Patent Publication Number: US-2023150566-A1

Title: Adjustment drive for a steering column, and steering column for a motor vehicle

Description:
PRIOR ART 
     The invention relates to an adjustment drive according to the features of the preamble of patent claim  1  and a steering column having such an adjustment drive. 
     Steering columns for motor vehicles have a steering shaft having a steering spindle, at the rear end of which facing the driver in the direction of travel a steering wheel is fitted in order for the driver to introduce a steering command. The steering spindle is rotatably supported about the longitudinal axis thereof in an adjustment unit which is retained by means of a carrier unit on the vehicle body. As a result of the fact that an inner covering pipe of the adjustment unit, also referred to a covering pipe for short, is received in a covering unit which is connected to the carrier unit and which is also referred to as a guiding case, outer covering pipe or boxed rocker so as to be able to be displaced in the manner of a telescope in the direction of the longitudinal axis, a length adjustment can be carried out. A height adjustment can be carried out by the adjustment unit or a covering unit which receives it being pivotably supported on the carrier unit. The adjustment of the adjustment unit in the longitudinal or vertical direction enables the adjustment of an ergonomically comfortable steering wheel position relative to the driver position in the operational position, also referred to as the driving or operating position, in which a manual steering action can be carried out. 
     It is known in the prior art, in order to adjust the adjustment unit relative to the carrier unit, to provide a motorized adjustment drive with a drive unit. The drive unit comprises an electric drive motor which—generally by means of a gear mechanism—is connected to a spindle drive which comprises a threaded spindle which is screwed into a spindle nut. As a result of the drive unit, the threaded spindle and the spindle nut can be rotatably driven with respect to each other about the spindle axis, whereby, depending on the rotation direction, they can be moved in translation toward each other or away from each other in the direction of the spindle axis. In an embodiment, a so-called rotary spindle drive, the threaded spindle can be rotatably driven about the spindle axis by means of the drive unit which is fixedly connected to the adjustment unit or the carrier unit and engages in the spindle nut, which is fitted in a fixed manner to the carrier unit or the adjustment unit with respect to rotation about the spindle axis. Axially, that is to say, in the direction of the spindle axis, the threaded spindle is supported by means of the coupling portion on the carrier unit or the adjustment unit, and the spindle nut is accordingly supported on the adjustment unit or the carrier unit so that a rotary drive of the threaded spindle brings about a translational adjustment of the carrier unit and adjustment unit relative to each other. 
     In an alternative embodiment, which is also referred to as an immersion spindle drive, the threaded spindle is coupled to the carrier unit or the adjustment unit with the coupling portion thereof in a non-rotatable manner with respect to rotation about the spindle axis thereof and the spindle nut is accordingly supported on the adjustment unit or the carrier unit so as to be able to be rotated but in a state fixed in the direction of the spindle axis. As in the first embodiment, the threaded spindle is supported by means of the coupling portion in the direction of the spindle axis axially on the carrier unit or the adjustment unit, and the spindle nut is accordingly supported on the adjustment unit or the carrier unit so that the threaded spindle can be displaced in a translational manner by the drive unit in the direction of the spindle axis. 
     In both embodiments, the spindle drive forms a motorized adjustment drive which is active between the carrier unit and adjustment unit and by means of which the adjustment unit can be displaced for adjustment relative to the carrier unit. A generic adjustment drive is described, for example, in DE 10 2017 207 561 A1. 
     In order to carry out a longitudinal adjustment of the adjustment unit in the direction of the longitudinal axis of the steering spindle, an adjustment drive can be arranged between the covering pipe of the adjustment unit and a covering unit which receives it in an axially longitudinally displaceable manner and which is connected to the carrier unit and wherein the spindle axis can be orientated substantially parallel with the longitudinal axis. 
     For height adjustment, an adjustment device may be arranged between the carrier unit and an adjustment unit which is supported thereon so as to be able to be vertically adjusted. 
     A motorized longitudinal and vertical adjustment can be formed individually or in combination on the steering column. 
     In order to improve the comfort when entering and leaving or in order to adjust the steering column into a storage position outside the manual operating range when moving into the autonomous driving mode during autonomous driving or to bring it back into the operating position from the storage position when changing into manual operation, a relatively rapid adjustment over relatively large adjustment paths is required. In this instance, collisions of the rapidly moved steering wheel with vehicle occupants may occur and injuries may result. A reduction of the adjustment speed in order to reduce the risk of injury is, however, not acceptable as a result of the associated comfort losses and also as a result of potential safety risks during autonomous driving. 
     With regard to the above-explained problem, an object of the present invention is to provide an adjustment drive with a higher safety level. 
     STATEMENT OF INVENTION 
     This object is achieved according to the invention with an adjustment drive having the features of claim  1 . Advantageous developments will be appreciated from the dependent claims. 
     In an adjustment drive for a steering column for a motor vehicle comprising a threaded spindle which engages with an outer thread in a spindle nut and a drive unit which is coupled to the threaded spindle or the spindle nut in such a manner that the threaded spindle and the spindle nut can be driven in rotation relative to each other, there is provision according to the invention for the drive unit to have a torque-transmitting slip clutch which is coupled to the threaded spindle or the spindle nut. 
     A slip clutch, which can also be synonymously referred to as a friction clutch, has friction faces which are connected to each other in a non-positive-locking or frictionally engaging manner and which are arranged on rotatably drivable drive elements by means of which the drive torque produced by an electric motor (drive motor) is coupled to the threaded spindle or the spindle nut. The slip clutch consequently forms a rotating clutch which is incorporated in the force or torque path between the motor which generates the drive torque and the spindle nut which is rotatably driven thereby or the threaded spindle. 
     As a result of the fact that the friction faces of the slip clutch are loaded with respect to each other with a predetermined coupling force, there is produced a defined friction force (adhesive force) by means of which a maximum drive torque or maximum torque is predetermined. If the drive torque applied exceeds this maximum drive torque, the friction faces slide through relative to each other since the effective static friction is exceeded and the torque transmission is interrupted. The adjustment is thereby stopped. 
     During normal operation of an adjustment drive, when the adjustable elements of the steering column can move relative to each other in an unimpeded manner, the drive torque which is introduced into the spindle drive, that is to say, by the motor into the spindle nut or the threaded spindle of the spindle drive, remains below the limit value predetermined by the maximum drive torque. The limit value is determined from the effective normal force which acts on friction faces and the static friction coefficient, that is to say, the borderline case represents the transition from static friction to sliding friction. However, if there is an obstacle in the adjustment path during adjustment, for example, if the steering wheel strikes a vehicle occupant, the drive torque increases. If, in this instance, the maximum drive torque of the slip clutch is exceeded, it slips through and the transmission of the drive torque is interrupted so that the adjustment is stopped. As a result of corresponding specification of the maximum drive force, a maximum permissible adjustment force is predetermined and the strength of the impact in the event of a collision with objects or vehicle occupants in the vehicle interior is thereby limited. The risk of injuries and damage is reduced and the safety level is increased. In this instance, it is advantageous for a high adjustment speed to continue to be maintained in order to ensure a high level of travel and operating comfort. 
     Preferably, the slip clutch is in the form of a dry clutch. As a result of this embodiment, the use of fluids can be prevented. In particular, a sealing of the slip clutch with respect to the environment can thereby be dispensed with. 
     Preferably, there may be provision for a motor of the drive unit to be coupled to the slip clutch. The drive torque is produced by the motor of the drive unit, preferably an electric drive motor, which is connected at the input side or drive side to the slip clutch, preferably by means of a gear mechanism. At the outlet or output side, in order to drive the spindle drive, the slip clutch is connected to the threaded spindle or the spindle nut, that is to say, consequently coupled. Consequently, the slip clutch is according to the invention incorporated in the drive train in order to transmit the drive torque between the motor and the spindle drive. 
     It is advantageous for the slip clutch to have at least two friction faces which contact each other in a frictionally engaging manner. These two friction faces which contact each other in a frictionally engaging manner form a friction pair. A friction clutch which has on rotating clutch components corresponding coupling faces which lie against each other in a non-positive-locking manner and which are in the form of friction faces is thereby formed. The friction faces may be opposite each other with respect to the rotation or coupling axis of the slip clutch axially, radially or obliquely. Such embodiments may be produced in a structurally simple and space-saving manner. 
     Preferably, the slip clutch has one or more friction pairs. In a very particularly preferred manner, the slip clutch has two friction pairs. 
     There may be provision for at least one of the friction faces to be configured in a friction-increasing manner. In order to produce a defined friction and a corresponding maximum torque, one or both friction faces may have a clutch lining or friction lining which may have a friction material, such as, for example, sintering materials, carbon fiber compound and/or ceramic materials which can be bound in a binding agent, such as synthetic resin or the like. It is thereby advantageously possible to carry out an optimized adaptation to the materials of the slip clutch, the response behavior and the desired transmission characteristic, the structural space available and the like. 
     It is possible for the friction faces to be formed or arranged on torque-transmitting coupling components and to be directly in abutment with each other. In addition, one or more friction elements may be arranged between friction faces, for example, friction or coupling plates which are supported in a floating manner and which are coupled only in a non-positive-locking manner in the torque path. In this instance, it is possible for a friction element to be configured in a friction-increasing manner, for example, with a friction liner, as set out above, or a friction element may be inserted in place of a friction liner between the friction faces and may, for example, be formed as a type of friction plate from an above-mentioned friction material. 
     With one or more interposed friction elements, a packet-like arrangement, as in multi-disk clutches known per se may be produced. 
     There may be provision for the slip clutch to have a pretensioning device, Using the pretensioning device, the friction faces may be loaded with respect to each other with a defined pretensioning force, the normal force. As a result of the level of the pretensioning force, the maximum drive torque may be predetermined and accordingly the adjustment force applied by the adjustment drive. 
     It may be advantageous for the pretensioning device to have a resilient element and/or an adjustable adjustment means. There may be provided as a resilient element, for example, a spring element which presses the friction faces resiliently one against the other. The level of the resilient force determines the pretensioning. In this instance, it is advantageous that tolerances are compensated for by the resilient force, whereby safe operation is ensured. 
     The adjustable adjustment means may be provided alternatively to or together with a resilient element and enables the adjustment and variable adaptation of the pretensioning force. There may, for example, be provided as an adjustment means an adjustment screw which can be tensioned with respect to a friction face in order to produce the pretensioning force of the non-positive-locking engagement at a defined level. Consequently, the response of the slip clutch, that is to say, the slipping through of the friction faces in order to interrupt the drive torque, can be adjusted in order to predetermine the maximum permissible adjustment force. 
     The combination of a resilient element with an adjustment means, for example, a pressure spring or spring plate, which can be inserted between an adjustment screw and a coupling component which has a friction face is advantageous. As a result of the fact that the adjustment screw is tensioned against the resilient element, the pretensioning force can be finely adjusted and can be introduced in a resiliently homogenized manner into the frictional engagement. This may be carried out, for example, during the assembly of the adjustment drive by the adjustment screw being rotated until the permissible adjustment force is reached. 
     Subsequently, there may additionally be provision for the adjustment screw to be fixed, for example, by means of caulking. 
     An advantageous embodiment may make provision for the slip clutch to be arranged coaxially with respect to the threaded spindle. As a result of an arrangement coaxially relative to the spindle axis of the threaded spindle, a particularly compact construction type can be achieved. Annular friction faces which completely or partially surround the threaded spindle may preferably be formed. The friction faces may have axial surfaces which can preferably be loaded with respect to each other by means of a pretensioning device in the direction of the spindle axis in order to predetermine a defined maximum drive torque. In this instance, at least one of the friction faces may be fitted or formed on the threaded spindle or the spindle nut. The drive torque which is directly applied to the spindle drive is thereby relevant for the response of the slip clutch so that the maximum drive torque cannot be impaired by means of friction at another location or the like. 
     An advantageous embodiment of the invention may make provision for the slip clutch to be connected to a gear wheel. The gear wheel is a gear element in the drive train between the motor and the threaded spindle or the spindle nut, for example, a gear, worm gear or the like. As a result of the connection to a gear wheel, the structural integration of the slip clutch into a gear mechanism of the drive unit can be improved and simplified. 
     In the last-mentioned embodiment, there may advantageously be provision for a rotationally drivable drive wheel of the drive unit to be in meshing engagement with the gear wheel. A worm which is connected to the motor and which engages in a gear wheel which is in the form of a worm gear may, for example, act as a drive wheel. 
     It is advantageously possible for the gear wheel to be connected to the spindle nut or the threaded spindle. The gear wheel may, for example, comprise a gear or a worm gear which is connected coaxially to the spindle nut in order to produce an immersion spindle drive in which the threaded spindle which is fixed with respect to rotation about the spindle axis passes coaxially through the spindle nut and the gear wheel. Alternatively, the gear wheel may be connected to the threaded spindle in order to form a rotation spindle drive in which the threaded spindle can be driven by the drive unit in a rotational manner relative to the steering column. 
     An advantageous development of the above-mentioned embodiment is that the slip clutch is integrated between a sprocket and a hub of the gear wheel. As a result, the slip clutch can advantageously be integrated in a particularly compact manner in the gear wheel, for example, a worm gear. In this instance, the hub may have at least one axial or radial friction face, preferably on the outer circumference thereof. The sprocket which is arranged coaxially with respect to the hub and which may, for example, have an externally circumferential worm gear arrangement has at least one corresponding friction face. 
     In an immersion spindle drive, the hub may have the inner thread of the spindle nut which may be integrally constructed with the hub, or in a threaded bush inserted into the hub. In a rotary spindle drive, the hub may be connected to the threaded spindle in a rotationally secure manner by means of a fixed connection or by means of integral construction. 
     It may be advantageous for the hub and the sprocket to have mutually opposing friction faces which are preferably directed axially counter to each other. The torque-transmitting frictional engagement may thereby be integrated in the gear wheel in a compact manner. An advantage of friction faces which are arranged axially with respect to the spindle axis is that a pretensioning device, which may, for example, comprise an adjustment screw and additionally or alternatively a resilient element, can be arranged axially with respect to the friction faces, whereby a production even with a relatively small diameter of the gear wheel is readily possible. An arrangement similar to that in a known construction type of a disk clutch with circular friction faces can be formed. 
     The integration of the slip clutch in the gearwheel is advantageous in many respects. A particularly compact construction type can thereby be achieved and a direct coupling to the threaded spindle or the spindle nut is enabled, whereby the response behavior of the adjustment drive is improved. It is further advantageous for the hub and the sprocket to be able to be constructed from different materials, for example, from steel, non-iron metals, or plastics materials, composite materials or the like. 
     Furthermore, the invention relates to a steering column for a motor vehicle, having a carrier unit which can be fitted to a vehicle body, and by means of which an adjustment unit in which a steering spindle is rotatably supported is retained, and having an adjustment drive which comprises a threaded spindle which engages with an outer thread in a spindle nut, and a drive unit which is coupled to the threaded spindle or the spindle nut in such a manner that the threaded spindle and the spindle nut can be driven so as to rotate relative to each other, wherein the adjustment drive is connected to the carrier unit or the adjustment unit, wherein the adjustment drive is in the form of an adjustment drive according to the invention. This drive may also have the above-mentioned advantageous developments individually or in combination. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Advantageous embodiments of the invention are explained in greater detail below with reference to the drawings, in which: 
         FIG.  1    shows a schematic, perspective view of a steering column according to the invention, 
         FIG.  2    shows another perspective view of the steering column according to the invention according to  FIG.  1    from another viewing angle, 
         FIG.  3    shows an adjustment drive according to the invention of a steering column according to  FIGS.  1  and  2    in a cut-away schematic view, 
         FIG.  4    shows the gear wheel of the adjustment drive as an exploded illustration, 
         FIG.  5    shows a longitudinal section along the spindle axis through the adjustment drive according to  FIG.  3   , 
         FIG.  6    shows an enlarged detailed illustration from  FIG.  5   . 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     In the various Figures, identical components are always given the same reference numerals and are therefore generally also only set out or mentioned once. 
       FIG.  1    shows a steering column  1  according to the invention as an oblique schematic perspective view from the top right toward the rear end, with respect to the travel direction of a vehicle which is not illustrated, wherein a steering wheel which is not illustrated in this instance is retained in the operating region.  FIG.  2    shows the steering column  1  as a view from the opposing side, that is to say, when viewed from the top right. 
     The steering column  1  comprises a carrier unit  2  which is in the form of a console, which has securing means  21  in the form of securing holes for fitting to a vehicle body which is not illustrated. The carrier unit  2  retains an adjustment unit  3  which is received in a covering unit  4 —which is also referred to as a guiding box or a boxed rocker. 
     The adjustment unit  3  has a covering pipe  31  in which there is rotatably supported about a longitudinal axis L a steering spindle  32  which extends axially in the longitudinal direction, that is to say, in the direction of the longitudinal axis L. At the rear end, there is formed on the steering spindle  32  a securing portion  33  to which a steering wheel which is not illustrated can be fitted. 
     In order to produce a longitudinal adjustment in the covering unit  4 , the adjustment unit  3  is received so as to be able to be displaced in the manner of a telescope in the direction of the longitudinal axis L in order to be able to position the steering wheel which is connected to the steering spindle  32  relative to the carrier unit  2  back and forth in the longitudinal direction, as indicated with the double-headed arrow parallel with the longitudinal axis L. 
     The covering unit  4  is supported in a pivot bearing  22  on the carrier unit  2  so as to be able to be pivoted about a horizontal pivot axis S which is located transversely with respect to the longitudinal axis L. In the rear region, the covering unit  4  is connected to the carrier unit  2  by means of an adjustment lever  41 . As a result of a rotational movement of the adjustment lever  41  by means of an adjustment drive  6  which is not illustrated (see  FIG.  2   ), the covering unit  4  can be pivoted relative to the carrier unit  2  about the pivot axis S which is located horizontally in the installation state, whereby an adjustment of a steering wheel which is fitted to the securing portion  33  can be carried out in the vertical direction H, which is indicated with the double-headed arrow. 
     A first adjustment drive  5  for longitudinal displacement of the adjustment unit  3  relative to the covering unit  4  in the direction of the longitudinal axis L has a spindle drive having a spindle nut  51  having an inner thread  74  which extends along an axis G and in which a threaded spindle  52  engages, that is to say, is screwed with the outer thread thereof into the corresponding inner thread  74  of the spindle nut  51 . The threaded spindle axis of the threaded spindle  52  is identical to the axis G and extends substantially parallel with the longitudinal axis L. 
     The spindle nut  51  is rotatably supported about the axis G in a bearing housing  53  which is securely connected to the covering unit  4 . In the direction of the axis G, the spindle nut  51  is axially supported on the covering unit  4  by means of the bearing housing  53 . The adjustment drive  5  is accordingly a so-called immersion spindle drive. 
     The threaded spindle  52  is connected to the adjustment unit  3  with a securing element  54  which is formed at the rear end thereof by means of a transmission element  34 , in a manner fixed in the direction of the axis G or the longitudinal axis L and fixed in position with respect to the rotation about the axis G. As a result of the rotationally drivable spindle nut  51  and the threaded spindle  52  which is fixed with respect to rotation, a so-called immersion spindle drive is produced. 
     The transmission element  34  extends from the adjustment unit  3  through a slot-like through-opening  42  in the covering unit  4 . In order to adjust the steering column  1  in the longitudinal direction, the transmission element  34  can be moved freely along in the through-opening  42  in the longitudinal direction. 
     The adjustment drive  5  has an electric drive motor  55  by means of which the spindle nut  51  can be driven in rotation with respect to the axis G relative to the fixed threaded spindle  52 . As a result—depending on the rotation direction of the drive motor  55 —the threaded spindle  52  can be displaced in the direction of the axis G in translation relative to the spindle nut  51  so that the adjustment unit  31  which is connected to the threaded spindle  52  is accordingly displaced relative to the covering unit  4  which is connected to the spindle nut  51  in the direction of the longitudinal axis L. The drive of the spindle nut  51  and the support of the spindle nut  51  in the direction of the axis G on the covering unit  4  is explained in detail below. 
     In  FIG.  2   , which shows a perspective view of the steering column  1  from the side located at the rear in  FIG.  1   , it can be seen how a second adjustment drive  6  for adjusting in the vertical direction H is fitted to the steering column  1 . This adjustment drive  6  comprises a spindle nut  61  in whose inner thread  74  a threaded spindle  52  engages along an axis G. The threaded spindle  52  is rotatably supported about the axis G in a bearing housing  63  which is secured to the covering unit  4  and axially supported on the covering unit  4  in the direction of the axis G and can be driven in rotation about the axis G by an electric drive motor  65  selectively in both rotation directions. Accordingly, the adjustment drive  6  is a so-called rotary spindle drive. 
     The spindle nut  61  which may be formed from plastics material or a non-ferrous heavy metal such as brass or the like, is fitted in a fixed manner with respect to a rotation about the axis G to one end of the two-armed adjustment lever  41  which is pivotably supported about a pivot bearing  23  on the carrier unit  2  and whose other arm is connected with the other end to the covering unit  4 . 
     By rotating the threaded spindle  61 —depending on the rotation direction of the drive motor  65 —the spindle nut  61  can be displaced in translation relative to the threaded spindle  52  in the direction of the axis G so that the covering unit  4  which is connected to the spindle nut  61  by means of the adjustment lever  41  together with the adjustment device  3  which is received therein can accordingly be adjusted upward or downward in a vertical direction H relative to the carrier unit  2 , as indicated with the double-headed arrow. 
       FIG.  3    shows the adjustment drive  5  which is in the form of an immersion spindle drive as a separate cut-away illustration, wherein the bearing housing  53  has been omitted for greater clarity.  FIG.  5    shows a longitudinal section along the spindle axis G and  FIG.  6    shows an enlarged detailed view therefrom. 
     The adjustment drive  5  has a gear wheel  7  which comprises a hub element  71  (=hub) which has the spindle nut  51  with the inner thread  511  and which is arranged in this instance coaxially with respect to the spindle axis G. The spindle nut  51  and the hub element  71  may preferably be constructed integrally as illustrated. However, it is also conceivable and possible for the spindle nut  51  to be formed from a metal material and for the hub element  72  which may be formed from a plastics material to be injected thereon with the plastics material injection-molding method. 
     A sprocket  72  having an externally circumferential tooth arrangement  721  is arranged coaxially with respect to the hub element  71  and connected by means of a slip clutch  8  according to the invention which will be explained in greater detail below. 
     A worm  56  which is connected to the motor shaft of the motor  55  is in toothed engagement with the tooth arrangement  721  of the sprocket  72 . 
     The slip clutch  8  according to the invention which is integrated in the gear wheel  7  is shown in the assembled state in  FIGS.  5  and  6    and as an exploded illustration pulled apart in the direction of the spindle axis G in  FIG.  4   . 
     The hub element  71  has a radially circumferential projection  711  which has at the axial sides thereof annularly circumferential axial friction faces  712  and  713 . The friction faces  712  and  713  are consequently formed by the shoulders of the circumferential projection  711 . In the installed state, the friction face  712  is axially opposite a corresponding, similarly annular axial friction face  722  internally on the sprocket  72 . The sprocket  72  has to this end an inwardly protruding portion which has a shoulder which comprises the friction face  722 . 
     Axially between the friction faces  712  and  722 , an annular-disk-like friction element  73  is arranged in a frictionally engaging manner. 
     At the side facing away from the friction face  722 , an annular adjustment screw  74  is screwed into a coaxial inner thread  723  of the sprocket  72 . The adjustment screw  74  has an axial friction face  741  which is axially opposite the friction face  713 . 
     An annular, axially resilient element  75 , for example, a helical spring, ondular washer or disk spring, or an O-ring formed from an elastomer material and a friction element  76  is arranged axially between the friction face  741  of the adjustment screw  74  and the friction face  713  of the hub element  71  and may be constructed in a similar manner to and substantially with the same function as the above-mentioned friction element  73 . 
     The friction elements  73  and  76  each have outwardly protruding pins  731  and  761  which are preferably arranged in a state distributed in a uniform manner over the circumference of the friction elements  73  and  76 . The sprocket  72  preferably has internal grooves  724  in which the pins  731  and  761  of the friction elements  73  and  76  engage and secure them with respect to the sprocket in a rotation direction about the spindle axis G. Consequently, the friction elements  73  and  76  cannot be rotated with respect to the sprocket  72  about the spindle axis G. 
     As a result of the adjustment screw  74 , the friction element  76  is loaded via the resilient element  75  with a defined pretensioning force with respect to the friction face  713  of the hub element  71 . At the same time, the hub element  71  is thereby pressed with the friction face  712  axially via the friction element  73  with the pretensioning force axially against the corresponding friction face  722  of the sprocket  77 . As a result, the hub element  71  is coupled via the friction faces  712  and  713  to the corresponding friction faces  722  and  741  on the sprocket  72  in a frictionally engaging, that is to say, non-positive-locking manner. As a result of the cooperation described, the slip clutch  8  according to the invention, by means of which the torque discharged by the motor  55  is transmitted to the gear wheel  7  and the threaded spindle  52  is formed. The maximum torque can be adjusted by screwing in or unscrewing the adjustment nut  74 , whereby the axial pretensioning force acting between the friction faces  712 ,  713 ,  722  and  741  can be adjusted. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Steering column 
           2  Carrier unit 
           21  Securing means 
           22 ,  23  Pivot bearing 
           3  Adjustment unit 
           31  Covering pipe 
           32  Steering spindle 
           33  Securing portion 
           34  Transmission element 
           4  Covering unit 
           41  Adjustment lever 
           42  Through-opening 
           5 ,  6  Adjustment drive 
           51 ,  61  Spindle nut 
           511  Inner thread 
           52 ,  62  Threaded spindle 
           53 ,  63  Bearing housing 
           54  Securing element 
           55 ,  65  Motor (drive motor) 
           56 ,  66  Worm 
           7  Gear wheel 
           71  Hub element 
           711  Projection 
           712  Friction face 
           713  Friction face 
           72  Sprocket 
           721  Tooth arrangement 
           722  Friction face 
           73 ,  76  Friction element 
           74  Adjustment screw 
           741  Friction face 
           75  Resilient element 
           8  Slip clutch 
         L Longitudinal axis 
         H Vertical direction 
         G Spindle axis (threaded spindle axis)