Patent Publication Number: US-2003226744-A1

Title: Conveyor

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to a conveyor for transporting articles such as letters, mail parcels, CDs, and other types of articles, in particular articles of a relatively light weight such as, e.g., of a maximum weight of about 8 kg, in particular a maximum weight of about 5 kg (although it will be understood that the principles of the invention can be utilized also in connection with articles of a higher weight than about 8 kg). In particular, the invention relates to a conveyor which allows a high degree of adaptability to requirements or constraints with respect to the space it can occupy and the path along which the conveyor units can travel.  
       BACKGROUND OF THE INVENTION  
       [0002] Conveyors for letters, mail parcels, CDs, videotapes, books and other relatively light articles should be designed so that they can relatively easily be installed and operated under widely varying space conditions, both with regard to the amount of space the conveyors can occupy and with respect to the three-dimensional configuration of the space available. Examples of conveyors in which the conveyor units are capable of travelling along a curving path extending between different vertical positions appear from AT Patent No. 347.333 and Swedish published patent application No. 92026749.  
       DISCLOSURE OF THE INVENTION  
       [0003] The present invention relates to such conveyors that are highly flexible with respect to their installation and operation and which permit the conveyor units thereof to follow paths in any desired direction; which can be installed and operated in a space comprising more than one floor and/or on walls; in which a loading station can be installed directly above or directly under a discharge station, and/or loading stations or discharge stations can be installed directly above each other; and which can be made economically and yet deliver a highly efficient and flawless operation.  
       [0004] The conveyor according to the invention comprises  
       [0005] a plurality of conveyor units;  
       [0006] endless carrier means to which each conveyor unit is connected, each of the conveyor units being connected to the endless carrier means;  
       [0007] an endless main track in which the carrier means is moving in a transport direction, said track comprising at least a first main track section, a second main track section, and a curving main track section being connected at a first end to the first main track section and at a second end to the second main track section;  
       [0008] each conveyor unit being pivotally connected to the carrier means so as to allow the conveyor unit to pivot relatively to the endless carrier means about a main axis of the conveyor unit, each conveyor unit comprising a first guide member and an article-supporting member, the article-supporting member extending away from the vertical plane of the carrier means;  
       [0009] first guiding means arranged to co-operate with the first guide member of each of said plurality of conveyor units when the conveyor unit moves along at least a part of the at least one curving section of said main track; and  
       [0010] means for counteracting or preventing the article supporting member from tilting from its position relative to the horizontal plane.  
       [0011] The means for counteracting or preventing the article supporting member from tilting preferably provide a force resulting in a torque balance around the main axis of each conveyor unit when the conveyor unit tends to tilt, e.g. by influence of gravity, for example when the conveyor unit moves along a curved path in a vertical plane, i.e. when it moves from one vertical level to another vertical level. It is desirable that the article-supporting member essentially maintains its horizontal orientation during operation of the conveyor. The means for counteracting or preventing the article-supporting member from tilting may comprise the first guiding means and the first guide member, in which case the first guiding means and the first guide member provide the torque balance mentioned above. In embodiments wherein the first guiding means and the first guide member do not constitute parts of the means for counteracting or preventing the article-supporting member form tilting, the guiding means and guide member may serve to guide the conveyor units along the main track. For example, if the carrier means comprise a chain with a plurality of rollers or wheels, such a roller or wheel may constitute the guide member, in which case the guiding means is constituted by a part of the main track which supports or guides the rollers or wheels. There may be provided a plurality of guide members and a plurality of guiding means.  
       [0012] The conveyor units may be supported on a single side of the carrier means.  
       [0013] The first and/or the second main track section may be substantially straight, in which case the conveyor according to the invention may reside in a single vertical plane. The fist and second main track sections may arranged in respective first and second levels, so that the transport directions of the first and second main track sections are of opposite directions. Thereby, the conveyor according to the invention may efficiently utilise space available in, e.g. a mail distribution centre, as the floor space required for the conveyor may be smaller than the floor space required for conveyors in which the conveyor units move in a single horizontal level.  
       [0014] In order to provide for compact design, the carrier means may reside in substantially a single vertical plane at the curving main track section.  
       [0015] The main axis is preferably substantially horizontal, and it is preferably substantially perpendicular to the main track section, so that the conveyor units and the associated article-supporting members extend substantially perpendicular to the main track section, thereby allowing for convenient loading and unloading of articles to/from the conveyor units.  
       [0016] In order to stabilize the conveyor units and the article-supporting members, i.e. in order to minimize any undesired tilting of the article-supporting members, there may be provided a plurality of guide members, such as a plurality of wheels for engaging with corresponding track sections. There may further be provided a plurality of guiding means.  
       [0017] The guiding means and may be arranged in a vertical plane which is parallel to the vertical plane of the carrier means. The article-supporting members of the conveyor units may be arranged between the vertical planes of the guiding means and the vertical plane of the carrier means when the conveyors units are moving along said curving main track section. Such arrangements are in particular useful in embodiments in which the guiding means and the guide members constitute parts of the means for counteracting or preventing the article-supporting members from tilting.  
       [0018] The first and the second main track sections will normally be horizontally oriented. The endless carrier means is normally, and in most cases preferably, a chain, but also other means, such as, e.g., a toothed belt or an endless series of connected carriages, which are capable of carrying the conveyor units in the manner disclosed herein are contemplated.  
       [0019] The main axis of each conveyor unit is preferably horizontal or substantially horizontal. In most cases, it will also be preferred that the main axis of each conveyor is substantially perpendicular to the transport direction.  
       [0020] The pivotal connection of each of the conveyor units with its article-supporting member, e.g. a cross-belt unit, extending away from the vertical plane of the carrier means permits a design where sections of the track can be mounted in an open space or on a wall as desired. Curvature of the track in a vertical direction, e.g. a vertical loop connecting two substantially straight track sections and reverting the direction of the track, can be compensated for by the conveyor units pivoting around their main axis, the maintaining or substantially maintaining the article-supporting member, e.g. a cross-belt unit, in its position relative to the horizontal plane, normally a position where the article-supporting surface of the member is horizontal, being achieved through the engagement of the guide member with the first guiding means. In most embodiments, the first guiding means will extend substantially parallel to at least a part of the main track. The first guiding means may in itself be a track with which the first guide member of each carrier unit engages, or it may comprise, e.g., protrusions such as pins or bars, which move along a path which normally extends substantially parallel to at least a part of the main track and which engage with the first guide member of each conveyor unit.  
       [0021] Torque-absorbing means may preferably be arranged in substantially the same vertical plane as the carrier means. This means that the torque-absorbing means will normally not extend further from the main track, normally in the direction opposite to the article-supporting member, than four times the breadth of the main track, preferably not further than three times and more preferred not further than twice the breadth of the main track. In an illustrative, but presently not preferred embodiment, the torque-absorbing means could be a relatively heavy counterweight placed on the opposite side of the carrier means relative to the article-supporting member of the conveyor unit; the counterweight could be adjustable to compensate for variations of the loading on the article-supporting member, the compensation being performed manually or preferably automatically, e.g. in response to a sensor detecting the angle of the article-supporting member of the conveyor unit relative to the horizontal plane and producing an output signal indicating the detected angle.  
       [0022] In practice, the torque absorbing means preferably constitutes an integrated part of the carrier means, such as, for each conveyor unit, at least one wheel or low friction slide part/slide member/slide shoe connected to or forming part of the carrier means and engaging with a track part, preferably a part of the main track, and co-operating with another engaging member, e.g. another wheel on a common axle, in such a manner that the torque is absorbed while permitting free movement of the total arrangement in the transport direction. An embodiment of such a torque-absorbing means is shown in the figures. It will be understood that in principle, to be comparable to the counterweight example above, such a torque-absorbing means consists of the combination of e.g. the wheel pair on the common axle and the track parts with which each wheel engages, and such a combination and similar combinations are covered by the term “torque absorbing means” herein.  
       [0023] The conveyor according to the invention may further comprise a second guiding means that extends substantially parallel to at least a substantial part of the main track, each conveyor unit comprises a second guide member adapted to co-operate with said second guiding means when the conveyor unit moves along at least a substantial part of straight sections of said main track, the second guiding means and the second guide members when co-operating are adapted to counteract or prevent the article-supporting member from tilting from its position relative to the horizontal plane.  
       [0024] To reduce the risk of tilting or instability in the transition from a straight to a curved main track section or vice versa, the first and second guide members of each conveyor unit and the first and second guiding means are preferably adapted so that the first guide member co-operates with the first guiding means when the conveyor unit enters the curving main track section and before the second guide member disengages with the second guiding means and so that the second guide member cooperates with the second guiding means when the conveyor unit leaves the curving main track section and before the first guide member disengages with the first guiding means. It will be understood that in some embodiments, the function of the second guide member can be performed by the first guide member so that both the first and the second guide members are constituted by one and the same guide member.  
       [0025] The second guide member and the second guiding means are preferably arranged in substantially the same vertical plane as the carrier means or in a vertical plane closely adjacent to said plane.  
       [0026] To further improve the stability when the first guide member travels through such parts of the first guiding means whose path may geometrically allow movement of the first guide member along their tangent (which means that in this situation there is no well-defined position of the first guide member in the first guide member), the first guide member of at least one conveyor unit and preferably of all the conveyor units may comprise two wheels arranged in substantially the same vertical plane with a spacing between them and arranged pivotally on a bar which is arranged on the conveyor unit such that said bar is allowed to pivot about a third axis that extends substantially midway between and substantially parallel to the axes about which said two wheels are allowed to pivot, said third axis being substantially parallel to the first axis of the conveyor unit, the first, second and third axes of the conveyor unit being arranged with a spacing between each axis, and the first guiding means comprising a track protruding from a vertical plane and comprising two parallel surfaces with which said two wheels engage with said track in between said two wheels when the first guide member engages with the first guiding means. At least one of said two wheels of the first guide member is advantageously resiliently biased against said track of the first guiding means when the first guide member engages with the first guiding means.  
       [0027] It will be understood that the above stability-increasing embodiment may be generalized to other designs which further increase the stability of the movement of the conveyor units by increasing the degree of definition with which the respective guide members engage with their guiding means.  
       [0028] In order to further increase the stability of the conveyor, it may comprise a third guiding means arranged in substantially in the same vertical plane as the first guiding means, each conveyor unit comprises a third guide member adapted to co-operate with said third guiding means, the second and third guiding means being adapted so that the third guide member co-operates with the third guiding means when the conveyor unit enters the curving main track section and before the second guide member disengages with the second guiding means and so that the second guide member co-operates with the second guiding means when the conveyor unit leaves the curving main track section and before the third guide member disengages with the third guiding means.  
       [0029] Furthermore, the conveyor may comprise a fourth guiding means arranged in substantially in the same vertical plane as the first guiding means, each conveyor unit comprises a fourth guide member adapted to co-operate with said fourth guiding means, the first, third and fourth guide members of each conveyor unit forming an obtuse-angled triangle in a vertical plane.  
       [0030] In one embodiment according to the invention, the conveyor comprises first, second and third guide members of each conveyor unit and first, second and third guiding means being adapted so that the first guide member co-operates with the first guiding means when the conveyor unit enters the curving main track section and before the second guide member disengages with the second guiding means, so that the third guide member co-operates with the third guiding means before the first guide member disengages with the first guiding means when the conveyor unit passes a given position along the curving main track section, and so that the second guide member co-operates with the second guiding means when the conveyor unit leaves the curving main track section and before the third guide member disengages with the third guiding means.  
       [0031] In a simple embodiment of the conveyor according to the invention, the first guide member comprises a wheel arranged on the conveyor unit such that said wheel is allowed to pivot about a second axis that forms the center axis of the wheel, said second axis being substantially parallel to the first axis of the conveyor unit and the first and second axes of the conveyor unit being arranged with a spacing between them, the first guiding means comprising a track that extends in the full length of the main track and in which track said wheel is moving, and the path of the track relatively to the main track is adapted to counteract or prevent the article-supporting member from tilting from its position relative to the horizontal plane.  
       [0032] Another embodiment based on the above basic design and having an increased stability compared thereto further comprises a second guiding means, at least one conveyor unit and preferably all conveyor units further comprising a second guide member adapted for engaging with said second guiding means, said second guide member comprising a wheel arranged on the conveyor unit such that said wheel is allowed to pivot about a third axis that forms the center axis of the wheel, said third axis being substantially parallel to the first axis of the conveyor unit and the first, second and third axes of the conveyor unit being arranged with a spacing in between each axis, the second guiding means extending substantially parallel to the main track over at least a substantial part of the curving main track section and comprising a track in which said wheel of the second guide member is moving, the path of the track of the second guide means relatively to the main track being adapted to counteract or prevent the article-supporting member from tilting from its position relative to the horizontal plane.  
       [0033] As apparent from the above discussion, each conveyor unit may be mounted on a shaft extending along the main axis. In order to provide a simple construction which does not require the presence of guide members, such as wheels, or guiding means, such as tracks, for counteracting or preventing the article-supporting member from tilting, the means for counteracting or preventing may be arranged to provide a torque for rotating said shaft, so that the article-supporting member may maintain its horizontal orientation or rotate around the main axis as a result of the torque being applied to the shaft. Such embodiments are not necessarily dependent from forces being applied to the article-supporting members through guide members and guiding means in order to counteract or prevent undesired tilting of the article-supporting members, but may rely solely on appropriately controlled, e.g. mechanically or electrically controlled, means for providing a torque to the shaft. The torque may be provided by a motor drive, e.g. comprising an electrically or hydraulically driven motor, which may be operatively connected to a control system for providing a control signal indicative of a horizontal position or of a tilting angle of at least one of the article supporting members and for control the motor in response to said signal. Electromagnetic means may also or alternatively be provided and controlled for providing the torque. The means for counteracting or preventing the article supporting member from tilting are directly coupled to the respective main axes of the conveyor units, or, alternatively, be coupled to the respective main axes of the conveyor units via a force transmission system. The means for counteracting or preventing the article-supporting member from tilting may be powered by an energy source system comprised in the conveyor. There may be provided battery units in one, more or all of the conveyor units. In case only some of the conveyor units are provided with battery units, electrical power may be transported from said selected conveyor units to other of the conveyor units via an electrical power transmission system. As an alternative to the electrical system described above, the first guiding means may comprise a toothed track with a plurality of teeth and interstices defined by respective side surfaces of the teeth, wherein the first guide member comprises a rotatable member with a plurality of radially extending engagement members, each of which is arranged so that the engagement member may co-operate with said side surfaces to provide a force resulting in a torque for preventing the article supporting member from tilting. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0034]FIG. 1 shows an overview of a conveyor system according to the invention,  
     [0035]FIG. 2 shows a cross-section of a profile forming the main track,  
     [0036] FIGS.  3 - 5  show cross-sections of embodiments of the main track equipped for operation,  
     [0037] FIGS.  6 - 9  show two embodiments of a drive chain,  
     [0038] FIGS.  10 - 26  show a number of different embodiments of arrangements for preventing the conveyor units from tilting when passing a vertical half-turn of the main track,  
     [0039] FIGS.  27 - 29  show a preferred embodiment of an arrangement for preventing the conveyor units from tilting when passing a vertical half-turn of the main track,  
     [0040] FIGS.  30 - 32  show the conveyor units, and  
     [0041] FIGS.  33 - 42  show an embodiment of an arrangement for preventing the conveyor units from tilting when passing a vertical half-turn of the main track. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
     [0042] An overview of a conveyor system according to a preferred embodiment of the invention is shown in FIG. 1. A carrier means to which the conveyor units  1  are connected moves in an endless main track  2  that have sections  3  that curves in the horizontal plane and section  4  that curves in the vertical plane forming vertical half-turns. For clarity, only a limited number of conveyor units are shown in FIG. 1. An endless belt extending to one side from the vertical plane of the carrier means constitutes the article-supporting surface  5  of each conveyor unit  1 . The endless belt or cross-belt may be moved in a direction perpendicular to the transport direction and articles may be discharged to either side of the conveyor unit  1 . The article-supporting surface  5  may also be constituted by other devices, such as, e.g., a tray that may be tiltable in both directions.  
     [0043] The control system of the conveyor system is adapted to control the loading of articles onto and discharging of articles from the conveyor system. The conveyor units  1  are label uniquely with an automatically readable identification mark, such as, e.g., a bar code label in order to identify the individual conveyor units  1 . The conveyor system further comprises reading means for reading the identification marks at least at one position along the track of the conveyor system, preferably at the loading stations and at the discharge stations  6 . A main control unit has means for communicating with the control units of each individual conveyor unit  1  which controls a drive unit on each conveyor unit  1  for driving the endless belt in a given direction to discharge articles from the article-supporting surface  5  at a predetermined discharge station  6 .  
     [0044] The conveyor system further comprises means for driving the conveyor units  1 , such as, e.g., a linear motor that is driving at least some of the conveyor units, one or more electro-motors driving the carrier means, etc, and it also comprises one or more loading stations for loading articles onto the article-supporting surfaces  5 .  
     [0045] A low radius of curvature of the curved main track sections  3 ,  4  make the conveyor system very flexible and allows for a very high spatial exploitation. The main track sections  4  that form half-turns in the vertical plane allow for discharge stations  5  to be stacked in the vertical direction. However, special arrangements must be made at the vertical half-turns  4  of the main track  2  to prevent the article-supporting surfaces  5  of the conveyor units  1  from tilting so as to prevent the articles transported by the conveyor units  1  from being unintentionally discharged.  
     [0046] The monorail sections forming the main track  2  on straight track sections and track sections curving in the horizontal plane are preferably made from a profile of extruded aluminum. A cross-section of a profile is shown in FIG. 2 and cross-sections of monorails mounted with a conveyor unit  1  and equipped for operation are shown in FIGS.  3 - 5 .  
     [0047] The profile forms a nearly closed room in which the drive chain can move. An opening  7  to one side enables the connection between the drive chain and the conveyor units  1 . The opening  7  is small compared to the total circumference of the profile, forming only about a tenth of the circumference so that the profile has a high stiffness about the longitudinal axis. The opening  7  is placed at one side of the profile to prevent dust and other impurities from settling into the interior of the profile and to enable vertical half-turns  4  of the main track  2 . A number of cavities  8  are formed within the profile to increase the longitudinal stiffness with low added material consumption. The profile furthermore has grooves  9  for assembling adjacent sections of the main track  2  and grooves  10  for mounting of cabling for power supply to the drive unit and for control of the conveyor and optionally for power supply to the conveyor units  1 . A shield for covering the cabling, control and power-supply units, etc. may be mounted on protrusions  11  placed on both sides of the profile.  
     [0048] Surfaces  12 ,  13  with which the support wheels  14 ,  15  engage are formed within the profile. The full surface of the profile is anodized after the extrusion for providing such a surface hardness that the aluminum can withstand the wear from the support wheels  14 ,  15 . Surfaces  16 ,  17  with which the directing wheels  18  may engage are also formed within the profile. The surfaces  19 ,  20  adjacent to the secondary support wheel  21  and the secondary directing wheel  22  are recessed one millimeter as compared to the similar surfaces  13 ,  16  adjacent to the support wheel  15  and the directing wheel  18  so that the secondary wheels  21 ,  22  do not engage under ordinary operation conditions. The different wheels and their intended operation will together with the drive chain be discussed in details later.  
     [0049] The profile has a groove  23  next to the conveyor units  1  in which a stationary part of a cordless energy transfer system may be mounted. The conveyor units  1  may have a demand for a power supply for driving, e.g., a cross-belt, a discharge device for a tilt tray and/or a control unit for controlling the operation of the conveyor unit  1 . Energy may be transferred to the conveyor unit  1  by means of a conductor rail on the main track  2  and a collector shoe on each conveyor unit  1  or by means of an inductive energy transfer system with a primary side on the main track  2  and a secondary side on each conveyor unit  1 . Alternatively, the conveyor unit may receive energy from the drive system by having a generator connected to a support wheel of the conveyor unit  1 .  
     [0050] Two rooms  24 ,  25  separated by a wall  26  give room for the power supply for the linear motor drive  27  and for cabling for the control units and means, respectively. The two types of cabling are separated to be in accordance with various safety regulations. An opening  27  between the two rooms  24 ,  25  and the covering shield  28  gives room for mounting of additional power supply units, control units, power supply for the energy supply system for the conveyor units  1 , etc. The drive chain is driven by means of a linear motor drive comprising a number of stator parts  29  placed along the main track  2  and reaction plates  30  mounted on the drive chain.  
     [0051] The use of a monorail on which the conveyor units  1  run is, especially when the monorail is made from one extruded profile, advantageous as compared to known systems having tracks on both sides of the article-supporting surface. The monorail is more flexible in installation and requires a simpler support arrangement than two parallel tracks and parts of the main track  2  may even be placed on a vertical wall. A monorail having room for the drive chain in the interior and with an opening in the side of the profile permits for vertical half-turns  4  in the path of the main track  2 .  
     [0052] The three configurations in FIGS.  3 - 5  shows different conveyor systems using a monorail. The ones shown in FIGS. 3 and 4 has the conveyor unit  1  and the article-supporting surface  5  arranged on one side of the main track  2 , thus permitting for vertical half-turns  4  of the path of the main track  2 . The configuration shown in FIG. 3 is suitable for conveying articles of a maximum weight of 5-8 kg, whereas the configuration shown in FIG. 4 is suitable for a maximum weight of 12 kg or more, depending on the size and shape of the protruding arm  31  with which the support wheel  15  engages. The arm  31  makes it possible to make the conveyor more stable in the transversal direction but is also more expensive to either have extruded on the profile or extruded separately and subsequently mounted on the profile. The configuration shown in FIG. 5 is a horizontal configuration which is very stable but does not permit for vertical half-turns  4 .  
     [0053] The drive chain on which the conveyor units are mounted is shown in two configurations in FIGS.  6 - 7  and FIGS.  8 - 9  for use in a purely horizontal configuration and in a configuration for vertical half-turns  4 , respectively. The drive chain comprises a plurality of longer links  32  carrying the reaction plates  30  for the linear motor drive coupled together by two shorter links  33  between each two longer links  32 . The short links are pivotally connected to one end of a longer link about pivot pins  34  being substantially vertical and carrying the directing wheels  18 ,  22  of a diameter of 70 mm for guiding the chain and delimiting slack in the transversal direction. Each pair of two shorter links  33  are pivotally interconnected with a common, substantially horizontal pivot pin  35  carrying the support wheels  14 ,  15 ,  36  of a diameter of 47 mm for supporting the conveyor unit  1 . Thus, the drive chain is flexible in the vertical plane as well as in the horizontal plane and the support wheels  14 ,  15  will run correctly without broadsiding through all curves of the main track  2  by forming the drive chain in this manner.  
     [0054] The weight of the conveyor unit  1  and the article supported thereon is during normal operation supported by the reaction force between the surface  13  on the monorail and the associated support wheel  15 . The torque caused by the vertical distance between the center of gravity of the conveyor unit  1  and the point of vertical support defined by the wheel  15  and the surface  13  is counteracted by the reaction force between the surface  12  and the associated support wheel  14 . Thus, the reaction force between the surface  13  and the wheel  15  counterbalances the gravity force on the conveyor unit  1  (and of the drive chain) as well as the reaction force caused by the torque. The pivot pin  35  carries the conveyor unit  1  and constitutes the main axis of the conveyor unit about which it pivots during passage of vertical half-turns  4 . The support wheel  36  is only engaging a surface  19  of the main track  2  during passage of vertical half-turns  4  where there is not enough room for the surface  13  with which the support wheel  15  engages on straight main track sections. The support wheel  36  is situated at a longer vertical distance from the center of gravity of the conveyor unit  1  and closer to the other support wheel  14  and the support of the conveyor unit  1  is therefore less stable when support wheel  36  takes over the function of support wheel  15  but this effect is counterweighted by the stabilizing influence of the guide means that engages with a guide track at the vertical half-turns  4  as will be described below.  
     [0055] The directing wheels  18 ,  22  are broad, 70 mm of diameter, so as to allow for as well broad as long reaction plates  30  with a small distance to the directing wheel  22 . The reaction plates  30  move as chords to the curve of the main track  2  during passage of vertical half-turns  4  and the dimensions can be made more suitable if the ends of the reaction plates  30  are able to enter the opening in which the directing wheels  18 ,  22  runs. The smaller diameter of 47 mm of the support wheels  14 ,  15 ,  36  is chosen as a compromise between limiting the dimensions of the extruded profile, which calls for smaller diameter, and minimizing the friction losses between wheels and track, which calls for larger diameter. At least the outer rim of the support wheels  14 ,  15 ,  36  and the directing wheels  18 ,  22  are suitably made from a plastics material of a suitable hardness, such as a hardness in the Shore A range, e.g. Shore 80-100 such as Shore 85-95. An example of such material is polyurethane Shore 90-92 plus/minus 3. It is preferred that the surface of the wheels  14 ,  15 ,  36 ,  18 ,  22 , primary the directing wheels  18 ,  22 , has a low friction coefficient, for which reason the plastics material may comprise about 15% by weight of polytetraflouroethylene.  
     [0056] The drive chain configuration for vertical half-turns as shown in FIGS.  8 - 9  has a directing wheel  18 ,  22  on each side of the vertical pivot pin  34  so that the chain may turn 180 degrees about a transversal axis in a vertical half-turn  4  and still operate in a main track as shown in FIGS.  3 - 5 . Furthermore, the longer links  32  has a second reaction plate  30  (not shown) mounted on the opposite side of the reaction plate  30  shown on FIGS.  8 - 9  for the same reason. The drive chain for purely horizontal operation it not equipped with these extra directing wheels  22  and reaction plates  30  and does not have the extra support wheel  36 .  
     [0057] The drive chains are designed for being flexible in the horizontal plane as well as in the vertical plane and having as long reaction plates  30  as possible to increase the efficiency of the linear drive by delimiting the number of air gaps between consecutive reaction plates  30 . The length of the reaction plates  30  is limited by the radius of curvature of the main track because the reaction plates  30  are stiff and move as chords to the curving path of the main track  2  and because the pivot axis of the directing wheels  18 ,  22  will be non-perpendicular to the curve of the main track during passage of a vertical half-turn  4  for which reason the directing wheels will broadside in their tracks. The broadsiding causes wear on directing wheels  18 ,  22  and tracks in the vertical half-turns  4  for which reason the surface of the wheels  18 ,  22  preferably are made from a low-friction material. The deviation from a right angle between the pivot axis of the directing wheels  18 ,  22  and the curve of the main track depends on the distance between two vertical pivot pins  34  of a longer link  32  and the radius of curvature of the main track section. The deviation should be less than 15 degrees and is in a preferred embodiment between 7 and 11 degrees, such as 9 degrees.  
     [0058] It is essential that the article-supporting surface  5  of the conveyor units  1  is kept horizontal during all operations of the conveyor including passage of vertical half-turns  4  so as to prevent conveyed articles from being accidentally dropped from the article-supporting surfaces  5 . The conveyor units  1  are connected to carrier means, the drive chain, with a horizontal pivot pin  35  about which the conveyor unit  1  may pivot. Each conveyor unit  1  is equipped with a first guide member or wheel  37  that engages with a first guiding means, a first track parallel to the main track when the conveyor unit  1  is moving on straight main track sections in order to prevent the conveyor unit  1  and thereby the article-supporting surface  5  from tilting. The first guide wheel  37  disengages with the first guide track when the conveyor unit  1  enters a vertical half-turn  4  and guide means for preventing the conveyor unit  1  from tilting during the passage of the vertical half-turn  4  of the main track  2  comes into action.  
     [0059] An embodiment having a first guide wheel  37  can be seen in FIGS.  10 - 12  and the first guide wheel  37  is also shown in the cross-sections in FIGS.  3 - 5 . A number of different embodiments of the guide means at vertical half-turns are shown in FIGS.  10 - 29 . The first guide wheel is arranged in the same horizontal plane as the support wheels  14 ,  15 ,  36  but with a horizontal distance in the transport direction of the conveyor to the pivot pin  35 . The first guide track is preferably displaced in the transverse direction of the conveyor relatively to the surface  13  on which the support wheels  15  engage so as to enable individual engagement of the two wheels  15 ,  37 .  
     [0060] One arrangement according to the invention for preserving a horizontal position of the article-supporting surface  5  of the conveyor units  1  at a vertically curving section  4  of the main track  2  is shown on FIGS.  10 - 12 .  
     [0061] The first guide wheel  37  of the conveyor section disengages with the first guide track when the conveyor unit  1  enters a vertical half-turn  4  of the main track  2  and a second guide wheel  38  engages with a second guide track  39 . The second guide wheel  38  and the second guide track  39  prevents, due to the design of the second guide track  39 , the conveyor unit  1  from tilting when it moves on vertical half-turns  4  of the main track  2 . The second guide wheel  38  disengages again with the second guide track  39  when the conveyor unit  1  leaves the vertical half-turns  4  of the main track section  2  and the first guide wheel  37  engages the fist guide track. Both guiding wheels  37 ,  39  are simultaneously engaging with their respective guide tracks over a short distance when the conveyor unit  1  moves into or out from the vertical half-turns  4  of the main track section  2  so as to ensure stability of the conveyor unit  1 .  
     [0062] A conveyor unit  1  according to the embodiment of the invention as shown in FIG. 10 is shown in FIGS. 11 and 12. The second guide wheel  38  and the second guide track  39  are arranged in a vertical plane parallel to the vertical plane of the carrier means  40  but displaced in the transverse direction so that the conveyor unit  1  passes between the two vertical planes when it moves on the vertical half-turns  4  of the main track  2 . With this arrangement, the path in the vertical plane of the second guide track  39  may cross the path of the curving main track section  4 .  
     [0063] The second guide wheel  18  is preferably mounted on the conveyor unit  1  with a resilient bar  41  as shown in FIG. 12 in order to obtain an improved stability of the conveyor system.  
     [0064] The article-supporting surface  10  is in the embodiment shown on FIG. 11 arranged in a horizontal plane below the main track  2  so that the articles that are discharged to that side pass under the main track  2 . An alternative embodiment where the article-supporting surface  5  is arranged in a horizontal plane above the main track  2  is shown on e.g. FIG. 16 which impose less restrictions on the height of the transported articles.  
     [0065] Another embodiment of the invention is shown in FIGS. 13 and 14. Instead of having a second guide wheel  38 , this embodiment has an arrangement with two wheels  42  arranged on a bar  43  that pivotally connected to the conveyor unit  1 . The two wheels  42  are engaging each side of a second guide track  39  and the wheels  42  are preferably resiliently biased towards the second guide track  39 .  
     [0066] Further modifications of the embodiment as shown in FIGS.  10 - 12  are shown in FIGS.  15 - 16  and FIGS.  17 - 18 . The embodiment shown in FIGS.  15 - 16  comprises a third guide wheel  44  and a third support track  45 .  
     [0067] The second and third guide wheels  38 ,  44  are positioned on the conveyor unit  1  so that they are not on line with the pivot pin  35 , the axis of the wheels  38 ,  44  and the pin  35  are rather forming an obtuse-angled triangle in a vertical plane, so as to prevent the conveyor unit  1  from having any positions along a vertical half-turn  4  of the main track  2  at which the unit  1  may rotate or tilt in a vertical plane. The embodiment shown in FIGS.  17 - 18  further comprises a fourth guide wheel  46  and a fourth guide track  47 , the fourth guide wheel  46  having substantially the same axis of rotation as the pivot pin  35  and the fourth guide track  47  follows the path of the main track  2  but in a vertical near the plane of the second and third guide tracks  39 ,  45 . The fourth guide wheel  46  substantially prevents the conveyor unit  1  from rotating or tilting in the plane of the tracks  2 ,  39 ,  45  during passage of a vertical half-turn  4  of the main track  2 , due so slack or clearance in the various wheels and tracks and/or in the carrier means  40 .  
     [0068] Yet another four embodiments of the invention are shown in FIGS.  19 - 20 , FIGS.  21 - 22 , FIGS.  23 - 24  and in FIGS.  25 - 26 . These embodiments all comprise a synchronizing means arranged in a vertical plane parallel to the vertical plane of the carrier means  40 . The synchronizing means comprises a plurality of coupling means  48 , such as, e.g. rods, adapted for engaging with a guide member  49 , such as, e.g. a member with a indentation, of each conveyor unit  1  when the conveyor unit  1  enters a vertical half-turn  4  of the main track  2 . The distance between immediately adjacent coupling means  48  is substantially equal to the distance between the guide members  49  of immediately adjacent conveyor units  1  and the path along which the synchronization means moves the coupling means  48  relative to the main track  2  adapts the arrangement for preventing the conveyor units  1  from tilting when they pass a vertical half-turn  4  of the main track  2 . The synchronization means are in all embodiments driven by the engagement with the conveyor units  1 . The embodiments further comprises a guide wheel  37  on each conveyor unit  1  which disengages with a guide track when the conveyor unit  1  enters a vertical half-turn  4  of the main track  2 .  
     [0069] One of these four embodiments of guide means at a vertical half-turn  4  of the main track  2  is shown in FIGS.  19 - 20 . The synchronization means comprises a disc member  50  arranged pivotally about a horizontal axis and the coupling means  44  are arranged on the disc member  50 . According to the second embodiment as shown in FIGS.  21 - 22 , the coupling means  48  comprises electromagnets  51  that are activated to couple with a magnetic component of the guide members  49  of each conveyor unit  1 . The magnetic component of the guide member  49  is arranged so that is may rotate about an axis  52  being parallel to the axis of the pivot pin  35 .  
     [0070] The embodiment shown on FIGS.  23 - 24  has a synchronization means comprising a non-moving part  53  with an outer surface  54  that defines a path or track for the synchronization wheels  55 . The synchronization wheels  55  each guides a coupling means  48  that is resiliently biased towards the surface  54  by a disc member  56  that also keeps the predetermined distance between the coupling means  48 .  
     [0071] The last of the four embodiments is shown in FIGS.  25 - 26 . The synchronization means of this embodiment is more flexible than the previous three and comprises a synchronization chain  57  on which the coupling means  48  are arranged. The synchronization chain  57  moves in a synchronization track  58  that may follow any path that the main track  2  may follow and does therefore not require that the vertical half-turn  4  of the main track  2  follows a semi-circular path.  
     [0072] A preferred embodiment of an arrangement for preventing the conveyor units  1  from turning or tilting when passing a vertical half-turn  4  of the main track  2  is shown in FIGS.  27 - 30 . The embodiment resembles the one shown in FIGS.  17 - 18  with the exception that it is not mandatory in the preferred embodiment that the fourth guide wheel  46  has substantially the same axis of rotation as the pivot pin  35 . The second  38 , third  44  and fourth guide wheels  46  are arranged in an obtuse-angled triangle in each their adjacent vertical planes as shown in FIG. 18 so that the respective guide tracks  39 ,  45 ,  47  may cross each other. The path of the guide tracks  39 ,  45 ,  47  is shown in FIG. 27. The guide tracks  39 ,  45 ,  47  are formed by rolling an aluminum bar  59  into the correct shape, fasten it to a support plate  60  and form the tracks  39 ,  45 ,  47  by milling the bar  59 . The bar with the tracks  39 ,  45 ,  47  is shown mounted together with the main track  2  at a vertical half-turn  4  of the main track  2  in FIGS. 28 and 29 as viewed from two sides.  
     [0073] The main track  2  at a vertical half-turn  4  of the main track  2  is formed from rolled profiles  61  shaped so that the drive chain is suitably supported when passing the half-turn  4 . The directing wheel  18 ,  22  of the drive chain being closest to the point of curvature is the one engaging the track. It has shown to be advantageous to obtain a low-friction contact between the directing wheel  18 ,  22  and the track at the vertical half-turn  4  because the wheel  18 ,  22  will broadside during the passage of the half-turn as described previously. A suitable way of decreasing the friction factor of the surface of the track is to provide it with a layer of polyamide such as polyamide  6  preferably by spraying it on the tracks. The milled profiles  61  forming the main track  2  are mounted on a support plate  62  as shown in FIGS.  28 - 29 .  
     [0074] Three conveyor units  1  are shown in FIGS.  30 - 32 , each being equipped with a cross-belt  63  forming the article-supporting surface  5  of the conveyor units  5 , a first guide wheel  37  and an opening  64  for engaging pivotally with the pivot pin  35 . The conveyor unit  1  shown in FIG. 30 is suitable for use in a system with vertical half-turns  4  of the main track  2  whereas the one shown in FIG. 31 is suited for a purely horizontal configuration. The part of the conveyor unit  1  carrying the cross-belt  63  is preferably hinged to a frame part of the conveyor unit  1  as shown in FIG. 32 so as to provide easy access to the interior of the frame part wherein the drive motor  65  for driving the cross-belt  63  is placed together with a control unit and an electric storage battery so that the power supply to the conveyor unit may be temporarily during its passage of the main track  2 .  
     [0075] A conveyor system having first guiding means comprising a toothed track  66  is shown in FIGS.  33 - 42 . The toothed track  66  comprises a plurality of teeth  67 , and interstices  68  are defined by respective side surfaces  69  of the teeth  67 . The first guiding member comprises a rotatable member  70  having four radially extending engagement members  71 , which can be seen in FIGS.  37 - 42 . Each of the radially extending engagement members  71  is arranged so that the engagement member  71  cooperates with the side surfaces  69  of the toothed track  66  as shown in FIG. 37. Thereby a force is provided resulting in a torque, which prevents the article supporting member from tilting and which ensures a substantial horizontal position of the article supporting surface  5  so as to prevent conveyed articles from being accidentally dropped from the article supporting surface  5 . As shown in FIGS. 38 and 42 a pin  72  extends from a link  73  of the drive chain perpendicular to the transport direction. The rotatable member  70  is adapted to be mounted on the pin  72 . Means are provided to fasten the rotatable member  70 , when the conveyor unit  1  is moved in a horizontal direction.