Abstract:
The invention relates to a drive acting on both sides for adjustment devices, including a rotationally mounted drive lever for producing a rotational movement that can be selectively effected in one direction of rotation or in the other direction of rotation, a drive wheel that can only be rotated when the drive lever is moved from a zero point position, at least a one-piece coupling element having two coupling areas for transmitting a torque to the drive wheel, in addition to a sliding block guide with areas for controlling the coupling element. The coupling element can be displaced radially and is tiltably mounted in a guide and coupled by both coupling areas in the zero point position of the drive lever to the drive wheel in an elastic manner so that the non-loaded coupling area is uncoupled from the drive wheel by the sliding block guide when the drive lever moves from the zero point position and remains uncoupled when the drive lever moves in the opposite direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of International application number PCT/DE00/01810, filed May 31, 2000, which in turn claims priority to German patent application number 199 27 033.3, filed Jun. 4, 1999. 
     FIELD OF INVENTION 
     The invention relates to a drive acting on both sides for adjustment devices. The invention is intended in particular for the adjustment of vehicle seat components such as seat backs and seat cushions. 
     BACKGROUND OF INVENTION 
     From DE 195 27 912 A1 a drive acting on both sides is known where a drive lever is mounted on a drive axle and supports swivel coupling elements whose free ends provided with toothed elements can be brought into engagement with the teeth mounted around the circumference of a drive wheel. The swivel coupling elements are associated with a sliding block guide which lifts the relevant unloaded coupling element from the gearing of the drive wheel according to the swivel direction of the drive lever. Spring areas are thereby provided on both sides and outside of the sliding block guide to exert on the coupling elements a force directed against the engagement direction of the teeth so as to prevent any so-called ratchet or rattling noises. 
     The drawback with the adjustment drive known from DE 195 27 912 A1 is in the first place the relatively large number of parts. In particular two independent coupling elements are required for an adjustment on both sides; this is linked with a correspondingly large structural space taken up by the adjustment drive. Secondly an expensive construction is required in order to guide the coupling elements in a defined manner and to prevent the teeth of the coupling elements from catching on the teeth of the drive wheel when the drive lever is returned to the zero point position. 
     SUMMARY OF THE INVENTION 
     Starting from the prior art the object of the invention is to provide a drive acting on both sides which produces a rotary movement, which uses a small number of individual parts, is simple to assemble and is characterised by a compact construction. The drive during use should operate substantially free of play and produce no switching or rattling noises. 
     The solution according to the invention proposes that the coupling element is mounted for both radial displacement and tilting movement in a guide whereby the two coupling areas are coupled spring-elastically to the drive wheel when the drive lever is in the zero point position. When the drive lever is moved out of the zero point position the non-loaded coupling area is uncoupled from the drive wheel through the sliding block guide and remains uncoupled when the drive lever is moved in the opposite direction, thus moved towards the zero point position. 
     Although play is required owing to the one-piece design of the coupling element, the drive appears subjectively free of play since the actual idling path which has to be covered up to rotation of the drive wheel is loaded through the spring-elastic pretension. This pretension can be produced for example by a coil spring which is supported on one side radially on the drive lever and on the other side on the coupling element, whereby the coupling area is pressed into the associated coupling area of the drive wheel. By way of example a coil spring presses the coupling element against the drive wheel whereby the movement of the drive lever overall is found to be free of play. Furthermore with the solution according to the invention no engagement click occurs since the coupling element is brought smoothly through the resilient tension into an entrainment position and not merely by applying a drive force. 
     In order to ensure a secure accurate guide of the coupling element in the radial direction a guide element is provided which is connected to the coupling element and which engages in a recess of the drive lever. This recess thereby extends in the radial direction and is formed for example as an oblong hole. 
     In an alternative embodiment the guide element is mounted on the drive lever and the radial recess which acts as a type of slide guide is worked into the coupling element. The guide element can be connected in one assembly process, for example by welding, riveting or screwing, to the drive lever or to the coupling element or can be formed by pressing or drawing. The recess is normally punched out. Similarly, the formation of the recess is possible in a re-shaping process. 
     Under certain circumstances it can be favourable in order to guarantee a uniform substantially non-jerky movement of the guide element to provide a slide block which connects the guide element to the radial recess. Through the slide block it is possible to produce a favourable friction pairing irrespective of the material of the guide element and of the component part enclosing the recess. 
     This recess is preferably dimensioned so that with maximum displacement of the coupling element towards the drive axle both coupling areas are not exactly coupled to the drive wheel. It is thereby reached that the play of the adjustment drive is minimal. 
     According to the invention a spring is mounted between a housing of the adjustment drive and the coupling element wherein the spring biases the coupling element radially in the direction of the drive wheel so that the two coupling areas in the zero point position of the drive lever reliably adjoin the drive wheel. 
     In one embodiment of the invention a spring is provided between the drive lever and the coupling element whereby it is favourable if the spring is mounted between the drive lever and a guide element of the coupling element. 
     In order to ensure a definite engagement of the coupling areas with the drive wheel it is advantageous to provide a keyed connection to transfer the force applied through the drive lever. To this end both the coupling areas of the coupling element and the circumference of the drive wheel are provided with teeth which are associated with each other so that the two toothed areas can engage in each other. The teeth of the drive wheel can thereby be formed as internal or external teeth. 
     Since the manufacture of the teeth, particularly internal teeth, is time-consuming and expensive, as an alternative to the keyed connection the coupling area is coupled in friction engagement with the drive wheel. The coupling areas of the coupling element and the circumference of the drive wheel are thereby designed accordingly, for example by coating with a friction lining so that the transfer of the required torque between the coupling element and drive wheel takes place through friction engagement. 
     The sliding block guide for controlling the coupling element is formed in one embodiment of the invention in a cover element of the drive. 
     In one development of the invention resilient elements are provided on both sides outside of the guide areas of the slide block and during movement of the drive lever towards the zero point position cause the coupling area which was previously loaded, to be lifted from the drive wheel. The spring force is thereby designed so that when the drive lever is moved out of the zero point position it allows the coupling areas to adjoin the drive wheel and at the same time guarantees that the coupling element is lifted when no force is introduced through the drive lever for adjusting the seat back for example. 
     For this, guide elements are arranged on the coupling element in the form of pins for example which are guided in the sliding block guide and which are in contact outside of the sliding block guide areas with resilient areas in order to engender the lifting movement of the coupling element in the absence of drive force so that no rattling or ratchet noises occur during the resetting movement. In a further development of the invention the guide elements have a non-rounded corner section so that during movement of the coupling element over the zero point position the reversing play is minimised. 
     A further development of the solution according to the invention is characterised in that a secondary force is exerted on the coupling element during an adjustment process such that when the drive lever is moved out of the zero point position torque is produced on the loaded coupling area in the direction of the drive wheel and when the drive lever is moved towards the zero point position torque is produced in the direction of lifting the loaded coupling area away from the drive wheel. 
     With this development of the solution according to the invention a more cost-effective play-free drive is provided which causes on one side a fixed coupling between the coupling area of the coupling element which is active in the relevant adjusting direction, and the drive wheel when the drive lever is moved out from the zero point position and on the other hand ensures a silent resetting of the drive lever back into the zero point position. 
     An advantageous development of this solution according to the invention for a drive where the coupling areas of the coupling element are formed as teeth, and toothing is provided on the circumference of the drive wheel associated with the teeth, is characterised in that when the drive lever is moved out from the zero point position torque is produced on the loaded coupling area in the sense of a toothed engagement and when the drive lever is moved in the direction of the zero point position torque is produced in the sense of separating the toothed engagement. 
     With this design of the invention on the one hand there is a fixed toothed engagement between the teeth of the coupling area of the coupling element which is active in the relevant adjusting direction and of the drive wheel when the drive lever is moved out from the zero point position and on the other it is guaranteed that the drive lever is reset into the zero point position without any ratchet noises. 
     A secondary force acts on the coupling element during an adjusting process to produce a torque in the direction of a reinforced coupling of the loaded coupling area and drive wheel when the drive lever is moved out of the zero point position. Likewise, a secondary force produces a torque in the direction of lifting the loaded coupling area from the drive wheel during movement of the drive lever in the direction of the zero point position. This secondary force is preferably achieved through axial tensioning of the coupling element, preferably outside of the zero point position of the drive lever. 
     This design ensures, in addition to a quasi play-free drive, a fixed coupling between the coupling element and drive wheel as well as the elimination of ratchet noises, that the coupling element in the zero point position is coupled with the drive wheel through the action of the spring acting on the coupling element and that the coupling element is fixed and thus no rattling or ratchet noises can occur. 
     Producing the axial tensioning of the coupling element is preferably carried out through at least one axially acting friction element which is mounted on the coupling element underneath the active line of the rotary point of the coupling element with the connection of the coupling areas to the drive wheel, is supported on a locally fixed part of the drive and in the zero point position of the drive lever is inactive so that the coupling areas are coupled to the drive wheel through the spring elastic connection of the coupling element with the drive lever. 
     Optionally two friction elements can be mounted on the coupling element one either side next to the drive lever and/or one friction element can be mounted centrally on the coupling element which engages through an opening in the drive lever. 
     The or each friction element is preferably formed as a leaf spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The idea on which the invention is based will now be explained with reference to the embodiments shown in the drawings in which: 
     FIG. 1 is a plan view of an adjustment drive with a coupling element mounted on the drive lever; 
     FIG. 2 shows an adjustment drive with an externally toothed drive wheel; 
     FIG. 3 shows an adjustment drive in the zero point position with a spring supported on the drive axle; 
     FIG. 4 shows an adjustment drive according to FIG. 3 in a slightly rotated position wherein the non-loaded coupling area is just lifted from the drive wheel; 
     FIG. 5 shows an adjustment drive according to FIG. 3 in a sectional view; 
     FIG. 6 shows an adjustment drive with springs fixed on the housing for the silent resetting of the coupling element back into the zero point position; 
     FIG. 7 shows an adjustment drive with a spring between the coupling element and the guide element mounted on the drive lever; 
     FIG. 8 shows an adjustment drive with friction elements mounted at the side of the drive lever on the coupling element; 
     FIG. 9 shows an adjustment drive with a friction element mounted centrally on the coupling element and engaging through an opening in the drive lever. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an adjustment drive with a housing secured against rotation where a drive lever  10  or component part  10  connectable to the drive lever is mounted rotatable on an axle  20 . A recess  11  is punched out in the drive lever  10 , which is shown shortened here, and a guide element  42  which is radially loaded by a spring  14  is guided in the recess. The guide element  42  which is here formed as a cylinder pin is moulded onto a coupling element  40  which is mounted between the housing  1  and drive lever  10 . 
     The spring  14  presses the guide element  42  radially outwards from the axle  20  whereby the coupling element  40  is removed from the axle  20  at the same time. In this embodiment the spring is formed as a compression spring  14 , but other types of springs, for example leaf or tensile springs as well as other spring elements such as rubber springs are fundamentally possible. 
     The radially displaceable coupling element  40  has two coupling areas  41   a  and  41   b  which are formed as teeth and which engage in corresponding internal teeth  31  of a drive wheel  30 . The drive wheel  30  is located substantially in a plane with the coupling element  40  and is connected rotationally secured to a gear element (not shown) which is in active connection with further gear elements or steps of the adjusting device. 
     In the illustration of FIG. 1 the drive lever  10  is located in its zero point position and the spring  14  presses the coupling element  40  radially outwards so that both coupling areas  41   a ,  41   b  are in positive engagement with the internal teeth  31  in the toothed sections  44   a ,  44   b . The radial mobility of the coupling element  40  is thereby not restricted by the dimensions of the recess  11  in the drive lever  10  so that a complete engagement of the teeth is possible in the direction of the drive wheel  30 . The recess  11  is dimensioned in the direction of the axle  20  so that when the guide element  42  stops against same the two coupling areas  41   a ,  41   b  are brought straight out of engagement with the internal teeth  31 . 
     When the drive lever  10  is operated in the direction of the arrow A a force acting in the direction of arrow A is exerted through the guide element  42  on the coupling element  40 . This force acts on the coupling area  41   b  which is supported on the corresponding toothed section  44   b  of the internal teeth  31  of the drive wheel  30  and turns the drive wheel  30  about the axle  20 . At the same time the coupling area  41   a  is lifted from the internal teeth  31  through a sliding block guide (not shown) so that the guide element  42  is moved in the recess  11  in the direction of the axle  20 . During resetting of the drive lever  10  into the zero point position the coupling area  41   b  slides over the internal teeth and the coupling area  41   a  is held by the corresponding slide block guide out of engagement with the internal teeth  31  until the starting position illustrated in FIG. 1 is occupied again. 
     Through the pressing of the coupling element  40  through the spring  14  in the zero point position both coupling areas  41   a ,  41   b  are in engagement with the internal teeth  31  so that no idling path has to be covered up to the engagement of the teeth. The spring  14  furthermore has the effect that despite a movement of the drive lever  10  without actual adjustment of the drive wheel  30 , the user does not have the feel of an idling path since when operating the drive lever  10  he is working against the resistance of the spring  14 . 
     A corresponding sliding block guide fixed on the housing and in which guide elements are guided so that the coupling areas  41   a ,  41   b  during resetting movement of the drive lever  10  into the zero point position do not engage with the internal teeth  31 , is not shown in FIG. 1 for reasons of clarity. The principle of the sliding block guide will be explained with reference to the following figures. 
     FIG. 2 shows an adjustment drive whose drive lever  10  is likewise mounted rotatable about an axle  20  and has a guide element  43  which is mounted in a radially extending recess  12  of the coupling element  40 ′. The drive wheel  30  is provided with external teeth  31 ′ into which the corresponding coupling areas  41   a ,  41   b  of the coupling element  40 ′ engage. A spring  15  is attached between the coupling element  40 ′ and the guide element  43  mounted on the drive lever  10  and loads the coupling element  40 ′ in the direction of the drive wheel  30  so that the two coupling areas  41   a ,  41   b  engage with the external teeth  31 ′ in the starting position. 
     Two guide elements  51   a ,  51   b  are fixed on the coupling element  40 ′ and are each guided in a sliding block guide  5   a ,  5   b  having sliding block guide areas  50   a ,  53   a and  50   b ,  53   b . Likewise a spring element  60   a  and  60   b  is associated with each guide element  51   a ,  51   b  wherein in the illustrated design the spring elements  60   a ,  60   b  are formed as a one-piece leaf spring. The spring elements  60   a ,  60   b  act against the spring  15 , that is they press the area of the coupling element  40 ′ which is unloaded when the drive lever  10  is returned, away from the external teeth  31 ′. However the spring elements  60   a ,  60   b  are designed and arranged so that in the zero point position of the drive lever  10  no force is exerted on the guide elements  51   a ,  51   b , or the force is less than that of the spring  15  so that the coupling element  40 ′ in the zero point position adjoins the external teeth  31 ′. On the other hand the force which is exerted on the guide elements  51   a ,  51   b  outside of the zero point position by the springs  60   a ,  60   b  is greater than the force emanating from the spring  15  so that the coupling area  41   a ,  41   b  which is unloaded at the time is lifted. 
     If the drive lever  10  is operated in the direction of the arrow A then a force is exerted through the guide element  43  on the coupling element  40 ′. The transition area between the sliding block guide areas  50   a ,  53   a  and  50   b ,  53   b  cause the coupling element  40 ′ to tilt about the coupling area  41   b  so that the coupling element  40  is moved outwards against the force of the spring  15  along the recess  12  whereby the coupling area  41   b  remains engaged with the external teeth  31 ′. At the same time the drive wheel  30  is driven correspondingly through the rotational movement of the coupling element  40  about the axle  20  and the guide element  51   b  is moved in the sliding block  5   b  spaced from the contour of the sliding block guide area  50   b . The spring element  60   b  is thereby pressed radially inwards through the force exerted on the drive lever  10 . 
     As soon as no more force is introduced through the drive lever  10  in the direction of arrow A the spring element  60   b  presses the guide element  51   b  radially outwards. The free space over the sliding block guide area  50   b  enables the coupling area  41   b  to be lifted from the external teeth  31 ′. Through the force of the spring element  60   b  the coupling element  40 ′ is turned about the guide element  51   a  which is located on the area  53   a  until the recess  12  stops the guide element  43 . During the resetting movement of the drive lever the two coupling areas  41   a ,  41   b  are therefore not in engagement with the external teeth  31 ′, namely the coupling area  41   b  as a result of the spring force of the spring element  60   b  and the coupling area  41   a  as a result of the sliding block guide area  53   a  which is spaced from the external teeth  31 ′. The resetting movement therefore takes place without any ratchet noise. 
     If the drive lever  10  is located in its zero point position then the spring  15  presses the coupling element  40 ′ back in the direction of the axle  20  and brings the coupling areas  41   a ,  41   b  into engagement with the external teeth  31 ′. 
     FIG. 3 shows an adjustment drive with an internally toothed drive wheel  30  wherein each one sliding block guide  5   a ,  5   b  fixed on the housing is in engagement with the guide elements  51   a ,  51   b  mounted on the coupling element  40  analogous with FIG.  2 . The spring  13  which is mounted here on the axle  20  of the drive presses the coupling element  40  radially outwards in the direction of the internal teeth  31  of the drive wheel  30 . The spring  13  thus acts corresponding to the spring  11  in FIG.  1 . Movement of the drive lever (not shown) counter-clockwise engenders a corresponding movement of the coupling element  40  and of the drive wheel  30 , as shown in FIG.  4 . 
     The coupling element  40  is thereby turned slightly about the axle  20  and the guide element  51   a  has been displaced along the shoulder between the sliding block guide area  50   a  and the sliding block guide area  53   a . The coupling area  41   a  has just been lifted from the internal teeth  31  of the drive wheel  30  against the externally acting force of the spring  13  as a result of the geometric dimensions of the sliding block guide  5   a . The coupling area  41   b  is thereby in engagement with the internal teeth  31  and drives the drive wheel  30 . 
     The coupling element  40  has a guide element  42  which corresponding to the arrangement in FIG. 1 is guided in a recess (not shown) through which the rotational movement about the coupling area  41   a  is restricted. 
     When resetting the drive lever into the zero point position the coupling area  41   b  remains in engagement with the internal teeth  31 . Since however the force through the spring still only presses the coupling element  40  radially outwards the coupling area  41   b  slides along on the internal teeth  31  so that ratchet noise is produced. The coupling area  41   a  which is relaxed during adjustment remains as a result of the sliding block guide  5   a  out of engagement with the internal teeth  31 . Only when or shortly before reaching the zero point position does the guide element  51   a  move from the area  53   a  towards the internal teeth  31  and come into engagement with same. 
     It is obviously possible to make the guide elements  51   a ,  51   b  fixed relative to the housing and to provide a sliding block guide on the coupling element  40 . 
     FIG. 5 shows a sectional view of FIG. 3 in which the arrangement of the drive lever  10  above the spring  13  and the coupling element  40  can be seen. The recess  11  is stamped or milled in the drive lever  10  whereby the guide element  42  which is formed integral with the coupling element  40  is guided in the recess. The recess  11  causes in interaction with the spring  13  a radially outwardly directed movement of the coupling element  40  and thus causes the coupling areas to adjoin the internal teeth  31  in the zero point position. 
     One variation of the invention in which there is likewise no ratchet noise when the drive lever  10  is reset is shown in FIG.  6 . The work principle thereby corresponds to that of the drive illustrated in FIG. 2, where the reference numerals correspond to those of FIG.  2  and the relevant elements have the same function as described in connection with FIG.  2 . As opposed to the drive according to FIG. 2 here according to FIG. 6 the coupling element  40  is located as with the drive according to FIG. 1 inside the drive wheel  30 . Guide elements  51   a ,  51   b  are formed on the coupling element  40  and engage in a sliding block guide  5   a ,  5   b.    
     In FIG. 7 the coupling element  40  has a recess  12  and the spring  15  is mounted on the guide element  43  which is formed on the drive lever. Guide elements  51   a  and  51   b  have corner sections  52   a  and  52   b . The spring  15  causes a constant abutment of the coupling element  40  against the internal teeth  31  if the coupling element  40  is located in the zero point position. A displacement out of the zero point position leads to a coupling area  41   a ,  41   b  lifting away from the internal teeth  31 . During resetting into the zero point position the coupling area  41   a ,  41   b  which is loaded during the adjustment slides along on the internal teeth  31  until the starting position shown in FIG. 7 is reached. 
     FIG. 8 shows a plan view of an adjustment drive which corresponds to the adjustment device according to FIG. 1 where the same reference numerals designate the same function elements of the adjustment drive according to FIG.  1 . 
     Differently from the design of the adjustment drive according to FIG. 1, in the adjustment drive illustrated in FIG. 8 however both coupling areas  41   a ,  41   b  of the coupling element  40  are provided with friction elements  45   a ,  45   b  which are arranged on either side of the drive lever  10 . The friction elements  45   a ,  45   b  can be fixed on the coupling areas  41   a ,  41   b  or are formed integral on the coupling area  41   a ,  41   b  and are located underneath the active line of the guide element  42  forming the rotational point of the coupling element  40  and of the toothed engagement of the toothed sections  44   a ,  44   b  of the coupling areas  41   a ,  41   b  with the internal teeth  31  of the drive wheel  30  so that a lever arm H is formed which during movement of the drive lever  10  out from the zero point position or during movement of the drive lever  10  into the zero point position cause a corresponding torque which acts on the relevant coupling area  41   a ,  41   b  of the coupling element  40 . 
     The friction elements  45   a ,  45   b  are supported on a locally fixed part of the adjustment drive, for example on the housing of the adjustment drive so that an axial tensioning of the coupling element  40  is caused and during movement of the drive lever  10  out from the zero point position causes a torque on the active side of the coupling element  40 , that is on the side which corresponds to the direction of rotation of the drive lever  10 , in the direction of a toothed engagement of the relevant coupling area  41   a  or  41   b  with the internal teeth  31  of the drive wheel  30 , whilst during resetting of the drive lever  10  into the zero point position torque arises on this coupling area  41   a ,  41   b  or the toothed section  44   a ,  44   b  associated therewith in the direction of lifting the relevant toothed section  44   a ,  44   b  out of the internal teeth  31  of the drive wheel  30 . 
     During movement of the drive lever  10  in the direction of arrow A, through the action of the axially tensioned friction element  45   b  the toothed section  44   b  of the coupling area  41   b  is pressed into the internal teeth  31  of the drive wheel  30  whilst during resetting of the drive lever  10  into the zero point position against the direction of arrow A the torque caused by the friction element  45   b  leads to the toothed section  44   b  lifting out from the internal teeth  31  of the drive wheel  30 . The toothed section  44   a  is lifted out from the internal teeth  31  of the drive wheel  30  during the movement of the drive lever  10  described above in the direction of the arrow A out from the zero point position or during the movement of the drive lever  10  in the direction of the zero point position analogous with the explanation on the adjustment drive according to FIG.  1 . 
     In the zero point position of the drive lever  10  the friction elements  45   a ,  45   b  are inactive, that is they are not axially tensioned and the spring  14  can press the coupling element  40  radially outwards and thus in the zero point position of the drive lever  10  both coupling areas  41   a  and  41   b  of the coupling element  40  are in engagement with the internal teeth  31  of the drive wheel  30 . 
     The arrangement of the friction elements  45   a ,  45   b  causes on the one hand a play-free adjustment drive which during resetting of the drive lever  10  into the zero point position causes no ratchet noises to arise and on the other a fixing of the coupling element  40  so that no rattling or ratchet noises can arise as a result of the required axial play. 
     The friction elements  45   a ,  45   b  are preferably formed as leaf springs which are supported on one side on the coupling areas  41   a  and  41   b  and on the other for example on the housing of the adjustment drive. 
     FIG. 9 shows an alternative to the arrangement according to FIG. 8 with a friction element  46  attached centrally on the coupling element  40 ′ and which engages through an opening  16  in the drive lever  10  and is likewise supported on a locally fixed part of the adjustment drive, preferably on the housing of the adjustment drive. 
     The action of this one central friction element  46  corresponds to the action of the two friction elements  45   a ,  45   b  mounted on the coupling element  40  at the sides of the drive lever  10 , according to FIG. 8, and is likewise mounted underneath the active line of the rotational point of the coupling element  40  formed by the guide element  42 , and the toothed engagement of the toothed sections  44   a ,  44   b  of the coupling areas  41   a ,  41   b  with the internal teeth  31 . The distance H′ between the friction element  46  and the guide element  42  which (distance) is greater in this embodiment compared with the embodiment of FIG. 8, forms the lever arm for the torque for lifting the coupling area  41   a  or  41   b  to reinforce the toothed engagement of each active coupling area  41   a ,  41   b  or for lifting the relevant toothed area  44   a ,  44   b  out from the internal teeth  31  during the return of the lever arm  10  into the zero point position. 
     As a further alternative a combination of the embodiments according to FIGS. 8 and 9 is possible, that is the arrangement of three friction elements  45   a ,  45   b  and  46 , of which two friction elements connected to the coupling element  40  are arranged at the side of the drive lever  10 , whilst a central friction element is provided corresponding to the friction element  46  according to FIG. 9 on the coupling element  40 . All the friction elements  45   a ,  45   b ,  46  are axially tensioned outside of the zero point position of the drive lever  10 , whilst in the zero point position the friction elements  45   a ,  45   b  and  46  are inactive so that the spring  14  presses the guide element  42  outwards, that is presses both toothed sections  44   a ,  44   b  of the coupling areas  41 ,  41   b  into the internal teeth  31  of the drive wheel  30 . 
     Also the central friction element  46  according to FIG. 9 consists preferably of a leaf spring which is fixed on the coupling element  40 , engages through the opening  16  of the drive lever  10  and is supported on a locally fixed element of the adjustment drive.