Electric axle drive train with grounding and adjustable bearing preload

Abstract

Electric axle drive train (1), in particular for driving a motor vehicle (2), comprising an electric machine (3) with a stator (4) and a rotor (5) which is rotatably mounted relative to the stator (4) and is connected to a transmission input shaft (7) of a transmission arrangement (6), and the transmission arrangement (6) further comprises a transmission output shaft (8) at which, during operation of the axle drive train (1), a different rotational speed than that of the transmission input shaft (7) is applied, and the rotor (5) has an axis of rotation (9) which is arranged coaxially with an axis of rotation (10) of the transmission input shaft (7) and the transmission output shaft (8), characterized in that a first electrically conductive contact body (11) is spring-loaded in the axial direction against the transmission input shaft (7) and a second electrically conductive contact body (12) is spring-loaded in the axial direction against the transmission output shaft (8), wherein the first contact body (11) and the second contact body (12) are arranged coaxially to the rotational axis (10) of the transmission input shaft (7), wherein the electrical machine (3) is designed as an axial flux machine, wherein the electrical machine (3) designed as an axial flux machine has a rotor shaft (13) designed as a hollow shaft, into which a bearing shaft (14) fixed relative to the stator (4) engages, wherein the rotor shaft (13) is rotatably mounted relative to the bearing shaft (14) by means of a rolling bearing (16) and the first contact body (11) is arranged in and / or on the bearing shaft (14) and a bearing preload device (20) is arranged on the bearing shaft (14), by means of which a bearing preload in the rolling bearing (16) can be adjusted, wherein the first contact body (11) is arranged on and / or in the bearing preload device (20).

Description

The present invention relates to an electric axle drive train, in particular for driving a motor vehicle, comprising an electric machine having a stator and a rotor which is mounted rotatably relative to the stator and is connected to a transmission input shaft of a transmission arrangement, and the transmission arrangement furthermore has a transmission output shaft, on which a rotational speed different from the transmission input shaft is present during operation of the axle drive train, and the rotor has an axis of rotation which is arranged coaxially with an axis of rotation of the transmission input shaft and of the transmission output shaft.Electric motors are increasingly used for the drive in motor vehicles in order to create alternatives to internal combustion engines that require fossil fuels. In order to improve the suitability of electric drives for all days and in addition to be able to offer users the usual riding comfort, considerable efforts have already been made.A detailed illustration of an electric drive is given in an article of the journal ATZ 113. Vol. 05 / 2011, pages 360-365 of Erik Schneider, Frank Fickl, Bernd Cebulski and Jens Liebold, with the title: High-Integration and Flexible Electric Drive Unit for E-vehicles. In this article, a drive unit for an axle of a vehicle is described, which drive unit comprises an electric motor which is arranged concentrically and coaxially with a bevel gear differential, wherein a shiftable 2-gear planetary gear set is arranged in the power train between electric motor and bevel gear differential, which is also positioned coaxially with the electric motor or the bevel gear differential or spur gear differential. The drive unit is of very compact construction and, owing to the shiftable 2-gear planetary gear set, allows a good compromise between climbing capability, acceleration and energy consumption. Such drive units are also referred to as E-axes.From DE 10 2010 048 837 A1, such a drive device with at least one electric motor and at least one planetary differential drivable with a rotor of the electric motor is known, wherein the planetary differential has at least one planet carrier operatively connected to a rotor of the electric motor, first planetary gears and second planetary gears rotatably mounted on the planetary carrier, as well as a first sun gear and a second sun gear, each of which is operatively connected to an output shaft of the planetary differential. The first planetary gears mesh with the first sun gear and each of the second planetary gears mesh with the second sun gear and with one of the first planetary gears. Furthermore, the sun gears are arranged coaxially with a rotation axis of the rotor.As a result of very strong voltage edges during the voltage supply of the electric drive, undesired electric currents are generated in the electric drive. In this case, the electrical currents flow via the housing, the shaft and also via the rotor bearings, which are damaged by mechanical wear and additionally also by the current passage. In order to avoid this, dissipation of this electrical energy is necessary. The dissipation of electrical currents via the shaft into the housing is already known from the prior art. 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. These grounding and / or insulating elements prevent the electric voltage induced in the mechanical structural elements of the electric motor from being discharged via the bearings or from being transmitted to the neighboring components of the electric motor.For example, document CN 104 037 985 A describes an antistatic discharge device for the rotor of an electric motor. The antistatic unloader includes a diverter pin disposed in a bore at the rear end of a rotor spindle of the motor. For grounding and for electric discharge of the rotor spindle, the lead pin is screwed to the housing of the electric motor.DE 10 2019 133 889 A1 shows an electric machine having a housing section, wherein the housing section delimits a housing space, having a rotor and having a shaft, wherein the shaft is electrically and transmission-connected to the rotor and wherein the shaft and the rotor are rotatably mounted in the housing space, wherein the shaft defines a main axis, having a discharge device for discharging an electrical charge and / or voltage from the rotor via the shaft to the housing section, wherein the discharge device has a first contact device for electrical and mechanical connection to the shaft and a second contact device for electrical and mechanical connection to the housing section, wherein the first and the second contact devices are rotatable relative to one another and electrically contact one another in an axial direction with respect to the main axis in a contact space, wherein the discharge device has a protective unit, wherein the contact space and the housing space are separated from one another in a sealing manner by the protective unit.DE 38 43 048 A1 shows a coaxial starter motor, in particular a coaxial starter motor with a planetary reduction gear.It is the object of the invention to provide an electric axle drive train, in particular for driving a motor vehicle, which provides improved grounding, and has a compact structure and a long service life.This object is achieved by an electric axle drive train, in particular for driving a motor vehicle, comprising an electric machine having a stator and a rotor which is mounted rotatably relative to the stator and is connected to a transmission input shaft of a transmission arrangement, and the transmission arrangement further has a transmission output shaft, on which a rotational speed different from the transmission input shaft is applied during operation of the axle drive train, and the rotor has a rotational axis which is arranged coaxially with a rotational axis of the transmission input shaft and the transmission output shaft, wherein a first electrically conductive contact body is applied to the transmission input shaft in an axially spring-loaded manner and a second electrically conductive contact body is applied to the transmission output shaft in an axially spring-loaded manner, wherein the first contact body and the second contact body are arranged coaxially with respect to the rotational axis of the transmission input shaft.This achieves the advantage that particularly reliable and low-loss grounding of the rotor can be provided. Furthermore, not only is the grounding of the rotor shaft to the housing of the stator detected, but at the same time also the grounding of the transmission output shaft, which significantly improves the EMC properties of the drive train.By applying spring force to the contact bodies, it is possible in particular to ensure a continuous electrical contact between the ground partners to be connected. The spring force application also allows the compensation of tolerances or wobble movements of the grounding partners. By the coaxial positioning of the contact bodies, a particularly low-friction grounding can furthermore be formed.First, the individual elements of the claimed subject matter of the invention are explained in the sequence in which they are stated in the set of claims and particularly preferred embodiments of the subject matter of the invention are described below.An electrically operable drive train comprises an electric machine and preferably a transmission arrangement coupled to the electric machine. The transmission arrangement and the electric machine form a structural unit. This can be formed, for example, by means of a drive train housing in which the transmission arrangement and the electric machine are accommodated jointly.The electric machine preferably has a motor housing and / or the transmission has a transmission housing, wherein the structural unit can then be effected by fixing the transmission with respect to the electric machine. The transmission housing is a housing for receiving a transmission. The object is to guide shafts present over the bearings and to provide the wheels (possibly cam disks) with the degrees of freedom under all loads that they require without hindering them in the rotational and possibly path movement, as well as to absorb bearing forces and supporting torques. A transmission housing can be formed in one or more shells, that is to say in an unshared or divided manner. The transmission housing should also be able in particular to damp noise and vibrations and also to absorb hydraulic fluid securely. The gear housing is preferably formed from a metallic material, in particular preferably from aluminum, grey cast iron or steel cast iron, in particular by means of a primary molding process such as casting or die casting.The motor housing encloses the electric machine. A motor housing can also accommodate the control and power electronics. The motor housing can furthermore also be a component of a cooling system for the electric machine and can be designed in such a way that hydraulic fluid can be supplied to the electric machine via the motor housing and / or the heat can be dissipated to the outside via the housing surfaces. In addition, the motor housing protects the electric machine and the optionally present electronics from external influences.A motor housing can be formed in particular from a metallic material. Advantageously, the motor housing can be formed from a metallic cast material, such as aluminum die casting, magnesium die casting, grey casting or steel casting.The electric machine serves for converting electrical energy into mechanical energy and / or vice versa, and it generally comprises a stationary part referred to as a stator, stator or armature, and a part referred to as a rotor or rotor and arranged movably, in particular rotatably, with respect to the stationary part.The electric machine of the axle drive train according to the invention is preferably designed as an axial flow machine. The magnetic flux in an electric axial flux machine (AFM) is directed in the air gap between stator and rotor axially to a direction of rotation of the rotor of the axial flux machine. There are different types of axial flow machines. One known type is a so-called I-arrangement, in which the rotor is arranged axially next to a stator or between two stators. Another known type is a so-called H-arrangement in which two rotors are arranged on opposite axial sides of a stator. The axial flow electric machine is preferably configured as an H-type.In principle, it is also possible for a plurality of rotor-stator configurations to be arranged axially next to one another as an I-type and / or H-type. In this context, it would also be possible to arrange both one or more rotor-stator configurations of the I-type and one or more rotor-stator configurations of the H-type next to one another in the axial direction. In particular, it is also preferable for the rotor-stator configuration of the H-type and / or of the I-type to be each of substantially identical design, such that they can be joined together in a modular manner to form an overall configuration. Such rotor-stator configurations can be arranged in particular coaxially with respect to one another and can be connected to a common rotor shaft or to a plurality of rotor shafts.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 30 kW, preferably greater than 50 kW and in particular greater than 70 kW. It is furthermore preferred that the electric machine provides rotational speeds greater than 5,000 U / min, particularly preferably greater than 10,000 U / min, very particularly preferably greater than 12,500 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 tractors.The transmission arrangement can be coupled in particular to the electric machine which is designed to generate a drive torque for the motor vehicle. The drive torque is particularly preferably a main drive torque, so that the motor vehicle is driven exclusively by the drive torque. The transmission arrangement is preferably designed as a planetary transmission. The planetary gear is most preferably wet running.A contact body can have a contour which forms a point-like contact with a contact structure abutting it. In particular, spherical shapes, spherical sections, pyramidal shapes or the like are suitable here. The point-like contact is particularly preferably effected in the axis of rotation of the transmission input shaft.A contact body can be acted upon by a force, in particular by a spring element. The spring element can be formed from one or more springs. The spring element preferably comprises a compression spring. Furthermore, however, it is also possible for the spring element to have a disk spring. The spring element is preferably configured such that it can exert an axial contact force on a contact body which is suitable for ensuring a secure electrical contact between the contact body and a bearing structure, in particular also in the presence of a fluid in the electrical contact region.According to the invention, it is provided that the electric machine is configured as an axial flow machine, whereby an axial drive train of particularly compact construction is made possible.According to the invention, it is further provided that the electric machine designed as an axial flux machine has a rotor shaft designed as a hollow shaft, into which a bearing shaft fixed with respect to the stator engages, wherein the rotor shaft is rotatably mounted with respect to the bearing shaft by means of a rolling bearing and the first contact body is arranged in and / or on the bearing shaft, which likewise contributes to an axially compact design.According to the invention, a bearing prestress device is arranged on the bearing shaft, by means of which device a bearing prestress in the rolling bearing can be set, wherein the first contact body is arranged on and / or in the bearing prestress device.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 an advantageous embodiment of the invention, it can be provided that the transmission arrangement is designed as a planetary transmission, wherein the transmission input shaft is a sun gear of the planetary transmission. The advantage of this embodiment is that particularly advantageous transmission ratios can be realized in a compact design in connection with the application in automobiles.The rolling bearing can be formed in one row or in multiple rows. Preferably, the rolling bearing is configured as a ball bearing.The inner ring can connect in particular the shaft receiving the rolling bearings to the rolling bearing or the rolling bodies. In this case, in particular, the shaft can be connected to the side of the lateral surface of the inner ring facing the shaft, where-when the inner ring raceway lies opposite this lateral surface-the rolling bodies of the rolling bearing roll. The inner ring can be formed from a metallic and / or ceramic material. It is conceivable in principle to configure the inner ring in one part or in multiple parts, in particular in two parts.The outer ring can connect in particular the bearing receiving the rolling bearings to the rolling bearing or the rolling bodies. In this case, in particular the bearing can be connected to the side of the lateral surface of the outer ring facing the bearing, wherein the outer ring raceway opposite this lateral surface rolls the rolling bodies of the rolling bearing. The outer ring can be formed from a metallic and / or ceramic material. It is conceivable in principle to form the outer ring in one part or in multiple parts, in particular in two parts.The rolling elements have the shape of a ball or a roller. 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. In an axial rolling bearing, the rolling bodies transmit the forces acting on the axial rolling bearing between the running disks. 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. According to a further preferred development of the invention, it can also be provided that the rolling bodies are manufactured from ceramic.The rolling bodies are preferably guided in a cage and spaced apart from one another. The cage can be embodied in one piece or in multiple pieces. The cage is configured in particular as a pocket cage.According to a further preferred development of the invention, it can also be provided that the rolling bearing is designed as a double-row angular contact ball bearing. The double-row angular contact ball bearing has a particularly suitable support base for absorbing tilting forces, as can occur during operation of the electric machine. The double row angular contact ball bearing can be formed in an O- or X-configuration. In the context of the invention, an O-arrangement is preferred.Furthermore, the invention can also be further developed to the effect that the transmission input shaft is designed as a hollow shaft, and the second contact body is positioned within the transmission input shaft, whereby an axle drive train of particularly compact axial construction can be realized. Furthermore, the grounding device with the second contact body within the hollow shaft can be protected from mechanical or chemical influences, which increases the operational reliability and service life of the axle drive train.In a likewise preferred embodiment variant of the invention, it can also be provided that the first contact body and / or second contact body are each integrated in a grounding element, which comprises a cylindrical housing in which a spring element is arranged, which is supported axially on the one hand on the housing and on the other hand is coupled to the first contact body or second contact body guided at least in sections in the housing. This can ensure that the contact bodies can be provided within a module in a prefabricated manner, which has a positive effect on the production and assembly costs.It can also be advantageous to further develop the invention to the effect that the grounding element has a piston which can be displaced axially in the housing and on which the spring element is supported axially on the one hand and on which the first contact body or the second contact body abuts axially on the other hand. The advantage that can be realized in this way is that the contact body can thus lie movably in or against the piston. For example, it is thus possible to rotatably mount a contact body designed as a cogel in or on the piston, which can contribute once again to a reduction in friction losses.According to a further preferred embodiment of the subject matter of the invention, it can be provided that the first contact body and / or the second contact body are / is designed as a ball. Finally, the invention can also be advantageously embodied to the effect that the first contact body and / or the second contact body are / is formed from a steel or from graphite.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 an electric axle drive train in a schematic axial sectional illustration, FIG. 2 shows an exposed detail view of the rotor shaft and the transmission input shaft in a schematic axial sectional illustration, FIG. 3 shows an electric axle drive train with current conducting paths pulling through the housing in a schematic axial sectional illustration, FIG. 4 shows a motor vehicle having an electric axle drive train in three different embodiments, each in a schematic block switching view.FIG. 1 shows an electric axle drive train 1, in particular for driving a motor vehicle 2, as is also shown by way of example in FIG. 4. As shown in FIG. 4 b, the axle drive train 1 can be arranged as one coaxial to the vehicle axle and can be configured to be operable by an electric machine 3. It is also possible for the axle drive train 1 to have two electric machines 3 and two transmission arrangements 6, wherein one electric machine 3 and one transmission arrangement 6 are assigned in pairs to a vehicle wheel of the motor vehicle 2, as is outlined in FIG. 4 in FIG. 4. It is preferable here for the electric machines 3 and gear mechanism arrangements 6 to be of substantially identical construction. Finally, FIG. c of FIG. 1 also shows an axle drive train 1 which has an arrangement which is axially parallel with respect to the vehicle axis.The axle drive train 1 comprises an electric machine 3 configured as an axial flux machine, having a stator 4 and a rotor 5 which is rotatably mounted relative to the stator 4 and is connected to a transmission input shaft 7 of a transmission arrangement 6. The electric machine 3 and the transmission arrangement 6 are structurally combined by the common housing 15. The transmission arrangement 6 further has a transmission output shaft 8, on which a rotational speed different from the transmission input shaft 7 is present during operation of the axle drive train 1. In the exemplary embodiment shown, the transmission arrangement 6 is designed as a wet-running planetary transmission, wherein the transmission input shaft 7 is a sun gear of the planetary transmission.Planetary gears 22 supported in a planetary gear carrier 24 are in driving engagement with the transmission input shaft 7, which are rotatably supported on the planetary gear shaft 25. Furthermore, the planetary gears 22 are in driving engagement radially on the outside with an internally toothed ring gear 23. The planet carrier 24 is connected in a rotationally fixed manner to the transmission output shaft 8, which is likewise designed as a hollow shaft. The transmission input shaft 7 is rotatably mounted with respect to the transmission output shaft 8 via the rolling bearing 26.The rotor 5 has an axis of rotation 9 which is arranged coaxially with an axis of rotation 10 of the transmission input shaft 7 and of the transmission output shaft 8.A first electrically conductive contact body 11 bears against the transmission input shaft 7 under spring force action in the axial direction, while a second electrically conductive contact body 12 bears against the transmission output shaft 8 under spring force action in the axial direction. As can be easily seen from FIG. 1, the first contact body 11 and the second contact body 12 are arranged coaxially to the axis of rotation 10 of the transmission input shaft 7.The electric machine 3 designed as an axial flux machine also has a rotor shaft 13 designed as a hollow shaft, in which a bearing shaft 14 fixed with respect to the stator 4 engages. The rotor shaft 13 is rotatably mounted with respect to the bearing shaft 14 by means of a rolling bearing 16. A bearing prestress device 20 is arranged on the bearing shaft 14, by means of which device a bearing prestress in the rolling bearing 16 can be adjusted, wherein the first contact body 11 is arranged on and / or in the bearing prestress device 20.The transmission input shaft 7 is also designed as a hollow shaft, wherein the second contact body 12 is positioned within the transmission input shaft 7.What can also be gathered from FIG. 1 is that the first contact body 11 and the second contact body 12 are each integrated in a grounding element 17, 27 which comprises a cylindrical housing 18 in which a spring element 19 is arranged, which is supported axially on the one hand on the housing 18 and is coupled on the other hand to the first contact body 11 or second contact body 12 guided at least in sections in the housing 18.As can be seen in the detailed view of FIG. 2, the grounding element 17, 27 has a piston 21 which can be displaced axially in the housing 18 and on which the spring element 19 is supported axially on the one hand and on which the first contact body 11 or the second contact body 12 abuts axially on the other hand. The first contact body 11 and the second contact body 12 are each formed as a ball, which can be formed from a steel or from graphite.The first spherical contact body 11 abuts a first contact disk 28, which is pressed into the transmission input shaft 7 designed as a hollow shaft and thus provides a mechanical as well as electrical contact between the contact body 11 and the transmission input shaft 7. Analogously, a second contact disk 29 is pressed into the transmission output shaft 8 and establishes the mechanical and electrical contact between the second contact body 12 and the transmission output shaft 8.The particular mode of operation of the grounding elements 17, 27 can also be easily understood once again with reference to the illustration of FIG. 3, in which the current conducting paths 30 are outlined as dashed lines through the axle drive train 1. It can be seen easily that, due to the use and arrangement of the two grounding elements 17, 27, substantially all the components are electrically conductively connected to the housing 15 of the axle drive train 1 and are grounded via this.In particular, the transmission output shaft is also correspondingly grounded, which leads to a marked improvement in the EMC properties of the drive train 1.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 Axle drive train 2 Motor vehicle 3 Electric machine 4 Stator 5 Rotor 6 Transmission arrangement 7 Transmission input shaft 8 Transmission output shaft 9 Rotational axis 10 Rotational axis 11 Contact body 12 Contact body 13 Rotor shaft 14 Bearing shaft 15 Housing 16 Rolling bearing 17 Grounding element 18 Housing 19 Spring element 20 Bearing biasing device 21 Piston 22 Planetary gears 23 Ring gear 24 Planetary gear carrier 25 Planetary gear shaft 26 Rolling bearing 27 Grounding element 28 Contact disk 29 Contact disk 30 Current conduction paths

Claims

An electric axle drive train (1), in particular for driving a motor vehicle (2), comprising an electric machine (3) having a stator (4) and a rotor (5) which is rotatably mounted relative to the stator (4) and is connected to a transmission input shaft (7) of a transmission arrangement (6), and the transmission arrangement (6) further has a transmission output shaft (8) on which, during operation of the axle drive train (1), a rotational speed different from the transmission input shaft (7) is applied, and the rotor (5) has an axis of rotation (9) which is arranged coaxially with an axis of rotation (10) of the transmission input shaft (7) and of the transmission output shaft (8), characterized in that a first electrically conductive contact body (11) bears, under spring force, against the transmission input shaft (7) in the axial direction and a second electrically conductive contact body (12) bears, under spring force, against the transmission output shaft (8), wherein the first contact body (11) and the second contact body (12) are arranged coaxially with respect to the rotational axis (10) of the transmission input shaft (7), wherein the electric machine (3) is designed as an axial flux machine, wherein the electric machine (3) designed as an axial flux machine has a rotor shaft (13) designed as a hollow shaft, into which a bearing shaft (14) fixed with respect to the stator (4) engages, wherein the rotor shaft (13) is rotatably mounted with respect to the bearing shaft (14) by means of a rolling bearing (16), and the first contact body (11) is arranged in and / or on the bearing shaft (14), and a bearing prestressing device (20) is arranged on the bearing shaft (14), by means of which bearing prestressing in the rolling bearing (16) can be adjusted, wherein the first contact body (11) is arranged on and / or in the bearing prestressing device (20).Axle drive train (1) according to Claim 1, characterized in that the transmission arrangement (6) is designed as a planetary transmission, wherein the transmission input shaft (7) is a sun wheel of the planetary transmission.Axle drive train (1) according to one of the preceding claims, characterized in that the transmission input shaft (7) is designed as a hollow shaft, and the second contact body (12) is positioned within the transmission input shaft (7).Axle drive train (1) according to one of the preceding claims, characterized in that the first contact body (11) and / or second contact body (12) are each integrated in a grounding element (17, 27) which comprises a cylindrical housing (18), in which a spring element (19) is arranged, which is supported axially on the one hand on the housing (18) and on the other hand is coupled to the first contact body (11) or second contact body (12), which is guided at least in sections in the housing (18).Axle drive train (1) according to one of the preceding claims, characterized in that the earthing element (17, 27) has a piston (21) which can be displaced axially in the housing (18) and on which, on the one hand, the spring element (19) is supported axially and on which, on the other hand, the first contact body (11) or the second contact body (12) bears axially.Axle drive train (1) according to one of the preceding claims, characterized in that the first contact body (11) and / or the second contact body (12) are / is designed as a ball.Axle drive train (1) according to one of the preceding claims, characterized in that the first contact body (11) and / or the second contact body (12) are / is formed from a steel or from graphite.

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