Patent Publication Number: US-2012037298-A1

Title: Tyre building drum for building an unvulcanised tyre

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a tyre building drum for building an unvulcanised tyre using rubber tyre components, wherein the tyre building drum comprises a drum shaft and two drum halves arranged at least almost substantially symmetrical with respect to the centre of the tyre building drum and around the drum shaft, which drum halves can be moved in axial direction along the drum shaft, wherein each drum half comprises a folding arm unit having folding arms that are connected to the drum half in question in a swivelling manner. 
     Such a tyre building drum is known from EP-B1-1.439.054. Said known tyre building drum comprises a base spindle for the folding units, about which base spindle a hollow second spindle for the bead core clamping units is coaxially arranged. The base spindle can be coupled to the hollow second spindle by means of a connectable coupling, so that both spindles can be coupled to each other (for moving the entire drum half) and be detached from each other. Although with respect to other tyre building drums that use a pneumatic drive for the folding arm carrier bodies, said known tyre building drum provides a considerable improvement regarding the synchronous lifting of the two folding arm units due to use of in particular the base spindle, the known tyre building drum has some drawbacks. For instance the connectable coupling is not only a relatively expensive and complex additional part, in case of failure it is also difficult to repair this connectable coupling. Furthermore due to use of the two coaxial spindles in the drum shaft, the tyre building drum is relatively long and bulky. 
     SUMMARY OF THE INVENTION 
     It is among others an object of the invention to provide a tyre building drum for building an unvulcanised tyre that is maintenance friendlier and that can be designed in a more compact manner. 
     For that purpose according to the invention a tyre building drum for building an unvulcanised tyre of the type mentioned in the preamble is characterised in that the tyre building drum comprises a main housing wherein the drum shaft projects from the main housing, wherein at least two pull/push rods are arranged in the drum shaft which pull/push rods are movable in axial direction and extend up into the main housing and in the main housing are coupled to a drive mechanism, wherein a first pull/push rod of the at least two pull/push rods is coupled to the folding arm unit of the first drum half, and a second pull/push rod of the at least two pull/push rods is coupled to the folding arm unit of the second drum half. The tyre building drum according to the invention is thus provided with an external actuator for a movement and turning up of the folding arms, which actuator is placed outside of the drum shaft. In that way, at least for these actions, it will no longer be necessary to place spindles in the hollow drum shaft, as a result of which the tyre building drum can be designed in a maintenance friendlier and more compact way. 
     A further advantage of the use of pull/push rods is that in that way a low play or even a play-free transmission from the drive mechanism to the folding arm units of the drum halves can be realised. 
     All other driven movements of the drum halves and/or parts thereof, for instance shoulder supports or bead clamping mechanisms, can on the one hand be driven by means of known actuators, such as one or more spindles or one or more pneumatically driven cylinders. On the other hand in one embodiment of the building drum according to the invention, one or more of these drives of the drum halves and/or parts thereof can also be driven by means of pull/push rods that are placed in the drum shaft. In an advantageous embodiment the at least two pull/push rods further comprise a third pull/push rod that is coupled to the first drum half, and a fourth pull/push rod that is coupled to the second drum half. 
     In one embodiment the at least two pull/push rods are placed concentrically in the drum shaft. The external drive mechanism is coupled to the drum halves and/or their folding arm units via the at least two tubular pull/push rods that are telescopically retractable in a concentric way, and are preferably placed substantially in the centre line of the drum shaft. 
     In the manufacturing of an unvulcanised tyre using rubber tyre components, it is common to let the drum shaft with the building drum halves rotate. In one embodiment the main housing comprises a bearing device, wherein with a first outer end the drum shaft is rotatably placed in the bearing device. In one embodiment the pull/push rods are substantially rotation-fixedly placed in the drum shaft. In that way the pull/push rods will rotate along with the drum shaft, as a result of which friction between the pull/push rods and the drum shaft is considerably reduced. 
     In order to couple the rotatable pull/push rods with a drive mechanism in the main housing, the at least two pull/push rods in one embodiment extend beyond the first outer end of the drum shaft and are rotatably coupled to the drive mechanism. The rotatable coupling ensures that the pull/push rods that are rotatable around the centre line of the drum shaft, can be coupled to a substantially stationary and/or fixed drive mechanism in the main housing. 
     In one embodiment the main housing comprises a drum shaft driving motor which by means of a transmission is coupled to the drum shaft, wherein the transmission is placed at a side of the bearing device that extends away from the drive mechanism. In one embodiment the transmission is placed outside of the main housing. Examples of such transmissions are a belt transmission, a gear wheel transmission, a quill drive motor, etc. The placement of the transmission according to this embodiment makes it easy to keep the transmission in good repair. 
     In one embodiment the transmission comprises a belt transmission. In that case the transmission from the motor to the drum shaft is ensured by a drive belt, such as a toothed belt. The drive belt can easily be removed from the drum shaft driving motor, be passed over the drum shaft, and be discharged at an outer end of the drum shaft that faces away from the bearing device. In a comparable manner a new drive belt can be supplied via the outer end of the drum shaft that faces away from the bearing device, be passed to the main housing along the drum shaft, and be placed over the drum shaft and the drum shaft driving motor. 
     In one embodiment the drive mechanism is provided with a separate drive for each pull/push rod of the at least two pull/push rods. In one embodiment the separate drives each preferably comprise a linear motor or a spindle drive. In that way all pull/push rods can be individually driven. 
     In an alternative embodiment the drive mechanism comprises a first spindle, wherein the first spindle comprises thread which on either side of the middle of the drive mechanism has an equally large pitch but of an opposite direction, wherein on the first spindle on either side of the middle of the drive mechanism a first flight nut and a second flight nut are arranged, wherein the first and second flight nut are coupled to the first pull/push rod and the second pull/push rod, respectively. In this way a swivelling of the folding arms of both first and second drum halves is driven via the first and second pull/push rod, respectively, that are both coupled to the first spindle. In that way an improvement regarding the synchronous lifting of the folding arms is achieved. 
     In one embodiment the first pull/push rod is coupled to the first flight nut in a substantially axially adjustable manner. In that way the position of the folding arms of the first drum half is adjustable, particularly with respect to the position of the folding arms of the second drum half. The second pull/push rod can be fixedly connected to the second flight nut. 
     In a further embodiment the drive mechanism comprises a second spindle, wherein the second spindle comprises thread which on either side of the middle of the drive mechanism has an equally large pitch but of an opposite direction, wherein on the second spindle on either side of the middle of the drive mechanism a third flight nut and a fourth flight nut are arranged, wherein the third and fourth flight nut are coupled to the third pull/push rod and the fourth pull/push rod, respectively. In this way a shifting of both first and second drum halves along the drum shaft is driven via the third and fourth pull/push rod, respectively, that are both coupled to the second spindle. In that way an improvement regarding the synchronous shifting of the drum halves is achieved. 
     In a further embodiment the third pull/push rod is coupled to the third flight nut in a substantially axially adjustable manner. In that way the position of the first drum half is adjustable, particularly with respect to the position of the second drum half. The fourth pull/push rod can be fixedly connected to the fourth flight nut. 
     In one embodiment the first spindle and the second spindle are each individually drivable. It is noted that for shifting the drum halves without turning up the folding arms, on the one hand the first and third pull/push rod are substantially jointly driven and on the other hand the second and fourth pull/push rod are substantially jointly driven. This can be achieved by driving the first and second spindle jointly. For turning up the folding arms without shifting the drum halves, on the one hand the first pull/push rod is moved with respect to the third pull/push rod and on the other hand the second pull/push rod is moved with respect to the fourth pull/push rod. This can be achieved by driving just the second spindle. 
     In an advantageous embodiment of a tyre building drum according to the invention each spindle is driven by its own servomotor. In particular two servomotors can be accurately synchronously driven for shifting the drum halves without turning up the folding arms. In this way also the first spindle can be accurately moved with respect to the second spindle for driving the folding arms. In this way a highly accurately functioning tyre building drum can be realised. 
     In a further advantageous embodiment of a tyre building drum according to the invention the spindles are driven by means of a hydraulic drive. Up until now it was not common to provide tyre building drums with a hydraulic drive because the fluids used for the hydraulic drive are not compatible with unvulcanised rubber. However, as the tyre building drum according to the invention uses a drive placed outside of the drum shaft, particularly placed in the main housing, use can be made here of a hydraulic drive. 
     In one embodiment the first spindle and the second spindle are placed substantially on either side of a centre line of the drum shaft. In that way the drive mechanism can be designed substantially compact, wherein the spindles and the pull/push rods placed in between them are situated substantially in one plane. 
     In one embodiment the drum shaft is provided with a first drum shaft slot for accommodating a first connecting part for connecting the first pull/push rod to the folding arm unit of the first drum half. 
     In one embodiment the drum shaft is provided with a second drum shaft slot for accommodating a second connecting part for connecting the second pull/push rod to the folding arm unit of the second drum half. 
     In a further embodiment the first drum shaft slot is adapted for accommodating a third connecting part for connecting the third pull/push rod to the first drum half. 
     In a further embodiment the second drum shaft slot is adapted for accommodating a fourth connecting part for connecting the fourth pull/push rod to the second drum half. 
     According to a second aspect the invention provides a tyre building drum for building an unvulcanised tyre using rubber tyre components, wherein the tyre building drum comprises a main housing having a drum shaft extending from the main housing and two first and second drum halves arranged at least almost substantially symmetrical with respect to the centre of the tyre building drum and around the drum shaft, which drum halves can be moved in axial direction along the drum shaft, wherein each drum half comprises a folding arm unit having folding arms, that are connected to the drum half in question in a swivelling manner, wherein the folding arm unit is movable in axial direction along the drum shaft, wherein the main housing comprises a bearing device and wherein with a first outer end the drum shaft is rotatably placed in the bearing device, wherein the main housing comprises a drum shaft driving motor which by means of a belt transmission is coupled to the drum shaft, wherein the belt transmission is placed near a side of the bearing device that faces a second outer end extending away from the first outer end. In one embodiment the belt transmission is placed outside of the main housing. The drive belt can then easily be removed from the drum shaft driving motor as described before, as a result of which building up the tyre building drum according to the invention and maintenance is made easier. 
     In one embodiment of the tyre building drums described above, the drum shaft comprises a first and a second drum shaft portion, wherein the first drum shaft portion comprises the first outer end that is rotatably placed in the bearing device, wherein a second outer end of the first drum shaft portion that extends away from the first outer end comprises a coupling member for substantially axially and detachably coupling the first drum shaft portion to the second drum shaft portion. In one embodiment the coupling member comprises a screwed joint or a flanged joint. In that way the main housing including the first drum shaft portion can be transported to clients as a unity, which makes transportation of the tyre building drum to the client easier. 
     According to a further aspect the invention provides a main housing with drive mechanism suitable and intended for a tyre building drum as described above. 
     According to a further aspect the invention provides for a use of a tyre building drum as described above for building an unvulcanised tyre using rubber tyre components, wherein during building the tyre a turning up of the folding arms of the two drum halves is driven by shifting at least the first and second pull/push rod, wherein the shifting of the first and second pull/push rod is driven by a drive mechanism placed in the main housing. 
     In one embodiment during building the tyre a shifting of the two drum halves is driven by shifting at least the third and fourth pull/push rod, wherein the shifting of the third and fourth pull/push rod is driven by a drive mechanism placed in the main housing. 
     In one embodiment for shifting the drum halves without turning up the folding arms, on the one hand the first and third pull/push rod are substantially jointly driven and on the other hand the second and fourth pull/push rod are substantially jointly driven. 
     In one embodiment for turning up the folding arms on the one hand the first pull/push rod is shifted with respect to the third pull/push rod and on the other hand the second pull/push rod is shifted with respect to the fourth pull/push rod. 
     The aspects and measures described in this description and the claims of the application and/or shown in the drawings of this application may where possible also be used individually. Said individual aspects may be the subject of divisional patent applications relating thereto. This particularly applies to the measures and aspects that are described per se in the sub claims. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       The invention will be elucidated on the basis of a number of exemplary embodiments shown in the attached drawings, in which: 
         FIG. 1  shows a schematic side view in cross-section of an exemplary embodiment of a tyre building drum according to the invention; 
         FIG. 2  shows a schematic side view in cross-section of the tyre building drum of  FIG. 1  in the condition in which rubber tyre components have been arranged on the drum halves; 
         FIG. 3  shows a schematic side view in cross-section of the tyre building drum of  FIG. 1  in the condition in which the drum halves are shifted towards each other; 
         FIG. 4  shows a schematic side view in cross-section of the tyre building drum of  FIG. 1  in the condition in which the folding arms have at least partially been lifted; and 
         FIG. 5  shows a schematic side view in cross-section of a second exemplary embodiment of a tyre building drum according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a side view in cross-section of an exemplary embodiment of a tyre building drum according to the invention. Building an unvulcanised tyre with rubber tyre components takes place on a first drum half  2  and a second drum half  3 , that have been arranged at least almost substantially symmetrical with respect to the centre of the tyre building drum and around the drum shaft  1 . The first and second drum halves  2 ,  3  can be moved in axial direction along the drum shaft  1 . Each drum half  2 ,  3  comprises a folding arm unit having folding arms  14 , that are connected to the drum half  2 ,  3  in question in a swivelling manner. Furthermore the drum halves  2 ,  3  are provided with a shoulder support  12  and a bead clamping mechanism  13 . 
     The tyre building drum comprises a hollow drum shaft  1  in which a transmission for the drive of the drum halves  2 ,  3  and of the folding arm units is accommodated. Said transmission comprises four pull/push rods  101 ,  103 ,  105 ,  107  that are concentrically placed in the drum shaft  1 . 
     The most central pull/push rod is the third pull/push rod  101 , which near its outer end situated within the drum shaft  1 , via a third connecting part or flight pin  102  is coupled to the first drum half  2  (not shown in detail). Around at least a portion of the central third pull/push rod  101  the tubular first pull/push rod  103  is placed, which near its outer end situated within the drum shaft  1  via a first connecting part or flight pin  104  is coupled to the folding arm unit  14  of the first drum half  2 . Around at least a portion of first pull/push rod  103  the tubular second pull/push rod  105  is placed, which near its outer end situated within the drum shaft  1  via a second connecting part or flight pin  106  is coupled to the folding arm unit of the second drum half  3 . Around at least a portion of the second pull/push rod  105  the tubular fourth pull/push rod  107  is placed, which near its outer end situated within the drum shaft  1  via a fourth connecting part or flight pin  108  is coupled to the second drum half  3 . 
     The four pull/push rods  101 ,  103 ,  105 ,  107  extend substantially parallel to the centre line h of the drum shaft  1  to within the main housing  4  of the tyre building drum. In the main housing  4  a drive mechanism  9 ,  10  is placed with which the pull/push rods  101 ,  103 ,  105 ,  107  are coupled as described below in more detail. 
     The drum shaft  1  comprises a first and a second drum shaft portion, wherein the first drum shaft portion also called driving shaft  5 , is rotatable in a bearing device  6  placed in the main housing  4 . The second drum shaft portion is connected to the first drum shaft portion via a coupling member that is adapted for substantially axially and detachably coupling the first drum shaft portion to the second drum shaft portion, via a flange coupling  15 . 
     The second drum shaft portion is provided with a first drum shaft slot for accommodating the first connecting part  104  for connecting the first pull/push rod  103  to the folding arm unit of the first drum half  2 . The first drum shaft slot may further be adapted for accommodating the third connecting part  102  for connecting the third pull/push rod  101  to the first drum half  2 . However in an alternative embodiment a separate drum shaft slot may have been provided for the third connecting part  102 . The second drum shaft portion furthermore also comprises a second drum shaft slot for accommodating the second connecting part  106  for connecting the second pull/push rod  105  to the folding arm unit of the second drum half  3 . The second drum shaft slot may furthermore be adapted for accommodating the fourth connecting part  108  for connecting the fourth pull/push rod  107  to the second drum half  3 . However in an alternative embodiment a separate drum shaft slot may have been provided for the fourth connecting part. 
     The driving shaft  5  is coupled to a drum shaft driving motor  8  by means of a belt transmission  7 , wherein the belt transmission  7  is placed at a side of the bearing device  6  extending away from the drive mechanism  9 ,  10 . As shown in  FIG. 1 , the belt transmission  7  is placed outside of the main housing  4 . 
     Furthermore within the main housing  4 , the drive mechanism  9 ,  10  is placed substantially in the extension of the drum shaft  1 , behind the bearing device  6 . The four pull/push rods  101 ,  103 ,  105 ,  107  extend beyond the bearing device  6  up into the drive mechanism  9 ,  10 . 
     The drive mechanism  9 ,  10  comprises a first spindle  115  and a second spindle  113 , which are placed substantially on either side of a centre line h of the drum shaft  1 . The first spindle  115  is provided with a thread which on either side of the middle of the drive mechanism has an equally large pitch but of an opposite direction. On the first spindle  115  on either side of the middle of the drive mechanism  10  a first flight nut  110  and a second flight nut  111  are arranged, wherein the first and second flight nut  110 ,  111  are connected to the first pull/push rod  103  and the second pull/push rod  105 , respectively. In the same way the second spindle  113  is also provided with thread which on either side of the middle of the drive mechanism has an equally large pitch but of an opposite direction. On the second spindle  113  on either side of the middle of the drive mechanism  9  a third flight nut  109  and a fourth flight nut  112  are arranged, wherein the third and fourth flight nut  109 ,  112  are connected to the third pull/push rod  101  and the fourth pull/push rod  107 , respectively. When driving the first  115  and second  113  spindle, respectively, the first and second flight nut  110 ,  111  and third and fourth flight nut  109 ,  112 , respectively, will be driven at substantially the same speed but in opposite direction. As the pull/push rods  101 ,  103 ,  105 ,  107  move along with the drum shaft  1  and the drive mechanism  9 ,  10  is substantially fixated in the main housing  4 , the pull/push rods  101 ,  103 ,  105 ,  107  are connected to the respective flight nuts,  109 ,  112 ,  110 ,  111  via a bearing. 
     Optionally the flight nuts  109 ,  110 ,  111  and  112  can be supported by linear guides. 
     The first spindle  115  and the second spindle  113  are each coupled to their own servomotor  116 ,  114  via a belt transmission. 
     The drive mechanism  9 ,  10  as discussed in detail above provides a considerable improvement in the synchronous movement of the drum halves  2 ,  3 , and of the folding arms  14  of the drum halves  2 ,  3 . For adjusting the position of the first drum half  2  and its folding arm mirror-symmetrically with respect to the position of the second drum half  3  and its folding arms, particularly mirror-symmetrically in perpendicular bisector plane v through the middle of the tyre building drum, the first pull/push rod  103  is substantially axially adjustably coupled to the first flight nut  110 , and the third pull/push rod  101  is substantially axially adjustably coupled to the third flight nut  109 . Optionally the second and/or fourth flight nut can also be substantially axially adjustably coupled to the pull/push rod in question. 
     In the embodiment as shown in  FIG. 1 , the drive mechanisms  9 ,  10  are placed on either side above and below the centre line h of the drum shaft  1 . In this embodiment the spindles  113 ,  115  are placed parallel to the pull/push rods  101 ,  103 ,  105 ,  107  on a so-called 12 o&#39;clock-6 o&#39;clock position. In an alternative embodiment the drive mechanisms can also be placed on other positions around the centre line h. For instance the drive mechanisms  9 ,  10  can be placed on either side of the centre line h in a substantially horizontal plane on a so-called 9 o&#39;clock-3 o&#39;clock position, or the drive mechanisms  9 ,  10  can be placed at an angle wherein the intersection of the centre line h and the first  115  and second  113  spindle with a plane perpendicular form the vertices of a triangle, for instance on a so-called 8 o&#39;clock-4 o&#39;clock position. 
     In an alternative embodiment the drive mechanism  9 ,  10  can be also be designed double. In that case each pull/push rod has its own spindle. Said four spindles can then be placed around the centre line h of the drum shaft  1 , for instance on a so-called 12 o&#39;clock-3 o&#39;clock-6 o&#39;clock-9 o&#39;clock position. 
     In use the tyre components t of rubber with the beads b are placed on the drum halves  2 ,  3  that are placed in a defined starting position, as shown in  FIG. 2 . 
     Subsequently the drum halves  2 ,  3  are shifted to the centre of the tyre building drum by shifting at least the third  101  and the fourth  107  pull/push rod, as shown in  FIG. 3 . For shifting the drum halves  2 ,  3  without turning up the folding arms  14 , on the one hand the third  101  and first  103  pull/push rod are substantially jointly driven and simultaneously the fourth  107  and second  105  pull/push rod are substantially jointly driven. This can be achieved by driving the first and second spindle  115 ,  113  at the same speed and in the same direction by means of the first and second servomotor  116 ,  114 . 
     Subsequently the folding arms  14  are activated to stitch the rubber components. For swivelling the folding arms  14  upwards, as shown in  FIG. 4 , on the one hand the first pull/push rod  103  is moved with respect to the third pull/push rod  101  and simultaneously the second pull/push rod  105  is moved with respect to the fourth pull/push rod  107 . When the drum halves  2 ,  3  are at a standstill during lifting the folding arms  14 , only the first spindle  115  needs to be driven. 
       FIG. 5  shows a second exemplary embodiment, wherein only the folding arms  34  of the first  22  and second  23  drum halves are driven by means of the first  203  and second  205  pull/push rods. The other movements of drum halves  22 ,  23  and/or other parts thereof but the folding arms  34  are driven in a conventional manner, for instance by means of a spindle or pneumatic conduit in the drum shaft  21 . 
     In this second exemplary embodiment the first  22  and second  23  drum half are each connected to a flight nut (not shown) via a connecting part  202 ,  208 , which flight nut is placed on a spindle  201  that is placed in the drum shaft  21  and extends up into the housing  24  as shown in  FIG. 5 . The connecting part  208  of the second drum half  23  extends through longitudinal slots (not shown) in the first  203  and second  205  pull/push rod, and is connected to the flight nut corresponding thereto on the spindle  201 . For driving the two drum halves  22 ,  23  the spindle  201  is conventionally connected to a motor (not shown) placed in the housing  24 . 
     Driving other movements in the drum halves, such as the movements of the shoulders supports  32  and the bead clamping mechanism  33 , takes place conventionally, for instance using pneumatic drive mechanisms in the drum halves  22 ,  23  in question. 
     The drive shaft  25  is coupled to a drum shaft driving motor  28  by means of a belt transmission  27 , wherein the belt transmission  27  is placed outside of the housing  24  at a side of the bearing device  26  that extends away from the drive mechanism  30 . 
     Furthermore within the housing  24  the drive mechanism  30  is placed substantially in the extension of the drum shaft  21 , behind the bearing device  26 . The two pull/push rods  203 ,  205  extend past the bearing device  26  up into the drive mechanism  30 . 
     The drive mechanism  30  comprises a first spindle  215  and a second spindle  213 , that are placed substantially on either side of a centre line h of the drum shaft  21 . Both spindles  215 ,  213  are provided with thread which on either side of the middle of the drive mechanism has an equally large pitch but of an opposite direction. The first pull/push rod  203 , near its outer end that extends in the housing, is provided with a first flight body  210 , which on either side of the first pull/push rod  203  is provided with flight nuts with which the flight body  210  is connected to the spindles  215 ,  213 . The second pull/push rod  205 , near its outer end that extends in the housing, is provided with a second flight body  211 , which on either side of the second pull/push rod  205  is provided with flight nuts with which the flight body  211  is coupled to the spindles  215 ,  213 . 
     Both spindles  215 ,  213  are rotation-fixedly connected and coupled to a motor  214  for a substantially simultaneous driving of the spindles. When driving the first  215 , and second  213  spindle, respectively, the flight nuts of the first flight body  210  and the flight nuts of the second flight body, respectively, will be driven at substantially equal speed but in opposite direction. As a result the turn up of the folding arms  34  can be carried out in a highly controlled manner. 
     As the pull/push rods  203 ,  205  move along with the drum shaft  21  and the drive mechanism  30  is substantially fixated in the main housing  24  and therefore does not move along with the drum shaft  21 , the pull/push rods  203 ,  205  are connected to the respective flight bodies  210 ,  211  via a bearing. 
     The above description is included to illustrate the operation of preferred embodiments of the invention and not to limit the scope of the invention. 
     Starting from the above explanation many variations that fall within the spirit and scope of the present invention will be evident to an expert. 
     Briefly summarised the invention provides a tyre building drum comprising a main housing having a drum shaft extending from the main housing and two first and second drum halves arranged at least almost substantially symmetrical with respect to the centre of the tyre building drum and around the drum shaft, which drum halves can be moved in axial direction along the drum shaft. Each drum half comprises a folding arm unit having folding arms that are connected to the drum half in question in a swivelling manner and wherein the folding arm unit can be moved in axial direction along the drum shaft. In the drum shaft at least two pull/push rods are arranged that are movable in axial direction and extend up into the main housing and in the main housing and on the one hand are coupled to a drive mechanism. On the other hand a first pull/push rod is coupled to the folding arm unit of the first drum half, and a second pull/push rod is coupled to the folding arm unit of the second drum half.