Shaft grounding assembly

Abstract

The invention relates to a shaft grounding assembly (10) for use with an electric apparatus (50) comprising a casing (52) and a shaft (54) mounted for rotation within the casing (52) The assembly (10) comprising a housing (12) being mountable to the casing (52) adjacent the shaft (54), at least one conductive arrangement (22) secured to the housing (12) for electrically contacting the shaft (54) to operatively conduct electric charge from the shaft (54) to the housing (12), and a seal (24) for contacting and sealing against the shaft (54) to operatively inhibit contaminants originating from the electric apparatus (50) to come into contact with the conductive arrangement (22).

Description

[0001] SHAFT GROUNDING ASSEMBLY

[0002] FIELD OF THE INVENTION

[0003] This invention relates, generally, to grounding devices for electric apparatuses. More particularly, but not exclusively, this invention relates to the protection of shaft grounding devices against excessive grease contamination.

[0004] BACKGROUND TO THE INVENTION

[0005] It is well known that a shaft of an electric motor incorporating a variable frequency drive (VFD) builds up an electric charge during use. Conventionally, this electric charge accumulates on the rotating shaft as the bearing lubrication acts as an insulator. However, when the electric charge on the shaft builds up to a level that exceeds the dielectric strength of the lubrication, it discharges in a short pulse through the motor bearings (i.e., the path of least resistance) to the motor housing, which is commonly grounded. After discharge, voltage again builds up on the shaft, and the cycle repeats itself. Initially, these discharges create a "frosted" or "sandblasted" effect on the surfaces of the bearings. Over time, this deterioration causes a groove pattern to form in the bearing races called "fluting”, which is an indication that the bearings have sustained severe damage. Eventually, the deterioration can lead to complete bearing failure, resulting in costly downtime and repairs.

[0006] One method known in the art and widely used to protect the bearings against fluting damage is to ground the motor shaft by means of a shaft grounding device. A shaft grounding device conducts the electric charge from the motor shaft to the motor housing and typically comprises a grounding body that is mounted to an axial end of the motor housing adjacent to the shaft. One or more conductive elements protrude from the grounding body to make electric contact with the shaft, thereby conducting electric charge from the shaft via the conductive element(s) and grounding body to the motor housing, which is commonly grounded. It will be appreciated that the conductive element(s) can be either in direct physical contact with the shaft or minimally spaced from the shaft to allow for indirect transfer of electric charge from the shaft to the conductive element(s).

[0007] However, a persistent challenge in industrial motor environments is the contamination of shaft grounding devices by lubricant. Excessive grease, typically originating from a motor bearing located near the shaft grounding device, can infiltrate the device and interfere with the electric contact between the conductive element(s) and the shaft, thereby diminishing the effectiveness of the grounding device.

[0008] OBJECT OF THE INVENTION

[0009] It is accordingly an object of the present invention to provide a shaft grounding assembly that will, at least partially, alleviate the abovementioned problems and / or that will be a useful alternative to existing shaft grounding devices of the type described above. SUMMARY OF THE INVENTION

[0010] According to a first aspect of the invention, there is provided a shaft grounding assembly suitable for use with an electric apparatus comprising a casing and a shaft mounted for rotation within the casing and extending from the casing, the shaft grounding assembly comprising:

[0011] - a housing being mountable to the casing adjacent to the shaft;

[0012] - at least one conductive arrangement secured to the housing and protruding from the housing to electrically contact the shaft, thereby operatively conducting electric charge from the shaft to the housing; and

[0013] - a seal for contacting and sealing against the shaft to operatively inhibit contaminants originating from the electric apparatus from coming into contact with the at least one conductive arrangement.

[0014] There is provided for the electric apparatus to comprise a motor, generator, or alternator.

[0015] The housing may define an inner surface facing the shaft when mounted to the casing, and an opposite outer surface. The conductive arrangement may project beyond the inner surface.

[0016] According to an example embodiment of the invention, the housing is annular, with the inner surface being a radial inner surface and the outer surface being a radial outer surface. There is provided for the housing to define at least one socket extending radially, at least partially, between the inner and outer surfaces, with the conductive arrangement secured in the socket. Preferably, the housing comprises a plurality of spaced apart sockets and a plurality of the conductive arrangements, wherein there is a conductive arrangement secured in each socket.

[0017] The conductive arrangement or each conductive arrangement, as the case may be, may comprise a multiplicity of filaments and / or a carbon brush.

[0018] There is further provided for the seal to be located adjacent to the housing. In a preferred embodiment, the seal is attached to the housing. Preferably, the seal may be held in axial compression by the housing. The seal may project beyond the inner surface.

[0019] The seal may be positioned adjacent to the conductive arrangement or conductive arrangements, as the case may be.

[0020] The shaft grounding assembly may be securable to the casing, with the seal positioned between an adjacent bearing of the electric apparatus and the conductive arrangement or conductive arrangements, as the case may be, wherein the seal operatively inhibits contaminants originating from the bearing from passing between the housing and shaft and coming into contact with the conductive arrangement or arrangements, as the case may be. The shaft grounding assembly may be mountable to either an internal or external side of the casing. There is provided for the seal to be resiliently deformable. Preferably, the seal comprises a polymeric material. Most preferably, the seal comprises polytetrafluoroethylene.

[0021] The seal may be annular, forming a circumferentially extending seal with the shaft. Preferably, the seal may comprise a rotary shaft seal.

[0022] The seal may define an opening for receiving the shaft therein, the opening being shaped and dimensioned for the seal to form an interference fit with the shaft when the shaft is received in the opening. Preferably, the opening is circular.

[0023] In an example embodiment of the invention, the seal is substantially frustoconical shaped in side view. Preferably, at least a portion of the seal may be curved. More preferably, a radially inner annular region of the seal extends at an acute angle with respect to an axis of the opening.

[0024] Preferably, at least an exposed portion of the seal may be sheet-like. More preferably, the entire seal may be sheet-like.

[0025] The shaft grounding assembly may comprise a single structure. Alternatively, the shaft grounding assembly may comprise separate structures, wherein the seal and conductive arrangement or conductive arrangements, as the case may be, are displaceable relative to each other. According to a second aspect of the invention, there is provided an electric apparatus, comprising:

[0026] - a casing;

[0027] - a shaft mounted for rotation within the casing and extending from the casing; and

[0028] - a shaft grounding assembly, as defined above, mounted to the casing wherein the seal is in contact with and seals against the shaft, and further wherein the seal is located axially between the casing and the conductive arrangement.

[0029] There is provided for the electric apparatus to comprise a motor, generator, or alternator.

[0030] According to a third aspect of the invention, there is provided a shaft sealing device for use in combination with a shaft grounding device on an electric apparatus comprising a casing and a shaft mounted for rotation within the casing and extending from the casing and wherein the shaft grounding device operatively conducts electric charge from the shaft to the casing, the shaft sealing device comprising:

[0031] - a frame defining an inner surface and an opposite outer surface, the frame being mountable to the casing wherein the frame is located axially between the casing and the shaft grounding device, with the inner surface facing the shaft; and

[0032] - a seal attached to the frame for contacting and sealing against the shaft to operatively inhibit contaminants originating from the electric apparatus from passing between the frame and shaft and coming into contact with the shaft grounding device. There is provided for the electric apparatus to comprise a motor, generator, or alternator.

[0033] According to an example embodiment of the invention, the frame is annular, with the inner surface being a radial inner surface and the outer surface being a radial outer surface.

[0034] The seal may be as defined hereinabove.

[0035] These and other features of the invention are described in more detail below.

[0036] BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0037] Embodiments of the invention are described below, by way of non-limiting examples only, and with reference to the accompanying schematic drawings in which:

[0038] Figure 1 is a perspective view of a shaft grounding assembly in accordance with one aspect of the invention;

[0039] Figure 2 is a plan view of the shaft grounding assembly of Figure 1 ;

[0040] Figure 3 is a section view along line Ill-Ill’ shown in Figure 2; Figure 4 is a cross-sectional view of part of an electric apparatus having the shaft grounding assembly of Figure 1 mounted to an outside thereof, in accordance with a further aspect of the invention;

[0041] Figure 5 is a cross-sectional view of part of an electric apparatus having the shaft grounding assembly of Figure 1 mounted to an inside thereof;

[0042] Figure 6 is a perspective view of a shaft sealing device, in accordance with a still further aspect of the invention; and

[0043] Figure 7 is a cross-sectional view of part of an electric apparatus having mounted thereto a shaft grounding device and the shaft sealing device of Figure 6.

[0044] DETAILED DESCRIPTION OF THE INVENTION

[0045] The invention described herein should not be construed as limited in scope by the specific embodiments disclosed herein, as the embodiments are provided to illustrate various aspects of the invention. It is to be appreciated that many variations and modifications of the embodiments are possible without departing from the scope of the appended claims.

[0046] Referring to the accompanying Figures, in which like numerals refer to like features, a shaft grounding assembly according to one aspect of the invention is generally indicated by reference numeral 10 in Figures 1 to 5, an electric apparatus according to a further aspect of the invention is generally indicated by reference numeral 50 in Figures 4, 5 and 7, and a shaft sealing device for use in combination with a shaft grounding device on an electric apparatus according to another aspect of the invention is generally indicated by reference numeral 100 in Figure 6.

[0047] The shaft grounding assembly 10 is suitable for use with the electric apparatus 50 that comprises a casing 52 and a shaft 54 mounted for rotation at least partially within the casing 52 and extending from the casing 52. It should be understood that the electric apparatus 50 could comprise a motor, generator, or alternator. In the example embodiment shown, the apparatus 50 is in the form of a motor 50.

[0048] Turning to Figures 1 to 3, the assembly 10 comprises an annular housing 12 that defines a radially inner surface 14 and an opposite radially outer surface 16. The housing 12 is mountable to the casing 52 around the shaft 54, with the radially inner surface 14 positioned adjacent to and facing the shaft 54 (as shown in Figures 4 and 5). The housing 12 is rigid and made from a conductive material, preferably a metal such as aluminium. The housing 12 includes first and second axially adjacent annular members 12.1 , 12.2, secured together by threaded fasteners 18, a method well known in the art.

[0049] The housing 12 defines a plurality of angularly-spaced sockets 20 that extend radially, at least partially, between the radial inner surface 14 and the radial outer surface 16. These sockets 20 are open in a radially inward direction and may either extend to or terminate before reaching the radial outer surface 16, depending on the application. The cross-sectional area of each socket 20 decreases in a radially outward direction, and the sockets 20 may have a stepped profile or taper along their lengths.

[0050] Each socket 20 contains a conductive arrangement 22 that projects radially inward beyond the radial inner surface 14 to make electrical contact with the shaft 54 when the assembly 10 is secured around the shaft 54. It should be appreciated that the conductive arrangements 22 could either be in direct physical contact with the shaft 54 or minimally spaced from the shaft 54 to still allow for the flow of electric current between the shaft 54 and conducive arrangements 22. Each conductive arrangement 22 comprises a multiplicity of carbon-containing filaments, which are press-fitted into the stepped or tapered sockets 20, securing them tightly.

[0051] An annular seal 24, in the form of a unitary rotary shaft seal, is attached to the housing 12. The seal 24 is axially compressed between the opposing first and second members 12.1 , 12.2, ensuring that it remains securely in place without partial disassembly of the housing 12. The seal 24 is located adjacent to the conductive arrangements 22 and extends radially inward beyond the radial inner surface 14 to contact and seal against the shaft 54. The seal 24 thus prevents contaminants (not shown) from passing through the assembly 10, specifically between the radial inner surface 14 and the shaft 54, in a direction from the casing 52 toward the assembly 10. The containments could include a lubricant 56 originating from excessive lubrication of an adjacent bearing 58 of the motor 50. Here, it is to be understood that the bearing 58 is located adjacent the assembly 10 and adjacent to where the shaft 54 extends from the casing 52. The purpose of the seal 24 is thus to prevent contaminants originating from the motor 50 to come into contact with the conductive arrangements

[0052] 22 and interfere with their operational efficiency.

[0053] The seal 24 defines a central circular opening 26 through which the shaft 54 is receivable. The opening 26 is dimensioned to create an interference fit with the shaft 54 when received therein so as to form a circumferential seal with the shaft 54. The seal 26 is firm yet resiliently deformable, and in a preferred embodiment, it is made from a polymeric material such as polytetrafluoroethylene (PTFE).

[0054] As shown in Figure 3, the seal 24 is generally flat or sheet-like, and presents a frustoconical shape. Specifically, the wall of the seal 24 is curved and diverges radially inward toward the axis A of the opening 26. The radially inner annular region of the seal 24 extends at an acute angle relative to the axis A. The seal 24 is further dimensioned so that the width of its wall in the radial direction exceeds its wall thickness in the axial direction by at least five times, and preferably by at least ten times. In an exemplary embodiment, the wall thickness in the axial direction is between 0.5 mm and 3 mm, preferably between 0.75 mm and 2 mm, and more preferably between 0.9 mm and 1 .5 mm.

[0055] The housing 12 further includes a plurality of angularly spaced mounting apertures 28 for mounting the assembly 10 to the casing 52 using fasteners and / or mounting brackets (not shown). The grounding assembly 10 is a single integrated entity.

[0056] Figure 4 illustrates a partial view of the motor 50, where the assembly 10 is mounted to an external side of the casing 52, specifically to an external surface 60 of a bearing housing 62 of the bearing 58, using fasteners 66 through apertures 28. The shaft 16 extends through the central opening 26, and both the conductive arrangements 22 and the seal 24 are in contact with the shaft 54. The seal 24 is positioned between the casing 52 and conductive arrangements 22. As the electric charge builds up on the shaft 54 during operation of the motor 50, it is conducted from the shaft 54 via the conductive arrangements 22 and the housing 12 to the casing 52. Furthermore, if lubricant 56 from the adjacent bearing 58 is discharged outwardly between the casing 52 and the shaft 54, the seal 24 will prevent the lubricant 56 from passing between the shaft 54 and the housing 12, thereby maintaining the integrity of the electrical contact between the shaft 54 and the conductive arrangements 22.

[0057] The configuration shown in Figure 5 is similar to that shown in Figure 4, except that the assembly 10 is mounted on an internal side of the casing 52, specifically to an internal surface 64 of the bearing housing 62. Here, if the lubricant 56 from the adjacent bearing 58 is discharged inwardly between the casing 52 and the shaft 54, the seal 24 will again prevent the lubricant 56 from interfering with the electrical contact between the shaft 54 and the conductive arrangements 22.

[0058] The sealing device 100, shown in Figure 6, comprises an annular frame 102 that defines a radially inner surface 104 and an opposite radially outer surface 106. The frame 102 is mountable to the casing 52 around the shaft 54, with the radially inner surface 104 positioned adjacent to and facing the shaft 54 (see Figure 7). The frame 102 is rigid and made from a conductive material, preferably a metal such as aluminium, and comprises axially adjacent first and second annular members 102.1 , 102.2, secured together by threaded fasteners 108. An annular seal 110 is attached to the frame 102, by being axially compressed between the opposing first and second members 102.1 , 102.2, ensuring that it remains securely in place without partial disassembly of the frame 102. The seal 1 10 projects radially inward beyond the radial inner surface 104 to contact and seal against the shaft 54. The seal 110 defines a central circular opening 112 through which the shaft 54 is receivable. It is to be understood that the seal 110 is similar to the seal 24 described earlier in terms of shape, configuration, and function.

[0059] The frame 102 also includes a plurality of angularly spaced mounting apertures 114 for mounting the device 100 to the casing 52.

[0060] Referring to Figure 7, the sealing device 100 is particularly useful in retrofitting applications for electric motors 50 that already have a conventional grounding device 150 installed. In this scenario, the sealing device 100 is mounted to the external side of the motor casing 52 so that it is positioned between the motor casing 52 and the grounding device 150. The sealing device 100 and the grounding device 150 are separate units. To secure the sealing device 100 in place, it is sandwiched between the motor casing 52 and the grounding device 150, with fasteners 152 extending through the apertures 114.

[0061] Thus, the invention teaches a grounding assembly 10 and sealing device 100 that effectively prevent excessive lubricant 56 from the electric motor 50 from contaminating the conductive arrangements located adjacent thereto, ensuring their operational efficiency remains uncompromised.

Claims

CLAIMS1. A shaft grounding assembly suitable for use with an electric apparatus comprising a casing and a shaft mounted for rotation within the casing and extending from the casing, the shaft grounding assembly comprising: a housing being mountable to the casing adjacent to the shaft; at least one conductive arrangement secured to the housing and protruding from the housing to electrically contact the shaft, thereby operatively conducting electric charge from the shaft to the housing; and a seal for contacting and sealing against the shaft to operatively inhibit contaminants originating from the electric apparatus from coming into contact with the at least one conductive arrangement.

2. The shaft grounding assembly as claimed in claim 1 , wherein the housing is annular and defines a radially inner surface facing the shaft when the housing is mounted to the casing, and an opposite radially outer surface.

3. The shaft grounding assembly as claimed in claim 2, wherein the housing defines at least one socket extending radially, at least partially, between the inner and outer surfaces, with the conductive arrangement secured in the socket.

4. The shaft grounding assembly as claimed in claim 1 , wherein the seal is attached to the housing with at least part of seal being held in axial compression by the housing.

5. The shaft grounding assembly as claimed in claim 1 , which is securable to the casing with the seal positioned between an adjacent bearing of the electric apparatus and the conductive arrangement, wherein the seal operatively inhibits contaminants originating from the bearing from passing between the housing and shaft and come into contact with the conductive arrangement.

6. The shaft grounding assembly as claimed in claim 1 , wherein the seal is resiliently deformable and comprises a polymeric material.

7. The shaft grounding assembly as claimed in claim 6, wherein the polymeric material comprises polytetrafluoroethylene.

8. The shaft grounding assembly as claimed in claim 1 , wherein the seal comprises an annular rotary shaft seal for forming a circumferentially extending seal with the shaft.

9. The shaft grounding assembly as claimed in claim 1 , wherein the seal is substantially frustoconical shaped in side view.

10. The shaft grounding assembly as claimed in claim 1 , wherein at least a portion of the seal may be curved, and a radially inner annular region of the seal extends at an acute angle with respect to an axis of an opening of the seal.11 . The shaft grounding assembly as claimed in claim 1 , wherein the seal is sheet- like.

12. The shaft grounding assembly as claimed in claim 1 , comprising a single structure.

13. An electric apparatus, comprising:- a casing;- a shaft mounted for rotation within the casing and extending from the casing; and- a shaft grounding assembly, as defined above, mounted to the casing wherein the seal is in contact with and seals against the shaft, and further wherein the seal is located axially between the casing and the conductive arrangement.

14. A shaft sealing device for use in combination with a shaft grounding device on an electric apparatus comprising a casing and a shaft mounted for rotation within the casing and extending from the casing and wherein the shaft grounding device operatively conducts electric charge from the shaft to the casing, the shaft sealing device comprising: a frame defining an inner surface and an opposite outer surface, the frame being mountable to the casing wherein the frame is located axially between the casing and the shaft grounding device, with the inner surface facing the shaft; and a seal attached to the frame for contacting and sealing against the shaft to operatively inhibit contaminants originating from the electric apparatus from passing between the frame and shaft and coming into contact with the shaft grounding device.

15. The shaft sealing device as claimed in claim 14, wherein the seal is as defined in any one of claims 1 -11.

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