Abstract:
A device for coupling a universal joint shaft to a power take-off shaft ( 1 ) of a tractor has a first coupling mechanism ( 48 ) that is non-rotatably arranged on the universal joint shaft. A second coupling mechanism ( 17 ) is non-rotatably arranged on the power take-off shaft ( 1 ) of the tractor. The first and second coupling mechanisms can be non-rotatably coupled to transmit torque. A coupling shield ( 7 ) is provided to couple the coupling mechanism ( 17, 48 ). The first coupling mechanism ( 48 ) is at least indirectly rotationally supported on the coupling shield ( 7 ). A locking mechanism ( 58, 58 ′) projects from the coupling shield ( 7 ). A bracket ( 2 ) is attachable on the rear of the tractor. The bracket ( 2 ) has at least one locking device ( 5, 5 ′) that is displaceable between an unlocked position and a locked position. The locking device ( 5, 5 ′) is displaced from the unlocked position into the locked position. The locking device ( 5, 5 ′) interacts with the locking members ( 58, 58 ′) of the coupling shield ( 7 ) to pull it towards the bracket ( 2 ). The first coupling mechanism ( 48 ) is transferred into a coupling position to couple it the second coupling mechanism ( 17 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to German Patent Application No. 102007043032.0 filed Sep. 11, 2007, which application is herein expressly incorporated by reference. 
       FIELD 
       [0002]    The disclosure relates to a device for coupling a universal joint shaft of an agricultural implement to a power take-off shaft of a tractor. The device has a first coupling mechanism that is non-rotatably arranged on the universal joint shaft. A second coupling mechanism is non-rotatably arranged on the power take-off shaft of the tractor. The first coupling mechanism can be coupled to the second coupling mechanism for torque transmission. Furthermore, a coupling shield is provided that at least indirectly rotatably supports the first coupling mechanism. A locking mechanism interacts with a locking device to transfer the coupling shield into a coupling position and to its locked position. 
       BACKGROUND 
       [0003]    A device is known from DE 33 02 379 C2. A retainer is provided on a carrier arm that is displaceably mounted, via a rolling contact bearing, onto an agricultural implement. A coupling sleeve is rotatably supported, by the rolling contact bearing, which is non-rotatably connected to a joint yoke of a universal joint of a universal joint shaft. The coupling sleeve can be non-rotatably connected to a coupling hub by longitudinal teeth. The coupling sleeve is again non-rotatably connected to a power take-off shaft of a tractor. The coupling sleeve has an inner circumferential groove. In the coupled position, locking balls are pushed by a releasing flange radially outwards into the inner circumferential groove. This prevents an unintended detachment of the coupling sleeve. Only by displacing the releasing flange can the locking balls move inward, so that the coupling sleeve can be pulled off the coupling hub. 
         [0004]    The tractor is reversed towards the implement until the coupling sleeve is connected to the coupling hub coupling the two. Beforehand, the carrier arm has to be exactly aligned to the height and the lateral position of the coupling hub. Thus, a direct coupling is enabled between the coupling sleeve and the coupling hub. 
         [0005]    EP 1 637 014 A1 shows a coupling frame that is attached on a three point linkage of a tractor. Coupling hooks for a three-point coupling of an implement are provided on the coupling frame. Furthermore, a coupling element is rotatably and axially displaceably supported on the coupling frame. The coupling element is connected to a universal joint shaft. Also, the coupling is connected to the power take-off shaft of the tractor. As soon as the implement is coupled to the coupling frame, the coupling element on the coupling frame is arranged in a centered position relative to the coupling element on the implement. The position relative to the three-point coupling of both coupling elements is identical. The coupling element on the coupling frame is then manually displaced by a moving lever axially in the direction toward the implement. The coupling elements, in the form of a jaw clutch coupling, are brought into engagement. 
         [0006]    EP 1 563 723 A1 shows a device for coupling a universal joint shaft to a power take-off shaft of a tractor. A yoke-like coupling element is provided on the tractor and is axially displaceable parallel to the rotational axis of the power take-off shaft of the tractor. In an extended position, a coupling element of the universal joint shaft can be inserted radially into the rotational axis between two arms of the yoke-like coupling element. The coupling element of the universal joint shaft has a groove and is formed rotationally symmetrically to the rotational axis. The yoke-like coupling element engages the coupling element. The yoke-like coupling element can be pulled axially towards the power take-off shaft. The coupling element of the universal joint shaft is also axially displaced. Due to the pulling, the coupling element of the universal joint shaft is pulled onto the power take-off shaft. The longitudinal teeth provide a non-rotatable connection between the power take-off shaft and the coupling element of the universal joint shaft. 
       SUMMARY 
       [0007]    It is an object of the disclosure to provide a device to couple a universal joint shaft to a tractor power take-off shaft. The device has a simple structure and ensures an easy and reliable coupling. 
         [0008]    The object is solved by a device comprising a first coupling mechanism that is non-rotatably arranged on the universal joint shaft A second coupling mechanism is non-rotatably arranged on the power take-off shaft of the tractor. The first and second coupling mechanisms are non-rotatably coupled together to transmit torque. A coupling shield is provided to couple the coupling mechanisms. The first coupling mechanism is at least indirectly rotationally supported by the coupling shield. The first coupling mechanism includes a locking mechanism. A bracket with at least one locking device is attachable on the rear of the tractor. The locking device is displaceable between an unlocking position and a locking position. The locking device is displaced from the unlocking position into the locking position. The interaction of the locking device with the locking mechanism causes the coupling shield to be pulled towards the bracket. As this occurs, the first coupling mechanism is transferred into a coupling position to couple with the second coupling mechanism. 
         [0009]    The tractor is initially reversed to couple with the implement. During this, the universal joint shaft is still not connected to the power take-off shaft. The universal joint shaft does not have to be exactly aligned with the power take-off shaft beforehand. Further, the driver does not have to drive exactly to the implement to couple the universal joint shaft to the power take-off shaft. The coupling shield is initially moved into a starting position where the locking device can grip the locking mechanism. The actual coupling process takes place by displacing the locking device. Accordingly, the locking device interacts with the locking mechanism so that the coupling shield is pulled towards the tractor. As this occurs, the universal joint shaft is coupled to the power take-off shaft or, as will be explained later, is transferred into a position, where a self-actuated coupling of the two coupling mechanism can take place. 
         [0010]    The locking device serves, besides the pulling of the coupling shield, also to lock the coupling mechanisms to each other to prevent an unintended detachment. 
         [0011]    A large advantage of the present disclosure is that no cumbersome components, such as coupling frames, are necessary. Further, the universal joint shaft is only moved in one direction. This applies to the approaching movement of the tractor as well as to the actual coupling process of the universal joint shaft to the power take-off shaft. A further coupling direction, transverse to the rotational axes, is not necessary. Thus, a simple coupling process is achieved. 
         [0012]    Preferably, the first coupling mechanism is represented by a first coupling element. It can be non-rotatably connected to the universal joint shaft. Furthermore, the second coupling mechanism is represented by a second coupling element. It can be non-rotatably connected to the power take-off shaft of the tractor. Thus, two coupling elements are provided that can be connected to a conventional power take-off shaft of a tractor and a conventional universal joint shaft. However, it is also possible for the first coupling mechanism to be represented by a joint yoke with a coupling sleeve. The coupling sleeve has a bore with inner longitudinal teeth. In this case, the second coupling mechanism can be represented by outer longitudinal teeth on the power take-off shaft. 
         [0013]    The locking mechanism may be provided by locking faces on the coupling shield. In this case, the locking device comprises, preferably, rocker levers. The levers are pivotable between an unlocking position and the locking position. The levers can engage behind the locking faces. The locking faces of the coupling shield can be gripped by the rocker lever and pulled close by engaging behind the locking faces. 
         [0014]    One or more hydraulic cylinders may be provided to actuate the rocker levers. The rocker levers may be rigidly connected to each other. Thus, they can be pivoted together. In this case, one hydraulic cylinder is sufficient to actuate the rocker levers. When, however, the rocker levers are separately pivotable and are not connected to each other, it may be necessary to provide two hydraulic cylinders. 
         [0015]    A simple constructed locking mechanism is provided insofar, as they comprise two studs projecting laterally from the coupling shield. 
         [0016]    The rocker levers can be locked mechanically in their locking position to prevent an unintended decoupling. A retaining arm is provided on at least one rocker lever to prevent the decoupling. The retaining arm can be pivoted between a retaining position and a releasing position. In the retaining position, the retaining arm engages with a first retaining face behind the rocker lever in its locking position and retains it in the locking position. 
         [0017]    The coupling shield is provided with a first centering mechanism to make the coupling process simple and secure. The first centering mechanism interacts with a second centering mechanism of the bracket. When the coupling shield is pulled toward the bracket, such that it is pulled into a tight position, a first rotational axis of the first coupling mechanism is aligned to a second rotational axis of the second coupling mechanism. The first centering mechanism may comprise first guide faces and the second centering mechanism may comprise second guide faces. 
         [0018]    Preferably, the first guide faces are formed by portions of the outer circumferential face of the coupling shield. The second guide faces are formed by guide arms projecting axially from the bracket. 
         [0019]    Four guide arms are provided and arranged in pairs opposite to each other. They are distributed around the second rotational axis. The distance at a right angle relative to the second rotational axis, increase between the second guide faces of two opposite guide arms in the direction towards the free ends of the guide arms. 
         [0020]    The guide arms and the guide faces form, preferably, a rough centering of the coupling shield relative to the bracket. Thus, when driving the tractor closer or when pulling the coupling shield close, initially the coupling shield is inserted between the guide arms and is centered roughly relative to the second rotational axis. 
         [0021]    For fine centering, the first centering mechanism is provided with a first centering face arranged concentrically to the rotational axis. The second centering mechanism includes a second centering face arranged concentrically to the second rotational axis. One of the centering faces is formed as an outer circumferential face. The other centering faces are formed as an inner circumferential face. At least one of the centering faces is formed conically. 
         [0022]    In a preferred embodiment, the first centering face may be formed by an outer face of the first coupling mechanism. The second centering face may be formed by a conical inner face of a centering sleeve on the second coupling mechanism. 
         [0023]    Thus, in the further coupling process, fine centering generally takes place by pulling close the coupling shield utilizing the locking device. 
         [0024]    Rough centering relative to the stationary components, namely the coupling shield and the bracket, takes places initially by driving the tractor close to the implement or by pulling close the coupling shield. When pulling the coupling shield close to the bracket, by the locking elements, fine centering relative to the rotating components takes place. For this, centering faces are provided on the coupling mechanism. 
         [0025]    Preferably, the first coupling mechanism is connected to a joint component of a joint of the universal joint shaft. The first coupling mechanism is rotatably supported on the coupling shield. 
         [0026]    The universal joint shaft is preferably a cardan joint shaft. The joint yoke of the cardan joint is indirectly supported, via the first coupling mechanism, on the coupling shield. 
         [0027]    Preferably, the first coupling mechanism has axially projecting first driving pawls. The second coupling mechanism has axially projecting second driving pawls. The driving pawls engage in a coupled condition of the two coupling mechanism, respectively, in gaps between the driving pawls of the other coupling element. 
         [0028]    A simple coupling mechanism is provided insofar, as one of the coupling mechanism is axially displaceable between a position, where it is pushed forward in direction towards the other coupling mechanism. For a retracted position, springs mechanism are provided to act upon the axially displaceable coupling mechanism with a force in the direction towards the pushed forward position. 
         [0029]    In this case, the second coupling mechanism is axially displaceably guided relative to the power take-off shaft. 
         [0030]    Therefore, in the coupling position of the first coupling mechanism, two positions of the two coupling mechanisms relative to each other are possible. The pawls of the first coupling mechanism engage, directly after the transferral of the first coupling mechanism into the coupling position, into gaps between the driving pawls of the second coupling mechanism. Thus, a direct non-rotatable connection is achieved. When the coupling mechanisms are arranged in a different rotational position relative to each other, the driving pawls rotate about each other at the ends and do not engage with the other. In the latter case, one of the coupling mechanisms is axially displaced. As soon as the power take-off shaft is rotated, the one coupling mechanism is rotated relative to the other coupling mechanism. Thus, the driving pawls of both coupling mechanisms can engage each other. 
         [0031]    In this case, it may, preferably, be provided that the second coupling mechanism is axially displaceably guided relative to the power take-off shaft. 
         [0032]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0033]    A preferred embodiment is described in detail in the following by the drawings. 
           [0034]      FIG. 1  is a perspective view of a device according to the disclosure in the unlocking position, where no universal joint shaft is coupled to the power take-off shaft. 
           [0035]      FIG. 2  is a perspective view of the device according to  FIG. 1  with the coupling shield in a locked position. 
           [0036]      FIG. 3  is an enlarged perspective view according to  FIG. 2  without the coupling shield. 
           [0037]      FIG. 4  is a side partial cross-sectional view of the locker levers including their actuation elements. 
           [0038]      FIG. 5  is a longitudinal sectional view through the second coupling element on the power take-off shaft. 
           [0039]      FIG. 6  is a longitudinal sectional view through the first and the second coupling element. 
           [0040]      FIG. 7  is a perspective view of the coupling shield with the first coupling element. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]      FIGS. 1 and 2  show perspective views of a device according to the disclosure at the rear of a tractor. 
         [0042]    In  FIG. 1  the power take-off shaft  1  of the tractor is shown, which is rotatable around a second rotational axis D 2 . A fastening frame  4  is provided at the rear of the tractor and is part of the tractor. A bracket  2  is mounted by fastening screws  3  to the frame  4 . The bracket  2  carries two rocker levers  5 ,  5 ′ that are pivotable around a first pivot axis S 1  between an unlocked position, shown in  FIG. 1  and a locked position, shown in  FIG. 2 . The rocker levers  5 ,  5 ′ have, respectively, a catching profile  6 ,  6 ′, interacting with locking faces on a coupling shield  7 , not shown here and described later. The rocker levers  5 ,  5 ′ are part of a locking device on the bracket  2 . The coupling shield  7  has a locking mechanism in the form of laterally projecting studs, which form locking faces. When pivoting the rocker levers  5 ,  5 ′ from the unlocking position into the locking position, the catching profiles  6 ,  6 ′ engage behind the studs on the coupling shield  7  and pull close to the bracket  2 , to couple, as later described, the universal joint shaft to the power take-off shaft  1 . 
         [0043]    The coupling shield  7  has a central bore  16  to rotationally support a joint yoke  15  of a universal joint of a universal joint shaft (here not completely shown). Initially, a rough pre-centering has to take place, to ensure a perfect coupling when moving the coupling shield  7  closer to the bracket  2 . To accomplish this, initial guide plates  19 ,  19 ′ are provided. The guide plates  19 ,  19 ′ are projectingly arranged to the rear on the side plates  21 ,  21 ′ of the bracket  2 . The guide plates  19 ,  19 ′ have guide faces  20 ,  20 ′ that oppose one another. Starting from the side plates  21 ,  21 ′, the length of the guide plates  19 ,  19 ′ increases in a direction towards the implement. Thus, initially a rough centering of the coupling shield  7  will take place, such that the laterally projecting studs are guided along the guide faces  20 ,  20 ′ of the guide plates  19 ,  19 ′ and are centered. 
         [0044]    A base plate  14 , which is part of the bracket  2 , is arranged at a right angle to the second rotational axis D 2  of the power take-off shaft  1 . Guide arms  10 ,  10 ′,  11 ,  11 ′ project from the base plate  14  to the rear in a direction towards the implement. The guide arms  10 ,  10 ′,  11 ,  11 ′ form, respectively, a second guide face  12 ,  12 ′,  13 ,  13 ′. The guide arms  10 ,  10 ′,  11 ,  11 ′ are distributedly arranged around the second rotational axis D 2  and are arranged in pairs opposite to each other. The second guide faces  12 ,  12 ′,  13 ,  13 ′ are arranged in pairs opposite to each other. The distance, at a right angle to the second rotational axis D 2  between the second guide faces  12 ,  12 ′,  13 ,  13 ′, increases in direction towards the free ends of the guide arms  10 ,  10 ′,  11 ,  11 ′. 
         [0045]    The coupling shield  7  has first guide faces  8 ,  8 ′,  9 ,  9 ′ on an outer circumference face. The first guide faces  8 ,  8 ′,  9 ,  9 ′ are arranged in pairs opposite to each other. When pulling the coupling shield  7  close or when driving the tractor close to the implement, the first guide faces  8 ,  8 ′,  9 ,  9 ′ abut, respectively, one of the second guide faces  12 ,  12 ′,  13 ,  13 ′. Thus, a rough centering of the coupling shield  7  takes place. 
         [0046]    As it is visible from  FIG. 1 , the device further comprises a second coupling mechanism in the form of a second coupling element  17  that has second driving pawls  18 . The second driving pawls  18  project at the end side in a direction of the second rotational axis D 2  and are formed with gaps between them. The second coupling element  17  can be coupled with first coupling mechanism in the form of a first coupling element, which will be described in detail later, and is correspondingly formed to the second coupling element  17 . In the locking position shown in  FIG. 2 , the driving pawls of both coupling elements engage each other, so that between these a rotationally fast connection is achieved. The first coupling element is rotatably connected with the joint yoke  15 . The second coupling element  17  is non-rotatably held via a bore  22  with longitudinal teeth on the power take-off shaft  1 , that also has longitudinal teeth. 
         [0047]      FIG. 3  shows an enlarged view of the device without the coupling shield. The rocker levers  5 ,  5 ′ are in the locking position. The two rocker levers  5 ,  5 ′ are connected to each other by a bridge  23  so that they are pivoted together. Thus, a single driving unit, e.g. a hydraulic cylinder, is sufficient, to pivot both rocker levers  5 ,  5 ′. In the present case a double acting hydraulic cylinder is used, as described in detail later. However, it is generally also possible, that the two rocker levers  5 ,  5 ′ are not connected to each other. Then, they may be pivoted together via a common actuation element by a single driving unit or by two separate driving units. 
         [0048]    The rocker levers  5 ,  5 ′ are pivotable around the first pivot axis S 1 . At their free ends  24 ,  24 ′, two retaining faces  25 ,  25 ′ are provided. They are directed to the rear in the locking position. On the bracket  2 , two retaining arms  27 ,  27 ′ are provided. A first retaining face  26 ,  26 ′ is formed on the two retaining arms  27 ,  27 ′. The rocker levers  5 ,  5 ′ have their second retaining faces  25 ,  25 ′ supported against the first retaining face  26 ,  26 ′ in the locked position. The rocker levers  5 ,  5 ′ are, therefore, prevented from transferring into the unlocked position. 
         [0049]    The retaining arms  27 ,  27 ′ can be transferred, by mechanism of a Bowden cable  28  from the above described retaining position around a second pivot axis S 2  into a released position. Here, the rocker levers  5 ,  5 ′ are released, so that they can be transferred from the locked position shown in  FIG. 3  to their unlocked position shown in  FIG. 1 . Instead of a Bowden cable, other actuation mechanism are possible, e.g. pneumatical or electro-mechanical components. 
         [0050]    The advantage of this mechanical locking of the rocker levers  5 ,  5 ′ is that with a hydraulic activation of the rocker levers  5 ,  5 ′ in the locking position no hydraulic pressure has to be exerted onto the driving hydraulic cylinder. Rather, the rocker levers  5 ,  5 ′ are held by mechanism of the mechanical locking in the locking position. This also leads to the fact, that even when the pressure decreases in the hydraulic system, the universal joint shaft remains securely coupled. 
         [0051]    Generally, in the present embodiment with a bridge  23  connecting the two rocker levers  5 ,  5 ′ to each other, it would be sufficient to provide one retaining arm  27 ,  27 ′. 
         [0052]    When moving the rocker levers  5 ,  5 ′ from their unlocked position into their locked position, the retaining arms  27 ,  27 ′ do not have to be externally actuated, to be transferred from the retaining position into the released position. For this, one actuation face  29 ,  29  is provided for each retaining arm  27 ,  27 ′. The free ends  24 ,  24 ′ of the rocker levers  5 ,  5 ′ abut the actuation faces  29 ,  29  and move the retaining arms  27 ,  27 ′ from their retaining position into the releasing position, until the retaining arms  27 ,  27 ′ engage behind the rocker levers  5 ,  5 ′. Thus, the first retaining faces  26 ,  26 ′ are self-actuated and transferred into their retaining position. For this, spring elements  30  are provided that act upon the retaining arms  27 ,  27 ′ to take up their retaining position. 
         [0053]      FIG. 4  shows, how the rocker levers  5 ,  5 ′ are actuated. A hydraulic cylinder  31  is pivotably mounted on the piston-side on the bracket  2  and is pivotably mounted on the piston-rod-side at the actuation element  32 . The actuation element  32  is again pivotably mounted around a third pivot axis S 3  on the bracket  2 . The hydraulic cylinder  31  is eccentrically attached to the third pivot axis S 3  at the actuation element  32 . The actuation element  32  has a first tooth portion  33  that is coaxially arranged to the third pivot axis S 3 . The first tooth portion  33  engages with a second tooth portion  34  of one of the rocker levers  5 . The second tooth portion  34  is coaxially arranged with the first pivot axis S 1 . Thus, by actuating the hydraulic cylinder  31 , the actuation element  32  is pivoted around the third pivot axis S 3 . Due to the teeth engagement of the two tooth portions  33 ,  34 , the rocker lever  5  is pivoted. The rocker lever  5  is, as already described, connected by the bridge  23  to the other rocker lever  5 ′, so that both rocker levers  5 ,  5 ′ are moved synchronously. If no bridge  23  is provided, a further actuation element has to be provided, which actuates the other rocker lever  5 ,  5 ′. Thus, either the two actuation elements, then provided, are non-rotatably connected to each other or a further hydraulic cylinder is necessary. 
         [0054]      FIGS. 5 and 6  show the coupling mechanism in detail. The coupling procedure will be described in detail. 
         [0055]      FIG. 5  shows the power take-off shaft  1  with its longitudinal teeth  35 . A circumferential groove  36  is formed in the longitudinal teeth  35 . The groove  36  is coaxially provided on the second rotational axis D 2 . The second coupling component  17  is pushed onto the power take-off shaft  1  by a hub  38 . The hub  38  has a central bore with longitudinal teeth corresponding to the longitudinal teeth  35  of the power take-off shaft  1 . 
         [0056]    The hub  38  of the second coupling element  17  has an elongated hole  39 . The hole  39  is axially limited and extends parallel to the second rotational axis D 2 . The elongated hole  39  is radially formed as a trough extending through opening. A locking ball  40  rests in the elongated hole  39 . The ball  40  engages in the groove  36  of the power take-off shaft  1 . A sleeve  41  is coaxially provided around the hub  38 . The sleeve  41  holds the locking ball  40  in the groove  36 . The sleeve  41  is rotatably held on the hub  38  and has a through opening across its circumference. Thus, the sleeve  41  can be rotated such, that the through opening aligns with the elongated hole  39  and the ball can radially leave the groove  36 . Thus, the second coupling element  17  can be removed from the power take-off shaft  1 . 
         [0057]    The second coupling element  12  further forms a driving portion  42 , in the form of a flange. The flange has driving pawls  18  axially projecting. The second coupling element  17  is acted upon by a spring  43  in the direction of an extended position. The spring  43  is supported on the one hand on the driving portion  42  of the second coupling element  17  and on the other hand on a retaining plate  44 . The retaining plate  44  has a central bore  45 . The bore cross-section corresponds to the longitudinal teeth  35  of the power take-off shaft  1 . Thus, the retaining plate  44  is rotationally securely held and is supported on the end of the longitudinal teeth and offers an axial abutment for the spring  43 . 
         [0058]    In  FIG. 5  the second coupling element  17  is shown in an intermediate position. In a completely extended position, not shown here, the second coupling element  17  is supported with a first abutment  46 , which is formed by the elongated hole  39 , on locking ball  40 , held in the groove  36 . In a completely retracted position, as shown in  FIG. 6 , the second coupling element  17  is supported with a second abutment  47 , which is formed by the elongated hole  39 , on the locking ball  40 . Thus, pre-defined positions of the second coupling element  17  are achieved. 
         [0059]      FIG. 6  shows the second coupling element  17  as well as the coupling shield  7  together with the first coupling element  48 . The first coupling element  48  has first driving pawls  49  pointing towards the second coupling element  17 . The first coupling element  48  is connected by a fastening screw  50  to the yoke  15  of a universal joint shaft, not shown here. The first coupling element  48  and also the joint yoke  15  are indirectly rotatably supported in the coupling shield  7  by a rolling contact bearing  51 . 
         [0060]    In the represented position, the first driving pawls  49  abut its first end faces  52  with second end faces  53  of the second driving pawls  18 . In this position, the second coupling element  17  is transferred into a retracted position. The second coupling element  17  is supported with the second abutment  47  on the locking ball  40 . The power take-off shaft  1  is rotated. This rotates the second coupling element  17  relative to the first coupling element  48 . The first driving pawls  18  reach a position where they align with the gaps between the first driving pawls. Thus, the first coupling element  17  is coupled to the second coupling element  48 . The second coupling element  17  is released into an intermediate position and the first driving pawls  49  engage the second driving pawls  18 . 
         [0061]    After the above described rough centering, a fine centering is carried out. For this, the first coupling element has a first centering face  56  that is provided on an outer circumferential face of the first coupling element  48 . The second coupling element  17  has a second centering face  55  in form of a conical inner face. Face  55  is formed by a centering sleeve  54 . The centering sleeve  54  rests externally on the driving portion  42  of the second coupling element  17 . When transferring the first coupling element  48  into the position shown in  FIG. 6 , the first coupling element  48 , with the first centering face  46 , is inserted into the centering sleeve  54 . It is centered by the second centering face  55 , in the form of a conical inner face. 
         [0062]      FIG. 7  shows a perspective view of the coupling shield  7 . The coupling shield has two laterally projecting studs  57 . The studs  57  form, respectively, with their outer circumferential face a locking face  58 . The studs  57  are engaged by the rocker levers  5 ,  5 ′ according to  FIGS. 1 to 3  and are pulled towards the bracket  2 . 
         [0063]    The coupling shield  7  is pivotably mounted on a pivot bearing  59 . The coupling shield can be pivoted around a vertical pivot axis S 4  relative to the pivot bearing  59 . The pivot bearing  59  is again pivotable on a retaining arm  60 , around a horizontal pivot axis S 5 . The retaining arm is mounted on an agricultural implement. The coupling shield  7  is freely adjustable relative to the implement and, thus, can be roughly pre-centered for a successful coupling process. 
         [0064]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.