Shaft grounding device, electric machine and kit-of-parts
Patent Information
- Application Number
- EP2024702479
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-02-28
- Filing Date
- 2024-01-18
- Publication Date
- 2026-01-07
AI Technical Summary
Existing shaft grounding systems for electric machines, particularly in motor vehicles, face issues with wear and tear, inefficiency, and the need for additional installation space, which can lead to bearing damage and electromagnetic interference.
A shaft grounding system featuring a rotatably mounted shaft with a first electrically conductive sleeve and a second sleeve containing an electrically conductive bridging sleeve and grounding rod, allowing for efficient conductivity and assembly, compatible with both dry and wet environments, and integrated into existing systems without size increase.
The system effectively dissipates electrical charges, reduces bearing damage, improves electromagnetic compatibility, and allows for easy integration and assembly, providing a wear-free and efficient solution for electric machines.
Smart Images
Figure DE2024100044_06092024_PF_FP
Abstract
Description
[0001] Shaft earthing, electrical machine and kit-of-parts
[0002] The present invention relates to a shaft grounding system for discharging an electrical charge and / or voltage from a shaft, in particular for a drive train of an electrically operated motor vehicle, comprising a rotatably mounted shaft into which a cavity extends axially from an end face having a shaft opening, and a first sleeve, which is rotationally fixed relative to the shaft and electrically conductive, passes through the shaft opening and extends axially into the cavity of the shaft. The invention further relates to an electrical machine and a kit of parts.
[0003] Electrical machines, especially electric traction machines, are electrically charged during operation by induced shaft voltages. In addition, circulating high-frequency currents can arise in high-power motors. Such electrical machines typically comprise a rotor and a rotor shaft, which are mounted in a housing via rolling bearings. Electrical discharges or circulating high-frequency currents can damage the rolling bearings. To prevent this, this electrical energy must be dissipated. For example, grounding brushes or radially acting sliding elements (carbon brushes) are known for grounding the rotor shaft. Axial-acting shaft grounding systems are also known.
[0004] The document DE 10 2020 119 719 A1 , which forms the closest prior art, discloses a discharge device for a motor for discharging an electrical charge and / or voltage from a rotor via a shaft to a housing, with a first contact module for electrical and mechanical connection to the shaft, with a second contact module for electrical and mechanical connection to the housing, wherein the first and the second contact module are rotatable relative to one another and are electrically connected to one another in an interior space sealed off from a housing space, wherein the interior space is filled with an electrically conductive liquid, wherein the first and the second contact module are electrically conductively connected to one another via the electrically conductive liquid.The invention is based on the object of creating a shaft grounding system that is characterized by easy installation, improved conductivity, and wear resistance. Furthermore, the object of the invention is to realize a correspondingly improved electrical machine. Finally, the object of the invention is also to provide a kit of parts that allows for simplified and convenient installation of a shaft grounding system.
[0005] This object is achieved by a shaft earthing system for discharging an electrical charge and / or voltage from a shaft, in particular for a drive train of an electrically operated motor vehicle, comprising a rotatably mounted shaft, into which a cavity extends axially from an end face having a shaft opening into the shaft, and a first sleeve, which is non-rotatable relative to the shaft and electrically conductive, passes through the shaft opening and extends axially into the cavity of the shaft, wherein a second electrically conductive sleeve is arranged in and / or on the first sleeve, in which second electrically conductive bridging sleeve is arranged, and the bridging sleeve has an electrically conductive earthing rod received therein and rotatably mounted relative to the bridging sleeve,wherein the earthing rod is electrically connected to the bridging sleeve on the one hand and is electrically connected to a third electrically conductive sleeve on the other hand, which in turn is connected to the shaft.
[0006] This makes it possible to provide an axially acting shaft grounding system that functions efficiently in dry and wet electrical machines and at all operating points. Existing installation space is utilized without increasing the axial or radial size of the electrical machine. Furthermore, the shaft grounding system can also be easily integrated into existing applications at a later date. This prevents bearing damage to the rotatably mounted shaft and improves electromagnetic compatibility (EMC), reducing or preventing interference with other devices. A further advantage of the shaft grounding system according to the invention is that it can be constructed as a standalone system and thus offered as a pre-assembled unit.
[0007] With regard to the fluids used and the structure of the shaft grounding unit, reference is made to the applicant's previously published DE 102022 113 208 A1, which is incorporated in its entirety by reference into the disclosure content of this application.
[0008] The fact that the electrically conductive sleeve is arranged in a rotationally fixed manner relative to the shaft means that the sleeve is fixedly positioned in the circumferential direction, while the shaft is rotatable relative to the rotationally fixed sleeve.
[0009] First, the individual elements of the claimed subject matter of the invention are explained in the order of their relevance or their mention in the set of claims, and particularly preferred embodiments of the subject matter of the invention are described below.
[0010] A rotor is the rotating part of an electrical machine. The rotor includes, in particular, a rotor shaft. The rotor shaft can be hollow, which reduces weight and allows for the supply of lubricant or coolant to the rotor body.
[0011] For the purposes of the invention, a rotor body is understood to mean the rotor without the rotor shaft. The rotor body is therefore composed, in particular, of a rotor core and the permanent magnets incorporated into the pockets of the rotor core or fixed circumferentially to the rotor core, as well as any axial cover parts for closing the pockets.
[0012] The rotor preferably has a plurality of rotor bodies. Particularly preferably, the rotor bodies are formed from essentially the same parts, in particular essentially identical. It is most preferred for the rotor body to be formed from identical, in particular essentially identical rotor laminations. The rotor body is therefore particularly preferably formed from a rotor lamination stack, which is composed of a plurality of laminated individual laminations or rotor laminations, generally made of electrical steel, which are layered and packaged one above the other to form a stack, the so-called rotor lamination stack. The individual laminations can be joined together in the rotor lamination stack by gluing, welding, or screwing. A rotor lamination stack can, in particular, also have permanent magnets introduced into the pockets of the rotor lamination stack or fixed circumferentially to the rotor lamination stack.It is possible for the rotor cores to be interlaced, meaning they are arranged at an angle to each other. This interlacing can be linear or V-shaped to avoid or at least reduce axial forces.
[0013] The term rotor magnet refers to the permanent magnets that are to be inserted into the pockets of the rotor core. Permanent magnets can preferably be inserted into the pockets of the rotor core. A single, larger rotor magnet designed as a bar magnet or several smaller permanent magnet elements can be provided per pocket.
[0014] A rotor lamination stack can, in particular, form a rotor body. A rotor lamination stack is understood to be a plurality of laminated individual laminations or rotor laminations, usually made of electrical steel, which are stacked and stacked together to form a so-called rotor lamination stack. The individual laminations can then be joined together in the lamination stack by gluing, welding, or screwing. A rotor lamination stack can, in particular, also have magnetic elements incorporated into the pockets of the rotor lamination stack or fixed circumferentially to the rotor lamination stack, as well as any axial cover parts for closing the pockets and the like.
[0015] The electric machine can be designed, in particular, as a rotary machine. The rotary machine can be configured, in particular, as a radial flux machine. A radial flux machine is characterized by the fact that the magnetic field lines in the air gap formed between the rotor and stator extend in a radial direction. The air gap is the gap between the rotor and stator. In a radial flux machine, this is a circular-ring-shaped gap in cross-section with a radial width corresponding to the distance between the rotor body and the stator body.
[0016] The electric machine is intended in particular for use within a drive train of a hybrid or fully electric motor vehicle. In particular, the electric machine is dimensioned such that vehicle speeds greater than 50 km / h, preferably greater than 80 km / h, and in particular greater than 100 km / h can be achieved. Particularly preferably, the electric motor has an output greater than 50 kW, preferably greater than 80 kW, and in particular greater than 150 kW. Furthermore, it is preferred that the electric machine provides rotational speeds greater than 8,000 rpm, particularly preferably greater than 12,000 rpm, and most preferably greater than 15,000 rpm.
[0017] For the purposes of this application, motor vehicles are defined as land vehicles that are propelled by mechanical power without being tied to railway tracks. A motor vehicle can, for example, be selected from the group of passenger cars (PK), trucks (HGVs), mopeds, light motor vehicles, motorcycles, buses (KOM), or tractors.
[0018] According to an advantageous embodiment of the invention, the first sleeve can have at least one passage opening on a circumferential surface through which a fluid can be guided from the sleeve into the cavity. The advantage of this embodiment is that the sleeve can thus provide cooling and / or lubrication of the shaft or components in the vicinity of the shaft. The first sleeve can therefore also be referred to as an oil lance.
[0019] Preferably, the third sleeve also has passages through which the fluid can flow, for example, in an axial direction. A further advantage of shaft grounding is that it can be used in wet electrical machines without requiring additional installation space. Due to the small dimensions of the second sleeve, the oil supply to the rotor is also not obstructed.
[0020] According to a further preferred development of the invention, it can also be provided that the shaft, starting from the cavity, has at least one through-channel extending in the radial direction through the shaft, via which the fluid can be led out of the cavity, whereby a hydraulic connection for cooling the rotor and / or stator of an electrical machine can be provided.
[0021] Furthermore, according to a similarly advantageous embodiment of the invention, the second sleeve can be fixed in the first sleeve by means of a press fit. The advantageous effect of this embodiment is that the first and second sleeves can thus be prefabricated in a manner that is favorable for production, which can correspondingly simplify subsequent assembly.
[0022] According to a further particularly preferred embodiment of the invention, it can be provided that the first sleeve, the second sleeve, the bridging sleeve, the third sleeve and the earthing rod are arranged coaxially to one another, which has proven to be advantageous in terms of assembly technology and particularly reliable in operation.
[0023] Furthermore, the invention can also be further developed such that the grounding rod protrudes axially from the second sleeve, and the third sleeve is arranged on the section protruding axially from the second sleeve by means of a press fit. The advantage of this configuration is that it can provide a particularly easy-to-assemble and reliable embodiment of a shaft grounding system. In principle, it would also be conceivable for the third sleeve to be arranged on the section protruding axially from the second sleeve by means of a positive fit, for example, by means of a spline.
[0024] The object of the invention is further achieved by an electric machine, in particular for a drive train of an electrically operated motor vehicle, comprising a stator and a rotor rotatable relative to the stator, wherein the rotor is coupled in a torque-transmitting manner to a shaft having a shaft earthing, wherein the shaft earthing is designed according to one of claims 1-6.
[0025] It may also be advantageous to further develop the invention in such a way that the first sleeve is electrically conductively and non-rotatably connected to a motor housing, which regularly leads to good earthing of the shaft.
[0026] According to a further preferred embodiment of the subject matter of the invention, the first sleeve can be connected to a fluid circuit of the electric machine, so that a fluid can be passed through the first sleeve and the shaft, thus allowing the rotor and / or the stator to be exposed to the fluid. This also allows the shaft grounding to be cooled, which is advantageous for its long-term operation.
[0027] Finally, the object of the invention can also be achieved by a kit of parts for producing a shaft earthing system, in particular for a shaft earthing system of an electrical machine, comprising a shaft into which a cavity extends axially into the shaft starting from an end face having a shaft opening; a first electrically conductive sleeve which can be inserted into the cavity; a second electrically conductive sleeve which can be arranged in and / or on the first sleeve, wherein an electrically conductive bridging sleeve is arranged in the second sleeve and the bridging sleeve has an electrically conductive earthing rod received therein and rotatably mounted relative to the bridging sleeve, wherein the earthing rod is electrically conductively connected to the bridging sleeve on the one hand and can be electrically conductively coupled to a third electrically conductive sleeve on the other hand, which third electrically conductive sleeve can in turn be coupled to the shaft;a third electrically conductive sleeve which can be positioned in an electrically conductive manner in the cavity and on the grounding rod;
[0028] This allows the components required for shaft grounding to be provided in a particularly convenient manner. The kit of parts can, for example, be a single packaging unit. Furthermore, it is possible to design the kit of parts as a collection of separate storage containers for storing the individual components or the respective component groups of the kit of parts.
[0029] The invention will be explained in more detail below with reference to figures without limiting the general inventive concept.
[0030] It shows:
[0031] Figure 1 shows a motor vehicle with an electric drive train in a schematic block diagram,
[0032] Figure 2 shows an electrical machine with a shaft earthing in a schematic axial section,
[0033] Figure 3 Detailed view of a shaft with a shaft earthing in an axial section view
[0034] Figure 4 is an axial section view of a shaft earthing in an exploded view,
[0035] Figure 5 shows a kit of parts for producing a shaft grounding system in a schematic representation. Figure 1 shows an electric machine 20 in a drive train 2 of an electrically operated motor vehicle 3. The electric machine 20—shown in Figure 2—comprises a stator 21 and a rotor 22 rotatable relative to the stator 21. The rotor 22 is coupled to a shaft 4 having a shaft grounding system 1 in a torque-transmitting manner.
[0036] Figure 3 shows a detailed view of the shaft grounding device 1 for discharging an electrical charge and / or voltage. The shaft grounding device has a rotatably mounted shaft 4, into which a cavity 7 extends axially from an end face 6 having a shaft opening 5. A first sleeve 8, which is non-rotatable relative to the shaft 4 and electrically conductive, passes through the shaft opening 5 and extends axially into the cavity 7 of the shaft 4. The electrically conductive sleeve 8 is thus non-rotatably arranged relative to the shaft 4.
[0037] The sleeve 8 is thus fixedly positioned in the circumferential direction, while the shaft 4 is rotatable relative to the non-rotatable sleeve 8. The first sleeve 8 is electrically conductively and non-rotatably connected to a motor housing 23 via a housing cover 25. The shaft 4 is mounted in the motor housing 23 via unspecified rolling bearings.
[0038] From the combination of Figure 1 and Figure 2 it can be seen that the first sleeve 8 is connected to a fluid circuit 24 of the electric machine 20, so that a fluid 14 can be passed through the first sleeve 8 and the shaft 4 and thus the rotor 22 and / or the stator 21 can be supplied with the fluid 14.
[0039] A second electrically conductive hollow cylindrical sleeve 9 is arranged in the first hollow cylindrical sleeve 8. An electrically conductive bridging sleeve 26 is arranged in this second sleeve, which has an electrically conductive grounding rod 10 received therein and rotatably mounted relative to the bridging sleeve 26. This grounding rod 10 is electrically conductively connected on the one hand to the bridging sleeve 26 and on the other hand to a third electrically conductive sleeve 11. The third hollow cylindrical sleeve 11 is in turn connected to the shaft 4. In the exemplary embodiment shown, the second sleeve 9 is fixed in the first sleeve 8 by means of a press fit. The first sleeve 8, the second sleeve 9, the bridging sleeve 26, the third sleeve 11 and the grounding rod 10 are arranged coaxially to one another.The grounding rod 10 protrudes axially from the second sleeve 9, with the third sleeve 11 being arranged in a rotationally fixed manner by means of a press fit on the section 16 protruding axially from the second sleeve 9. In principle, it would also be conceivable for the third sleeve 11 to be arranged in a rotationally fixed manner by means of a positive fit, for example, by means of a spline, on the section 16 protruding axially from the second sleeve.
[0040] The first sleeve 8 has at least one passage opening 13 on a lateral surface 12, through which a fluid 14 can be guided from the sleeve 8 into the cavity 7. The shaft 4, in turn, has at least one through-channel 15 extending radially through the shaft 4 from the cavity 7, through which the fluid 14 can be guided out of the cavity 7. In the embodiment shown, the first sleeve 8 thus functions as an oil lance. This is sealed from the environment by the seal 17 in the first sleeve 8.
[0041] For electrical contact, the interior of the second sleeve 9 is filled with an electrically conductive liquid 19, wherein the grounding rod 10 and the second sleeve 9 are electrically conductively contacted with one another via the interposition of the electrically conductive liquid 19 and a sleeve 26 arranged coaxially between the grounding rod 10 and the second sleeve 9. For example, the electrically conductive liquid is in the form of an electrically conductive oil, grease, liquid metal, or ionic liquid. The structure described below also makes it possible to use oils and greases without any special conductive components. The second sleeve 9 is separated in a fluid-tight manner from the first sleeve 8 or from the area surrounding the shaft grounding 1, thus preventing the electrically conductive liquid 19 from escaping from the second sleeve 9.This creates a shaft earthing system 1 that can be inserted axially into the shaft and operates effectively, and can be used in both dry and wet environments. Since the electrical contact between the second sleeve 9 and the earthing rod 10 is established via the electrically conductive fluid 19 and the sleeve 26 arranged coaxially between the earthing rod 10 and the second sleeve 9, the shaft earthing system 1 operates with low friction and wear during operation of the electrical machine 20, preventing conductive abrasion. Furthermore, the electrically conductive fluid 19 enables position-independent operation, as the electrically conductive fluid 19 always ensures contact between the bridging sleeve 26 and the earthing rod 10.
[0042] In a preferred embodiment of the invention, the annular gap has a single annular gap width, i.e., a distance between the inner circumference of the bridging sleeve 26 and the outer circumference of the grounding rod 10 of less than 200 µm, preferably less than 60 µm, and especially less than 30 µm. The small annular gap width increases the electrical conductivity and / or reduces the electrical resistance of the shaft grounding 1.
[0043] Preferably, the electrically conductive liquid - the fluid 14 - is itself electrically conductive and / or is mixed with electrically conductive additives. In principle, the electrically conductive liquid can be in the form of a liquid metal or an ionic liquid or the like. However, the electrically conductive liquid is preferably in the form of a conductive oil or grease. Although conductive oils and greases generally have a lower conductivity than, for example, liquid metal, they are less toxic and / or easier to handle. By using the bridging sleeve 26, the lower conductivity can be compensated, so that the bridging sleeve 26 enables a technically sensible use of conductive oils and / or greases. In particular, an electrically conductive liquid with an electrical conductivity at 25°C of greater than 10,000 nS / m, in particular greater than 30,000 nS / m, is used, such as, for example,Oils or greases with conductive additives or other oil-free and / or grease-free fluids. These are particularly suitable for low-resistance applications.
[0044] For lower transmission resistance requirements, i.e., for higher transmission resistances, even conventional gear oils can be used thanks to the bridging sleeve. In an alternative embodiment, the electrically conductive fluid then has an electrical conductivity at 25°C of less than 500 nS / m, preferably less than 300 nS / m, and in particular less than 100 nS / m or less than 50 nS / m. On the other hand, it is preferred that the electrical conductivity is greater than 1 nS / m, preferably greater than 5 nS / m, and in particular greater than 10 nS / m. With this electrical conductivity, it is possible to use conventional gear oils, eliminating the need for specially adapted fluids, in particular those with conductive additives. This is made possible by the use of the bridging sleeve 26 and the resulting reduced annular gap width.
[0045] The current path through the shaft earthing 1 is indicated by the dashed line in Figure 3.
[0046] The grounding rod 10 is designed as a metallic, in particular electrically conductive, cylindrical rod which is pushed so far in the axial direction into the second sleeve 9 that the grounding rod 10 is partially immersed in the electrically conductive liquid 19 in the axial region of the second sleeve 9. An electrical path is thus created between the grounding rod 10 and the second sleeve 9 via the electrically conductive liquid 19, the bridging sleeve 26 and the second sleeve 9. The induced currents are safely diverted to ground via this path of least resistance, so that the currents are diverted away from the bearing through this system. The bearings of connected assemblies, such as gearboxes, etc., can also be protected in this way. The shaft grounding 1 has a seal 18, with the grounding rod 10 being sealingly guided in the second sleeve 9 via the seal 18.The earthing rod 10 is in radial contact with the second sleeve 9 via the seal 18.
[0047] For assembly, a kit of parts 30 for producing the shaft earthing 1 can be provided, as outlined in Figure 5.
[0048] The kit of parts 30 comprises a shaft 4, into which a cavity 7 extends axially from an end face 6 having a shaft opening 5. The kit of parts 30 also has a first electrically conductive sleeve 8, which can be inserted into the cavity 7, and a second electrically conductive sleeve 9, which can be arranged in and / or on the first sleeve 8, wherein the second sleeve 9 has an electrically conductive grounding rod 10 received therein and rotatably mounted relative to the second sleeve 9, wherein the grounding rod 10 is electrically conductively connected to the second sleeve 9. An electrically conductive bridging sleeve 26 is arranged in the second sleeve 9 and is designed to be rotatable relative to the electrically conductive grounding rod 10.
[0049] Furthermore, the kit of parts 30 has a third electrically conductive sleeve 11, which can be positioned in the cavity 7 and on the grounding rod 10. This assembly can also be clearly understood from Figure 4.
[0050] The invention is not limited to the embodiments illustrated in the figures. The above description is therefore not to be considered restrictive, but rather explanatory. The following claims are to be understood in such a way that a stated feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. Where the claims and the above description define 'first' and 'second' features, this designation serves to distinguish between two similar features without establishing a priority.
[0051] List of reference symbols
[0052] 1 shaft grounding
[0053] 2 Drivetrain
[0054] 3 Motor vehicle
[0055] 4th wave
[0056] 5 Shaft opening
[0057] 6 Front side
[0058] 7 Cavity
[0059] 8 first sleeve
[0060] 9 second sleeve
[0061] 10 Earthing rod
[0062] 11 third sleeve
[0063] 12 Shell surface
[0064] 13 Passage opening
[0065] 14 Fluid
[0066] 15 Through channel
[0067] Section 16
[0068] 17 Seal
[0069] 18 Seal
[0070] 19 Liquid
[0071] 20 machines
[0072] 21 Stator
[0073] 22 Rotor
[0074] 23 Engine housing
[0075] 24 Fluid circuit
[0076] 25 housing cover
[0077] 26 Bridging sleeve
[0078] 30 kit of parts
Claims
Claims 1. Shaft earthing (1) for discharging an electrical charge and / or voltage from a shaft (4), in particular for a drive train (2) of an electrically operable motor vehicle (3), comprising a rotatably mounted shaft (4), into which a cavity (7) extends axially from an end face (6) having a shaft opening (5), and a first sleeve (8) that is non-rotatable relative to the shaft (4) and electrically conductive, passes through the shaft opening (5) and extends axially into the cavity (7) of the shaft (4), characterized in that a second electrically conductive sleeve (9) is arranged in and / or on the first sleeve (8), in which sleeve, in turn, an electrically conductive bridging sleeve (26) is arranged, and the bridging sleeve (26) has an electrically conductive earthing rod (10) received therein and rotatably mounted relative to the bridging sleeve (26).wherein the earthing rod (10) is on the one hand electrically conductively connected to the bridging sleeve (26) and on the other hand electrically conductively connected to a third electrically conductive sleeve (11), which in turn is connected to the shaft (4).
2. Shaft earthing (1) according to claim 1, characterized in that the first sleeve (8) has at least one passage opening (13) on a lateral surface (12), via which a fluid (14) can be guided from the sleeve (8) into the cavity (7).
3. Shaft earthing (1) according to claim 1 or 2, characterized in that the shaft (4) has, starting from the cavity (7), at least one through-channel (15) extending in the radial direction through the shaft (4), via which the fluid (14) can be led out of the cavity (7) can.
4. Shaft earthing (1) according to one of the preceding claims, characterized in that the second sleeve (9) is fixed in the first sleeve (8) by means of a press fit.
5. Shaft earthing (1) according to one of the preceding claims, characterized in that the first sleeve (8), the second sleeve (9), the bridging sleeve (26), the third sleeve (11) and the earthing rod (10) are arranged coaxially to one another.
6. Shaft earthing (1) according to one of the preceding claims, characterized in that the earthing rod (10) projects axially from the second sleeve (9) and the third sleeve (11) is arranged on the section (16) projecting axially from the second sleeve (9) by means of a press fit or a form fit.
7. Electrical machine (20), in particular for a drive train (2) of an electrically operated motor vehicle (3), comprising a stator (21) and a rotor (22) rotatable relative to the stator (21), wherein the rotor (22) is coupled in a torque-transmitting manner to a shaft (4) having a shaft earthing (1), characterized in that the shaft earthing (1) is designed according to one of claims 1 -6.
8. Electrical machine (20) according to claim 7, characterized in that the first sleeve (8) is electrically conductively and non-rotatably connected to a motor housing (23).
9. Electrical machine (20) according to claim 7 or 8, characterized in that the first sleeve (8) is connected to a fluid circuit (24) of the electrical machine (20), so that a fluid (14) can be passed through the first sleeve (8) and the shaft (4) and thus the rotor (22) and / or the stator (21) can be acted upon by the fluid (14).
10. Kit-of-parts (30) for producing a shaft earthing (1), in particular for a shaft earthing (1) of an electrical machine (20), comprising - A shaft (4) into which a cavity (7) extends axially from an end face (6) having a shaft opening (5) into the shaft (4), - A first electrically conductive sleeve (8) which can be inserted into the cavity (7), - A second electrically conductive sleeve (9) which can be arranged in and / or on the first sleeve (8), wherein an electrically conductive bridging sleeve (26) is arranged in the second sleeve (9) and the bridging sleeve (26) has an electrically conductive earthing rod (10) received therein and rotatably mounted relative to the bridging sleeve (26), wherein the earthing rod (10) is, on the one hand, electrically conductively connected to the bridging sleeve (26) and, on the other hand, can be electrically conductively coupled to a third electrically conductive sleeve (11), which in turn can be coupled to the shaft (4), A third electrically conductive sleeve (11) which can be positioned in an electrically conductive manner with the shaft (4) in the cavity (7) and on the earthing rod (10).