ASSEMBLY

TECHNICAL FIELD OF THE INVENTION [0001] The present invention relates to electric machines such as electric motors or generators. It relates in particular, albeit not exclusively, to electric motors for driving vehicles, in particular electric motors for rail vehicles.

[0002] In principle, the present invention can be applied to any electric machine. However, the benefits are particularly important in the case of permanent magnet- excited electric machines in which high magnetic forces are generated not only when the machine is powered, but permanently.

BACKGROUND ART

[0003] Electric machines for driving vehicles are usually provided with a stator comprising a bearing shield and a fixed labyrinth seal ring, a rotor comprising a rotor shaft and a rotatable labyrinth seal ring mounted on the rotor shaft and facing the fixed labyrinth seal ring at a distance of the fixed labyrinth seal ring so as to form an annular labyrinth between the fixed labyrinth seal ring and rotatable labyrinth seal ring. One or more bearing assemblies are releasably mounted between the bearing shield and the rotor shaft, for guiding a rotation motion of the rotor about the revolution axis and for preventing an axial motion of the rotor relative to the stator.

[0004] Such bearings of electric machines are heavily used and must be regularly serviced or replaced. Changing the bearings is relatively complex and involves the risk of damage to components of the electric machine. Usually, the rotor must be dismantled during the bearing change and then reassembled. This may be a difficult task, particularly in the case of permanent magnet-excited electric machines, because of the high magnetic forces occurring in the air gap between the rotor and the stator.

[0005] It has been proposed in EP 2 610 514 to build an electric machine, in which the labyrinth rings can be moved relative to one another until they touch one another in a locking position, so that the rotor can be fixed in the axial and radial directions for exchanging the bearings. The labyrinth rings have conically shaped surfaces, which touch each other in the locking position so that a positive connection between the labyrinth rings can be achieved in the locking position, which ensures that the rotor is fixed exactly in the centre of the axis of rotation. The fixed labyrinth ring is connected to the bearing shield by bolts during normal operation of the electric machine. These bolts are loosened for the purpose of transferring the labyrinth ring to the locking position for changing the bearing. The fixed labyrinth ring can be fixed in the locking position by means of at least one locking bolt, preferably three locking bolts arranged at an angle. This ensures a secure fixation of the position of the motor during the bearing change with comparatively little effort. In the maintenance position, the bearings of the electric machine can be serviced or replaced quickly and easily.

[0006] One disadvantage of this solution, however, is that the fixation is quite weak, relying on compressive loads on the bolts to form a friction joint in the labyrinth, which provides only reduced contact surfaces. Moreover, grease is usually present in the labyrinth, which further reduces this friction joint. Additionally, this solution is sensitive to the way the locking bolts are tightened and the bolts need to be tightened crosswise with small increments in order to prevent jamming of the rotor. Last but not least, this solution requires specific labyrinth seal rings.

SUMMARY OF THE INVENTION

[0007] The invention aims to overcome at least some of the drawbacks of the prior art and to provide an electric machine that can be easily maintained.

[0008] According to a first aspect of the invention, there is provided an electric machine, comprising: a stator, a rotor and a bearing assembly. The stator is provided with a bearing shield assembly comprising a bearing shield and a fixed labyrinth seal ring having a tortuous labyrinth surface. The rotor is rotatable about a revolution axis relative to the stator and comprises a rotor shaft and a rotatable labyrinth seal ring mounted on the rotor shaft. The rotatable labyrinth seal ring has a tortuous labyrinth surface facing the tortuous labyrinth surface of the fixed labyrinth seal ring at a first axial distance of the tortuous labyrinth surface of the fixed labyrinth seal ring so as to form an annular labyrinth between the tortuous labyrinth surface of the fixed labyrinth seal ring and the tortuous labyrinth surface of the rotatable labyrinth seal ring. The bearing assembly is releasably mounted between the bearing shield assembly and the rotor shaft, for guiding a rotation motion of the rotor about the revolution axis, The bearing shield assembly is provided with a fixed contact face, and the rotor further comprises an opposite contact face axially facing the fixed contact face of the bearing shield assembly at a second axial distance of the fixed contact face of the bearing shield assembly, the second axial distance being shorter than the first axial distance.

[0009] When the releasable bearing is loosened, it becomes possible to translate the rotor relative to the stator until the fixed contact face reaches the opposite contact face, which happens before the rotatable labyrinth ring contacts the fixed labyrinth seal ring. The fixed contact face and opposite contact face provide large and clean surfaces for locking the rotor to the stator.

[0010] In an embodiment, the bearing assembly comprises a bearing cartridge, an outer race ring fitted into the bearing cartridge, and an inner race ring fitted onto the rotor shaft. Preferably, the bearing cartridge is provided with mounting through holes parallel to the revolution axis and aligned with threaded holes of the bearing shield assembly, the bearing assembly comprising fastening bolts inserted into the mounting through holes of the bearing cartridge and screwed into the threaded holes of the bearing shield assembly to fasten the bearing cartridge to the bearing shield assembly. The fastening bolts can be loosened to dismount the bearing assembly.

[0011] In an embodiment, the bearing cartridge bears on an outer face of the bearing shield while the fixed labyrinth seal ring bears an inner face of the bearing shield, opposite to the outer face.

[0012] In a preferred embodiment, the bearing cartridge is provided with clearance through holes, which are parallel to the revolution axis and aligned with through holes of the bearing shield assembly, and which can be aligned with threaded holes of the rotor in an indexed angular position of the rotor relative to the stator, the clearance through holes having a diameter greater than the through holes of the bearing shield assembly and threaded holes of the rotor, so that in the indexed angular position, locking bolts can be inserted into the clearance through holes and in the through holes of the bearing shield assembly and screwed into the threaded holes of the rotor, with a cylindrical bolt head of the locking bolts bearing axially against an edge of the through holes of the bearing shield assembly. Once the bolt head of one of the locking bolts reaches the edge of the through hole of the bearing shield assembly, subsequent tightening of the bolt will result in a translation movement of the rotor relative to the stator parallel to the bolt axis, i.e. parallel to the revolution axis, until a contact is established between the contact face of the bearing shield assembly and the opposite contact face of the rotor. This translation is guided by the radial contact between the bolt shanks and the inner wall of the through holes of the bearing shield assembly. Once the contact between the contact faces is established, the locking bolts are further tightened until the tensile loaded locking bolts provide sufficient pressure between the contact faces and the rotor is locked by the frictional engagement of the contact faces.

[0013] In one embodiment, a thrust washer releasably is fixed at an end of the rotor shaft so as to axially bear against the inner race ring of the bearing assembly. An opposite axial end of the inner race ring may bear against a shoulder of the rotor shaft or another axial stop.

[0014] In one embodiment, the bearing assembly comprises rolling elements between the outer race ring and inner race ring. The rolling elements may include balls, cylindrical, tapered or barrel-shaped rollers, arranged in one or several rows.

[0015] In one embodiment, an inner bearing space filled with lubricant is formed between the inner race ring and outer race ring and sealed by the annular labyrinth formed between the fixed labyrinth seal ring and rotatable labyrinth seal ring. The lubricant is preferably grease.

[0016] In one embodiment, an outer bearing cover is releasably fixed to the bearing shield assembly. The outer bearing cover may close the inner lubrication space of the bearing, which extends from the outer bearing cover to the labyrinth seal rings. The outer bearing preferably covers the mounting through holes and/or the clearance through holes of the bearing cartridge. [0017] In one embodiment, the fixed contact face is made of an electric insulation material. Alternatively, or additionally, the opposite contact face is made of an electric insulation material. This ensures that no current will flow through the contact faces when they are in contact. [0018] In one embodiment, the fixed contact face is made in one piece with the fixed labyrinth seal ring. Alternatively, the fixed contact face is formed on the bearing shield.

[0019] In a preferred embodiment, the fixed contact face includes at least a planar portion, which faces a planar portion of the opposite contact face at said second axial distance.

[0020] Advantageously, the fixed labyrinth seal ring and rotatable labyrinth seal ring are provided with interleaved tubular ribs aligned with the revolution axis, the labyrinth being formed by a continuous annular space between said interleaved tubular ribs. [0021] In embodiment, the electric machine is a drive motor of a rail vehicle .

BRIEF DESCRIPTION OF THE FIGURES

[0022] Other advantages and features of the invention will then become more clearly apparent from the following description of a specific embodiment of the invention given as non-restrictive examples only and represented in the accompanying drawings in which:

- figure 1 is an isometric view, from a drive side, of an electric machine according to an embodiment of the invention;

- figure 2 is an isometric view of the electric machine of figure 1, from a non-drive side during a step of disassembly of a first bearing assembly on the non-drive side of the electric machine;

- figure 3 is a front view of the electric machine of figure 1, from the non drive side, without an outer bearing cover; - figure 4 is an axial section of the electric machine of figure 1 by a section plane IV-IV illustrated in figure 3;

- figure 5 is an axial section of the electric machine of figure 1 by a section plane V-V illustrated in figure 3; - figure 6 illustrates a detail of the electric machine of figure 1, during a step of disassembly of the first bearing assembly on the non-drive side of the electric machine, in the section plane V-V illustrated in figure 3;

- figure 7 illustrates a detail of the electric machine of figure 1, during another step of disassembly of the first bearing assembly on the non- drive side of the electric machine, in the section plane V-V illustrated in figure 3;

- figure 8 illustrates a detail of the electric machine of figure 1, during the step of disassembly of figure 7, in the section plane IV-IV illustrated in figure 3; - figure 9 illustrates a detail of the electric machine of figure 1, during a step of disassembly of a second bearing assembly on the drive side of the electric machine, in the section plane V-V illustrated in figure 3;

- figure 10 illustrates a detail of the electric machine of figure 1, during another step of disassembly of the second bearing assembly on the drive side of the electric machine, in the section plane V-V illustrated in figure 3.

[0023] Corresponding reference numerals refer to the same or corresponding parts in each of the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS [0024] With reference to figures 1 to 5, an electric machine 10, e.g. a drive motor of a rail vehicle, comprises a stator 12, a rotor 14, a first bearing assembly 16 at a non-drive end of the electric machine 10 and a second bearing assembly 116 at a drive end of the electric machine 10 for guiding a rotation motion of the rotor 14 relative to the stator 12 about a revolution axis 100 of the electric machine 10.

[0025] The stator 12 is provided with stator windings 18 housed in a stator housing 20, which comprises a stator frame 22 a first bearing shield assembly 24 at the non-drive end of the electric machine 10 and a second bearing shield assembly 124 at the drive end of the electric machine 10. The first bearing shield assembly 24 is annular and includes a bearing shield 26 and a fixed labyrinth seal ring 28. The second bearing shield assembly 124 is annular and includes a bearing shield 126 and two fixed labyrinth seal rings 128, 228. [0026] The rotor 14 is centred on the revolution axis 100 and comprises a set of rotor windings or permanent magnets 30, a rotor shaft 32, a first rotatable labyrinth seal ring 34 mounted on the rotor shaft 32 and facing the fixed labyrinth seal ring 28 of the first bearing shield assembly 24, and a pair of second rotatable labyrinth seal rings 134, 234 mounted on the rotor shaft 32 and facing the two fixed labyrinth seal rings 128, 228 of the second bearing shield assembly 124. The fixed labyrinth seal rings 28, 128, 228 and rotatable labyrinth seal rings 34, 134, 234 are provided with interleaved tubular ribs 36, 38, 136, 138, 236, 238 aligned with the revolution axis 100 and a tortuous path is formed by a continuous annular space between the interleaved tubular ribs 36, 38, 136, 138, 236, 238. [0027] In the axial direction parallel to the revolution axis 100, a minimal distance D1 separates the tortuous labyrinth surface formed by the tubular ribs 38 of rotatable labyrinth seal ring 34 from the tortuous labyrinth surface formed by the tubular ribs 36 of the fixed labyrinth seal ring 28.

[0028] Remarkably, the bearing shield assembly 24 is provided with one or more fixed contact faces 40 and the rotor 14 further comprises one or more opposite contact faces 42 axially facing the fixed contact face(s) 40 of the bearing shield assembly 24 at an axial distance D2 of the fixed contact face of the bearing shield assembly, with D2 < Dl. The fixed contact face(s) and opposite contact face are planar faces perpendicular to the revolution axis. [0029] A minimal distance Dll separates, in the axial direction parallel to the revolution axis 100, the tortuous labyrinth surfaces formed by the tubular ribs 138, 238 of the rotatable labyrinth seal rings 134,234 from the tortuous labyrinth surfaces formed by the tubular ribs 136,138 of the fixed labyrinth seal rings 128, 228.Dll is such that andD2<Dll and is preferably equal toDl.

[0030] Similarly, the bearing shield assembly 124 is provided with one or more fixed contact faces 140 and the rotor 14 further comprises one or more opposite contact faces 142 axially facing the fixed contact face(s) 140 of the bearing shield assembly 124 at an axial distance D22 of the fixed contact face of the bearing shield assembly, with D12 < Dll. The fixed contact face(s) and opposite contact face are planar faces perpendicular to the revolution axis. Preferably,D12 is equal to D2.

[0031] The first bearing assembly 16 at the non-drive end of the electric machine 10 is mounted between the first bearing shield assembly 24 and the rotor 14 while the second bearing assembly 116 is mounted between the second bearing shield assembly 124 and the rotor 14 for guiding a rotation motion of the rotor 14 about the revolution axis 100.

[0032] More specifically, the first bearing assembly 16 comprises a bearing cartridge44, an outer race ring46 force fitted into the bearing cartridge 44, an inner race ring48 force fitted onto the rotor shaft 14 and rolling elements 50 between the inner race ring 48 and outer race ring 46. In this embodiment, the rolling elements 50 are balls, and the inner and outer race rings 48,46 have a concave cross-section. The inner race ring 48 bears axially against a shoulder 52 of the rotor shaft 14. A thrust washer 54 is releasably fixed at an end 56 of the rotor shaft 32 by means of bolts 58 so as to axially bear against the inner race ring 48 of the bearing assembly 16.

[0033] As depicted in figure 4, the bearing cartridge 44 bears axially against an outer side 60 of the bearing shield 26 and is provided with a set of several, preferably three or more, mounting through holes 62 distributed over the circumference of the bearing cartridge 44 and extending in a direction parallel to the revolution axis 100. The mounting through holes 62 are aligned with intermediate holes 64 of the bearing shield 26 and threaded holes 65 of fixed labyrinth seal ring 28. Fastening bolts 66 are inserted into the mounting through holes 62 of the bearing cartridge and screwed into the threaded holes 64 of the bearing shield 26 to fasten the bearing cartridge 44 to the bearing shield 26.

[0034] The bearing cartridge 44 is further provided with a set of several, preferably three or more, clearance through holes 68 distributed over the circumference of the bearing cartridge 44, illustrated in figure 5, which are parallel to the revolution axis 100 and aligned with through holes 70, 72 of the bearing shield assembly 24, in this embodiment through the bearing shield 26 and the fixed labyrinth seal ring 28.

[0035] These clearance through holes 68 are aligned with threaded holes 74 of the rotor 14 when the rotor 14 is placed in an indexed angular position relative to the stator 12. The clearance through holes 68 have a diameter greater than the through holes 70, 72 of the bearing shield assembly and threaded holes 74 of the rotor.

[0036] An outer bearing cover 76 is releasably fixed to the bearing shield 26 by means of bolts 78 and covers the mounting through holes 62 and the clearance through holes 68 of the bearing cartridge 44. A sealed lubrication volume 80 is formed between the inner and outer race rings 48, 46, closed at one axial end by the outer bearing cover 76 and sealed at the opposite axial end by the labyrinth seal formed by the fixed labyrinth seal ring 28 and rotatable labyrinth seal ring 34. This lubrication volume can be filled with a lubricant, preferably grease.

[0037] Similarly, the second bearing assembly 16 at the drive end of the electric machine comprises a bearing cartridge 144 integral with the fixed labyrinth seal ring 228, an outer race ring 146 force fitted into the bearing cartridge 144, an inner race ring 148 force fitted onto the rotor shaft 14 and rolling elements 150 between the inner race ring 148 and outer race ring 146. In this embodiment, the rolling elements are cylindrical rollers and the races of the inner race ring 148 and outer race ring 46 are cylindrical. The inner race ring 148 bears axially against a shoulder 152 of the rotor shaft 14. [0038] As depicted in figure 4, the bearing cartridge 144 bears axially against an outer side 160 of the bearing shield 126 and is provided with a set of several, preferably three or more, mounting through holes 162 distributed over the circumference of the bearing cartridge 144 and extending in a direction parallel to the revolution axis 100. The mounting through holes 162 are aligned with intermediate through holes of the bearing shield 126 and threaded holes 165 of the fixed labyrinth seal ring 128. Fastening bolts 166 are inserted into the mounting through holes 162 of the bearing cartridge 144 and screwed into the threaded holes 164 of the bearing shield 126 to fasten the bearing cartridge 144 to the bearing shield 126.

[0039] The bearing cartridge 144 is further provided with a set of several, preferably three or more, clearance through holes 168 distributed over the circumference of the bearing cartridge 144, illustrated in figure 5, which are parallel to the revolution axis 100 and aligned with through holes 170, 172 of the bearing shield assembly 124, in this embodiment through the bearing shield 126 and the fixed labyrinth seal ring 128. These clearance through holes 168 can be aligned with threaded holes 174 of the rotor 14 when the rotor 14 is in an indexed angular position relative to the stator 12. The clearance through holes 168 have a diameter greater than the through holes 170, 172 of the bearing shield assembly and threaded holes 174 of the rotor.

[0040] A sealed lubrication volume 180 is formed between the inner and outer race rings 48, 46, sealed at the one axial end by the labyrinth seal formed by the fixed labyrinth seal ring 128 and rotatable labyrinth seal ring 134 and sealed at the opposite axial end by the labyrinth seal formed by the fixed labyrinth seal ring 228 integral with the bearing cartridge 144 and rotatable labyrinth seal ring 34. This lubrication volume can be filled with a lubricant, preferably grease.

[0041] To dismount the first bearing assembly 16 at the non-drive end of the electric machine 10, it is first necessary to unscrew the bolts 78 and remove the outer bearing cover 76, which gives access to the clearance through holes 68 of the bearing cartridge 44 and to the mounting through holes 62 of the bearing cartridge 44. [0042] The rotor 14 is rotated to the indexed angular position, and locking bolts 82 are inserted into the clearance through holes 68 (see figure 2) and in the through holes 70, 72 of the bearing shield assembly 24 . The locking bolts 82 are screwed into the threaded holes 74 of the rotor 14, until a cylindrical bolt head 84 of the locking bolts 82 bears axially against an edge 86 of the through holes 70 of the bearing shield 26 as illustrated in figure 6.

[0043] Once the bolt heads 84 of the locking bolts 82 reach the edges 86 of the through holes 70 of the bearing shield 26, the shanks of the locking bolts 82 are in contact with the inner walls of the through holes 70, 72 of the bearing shield 24 and fixed labyrinth seal ring 28 and ensure a centred positioning of the rotor 14 relative to the revolution axis 10 independent from the bearing assembly 16. The mounting bolts 66 can therefore be at least partially unscrewed to allow a limited translation movement of the rotor 14 relative to the stator 12.

[0044] Subsequent tightening of the locking bolts 82 results in a translation movement of the rotor 14 relative to the stator 12 parallel to the locking bolt axis, i.e. parallel to the revolution axis 100, until a contact is established between the contact face 40 of the bearing shield assembly 24 and an opposite contact face 42 of the rotor 14, as illustrated in figure 7. This translation is guided by the radial contact between the shanks of the locking bolts 82 and the inner walls of the through holes 70, 72 of the bearing shield assembly 24 and/or by the rolling bodies 50 and inner race ring 48. Once the contact between the contact faces 40, 42 is established, the locking bolts 82 are further tightened until the tensile loaded locking bolts 82 provide sufficient pressure between the contact faces 40, 42 and the rotor 14 is locked by the frictional engagement of the contact faces 40, 42. Remarkably, no contact occurs between the interleaved ribs 36, 38 of the fixed labyrinth seal ring and movable labyrinth seal ring, because D2<D1. Similarly, no contact occurs between the interleaved ribs 136, 138, 236, 238 of the fixed labyrinth seal rings 128, 228 and movable labyrinth seal rings 134, 234, because D2<D11. During the translation movement of the rotor, the cylindrical roller 150 slide on the race of the inner race ring 148.

[0045] If they have not been completely removed at the end of the step of figure 5, the mounting bolts 66 can be fully unscrewed and removed as well as the bolts 58. The bearing cartridge 76, outer race ring 46 and rolling bodies 50 can be removed before a ring puller is inserted to reach the remote end face of the inner ring 48 and pull the inner race ring 48.

[0046] To dismount the second bearing assembly 116 at the drive end of the electric machine 10, it is first necessary to unscrew the bolts 78 and remove the outer bearing cover 76 at the non-drive end of the electric machine 10, as illustrated in figure 9, which gives access to the clearance through holes 68 of the bearing cartridge 44 and to the mounting through holes 62 of the bearing cartridge 44.

[0047] The rotor 14 is rotated to the indexed angular position, and, optionally, a threaded indexing rod 90 is inserted into the clearance through holes 68 and in the through holes 70, 72 of the bearing shield assembly 24 and screwed into the threaded holes 74 of the rotor 14, as illustrated in figure 9.

[0048] Once the indexing rod 90 has is in place, the shanks of the indexing rod 90 are in contact with the inner walls of the through holes 70, 72 of the bearing shield 24 and fixed labyrinth seal ring 28 and ensure a centred positioning of the rotor 14 relative to the revolution axis 10 independent from the bearing assembly 16. The mounting bolts 66 can therefore be at least partially unscrewed to allow a limited translation movement of the rotor 14 relative to the stator 12.

[0049] Once these preliminary steps have been completed at the non-drive end of the electric machine 10, the actual operation at the drive end can start. Locking bolts 182 are inserted into the clearance through holes 168 and in the through holes 170, 172 of the bearing shield assembly 124 and screwed into the threaded holes 174 of the rotor 14, until a cylindrical bolt head 184 of the locking bolts 182 bears axially against an edge 186 of the through holes 170 of the bearing shield 126 as illustrated in figures 9.

[0050] Once the bolt heads 184 of the locking bolts 182 have reached the edges 186 of the through holes 170 of the bearing shield 126, the shanks of the locking bolts 182 are in contact with the inner walls of the through holes 170, 172 of the bearing shield 124 and fixed labyrinth seal ring 128 and ensure a centred positioning of the rotor 14 relative to the revolution axis 10 independent from the bearing assembly 116. The mounting bolts 166 can therefore be at least partially unscrewed to allow a limited translation movement of the rotor 14 relative to the stator 12.

[0051] Subsequent tightening of the locking bolts 182 results in a translation movement of the rotor 14 relative to the stator 12 parallel to the locking bolt axis, i.e. parallel to the revolution axis 100, until a contact is established between the contact face 140 of the bearing shield assembly 124 and an opposite contact face 142 of the rotor 14, as illustrated in figure 10. This translation is guided by the radial contact between the shanks of the locking bolts 182 and the inner walls of the through holes 170, 172 of the bearing shield assembly 124 and/or by the rolling bodies 150 and inner race ring 148. Once the contact between the contact faces 140, 142 is established, the locking bolts 182 are further tightened until the tensile loaded locking bolts 182 provide sufficient pressure between the contact faces 140, 142 and the rotor 14 is locked by the frictional engagement of the contact faces 140, 142. Remarkably, no contact occurs between the interleaved ribs 136, 138, 236, 238 of the fixed labyrinth seal ring and movable labyrinth seal ring, because of the fixed labyrinth seal rings 128, 228 and movable labyrinth seal rings 134, 234, because D12<D11. Remarkably, the bearing assembly 16 at the non-drive end of the electric machine 10 moves with the rotor 14 and the thrust washer 54 and pushes the fixed labyrinth seal ring 28 away from the bearing shield 26, while the relative angular position between both parts is maintained by the indexing rod 90. The movable labyrinth seal ring 223 can then be pulled out as shown in figure 9, and the bearing cartridge 176, outer race ring 146 and rolling bodies 150 can be removed before a ring puller is inserted to reach the remote end face of the inner ring 148 and pull the inner race ring 148.

[0052] The bearing shields 26, 126 are preferably metallic. The fixed labyrinth seal rings 28, 128, 228 are preferably made of an electric insulation material, so that no electric path is generated when the contact surfaces 40, 42, 140, 142 touch each other.

[0053] As a variant, the subsequent tightening of the locking bolts 82 after the step of figure 4 may result in an elastic deformation of the bearing shield 26, which results in a contact between the contact faces 40, 42 without translation of the rotor 14 relative to the stator 12. Alternatively, a deformation of the rotor press plate in which the threaded holes 74 are located can take place.

[0054] A clearance may be preserved between the shanks of the locking bolts 82 and the through holes 70, 72 of the bearing shield 24 and fixed labyrinth seal ring28, in which case the centred positioning of the rotor 14 relative to the revolution axis 10 when the mounting bolts 66 are removed is provided by the bearing assembly16.