Roller set and bearing with electrical shunt roller

Abstract

A roller set for a rolling element bearing, the roller set comprising a plurality of rolling elements configured to occupy a radial gap between an inner ring and an outer ring configured for relative rotation therebetween. A majority of the plurality of rolling elements having a matching construction that is electrically insulating, either of a metal construction and coated in a wear-resistant coating or constructed of ceramic. A minority of the plurality of rolling elements does not have the matching construction of the majority of the plurality of rolling elements, and is instead electrically conductive to act as an electrical shunt for stray current passing between the inner and outer rings.

Description

Attorney Docket No. 023644-0010-WO01BEARING WITH ELECTRICAL SHUNT ROLLERBACKGROUND

[0001] The present invention relates to rolling element bearings and techniques for combatting premature wear due to stray current passage through the bearing.SUMMARY

[0002] In one aspect, the disclosure provides a roller set for a rolling element bearing, the roller set comprising a plurality of rolling elements configured to occupy a radial gap between an inner ring and an outer ring configured for relative rotation therebetween. A majority of the plurality of rolling elements having a matching construction that is electrically insulating, either of a metal construction and coated in a wear-resistant coating or constructed of ceramic. A minority of the plurality of rolling elements does not have the matching construction of the majority of the plurality of rolling elements, and is instead electrically conductive to act as an electrical shunt for stray current passing between the inner and outer rings.

[0003] In another aspect, the disclosure provides a rolling element bearing comprising the roller set, along with an outer ring defining a first raceway receiving the plurality of rolling elements, and an inner ring defining a second raceway receiving the plurality of rolling elements.

[0004] Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a perspective view of a rolling element bearing, including an inner ring, a roller set, and a partially sectioned outer ring.

[0006] FIG. 2 is a set of photographs illustrating rolling element wear after 4 million revolutions, after 8 million revolutions, and after 21 million revolutions (failure). The rolling elements are from a uniform diamond-like carbon coated tapered roller bearing (TRB) roller set under axial load, without any designated electrical shunt roller.Attorney Docket No. 023644-0010-WO01

[0007] FIG. 3 is a set of photographs illustrating rolling element wear after 4 million revolutions, after 8 million revolutions. The rolling elements are from a TRB roller set under axial load including a single designated electrical shunt roller.

[0008] Before any embodiments of the invention are explained in detail, it is to be understood that the invention 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 invention 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 DESCRIPTION

[0009] FIG. 1 illustrates a bearing 4 configured to support a shaft (not shown) for rotation about a central axis A. The bearing 4 includes an inner ring 8, a roller set including a plurality of rolling elements 12, and an outer ring 20. The inner ring 8 defines a bore to receive a portion of the shaft. The bearing 4 is press-fit on the shaft, thereby enabling the inner ring 8 to co-rotate with the shaft. The rolling elements 12 are positioned between a raceway of the inner ring 8 and an opposite raceway of the outer ring 20 and may be tapered rollers, cylindrical rollers, or any other type of rolling element. The rolling elements 12 provide a rolling interface to reduce friction between the inner ring 8 and the outer ring 20. The outer ring 20 is coupled to a housing (not shown) so that the outer ring 20 does not rotate with the inner ring 8 and the shaft. In other configurations, the bearing 4 may support relative rotation between inner and outer members in another way (e.g., outer ring 20 fixed to a rotating component, or both the inner and outer rings 8, 20 are configured as rotatable - with differential rotation).

[0010] During operation of the bearing 4 in various environments, stray voltage can manifest in unplanned electrical currents flowing through the bearing 4, and this can cause pitting, fluting, and spalling damage to the rollers or ring raceway contacting surfaces. This in turn leads to increased vibration and friction, which ultimately results in premature bearing failure. It occurs when the circulating current induced on a motor or shaft is discharged through the bearing4, flowing through the rolling elements 12, raceways, and a lubricant that may be present in the bearing 4. The breakdown of the lubricant leads to electrical arcing, melting the point of contactAttorney Docket No. 023644-0010-WO01 between the rolling elements 12 and raceways, eroding the surface. Electric current can also produce thermal damage on the surface, due to localized heat that will produce further degradation. This problem has been the focus of several studies and has been addressed in the industry using different methods. For example: installations of grounding brushes or rings on the shaft, used to redirect stray currents away from bearings and into the ground. The grounding brushes can prevent electrical damage to bearings; however, they may require periodic inspections and additional installation and maintenance costs. Therefore, complementary solutions such as dielectric or insulating coatings are also implemented. Dielectric coatings are applied between the outer ring and the housing or between the inner ring and the shaft to prevent electrical currents from passing through the bearing. These electrically insulating surfaces may protect the system, even though the coatings can wear off over time, requiring reapplication.

[0011] Another approach to preventing the current from damaging the bearing is to use hybrid bearings that integrate metal parts (e.g., rings 8, 20) with rolling elements 12 of ceramic or other materials inherently resistant to electrical currents. Nevertheless, this alternative can be more expensive than standard bearings. These approaches try to deviate or prevent the electric current from flowing through the rolling element bearing 4. As described in detail below, the present disclosure proposes a new bearing configuration to control and direct the current path inside the bearing assembly 4 if a circulating current is present in the system. Focusing the current through one (or a small minority) conductive rolling element(s) 12 - referred to herein as a shunt roller(s) - can reduce the harmful impact of electrical currents on roller and / or ring contacting surfaces and therefore bearing life attributed to many current flow points, improving overall bearing reliability. This novel approach to bearing construction can be used in different applications and levels of stray current. Combining this approach with a dielectric coating on the mounting surfaces provides an additional layer of protection.

[0012] In one aspect, the disclosure proposes that the bearing 4 is provided with a majority of (e.g., all but one) rolling elements 12 as electrically insulating (e.g., coated with an enhanced tribological wear resistance coating such as diamond-like carbon (DLC)). For example, a common DLC roller coating used in industry was found to have electrical resistivity on the order of 3x1 O'6Q.m using a four-point probe measurement technique. The remaining rolling element(s) 12 is significantly less resistant to electrical conduction - for example bearing steelAttorney Docket No. 023644-0010-WO01 without the DLC coating - to short-circuit the electric passage of stray current to an intended path of low electrical impedance. For example, an uncoated steel surface has a much lower resistivity on the order of lxlO-7.m by the same measurement technique. Thus, the majority of the rolling element(s) 12 (those not designated for electrical current passage) can have electrical resistivity an order of magnitude greater than the designated minority of the rolling elements 12 within the roller set. By concentrating the electrical current to the designated rolling element(s) as an effective shunt, the damage to the overall bearing assembly is minimized. When the bearing has a 360-degree load zone, the designated shunt roller may be limited to a single roller of the roller set of rolling elements 12. In cases where the load zone is less than 360-degrees, two or three shunt rollers may be provided within the bearing 4, with the remaining majority of rolling elements 12 being significantly more resistant to electrical conduction (e.g., DLC coated). This strategy is to assure that under various mechanical loading configurations, at least one shunt roller is present in contact between the rings 8, 20 to direct the electrical current passage through. The rings 8, 20 can have relatively low electrical resistivity, like that of the selected few shunt rolling elements 12 (e.g., uncoated bearing steel).

[0013] In an alternative construction of the bearing 4, the designated shunt roller(s) is / are coated with a conductive and tribologically wear resistant coating such as chromium, cadmium, chromium nitride, etc. that allows for easy electric current passage while at the same time providing tribological wear protection to the contact surfaces. The other rolling elements will be coated with a less conductive coating such as DLC. In yet another alternative construction of the bearing, the majority of the rolling elements 12 exclusive of the shunt roller(s) are non- conductive rollers made of a bulk technical ceramic such as silicon nitride (SisN4).

[0014] In some constructions, a reinforced bearing cage 24 with embedded graphene or conductive particles is provided to reduce friction and dissipate the electric current using the bearing 4 according to one of the above-described configurations with shunt roller(s). Aspects of the present disclosure can be applied to tapered roller bearings, cylindrical roller bearings, spherical roller bearings, needle roller bearings, thrust roller bearings, and ball bearings. Furthermore, bearings of the present disclosure may use an additional electrical current protection such as a polymeric coating or aluminum oxide coating deposited on the bearing assembly bore (in contact with the shaft) or outer ring outside diameter (in contact with theAttorney Docket No. 023644-0010-WO01 mounting housing) to minimize the circulating current using a bearing according to any of the preceding configurations.

[0015] The present disclosure proposes a new bearing design to short-circuit the passage of stray electric current to an intended path of low electrical impedance through the operating rolling element bearing. To the contrary, prior efforts have focused on preventing current flow through the bearing by redirecting current external to the bearing or preventing its flow entirely via electrically resistive coatings. According to the present disclosure, the current is intentionally directed through a limited pathway of the rolling bearing assembly, minimizing the damage to the overall bearing assembly. The limited damage allowed by current flow through a minimal quantity of shunt rollers may be repaired or minimized by the presence of DLC or other tribologically wear resistant rollers in the assembly. For example, if a small pit is created by current flow through a shunt roller, the neighboring DLC-coated rollers may mechanically polish the small damage to render it harmless and prevent propagation of that same damage spot to larger damage areas. Laboratory and field experience has shown that the use of an electrically insulating tribologically wear resistant coating such as DLC on all rolling element contacts may accelerate bearing damage due to inevitable small locations of current leakage through coating defects across many multiple contact locations in the bearing assembly, limiting the life of the bearing assembly dramatically. On the other hand, if the current is free to follow any path of least resistance outside of this invention, there can be multiple locations and types of damage resulting in a lower overall useful service life of the bearing assembly as shown by field experience and laboratory results.

[0016] Consistent with the preceding description, a bearing 4 of the present disclosure may conform to any one of the following constructions such that electric current passage through an assembled rolling element bearing in use is diverted around a majority of the rolling elements of the bearing’s roller set.

[0017] In a first example, a single uncoated, electrically conductive rolling element 12 is provided in the bearing 4 while the rest of the rolling elements 12 are coated with an enhanced tribological wear resistant coating (but less electrically conductive than an uncoated rollingAttorney Docket No. 023644-0010-WO01 element) such as diamond-like carbon (DLC) or similar when the bearing has a 360-degree load zone, typically under thrust or axial loading conditions in use.

[0018] In a second example, two or three uncoated, electrically conductive rolling elements 12 are provided in the bearing 4 while the rest of the rolling elements 12 are coated with an enhanced tribological wear-resistant coating (but less electrically conductive than an uncoated rolling element) such as DLC or similar when the bearing has a load zone less than 360-degrees, typically under radial loading in use. The uncoated electrical shunt rollers are positioned diametrically opposite (for two rollers) or 120-degrees separate (for three rollers) from each other. The second example, including greater than 1 electrical shunt rollers can also be used for thrust or axial loading conditions.

[0019] In a third example, the bearing 4 is provided as described in the first or second example, except that the electrically conductive shunt rolling element(s) 12 is / are coated with an electrically conductive and wear-resistant coating such as chromium, cadmium, chromium nitride, etc. that withstands the electric current passage while still providing some measure of tribological wear resistance (may be less effective in tribological performance than the other coated rollers that have less electrical conductivity). The other (majority of) rolling elements 12 are coated with a less electrically conductive but more tribologically advantageous coating such as DLC.

[0020] In a fourth example, the bearing 4 is provided as described in the first or second example, except that the electrically conductive shunt roller(s) 12 is / are combined with nonconductive rollers 12 made of a bulk technical ceramic such as silicon nitride (Si3N4).

[0021] In the bearings 4 of any of the above examples, the cage 24 may be constructed with embedded conductive particles (e.g., graphene) that render the cage 24 electrically conductive to help direct stray electrical current between the bearing rings 8, 20 through the electrically conductive roller(s) 12.

[0022] In the bearings 4 of any of the above examples, additional electrical current protection may be provided by way of a coating (polymeric or aluminum oxide) provided at the innermost and / or outermost bearing surface. In particular, the coating may be deposited on the bearingAttorney Docket No. 023644-0010-WO01 assembly bore (in contact with the shaft) or outer ring outside diameter (in contact with the mounting housing) to minimize the circulating current.

[0023] In addition to the tapered roller bearing 4 illustrated in FIG. 1, aspects of the present disclosure can also equally be applied in other bearing types, including cylindrical roller bearings, spherical roller bearings, needle roller bearings, thrust roller bearings, and ball bearings. The bearing constructions of the present disclosure can reduce the risk of electrical damage and extend the life of the component, focusing on the effect of electric current on only one path inside the bearing assembly instead of many. In addition, the presence of majority DLC (or tribological wear resistant coating) or ceramic rollers will help repair and minimize any roller bearing damage experienced due to current passage through the conductive rolling element path from ring to ring.

[0024] Bearings of conventional construction and that of the present disclosure have been tested under different levels of electric current to evaluate the influence of electric current on bearing surface damage. In the current testing, tapered roller bearing (TRB) assemblies M86600 series are tested under an axial load of 6.23 kN, held at a constant speed of 2,700 RPM. In the test configuration, the inner ring is rotated, while the outer ring is stationary. SAE 75W-80 API GL-4 semi-synthetic gear oil is used as a lubricant. Different levels of direct current were applied to the bearing using a high-accuracy source meter. The test ran until a maximum threshold of torque, temperature, or vibration was reached. During this process, the tests were interrupted after 4 and 8 million revolutions to evaluate damage initiation and progression. The rolling elements were numbered for tracking. The vibration threshold of the machine was set to a predetermined value at the start of the test.

[0025] In some test, all the rolling elements were coated with a diamond-like carbon coating (DLC) that provides adhesive wear resistance, mitigates micropitting, and increases fatigue life. Thus, the bearing did not include the advancements of the present disclosure. Results show that the application of electric current can induce fluting damage in the DLC-coated rollers during the first early cycles of testing. The coating degradation increased over time under the influence of electric current. Greater coating damage was observed with increasing electric current. FIG. 2 illustrates photographs of rolling elements subjected to the above described testing, wherein allAttorney Docket No. 023644-0010-WO01 the rolling elements are DLC-coated rollers, and the test was conducted with 1 ampere of current between the inner and outer rings (through the collective roller set). In row (a) of FIG. 2, some of the rolling elements with greater coating damage are shown after 4 million revolutions. Few rollers showed spots of damage, while 2 out of 18 rollers were found with early stages of fluting, such as the roller labeled as roller #1. The fluting was located mainly on the top and middle body of the rollers. After the photographs of row (a) of FIG. 2, the bearing was reassembled with the same rollers and subjected to continued testing to evaluate damage progression. Row (b) of FIG. 2 illustrates these same rollers after another 4 million revolutions (total of 8 million revolutions). After 8 million revolutions, roller #1 demonstrated an observable increase in the coating damage characterized by a greater fluting region. Fluting was also found in more spots on the coated rollers as seen in rollers #9 and #13. After the photographs of row (b), the bearing was reassembled and subjected to continued testing (until failure). Bearing failure occurred at 21 million revolutions, and photographs of the same four rollers are shown in row (c) of FIG. 2. Here, there are more pronounced signs of fluting in the rollers over time, increasing the fluting region to a larger area, in some cases covering the majority of the roller body. The prominent fluting on the roller's body, suggests that electric current may compromise the coating's protective capabilities, exposing the steel substrate and leaving it susceptible to further damage, accelerating bearing failure. Furthermore, the presence of electric current led to greater coating damage and a decrease in bearing life. High vibration due to bearing spalling shut off the test after 21 million revolutions. According to similar test results without the application of electrical current, the same bearing is expected to run longer than 150 million revolutions. On DLC-coated rollers, the decrease in performance under electric current may be exacerbated by increased current density at initial spots of coating damage where steel was exposed.

[0026] To prove the efficacy of the present disclosure, additional tests were conducted using the same bearing construction and test procedures, with the exception of a designated conductive path through the roller set as provided by way of a single rolling element of the DLC-coated roller set being left uncoated. The bare steel is more electrically conductive than the DLC coating, and this effectively forces the electrical current to flow through the path of less resistance - avoiding conduction through the majority of the (DLC-coated) rolling elements. The test was conducted under the same electrical current of 1 ampere. FIG. 3 shows the photographic results obtained after 4 million (row (a)) and 8 million (row (b)) revolutions of testing with theAttorney Docket No. 023644-0010-WO01 bearing having the one uncoated rolling element. It was observed that after the first inspection at 4 million revolutions, minimal damage was found on the coated rollers - only a few wearlines. However, the uncoated roller labeled as roller #1, showed discoloration (black / brown color), along with early stages of fluting. Similarly, after 8 million revolutions, the uncoated roller #1 was found with a clear fluting pattern. The longitudinal grooves covered a vast majority of the roller body, reflecting the great effect of electrical discharge on this roller, through which all the electrical energy is being conducted. In contrast, the DLC-coated rollers after 8 million revolutions did not show any signs of fluting as in previous tests (FIG. 2) where more visible wear lines were found near the roller's small end and mid-body. This test was conducted to bearing failure by vibration caused by surface damage within the bearing, and it reached 119 million revolutions - far exceeding the test from FIG. 2 that failed at 21 million revolutions. These unexpected results confirm that replacing a single tribologically-advantaged DLC-coated roller with a tribologically-disadvantaged electrical shunt roller under thrust loading in a TRB can reduce damage in the bearing system, and reduce the risk of early and rapid bearing failure by stray currents. While the test example for the configuration of FIG. 2 and FIG. 3 was conducted using DC electrical current passage conditions, aspects of the present disclosure may equally apply to AC electrical passage conditions as well.

[0027] In some aspects, the present disclosure provides a method of machine operation and maintenance in which a bearing of the present disclosure is assembled and put into initial service with a limited number of electrical shunt roller(s). At a fraction of the service life of the majority of the rolling elements, the bearing is serviced by replacement of only the electrical shunt roller(s). The bearing is then reassembled and put back into service with the original rollers and the electrical shunt roller(s). The electrical shunt roller(s) can be replaced once or multiple times during the overall service life of the bearing. Even with one or more services to replace an electrical shunt roller(s), there is an advantage in time and / or cost as compared to a replacement of the full roller set or the entire bearing.

[0028] Various aspects of the present disclosure are set forth in the following claims.

Claims

Attorney Docket No. 023644-0010- WOO 1CLAIMSWhat is claimed is:

1. A roller set for a rolling element bearing, the roller set comprising a plurality of rolling elements configured to occupy a radial gap between an inner ring and an outer ring configured for relative rotation therebetween, wherein a majority of the plurality of rolling elements having a matching construction that is electrically insulating, either of a metal construction and coated in a wear-resistant coating or constructed of ceramic, and wherein a minority of the plurality of rolling elements does not have the matching construction of the majority of the plurality of rolling elements, and is instead electrically conductive to act as an electrical shunt for stray current passing between the inner and outer rings.

2. The roller set of claim 1, wherein the minority of the plurality of rolling elements includes a single rolling element providing the electrical shunt.

3. The roller set of claim 1, wherein the minority of the plurality of rolling elements includes two diametrically opposed rolling elements providing the electrical shunt.

4. The roller set of claim 1, wherein the minority of the plurality of rolling elements includes three equally spaced rolling elements providing the electrical shunt.

5. The roller set of claim 1, wherein the majority of the plurality of rolling elements have a diamond-like carbon (DLC) coating.

6. The roller set of claim 1, wherein the majority of the plurality of rolling elements have a silicon nitride ceramic construction.

7. The roller set of claim 1, wherein the minority of the plurality of rolling elements includes an electrically conductive wear-resistant coating.Attorney Docket No. 023644-0010- WOO 18. The roller set of claim 7, wherein the electrically conductive wear-resistant coating is chromium, cadmium, or chromium nitride.

9. The roller set of claim 1, wherein the majority of the plurality of rolling elements have an electrical resistivity that is an order of magnitude greater than an electrical resistivity of the minority of the plurality of rolling elements.

10. A rolling element bearing comprising: an outer ring defining a first raceway receiving the plurality of rolling elements; an inner ring defining a second raceway receiving the plurality of rolling elements; and the roller set of claim 1, including the plurality of rolling elements.

11. The rolling element bearing of claim 10, wherein the minority of the plurality of rolling elements includes a single rolling element providing the electrical shunt.

12. The rolling element bearing of claim 11, wherein the rolling element bearing has a 360- degree load zone.

13. The rolling element bearing of claim 10, wherein the rolling element bearing has a load zone of less than 360 degrees.

14. The rolling element bearing of claim 13, wherein the minority of the plurality of rolling elements includes two diametrically opposed rolling elements providing the electrical shunt.

15. The rolling element bearing of claim 13, wherein the minority of the plurality of rolling elements includes three equally spaced rolling elements providing the electrical shunt.

16. The rolling element bearing of claim 9, wherein the majority of the plurality of rolling elements have a diamond-like carbon (DLC) coating.Attorney Docket No. 023644-0010- WOO 117. The rolling element bearing of claim 10, wherein the majority of the plurality of rolling elements have a silicon nitride ceramic construction.

18. The rolling element bearing of claim 10, further comprising a cage including a plurality of bridges interspersed between adjacent ones of the plurality of rolling elements, wherein the cage has a base material that is electrically insulating and includes embedded electrically conductive particles to enhance electrical conductivity.

19. The rolling element bearing of claim 18, wherein the embedded electrically conductive particles of the cage include graphene.

20. The rolling element bearing of claim 10, wherein the majority of the plurality of rolling elements have an electrical resistivity that is an order of magnitude greater than an electrical resistivity of the inner ring and the outer ring.

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