Shaft grounding, electric machine and electrically operated axle drive train

Abstract

The invention relates to a shaft grounding device (1) for dissipating an electrical charge and / or voltage from a shaft (4), in particular for a drive train of an electrically operated motor vehicle (3), comprising a rolling bearing (5) arranged on the shaft (4) with an inner ring (6) and an outer ring (7) as well as rolling elements (8) arranged to roll between the inner ring (6) and the outer ring (7), wherein the shaft grounding device (1) has an inner contact ring (9) and an outer contact ring (10) arranged coaxially thereto, which are connected to each other via a plurality of spring elements (11) acting elastically in the axial direction, wherein the inner contact ring (9) bears against the inner ring (6) under spring force in the axial direction and the inner contact ring (9) bears against an end face (12) of the inner ring (6).

Description

The present invention relates to a shaft ground for discharging an electrical charge and / or voltage from a shaft, in particular for a drive train of an electrically operable motor vehicle, comprising a rolling bearing arranged on the shaft and having an inner ring and an outer ring and rolling bodies arranged rolling between the inner ring and the outer ring. The invention further relates to an electric machine and an electrically operable axle drive train.Electric machines, in particular electric traction machines, are electrically charged during operation by induced wave voltages. In addition, circulating high-frequency currents can arise in motors of high power classes. Such electric machines generally have a rotor and a rotor shaft, which are mounted in a housing via rolling bearings. Owing to electrical discharges or the circulating high-frequency currents, damage to the rolling bearings can occur. In order to avoid this, dissipation of this electrical energy is necessary. For this purpose, for example, grounding brushes or radially acting grinding elements (carbon brushes) are known, which serve for grounding the rotor shaft. Furthermore, axially acting systems for shaft grounding are known.The publication DE 10 2020 119 719 A1, for example, discloses a discharge device for a motor for discharging an electrical charge and / or voltage from a rotor via a shaft to a housing, having a first contact module for electrical and mechanical connection to the shaft, having a second contact module for electrical and mechanical connection to the housing, wherein the first and the second contact modules are rotatable relative to one another and are electrically connected to one another in an interior space sealed with respect to 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.For the bearings of an electric motor, in addition to the mechanical wear due to shaft rotation, electric currents flowing from the rotor shaft to ground via the bearings are therefore a further cause of wear. Stresses of some magnitude can overcome the isolation properties of bearing lubrication, creating sparks which can lead to pitting, corrugated surface, fusion craters and eventually to premature failure of the bearings and motor. A bearing voltage, i.e. the voltage between the two bearing shells, usually correlates with a shaft voltage, i.e. the voltage between the shaft and ground.Due to capacitive coupling between the stator winding and the rotor and the high-frequency actuation of the stator phases, a voltage potential is formed on the rotor bearings, which voltage potential can damage the bearings by means of so-called discharge currents via the bearings. As countermeasures, for example, the bearings are insulated (for example by means of ceramic balls) or the shaft is grounded by means of a sliding ground contact (electrically parallel to the bearings).Furthermore, DE 10 2017 109 049 A1 already discloses a device and a method for compensating a wave voltage on a wave. In this case, by means of a compensation circuit for generating a compensation voltage, the compensation voltage is coupled directly or indirectly to the rotor shaft via a coupling element. A detection of the shaft voltage takes place directly or indirectly on the rotor shaft via a tapping element.EP 1 455 434 A1 discloses an electric motor having a stator and a rotor which is rotatably mounted in or on the stator by means of a bearing arrangement. For electrical insulation, at least one sleeve and / or plastic layer electrically insulating the bearing arrangement with respect to the stator or rotor and / or the rotor shaft with respect to the rotor is provided. As a result, capacitive displacement currents through the bearings are prevented, which primarily arise at high pulse or switching frequencies. These can lead to a reduction in the service life of the bearingsDE 20 2017 107 755 U1 describes a device for reducing harmful bearing voltages in an electric machine fed by an intermediate circuit voltage of an intermediate circuit, comprising a stator having windings and insulated from earth and a rotor and a motor shaft, wherein furthermore a rotor-side bearing and a stator-side bearing are each designed insulated from earth and wherein the rotor and the stator are electrically connected to one another via a bypass capacitance with a predetermined capacitance.DE 299 16 854 U1 discloses a rolling bearing which has a so-called current bridge between the inner ring and the outer ring in order to prevent damage due to current passage.DE 10 2017 122 466 A1 relates to a drive device which has a rotatable rotary shaft, a rotor which is connected to the rotary shaft in a rotationally fixed manner, and at least one balancing disk. In order to make it possible to produce the drive device without external alignment devices, it is proposed that the balancing disk has direct contact with the rotary shaft and is connected thereto in a rotationally fixed manner, wherein the balancing disk and the rotor lie next to one another with at least one partial region in a fixing plane orthogonal to the rotary shaft.In the field of electric mobility, such electric machines are also frequently integrated into an axle drive train, which forms a structural unit consisting of power electronics (inverter), electric machine and transmission. This structural unit must be connected to the vehicle ground for potential compensation. This is generally effected via a conductor which is located between the body and the housing of the structural unit.In such final drive trains, usually also a grounding element is located in the gear set of the transmission and / or directly on the rotor shaft of the electric machine, in order to be able to secure the bearings located there particularly well against harmful currents or discharges with the structural proximity. These grounding elements are usually electrically conductively connected to the housing of the axle drive train. For example, DE 10 2020 111 929 A1 and DE 10 2020 101 417 A1 show a wave ground for the dissipation of an electric charge and / or voltage from a wave.In principle, such grounding elements have the problem that dry running grounding elements generate electrically conductive abrasion, which can thus lead to short circuits in the electric machine. Wet running grounding elements, on the other hand, often have the problem that they do not function efficiently under oil at all operating points.It is therefore the object of the invention to reduce or completely avoid the disadvantages known from the prior art and to provide a shaft grounding which is of simple construction and has a high operational reliability and also durability. It is also the object of the invention to realize an improved electric machine and an improved electrically operable axle drive train.This object is achieved by a shaft grounding for discharging an electrical charge and / or voltage from a shaft, in particular for a drive train of an electrically operable motor vehicle, comprising a rolling bearing arranged on the shaft, having an inner ring and an outer ring, and rolling bodies arranged rolling between the inner ring and the outer ring, wherein the shaft grounding has an inner contact ring and an outer contact ring arranged coaxially thereto, which are connected to one another via a plurality of spring elements acting elastically in the axial direction, wherein the inner contact ring bears against the inner ring under the action of a spring force in the axial direction.This makes it possible to provide an axially acting system for shaft grounding, which functions efficiently in dry and wet electric machines and at all operating points. In this case, an existing installation space, for example in an electric machine or a transmission arrangement, can be used without enlarging the electric machine axially or radially. The wave grounding can also be integrated later in existing applications in a simple manner. In this way, on the one hand, bearing damage to the rotatably mounted shaft can be prevented and, on the other hand, electromagnetic compatibility (EMC) can be improved in order to reduce or prevent the interference with other devices.It is also provided that the inner contact ring abuts an end face of the inner ring. The advantage of this embodiment is that a particularly large and generally already ground contact surface can be provided as a result.A further advantage of the wave grounding according to the invention is that the wave grounding can be constructed as an independent system and thus be offered as a prefabricated unit.The wave grounding according to the invention can be used as dry-running and / or wet-running wave grounding.A further advantage of the wave grounding according to the invention is that the individual components of the wave grounding are simple to produce and have no geometries that make it possible to make a manufacturing effort, which also has a positive impact on the cost price of the wave grounding.It has also proven advantageous that only comparatively low axial spring forces are required in order to produce the electrical sliding contact between the inner contact ring and the inner ring of the rolling bearing, so that the shaft ground can also be operated with very low friction.The shaft grounding according to the invention is designed in particular to run in a wet chamber filled with oil, for example, and nevertheless have a low electrical resistance and thus ensure good electrical dissipation. The good conductivity, especially also in the wet chamber, is also achieved, inter alia, by the comparatively large circumferential contact surface of the inner contact ring.By arranging the inner and outer contact rings via the spring elements, both a radial tolerance chain, for example between a housing and the rotor shaft of an electric machine, and an axial tolerance chain can be compensated.According to the invention, the shaft ground has a rolling bearing. A rolling bearing can be used in particular to enable rotational movements with as low friction losses as possible. Roller bearings can be used in particular for fixing and / or mounting axles and shafts, wherein, depending on the design, they absorb radial and / or axial forces and at the same time enable the rotation of the shaft or of the components mounted in this way on an axle. The terms rolling bearing and rolling bearing are used as synonyms in this application.For this purpose, rolling bodies are preferably arranged between an inner ring and an outer ring of the rolling bearing. Between these three main components inner ring, outer ring and the rolling elements, rolling friction generally occurs mainly within the rolling bearing. Since the rolling bodies in the inner and outer rings can preferably roll on hardened steel surfaces with optimized lubrication, the rolling friction of such bearings is relatively low.Particularly preferably, a rolling bearing in connection with this invention is configured as a single- or multi-row rolling bearing.The inner ring can be formed from a metallic and / or ceramic material. It is conceivable in principle to form the inner ring in one part or in multiple parts, in particular in two parts.The inner ring may have an inner ring recess. In an inner ring recess, in particular a cover disk, sealing disk and / or seal can be arranged in particular in a force-fit and / or form-fit manner. The inner ring recess is preferably formed as a circumferential groove in the inner ring.The outer ring can be formed from a metallic and / or ceramic material. It is conceivable in principle to configure the outer ring in one part or in multiple parts, in particular in two parts.The outer ring may have an outer ring recess. In an outer ring recess, in particular a cover disk, sealing disk and / or seal can be arranged in particular in a force-fit and / or form-fit manner. The outer ring recess is preferably formed as a circumferential groove in the outer ring.The rolling elements have the shape of a ball or a roller depending on the type of rolling bearing. They roll on the raceways of the rolling bearing and have the task of transmitting the force acting on a radial rolling bearing from the outer ring to the inner ring and vice versa. Roller-shaped rolling elements are also referred to as roller rolling elements and spherical rolling elements as bearing balls.Roller-shaped rolling bodies can be selected, for example, from the group of symmetrical pendulum rollers, asymmetrical pendulum rollers, cylindrical rollers, needle rollers and / or conical rollers.The rolling bodies can be guided in a cage and be spaced apart from one another.The rolling bodies can roll within the rolling bearing, in particular on the inner ring raceway of the inner ring. For this purpose, the surface of the inner ring raceway can advantageously be designed to be correspondingly abrasion-resistant, for example also by a corresponding surface treatment method and / or by applying a corresponding additional material layer. The inner ring raceway can be planar or profiled. A profiled configuration of the inner ring raceway can serve, for example, for guiding the rolling bodies on the inner ring raceway. A planar formation of the inner ring raceway, on the other hand, can allow for a certain axial displaceability of the rolling bodies on the inner ring raceway, for example.The rolling bodies can roll within the rolling bearing, in particular on the outer ring raceway of the outer ring. For this purpose, the surface of the outer ring raceway can advantageously be designed to be correspondingly abrasion-resistant, for example also by a corresponding surface treatment method and / or by applying a corresponding additional material layer.The outer ring raceway can be planar or profiled. A profiled configuration of the outer ring raceway can serve, for example, for guiding the rolling bodies on the outer ring raceway. A planar formation of the outer ring raceway, on the other hand, can allow for a certain axial displaceability of the rolling bodies on the outer ring raceway, for example.A rolling bearing can have a cage, wherein the cage guides the rolling bodies. The cage is designed such that the rolling element balls and / or the rolling element rollers are spaced apart from one another, so that, for example, the friction and heat generation of the rolling elements is kept as low as possible. Furthermore, the cage holds the rolling element balls and / or rolling element rollers at a fixed distance from one another during rolling, as a result of which a uniform load distribution can be achieved. The cage is embodied in multiple pieces, with a first cage half and a second cage half. The first cage half and / or the second cage half are / is embodied in a ring-shaped manner so as to be closed all around. Preferably, the first cage half and the second cage half are substantially identical.A rolling bearing may have a seal to prevent lubricant from leaking out of the rolling bearing or dirt or moisture from entering the rolling bearing. For this purpose, the seals used can be provided with one or more sealing lips which can bear against a component of the rolling bearing. These are designed in such a way that they seal the bearing as much as possible over the entire service life, on the one hand, and the friction due to the bearing seal is not too high, on the other hand.Further advantageous embodiments of the invention are specified in the dependent claims. The features listed individually in the dependent claims can be combined with one another in a technically expedient manner and can define further embodiments of the invention. In addition, the features specified in the claims are more precisely described and explained in the description, further preferred embodiments of the invention being presented.According to a further preferred development of the invention, it can also be provided that the inner contact ring is of annular disk-like design, wherein an end face of the contact ring bears against the end face of the inner ring, which has likewise proven to be particularly advantageous with regard to good axial contact.Furthermore, according to a likewise advantageous embodiment of the invention, it can be provided that the annular-disk-like inner contact ring has a circumferential contour deviating from the circular shape on its inner circumferential surface. The special circumferential contour of the inner contact ring can limit the thickness of the lubricating film between the inner contact ring and the inner ring of the rolling bearing. Thus, a constant derivation can be ensured even at higher rotational speeds. At the same time, the system thus operates with very low friction and no conductive abrasion occurs.According to a further particularly preferred embodiment of the invention, it can be provided that the spring elements are designed as spring tongues. In this way, spring elements that can be produced particularly easily and cost-effectively can be provided from a manufacturing standpoint.The spring tongues can be connected in particular in a materially integral manner to the inner contact ring and / or the outer contact ring, for example by welding, soldering or adhesive bonding.Furthermore, the invention can also be further developed to the effect that the spring tongues have a curvature, wherein the opening of the curvature faces the rolling bearing.In a likewise preferred embodiment variant of the invention, it can also be provided that the outer contact ring has an outer diameter which substantially corresponds to the outer diameter of the outer ring. This can achieve the effect that the rolling bearing and the outer contact ring can be combined to form a module and / or the rolling bearing and the outer contact ring can be pushed into a common bearing seat.It can also be advantageous to further develop the invention to the effect that the outer contact ring has a contour which is U-shaped in cross section, wherein the spring elements are supported on a free leg of the contact ring. Due to the U-shaped contour of the outer contact ring, the spring elements are supported over a comparatively large surface, whereby the load can be distributed more uniformly. The U-shaped contour of the outer contact ring and the support of the spring elements on a free leg can furthermore lead to an increased rigidity of the system. This means that the system is less susceptible to deformations or compliances under load.The object of the invention is furthermore achieved by an electric machine, in particular for a drive train of an electrically operable motor vehicle, comprising a stator and a rotor with a rotor shaft which is rotatably mounted relative to the stator, wherein the electric machine has a shaft ground according to one of the preceding claims 1-8, wherein the inner ring of the rolling bearing is connected to the rotor shaft in a rotationally fixed manner and the outer contact ring is connected to a connection structure of the electric machine.The electric machine can be designed in particular as a rotary machine. The rotary machine can be configured in particular as a radial flow machine. A radial flux machine is characterized in that the magnetic field lines extend in the radial direction in the air gap formed between the rotor and the stator. The air gap is the gap existing between the rotor and the stator. In a radial flux machine, this is a gap having an annular cross section and a radial width corresponding to the distance between the rotor body and the stator body.The electric machine is provided in particular for use within a drive train of a hybrid or fully electrically driven 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 a power greater than 50 kW, preferably greater than 80 kW and in particular greater than 150 kW. It is furthermore preferred that the electric machine provides rotational speeds greater than 8,000 U / min, particularly preferably greater than 12,000 U / min, very particularly preferably greater than 15,000 U / min.Motor vehicles in the sense of this application are land vehicles which are moved by machine power without being bound to railway tracks. A motor vehicle can be selected, for example, from the group of passenger cars (passenger cars), trucks (trucks), small-size trucks, light-weight trucks, bicycles, buses (COM) or tractorsFinally, the object of the invention can also be achieved by an electrically operable axle drive train comprising an electric machine comprising a stator and a rotor, which is rotatably mounted relative to the stator and has a rotor shaft, and a transmission arrangement coupled to the rotor of the electric machine in a torque-transmitting manner, wherein a shaft grounding according to one of the preceding claims 1-8 is arranged on a shaft lying in the torque flow of the axle drive train.The invention will be explained in more detail below with reference to figures without limiting the general concept of the invention.It shows: FIG. 1 shows a shaft ground in an axially cut exploded view, FIG. 2 shows a shaft grounding in a state installed on a rotor shaft in an axial sectional view, FIG. 3 shows an electrically operable axle drive train in a schematic block switching illustration.FIG. 1 shows a shaft ground 1 for the dissipation of an electrical charge and / or voltage from a shaft 4, in particular for a drive train of an electrically operable motor vehicle 3, as is also outlined in FIG. 3.The shaft ground 1 has a rolling bearing 5 arranged on the shaft 4 with an inner ring 6 and an outer ring 7 and rolling bodies 8 arranged rolling between the inner ring 6 and the outer ring 7.The shaft ground 1 furthermore has an inner contact ring 9 and an outer contact ring 10 which is arranged coaxially thereto and which are connected to one another via a plurality of spring elements 11 which act resiliently in the axial direction, wherein the inner contact ring 9 bears against the inner ring 6 under the action of a spring force in the axial direction. The inner contact ring 10 is here embodied in the manner of an annular disk, wherein an end face 16 of the contact ring 10 abuts the end face 12 of the inner ring 6. Since the inner ring 6 is already finish-ground on the end face as a result of the production, no further component is required on the rotating side.The annular disk-like contact ring 10 has a circumferential contour on its inner circumferential surface 17 that deviates from the circular shape, whereby the lubricating film between the inner contact ring 10 and the inner ring 6 can be influenced.The spring elements 11 are designed as spring tongues 13, wherein the spring tongues 13 have a curvature 18 and the opening 19 of the curvature 18 faces the rolling bearing 5, which can be easily seen with reference to FIG. 2.What can also be gathered from FIG. 2 is that the outer contact ring 10 has an outer diameter which substantially corresponds to the outer diameter of the outer ring 7. The outer contact ring 10 also has a contour which is U-shaped in cross section, wherein the spring elements 11 are supported on a free leg 15 of the contact ring 10. In the exemplary embodiment shown, the spring tongues 13 are welded or soldered to the outer contact ring 10. The rolling bearing 5 is secured axially via the sleeve 21, against which the seal 20 also bears.The outer contact ring 10 and the inner contact ring 9 are first connected to one another coaxially via the spring tongues 13 and thus form a common structural unit which is inserted together with the rolling bearing 5 in the connection structure 34 and is axially secured via the clamping ring 22. Via the path of the lowest electrical resistance, the induced currents can be safely diverted to earth (housing), that is to say via the outer contact ring 10, the spring elements 11, the inner contact ring 9 and the inner ring 6. The bearings of the connected assemblies are thus also protected reliably.It is understood that a lower electrical resistance is present between the outer contact ring 10, via the spring elements 11, the inner contact ring 9 to the inner ring 6, than between the inner ring 6 and the outer ring 7 of the rolling bearing 5.FIG. 3 shows an electric machine 30 in a drive train of an electrically operable motor vehicle 3, comprising a stator 31 and a rotor 32 with a rotor shaft 33, which rotor is rotatably mounted relative to the stator 31, wherein the electric machine 30 has a shaft ground 1, the contact ring 10 of which is connected to the rotor shaft 33 in a rotationally fixed manner.Finally, FIG. 3 also shows an electrically operable axle drive train 40 comprising the electric machine 30 with a stator 31 and a rotor 32 with a rotor shaft 33 rotatably mounted relative to the stator 31, as well as a transmission arrangement 41 coupled to the rotor 32 of the electric machine 30 in a torque-transmitting manner, wherein a shaft ground 1 is arranged on a shaft 42 lying in the torque flow of the axle drive train 40. In the example shown, the transmission arrangement 41 is designed as a planetary transmission.The invention is not limited to the embodiments shown in the figures. The foregoing description is, therefore, not to be considered as limiting, but illustrative. The following claims should be understood to mean that a said feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. If the patent claims and the above description define "first" and "second" features, this designation serves to distinguish two features of the same type without specifying a ranking.List of reference characters1 Shaft grounding 3 Motor vehicle 4 Shaft 5 Rolling bearing 6 Inner ring 7 Outer ring 8 Rolling bodies 9 Inner contact ring 10 Outer contact ring 11 Spring elements 12 End side (from 6) 13 Spring tongues 15 Leg 16 End side (from 9) 17 Lateral surface 18 Curvature 19 Opening 20 Seal 21 Sleeve 22 Clamping ring 30 Electric machine 31 Stator 32 Rotor 33 Rotor shaft 34 Connection structure 40 Axle drive train 41 Transmission arrangement 42 Shaft

Claims

Shaft grounding (1) for discharging an electrical charge and / or voltage from a shaft (4), in particular for a drive train (40) of an electrically operable motor vehicle (3), comprising a rolling bearing (5) arranged on the shaft (4) and having an inner ring (6) and an outer ring (7) and rolling bodies (8) arranged rollingly between the inner ring (6) and the outer ring (7), wherein the shaft grounding (1) has an inner contact ring (9) and an outer contact ring (10) arranged coaxially thereto, which are connected to one another via a plurality of spring elements (11) acting resiliently in the axial direction, wherein the inner contact ring (9) bears against the inner ring (6) under the action of a spring force, characterized in that the inner contact ring (9) bears against an end side (12) of the inner ring (6).Shaft grounding (1) according to claim 1, characterised in that the inner contact ring (9) is designed in the manner of an annular disc, wherein an end face (16) of the inner contact ring (9) abuts the end face (12) of the inner ring (6).The shaft grounding (1) according to claim 2, characterized in that the ring-disk-like inner contact ring (9) has a circumferential contour deviating from the circular shape on its inner circumferential surface (17).Shaft grounding (1) according to one of the preceding claims, characterized in that the spring elements (11) are designed as spring tongues (13).Shaft grounding (1) according to claim 4, characterised in that the spring tongues (13) have a curvature (18), wherein the opening (19) of the curvature (18) faces the rolling bearing (5).The shaft grounding (1) according to any one of the preceding claims, characterized in that the outer contact ring (10) has an outer diameter which substantially corresponds to the outer diameter of the outer ring (7).The shaft grounding (1) according to one of the preceding claims, characterized in that the outer contact ring (10) has a contour which is U-shaped in cross section, wherein the spring elements (11) are supported on a free leg (15) of the contact ring (10).Electric machine (30), in particular for a drive train (40) of an electrically operable motor vehicle (3), comprising a stator (31) and a rotor (32), which is rotatably mounted relative to the stator (31) and has a rotor shaft (33), wherein the electric machine (30) has a shaft grounding (1) according to one of the preceding claims 1 - 7, wherein the inner ring (6) of the rolling bearing (5) is connected in a rotationally fixed manner to the rotor shaft (33) and the outer contact ring (10) is connected to a connection structure (34) of the electric machine (30).Electrically operable axle drive train (40) comprising an electric machine (30) comprising a stator (31) and a rotor (32), which is rotatably mounted relative to the stator (31) and has a rotor shaft (33), and a transmission arrangement (41), which is coupled to the rotor (32) of the electric machine (30) in a torque-transmitting manner, wherein a shaft grounding (1) according to one of Claims 1 - 7 is arranged on a shaft (42) lying in the torque flow of the axle drive train (40).

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