Abstract:
A conveyor system includes a plurality of end-to-end conveying modules, a main frame, a control unit, and a guide unit. The conveyor system influences spacing between successive products during transport thereof in a direction of transport between a charge end and a discharge end. Each of the conveying modules includes a frame, a plurality of parallel endless conveyor belts which pass over rotatable pulleys provided at a front side and a rear side of the conveying module, and a drive unit including a motor for driving each of the parallel endless conveyor belts of the conveying module. The conveying modules are connected to the main frame. The control unit controls the drive unit of each of the conveying modules individually. The guide units guides the conveying modules that are positioned between other of the conveying modules in a horizontal direction transverse to the direction of transport.

Description:
BACKGROUND OF THE INVENTION 
   I. Field of the Invention 
   The present invention relates to a conveyor system for influencing the spacing between successive products during transport thereof in a direction of transport between a charge end and a discharge end. 
   II. Description of Related Art 
   U.S. Pat. No. 5,070,995 discloses a conveyor system which makes it possible to influence the spacing between successive objects during transport thereof in a direction of transport between a charge end and a discharge end. Said conveyor system comprises five successive conveying modules each comprising two pulleys, one of which is driven by an individually controllable driving motor, over which two pulleys an endless conveyor belt is passed, which conveyor belt extends the full width of the conveying module. Randomly spaced objects being supplied upstream of the conveyor system by a supply conveyor can be spaced a desired (constant) distance apart upstream of the conveyor system by suitably controlling the various driving motors of the respective conveying modules. The conveying modules are relatively short in length, in the sense that, judging from the schematic FIG. 1 of the aforesaid U.S. patent, said length is in any case even shorter than the width of the conveyor belts. With such conveyor belts exhibiting a relatively small length-width ratio there is a risk that the conveyor belt will not run true on the pulleys and consequently tends to move off the pulleys in lateral direction. This risk occurs in particular upon acceleration or deceleration of the conveyor belts or when the conveyor belts are subjected to varying loads. It is precisely these conditions that occur with the conveyor system according to U.S. Pat. No. 5,070,995. Another important drawback of the conveyor system according to U.S. Pat. No. 5,070,995 is the fact that if a conveyor belt should fall out of action, which may occur in particular if the belt should break, the entire conveyor system cannot be used until said conveyor belt has been exchanged, which, depending on the construction of the conveying modules that is not described in detail in U.S. Pat. No. 5,070,995, may take up a considerable amount of time. With certain applications such a temporary failure of the conveyor belt, and consequently of the conveyor system as a whole, is unacceptable. 
   BRIEF SUMMARY OF THE INVENTION 
   The object of the present invention is in the first place to provide a conveyor system of the kind referred to in the introduction which provides a solution or at least an improvement as regards the problems as described above regarding the true running of the conveyor belts. In order to accomplish that object, the conveyor system according to the invention comprises a number of end-to-end conveying modules, a main frame, to which the conveying modules are connected, each conveying module comprising a frame and a number of parallel endless conveyor belts which are passed over pulleys provided at the front side and the rear side of the conveying module in question, which pulleys are rotatable with respect to the frame, as well as drive means comprising a motor for driving all the conveyor belts of the module, which conveyor system further comprises control means for individually controlling the drive means of each conveying module. A major advantage of using a number of side-by-side endless conveyor belts instead of one single, wide endless conveyor belt is that the length-width ratio of the conveyor belts is increased in such a manner that the behaviour of the individual conveyor belts as regards the true running thereof is significantly improved. A very important additional advantage is the fact that if one of the conveyor belts should break, the conveyor system as a whole can nevertheless continue to function, assuming that the objects to be processed have a width greater than the spacing between conveyor belts positioned adjacent to each other. Repairing or exchanging the conveyor belt in question can take place at a suitable later point in time, when the conveyor system is not required to function. 
   From a viewpoint of preventive or corrective maintenance it is preferable if the conveyor system according to the invention comprises guide means for guiding at least conveying modules that are positioned between other conveying modules in a horizontal direction transversely to the direction of transport. In this way a conveying module can readily be made available to one side for maintenance without there being a need to use the floor space that is present under the conveying modules, which is usually taken up by cable ducts, which makes it difficult or even impossible to use a hand truck or the like onto which a conveying module can be lowered after being detached from the main frame. 
   Although it is possible within the framework of the present invention that the guide means comprise guides which are rigidly connected to the main frame, it is preferable if the guide means comprise guides mounted to each conveying module for guiding an adjacent conveying module. This makes it possible to use a very simple construction for the main frame, in spite of the possibility to move individual intermediate conveying modules transversely to the direction of transport. 
   As regards the most downstream and/or the most upstream conveying module, which cannot be regarded as conveying modules positioned between other conveying modules, of course, on the sides of which legs of the main frame will generally be mounted in practice, which legs interfere with the guiding of the most downstream and/or the most upstream conveying module in lateral direction from their active position, it is advantageous if the most downstream and/or the most upstream conveying module is (are) pivotable about a horizontal pivot axis that extends transversely to the direction of transport. Said pivotability makes it possible to use smaller dimensions for the most downstream and/or the most upstream conveying module, so that it will nevertheless be possible to guide the most downstream and/or the most upstream conveying module sideways past a leg of the main frame and thus pass the leg in question. 
   The most downstream and/or the most upstream conveying module preferably engage(s) an adjacent conveying module upon pivoting, so that few if any adaptations of the main frame are required for realising the aforesaid pivotability. The most downstream and/or the most upstream conveying module pivots about the point of engagement with the adjacent conveying module in question, as it were, and can be moved sideways, being suspended from said conveying module, in a manner comparable to the manner in which the intermediate conveying modules are moved. 
   In particular in order to enable a sufficient degree of pivoting, it is preferable if the conveying modules are at least substantially triangular in shape when seen in side view. 
   The use of a number of parallel conveyor belts for each conveying module presents this problem that each of said conveyor belts must be maintained at the correct tension in use. Naturally it is possible, also within the scope of the present invention, to provide separate tensioning means for each of said conveyor belts for this purpose. The drawback of such an embodiment, however, is that the conveying modules will become relatively complex in that case on account of all the features that are required for individually tensioning the parallel conveyor belts. It is quite preferable within this framework if a tensioning roller is provided for each conveyor belt of each conveying module, which tensioning roller abuts against the associated conveyor belt over part of its circumference, wherein the tensioning rollers of each conveying module are pivotable about a pivot axis with respect to a tensioning frame that is kept under tension with respect to the frame, and wherein the tensioning rollers are of a springy type, so that at least the part of the circumference of the tensioning rollers that abuts against the associated conveyor belts can spring inwards and outwards under the influence of the tensile force in the associated conveyor belt. An important advantage that is obtained in this manner is the fact that relatively simple features suffice for maintaining the parallel conveyor belts at the correct tension. The pivot axes for each tensioning roller need not necessarily be coaxial, although this may be preferable according to a preferred embodiment yet to be discussed. It should be realised within the framework of the present preferred embodiment that even if the parallel conveyor belts would have the same length upon installation of the conveyor system, said length will change in practice due to be elongation of the conveyor belts, partly in dependence on the loads to which the individual parallel conveyor belts are subjected in use. 
   With regard to the above preferred embodiment, the present invention further recognises that this principle can be used not only in a conveyor system for influencing the spacing between successive objects during transport thereof in a direction of transport between a charge end and a discharge end, but in general also in a conveyor system for transferring objects during transport thereof in a direction of transport between a supply conveyor and a discharge conveyor, which conveyors extend at an angle ranging between 0° and 90° relative to each other. According to the present invention, such a conveyor comprises a conveying module with a frame and a number of parallel endless conveyor belts which are passed over pulleys provided at ends of the conveying modules, which pulleys are rotatable with respect to the frame, wherein individual pulleys are provided for each conveyor belt at various longitudinal positions of the conveying module, at least at the discharge end, as well drive means for driving the conveyor belts, wherein a tensioning roller is provided for each conveyor belt, which tensioning rollers abut against the respective associated conveyor belts over part of their circumference, wherein said tensioning rollers can pivot about a pivot axis with respect to a tensioning frame that is kept under tension with respect to the frame, and wherein the tensioning rollers are of a springy type, so that at least the part of the circumference of the tensioning rollers that abuts against the associated conveyor belts can spring inwards and outwards in radial direction with respect to the associated axes of rotation under the influence of the tensile force in the associated conveyor belt. The invention makes it possible to use a simpler construction for such conveyor systems than for prior art conveyor systems having the same functionality. 
   Preferably, the tensioning rollers abut against the associated conveyor belts below the central axes of the pulleys. 
   Alternatively, a pulley itself may function as a tensioning roller, so that the conveying module can have relatively limited overall height as the endless conveyor belt in that case only needs to comprise a forward portion (in the direction of transport) and a parallel return portion a short distance below said forward portion. In such a preferred embodiment, the other pulley would be used for driving the parallel conveyor belts. 
   A very advantageous preferred embodiment is characterized in that the tensioning rollers of each conveying module are formed by longitudinal portions of an at least originally common tensioning roller having a length such that it extends over at least two adjacent conveyor belts and preferably over all the adjacent conveyor belts of the associated conveying module. The fact is that such tensioning rollers can be produced at relatively low cost because there is no need for every conveyor belt to have its own tensioning roller to be provided on the common axis of rotation. 
   In order to prevent adjacent tensioning rollers from influencing each other by springing inwards and outwards, it is advantageous if the common tensioning roller between the longitudinal portions has a reduced diameter. 
   As regards the use of springy type tensioning rollers according to the above-described preferred embodiments, it generally applies that it is furthermore preferable if one common axis of rotation forms the axis of rotation of all the tensioning rollers, so that one and the same, common axis of rotation element can be used for all the tensioning rollers of a conveying module. 
   The present invention furthermore relates to a method of maintaining a conveying module having a broken conveyor belt positioned between other conveyor belts, which conveying module forms part of a system according to the invention comprising a number of conveying modules as described above, which method comprises the steps of:
         removing the broken conveyor belt, if it is still present,   moving at least one conveyor belt one position over the pulleys to the original position of the broken conveyor belt, in such a manner that an empty position where no conveyor belt is present is created on the outer side of the module.
 
In this way it is readily possible to fill the position of a broken conveyor belt with an adjacent conveyor belt, insofar as this should be necessary, by moving adjacent conveyor belts towards the original position of the broken conveyor belt. An empty position is (temporarily) created on one side of the conveying module in that case, which is less objectionable than an empty position between the outer conveyor belts of a conveying module. Said empty position may be filled with a new conveyor belt at a later point in time, for example when sufficient time is available.
       

   More specifically, it is preferable in that case if the method comprises the steps of:
         clearing at least one transverse side of the conveying module,   moving all the conveyor belts between the cleared transverse side and the original position of the removed conveyor belts one position over the pulleys in the direction of the original position of the removed conveyor belt,   adding a conveyor belt over the pulleys on the cleared transverse side from said cleared transverse side.
 
Said clearing of the transverse side of the conveying module can take place in a very suitable manner by moving the conveying module sideways, making use of the guide means as already explained above in the description of certain preferred embodiments of the conveyor system according to the invention.
       

   As an alternative solution to the problem regarding the true running of a conveyor belt over pulleys as described in the introduction, the present invention also relates to a conveyor system for influencing the spacing between successive objects during transport thereof in a direction of transport between a charge end and a discharge end, comprising a number of end-to-end conveying modules, a main frame, to which the conveying modules are connected, each conveying module comprising a frame and at least one endless conveyor belt passed over pulleys at the front side and the rear side of the conveying module in question, which pulleys are rotatable with respect to the frame, as well as drive means with a motor for driving the conveyor belt(s) of the module, said conveyor system further comprising control means for individually controlling the drive means of each conveying module, wherein said at least one conveyor belt is provided with inwardly extending grooves and wherein at least one of said pulleys is circumferentially provided with ribs that fall into said grooves, or wherein at least one of said pulleys is circumferentially provided with grooves and said at least one conveyor belt is provided with inwardly extending ribs that fall into said grooves. Providing said at least one conveyor belt with said grooves or ribs and providing at least one of said pulleys with said ribs or grooves, respectively, achieves that the conveyor belt is retained in lateral direction, as it were, so as to improve the true running of the conveyor belt. 
   It has become apparent that a favourable effect on the behaviour of the of the conveyor belt as regards the true running thereof is obtained in particular if said grooves and ribs extend in a direction perpendicular to the direction of transport, which advantageously imparts rigidity to the conveyor belt(s) in a direction transversely to the direction of transport, which has an additional favourable effect on the true running thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail hereinafter by means of a description of a number of embodiments of the present invention, in which reference is made to the following figures: 
       FIG. 1  shows a conveyor system according to the invention, which functions as a transfer conveyor between a supply conveyor and a discharge conveyor; 
       FIG. 2  shows the conveyor system of  FIG. 1 ; 
       FIG. 3  shows a conveying module that forms part of the conveyor system of  FIGS. 1 and 2 ; 
       FIGS. 4   a  and  4   b  show in side elevation two successive situations during use and during (temporary) (partial) removal, respectively, of an intermediate conveying module; 
       FIGS. 5   a  and  5   b  show in side elevation two successive situations during use and during (temporary) (partial) removal, respectively, of a conveying module positioned at an end of the conveyor system; 
       FIG. 6  shows a part of the conveyor system during (partial) removal of an intermediate conveying module as well as a conveying module positioned at an end of the conveyor system; 
       FIG. 7  is a view of the conveying module of  FIG. 3  with partially cutaway conveyor belts; 
       FIG. 8  shows tensioning features for a conveying module as shown in  FIG. 3 ; 
       FIGS. 9   a  and  9   b  show two successive situations during repair of an intermediate conveyor belt of a conveying module; 
       FIG. 10  shows alternative features for tensioning the conveyor belts of a conveying module; 
       FIG. 11  shows a tensioning frame for a conveying module fitted with a tensioning roller; 
       FIG. 12  shows a tensioning frame identical to the tensioning frame of  FIG. 11 , in which different tensioning rollers are used; 
       FIG. 13  shows the tensioning frame of  FIG. 11 , in which a common tensioning roller is used; 
       FIG. 14  shows a conveyor system according to the invention at a position where a supply conveyor joins a main conveyor at a non-perpendicular angle; 
       FIG. 15  shows a conveying module with a partially cutaway conveyor belt; and 
       FIG. 16  shows a conveying module that forms part of a conveyor system according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a conveyor system  1  comprising a supply conveyor  2 , a transfer conveyor  3  according to the invention and a discharge conveyor  4 , which are positioned end-to-end with respect to one another. The transfer conveyor  3  is capable of influencing the spacing between parcels  5  being supplied to the transfer conveyor  3  by the supply conveyor  2 , i.e. enlarge or reduce said spacing so that the products  5  will in any case arrive on the discharge conveyor  4  with a specified spacing between them. Said minimum spacing is required in order to enable automated sorting means (not shown) to sort the parcels  5  sideways onto one of the sorting conveyor is  6   a ,  6   b ,  6   c ,  6   d . Such sorting means may for example consist of a pusher on the side of the discharge conveyor remote from the sorting conveyors  6   a - 6   d , or of shoes forming part of the discharge conveyor  4 , which can move along slats which extend transversely to the direction of transport and which form part of the discharge conveyor  4  as described in European patent application EP 602 694. The transfer conveyor functions not only to ensure that the spacing between products  5  on the discharge conveyor  4  in any case is larger than a predetermined minimum required spacing, but also to ensure that said value is as close to the minimum required value as possible with a view to utilising the sorting capacity on the discharge conveyor  4  as optimally as possible. 
     FIG. 2  is a more detailed view of the intermediate conveyor  3  in isolated position. The intermediate conveyor  3  comprises a main frame substantially consisting of four legs on the corners of the frame, two opposed longitudinal girders  8 , which each interconnect two legs  7  on one longitudinal side of the main frame, and two cross girders  9 , which each interconnect two legs  7  at the ends of the intermediate conveyor  3  at the bottom side thereof. In the present example, the intermediate conveyor  3  furthermore comprises eight conveying modules  10 , of which the conveying module  10 ′ that is positioned between other conveying modules  10  has been extended sideways in part in a manner yet to be described in more detail. 
     FIG. 3  shows an intermediate conveying module  10 . Each conveying module  10  comprises a frame consisting of a folded steel plate  11  (see  FIG. 8 ) having a vertical front side  12 , a substantially horizontal upper side  13  and a vertical rear side  14 . Each conveying module  10  furthermore comprises a number of parallel endless conveyor belts  15  (twenty in this case). Each conveyor belt  15  is passed over a common cylindrical pulley  16  at the front side of the associated conveyor module  10 , which pulley has a length equal to the width of all the endless conveyor belts  15 , and over individual pulleys  17  at the rear side, which pulleys have a width substantially equal to that of the associated conveyor belts  15 . With reference furthermore to  FIG. 8 , the individual pulleys  17  are barrel-shaped, as it were, with a slightly convex outer surface that is to have a positive effect on the true running of each conveyor  15  on the associated pulley  17 . 
   The pulleys  17  are individually supported at the ends of a support member  18  that has a substantially square, hollow cross-section over the larger part of its length, viz. with the exception of that part of the length of the support member  18  where the pulley  17  is provided. Each support member  18  extends into a correspondingly shaped recess  19  in a tensioning frame  20  with the front part thereof. As a result, the support member  18  can only move in the directions indicated by the double arrow  21  with respect to the tensioning frame  20 . A compression spring  22  acts between vertical stop walls that extend transversely to the direction of transport to bound the cavity in the support member as well as in the recess  19 , which compression spring functions to maintain the required tension in the conveyor belt  15  that is passed over the pulley  17 . 
   The tensioning frame  20  is at both ends provided with a horizontal guide strip  23  that extends laterally outwards, which strip falls within a guide groove  24 , which has a length greater than that of the guide strip  23  and which is provided with a constructional member  25 , which, seen in transverse direction, is mounted to the frame  11  at both ends of the parallel conveyor belt  15 . A bore  26  is present in the part of the constructional member  25  that is positioned at the front side, within which bore a shaft member for the common pulley  16  can rotate. To that end, a gear  27  is provided coaxial with the common pulley  16  at the outer side of the constructional member  25 , which gear  27  is rigidly connected to the shaft member in question for joint rotation. An endless toothed belt  29  is passed over the gear  27  and over the gear  28 . The gear  28  is driven by an electric motor  30 , which is mounted to the inner side of the vertical mounting plate  31 , which is in turn mounted to the frame  11 . The toothed belt  29  is correctly tensioned by means of a tensioning roller  32 , which presses against the outer side of the toothed belt  29  and which is rotatably supported in one end of the right-angled arm  33 , which can pivot about a pivot axis  34  with respect to the frame  11 . The pivot axis  34  extends in the longitudinal direction of the conveying module  10  (i.e. transversely to the direction of transport for the transfer conveyor  3 ) in the corner of the right-angled shape of the arm  33 . An adjusting screw  35  is provided for pivoting the right-angled arm  33  about the pivot axis  34 , thereby increasing or decreasing the tension of the toothed belt  29 , which adjusting screw engages the end of the right-angled arm  33  that is positioned opposite the tensioning roller  32 . The (at least substantially) horizontal part of the right-angled arm  33  that is shown in  FIG. 3  extends within a housing  36  which is rigidly connected to the frame  11  and in which the right-angled arm  36  is mounted to enable pivoting thereof about the pivot axis  34 . 
   In an alternative embodiment it is conceivable that the tensioning frame forms part of the frame  19  or is at least rigidly connected thereto, or that only the transverse stop walls within the recesses  19  on the side of the support member  19  remote from the pulleys  19  are movable together with the wedge element  41 . 
   The electric motors  30  of each conveying module  10  are individually controlled by control means (not shown), for example on the basis of measurements by means of a light screen or a camera, for example, of the spacing between parcels  5  on the supply conveyor  2  and/or on the transfer conveyor  3  itself. As a result, the individual parcels  5  can move according to an individual velocity profile on the transverse conveyor  3 , thus making it possible to change the spacing between the parcels  5  and to present parcels  5  to the discharge conveyor  4  with a desired spacing between them. 
   An important advantage of the transfer conveyor  3  as described so far is the fact that a number of parallel conveyor belts  15  are used rather than a single, wide conveyor belt as in the prior art. The first advantage that is achieved in this manner is that the behaviour of the individual conveyor belts as regards the true running thereof is much more favourable because of the relatively great length-width ratio of the endless conveyor belt  15 . A further important advantage that is achieved is that in the unhoped-for event that the operation of one of the endless conveyor belts  15  is halted, for example because the belt has broken, this does not automatically mean that the operation of the entire conveying module  10  and thus of the transfer conveyor  3  is halted, which might lead to considerable consequential damage. 
   At the front side, the tensioning frame  20  is provided with a wedge element  41  at two longitudinal positions (also refer to  FIG. 7 ), which wedge element mates with a counter wedge element  42 . The counter wedge element  42  engages around the upper side and the bottom side of the associated wedge element  41  and can be laterally reciprocated with respect to the frame  11 , or more specifically with respect to the tensioning frame  20 , in the directions indicated by the double arrow  43 . The counter wedge element  42  is to that end provided with internal screw thread at the front side thereof, which screw thread mates with the external screw thread  44  on the tensioning shaft  45 , the ends of which are supported in the frame via construction members  25 , at the rear side of bores  26  thereof. To rotate the tensioning shaft  45  in the directions indicated by the double arrow  46 , the construction member  25  that is shown in  FIG. 8  is externally provided with a hexagonal engagement portion  47 . 
   When the counter wedge element  42  is moved to the right, seen in  FIG. 8 , the tensioning frame  20  is urged away from the tensioning shaft  45  on account of the interaction between the counter wedge element  42  and the wedge element  41 , as a result of which the tensioning rollers  17  will press against the inner side of the conveyor belt  15  with increased force so as to tension said conveyor belts. Also the compression springs  22  will be loaded for pressure in that case. On the other hand, when the counter wedge element  42  is moved to the left, seen in  FIG. 8 , space will be cleared for the tensioning frame  20  to move in the direction of the tensioning shaft  45 , which takes place under the influence of the compression springs  22  as long as there is tension on the conveyor belts  15  and which can be effected manually by pushing the tensioning frame  20  forward (seen in the direction of transport) when there is no tension on the conveyor belts  15 , or by spring means (not shown) acting between the tensioning frame  20  and the frame  11 , which spring means tend to push the tensioning frame forward. Such spring means may be provided in the rear side of guide grooves  24  and press against the end faces  46  of the guide strips  23 . 
   The manner in which individual conveying modules  10  can be detached from the remaining part of the transfer conveyor  3 , for example for carrying out maintenance work, will now be explained with reference in particular to  FIGS. 4   a - 6 . In the first place it is important in this connection to know that in the position of use the ends of the conveying modules  10  are mounted to the underside of the longitudinal girders  8  by means of bolts  51  that engage in screwed holes  52  of the two housings  36  at the ends of each conveying module  10 . Furthermore it is important to note that each conveying module  10  positioned between other conveying modules  10  is provided, both at the front side  12  and at the rear side  14 , with a U-shaped section  53 ,  54 , whose open sides are directed to the front and to the rear, respectively. The rear section  53  is connected to the frame  11  at a slightly higher position than the front section  54 , the difference in height amounting to half the height of the sections  53 ,  54 . The dimensions of each conveying module  10 , the spacing between conveying modules and the length of the legs of the sections  53  and  54  have been selected such that the upper leg of a front section  54  is positioned between the legs of the rear section  53  of a conveying module positioned upstream thereof, without said legs making contact with each other in the position of use, whilst (logically) the upper leg of the front section  54  extends between the two legs of the rear section  53  of a downstream conveying module  10 . 
   Unscrewing the bolt  51  ( FIG. 4   b ) will lead to the conveying module in question moving downwards as indicated by the arrows  55 , as a result of which the legs of the section  53  will be supported on the legs of the section  54  of the upstream conveying module  10 , whilst the upper leg of the front section  54  will be supported on the lower leg of the section  53  of the downstream conveying module  10 . The legs of the sections  53 ,  54  of the adjacent conveying modules  10  on which the aforesaid legs of the sections  53 ,  54  of the conveying module in question will be supported can subsequently function to guide the movement in the lateral direction of the conveying module in  10  under the longitudinal girder  8 , as shown for one conveying module  10 ′ in  FIG. 2 . If desired, the conveying module  10 ′ can thus be removed in its entirety and be exchanged for another conveying module. Alternatively it is also readily possible to arrange for the conveying module  10 ′ to have a free end on the side of the conveying module  10  remote from the electric motor  30  for the purpose of mounting a conveyor belt  15 , which will be discussed in more detail yet with reference to  FIGS. 9   a  and  9   b.    
   It is not possible for at least two reasons to remove the conveying modules  10  that are positioned at the front side and the rear side of the transfer conveyor  3  in the same manner as the intermediate conveying modules  10  as explained with reference to  FIGS. 4   a  and  4   b . In the first place, a leg  7  physically stands in the way. Furthermore, there is only one conveying module adjacent to the conveying modules  10  in question, so that said conveying modules cannot be supported on an adjacent conveying module  10  at the respective end of the transfer conveyor  3 . 
     FIG. 5   a  shows the most downstream conveying module  10  of the transfer conveyor  3 . This conveying module  10 , too, is provided with a rear section  56  at the rear side  12 , although said section  56  is provided at a lower position than the rear sections  53  of the intermediate conveying modules  10 , and in such a manner that the upper leg rather than the lower leg of the rear section  56  extends between the legs of the front section  54  of the conveying module  10  positioned upstream thereof. In addition to that, the most downstream conveying module is provided with a U-shaped section  57  at the front side  14  thereof, with this difference that the open side of said U-shaped section  57  is directed upwards, as a result of which the U-shaped section  57  takes up less space, seen in the direction of transport. 
   As with the intermediate conveying modules  10 , the associated bolts  51  are unscrewed for removing the most downstream conveying module and  10 , as a result of which the upper leg of the rear section  56  of the most downstream conveying module  10  will be supported at the rear side on the lower leg of the section  54  of the conveying module  10  positioned upstream thereof. The two legs in question, like all the other legs of the other sections  53 ,  54 ,  56 ,  57  for that matter, are hook-shaped at their ends, causing the legs in question to interlock and form a hinge, as it were, about which the most downstream conveying module  10  can pivot downwards in the direction indicated by the arrow  59  until the moment when the back of the front section  57  of the most downstream conveying module  10  abuts against the cross girder  9 , which functions as a fixed guide. As  FIG. 5   b  shows, the most downstream conveying module  10  just extends on the inner side of the leg  7  in that pivoted position, as a result of which the most downstream conveying module  10  can be moved over the front section  54  of the upstream conveying module  10  and the fixed guide  9 , perpendicular to the plane of drawing. In order to enable the most downstream conveying module  10  to pivot to a sufficient degree in the direction indicated by the arrow  59 , it is important that the conveying modules  10  are triangular in shape, seen in perpendicular a view, which triangular shape is defined in particular by the shape of the mounting plate  31 , which has a triangular shape with the vertex facing downwards. 
     FIG. 6  shows by way of illustration the transfer conveyor  3  in the condition in which the most upstream conveying module  10  has been (temporarily) pivoted downwards in a manner comparable to the manner as described for the most downstream conveying module  10  with reference to  FIGS. 5   a  and  5   b,  whilst an intermediate conveying module  10 ′ is shown to be extended. 
     FIGS. 9   a  and  9   b  show how it is possible in a relatively simple manner to exchange a (broken intermediate) conveyor belt  15  (which has already been removed in  FIG. 9   a ) at the location indicated at  61 , in which connection it is noted that failure of the conveyor belt  15  need not necessarily lead to failure of the entire conveying module  10  and thus of the entire transfer conveyor  3 , so that it is possible to carry out such an exchange at a suitable point in time, when the transfer conveyor  3  is not required to function. The tension on the conveyor belts  15  is released ( FIG. 9   a ) by suitably turning the tensioning shaft  45  as explained in the foregoing, in particular with reference to  FIG. 8 . In this way it becomes possible to move the conveyor belts  15  that are positioned at the end of the open position  61  remote from the electric motor  30  in  FIG. 9   a  in the direction of the free position  61  as indicated by the arrows  62 , thus creating a free position for a conveyor belt  15  at the end remote from the electric motor  30  ( FIG. 9   b ). To add a new conveyor belt  15  at the location of the free position, it is important that a free end is created at the conveying module  10 . The conveying module is to that end extended sideways, as explained with reference to  FIGS. 4   a - 5   b,  for example. Thus a possibility is created to fit a new conveyor belt  15  and subsequently, in random order, return the conveying module to its operative position and tension the conveyors belts  15  again by suitably turning the tensioning shaft  45 . 
     FIG. 10  shows the principle of an alternative embodiment for a conveying module  71 . The conveying module  71  comprises a number of parallel endless conveyor belts  72 , which are passed, in the form of a triangle, over two common pulleys  73 ,  74  positioned at the same vertical level, and a tube motor  75  positioned under the common pulleys  73 ,  74 , centrally between said pulleys. A tensioning unit  76  is provided for each conveyor belt  72 . Each tensioning unit  76  has a tensioning roller  77  that presses against the outer side of the associated conveyor belt  72  at a position between the common pulley  74  and the tube motor  75 . Each tensioning unit  76  further comprises a fixed arm  78  that is fixedly connected to the frame (module) of the conveying module  71  and two pivoting arms  79  capable of reciprocating pivoting movement about the pivot axis  80  in the direction indicated by the arrow  81 , between which the tensioning roller  77  is rotatably mounted. The pivoting arms  79  are interconnected by means of a rod  82  above the pivot axis  80  and below the tensioning roller  77 . A tension spring  83  is operative between the end of the fixed arm  78  remote from the pivot axis and the rod  82 , which tension spring  83  tends to press the tensioning roller  77  against the outer side of the associated conveyor belt  72 . 
   The tensioning units  76  are disposed side by side. To release the conveyor belts  72 , it is preferable if the adjacent tensioning units  76  are mounted to a separate tensioning frame, which can be detached from the frame of the conveying module  71  for moving the tensioning roller  77  and the conveyor belts  72  apart. 
     FIG. 11  shows a tensioning frame  91  that can function as an alternative to the tensioning frame  20  in  FIG. 8 . Insofar as applicable, like parts are indicated by the same numerals. A pulley  92  is provided for each of the parallel conveyor belts  15  (only one pulley being shown in  FIG. 11 ), which pulley is mounted, being rotatable about its central axis  96 , between two rearwardly extending legs of a U-shaped section  93 , the web of which U-shaped is mounted to the rear side  94  of the tensioning frame  91 , which is to that end provided with screwed holes  95 . 
   The pulleys  92 , which may be produced by means of injection-moulding, for example, each comprise an inner ring  97  and an outer ring  98 , which are both made of a relatively hard material, for example a metal. The outer ring  98  has a slightly convex surface so as to make it easier for the associated conveyor belt  15  to run true. The inner ring  97  and the outer ring  98  are interconnected via leaf springs  99  made of a plastic material or a metal, for example. The leaf springs  99  extend substantially tangentially from the inner ring  97  and join the inner side of the outer ring  98  at an angle of approximately 45°. In unloaded condition, the central axis of the outer ring  98  coincides with the central axis  96  of the inner ring  97 . In loaded condition, however, the outer ring  98  will move in the direction indicated by the arrow  100  with respect to the central axis  96  under the influence of the tensile force in the associated conveyor belt  15 , against the spring action of the leaf springs  99 , which will deform, of course, and thus enable movement of the outer ring  98 . The important advantage that is achieved in this manner is the fact that a much simpler construction can be used for the tensioning frame  91  than for the tensioning frame  20  according to  FIG. 8 , for example, and that furthermore fewer separate additional features are required in order to ensure the required tensile force in the various conveyor belts  15 . 
   A similar advantage applies with regard to the pulleys  110 ,  120  in  FIG. 12 . The pulleys  110 ,  120  each have an inner ring  111 ,  121  and an outer ring  112 ,  122 . The space between the respective inner rings  111 ,  121  and outer rings  112 ,  122  is filled with a foam-like material  113 ,  123 , for example polyurethane. Because of its nature, the foam-like core  113 ,  123  can deform under the influence of external forces being exerted on the outer ring  112 ,  122 , as a result of which the central axis of the outer ring  112 ,  122  will be shifted with respect to the central axis of the inner ring  111 ,  121  in the direction of the force being exerted thereon. Air chambers  114 ,  124  are provided in the foam-like core  113 ,  123  to make it easier for said core to deform. 
     FIG. 13  shows a further alternative for using the pulleys  92 ,  110  and/or  120  according to  FIGS. 11 and 12 . In  FIG. 13  one common pulley  130  having a common axis of rotation is used instead of a number of individual pulleys. Said common pulley  130  comprises a single shaft member  131  extending the full length of the tensioning frame  91 , around which a cylindrical foam core  132  is provided, which is fixedly connected to the shaft member  131 . A hard shell  133  has furthermore been provided over the entire length of the common pulley  130  at the time of the original manufacture thereof. The diameter of the common pulley  130  is reduced at regular intervals, viz. at positions between the eventual power will conveyor belts  15 . To that end not only parts of the shell  133  have been removed, but also parts of the cylindrical foam core  132 , as indicated at  134 . By locally reducing the diameter at regular intervals, parts that can function more or less as individual pulleys are formed between the reduced areas, in the sense that local inward springing of a part of the shell  133  will not result in inward springing of an adjacent part of the shell  133  for an adjacent conveyor belt  15  as well. This also depends on the nature of the material of the cylindrical foam core  132 , of course, and on the degree to which the diameter has been reduced at the location indicated at  134 . It is even conceivable in that connection that material is only removed from the shell  133  and not from the cylindrical foam core  132  when the diameter is being locally reduced. Alternatively it is conceivable within the framework of the present invention to provide the shell  133  only at discrete positions, so that the shell  133  need not be removed at a later stage. 
     FIG. 14  shows a conveyor system in which advantageous use can be made of conveying rollers as explained with reference to  FIGS. 11 and 12 . The conveyor system  141  comprises a supply conveyor  142  having a direction of transport  143  and a discharge conveyor  144  having a direction of transport  145 . The direction of transport  143  extends at an angle β of approximately 45° with respect to the discharge conveyor  144 , with a transfer conveyor  146  being provided in the transition area between the supply conveyor  142  and the discharge conveyor  144 . A specialist term for such a transfer conveyor  146  is “junction”. The transfer conveyor  146  is at least substantially triangular in shape, seen in top plan view, so as to enable the transfer conveyor  146  to join the end side of the supply conveyor  142  on the one hand and the longitudinal side of the discharge conveyor  144  on the other hand. 
   The transfer conveyor  146  comprises a number of endless conveyor belts  147  extending in the direction of transport  143 , which are each passed over a common pulley  148  on the side of the supply conveyor  142  and over individual pulleys  149  on the side of the discharge conveyor  144 . Because of the triangular shape of the transfer conveyor  146 , the individual conveyor belts  147  each have their own individual length, which length is different from that of the other conveyor belts  147 . The individual pulleys  149  are mounted between two legs of a U-shaped section  150  in such a manner as to be rotatable about their central axes, which U-shaped section  150  is provided with a tensioning arm  151  at the end thereof, via which arm  151  the position of said section can be adjusted by means of an adjusting screw  152 . When a conveyor belt  147  is to be rendered tensionless, for example for carrying out maintenance work, the tensioning arm  151  can be moved in the direction of the supply conveyor  142 . The pulley  149  can have a configuration identical to that of the pulley  92  or of the pulley  120 , in any case having a hard outer ring/shell and a springy core, so that the outer ring/shell can spring inwards in the direction of the supply conveyor  142 , with the central axis of the outer ring/shell being shifted with respect to the original position when the pulley  149  is not loaded. An important constructional advantage that is achieved in this manner is that no additional spring means are required in the transfer conveyor  146  for ensuring the required tensile force in the conveyor belt  147 . 
     FIGS. 15 and 16  show two conveying modules  160 ,  170 , which can function as alternatives to the conveying module  10 , as explained in the foregoing in particular with reference to  FIG. 3 . Like parts will be indicated by the same numerals. One relatively wide conveyor belt  161 ,  171  is used with the conveying modules  160 ,  170  instead of a number of parallel, relatively narrow conveyor belts  15 , which conveyor belts  161 ,  171  are passed over a front pulleys  162 ,  172 , respectively, and rear pulleys  163 ,  173 , respectively. 
   In order to obtain an improved behavior as regards the true running of the relatively short, wide conveyor belts  161 ,  171 , grooves  164 ,  174  are formed in the pulleys  162 , 163  and  172 ,  173 , into which inwardly extending ribs  165 ,  175  of the associated conveyor belts  161 ,  171  extend. Because of the engagement between the grooves  164 ,  174  and the ribs  165 ,  175 , the associated conveyor belts  161 ,  171  do not exhibit any tendency, or at least to a significantly reduced extent, to move sideways off the associated pulleys  162 ,  163  and  172 ,  173 , in spite of their relatively short length. Thus they exhibit an improved behavior as regards the true running thereof. The difference between the conveying module  160  and the conveying module  170  is that the grooves  164  and the ribs  165  of the conveying module  160  extend transversely to the direction of transport, whilst the grooves  174  and the ribs  175  of the conveying module  170 , on the other hand, extend parallel to the direction of transport.