in box  41 , this semi-finished bearing component is preferably a bearing ring and all surfaces are finished to final dimensions with the possible exception of the raceway which may need final finishing after the insulating coating arrangement  30  is applied. Preferably, these bearing components are washed or cleaned as indicated in box  42 . This cleaning is by known processes, using detergents and/or solvents. If necessary, the surface of the bearing components to be coated can be roughened as indicated in box  43 . Preferably, roughening is done only on the areas to be coated and not the raceways  22 ,  24 , and a surface roughness of Ra≧6 μm is provided to enhance bonding of high purity aluminum onto the surfaces of the bearing component to be coated. A final cleaning step can be provided as indicated in box  44  which can include rinsing with a solvent and/or detergent to remove any impurities including surface oils which could prevent proper adhesion of the coatings to be applied. As indicated in box  45 , preferably the raceways  22 ,  24  of the bearing components in the form of bearing rings  12 ,  14  are masked. The masking can be clone with a polyester film or tape and an appropriate adhesive, which can be silicone based, or can be done with caps or plugs which are resistant to the electroplating chemicals and/or the anodizing acid. The mask is applied in a known manner to areas that are not to be coated or treated, such as the raceways  22 ,  24 . 
         [0040]    As shown in box  46 , an anode is applied to each of the bearing components, in the form of the outer and inner rings  12 ,  14 , and the bearing components are electroplated with high density pure aluminum, as shown in box  47 , preferably to a thickness of at least about 50 μm (0.002 in). The coated thickness may vary between 25 and 200 μm depending on the application and the further processing to be carried out. The electroplating is carried out using a non-acquiesce electrolyte, such as toluene, in an inert environment which allows the pure aluminum to be deposited onto the steel bearing components in a highly pure and dense state without inclusions or voids. As shown in box  48 , preferably the thickness of the aluminum is verified. 
         [0041]    This highly pure aluminum is then hard coat anodized as indicated in box  49 , preferably to 50% to 60% of the thickness of the high purity aluminum layer  32 , resulting in the layer of pure aluminum  32  as well as the anodized insulating layer  34  as shown in  FIG. 3  which are formed on the steel bearing component in the form of the bearing rings  12 ,  14 . Preferably, the anodization is a sulfuric acid anodization or alternately oxalic acid anodization with the bearing component forming the anode of the electrical circuit in the anodization bath. As shown in box  50 , optionally a dye may be applied to the anodized surface in order to provide a color of choice to the bearing component. The anodized insulating layer  34  can optionally be sealed. 
         [0042]    As shown in box  51 , preferably the total thickness of the anodized layer  34  and the pure aluminum layer  32  is non-destructively verified. The total layer thickness for each layer is preferably in range of 50-200, and more preferably μm 40-75 μm. Preferably, the total coating thickness is 50-400 μm and more preferably in the range of 75-125 μm. In one embodiment, the total layer thickness is nominally 100 μm (0.004 in). 
         [0043]    As shown in box  52 , the raceway masking can then be removed and any final finishing or machining of the raceways  22 ,  24  can then be performed. Finally, as indicated in box  53 , preferably electrical testing of the coating is performed to ensure that the insulating layer has a resistance of at least 10 MΩ at 50 VDC more preferably a resistance of at least 50 MΩ at 1000 VDC. 
         [0044]    This arrangement provides for an EDM resistant rolling bearing component and more preferably a EDM resistant roiling bearing assembly such as the rolling bearing  10  shown in  FIG. 1  in which one or both of the rings  12 ,  14  include the insulating coating arrangement  30 . 
         [0045]    The embodiments described above are considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore to be embraced therein.