Abstract:
Device ( 1 ) for gripping an object such as a blow-molding container preform ( 3 ), comprising: a guiding body ( 12 ); a stop body ( 13 ), mounted so as to move axially relative to the guiding body ( 12 ) along an axis ( 4 ) of the device, intended to be aligned with the object ( 3 ) to be gripped; a mandrel support ( 7 ), equipped with a gripping mandrel ( 15 ), able to move axially relative to the guiding body ( 12 ) and having a plunge-in travel ( 8 ) going from a set-back position to a plunged-in position in which the mandrel ( 15 ) is capable of gripping the object ( 3 ). The stop body ( 13 ) has, together with the mandrel support ( 7 ), a system of support stops ( 18, 19 ) arranged so that the mandrel support ( 7 ) drives the stop body ( 13 ) axially onto a terminal portion of the plunge-in travel ( 8 ). The device ( 1 ) includes a return means ( 34, 36 ) mounted between the guiding body ( 13 ) and the stop body, for the purpose of driving the stop body along an axial set-back direction opposite direction of plunge-in travel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/FR2011/050600, filed on Mar. 22, 2011, which claims priority from French Patent Application No. 1052302, filed on Mar. 29, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     OBJECT OF THE INVENTION 
     The invention relates to the field of devices for gripping objects, particularly preforms for the production of blow-moulded containers such as bottles. 
     The invention also relates to object conveyance systems which make use of such gripping devices, particularly for conveying thermoplastic preforms through a oven. 
     STATE OF THE PRIOR ART 
     Such preforms are generally obtained by injection. They have a tubular cylindrical body that is closed at one of its axial ends, the other end extending out in a neck which is also tubular. The neck is generally injected so that it already has its final shape, while the body of the preform undergoes relatively significant reshaping to form the final container after the blowing operation. The neck of the preform often has a supporting collar which cooperates with a clamp or guide rail. The thinned edge at the axial open end of the neck is called the “mouth” of the container. The axial distance between the mouth end and the bottom of the collar is generally a very specific distance required by the container cap. 
     The blow-moulding operation that shapes the body of the preform must bring the body to a temperature greater than the glass transition temperature of the material. The preform is thermally prepared by being circulated inside an oven. The oven uses infrared lamps, for example, to heat the preform as it is advanced by a conveyance system. 
     Gripping devices intended for conveyance systems that pass through an oven are generally not simple clamps because the gripping device must allow advancing the object rotationally around an axis. In particular, the axis of the preform must be aligned with the axis of the gripping device during conveyance of the preform within the oven, as a disparity during heating could result in preform deformations. 
     Patent application WO 2009/018952 describes a preform gripping device for blow-moulding machines. The device comprises a mandrel which is inserted into the neck. The mandrel has radially elastic jaws that hold the preform by friction. The mandrel described additionally has a shoulder that is in a fixed position relative to the mandrel head. The shoulder provides support contact for the mouth of the preform neck. This shoulder acts to seat the preform to ensure that the mouth of the gripped preform and the axis of the mandrel are properly perpendicular. However, such a device has a complex sequence for releasing the held object because of the friction of the mandrel against the neck. A mechanical part called a stripper must hold the preform at the end of its conveyance while the mandrel is axially withdrawn from the neck. As the shoulder of the device is pressing against the mouth of the preform, the stripper must either press against the outer contours of the neck, which are less precise than the mouth, or the stripper and the shoulder serving to seat the preform share the radial surface at the end of the mouth. 
     Also, preforms that are to travel through an oven are brought to a loading position for the conveyance system through the oven by guide rails, wheels having the traditional clamps, or by notched wheels. The pressure of the preform collar against the supporting surface of the rail or the holding surface of the preform clamp is generally only due to the weight of the preform. The very high production rates of blow-moulding machines cause the preforms to oscillate continuously around their position of equilibrium. It is therefore necessary for the gripping means in the retracted state to be at least two millimeters above the mouth of the preform neck, to allow the preform to reach the position where it is to be grasped without the small oscillations of the preform causing it to knock against the gripping means. 
     It is also necessary to stabilize the preform while the mandrel is introduced into the preform neck. 
     Patent application FR 2794109 describes a preform conveyance system for blow-moulding thermoplastic containers. The gripping system comprises a stabilizer tube which the radially elastic jaws of the mandrel rub against before entering the neck of the preform. The stabilizer tube is thus carried along as the mandrel descends and presses the preform against the support for the preform collar. This movable stabilizer tube improves the alignment of the preform axis and the mandrel axis. 
     When the mandrel is retracted from the preform neck, the stabilizer tube is also lifted by the friction of the mandrel in the stabilizer tube. This holds the tube in position above the mouth while awaiting the arrival of a new preform. The tube can also serve as a stripper, holding the neck of the preform in place as the mandrel is retracted. 
     However, one disadvantage of the device described in FR 2794109 is that the repeated friction of the mandrel every one or two seconds in the same stabilizer tube may require lubrication to prevent the production of metal filings. Improving the safety of container manufacturing requires reducing the risk of abrasion or contamination around the container. 
     TECHNICAL PROBLEM SOLVED BY THE INVENTION 
     The invention proposes a device for gripping objects such as a blow-moulding container preform, as well as a system for conveying objects, which overcome the above disadvantages or satisfy the above needs. 
     One aim of the invention is to control the alignment of the object with the axis of the gripping device, and to allow the arrival of a new preform, while improving the safety of the gripping operation. 
     GENERAL DESCRIPTION OF THE INVENTION 
     In one embodiment, the device for gripping an object such as a blow-moulding preform comprises:
         a guiding body;   a stop body, mounted so as to move axially relative to the guiding body along an axis of the device;   a mandrel support, equipped with a gripping mandrel, able to move axially relative to the guiding body and having an insertion stroke from a retracted position to an insertion position in which the mandrel is capable of gripping the object.       

     The stop body has, together with the mandrel support, a system of support stops arranged so that the mandrel support drives the stop body axially over an end portion of the insertion stroke. 
     The device comprises a return means mounted between the guiding body and the stop body, for the purpose of driving the stop body in an axial retraction direction that is opposite the direction of the insertion stroke. 
     One can see that, for the end portion of the insertion stroke, the return means holds the stop body firmly against the system of support stops. The stop body and the mandrel are therefore solidly attached during the end portion of the insertion stroke, regardless of whether the mandrel support is moving towards the insertion position or is returning from it. Additionally, the stop body is mounted to move in translation along the axis of the device, and may serve as a mechanical reference for aligning the object to be grasped along the axis of the device. In addition, the return means forces the stop body to retract. This allows new preforms to arrive in a “ready to be grasped” position, without risk of becoming stuck. The movement of the stop body occurs without friction with the mandrel. This improves the safety of the operation. 
     Advantageously, the stop body and the guiding body comprise a system of guiding stops that limit the axial movement of the stop body in the retraction direction. This allows the stop body to act as a stripper. With the system of guiding stops, the stop body can hold the preform in place during retraction of the gripping mandrel. 
     In a first particular embodiment, the stop body and the guiding body are of non-magnetic material. The return means comprises at least one permanent magnet mounted onto one of either the guiding body or stop body and an element of magnetic material mounted on the other of the guiding body or stop body. The magnetic element and the permanent magnet are arranged so that they apply an axial mechanical force to the stop body pressing on the guiding body, said mechanical force being oriented in the stop body retraction direction. 
     Advantageously, the return means comprises at least one pair of return magnets consisting of a first permanent return magnet mounted on the guiding body and a second permanent return magnet mounted on the stop body. The magnetic axes of the first and second permanent return magnets are perpendicular to the axis of the device and are located in the same radial half-plane. The magnetic axis of the first permanent return magnet is offset along the axis of the device relative to the magnetic axis of the second magnet, in the insertion direction if the two permanent return magnets are mutually attracting and in the retraction direction if the two permanent return magnets are mutually repelling. When the two return magnets are attracted to each other, the attraction has the effect of reducing the axial offset. When the two magnets repel each other, the magnetic repulsion has the effect of increasing the axial offset. In either case, the effect of the pair of return magnets is to propel the stop body in the axial direction of retraction. 
     The device advantageously comprises a plurality of pairs of return magnets, the first return magnets of each pair being identical and the second return magnets of each pair being identical, the half-planes being uniformly distributed around the axis of the device, and the axial offset distances for each pair being identical. The symmetry of the distribution of the pairs of return magnets around the axis allows the radial forces applied by each pair of return magnets to compensate for each other. Only the axial component of the forces applied by the pairs of return magnets is added on, generating a retraction force on the stop body. 
     In a first variant, the stop body has a radially extending seating surface located at the insertion end of the stop body. The device additionally comprises a removable adapter washer intended to be attached to said seating surface. This allows the gripping device to adapt to different objects, particularly preforms having a different height between the mouth and the collar underside. 
     Advantageously, the device comprises a plurality of attachment magnets for attaching the adapter washer to the stop body. This reduces the time required to convert the gripping device. This is particularly useful for blow-moulding facilities where the production rate may reach 34 containers per second, while the time required to heat a preform above its glass transition temperature is a few dozen seconds. In this type of facility, the number of preform gripping devices to be conveyed through the oven amounts to several hundred. One can see that reducing the time to convert each of the gripping devices reduces the cost of changing the container series. 
     Advantageously, the attachment magnets of the stop body are arranged around the axis of the device so as to come in between the second return magnets. Thus the magnetic field created in the stop body by the attachment magnets attaching the adapter washer creates little interference with the return magnets. 
     In a second variant, the device comprises a selective means of axial indexing between the mandrel support and the gripping mandrel. Said selective means defines several extreme positions of the gripping mandrel for the same insertion position of the mandrel support. 
     It is particularly advantageous to combine this second variant with the above first variant. The distance between the different extreme positions of the gripping mandrel can correspond to the thickness of the adapter washer. Thus the gripping mandrel is inserted into the neck to the same distance from the mouth, even if the preform series to be held is changed. 
     Advantageously, the selective axial indexing means comprises a groove having two parallel walls which a pin slides between. At least a portion of the groove is helicoid. The groove is arranged in the mandrel support and the pin is attached to the mandrel. Alternatively, the groove is arranged in the mandrel and the pin is attached to the mandrel support. By causing the gripping mandrel to pivot around the axis of the device while the mandrel support remains fixed, the axial position of the mandrel relative to the mandrel support can be varied. 
     Advantageously, the groove is closed at its two ends. The device additionally comprises at least one pair of indexing magnets mounted so they are repelling, one on the mandrel support and the other on the gripping mandrel, such that the pin is pressed against one of the two ends of the groove. The closed ends of the groove act as angular stops. The angularly indexing magnets together with the angular stops form an angularly bistable assembly that the helicoid groove converts into an axially bistable device. 
     In a third variant, the stop body comprises an external angular indexing means with the guiding body and an internal angular indexing means with the mandrel support. The guiding body is assembled to rotate freely in the main body of the device. The main body of the device may, for example, be made to move translationally along a conveyance path which passes through an oven for example. The mandrel support may be made to rotate. This allows pivoting the held object (the preform) around the axis of the gripping device while it passes through the oven. 
     Advantageously, the indexing magnet or magnets mounted on the mandrel support are each facing one of the first return magnets when the mandrel support is in the insertion position and have the same magnetic orientation as said first return magnet. Thus the indexing magnets contribute to reinforcing the return force generated by the return magnets. 
     In a second particular embodiment, the device comprises a selective angular indexing means between the stop body and the mandrel support, having a plurality of angular positions in each of which the axial movement is limited by a system of support stops which allow driving the stop body axially over a different portion of the insertion stroke of the mandrel support, different for each of the angular positions. This second embodiment allows varying the end of the stop body that acts as seating while keeping the same insertion stroke for the mandrel support. This allows the gripping device to adapt to different neck heights with no need to add an additional part. This is an alternative solution to adding an adapter washer, reducing the number of part references to be kept in stock. 
     Advantageously, the device comprises a selective angular indexing means between the guiding body and the stop body, having a plurality of angular positions in each of which the axial movement of the stop body relative to the guiding body is limited by a system of axial guiding stops defining a retracted position of the stop body that is different for each of said angular positions. This allows having several retraction distances for the stop body acting as a stripper, for the same retraction position of the mandrel support. Thus the retracted position of the stop body acting as a stripper can be adjusted to the same distance above the mouth for different neck heights. 
     In another aspect, the invention also relates to a system for conveying objects, comprising a conveyor that conveys a plurality of the above gripping devices. The system has a loading area and a means of bringing objects to be loaded on a loading seat. The axis of a gripping device is perpendicular to the path said gripping device follows as it passes through the loading area. Said gripping device cooperates with a fixed cam system to drive the mandrel support axially. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The invention will be better understood by examining a detailed description of a few embodiments provided as non-limiting examples, illustrated by the attached drawings in which: 
         FIG. 1  is a longitudinal cross-section of a first embodiment of a gripping device in a retracted position, equipped for a short neck, 
         FIG. 2  is a partial view along plane II-II of  FIG. 1 , 
         FIG. 3  is a longitudinal cross-section of the first embodiment in the insertion position, for a short neck, 
         FIG. 4  is a transverse cross-section along plane IV-IV of  FIG. 1 , 
         FIG. 5  is a partial view of the mandrel support and the mandrel along arrow F 5  in  FIG. 4 , 
         FIG. 6  is a partial view of the mandrel support and the mandrel along arrow F 6  in  FIG. 4 , 
         FIG. 7  is a transverse cross-sectional view along plane VII-VII of  FIG. 3 , 
         FIG. 8  is a longitudinal cross-section of the first embodiment in the retracted position, for a long neck, 
         FIG. 9  is a longitudinal cross-section of the first embodiment in the insertion position, for a long neck, 
         FIG. 10  is a longitudinal cross-section of a second embodiment in the retracted position, for a short neck, 
         FIG. 11  is a longitudinal cross-section of the second embodiment in the insertion position, for a short neck, 
         FIG. 12  is a longitudinal cross-section of the second embodiment in the retracted position, for a long neck, 
         FIG. 13  is a longitudinal cross-section of a second embodiment in the insertion position, for a long neck. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A conveyance system  1 , illustrated in  FIG. 1 , comprises a device  2  for gripping a preform  3 . The conveyance system  1  also comprises a conveyor rail  5  extending along a conveyance path  5   a . A loading area  61  of the conveyance system  1  extends from a loading start position illustrated in  FIG. 1  to a loading end position illustrated in  FIG. 3 . The conveyance path  5   a  is horizontal in the loading area  61 . The gripping device  2  has a gripping device axis  4  which is perpendicular to the conveyance path  5   a.    
     The conveyance system  1  also comprises a fixed cam system  6  which causes a displacement  6   a  allowing the mandrel support  7  to rise or descend. The high position of the fixed cam system  6  illustrated in  FIG. 1 , and the low position illustrated in  FIG. 3 , define the insertion stroke  8 . The insertion direction is from top to bottom. 
     The conveyance system  1  also comprises a loading seat  9  for the preform  3 , moved by conveyor means (not represented) along a preform  3  conveyor path  10 . The path  10  runs parallel to the conveyor rails  5 , inside the loading area  61 . The horizontal speed of the loading seat  9  is identical to the speed at which the gripping device  2  is conveyed along the conveyor rails  5 . This allows the axis  4  of the gripping device  2  to remain aligned with the axis of the preform  3  throughout the entire loading area  61 . 
     The gripping device  2  comprises a main body  11  equipped with rollers  11   a  adapted to the conveyor rails  5  in order to guide the main body  11  in its translational movement. 
     The gripping device  2  also comprises a guiding body  12  and a stop body  13 , both substantially tubular and coaxial to the axis  4  of the device  2 . The guiding body  12  is mounted to allow rotation within the main body  11  while being prevented from translational movement in the axial direction by a rotating stop  11   b . In other words, the axial position of the main body  11  upwards and downwards at any given moment is determined by the conveyor rail  5 . 
     The stop body  13  is mounted to allow axial movement inside the guiding body  12  while being attached to the guiding body  12  so that they rotate together, by an external angular indexing means  14 . The external indexing means  14  may comprise, as illustrated in  FIG. 1 , a groove  14   a  running longitudinally in the stop body  13  and a pin  14   b  that fits into the groove  14   a  and is attached to the guiding body  12 . 
     The mandrel support  7  is solidly attached, translationally and rotationally, to a gripping mandrel  15  by an indexing selection means  16  comprising a dowel  16   a  and a pair  17  of indexing magnets which operate as detailed below for  FIGS. 4 to 6 . 
     The mandrel support  7  comprises three angular indexing tongues  18  sliding vertically along three longitudinal grooves  19  in the stop body  13 . The longitudinal grooves  19  form an internal angular indexing means  19  of the stop body  13  with the mandrel support  7 . 
     A drive shaft  20  is attached to a notched wheel  21  cooperating with a chain, not represented. The mandrel support  7  comprises an axially telescoping means  22  which ensures angular indexing between the mandrel support  7  and the drive shaft  20 . The angular indexing telescoping means  22  may comprise a radial dowel sliding in longitudinal grooves. 
     It is understood that the external indexing means  14 , the internal indexing means  19 , the selective indexing means  16 , and the means  22  act such that the assembly formed by the guiding body  12 , the stop body  13 , the mandrel support  7 , the gripping mandrel  15 , and the notched wheel  21 , form an assembly which rotates as one unit. The notched wheel  21  and the guiding body  12  move parallel to each other along the conveyor rail  5 . 
     Next will be described an axial stop system and a return means which allow determining the axial position of the mandrel support  7 , the gripping mandrel  15 , and the stop body  13 . 
     The cam system  6 , which is fixed relative to the conveyance system  1 , comprises a surface which a roller  23  of the mandrel support rests against. A supporting spring  24 , which is a coil spring, is arranged axially between a shoulder  25  of the mandrel support  7 , facing towards the top of the device, and a cover  26  attached to the stop body  13 . The supporting spring  24  is assembled in its compressed state and applies a vertical downwards force to the roller  23 , using the conveyor rail  5  as support. 
     The guiding body  12  has a lower counterbore  28  forming a radial surface facing towards the bottom of the guiding body  12 , and an upper counterbore  29  forming a radial surface facing towards the top of the guiding body  12 . The guiding body  12  also has a guide bore  30  extending axially between the two lower  28  and upper  29  counterbores. The stop body  13  has an outer diameter  31  adjusted to the guide bore  30 , guaranteeing good coaxiality between the guiding body  12  and stop body  13 . The stop body  13  comprises a shoulder  32  that cooperates with the lower counterbore  28 . A washer  38  extends radially beyond the diameter  31  and thus provides a shoulder  33  that cooperates with the upper counterbore  29 . 
     The guiding body  12  also comprises four first permanent magnets  34  having a radial magnetic axis distributed every  90  degrees within a first horizontal plane  35 . The stop body  13  comprises four second permanent magnets  36  each having a radial magnetic axis parallel to the magnetic axis of one of the four first magnets  34  and thus forming four pairs  34 - 36  of return magnets. The magnetic axes of the first and second return magnets of each pair are located in the same radial half-plane. The magnetic axes of the four second return magnets  36  are all located in a second horizontal plane  37  that is offset downwards by an axial distance “d 1 ”. 
     The gripping mandrel  15  is attached inside the mandrel support  7  such that the lower end of the gripping mandrel  15  is slightly retracted relative to the lower end of the adapter washer  39 . Thus, when the mandrel support  7  is in a retracted position, as illustrated in  FIG. 1 , the lower end of the adapter washer  39  is horizontal and located at a distance “e 1 ” above the mouth  43  of the preform  3 . 
     The gripping mandrel  15  comprises a ring  45  received in an annular groove and pressed radially outwards by an O-ring  46 . The stop body  13  has an inner diameter  47  that is greater than the extreme radial dimensions of the three-section ring  45  so that there is no friction between the gripping mandrel and the stop body  13  when the mandrel support is in the retracted position. 
     As illustrated in  FIGS. 1 and 2 , the gripping device  2  also comprises an adapter washer  39  pressing against a radially extending seating surface  40  of the stop body  13 , and located at the insertion end of the stop body  13 . The adapter washer  39  is attached to the stop body  13  by four pairs of attachment magnets  41 , one of the attachment magnets of each pair being attached (embedded or glued) to the stop body  13  and level with the seating surface  40 , and the other attachment magnet  41  of each of the four pairs being attached to the adapter washer  39 . Each of the attachment magnets  41  is a permanent magnet having a vertical magnetic axis, the magnets of each pair being arranged so as to be attracted to each other. 
     Advantageously, the four pairs of attachment magnets  41  are angularly arranged so they alternate with the four pairs  34 - 36  of permanent return magnets, as illustrated in  FIG. 2 . 
     One can see that it is particularly fast to install the adapter washer  39  on the end of each of the gripping devices of the conveyance system  1 . Removal of the adapter washer  39  is also simple, because of a lip  42  on the adapter washer  39  extending axially around the stop body  13 . The operator takes hold of the lip  42  and pivots the adapter washer  39  around the axis  4  of the device until the two magnets  41  of each pair of attachment magnets are moved apart from each other. 
     As illustrated in  FIG. 3 , when the gripping device  2  reaches the end of the loading area, the roller  23  integrally attached to the mandrel support  7  is at the bottom of the displacement distance  6   a  defined by the fixed cam system. The supporting spring  24  presses axially against the conveyor rail  5  via the bearing rollers  11   a , the main body  11 , the rotating stop  11   b , the guiding body  12 , and the cover  26  for the guiding body  12 . 
     The insertion stroke  8  of the mandrel support  7  comprises a first portion in which the indexing tongue  18  slides along the longitudinal groove  19  to the bottom  44  of the longitudinal grooves  19 . The insertion stroke  8  then continues in the insertion direction in a terminal portion of the insertion stroke  8  during which the indexing tongues  18  of the mandrel support  7  move the stop body  13  along, until the supporting washer  38  of the stop body  13  comes into contact with the upper counterbore  29  of the guiding body  12 . 
     The axial distance between the shoulders  32  and  33  is greater than the axial distance between the counterbores  28  and  29  by a value equal to a distance “e 2 ”. “e 2 ” corresponds to the axial play between the stop body  13  and the guiding body  12 . “e 2 ” also corresponds to the terminal portion of the insertion stroke  8 . During the terminal portion e 2  of the insertion stroke  8 , the second horizontal plane  37  of the two repelling magnets moves away to a distance “d 2 ” from the first horizontal plane of the magnetic axes of the first repelling magnets  34  attached to the repelling body  12 . The sizes of the magnets  34 ,  36  are chosen so that the repelling force is at its maximum between d 1  and d 2 . 
     The terminal portion e 2  of the insertion stroke  8  occurs without significant friction because the first and second magnets  34 ,  36  are magnetically oriented to be attracting; they are maintained at a radial distance from each other by the adjustment of the diameter  31  in the bore  30 . This adjustment can be lubricated with no risk of contaminating the containers. 
     The preform  3  has an inside diameter  27  less than the extreme radial dimensions of the three-section ring  45 , so that introducing the gripping mandrel  15  inside the diameter  27  of the preform  3  causes compression of the O-ring  46 . 
     When the mandrel support  7  has reached the fully inserted position, the lower surface  60  of the adapter washer  39  comes into contact with the mouth  43  of the preform  3 . Thus the conveyance system  1  can continue along the conveyance path while the loading seat  9  withdraws from below the collar of the preform  3 . The preform  3  is firmly held in alignment with the axis  4  of the gripping device  2  by the friction of the three-section ring  45  against the preform, and by the fact that the stop body  13  and the adapter washer  39  are pressed tightly in the retraction direction against the tongue  18  of the mandrel support  7  by the four pairs of return magnets  34 - 36 . Thus, in spite of the vibrations caused by high production speeds, the lower surface  60  of the adapter washer  29  behaves as a fixed seating relative to the gripping mandrel  15 . The distance “e 1 ” that the gripping device  2  retreats above the mouth  43  of the preform  3  is equal to the axial play “e 2 ” between the stop body  13  and the guiding body  12 . 
     The selective axial indexing means  16  between the mandrel support  7  and the gripping mandrel  15  will now be described, with reference to  FIGS. 4 to 6 . 
     The gripping mandrel  15  comprises a shaft  48  received in a skirt  49  of the mandrel support  7 . The transverse dowel  16   a  fits tightly in the shaft  48  and enters two diametrically opposite grooves  50  arranged in the skirt  49  of the mandrel support  7 . The ends of the dowel  16   a  form two pins  51  sliding in an adjusted manner between the two parallel walls of each of the grooves  50 . Each of the grooves  50  comprises two closed ends  52  axially distanced by a distance S and connected by a helicoid portion  53 . Thus when the gripping mandrel  15  pivots around the axis  4  of the device inside the mandrel support  7 , the axial position of the gripping mandrel  15  is offset by the distance S relative to the mandrel support  7 . 
     Two first permanent indexing magnets  54  are mounted in the skirt  49  of the mandrel support  7 , and two second permanent indexing magnets  55  are mounted in the shaft  48  of the mandrel  15  so that they face the first magnets  54  when the pin  51  is in the middle between the two ends  52  and in the middle of S, in the helicoid portion  53  of the grooves  50 . The first and second indexing magnets have radially oriented magnetic axes that are inversely magnetic to each other such that they repel each other. The second indexing magnets  55  therefore tend to move away from the first indexing magnets  54  until the pins  51  come into contact with one of the two ends  52  of the grooves  50 . In other words, the pairs of indexing magnets  54 - 55  behave as an angularly bistable mechanism. The helicoid portion  53  converts this angularly bistable mechanism into an axially bistable mechanism. Such a mechanism allows very quickly changing the preform  3  series to be handled. The operator simply pivots each of the gripping mandrels  51  of each of the gripping devices  2  of the conveyance system  1  so that the gripping mandrel  15  assumes a different extreme position of the mandrel for the same insertion position of the mandrel support  7 . 
     As illustrated in  FIG. 7 , the first indexing magnets  54  of the mandrel support  7  are axially positioned so that they substantially face the first return magnets  34  of the guiding body  12 . In addition, the six permanent magnets constituting the first two indexing magnets  54  and the pairs  34 - 36  of return magnets positioned facing the first two magnets  54  are all oriented in the same magnetic direction. Thus the magnetic effects of the above six magnets mutually reinforce each other. In other words, the three magnets consisting of one of the first indexing magnets  54  and the two magnets of the pair of return magnets  34 - 36  positioned facing said first indexing magnet  54 , are in a north-south orientation in the centripetal direction on the left in  FIG. 7 , while the three other diametrically opposite magnets are in a north-south orientation in the centrifugal direction. This arrangement reinforces the return force when the mandrel support  7  is in the inserted position, as well as reinforcing the remaining of the gripping mandrel  15  in the selected axial indexing. 
     The two other pairs of repelling magnets that do not correspond to any first indexing magnets are both in a north-south orientation in the centripetal direction. Thus the magnetic fields created by the two pairs illustrated at the top and bottom of  FIG. 7  have an impact on the second indexing magnets  55 , compensating for each other. 
     The advantages of the adapter washer  39  and the selective axial indexing means  16  will now be illustrated with reference to  FIGS. 8 and 9 . The same conveyance system  1  as the one described for the above figures allows a rapid change of series to adapt to preforms  3   a  having a reference height H 2  between the mouth  43  and the loading seat  9  that is different from preform  3 . The axial thickness of the indexing washer  39  corresponds to the height difference “H 2 −H 1 ”. In addition, the distance S caused by the change in axial indexing of the gripping mandrel  15  is also equal to the difference “H 2 −H 1 ”. The seating surface  40  is then situated at a distance “e 1 ” above the mouth  43  of the preform  3 . Therefore, regardless of whether it is preform  3  or  3   a , the gripping device  2  presses against the mouth  43  or  43   a  when in the inserted position, and rises by the same distance e 1 =e 2  above said mouth in the retracted position. 
     The seating surface  40  at the end of the stop body  13  behaves as a fixed seat relative to the gripping mandrel  15 , even when the loading seat  9  has been removed. 
     When the mandrel support  7  descends into the inserted position, the three-section ring  45  penetrates to the same depth inside the inner diameter  27   a  of the preform  3   a  as in the short neck configuration illustrated in  FIGS. 1 to 7 . 
     A second embodiment of the invention will now be illustrated with reference to  FIGS. 10 to 13 . The advantage of this embodiment is that when changing the series from preforms  3  having a neck height H 1  to preforms  3   a  having a neck height H 2 , no inventory of additional adapting parts is required. 
     The conveyance system  101  differs from the conveyance system  1  described above in the selective axial indexing means  116  between the mandrel support  107  and the gripping mandrel  115 . It also differs in the fact that the external angular indexing means  114  between the stop body  113  and the guiding body  112  is a selective means having two relative angular positions that are selectable, and in the fact that the angular indexing means  119  between the stop body  113  and the mandrel support  107  is a selective means also having two relative angular positions that are selectable. In addition, each of the gripping devices  102  has a repelling means  134  assembled between the guiding body  112  and the stop body  113  which is in the form of an elastic coil spring  134 . 
     As illustrated in  FIG. 10 , the selective external angular indexing means  114  consists of two catches  161  sliding in two longitudinal grooves  162  that are diametrically opposite. By pulling downwards on the stop body  113 , the operator causes the catches  161  to exit the grooves  162 , pivots the stop body  113  by 180 degrees relative to the guiding body  112 , and reinserts the catches in the corresponding grooves  162 . 
     In one of the relative angular positions illustrated in  FIG. 10 , the guiding body and stop body  112 ,  113  have a first axial retraction stop system  163 . In the second selected angular position illustrated in  FIG. 12 , the stop body and guiding body  112 ,  113  have a second axial retraction stop system  164 . The first and second axial retraction stop systems  163  consist of a catch  165  protruding from the stop body  113  which comes into contact either with a first retraction stop  166  in the axial indexing illustrated in  FIG. 10 , or with a second stop  167  of the guiding body  112  when the stop body  113  is in the angular indexing illustrated in  FIG. 12 . 
     Similarly, the selective angular means  119  comprises a catch  168  on the mandrel support  107 , sliding in two grooves  169  arranged inside the stop body  113 . The operator pulls the stop body  113  downward to pivot it by 180 degrees while the guiding body  112  and the mandrel support  107  remain unmoving. The changes in angular indexing of the means  114  and  119  are therefore simultaneous. 
     The gripping device  102  has a first axial support stop system  170 , illustrated in  FIG. 11 , which is active when the stop body  113  is in the angular position illustrated in  FIGS. 10 and 11 , and a second axial support stop system  171 , illustrated in  FIG. 13 , when the stop body  113  is in the second angular indexing illustrated in  FIGS. 12 and 13 . 
     The first and second axial support stop systems  170 ,  171  are formed by the fact that the catch  168  on the mandrel support  107  carries the stop body  113  along with it once it comes into contact with the bottom of one of the grooves  169 . 
     The selective axial indexing means  116  between the mandrel  115  and the mandrel support  107  also relies on a catch pressing against a different axial stop  173  depending on whether the mandrel  115  is pivoted by 180 degrees relative to the mandrel support  107  ( FIGS. 10 and 11 ). 
     The axial distance between the first stop  166  and the second stop  167  of the guiding body  112  is equal to the difference “H 2 −H 1 ” between the neck heights of the preforms  3  and  3   a  ( FIGS. 10 and 12 ). Thus the extreme retracted position of the stop body  13  has its lower extreme surface  160  at the same distance above the mouth  43  or  43   a  of the preforms  3 ,  3   a.    
     The axial distance between the bottoms of the two grooves  169  inside the stop body  112  is also equal to the difference “H 2 −H 1 ”, such that the lower surface  160  of the stop body  113  is perfectly in contact with the respective mouths  43  and  43   a  of the preforms  3 ,  3   a  ( FIGS. 11 and 13 ). 
     Advantageously, the difference in axial distance between the stops  173  of the mandrel support  107  is also equal to the difference “H 2 −H 1 ” such that the gripping mandrel  115  penetrates into the mouth  43  or  43   a  by the same respective distance ( FIGS. 11 and 13 ). 
     In one variant, the conveyance systems  1  or  101  may be equipped with fixed cam systems having two parallel surfaces ensuring the insertion and retraction of the mandrel supports  7 ,  107  with no need for supporting springs  24 ,  124 . The angular indexing means, which do not constrain the relative axial movement, may consist of ribs or of combined translational guide surfaces having one or more angular periodicities around the axis  4  of the device. For example, the bores  30  and the guide diameter  31  may have a square, rectangular, or elliptical cross-section. 
     In another variant, the gripping device  102  may have only the selective angular indexing means  119  which allows the lower surface  160  of the stop body  113  to act as a seat for the two types of preforms  3 ,  3   a . In this case, the height e 1 , e 2  required for the stripping is different for the two types of preforms  3 ,  3   a , and the penetration of the mandrel into the neck is also different. 
     In another variant, the gripping device  102  may have a second selective angular indexing system, either between the mandrel  115  and the mandrel support  107 , or between the stop body and guiding body  112 ,  113 . 
     Similarly, the gripping device  2  may not have a selective axial indexing means  16  for the gripping mandrel  15 . 
     In another variant, the gripping device  2  may be equipped with a return means between the guiding body and stop body  12 ,  13 , using an elastic coil spring. 
     In another variant, the gripping device has a selective axial indexing means based on repelling magnets, not between the mandrel  15  and the mandrel support  7 , but between the mandrel support  7  and the stop body  13 . This eliminates the need for the adapter washer  39 .