Abstract:
A workpiece carrier ( 10 ) for conveying one or more workpieces with a belt conveyor ( 12,18 ) is described, which reduces the production cycle time of an industrial manufacturing plant. The workpiece carrier ( 10 ) includes energy storage devices ( 24,32,38 ), into which energy of motion of the workpiece carrier ( 10 ) is transferred when the workpiece carrier is braked against or stopped on the belt conveyor ( 12,18 ). The energy storage devices each include a coupling element ( 28,36 ), advantageously a friction wheel  28  or contacting piece  36 , urged into continuous contact with the conveyor belt, and a spring device, advantageously a spiral spring  30  or a coil spring  34 , connected with the friction wheel and put under tension when the workpiece carrier ( 10 ) is braked. If the workpiece carrier is no longer braked or stopped, the spring device is released and delivers stored energy by driving the friction wheel or contacting piece, where by the workpiece carrier ( 10 ) is additionally accelerated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a workpiece carrier and, more particularly, to a workpiece carrier for transport by at least one belt conveyor, especially a double belt conveyor. 
     2. Related Art 
     A workpiece conveyor for conveying one or more workpieces by means of a belt conveyor, especially a double belt conveyor, is described in European Patent Document EP 37 361 B1. This sort of workpiece carrier is used in industrial assembly and manufacturing plants. The workpiece carrier or support is placed on the belt conveyor and is conveyed by a frictional connection between the belt conveyor and the workpiece carrier. These workpiece carriers are brought to a stop in front of transfer points to other belt conveyors or processing stations. This usually happens by means of a stopping device having a traveling holding element that can travel into the motion path of the workpiece carrier and thereby stop it. If the workpiece carrier is stopped, the belt conveyor slides past the workpiece carrier under the workpiece carrier. If the workpiece carrier is no longer braked, i.e. the holding element of the stopping device releases the workpiece carrier so that it can travel over the motion path. The workpiece carrier still requires a predetermined time interval in order to reach the processing station or the transfer point by means of the belt conveyor. Still more time can be lost by the motion of the belt conveyor under the workpiece carrier during travel. In any case the total machine cycle time for the concerned fabrication and assembly unit increases. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved workpiece carrier for a belt conveyor of the above-described kind, which does not suffer from the above-mentioned disadvantages. 
     According to the invention the workpiece carrier for conveying one or more workpieces with at least one belt conveyor comprises at least one energy storage means, which stores energy transferred to it when the workpiece carrier is stopped on or braked against the at least one belt conveyor and which releases the stored energy when the workpiece carrier is no longer braked or stopped on the at least one belt conveyor, whereby the workpiece carrier experiences an additional acceleration. 
     The workpiece carrier according to the invention has the advantage that during travel, i.e. when the workpiece carrier is no longer braked, less time is lost. 
     In an especially simple and advantageous embodiment of the invention a spiral spring is put under tension or a coil spring is compressed by means of the conveyor motion of the belt conveyor via a coupling element, advantageously a friction wheel or contacting piece, when the workpiece carrier is braked against or stopped on the belt conveyor. In another preferred embodiment the friction properties of the workpiece carrier on the conveyor belt may be optimized by means of a layer or coating with anisotropic friction properties. The amount of friction between the coupling element and the belt conveyor may be improved by a pressing spring element acting on the coupling element. If the pressing spring is dimensioned so that the workpiece carrier contacts on the belt conveyor only via the energy storing means, all undesirable friction may be eliminated. 
     Further features and advantages are described in the detailed description and claims appended hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiments, with reference to the accompanying figures in which: 
     FIG. 1 is a partially cutaway top plan view of a workpiece carrier on a belt conveyor, cutaway to show the energy storage means of the invention; 
     FIG. 2 is a side view of the device shown in FIG. 1 with a first embodiment of the energy storage means; 
     FIG. 3 is a side view of another embodiment of a workpiece carrier according to the invention with a second embodiment of the energy storage means; and 
     FIG. 4 is a side view of an additional embodiment of a workpiece carrier according to the invention with a third embodiment of the energy storage means. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A workpiece carrier  10  is shown in FIG. 1, which is placed on a first belt conveyor  12 , of which only one segment is shown. The belt conveyor  12  has two parallel extending belts  14  and thus is a so-called double belt conveyor. Alternatively the belt conveyor  12  could be a belt conveyor which has only one belt of a width such that the workpiece carrier  10  can be transported on it. The transport direction  16  of the belt conveyor  12  is shown with an arrow in FIG. 1. A second belt conveyor  18  is indicated at right angles to the first belt conveyor  12  with dashed lines in FIG.  1 . The workpiece carrier  10  can similarly be conveyed along the second belt conveyor  18 . A workpiece  20  is only shown symbolically on the workpiece carrier  10 . Also it can carry several workpieces. A stopping device  22  with a travelling holding element  23  is arranged upstream of the workpiece carrier  10  in the conveying direction  16 . The stopping device  22  acts to halt the work piece carrier  10 , which will be explained in more detail later. 
     Dashed boxes show several energy storage devices  24  that are attached to the workpiece carrier  10  in FIG.  1 . In the instant embodiment two energy storage means  24  are arranged one after the other on each belt  14 , so that they can cooperate with the respective belts  14  of the belt conveyor  12  or  18 . Also four energy storage means  24  are provided for each of the two conveying directions  16  at right angles to each other. Two energy storage means  24  arranged one after the other are spaced so that, when they are not in use, they do not prevent the transport of the workpiece carrier  10  on the respective belt conveyor  12 ,  18 , i.e. the outer spacing of two of the energy storage means  24  arranged behind each other which are used on the belt conveyor  12  is less than the smallest distance between the belts  14  of the belt conveyor  18 , and vice versa. In any case it is important that the workpiece carrier  10  has at least one energy storage means  24 . It is however advantageous when at least two energy storage means  24  are provided for each transport direction  16  for the workpiece carrier  10 . Herein an energy storage means  24  is provided for each belt  14  of the belt conveyor  12 ,  18 . 
     FIG. 2 the workpiece carrier is cutaway to show a cavity  26 . A first possible embodiment of an energy storage means  24  is arranged in this cavity  26  and shown in simplified form in FIG.  2 . The energy storage means  24  has a rotatable friction wheel  28  operating as a coupling element, which is effectively connected with the belt conveyor  12 ,  18 . The energy storage means  24  has a spring device comprising a spiral spring  30 , which is in effective connection with the friction wheel  28 , i.e. the spiral spring  30  is tensioned by means of the friction wheel  28 . A pressing spring  31  is arranged in the workpiece carrier. It is connected with a central part of the spiral spring  30  so that the energy storage means  24  is pressed on the belt conveyor  12 ,  18  by the pressing spring, whereby the friction between the friction wheel  28  and the respective belt  14  is increased. 
     The workpiece carrier  10  is shown in the stopped position on the continuously travelling belt conveyor  12  because of the action of the holding element  23  of the stopping device  22 , which extends into the motion path of the workpiece carrier  10 . The holding element  23  travels below the conveyor plane in which the belt  14  of the belt conveyor  12 ,  18  moves by means of pneumatic operation of the stopping device  22 , so that the workpiece carrier  10  travels further. 
     In normal operation the workpiece carrier  10  for transport by the conveyor belt  12 ,  18  has the same speed as the belt conveyor belt  12 , 18 . A workpiece carrier  10  is halted, for example by the holding element  23  of the stopping device  22 , in front of or immediately upstream of a work station or processing station or a transfer point at which the workpiece carrier  10  is transferred, for example from the belt conveyor  12  to the belt conveyor  18 . This is necessary when an additional workpiece carrier  10  is immediately found in a processing station or at a transition point from one belt conveyor  12  to another belt conveyor  18 . Thus several workpiece carriers  10  can be found one after the other on a belt conveyor  12 . If the processing station or the transition point is again free, the next workpiece carrier  10  can arrive. The time that is required for the workpiece carrier  10  to arrive in the work or processing station is however lost, since no processing can occur in this time, whereby the product cycle time of the entire manufacturing operation increases. 
     When the workpiece carrier  10  according to the invention is located immediately in a waiting position, i.e. its travel on the belt conveyors  12 ,  18  is halted, so that the speed of the workpiece carrier  10  is zero. The speed of the belt  14  of the belt conveyor  12 ,  18  however is not changed. When the belt  14  moves in the transport or conveying direction  16  indicated in FIG. 2, i.e. to the right in the drawing, the friction wheel  28  is rotated by the conveying motion in a counterclockwise direction. The spiral spring  30  is put under tension because of this motion, so that it receives energy. So that the spiral spring  30  is not put under too much tension by rotation of the friction wheel  28 , the friction wheel  28  is pressed by the pressing spring  31  against the belt  14 . When the workpiece carrier  10  is released by the holding element  23 , i.e. when the motion of the workpiece carrier is no longer braked, the spiral spring  30  can loose its tension, thus giving up some of the energy that it received. In FIG. 2 one sees that the friction wheel rotates in a clockwise sense because of that. The workpiece carrier  10  thus experiences an additional acceleration in the conveying direction  16 . Preferably this additional acceleration is large enough so that the speed of the workpiece carrier  10  is larger than that of the belt conveyor  12 . In this way the cycle time of the manufacturing operation or plant can be strongly reduced. The principle of reduction of the operating time or cycle time is thus based on the fact that energy is transferable into the workpiece carrier from a workpiece carrier braked relative to the belt conveyor. In a workpiece carrier  10  no longer braked relative to the belt conveyor  12 ,  18  the received energy is again released so that the workpiece carrier  10  experiences the additional acceleration. The transfer of the energy occurs by means of the conveyor motion of the belt conveyor  12 ,  18 . The friction wheel  28  operates as a coupling element, the spiral spring as the energy storage means. 
     A transfer of energy occurs in this case not only in a workpiece carrier  10  whose speed is zero. As soon as the speed of the workpiece carrier  10  drops below that of the belt conveyor  12 ,  18 , energy is transferred into the energy storage means  24 . A reduced speed can occur when several workpiece carriers  10  are blocked one after the other on the belt. It is also possible for energy to be transferred into an energy storage device  24  in another form when the workpiece carrier  10  is halted. This occurs by means of the conveyor motion of the belt conveyor  12 , 18 . This can occur, for example, by means of additional devices arranged beside or below the belt conveyor  12 ,  18 , which build up the tension in a spiral spring  30  by a suitable mechanism. This received energy can also be delivered in the other form. It would also be possible to provide a separate block, in which a coupling element is braced, in order to deliver energy from the energy storage means to the block. The transfer of energy by the conveying motion of the belt conveyor  12 ,  18  is however the simplest method. 
     A second embodiment of a workpiece carrier  10  is shown in FIG. 3 including a second embodiment of the energy storage means  32 . This energy storage means  32  is accommodated in a cavity  26  provided in the workpiece carrie  10 . The energy storage means  32  comprises a coil spring  34  acting essentially parallel to the belt  14  and a contacting piece  36  acting as a coupling element and connected with the coil spring  34  arranged in a receptacle  33 . The receptacle  33  is open on its side facing the belt  14  and is pressed by means of a pressing or compressed spring  31  against the belt conveyor  12 ,  18 . The receptacle  33  is arranged so that it is movable essentially perpendicular to the conveying direction  16  of the belt conveyor  12 ,  18 . If the workpiece carrier  10  is braked against the belt conveyor  12 ,  18 , the contacting piece  36  is pushed by the belt  14  in the direction of conveying  16 . Because of that the coil spring  34  is compressed, i.e. it stores energy. If the workpiece carrier  10  is no longer braked, the coil spring  34  is released from tension and gives up the energy it received, whereby the workpiece carrier  10  is accelerated relative to the belt conveyor  12 ,  18 . 
     An elastic layer extends or projects from the workpiece carrier  10 , which cooperates with the belt conveyor  12 , 18 . This elastic layer or coating comprises bristles  38  in the embodiment shown. Because of motion of the belt conveyor  12 , 18  in the conveying direction  16  of the belt conveyor  12 ,  18  the bristles  38  are oriented at an angle α, which is less than 90°. Thus the layer or coating provided between the workpiece carrier  10  and the belt conveyor  12 ,  18  by the bristles  38  has a first friction coefficient μ 1  at a speed of the workpiece carrier  10  which is less than the speed of the belt conveyor  12 ,  18 . The ends of the bristles are bent toward the direction of transport  16  because of the higher speed of the belts  14  relative to the workpiece carrier  10 . If the workpiece carrier  10  is released or no longer braked, the bristles  38  bend back into their initial positions because of their elastic properties and the workpiece carrier  10  experiences an additional acceleration. Ideally the speed of the workpiece carrier  10  is raised relative to the speed of the belt conveyor  12 ,  18 . That means that the layer or coating, e.g. provided by bristles  38 , has a second friction coefficient μ 2  that is less than the first friction coefficient μ 1  when the workpiece carrier speed of the workpiece carrier  10  is higher than the speed of the belt conveyor  12 ,  18 . Thus the workpiece carrier essentially has anisotropic friction properties in this situation. Ideally the bristles  38  however are used in combination with the energy storage means  24  or  32 . The friction between the workpiece carrier  10  and the belt  14  of the belt conveyor  12 ,  18  acts oppositely to an acceleration caused by the energy storage means  24 , 32  when the workpiece carrier  10  is released or when it is no longer braked. Use of the bristles  38  reduces the friction reducing the acceleration. 
     The invention avoids the friction problems in the simplest possible manner because the workpiece carrier  10  is in contact with the belts  14  of the belt conveyor  12 , 18  only by means of a friction wheel  28  of the energy storage means  24 . The sum of the friction forces of the pressing springs  31  of the four energy storage means  24  provided for each transport direction  16  must be greater than the operational weight of the workpiece carrier  10  including that of the workpiece carrier being conveyed in operation. 
     The disclosure in German Patent Application 198 26 863.7-22 of Jun. 17, 1998 is incorporated here by reference. This German Patent Application describes the invention described hereinabove and claimed in the claims appended hereinbelow and provides the basis for a claim of priority for the instant invention under 35 U.S.C. 119. 
     While the invention has been illustrated and described as embodied in a workpiece carrier, especially for a belt conveyor, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 
     What is claimed is new and is set forth in the following appended claims.