Rotor shaft and electric machine

Abstract

The invention relates to a rotor shaft (1) for a rotor (17) of an electric machine (15). The rotor shaft (1) comprises a hollow shaft (2) extending along an axial direction (A) from a first axial end section (3a) to a second axial end section (3b). Furthermore, the rotor shaft comprises an extension element (4), preferably sleeve-like and necessarily separate from the hollow shaft (2). The rotor shaft (1) comprising the hollow shaft (2) and the extension element (4) is thus formed in at least two parts. The extension element (4) is rotatably connected to the hollow shaft (2) at its first end section (3a) by means of a first bearing arrangement (5a) and extends axially beyond the first end section (3a) of the hollow shaft (2) to extend it axially.The rotor shaft (1) further comprises a second bearing arrangement (5b) arranged externally on the second end section (3b) of the hollow shaft (2) and rotatably or non-rotatably connected to it.

Description

[0001] The invention relates to a rotor shaft for a rotor of an electrical machine and to an electrical machine with such a rotor shaft.

[0002] Conventional rotor shafts for electrical machines are often designed in the form of a series of several cylinders with different diameters and each with different functional elements for forming bearing points, forming a joint seat for rotor laminations and balancing discs, accommodating cable guides in the case of a separately excited synchronous machine, and the like.

[0003] This geometric arrangement of the individual functional elements results in a comparatively high installation space requirement with regard to the axial longitudinal extension of the rotor shaft.

[0004] Conventional rotor shafts are often designed as hollow shafts and have two axially spaced bearing devices for rotatably supporting the rotor shaft.

[0005] For the sake of simplicity, both storage facilities are often designed identically or at least have the same dimensions, even if an individual design of the two storage facilities may be necessary due to different loads on the two storage facilities.

[0006] Against this background, it is an object of the present invention to demonstrate new approaches in the development of rotor shafts for electrical machines and their rotors. In particular, an improved embodiment of a rotor shaft is to be created that is characterized by a reduced installation space requirement compared to conventional rotor shafts, while simultaneously maintaining a simple technical design and thus reducing manufacturing costs.

[0007] This problem is solved by the subject matter of the independent patent claims. Preferred embodiments are the subject matter of the dependent patent claims.

[0008] The basic idea of ​​the invention is therefore not to design two bearing devices for rotatably supporting a rotor shaft identically, but rather to connect one of the two bearing devices to the hollow shaft via an extension element formed separately from the actual hollow shaft. Said extension element is thus designed to be rotatable relative to the actual hollow shaft using these bearing devices, so that the extension element can be fixed in place, for example, to a stator or to a housing of the electrical machine.

[0009] With the help of said extension element, the hollow shaft can be attached in a mechanically rigid and stable manner to a stationary component of an electrical machine, such as the stator or housing of the electrical machine, and rotatably mounted. This is accompanied by an advantageous reduction in undesirable bending of the hollow shaft during operation of the rotor shaft. This, in turn, advantageously reduces the mechanical stress occurring on both the low-load and the loaded, i.e., load-transmitting side, and thus in both bearing devices of the rotor shaft. In principle, this is accompanied by the advantageous possibility of dimensioning the bearing device connected to the extension element smaller than the other of the two bearing devices. Finally, the extension element can be manufactured independently of the rotor shaft and is therefore more cost-effective to produce than a conventional rotor shaft with a conventional bearing journal.

[0010] Following the basic idea of ​​the invention, a rotor shaft according to the invention for a rotor of an electrical machine comprises a hollow shaft extending along an axial direction from a first axial end section to a second axial end section. The rotor shaft further comprises an extension element, preferably sleeve-like, which is necessarily formed separately from the hollow shaft. The rotor shaft comprising the hollow shaft and the extension element is therefore formed in at least two parts. The extension element is rotatably connected to the hollow shaft at its first end section by means of a first bearing device and extends axially beyond the first end section to axially extend the hollow shaft. The rotor shaft further comprises a second bearing device arranged externally on the second end section of the hollow shaft and connected to it in a rotatable or rotationally fixed manner.

[0011] According to an advantageous further development, a bearing shield projecting radially inwards into the interior space can be formed or arranged, in particular integrally, axially in the region of the first end section on the hollow shaft, on which bearing shield the first bearing device is supported and / or fastened.

[0012] According to a further advantageous development, an electrical line or cable feedthrough is provided axially in the region of the first end section on the hollow shaft and on the extension element, in particular for providing an electrical connection from the stationary extension element to the rotating hollow shaft and thus for transmitting electrical energy from the extension element to the hollow shaft. The design of the rotor shaft according to the invention with a stationary extension element simplifies the construction of such a line or cable feedthrough, the routing of electrical lines or cables to the rotating hollow shaft, including their fluidic sealing, since the sealing can be effected on the stationary extension element.

[0013] Said electrical line or cable feedthrough can be used in particular to supply electrical energy to electrically energizable rotor coils arranged on the rotor to generate a magnetic rotor field when the rotor shaft according to the invention is used as part of a rotor in an electrical machine. By means of the electrical line or cable feedthrough, electrical energy is thus transferred from the stationary stator system to the rotor system rotating during operation of the electrical machine. The installation space available in the first end section or in the region of the extension element can be effectively used to supply electrical energy or power to components arranged on the rotor shaft, in particular said rotor coils of a rotor.

[0014] Particularly preferably, the electrical line or cable feedthrough for wireless electrical energy transmission can be designed as an electrical rotary transformer. In such a rotary transformer, the energy is transferred from the stationary extension element to the rotatable or rotating hollow shaft inductively and thus wirelessly. In an electrical rotary transformer, the wireless electrical connection between the stationary extension element and the rotating hollow shaft prevents undesirable wear effects, such as those that occur in a wired electrical connection, such as a wired sliding contact, due to the resulting sliding friction.

[0015] In a further preferred embodiment, the rotary transformer comprises a rotary transformer rotor that is non-rotatably connected to the hollow shaft, and a rotary transformer stator that is axially spaced from the rotary transformer rotor and inductively coupled to it, which stator is fixedly or non-rotatably connected to the extension element. Such a rotary transformer stator and such a rotary transformer rotor can each each have at least one electrically energizable rotary transformer stator coil or rotary transformer rotor coil, respectively, which generate a magnetic field when energized.The rotary transformer stator coil and the rotary transformer rotor coil must be magnetically coupled to each other so that when the rotary transformer stator coil is energized, an alternating voltage and thus an alternating current are induced according to the law of induction due to the existing magnetic coupling with the rotary transformer rotor coil. After rectification, this alternating current can be used to energize the rotor of the electric machine. The rotor can thus be driven through magnetic interaction with the stator of the electric machine. This operating principle is particularly applied in separately excited synchronous machines, so the use of the rotary transformer described above in a separately excited synchronous machine proves particularly advantageous.

[0016] In a particularly preferred embodiment, the rotary transformer can be arranged axially on a side of the first bearing device and / or the bearing plate facing away from the interior, i.e., outside the interior. This variant allows the worker particularly good access to the rotary transformer, making it particularly easy to install and remove.

[0017] In an alternative to the previously explained embodiment, but also particularly preferred, the rotary transformer can be arranged axially on a side of the first bearing device and / or the bearing shield facing the interior, i.e. in the interior space surrounded by the hollow shaft. Particularly preferably, the rotary transformer can be arranged, most preferably entirely, in the interior space. This variant requires particularly little installation space, since the interior space present in the hollow shaft can be used to accommodate the rotary transformer. In this way, no further installation space outside the hollow shaft is required to accommodate the rotary transformer. Consequently, the entire rotor shaft with the rotary transformer also requires particularly little installation space.

[0018] In another preferred development of the rotor shaft according to the invention, the electrical line or cable feedthrough is designed for wired electrical energy transmission and can for this purpose have at least one slip ring as part of a sliding contact, which is preferably connected in a rotationally fixed manner to the extension element or to the hollow shaft.

[0019] In a further preferred embodiment of the rotor shaft according to the invention, the first bearing device is supported radially outwardly on the hollow shaft and radially inwardly on the extension element, in particular on a peripheral wall of the extension element. In this way, the hollow shaft can be mechanically stiffened and also rotatably mounted in a particularly stable manner via the first bearing device.

[0020] According to a further advantageous development of the rotor shaft according to the invention, a coupling device for drivingly coupling the rotor shaft to an external component can be formed in the second end section of the hollow shaft. In this case, the coupling device forms an output element for transmitting the rotational movement of the rotor shaft to the external component through said drive connection. Said coupling device or said drive element can be formed, in particular, by a surface structure provided on the outer or inner circumference of the rotor shaft.

[0021] A diameter of the hollow shaft, measured in the radial direction, can expediently be reduced in the second end section. Thus, the hollow shaft has a diameter in its second end section that is smaller than in at least one axial section different from the second end section. This axial section can, in particular, comprise the first end section. Said axial section can be complementary to the second end section.

[0022] In a further preferred embodiment, an extension of the second end section of the hollow shaft, measured along the axial direction, can be greater than a projection of the extension element beyond the first end section of the hollow shaft, measured along the axial direction. The rotor shaft is thus particularly compact axially in the region of the first axial end section. This embodiment proves to be particularly advantageous in combination with a coupling device, as already explained above, provided in the second end section for drivingly coupling the rotor shaft to an external component, if this coupling device is arranged outside, preferably on the outer circumference, of the second end section.

[0023] In a further preferred embodiment, the first bearing device is arranged radially between the hollow shaft and the extension element. This allows the installation space required for the rotor shaft in the radial direction to be kept small. In both variants explained above, the entire rotor shaft requires particularly little installation space in the axial direction compared to conventional rotor shafts with dual bearings.

[0024] In a further preferred embodiment, the first bearing device is arranged axially in the region of the first end section in an interior space that is at least radially delimited by the hollow shaft. This variant also reduces the installation space required by the rotor shaft in the radial direction.

[0025] Particularly expediently, the first and / or second bearing device can be designed as a radial bearing.

[0026] In a preferred embodiment, the first bearing device is also connected to the extension element in a rotationally fixed manner and is rotatably connected to the hollow shaft. In an alternative variant, the first bearing device can be connected to the hollow shaft in a rotationally fixed manner and also rotatably connected to the extension element. The person skilled in the art can thus select and implement the most advantageous variant for the specific application.

[0027] In another preferred embodiment, the second bearing device has a greater spatial extent than the first bearing device. Alternatively or additionally, in this variant, the second bearing device is designed to absorb greater bearing forces than the first bearing device.

[0028] The invention further relates to an electrical machine. The electrical machine according to the invention comprises a housing surrounding a housing interior, in particular at least radially. Furthermore, the machine comprises a stator arranged in the housing interior in a fixed position relative to the housing, and also a rotor arranged at least partially in the housing interior and rotatable relative to the stator and magnetically coupled to it, in particular drivable by it by means of magnetic interaction. According to the invention, the rotor comprises a rotor shaft according to the invention as presented above, on which magnetic field generating elements for generating a magnetic field are arranged in a rotationally fixed manner. The advantages of the rotor shaft according to the invention explained above are therefore transferred to the electrical machine according to the invention. Said magnetic field generating elements can be formed in particular by electrically energizable rotor coils or by permanent magnets.In the machine according to the invention, the extension element of the rotor shaft is firmly, in particular rigidly, connected to the housing and / or the stator. Furthermore, the hollow shaft is rotatably mounted on the housing and / or the stator by means of the second bearing device.

[0029] Further important features and advantages of the invention emerge from the subclaims, from the drawings and from the associated description of the figures based on the drawings.

[0030] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0031] Preferred embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description, wherein the same reference numerals refer to the same or similar or functionally identical components.

[0032] They show, schematically: Fig. 1 a first example of a rotor shaft according to the invention, Fig. 2 a first development of the rotor shaft of the Fig. 1 in the area of ​​the extension element with an electrical line or cable feedthrough for wired energy transmission from the stationary extension element to the rotor shaft. Fig. 3, Fig. 4 a second or third development of the rotor shaft of the Fig. 1 in the area of ​​the extension element with an electrical line or cable feedthrough for wireless energy transmission from the stationary extension element to the rotor shaft.

[0033] The Fig. 1 shows in a roughly schematic sectional view an example of an electrical machine 15 according to the invention with a rotor shaft 1 according to the invention, which in Fig. 1 is shown greatly enlarged to clarify the facts.

[0034] The electrical machine 15 according to the invention comprises Fig. 1 shows a highly simplified housing 18 which surrounds a housing interior 19. In the housing interior 19, a stator 16 is arranged fixed to the housing 18, which stator can be attached to the housing 18 and in Fig. 1 is only shown schematically.

[0035] Furthermore, the electric machine 15 comprises a rotor 17 arranged in the housing interior 19 and rotatable relative to the stator 16 about a predetermined rotational axis D, which rotor comprises a rotor shaft 1 according to the invention. The stator 16 and the rotor 17 can each have electrically energizable stator and rotor coils (not shown), respectively, which generate a magnetic field when energized.

[0036] The rotor shaft 1 comprises a hollow shaft 2, on the outer circumference 23 of which rotor coils 24 (in Fig. 1 only roughly schematically indicated) can be arranged, by means of which a magnetic rotor field can be generated. The use of rotor coil 24 is particularly advantageous when the electrical machine 15 is a separately excited electrical synchronous machine 15a. Through magnetic interaction of the magnetic stator field with the magnetic rotor, the rotor 17 including the rotor shaft 1 can be driven and thus set into a rotary movement about the axis of rotation D relative to the stator 16 or the housing. Instead of rotor coils 24, permanent magnets (not shown) can also be used.

[0037] How Fig. As can be seen in Figure 1, the rotor shaft 1 is constructed in several parts and comprises at least one hollow shaft 2 and an extension element 4 formed separately from the hollow shaft 2, both of which extend along an axial direction A and can both be arranged coaxially to one another, as shown. The hollow shaft 2 delimits an interior space 8 of the rotor shaft 1.

[0038] In the example scenario, a common central longitudinal axis M of the hollow shaft 2 and the extension element 4 form a rotation axis D of the rotor shaft 1. A radial direction R extends perpendicular to the axial direction A away from the central longitudinal axis M or the rotation axis D. A circumferential direction U runs perpendicular to both the axial direction A and the radial direction R around the central longitudinal axis M or the rotation axis D.

[0039] How Fig. 1, the extension element 4 of the rotor shaft 1 is rigid and thus firmly connected to the housing 18 and in this way - indirectly via the housing 18 - also firmly connected to the stator 16 of the electric machine 15. In a variant not shown, a direct, fixed or rigid connection of the extension element 4 to the stator 16 is also conceivable. Both connection variants can be realized, for example, by means of a clamp connection (not shown).

[0040] The hollow shaft 2 extends along the axial direction A from a first axial end portion 3a to a second axial end portion 3b. The extension element 4 can be Fig. 1 be sleeve-like and, analogous to the hollow shaft 2, also surround an element interior 27 which can communicate fluidically with the interior 8. The extension element 4 is rotatably connected to the first end section 3a of the hollow shaft 2 by means of a first bearing device 5a and extends axially beyond the first end section 3a for the axial extension of the hollow shaft 2, so that the extension element 4 axially extends the hollow shaft 2. The first bearing device 5a can be arranged axially in the region of the first end section 3a in the interior 8 delimited by the hollow shaft 2, as shown. Furthermore, the first bearing device 5a is arranged radially between the hollow shaft 2 and the extension element 4, as shown. The first bearing device 5a is supported radially on the outside on an inner circumferential side 25 of the hollow shaft 2 and radially on the inside on an outer circumferential side 30 of a circumferential wall 6 of the extension element 4.

[0041] Furthermore, the rotor shaft 1 comprises a second bearing device 5b arranged on the outer circumference 23 of the second end section 3b of the hollow shaft 2 and rotatably connected thereto. The hollow shaft 2 is rotatably mounted on the housing 18 by means of the second bearing device 5b, for which purpose the second bearing device 5b can be fixedly connected to the housing 18.

[0042] The two bearing devices 5a, 5b can each be designed as radial bearings. The second bearing device 5b can have a larger spatial extent or spatial extension than the first bearing device 5a (not shown) and can also be designed to absorb greater bearing forces than the first bearing device 5a.

[0043] Furthermore, a coupling device 7 for drivingly coupling the rotor shaft 1 to an external component can be formed in the second end section 3b of the hollow shaft 2. In the example of Fig. 1, the coupling device 7 is formed by a toothed structure 26 extending along the circumferential direction U on the outer circumference 23 of the hollow shaft 2, or by another suitable surface structure. The coupling device 7 forms an output element for transmitting the rotational movement of the rotor shaft 1 to said external component by means of said drive connection.

[0044] It is advisable to use the Fig. 1, a diameter D of the hollow shaft 2 measured along the radial direction R can be reduced in its second end section 3b. A value D2 of the diameter D of the hollow shaft 2 in its second end section 3b is therefore smaller than a value D1 of the diameter D in the region of the first end section D1 and also smaller than a value D0 of the diameter D of the hollow shaft 2 in an axial region 3c between the two axial end sections 3a, 3b.

[0045] Furthermore, an extension E2 of the second end portion 3b of the hollow shaft 2 measured along the axial direction A can be greater than a projection E1 of the extension element 4 measured along the axial direction A beyond the first end portion 3a of the hollow shaft 2, as shown in the example.

[0046] Axially in the region of the first end section 3a, an annular bearing shield 9 projecting inwards into the interior 8 along the radial direction R can be integrally formed on the hollow shaft 2, on which the first bearing device 5a is supported and can also be fastened.

[0047] In the example scenario of Fig. 1, the first bearing device 5a is non-rotatably connected to the extension element 4 and rotatably connected to the hollow shaft 2.

[0048] In an alternative variant, the first bearing device 5a can also be connected in a rotationally fixed manner to the hollow shaft 2 and correspondingly rotatably connected to the extension element 4.

[0049] The Fig. 2 shows a first development of the rotor shaft of the Fig. 1. In this further development, an electrical line or cable feedthrough 10 is formed axially in the area of ​​the first end section 3a on both the hollow shaft 2 and the extension element 4 for transmitting electrical energy from the extension element 4 to the rotor shaft 2, which is rotatable relative to the extension element 4. The electrical line or cable feedthrough 10 serves in the example scenario to connect the electrical power supply 10 to the rotor 17 (in Fig. 2 not shown, cf. Fig. 1) arranged, electrically energizable rotor coils 24 for generating a magnetic rotor field with electrical energy. By means of the electrical line or cable feedthrough 10, electrical energy is thus transferred from the stationary stator system to the magnetic field generated during operation of the electrical machine 15 (cf. Fig. 1) rotating rotor system.

[0050] In the example of Fig. 2, the line or cable feedthrough 10 is designed for wired electrical energy transmission. For this purpose, the line or cable feedthrough 10 has two sliding contacts 14a, 14b arranged at an axial distance from one another. Each of the two sliding contacts 14a, 14b comprises an electrically conductive slip ring 20a or 20b arranged on the outer circumferential side 30 of the extension element 4 and extending along the circumferential direction U. In addition, each of the two sliding contacts 14a, 14b comprises an electrically conductive brush 21a or 21b which interacts with the respective slip ring 20a or 20b and rotates with the hollow shaft 2, thus slidingly abutting the respective slip ring 20a, 20b to form an electrically conductive connection during the rotational movement of the hollow shaft 2.By means of the two sliding contacts 14a, 14b, electrical lines 22a, 22b arranged in a fixed position relative to the electrical connecting element 4 can be electrically connected to electrical lines 23a and 23b, respectively, which rotate with the hollow shaft 2.

[0051] The Fig. 3 shows a second development of the rotor shaft 1 of the Fig. 1. According to this third development, the electrical line or cable feedthrough 10 for wireless electrical energy transmission can be designed as an inductive rotary transformer 11 of the electrical machine 15, which comprises a rotary transformer rotor 12 connected in a rotationally fixed manner to the rotatable hollow shaft 2 and a rotary transformer stator 13 connected in a rotationally fixed manner to the extension element 4 and inductively coupled to the rotary transformer rotor 12, such that the rotary transformer rotor 12 is rotatable relative to the rotary transformer stator 13. In such a rotary transformer 11, the energy is transferred from the rotary transformer stator 13 to the rotary transformer rotor 12 inductively.The alternating magnetic field generated by the rotary transformer stator 13 by appropriate electrical current supply to its rotary transformer stator coil (not shown) induces an alternating electrical current in a rotary transformer rotor coil (not shown) of the rotary transformer rotor 12 by axial magnetic coupling, with which the rotor coils 24 (cf. Fig. 1) the electric machine 15 can be supplied with electrical current. The rotary transformer 11 can be designed as shown in Fig. 3 shown axially on a side 28 of the bearing plate 9 facing away from the interior 8 and thus also of the first bearing device 5a, i.e. outside the interior 8.

[0052] The Fig. 4 illustrates a third development of the example of Fig. 1. The third training course is an alternative version of the second training course of Fig. 3 and differs like the Fig. 3 and Fig. 4, the second development is distinguished by the fact that the rotary transformer 11 is arranged axially on a side 29 of the first bearing device 5a, and thus also of the bearing plate 9, facing the interior space 8. The rotary transformer 11 is thus arranged in the interior space 8.

[0053] In the examples of Fig. 3 and Fig. 4, the transmission of electrical energy from the two electrical lines 22a, 22b to the two electrical lines 23a and 23b is therefore wireless.

Claims

[1] Rotor shaft (1) for a rotor (17) of an electrical machine (15), - with a hollow shaft (2) extending along an axial direction (A) from a first axial end section (3a) to a second axial end section (3b), - with an extension element (4) which is designed separately from the hollow shaft (2) and is preferably sleeve-like and which is rotatably connected to the first end section (3a) of the hollow shaft (2) by means of a first bearing device (5a) and extends axially beyond the first end section (3a) of the hollow shaft (2) for the axial extension of the hollow shaft (2), - with a second bearing device (5b) arranged externally on the second end section (3b) of the hollow shaft (2) and rotatably or non-rotatably connected thereto. [2] Rotor shaft according to claim 1, characterized bythat a bearing plate (9) projecting radially inwards into the interior (8) is formed, in particular integrally, axially in the region of the first end section (3a) on the hollow shaft (2), on which bearing plate the first bearing device (5a) is supported. [3] Rotor shaft according to claim 1 or 2, characterized by that an electrical line and / or cable feedthrough (10), in particular for transmitting electrical energy from the extension element (4) to the rotor shaft (2), is provided axially in the region of the first end section (3a) on the hollow shaft (2) and on the extension element (4). [4] Rotor shaft according to claim 3, characterized by that the electrical line or cable feedthrough (10) for wireless, in particular inductive, electrical energy transmission is designed as part of a rotary transformer (11) or is such a rotary transformer (11) or comprises such a rotary transformer (11). [5] Rotor shaft according to claim 4, characterized bythat the rotary transformer (11) comprises a rotary transformer rotor (12) which is connected in a rotationally fixed manner to the hollow shaft (2) and a rotary transformer stator (13) which is arranged axially at a distance from the rotary transformer rotor (12) and inductively coupled thereto and which is fixedly, in particular in a rotationally fixed manner, connected to the extension element (4). [6] Rotor shaft according to claim 5, in particular when referring back to claim 2, characterized by that the rotary transformer (11) is arranged axially on a side (28) of the first bearing device (5a) and / or the bearing plate (9) facing away from the interior (8). [7] Rotor shaft according to claim 5, in particular when referring back to claim 2, characterized by that the rotary transformer (11) is arranged axially on a side (29) of the first bearing device (5a) and / or the bearing plate (9) facing the interior (8). [8] Rotor shaft according to claim 5 or 7, characterized bythat the rotary transformer (11) is arranged in the interior space (8) delimited by the hollow shaft (2). [9] Rotor shaft according to one of claims 1 to 3, characterized by that the electrical line or cable feedthrough (10) is designed for wired electrical energy transmission and preferably has at least one slip ring (21a, 21b) as part of a sliding contact (14a, 14b), which is particularly preferably connected in a rotationally fixed manner to the extension element (4) or to the hollow shaft (2). [10] Rotor shaft according to one of the preceding claims, characterized by that the first bearing device (5a) is supported radially outwardly on the hollow shaft (2) and radially inwardly on the extension element (4), in particular on a peripheral wall (6) of the extension element (4). [11] Rotor shaft according to one of the preceding claims, characterized bythat a coupling device (7) for drivingly coupling the rotor shaft (1) to an external component is formed in the second end section (3b) of the hollow shaft (2). [12] Rotor shaft according to one of the preceding claims, characterized by that a diameter (D) of the hollow shaft (2) is reduced in the second end section (3b). [13] Rotor shaft according to one of the preceding claims, characterized by that an extension (E2) of the second end portion (3b) of the hollow shaft (2) measured along the axial direction (A) is greater than a projection (E1) of the extension element (4) measured along the axial direction (A) beyond the first end portion (3a) of the hollow shaft (2). [14] Rotor shaft according to one of the preceding claims, characterized by that the first bearing device (5a) is arranged radially between the hollow shaft (2) and the extension element (4). [15] Rotor shaft according to one of the preceding claims, characterized bythat the first bearing device (5a) is arranged axially in the region of the first end section (3) in an interior space (8) delimited by the hollow shaft (2). [16] Rotor shaft according to one of the preceding claims, characterized by , that - the first bearing device (5a) is connected to the extension element (4) in a rotationally fixed manner and is connected to the hollow shaft (2) in a rotationally fixed manner; or that - the first bearing device (5a) is connected to the hollow shaft (2) in a rotationally fixed manner and is connected to the extension element (4) in a rotationally fixed manner. [17] Rotor shaft according to one of the preceding claims, characterized by , that - the second storage device (5b) has a greater spatial extent than the first storage device (5a); or / and that - the second bearing device (5b) is designed to absorb greater bearing forces than the first bearing device (5a). [18] Electric machine (15), - with a housing (18) surrounding a housing interior (19), - with a stator (16) arranged in a fixed position relative to the housing (18) in the housing interior (19) and with a rotor (17) arranged at least partially in the housing interior (19) and rotatable relative to the stator (16) and magnetically coupled thereto, in particular drivable by the stator by means of magnetic interaction, - wherein the rotor (17) comprises a rotor shaft (1) according to one of the preceding claims, on which magnetic field generating elements (24) for generating a magnetic rotor field are arranged in a rotationally fixed manner, - wherein the extension element (4) of the rotor shaft (1) is firmly, in particular rigidly, connected to the housing (18) and / or the stator (16) and the hollow shaft (2) is rotatably mounted on the housing (18) and / or the stator (16) by means of the second bearing device (5b).

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