Patent Publication Number: US-2023145716-A1

Title: Aircraft turbomachine

Description:
TECHNICAL FIELD 
     This present disclosure relates to the field of aircrafts, and more particularly a turbomachine that can be used for aeronautical propulsion. 
     PRIOR ART 
     The turbomachines used for the propulsion of aircrafts, for example turbojet engines, generally comprise a casing in which a fan providing most of the thrust, a compressor and a turbine are housed. The compressor supplies compressed air to a combustion chamber and the combustion gases produced in said chamber set in motion the turbine, which in turn drives the fan. 
     To maximize the efficiency of the turbine, it is desirable that it rotates as quickly as possible. Conversely, the speed of the fan is limited by the speed of rotation at the blade tip, which must most often remain lower than the speed of sound. 
     To overcome this problem, turbomachines equipped with a reduction gear placed in the transmission chain between the turbine and the fan have been known. A reduction gear allows rotating the turbine and the fan at different speeds. 
     There is nevertheless still a need to improve aircraft turbomachines. 
     DISCLOSURE OF THE INVENTION 
     To this end, the present disclosure relates to an aircraft turbomachine comprising a casing, a fan, a compressor and a turbine, and an epicyclic gear train comprising an input driven in rotation by the turbine, a first output stage configured to drive in rotation the compressor and a second output stage coupled to the first output stage and configured to drive in rotation the fan, the compressor being driven in rotation by the ring gear of the first output stage. 
     The casing designates a generally fixed portion of the turbomachine, the fan, the compressor and the turbine comprising bladed wheels configured to rotate relative to the casing. The casing may comprise an inlet casing between the fan and the compressor. 
     In the present disclosure, “axis of the turbomachine” refers to its axis of symmetry or quasi-symmetry, which forms the axis of rotation at least of the turbine, and generally also of the compressor and of the fan. The axial direction corresponds to the direction of the axis of the turbomachine and a radial direction is a direction perpendicular to this axis and intersecting this axis. Similarly, an axial plane is a plane containing the axis of the turbomachine and a radial plane is a plane perpendicular to this axis. A circumference is understood as a circle belonging to a radial plane and whose center belongs to the axis of the turbomachine. A tangential or circumferential direction is a direction tangent to a circumference; it is perpendicular to the axis of the turbomachine but does not pass through the axis. 
     Unless otherwise specified, the adjectives “front” and “rear” are used in reference to the axial direction, it being understood that the inlet of the turbomachine is located on the front side of the turbomachine, while its outlet is located on the rear side. The adjectives “upstream” and “downstream” are used in reference to the normal flow direction of the gases in the turbomachine. 
     Finally, unless otherwise specified, the adjectives “internal (inner)” and “external (outer)” are used in reference to a radial direction so that the internal portion of an element is, along a radial direction, closer to the axis of the turbomachine than the external portion of the same element. 
     An epicyclic gear train comprises an internal planetary gear, also known as central pinion or sun gear and simply called “sun gear” thereafter, one or several planet gears in revolution around the sun gear and a ring gear, sometimes called external planetary gear, surrounding the planet gear(s). Thereafter, without loss of generality, we will speak equally of one or several planet gear(s). The planet gears may be rotatably mounted on a planet carrier, the planet carrier being configured to synchronize the revolution of the different planet gears relative to the sun gear or to the ring gear. 
     Unless explicitly stated otherwise or apparent from the context, thereafter, the references to the radii and diameters of the wheels are understood as references to the pitch radii and diameters, respectively. The pitch circle is such that the pitch circles of two meshing gears have the same tangential speed. 
     The mechanical connection between the components of the epicyclic gear train (sun gear, planet gears, planet carrier, ring gear) and the members of the turbomachine (fan, compressor, turbine) can be ensured by means of shafts secured in rotation to said members. 
     Thanks to the fact that the fan and the compressor are driven in rotation by two different outputs of the epicyclic gear train, it is possible to rotate not only the fan and the turbine, but also the compressor and the turbine, at different speeds. Several factors make it necessary to increase the radius of the compressor: the integration of the epicyclic gear train, provided for the fan, in the vicinity of the compressor, but also the aerodynamic stresses of the flowpath and the integration of other elements of the turbomachine such as bearings, shafts, trunnions and ventilation. In doing so, the speed of rotation of the compressor must be reduced to prevent the tangential speed at the compressor blade tip from exceeding the applicable limits. The use of two different outputs therefore allows limiting the rotation speed (rpm) of the compressor while having the highest possible rotation speed for the turbine and a relatively low rotation speed for the fan, with a view, for example, to increasing the radius of the fan. 
     The second output stage is coupled to the first output stage, which means that there is a functional connection between the first output stage and the second output stage, allowing the transmission of a torque between these two output stages. Thanks to the fact that the epicyclic gear train is a multi—stage epicyclic gear train comprising at least a first output stage and a second output stage coupled to each other, the turbomachine can maintain good compactness despite a potentially very high turbine— fan reduction ratio (typically above 5). By comparison, a single—stage epicyclic gear train with a comparable reduction ratio would be radially bulky and would require either providing a radially outward offset, sometimes called “gooseneck”, and an axial elongation of the turbomachine, or placing the compressor to a larger radius and therefore limiting its speed, which affects its efficiency. 
     Finally, thanks to the fact that the compressor is driven in rotation by the ring gear of the first output stage, it is possible to obtain a reduction ratio between the compressor and the turbine proportional to the ratio of the radii of the sun gear and of the ring gear of the first output stage, while maintaining a slower output on the ring gear of the second stage. By contrast, if the compressor was driven by the sun gear of the first output stage, the input driven by the turbine should be the internal planetary gear of the second stage. The second output stage driving the fan in rotation, this configuration would leave only one stage to achieve the reduction ratio between the fan and the turbine:this would therefore bring the architecture back to space requirement problems described above. Moreover, the fact of not using the inner planetary gears of all the stages facilitates the integration of inner abutments to the epicyclic gear train in order to take up the inner forces when using herringbone teeth. 
     Together, these characterstics improve the integration of the compressor and the overall efficiency of the turbomachine. 
     In some embodiments, the fan is driven in rotation by the ring gear of the second output stage. Thus, two stages of the epicyclic gear train are used to obtain relatively high reduction ratios with a reasonable radial space requirement. 
     In some embodiments, the epicyclic gear train comprises at least one planet gear comprising a first wheel and a second wheel secured in rotation to each other, the first wheel belonging to the first output stage and meshing with the ring gear of the first output stage and the second wheel belonging to the second output stage. The first wheel and the second wheel can belong to the same part forming the planet gear. 
     If necessary, the second wheel can mesh with the ring gear of the second output stage. 
     In some embodiments, the ratio between the diameter of the first output wheel and the diameter of the second output wheel is comprised between 0.5 and 10. 
     In some embodiments, the epicyclic gear train comprises a planet carrier fixed relative to the casing. The planet carrier being fixed, the ring gear of each output stage and the sun gear are driven in rotation. 
     Thus, in some embodiments, the input of the epicyclic gear train comprises a sun gear of the epicyclic gear train. In other words, the turbine is configured to drive in rotation the sun gear of the epicyclic gear train. This choice allows obtaining a reduction in the speed of rotation while limiting the complexity and the space requirement. 
     In some embodiments, the compressor is supported relative to the casing by at least one bearing. For example, the bearing can be a ball bearing or a roller bearing. Several bearings can also be provided, at least one, several or all of the bearings being arranged between the compressor and the casing. 
     In some embodiments, the turbomachine further comprises a bearing arranged between a shaft of the compressor and a shaft of the turbine. Such a bearing, installed between two rotating portions, is generally called intershaft bearing. For example, the intershaft bearing may be a roller bearing. Several bearings can also be provided, at least one, several or all of the bearings being arranged between a shaft of the compressor and a shaft of the turbine. An intershaft bearing arranged in this way allows limiting the space requirement of the stator portions under the compressor. 
     In some embodiments, three elements among a fan shaft, a compressor shaft, a turbine shaft and the casing comprise a locally more flexible portion able to accommodate axial displacements of said elements. The portion is said to be locally more flexible in the sense that it locally has less rigidity than the surrounding portions. Said portion therefore forms an area of preferential and facilitated deformation to accommodate the displacements. Indeed, the epicyclic gear train being hyperstatic, the relative displacements of the input and the output stages, in particular in the axial direction, tend to cause overloads in the components of the epicyclic gear train. The respective locally more flexible portions allow reducing, if not avoiding, these overloads. The fourth element among the aforementioned elements can be relatively more rigid than the locally more flexible portions. The component of the epicyclic gear train coupled to this element is called “master” insofar as it imposes its displacements on the other components of the epicyclic gear train, called “slaves” and which, through the locally more flexible portions, do not cause displacement of the corresponding turbomachine members. 
     In some embodiments, said portion has meanders in axial section. The axial section is a section in an axial plane as defined above. The meanders define a non-purely axial shape, for example having a non-zero radial component. The meanders can comprise sinuosities, broken line, zigzag, chicane shapes, etc. 
     In some embodiments, said fan is the only fan of the turbomachine. The turbomachine therefore does not comprise any other fan. 
     In some embodiments, said compressor is a low-pressure compressor and the turbomachine further comprises a high-pressure compressor downstream of the low-pressure compressor, the casing comprising an inter-compressor casing between the low-pressure compressor and the high-pressure compressor. 
     In some embodiments, a fan shaft is supported relative to the inlet casing by at least one bearing. Optionally, the fan shaft is supported relative to the inlet casing by at least two bearings. Optionally, these two bearings comprise at least one ball bearing and at least one roller bearing. 
     Furthermore, the present disclosure also relates to an aircraft turbomachine comprising a casing, a fan, a compressor and a turbine, and an epicyclic gear train comprising an input driven in rotation by the turbine, a first output stage and a second output stage coupled to the first output stage, in which the turbine is configured to drive in rotation a sun gear of the first output stage, the fan is driven in rotation by the ring gear of the second output stage, and the compressor is driven in rotation by the ring gear of the first output stage or by the sun gear of the second output stage. 
     This turbomachine may have all or part of the characteristics detailed elsewhere in the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the object of the present disclosure will emerge from the following description of embodiments, given by way of non-limiting examples, with reference to the appended figures. 
         FIG.  1    is a schematic overview of a turbomachine according to one embodiment. 
         FIG.  2    is a diagram of an epicyclic gear train according to one embodiment. 
         FIG.  3    is part of a longitudinal section of the turbomachine according to a first embodiment. 
         FIG.  4    is part of a longitudinal section of the turbomachine according to a second embodiment. 
         FIG.  5    is a longitudinal and perspective sectional view of part of the turbomachine according to the second embodiment. 
         FIG.  6    is part of a longitudinal section of the turbomachine according to a third embodiment. 
         FIG.  7    is part of a longitudinal section of the turbomachine according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An aircraft turbomachine  10  according to one embodiment is schematically represented in  FIG.  1   , in longitudinal half-section. In this case, the turbomachine  10  is a two-spool, dual-flow turbojet engine. Indeed, the turbomachine  10  comprises a fan  20 , preferably a single fan, an inner casing  30  disposed downstream of the fan  20  and separating a primary flowpath  12  from a secondary flowpath  14 . A low-pressure compressor (LP compressor)  50 , a high-pressure compressor (HP compressor)  60 , a combustion chamber  70 , a high-pressure turbine (HP turbine)  80  and a low-pressure turbine (LP turbine)  90  are arranged in the primary flowpath  12 , from upstream to downstream. Because the turbomachine  10  is a two-spool turbomachine, it includes two kinematically independent rotating assemblies, namely on the one hand a high-pressure body (HP body), comprising the HP compressor  60  and the HP turbine  80 , and on the other hand a low-pressure body (LP body) comprising the LP compressor  50  and the LP turbine  80 . Each compressor  50 ,  60  is directly or indirectly driven by the turbine  80 ,  90  of the corresponding body, the turbines  80 ,  90  being set in motion by the combustion gases coming from the combustion chamber  70 . 
     However, the present disclosure can be transposed to the case of a single-spool turbomachine. The single body would have the function of the HP body for the operation of the turbomachine, but its role in relation to the epicyclic gear train described below would be that of the LP body. 
     The casing of the turbomachine further comprises, in this embodiment, an inlet casing  32  between the fan  20  and the LP compressor  50 , an inter-compressor casing  34  between the LP compressor  50  and the HP compressor  60 , an inter-turbine casing  36  between the HP turbine  80  and the LP turbine  90 , and a turbine rear casing  38  (or turbine rear frame) downstream of the LP turbine  90 . The casing elements are fixed in the reference frame of the aircraft, and the rotating portions, namely the movable bladed wheels of the fan  20 , of the compressors  50 ,  60  and of the turbines  80 ,  90 , rotate relative to the casing. 
     The rotation of the HP turbine  80  drives the HP compressor  60  via a HP shaft  82 . The HP compressor  60  and the HP turbine  80  are therefore kinematically dependent on each other and, particularly here, rotate at the same speed. The HP shaft  82  can be supported relative to the casing by at least one bearing, in this case a first bearing  84 , typically a ball bearing, relative to the inter-compressor casing  34  and a second bearing  86 , typically a roller bearing, relative to the inter-turbine casing  36 . 
     Furthermore, in this embodiment, the LP turbine  90  drives in rotation the LP compressor  50 . The LP turbine  90  also drives in rotation the fan  20 . More specifically, the turbomachine  10  comprises a transmission, here an epicyclic gear train  40 , coupled to LP turbine  90  via LP turbine shaft  92 . In this embodiment, the LP turbine shaft  92  is arranged coaxially inside the HP shaft  82 . Bearings  94 ,  96  can be provided to support the LP turbine shaft  92 . 
     Furthermore, as illustrated in  FIG.  1   , the epicyclic gear train  40  is further coupled to the fan  20  and to the LP compressor  50  in order to modify the rotational speed transmission ratio between the LP turbine  90  and the fan  20  on the other hand, the LP compressor  50  on the other hand. The epicyclic gear train  40  therefore forms a reduction gear between the LP turbine  90  and the fan  20  on the one hand, the LP compressor  50  on the other hand. 
     In other words, as illustrated in  FIG.  1   , the epicyclic gear train  40  comprises an input driven in rotation by the LP turbine  90 , a first output stage configured to drive in rotation the LP compressor  50  and a second output stage coupled to the first output stage and configured to drive in rotation the fan  20 . 
     The structure of the epicyclic gear train  40  is represented in detail in the diagram of  FIG.  2   . 
     The epicyclic gear train  40  comprises a sun gear  49 . In this embodiment, the sun gear  49  is driven in rotation by the LP turbine  90 . More specifically, the sun gear  49  can be driven in rotation, even secured in rotation, to the LP turbine shaft  92 . Thus, in this embodiment, the input of the epicyclic gear train  40  comprises the sun gear  49 . 
     The sun gear  49  meshes with at least one planet gear  41 . The planet gear  41  rotates on itself. The planet gear  41  follows a movement of revolution around the sun gear  49  but, as the sun gear  49  itself rotates, the planet gear  41  can have a fixed axis of rotation in the reference frame of the turbomachine  10 , as will be illustrated thereafter. 
     The planet gear  41  comprises a first wheel  41   a  and a second wheel  41   b . The first wheel  41   a  and the second wheel  41   b  are coupled to each other, and more specifically here secured in rotation to each other. The first wheel  41   a  belongs to the first output stage  47  of the epicyclic gear train  40 . The second wheel  41   b  belongs to the second output stage  48  of the epicyclic gear train  40 . 
     The planet gear  41  is rotatably mounted on a planet carrier  43 . The planet carrier  43  is here fixed relative to the casing  30 , for example, as schematized in  FIG.  1   , fixed on the inter-compressor casing  34 . 
     The epicyclic gear train  40  furthermore comprises two ring gears  42 ,  45 . The first ring gear  45 , or ring gear of the first output stage  47 , meshes with the planet gear  41 , more particularly its first wheel  41   a . Furthermore, the first ring gear  45  is configured to drive in rotation the LP compressor  50 . More specifically, the first ring gear  45  can drive in rotation, even be secured in rotation to a LP compressor shaft  52  itself secured in rotation to the LP compressor  50 . 
     The second ring gear  42 , or ring gear of the second output stage  48 , meshes with the planet gear  41 , more particularly its second wheel  41   b . Furthermore, the second ring gear  42  is configured to drive in rotation the fan  20 . More specifically, the second ring gear  42  can drive in rotation, even be secured in rotation to a fan shaft  22  itself secured in rotation to the fan  20 . 
     The fan shaft  22  can be supported relative to the casing  30 , in particular relative to the inlet casing  32 , by at least one bearing. In this case, as illustrated in  FIG.  1   , the at least one bearing can comprise at least two bearings, namely at least one roller bearing  24  and at least one ball bearing  26 . 
     The planet carrier  43  being fixed relative to the casing, the rotation of the sun gear  49  is transmitted to the ring gears  42 ,  45  via the planet gear  41 . 
     Thus, in operation, the rotation of the LP turbine  90  is transmitted, via the LP turbine shaft  92 , to the sun gear  49 . The rotation of the sun gear  49  drives in rotation the planet gear  41 . The first output stage  47  drives in rotation the LP compressor  50  via the first wheel  41   a , the first ring gear  45  and the LP compressor shaft  52 . The second output stage  48  drives in rotation the fan  20  via the second wheel  41   b , the second ring gear  42  and the fan shaft  20 . 
     By noting R i  the functional radius of a component i with respect to its axis of rotation (for example R 42  for the inner radius of the second ring gear  42 ) and Ω i  the speed of rotation of the component i, the relations R 49 Ω 49 =R 41a Ω 41 , R 41a Ω 41 =R 45 Ω 45 , and R 41b Ω 41 =R 42 Ω 42  are obtained, from which it is derived that the speed of rotation of the first ring gear  45 , and therefore of the LP compressor shaft  52 , is equal to Ω 45 =(R 49 /R 45 )Ω 49 , and that the speed of rotation of the second ring gear  42 , and therefore of the fan shaft  22 , is equal to Ω 42 =(R 49 /R 41a )(R 41b /R 42 )Ω 49 . 
     For the dimensioning of the relative speeds of the fan  20  and of the LP compressor, the planet gear  41  can be designed so that the ratio between the diameter of the first output wheel and the diameter of the second output wheel is comprised between 0.5 and 10. 
     The configuration of the epicyclic gear train  40  being given as above, its detailed implementation can be designed by the person skilled in the art according to his general knowledge. 
     Furthermore, although an epicyclic gear train  40  has been described here, the sun gear  49  of which forms the input and the ring gears  42 ,  45  form the outputs, it is possible to choose otherwise the components of the epicyclic gear train  40  forming the input and the outputs, for example according to the desired speed reduction ratios. These changes can be made even if it means adding if necessary an additional wheel to the planet gear  41  in order to offer more freedom of design. 
     The practical integration of the epicyclic gear train  40  in a turbomachine is illustrated, according to a first embodiment, in  FIG.  3   . The presence of the epicyclic gear train  40  and of separate shafts for the LP turbine  90  and the LP compressor  50  indeed imposes to review the architecture of the turbomachine, particularly the location of the bearings. 
     Thus, in this first embodiment, the turbomachine  10  comprises at least one bearing configured to support the LP compressor  50  relative to the casing  30 . In this case, two bearings  54 ,  56  are provided. The bearings  54 ,  56  are here arranged between the inter-compressor casing  34  and the LP compressor shaft  52 . More specifically, the bearings  54 ,  56  are arranged on the portion of the casing  30  on which the planet carrier  43  is fixed. 
     In this embodiment, the bearing  54  (third bearing) can be a ball bearing, while the bearing  56  (fourth bearing) can be a roller bearing. However, the bearings  54 ,  56  can be interchanged and/or be of another type, even if it is desirable that at most one of the bearings is a ball bearing, so as to avoid a hyperstatic mounting of the bearings. 
     The presence of two bearings ensures the holding and prevents the swiveling of the LP compressor  50 , that is to say the risk that a single bearing behaves like a swivel in case of an excessive off-center radial load, which would result in no longer ensuring the coaxiality of the rotor of the LP compressor  50  with its stator, and therefore in damaging it. 
     Furthermore, to avoid a hyperstatic mounting of the epicyclic gear train  40 , it is possible to provide that three elements among the fan shaft  22 , the LP compressor shaft  52 , the LP turbine shaft  92  and the casing  30  (in this case the inter-compressor casing  34 ) comprise a locally more flexible portion able to accommodate axial displacements of said elements. In this case, the fan shaft  22  is designed relatively rigid or stiff, while locally flexible portions  34   a ,  52   a ,  92   a  are provided respectively on the inter-compressor casing  34 , the LP compressor shaft  52  and the LP turbine shaft  92 . 
     As illustrated in  FIG.  3   , said locally flexible portions are provided on the shafts between the epicyclic gear train  40  and the bearings supporting the corresponding members of the turbomachine, for example the bearings  54 ,  56 . Thus, the relative axial and/or radial displacements within the epicyclic gear train  40 , even the axial and/or radial displacements of the fan shaft  22 , are absorbed by the locally flexible portions  34   a ,  52   a ,  92   a  and do not disturb the operation of the bearings nor, more generally, the dynamics of the turbomachine  10 . 
     In this embodiment, at least one of said locally flexible portions  34   a ,  52   a ,  92   a , here all of them, have meanders in axial section. The meanders are here formed by an axial succession of radially increasing and decreasing sections. The meanders provide flexibility without compromising the mechanical strength of the turbomachine  10 . 
     When an element is supported by both a ball bearing and a roller bearing, it is possible to provide, as illustrated in  FIG.  1   , that the ball bearing is closer to the epicyclic gear train  40  than the roller bearing, in order to minimize the axial clearances inside the epicyclic gear train  40 . This property can be provided independently for each element. This results in an optimized support of the epicyclic gear train. 
       FIGS.  4  to  7    show the turbomachine in other embodiments. In these figures, the elements corresponding or identical to those of the first embodiment will receive the same reference sign and will not be described again. 
     In the second embodiment, illustrated in  FIGS.  4  and  5   , which shows one variant of the first embodiment, the bearings  54 ,  56  are arranged between the LP compressor shaft  52  and a portion of the inter-compressor casing  34  independent of the portion on which the planet carrier  43  is fixed. This configuration allows closing the clearances of the bearings  54 ,  56 , and more particularly of the ball bearing  54 : indeed, the rotor is located radially inside the stator. Under centrifugal force, the rotor/stator clearance closes. 
       FIG.  5    illustrates more particularly the arrangement of the planet gears  41 , here four in number (three of which are visible in the figure), on the planet carrier  43  and the holding of the planet carrier  43  relative to the inter-compressor casing  34 . In the view represented, the inter-compressor casing  34  and the planet carrier  43  are integrally formed, but alternatively, it is possible to provide a fixing between the inter-compressor casing  34  and the planet carrier  43 , for example to facilitate the mounting of the epicyclic gear train  40 . 
     In the third embodiment, illustrated in  FIG.  6   , the bearings  54 ,  56  are arranged between the LP compressor shaft  52  and the inter-compressor casing  34 . However, the planet carrier  43  is for its part fixed not on the inter-compressor casing  34 , but on the inlet casing  32 . To do so, for example, the support of the planet carrier  43  passes between two consecutive planet gears  41 . This frees up space to position the bearings  54 ,  56  closer to the axis of rotation X, which urges them less mechanically. In addition, it allows freeing up space under the LP compressor  50 , which facilitates the integration under the LP compressor  50  and allows lowering the bores, thus reducing the mass of the disks. 
     In addition, the inlet casing  32  being positioned forwardly relative to the planet carrier  43 , the junction between the planet carrier  43  and the inlet casing  32  forms, as illustrated in  FIG.  6   , a return bend  33 . The return bend can have a portion extending axially forwards and radially outwards. The return bend  33  can form a locally more flexible portion able to accommodate axial displacements of the epicyclic gear train  40  relative to the inlet casing  32 . In addition, the fact that the part linking the planet carrier  43  to the inlet casing  32  is perforated at the level of the planet gears  41  allows accommodating radial displacements of the epicyclic gear train  40  relative to the inlet casing  32 . Thus, in these embodiments, the casing  30  can be devoid of meanders. This allows reducing the length and space requirement of the engine. 
     In the fourth embodiment, illustrated in  FIG.  7   , which shows one variant of the third embodiment, the third bearing  54  supports the LP compressor  50  relative to the casing  30 , here relative to the inlet casing  32 . The fourth bearing  56 , in this case the roller bearing, is arranged between the shaft of the LP compressor  52  and the LP turbine shaft  92 . The fourth bearing  56  therefore forms an intershaft bearing. As indicated above, the integration of an intershaft bearing allows limiting the space requirement of the stator parts under the LP compressor  50 . 
     Throughout the present disclosure, when it comes to the driving, this driving may be direct, that is to say particularly without intermediate transmission stage. 
     Although the present description refers to specific exemplary embodiments, modifications can be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual characteristics of the different illustrated or mentioned embodiments can be combined in additional embodiments. Accordingly, the description and drawings should be considered in an illustrative rather than restrictive sense.