FIELD
The present disclosure relates to the field of roller bearings.
BACKGROUND
Often it is found that the bearings used in electrical equipment such as electric motors and power generators are subject to electrical pitting. There is a high possibility of electric current passing through the bearings used in such electrical 5 equipment. This results in sparks that damage the rolling element surface ultimately leading to bearing failure.
A simple solution to prevent this is by electrically insulating the bearing by providing an insulating coating over the bearing surface. The commonly used coatings that are used in the state-of-the-art are ceramic based or resin based 10 coatings. Ceramic coatings are very expensive and take a long time for manufacture. Besides, the grinding of ceramics is costly and due to their hardness the grinding requires the use of diamond wheels. Also, most ceramic based coatings like alumina coatings are porous in nature, due to which they allow passage of current at higher potential differences and cause sparks. 15
Resin based coatings are a cheaper and an effective substitute to these ceramic based coatings. The most commonly used resin based coating is a polyphenylene sulphide based coating. It shows good electrical insulation properties. However, currently, all resin based coatings, including polyphenylene sulphide coatings, are used for electrical insulation on stationary parts only and are not suitable for 20 movable parts as the coated surfaces have a high co-efficient of friction. The high co-efficient of friction leads to abrasive wear of the bearing components. So, these coatings are not suitable for application on bearing components which are subject to continuous motion. Also, many coatings cannot withstand a very high potential difference, thereby, limiting the service life of the bearings. 25
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Hence, there is a felt need to provide electrically insulated bearings which can withstand a high potential difference and has an increased service life.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows. 5
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
An object of the present disclosure is to provide an electrically insulated rolling element bearing.
Another object of the present disclosure is to provide an electrically insulated 10 rolling element bearing that has a high performance with low noise and low friction.
Still another object of the present disclosure is to provide an electrically insulated rolling element bearing that has an increased service life.
Other objects and advantages of the present disclosure will be more apparent from 15 the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
An electrically insulated low friction rolling element bearing is disclosed herein. The roller element bearing comprises an inner ring, an outer ring and rolling 20 elements which are in rolling contact with the raceways of the inner ring and the outer ring. At least one of an outer surface of the outer ring and the raceway of at least one of the rings is coated with a layer of a cured epoxy composition. In an embodiment, the layer of cured epoxy composition is achieved by spray or brush coating. 25
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The epoxy composition in an uncured form comprises 100 parts of an epoxy resin, 5 to 20 parts of a hardener, 5 to 20 parts of at least one filler and 5 to 20 parts of an oil selected from perfluoropolyether oil and highly refined mineral oil.
Typically, the layer of cured epoxy composition has a thickness in the range of 150 μm to 200 μm. 5
Typically, the epoxy resin is at least one selected from the group consisting of Bisphenol-A diglycidyl ether polymer, Bisphenol-F diglycidyl ether polymer, and Novolac epoxy resin. Typically, the hardener is selected from the group consisting of polyfunctional amines, acids, acid anhydrides, phenols, alcohols, and thiols.
Typically, the filler is at least one selected from the group consisting of talc and 10 graphite.
Typically, the bearings that are coated with the epoxy composition of the present disclosure are selected from the group consisting of a deep groove ball bearing, cylindrical roller bearing, spherical roller bearing, Y-bearing, tapered roller bearing, self-aligning ball bearing, thrust ball bearing, cylindrical roller thrust 15 bearing, tapered roller thrust bearing, spherical roller thrust bearing, angular contact ball bearing, magnetic bearing, gear bearing, needle roller bearing, needle roller thrust bearing, angular contact thrust ball bearing, combined cylindrical/taper roller bearing, double row angular contact bearing, and combined needle roller bearing. 20
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The electrically insulated low friction rolling element bearing of the present disclosure will now be described with the help of the accompanying drawing, in which:
Figure 1 illustrates a schematic cross-sectional view of a cylindrical roller bearing 25 with the outer surface of the outer ring coated with a layer of a cured epoxy composition (A);
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Figure 2 illustrates a schematic cross-sectional view of a ball bearing depicting the raceways of the inner ring and the outer ring, each coated with a layer of a cured epoxy composition, the layers indicated by A;
Figure 3 illustrates the surface microstructure of a layer of cured epoxy composition coated over an outer surface of the outer ring of a cylindrical roller 5 bearing observed using an optical microscope;
Figure 4 illustrates a schematic representation of a set-up to determine the electrical resistance of a coated component of a rolling element bearing coated with a layer of a cured epoxy composition;
Figure 5 illustrates the variation of co-efficient of friction of the epoxy coating of 10 the present disclosure with the number of cycles measured using a ball on disc tribometer;
Figure 6 illustrates the number of cycles withstood (wear life) by the rotating disk coated with the epoxy composition of the present disclosure against the applied contact stress measured using a ball on disc tribometer; and 15
Figure 7 illustrates an FTIR graph of the cured epoxy composition coated on a surface of a rolling element bearing.
DETAILED DESCRIPTION
Rolling element bearings with alumina or polyphenylene sulphide coatings are commonly used as electrically insulated bearings. However, these bearings do not 20 have the performance characteristics at high potential differences and hence do not have a long service life. The bearings with resin based (polyphenylene sulphide) coatings also do not last long because only their stationary parts can be coated with the resin. As these resins have a high co-efficient of friction which may lead to abrasion wear, they are not suitable for movable parts. 25
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The present disclosure, therefore, envisages a rolling element bearing having a long service life which is coated with a substance that insulates it electrically even at very high potential differences.
Such an electrically insulated rolling element bearing is being disclosed herein. The rolling element bearing comprises an inner ring, an outer ring and rolling 5 elements which are in rolling contact with the raceways of the inner ring and the outer ring. At least one of an outer surface of the outer ring and the raceway of at least one of the rings is coated with a layer of a cured epoxy composition. In an embodiment, the coating is performed by spray or brush coating.
The bearings that are coated with the epoxy composition of the present disclosure 10 are selected from the group consisting of a deep groove ball bearing, cylindrical roller bearing, spherical roller bearing, Y-bearing, tapered roller bearing, self-aligning ball bearing, thrust ball bearing, cylindrical roller thrust bearing, tapered roller thrust bearing, spherical roller thrust bearing, angular contact ball bearing, magnetic bearing, gear bearing, needle roller bearing, needle roller thrust bearing, 15 angular contact thrust ball bearing, combined cylindrical/taper roller bearing, double row angular contact bearing and combined needle roller bearing.
Figure 1 illustrates a schematic cross-sectional view of a cylindrical roller bearing with its outer ring coated with a layer of a cured epoxy composition, the layer indicated by A. 20
Figure 2 illustrates a schematic cross-sectional view of a ball bearing depicting the raceways of the inner ring and the outer ring each coated with a layer of a cured epoxy composition, the layers indicated by A.
Figure 3 illustrates the surface microstructure of a layer of cured epoxy composition coated over an outer surface of the outer ring of a cylindrical roller 25 bearing observed using an optical microscope.
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The epoxy composition in an uncured form comprises 100 parts of an epoxy resin, 5 to 20 parts of a hardener, 5 to 20 parts of at least one filler, and 5 to 20 parts of an oil selected from the group consisting of perfluoropolyether (PFPE) oil and highly refined mineral oil. The thickness of the layer is in the range of 150 μm to 200 μm. 5
The epoxy resin is at least one selected from the group consisting of Bisphenol-A diglycidyl ether polymer, Bisphenol-F diglycidyl ether polymer, and Novolac epoxy resin. In a preferred embodiment, the epoxy resin is Bisphenol-A diglycidyl ether polymer.
The hardener is at least one selected from the group consisting of polyfunctional 10 amines, acids, acid anhydrides, phenols, alcohols, and thiols. In a particular embodiment, the hardener used is a combination of 2,4,6-tris(dimethylaminomethyl)phenol and triethylenetetramine.
The filler is at least one selected from the group consisting of talc and graphite. In a specific embodiment, the filler used is talc. 15
A preferred epoxy composition in an uncured form is 100 parts of Bisphenol-A diglycidyl ether polymer, 10 parts of a combination of 2,4,6-tris(dimethylaminomethyl)phenol and triethylenetetramine as the hardener, 5 parts of talc filler and 7.5 parts of PFPE oil.
The present disclosure is further described in light of the following laboratory 20 experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.
EXPERIMENTS 25
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Experiment 1
A cylindrical roller bearing was coated with an uncured epoxy composition on the outer surface of the outer ring, and the raceways of both the rings. The uncured epoxy composition used is shown in Table-I.
Ingredient
Parts by weight
Bisphenol-A diglycidyl ether polymer
100
Hardener – HY 991 which is a combination of 2,4,6-tris(dimethylaminomethyl)phenol and triethylenetetramine
10
Talc powder
5
PFPE oil
7.5
Table-1 – Ingredients of the epoxy composition used in Experiment 1. 5
The uncured epoxy composition was applied over the components of the cylindrical roller bearing by brush coating. A spin coater was used to uniformly coat the rings at 1200 rpm. The components were then subjected to curing at room temperature for 24 hours to form a layer of cured epoxy composition. The thickness of the layer of the cured epoxy composition was found to be 150μm. 10
Experiment 2
The cured epoxy composition prepared in Experiment 1 was tested for electrical resistance.
Figure 4 illustrates a schematic representation of a set-up to determine the electrical resistance of a coated component of a rolling element bearing coated 15 with a layer of a cured epoxy composition. One side of a bearing component was coated with a layer of a cured epoxy composition similar to Experiment 1 and the other side was left uncoated. A very high potential difference of 1000 V DC supply was applied across the coated and the uncoated surfaces of the bearing
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component. The value of electrical resistance obtained was around 2000 MΩ. The exceptionally high value of the electrical resistance even at very high potential differences indicates that the bearing can withstand very high potential differences.
Experiment 3 5
The outer surface of the outer ring of a spherical roller bearing was coated with the epoxy composition similar to a method in Experiment 1. The coated portion was checked for breakdown voltage by noting the electric current passing across the coated portion while increasing the potential difference. It was seen that at a potential difference of 3 kV, current was observed across the coated portion of the 10 spherical roller bearing and therefore, 3 kV was the breakdown voltage of the cured epoxy coating. This exhibits the high electrical resistance shown by spherical roller bearing coated with a layer of the cured epoxy composition of Experiment 1.
Experiment 4 15
The epoxy coating with the composition as disclosed in Experiment 1 was tested for co-efficient of friction and wear life using a ball on disc tribometer - UMT-CETR (Universal Material tester). The test involved a ball shaped specimen that was allowed to slide against a rotating disk. The rotating disk was coated with the epoxy composition of Experiment 1 and was allowed to cure. A load was applied 20 vertically downwards with a motor driven carriage that used the force/load sensor for feedback to maintain a constant load. Using the UMT-CETR, the actual dynamic normal load, friction force, the coefficient of friction, and the depth of wear were recorded. Two loads were applied which generated contact stresses of 2.5 GPa and 3.0 GPa respectively. The sliding speed of the rotating disc was 25 maintained at 1.25 mps (metres per second) and the number of cycles was kept upto 100000 cycles.
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Figure 5 illustrates the co-efficient of friction measured against number of cycles using the ball on disc tribometer. The co-efficient of friction of the epoxy coating composition coated on the disk was found to be in the range of 0.1 to 0.32 even up to 1,00,000 cycles. This proves that the epoxy coating of the present disclosure has a low co-efficient of friction. 5
Figure 6 illustrates the number of cycles withstood (wear life) by the rotating disc coated with the epoxy composition of the Experiment 1 against the applied contact stresses (2.5 GPa and 3 GPa loads). From Figure 6, it can be seen that the cured epoxy composition could withstand a stress of 2.5 GPa and 3 GPa for even up to 1,00,000 cycles. 10
Figure 7 illustrates an FTIR graph of the cured epoxy composition coated on a surface of a rolling element bearing. Table 2 specifies the various peaks observed in Figure 7 and their interpretations.
Major Characteristic Absorptions (cm-1)
Functional Group
Description of Functional Group
Remarks
3380
N-H Stretch
Primary and Secondary Amines
Cross Linked Epoxy
2918
C-H Stretch
Alkyl
Epoxy
1657
C=C Stretch
Alkenes
Epoxy
1605
N-H Bend
Primary Amines
Hardener
1505
N-O Asymmetric Stretch
Nitro Compounds
Hardener
1233,1179,1110 and 1028
C-N Stretch
Aliphatic Amines
Hardener
825 and 760
N-H Wag
Primary and Secondary Amines
Cross Linked Epoxy
Table-2 –FTIR peaks and their interpretations
From Table 2, it can be confirmed that the epoxy composition coated onto the 15 surface of the rolling element bearing is a cured epoxy composition as seen from the characteristic peaks indicating the cured epoxy resin and the hardener.
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The electrically insulated rolling element bearing of the present disclosure is, thus, found to resist very high voltage electric currents across it and has a long service life. The low co-efficient of friction of the cured epoxy composition of the present disclosure enables the use of the composition even on movable parts.
TECHNICAL ADVANCES AND ECONOMICAL SIGNIFICANCE 5
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of an electrically insulated rolling element bearing that:
 has excellent resistance to very high voltage electric currents;
 has a long service life; and 10
 has a very low co-efficient of friction.
The foregoing description of the specific embodiments so fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and 15 modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will 20 recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the 25 exclusion of any other element, integer or step, or group of elements, integers or steps.
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The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context 5 for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions 10 or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the components and 15 component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure will be apparent to those skilled in the art from the disclosure herein, 20 whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

WE CLAIM:
1. An electrically insulated low friction rolling element bearing, comprising an inner ring, an outer ring and rolling elements which are in rolling contact with the raceways of the inner ring and the outer ring, wherein at least one of 5
an outer surface of the outer ring, and
the raceway of at least one of the rings
is coated with a layer of a cured epoxy composition, said composition in an uncured form comprises 100 parts of an epoxy resin, 5 to 20 parts of a hardener, 5 to 20 parts of a filler and 5 to 20 parts of an oil selected from 10 the group consisting of perfluoropolyether (PFPE) oil and highly refined mineral oil.
2. The bearing as claimed in claim 1, wherein said layer is of thickness in the range of 150 μm to 200 μm. 15
3. The bearing as claimed in claim 1, wherein said epoxy resin is selected from the group consisting of Bisphenol-A diglycidyl ether polymer, Bisphenol-F diglycidyl ether polymer, and Novolac epoxy resin.
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4. The bearing as claimed in claim 1, wherein said epoxy resin is Bisphenol-A diglycidyl ether polymer.
5. The bearing as claimed in claim 1, wherein said hardener is at least one selected from the group consisting of polyfunctional amines, acids, acid 25 anhydrides, phenols, alcohols, and thiols.
6. The bearing as claimed in claim 1, wherein said hardener is a combination of 2,4,6-tris(dimethylaminomethyl)phenol and triethylenetetramine.
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7. The bearing as claimed in claim 1, wherein said filler is at least one selected from the group consisting of talc and graphite.
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8. The bearing as claimed in claim 1, wherein said bearing is one selected from the group consisting of a deep groove ball bearing, cylindrical roller bearing, spherical roller bearing, Y-bearing, tapered roller bearing, self-aligning ball bearing, thrust ball bearing, cylindrical roller thrust bearing, 5