Abstract:
The platform truck apparatus is provided with: a narrow platform truck for transporting a car body; a left sub-platform truck, which is attached along the left side of the narrow platform truck and is pulled by the narrow platform truck; a left parts shelf carried on the left sub-platform truck; a right sub-platform truck, which is attached along the right side of the narrow platform truck and is pulled by the narrow platform truck; and a right parts shelf carried on the right sub-platform truck. Because the left sub-platform truck is moved backward relative to the narrow platform truck when a car body is loaded onto the narrow platform truck, the left parts shelf does not interfere with the loading of the car body.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method of transferring a workpiece, typically, a vehicle body, an apparatus for transporting the workpiece, and a cart apparatus for use of the workpiece transfer method and the workpiece transportation apparatus (platform truck apparatus, workpiece transfer method, and workpiece conveyor equipment). 
       BACKGROUND ART 
       [0002]    In production lines of vehicles, vehicle bodies are transported using carts, and parts are assembled to the vehicle bodies which are being transported, manually or using machines. A method of transporting a vehicle body for this purpose is known, e.g., in Patent Document 1. 
         [0003]      FIG. 19  shows a vehicle production line disclosed in Patent Document 1. Referring to  FIG. 19 , a plurality of carts  202  are provided in a closed first transportation path  201 . A vehicle body  203  is placed on each of the carts  202 . 
         [0004]    In the drawing, at a point P 1 , serving carts  204 ,  205  for supplying (distributing) parts are coupled to front and rear portions of the cart  202 . Parts to be assembled to the vehicle body  203  are placed on these serving carts  204 ,  205 . During movement of the cart  202  from the point P 1  to a point P 2 , the parts are assembled to the vehicle body  203 . After the service carts  204 ,  205  become vacant, at the point P 2 , the service carts  204 ,  205  are removed from the cart  202 . 
         [0005]    At a point P 3 , the vehicle body  203  is picked up from the cart  202  using a workpiece transfer apparatus  216  such as a lifting apparatus. After the cart  202  becomes vacant, the cart  202  moves along the first transportation path  201  counterclockwise in the drawing, and returns to the point P 1 . Some of the parts are partially assembled in the first transportation path  201 , and the remaining parts are assembled in a second transportation path  211 . 
         [0006]    A plurality of carts  212  are provided also in the second transportation path  211 . At a point P 4 , serving carts  214 ,  215  are coupled to front and rear portions of the cart  212 . Then, at the point P 4 , the vehicle body  203  is placed on the cart  212  by the workpiece transfer apparatus  216 . During movement from the point P 4  to a point P 5 , parts (remaining parts) are assembled to the vehicle body  203 . 
         [0007]    That is, the parts from the serving cart  214  are assembled to the front portion of the vehicle body  203 , and the parts from the serving cart  215  are assembled to the rear portion of the vehicle body  203 . Since the center of the vehicle body  203  is far from the serving carts  214 ,  215 , the flow line of the operator becomes long. Therefore, while there is a need to improve the productivity, in the case of assembling parts at the center of the vehicle body  203 , it is desirable to shorten the flow line of the operator. 
         [0008]    Further, each of the carts  202 ,  212  is equipped with a drive source, and can travel by itself using the own drive source. At the point P 3 , in the state where the cart  102  is stopped, the vehicle body  203  is picked up therefrom. At the point P 4 , in the state where the cart  212  is stopped, the vehicle body  203  is placed on the cart  212 . In the meanwhile, the other carts  202 ,  212  move at low speed. In order to avoid collisions between the carts  202 ,  212 , a certain interval needs to be kept between the adjacent carts  202 , and likewise, a certain interval needs to be kept between the adjacent carts  212 . 
         [0009]    For this purpose, in order to improve the productivity for certain lengths of the transportation paths  201 ,  211 , it is desirable to position the carts  202 ,  212  more densely. 
       PRIOR ART REFERENCE 
     Patent Document 
     Patent Document 1: 
     International Publication WO2013/042511 
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
       [0010]    An object of the present invention is to provide a cart apparatus, a workpiece transfer method, and a workpiece transportation apparatus in which, in the case of assembling parts at the center of a workpiece such as a vehicle body, it is possible to shorten the flow line of an operator, and it is possible to densely position carts in a transportation path. 
         [0011]    In the invention described in claim  1 , a cart apparatus for transporting a workpiece is provided. The cart apparatus includes a cart configured to travel on a closed transportation path, for transporting the workpiece placed on the cart, a left sub-cart provided along a left side of the cart, and towed by the cart, a left parts shelf placed on the left sub-cart, a right sub-cart provided along a right side of the cart, and towed by the cart, and a right parts shelf placed on the right sub-cart. The left parts shelf is transferred from outside of the closed transportation path to the left sub-cart, and the right parts shelf is transferred from the outside of the closed transportation path to the right sub-cart. 
         [0012]    In claim  2 , preferably, at least one of the left sub-cart and the right sub-cart is attached to the cart movably in a travel direction. 
         [0013]    In claim  3 , preferably, each of the left parts shelf and the right parts shelf has a caster, upper surfaces of the left sub-cart and right sub-cart are at same level with an upper surface of the cart, and the cart includes a guide rail configured to guide the left parts shelf when the left parts shelf moves toward the right sub-cart or guide the right parts shelf when the right parts shelf moves toward the left sub-cart. 
         [0014]    In the invention described in claim  4 , a method of transferring a workpiece by loading the workpiece on or unloading the workpiece from a cart configured to travel on a closed transportation path is provided. The method includes the step of using a cart apparatus including the cart, a left sub-cart provided along a left side of the cart and towed by the cart, a left parts shelf placed on the left sub-cart, a right sub-cart provided along a right side of the cart and towed by the cart, a right parts shelf placed on the right sub-cart. The left sub-cart is attached to the cart movably in a travel direction. Further, the method further includes the step of, at time of transferring the workpiece, moving a sub-cart positioned outside the transportation path backward to load the workpiece on or unload the workpiece from the cart, from outside of the transportation path. 
         [0015]    In claim  5 , preferably, at the time of replacing a parts shelf positioned inside the transportation path, among the left parts shelf and the right parts shelf, the left sub-cart is positioned on a left side of the cart and the right sub-cart is positioned on a right side of the cart, and the parts shelf positioned inside the transportation path is moved to the sub-cart positioned outside the transportation path via the cart. 
         [0016]    In the invention described in claim  6 , a method of transporting a workpiece by a cart configured to move along a closed transportation path is provided. The cart is moved by a drive source positioned outside the cart. The transportation path includes a first low speed movement path, a first high speed movement path extending from the first low speed movement path and being configured to move the cart at high speed in comparison with the first low speed movement path, a second low speed movement path extending from the first high speed movement path and being configured to move the cart at low speed in comparison with the first high speed movement path, and a second high speed movement path extending from the second low speed movement path to the first low speed movement path and being configured to move the cart at high speed in comparison with the second low speed movement path. A first workpiece transfer apparatus extends to the first high speed movement path and is configured to transfer the workpiece from outside of the transportation path to the cart, and a second workpiece transfer apparatus extends to the second high speed movement path and is configured to transfer the workpiece on the cart to the outside of the transportation path. The method includes the steps of, in the first low speed movement path, arranging a plurality of the carts in series to contact one another, and placing parts to be attached to the workpiece, on the cart, in the first high speed movement path, allowing one of the carts to travel at high speed, and stop at one intermediate position, for loading the workpiece on the cart by the first workpiece transfer apparatus, in the second low speed movement path, arranging a plurality of the carts in series to contact one another, and attaching the parts to the workpiece, and in the second high speed movement path, allowing one of the carts to travel at high speed, and stop at one intermediate position, for moving the workpiece on the cart to the outside of the transportation path, by the second workpiece transfer apparatus. 
         [0017]    In the invention described in claim  7 , a workpiece transportation apparatus for transporting a workpiece by a cart is provided. The workpiece transportation apparatus includes a drive source provided outside the cart and configured to move the cart, a transportation path, and a control unit configured to control the drive source. The transport path includes a first low speed movement path configured to allow the cart to move at low speed to perform operation of placing parts to be attached to the workpiece, on the cart, a first high speed movement path extending from the first low speed movement path and being configured to allow the cart to move at high speed in comparison with the first low speed movement path, a second low speed movement path extending from the first high speed movement path and keeping movement of the cart at low speed in comparison with the first high speed movement path to perform operation of attaching the parts to the workpiece, and a second high speed movement path extending from the second low speed movement path to the first low speed movement path and being configured to allow the cart to move at high speed in comparison with the second low speed movement path. The drive source is controlled by the control unit to: in the first low speed movement path and the second low speed movement path, arrange a plurality of the carts in series to contact one another, in the first high speed movement path, allow one of the carts to travel at high speed, and stop at one intermediate position, and in the second high speed movement path, allow one of the carts to travel at high speed, and stop at one intermediate position. 
       Advantages of the Invention 
       [0018]    In the invention according to claim  1 , the workpiece is placed on the cart. The left parts shelf is provided along the left side of the cart, and the right parts shelf is provided along the right side of the cart. Since the distance from the center of the workpiece to the left parts shelf and the right parts shelf becomes short, it is possible to shorten the flow line of the operator. Further, parts are supplied only from the outside of the transportation path. Since it is not necessary for the operator to enter the inside of the closed transportation path, it becomes possible to supply and/or distribute the parts very easily, and further improvement in the operating efficiency is achieved. 
         [0019]    In claim  1 , in the case where a tall parts shelf is provided on a side of the cart, it is necessary to lift the workpiece to a high position to avoid contact between the workpiece and the parts shelf when the workpiece is transferred from the cart. In such a situation, it takes considerable time for the transfer operation, and the productivity may be adversely affected. Thus, in the invention according to claim  2 , the parts shelf can be retracted at the time of the transfer operation. As a result, it becomes possible to perform the transfer operation without significantly lifting the workpiece. Therefore, the time required for the transfer is reduced, and hence, it is possible to improve the productivity. 
         [0020]    In the invention according to claim  3 , it is possible to easily move the parts shelf positioned inside the closed transportation path to the outside of the transportation path via the cart. In the case where the parts shelf is positioned outside the transportation path, for example, replacement of the parts shelf can be performed easily, and improvement in the workability is achieved. 
         [0021]    In the invention according to claim  4 , the parts shelf can be retracted at the time of the transfer operation. As a result, it becomes possible to perform the transfer operation without significantly lifting the workpiece. Therefore, the time required for the transfer is reduced, and hence, it is possible to improve the productivity. 
         [0022]    In the invention according to claim  5 , it is possible to easily move the parts shelf positioned inside the closed transportation path to the outside of the transportation path via the cart. In the case where the parts shelf is positioned outside the transportation path, for example, replacement of the parts shelf can be performed easily, and improvement in the workability is achieved. 
         [0023]    In the invention according to claim  6 , the cart is stopped in the first high speed movement path, and the workpiece is loaded on this cart from the outside, the cart is stopped in the second high speed movement path, and the workpiece is moved to the outside from this cart. In the first and second low speed movement paths, the carts move forward continuously without any stop. Therefore, the carts can be positioned in series densely in the first and second low speed movement paths. 
         [0024]    In the invention according to claim  7 , the first low speed movement path, the first high speed movement path, the second low speed movement path, and the second high speed movement path form the closed transportation path. In the first low speed movement path, the parts are placed on the cart, in the first high speed movement path, the workpiece is placed on the cart, in the second low speed movement path, the parts are assembled to the workpiece, and in the second high speed movement path, the workpiece is taken out from the cart. In the first and second low speed movement paths, the carts move forward continuously without any stop. Therefore, the carts can be positioned in series densely in the first and second low speed movement paths. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a perspective view of a cart including a cart apparatus according to the present invention; 
           [0026]      FIG. 2  is a plan view of the cart shown in  FIG. 1 ;  FIG. 3  is a plan view of a circular cart shown in  FIG. 2 ; 
           [0027]      FIG. 4  is a view shown in a direction indicated by an arrow  4  in  FIG. 3 ; 
           [0028]      FIG. 5  is a plan view of a constricted cart shown in  FIG. 2 ; 
           [0029]      FIG. 6  is a view shown in a direction of a line  6 - 6  in  FIG. 5 ; 
           [0030]      FIG. 7  is a diagram showing a closed transportation channel; 
           [0031]      FIG. 8  is a view showing third and fourth parts shelves shown in  FIG. 1 ; 
           [0032]      FIG. 9  is a view showing the third and fourth parts shelves shown in  FIG. 5 , viewed in a direction indicated by an arrow  9 ; 
           [0033]      FIG. 10  is a view showing the relationship between the constricted cart and the third parts shelf; 
           [0034]      FIG. 11  is a view cut along a line  11 - 11  in  FIG. 10 ; 
           [0035]      FIG. 12  is an enlarged view of a portion  12  in  FIG. 10 ; 
           [0036]      FIG. 13  is a view showing a procedure of loading a right parts shelf as the fourth parts shelf on a right sub-cart; 
           [0037]      FIG. 14  is a view showing a procedure of loading a left parts shelf as the third shelve on a left sub-cart; 
           [0038]      FIG. 15  is a view showing movement of the left-sub cart; 
           [0039]      FIG. 16  is a view showing a step of moving a workpiece and returning of the left sub-cart; 
           [0040]      FIG. 17  is a view showing the procedure of assembling parts to the workpiece; 
           [0041]      FIG. 18  is a view showing a modified example of the left sub-cart; and 
           [0042]      FIG. 19  is a diagram showing a conventional vehicle production line. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0043]    Hereinafter preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, “Fr” stands for “Front”, and “Rr” stands for “Rear”. 
       EMBODIMENTS 
       [0044]    Referring to  FIGS. 1 to 4 , a cart  10  includes a cart apparatus  20  and a circular cart  110  provided on a front side of the cart apparatus  20 . A first parts shelf  111  and a second parts shelf  112  are placed on the circular cart  110 . 
         [0045]    The cart apparatus  20  includes a constricted cart  30 , a left sub-cart  90 , a third parts shelf (left parts shelf)  85 , a right sub-cart  100 , and a fourth parts shelf (right parts shelf)  80 . The constricted cart  30  transports a vehicle body  11  as a workpiece, and the central portion of the constricted cart  30  is constricted in a curved shape. The left sub-cart  90  is provided along the left side of the constricted cart  30 , and towed by the constricted cart  30 . The third parts shelf  85  is placed on the left sub-cart  90 . The right sub-cart  100  is provided along the right side of the constricted cart  30 , and towed by the constricted cart  30 . The fourth parts shelf  80  is placed on the right sub cart  100 . The left and right sub-carts  90 ,  100  are attached to the left and right sides of the constricted cart  30 , respectively. The upper surface of the left right sub-cart  90 ,  100  and the upper surface of the constricted cart  30  are at the same level (height). A reinforcement ring plate  113  is provided on a lower surface of a table of the circular cart  110 . 
         [0046]    As shown in  FIG. 4 , the circular cart  110  includes a framework  121  formed by combining steel members, and a plurality of free wheels  123 . The circular cart  110  is a vehicle which can freely travel on a factory floor  13 . The circular cart  110  includes a post  124  and a center pin  125  at its central position. The post  124  protrudes upward, and the center pin  125  extends downward. A plurality of brackets  126  protrude downward from the framework  121 . Each of the brackets  126  has a free roll  127  which rotates horizontally. A plurality of rails  114  in the form of grooves are provided on the upper surface of the factory floor  13 . The free rolls  127  are guided by the rails  114  for allowing the circular cart  110  to travel in a direction toward the backside, or toward the front side of the drawing sheet of  FIG. 4 . It should be noted that, since the circular cart  110  is not equipped with any drive source, the circular cart  110  cannot travel by itself. 
         [0047]    Though a wooden floor  122  of the circular cart  110  is reinforced by the reinforcement ring plate  113  (see  FIG. 1 ), the wooden floor  122  is overhung by a distance “a”. 
         [0048]    As shown in  FIGS. 5 and 6 , the constricted cart  30  includes a framework  31  formed by combining steel members and a wooden floor  32  attached to the upper surface of the framework  31 . The framework  31  includes a cross member  33 . Columns  34  are provided to stand upright using this cross member  33 . Receivers  35  for receiving the vehicle body and positioning pins  36  for horizontally positioning the vehicle body are provided at these columns  34 . Guide rails  40  are provided between the columns  34 . The guide rails  40  protrude toward each other. 
         [0049]    A front pole  37  extends forward from the framework  31 . A hole  38  is provided at the front end of the front pole  37 , and the center pin  125  ( FIG. 4 ) extending downward from the central position of the circular cart  110  is fitted into this hole  38 . Further, a rear pole  39  extends backward from the framework  31 . A bifurcated portion  41  is provided at the rear end of this rear pole  39 , and the center pin  125  is fitted to the bifurcated portion  41 . At the time of forward movement (Fr), the center pin  125  pushes the bifurcated portion  41 . Further, when the forward movement speed of the bifurcated portion  41  is larger than the forward movement speed of the center pin  125 , the bifurcated portion  41  is removed from the center pin  125 . 
         [0050]    Pinch rolls  15  as drive sources are provided on the factory floor  13 . The pinch rolls  15  are rotated by motors, etc., respectively. The front pole  37  and the rear pole  39  are sandwiched between the pinch rolls  15 , and forward movement of the constricted cart  30  is actuated to move the constricted cart  30 . When the pinch rolls  15  are stopped, movement of the constricted cart  30  is stopped. The low speed positive rotation, stop, and high speed positive rotation of the pinch rolls  15  are controlled altogether by a control unit  17 . 
         [0051]    The front pole  37  extends to the center of circular cart positioned on the front side, and the rear pole  39  extends to the center of the circular cart positioned on the backside. Therefore, both of the front pole  37  and the rear pole  39  are elongated members. Since the front pole  37  and the rear pole  39  are elongated members, the front pinch rolls  15  and the rear pinch rolls  15  can be positioned remotely, and it is possible to reduce the number of drive sources. 
         [0052]    As shown in  FIG. 6 , a plurality of free wheels  42  are attached to the framework  31  for allowing the constricted cart  30  to travel on the factory floor  13 . As shown in  FIG. 5 , the framework  31  extends from the wooden floor  32  in the forward and backward travel directions of the constricted cart  30  by the distance “b”. 
         [0053]    As shown in  FIG. 2 , the center pin  125  shown in  FIG. 4  is fitted in the hole  38  formed at the front end of the front pole  37 . Thus, the cart  10  takes its final form shown in  FIG. 2 . In the normal state, the circular cart  110  is not removed from the constricted cart  30 . 
         [0054]    Next, a specific example of the above workpiece transportation apparatus using the cart  10  including the constricted cart  30  will be described. 
         [0055]    As shown in  FIG. 7 , a workpiece transportation apparatus  60  includes drive sources (e.g., pinch rolls  15  shown in  FIG. 5 ), a closed transportation path  65 , and the control unit  17 . The drive sources are positioned outside the carts  10 , and move the carts  10 . The closed transportation path  65  includes a first low speed movement path  61 , a first high speed movement path  62 , a second low speed movement path  63 , and a second high speed movement path  64 . In the first low speed movement path  61 , the carts  10  move at low speed for placing parts to be attached to a workpiece (e.g., vehicle body  11  shown in  FIG. 1 ), on the carts  10 . The first high speed movement path  62  extends from the first low speed movement path  61 . In the first high speed movement path  62 , the carts  10  move at high speed in comparison with the first low speed movement path  61 . The second low speed movement path  63  extends from this first high speed movement path  62 . In the second low speed movement path  63 , the carts  10  keep their movement at low speed in comparison with the first high speed movement path  62  for attaching the parts to the workpiece. The second high speed movement path  64  extends from the second low speed movement path  63  to the first low speed movement path  61 . In the second high speed movement path  64 , the carts  10  move at high speed in comparison with the second low speed movement path  63 . 
         [0056]    In the drawing, reference numerals  22  denote joint portions provided in the first high speed movement path  62  and/or the second high speed movement path  64 . Extension paths  23  extend from the joint portions  22  to the outside of the transportation path  65 , up to cart buffers  24  where spare carts  10  are present. Therefore, in the event that a failure occurs in any of the carts  10 , the cart  10  having the failure is guided from the joint portion  22  to the extension path  23 , and the cart  10  is retreated (turned out) to the cart buffer  24 . In return, a spare cart  10  is sent back from the cart buffer  24  to the transportation path  65 . By providing the joint portion  22 , the cart  10  having the failure can be replaced with the spare cart  10  efficiently. 
         [0057]    In terms of speed range, the workpiece transportation apparatus  60  is divided into a first low speed area  66  chiefly made up of the first low speed movement path  61 , a first high speed area  67  chiefly made up of the first high speed movement path  62 , a second low speed area  68  chiefly made up of the second low speed movement path  63 , and a second high speed area  69  chiefly made up of the second high speed movement path  64 . 
         [0058]    Further, the first low speed area  66  includes a parts supply area  71 . In the first high speed area  67 , a first workpiece transfer apparatus  72  extends from the outside, and a front end of the first workpiece transfer apparatus  72  is overlapped with the first high speed movement path  62  in a plan view. The second low speed area  68  includes a work area  73  for assembling parts. In the second high speed area  69 , a second workpiece transfer apparatus  74  extends from the outside, and a front end of the second workpiece transfer apparatus  74  is overlapped with the second high speed movement path  64  in a plan view. 
         [0059]    Each of the first workpiece transfer apparatus  72  and the second workpiece transfer apparatus  74  includes a lift hanger  75  for catching the vehicle body  11  to move it up/down, and a ceiling rail  76  for guiding the lift hanger  75  in a direction perpendicular to the first high speed movement path  62  or the second high speed movement path  64 . 
         [0060]    Under the control of the control unit  17 , in the first low speed movement path  61  and the second low speed movement path  63 , a plurality of carts  10  are arranged in series to contact one another, in the first high speed movement path  62 , one cart  10  travels at high speed, and the cart  10  is stopped at one intermediate position (position overlapped with the first workpiece transfer apparatus  72 ), and in the second high speed movement path  64 , one cart  10  travels at high speed, and the cart  10  is stopped at one intermediate position (position overlapped with the second workpiece transfer apparatus  74 ). The travel direction of the cart  10  is determined by the circular cart  110  shown in  FIG. 5 . The following constricted cart  30  follows the circular cart  110 . Therefore, the cart  10  is guided along the rails  114  (see  FIG. 4 ) to make turns, or move straight. 
         [0061]    Movement of the cart  10  will be described again using notations P 1  to P 7  shown in  FIG. 7 . 
         [0062]    A group of carts  10  moves at low speed between the points P 1  and P 2 . The carts  10  are coupled in series densely between the points P 1  and P 2 . While the carts  10  move at low speed from the point P 1  to the point P 2 , parts are supplied to the first to fourth parts shelves  85 ,  80 ,  111 , and  112 . Instead of placing the parts on the first to fourth parts shelves  85 ,  80 ,  111 , and  112 , the parts may be supplied to the carts  10  by replacing the parts shelves  80 ,  85 ,  111 , and  112  with other parts shelves. 
         [0063]    At the point P 2 , only the speed of the leading cart  10  is switched from the low speed to the high speed. In this manner, only one cart  10  arrives at the point P 3  in a short period of time, and the cart  10  is stopped at the point P 3 . The first workpiece transfer apparatus  72  loads the vehicle body  11  on the cart  10 . Next, the cart  10  moves to the point P 4  at high speed. Since the preceding cart  10  at the point P 4  moves at low speed, as shown by an imaginary line, the next following cart  10  catches up with this preceding cart  10 . Then, a center pin  25  of the next following cart  10  is fitted into the bifurcated portion  41  of the rear pole  39  of the preceding cart  10 . Thereafter, the carts  10  move together at low speed. Consequently, a plurality of the carts  10  are coupled together, and arranged densely between the point P 5  and the point P 6 . In the work area  73 , parts are assembled to the vehicle body  11 . 
         [0064]    It should be noted that, in the work area  73 , so called cell production is performed. The “cell production” herein means a production system where parts shelves are provided around the workpiece. In this state, assembling operation is performed from the start to the end. However, normal production may be performed in the work area  73 . The “normal production” herein means a production system where parts are picked up from parts shelves placed on the floor, and assembled to a workpiece which is being transported. 
         [0065]    In  FIG. 7 , though the second low speed movement path  63  in the work area  73  has a U-shape, for the purpose of increasing the length of the transportation path, the second low speed movement path  63  may have a W shape, or may have more serpentine curves. Likewise, though the illustrated first low speed movement path  61  in the parts supply area  71  has a U-shape, for the purpose of increasing the length of the transportation path, the first low speed movement path  61  may have a W shape, or may have a larger number of serpentine curves. That is, the length of the transportation path of the second low speed movement path  63  should be determined based on the number of operation steps, and the length of the transportation path of the first low speed movement path  61  should be determined based on the number of assembling parts. 
         [0066]    Further, in the work area  73 , an operator rides on the circular cart  110  shown in  FIG. 1 , and assembles parts on the first parts shelf  111  to the front portion of the vehicle body  11 . Parts stored on the second parts shelf  112  are assembled to the rear portion of a preceding vehicle body. The operator rides on the constricted cart  30 , and assembles parts on the third parts shelf  85  to the left side of the vehicle body  11 , and assembles parts on the fourth parts shelf  80  to the right side of the vehicle body  11 . 
         [0067]    The walking surface of the circular cart  110  and the walking surface of the constricted cart  30  are wooden floor surfaces. The wooden floor has a significantly large frictional resistance coefficient in comparison with the normal steel floor. Therefore, if the wooden floor surface is adopted, even in the case where oil or grease is adhered to the floor surface, the operator does not slip easily. 
         [0068]    At or before the point P 6  in  FIG. 7 , operation of assembling parts to the vehicle body has been finished. Therefore, at the point P 6 , the speed of only the preceding cart  10  is switched from low speed to high speed. Then, only one cart  10  arrives at the point P 7  in a short period of time, and the cart  10  is stopped at the point P 7 . By the second workpiece transfer apparatus  74 , the vehicle body  11  is picked up from the preceding cart  10 . Then, the preceding vacant cart  10  moves to the point P 1  at high speed. At the point P 1 , since the preceding cart  10  travels at low speed, the following cart  10  catches up with this preceding cart  10 . Then, the center pin  25  of the next following cart  10  is fitted into the bifurcated portion  41  of the rear pole  39  of the preceding cart  10 . Thereafter, the carts  10  move together at low speed. Consequently, a plurality of the carts  10  are coupled together, and arranged densely between the point P 1  and the point P 2 . Parts are supplied to the carts  10  in the parts supply area  71 . 
         [0069]    In  FIG. 7 , for the sake of simplicity, the illustrated work area  73  is a relative small area. However, the work area  73  may be a large area, e.g., in the case where a large number of parts are present. In such a case, by configuring the second low speed movement path  63  to have a serpentine pattern, it is possible to suppress increase in the size of the work area  73 . 
         [0070]    A large number of carts  10  are arranged densely in the second low speed movement path  63 . The same applies to the first low speed movement path  61 . Consequently, in comparison with the conventional technique where the carts are spaced from one another, in the embodiment of the present invention, a larger number of carts  10  can be positioned in the work area  73  and the parts supply area  71 . Therefore, significant improvement in the productivity per unit area of the working floor is achieved. 
         [0071]    In the embodiment of the present invention, the cart  10  includes the circular cart  110  and the constricted cart  30 . However, the cart  10  may be simply a normal cart without having this structure. Instead of the pinch roll, a dog chain, a chain with an attachment may be used. The type and form of the drive source can be determined arbitrarily. Further, the workpiece may not be the vehicle body as long as the workpiece is a casing before assembling of parts to the workpiece. The workpiece may be of any type, or may have any structure. However, it should be noted that the present invention is suitably applicable to vehicle production lines where improvement in the efficiency of producing the workpiece is strongly desired. 
         [0072]    On outside  78  of the closed transportation path  65 , the operator can freely approach the cart  10 . Since no overbridges bridging over the transportation path  65  are provided, and no tunnels passing under the transportation path  65  are provided, the operator cannot go inside  79  of the transportation path  65  freely. In  FIG. 5 , the left-sub cart  90  is positioned on the outside  78  of the transportation path  65 , and the right-sub cart  100  is positioned on the inside  79  of the transportation path  65 . There are some problems in operation of loading the fourth parts shelf (right parts shelf)  80  on the right sub-cart  100 , and operation of unloading the fourth parts shelf (right parts shelf)  80  from the right sub-cart  100 . The present invention is directed to provide solutions to address the problems. Hereinafter, the details of the solution will be described in detail. 
         [0073]    As shown in  FIG. 8 , the fourth parts shelf (right parts shelf)  80  includes a frame plate  82  equipped with casters  81 , a frame  83  provided upright on this frame plate  82 , and free rolls  84  provided respectively at the left and right ends of the frame plate  82 . The free rolls  84  contact the guide rails  40  indicated by imaginary lines, and the free rolls  84  rotate freely. Consequently, the fourth parts shelf  80  travels on the constricted cart  30  in a direction toward the backside, or toward the front side of the drawing sheet. The third parts shelf  85  may have the same structure as the fourth parts shelf  80 . However, the free rolls  84  may be omitted. 
         [0074]    As shown in  FIG. 9 , the left sub-cart  90  is a cart having a frame plate  92  which is equipped with casters  91 , and travels on the factory floor  13 . The third and fourth parts shelves (left and right parts shelves)  85 ,  80  are placed on the frame plate  92 . Since the right sub-cart  100  has the same structure as the left sub-cart  90 , description of the structure is omitted. 
         [0075]    As shown in  FIG. 10 , an elongated rail  93  is provided along the left side of the constricted cart  30 . A front slider  94  and a rear slider  95  are movably fitted to this elongated rail  93 . The left sub-cart  90  is fixedly coupled to the front slider  94 . 
         [0076]    As shown in  FIG. 11 , the front slider  94  is fitted to the elongated rail  93  through steel balls  96 , and moves slightly in the direction toward the backside, or toward the front side of the drawing sheet. It should be noted that movement of the front slider  94  can be prevented by inserting a lock pin  97  into the front slider  94  and the elongated rail  93 . The lock pin  97  is coupled to the constricted cart  30  using a chain  97   a.    
         [0077]    As shown in  FIG. 12 , the rear slider  95  is movably fitted to the elongated rail  93  as well. A fitting hole  98  is formed in the rear slider  95 , and the left sub-cart  90  has a fitting pin  99 . When the fitting pin  99  is fitted into the fitting hole  98 , the left sub-cart  90  and the rear slider  95  are coupled together. 
         [0078]    In the parts supply area  71  shown in  FIG. 7 , parts are supplied to the cart  10 . 
         [0079]    As shown in  FIG. 13 , the left sub-cart  90  is provided along the left side of the constricted cart  30 , and the right sub-cart  100  is provided along the right side of the constricted cart  30 , on both sides of the guide rails  40 . In this state, the constricted cart  30  moves forward at low speed while towing the left and right sub-carts  90 ,  100 . 
         [0080]    In the parts supply area  71  ( FIG. 7 ), the operator carries the fourth parts shelf (right parts shelf)  80  using a hand cart  102 . This hand cart  102  is positioned next to the left sub-cart  90 . In this state, as indicated by an arrow ( 1 ), the right parts shelf  80  is transferred from the hand cart  102  to the left sub-cart  90 . Further, as indicated by an arrow ( 2 ), the right parts shelf  80  is guided by the guide rails  40  to move on the constricted cart  30 , and the right parts shelf  80  is transferred to the right sub-cart  100 . That is, the right parts shelf  80  is transferred to the right sub-cart  100  without requiring the operator to enter the inside  79  ( FIG. 7 ). 
         [0081]    Then, as shown in  FIG. 14 , the operator transports the left parts shelf  85  using the hand cart  102 . This hand cart  102  is positioned next to the left sub-cart  90 . In this state, as indicated by an arrow ( 3 ), the left parts shelf  85  is transferred from the hand cart  102  to the left sub-cart  90 . In this manner, the parts shelves  85 ,  80  are placed on the left and right sides of the constricted cart  30 . 
         [0082]    Since the hand cart  102  is no longer required, the hand cart  102  is returned to the stand-by position. The left sub-cart  90  is towed by the constricted cart  30  using the front slider  94  and the rear slider  95  through the elongated rail  93 . As shown in  FIG. 7 , in the parts supply area  71 , since the first low speed movement path  61  is curved, a centrifugal force is applied to the left sub-cart  90  in a direction away from the constricted cart  30 . However, as shown in  FIG. 14 , since the left sub-cart  90  is coupled to the constricted cart  30  by the front and rear sliders  94 ,  95 , the left sub-cart  90  makes turns without being detached from the constricted cart  30 . 
         [0083]    Referring to  FIG. 7 , in the first high speed area  67 , the cart  10  moves at high speed along the straight first high speed movement path  62 . As shown in  FIG. 15( a ) , the lock pin  97  is pulled out to allow movement of the left sub-cart  90 . Then, as indicated by an arrow ( 4 ), the constricted cart  30  moves forward at high speed. Consequently, as shown in  FIG. 15( b ) , the left sub-cart  90  is left, and moves relatively to the rear end of the constricted cart  30 . Specifically, the fitting pin  99  is detached from the rear slider  95 , and the front slider  94  is retreated backward relatively to the backward retreat limit position. Consequently, the left side of the constricted cart  30  is almost opened. 
         [0084]    As shown in  FIG. 16( a ) , by the lift hanger  75 , the vehicle body  11  is carried into the constricted cart  30 , as indicated by an arrow ( 5 ). As shown in  FIG. 1 , front and rear lower portions of the vehicle body  11  are cut away (free of parts). Therefore, in  FIG. 16( a ) , the vehicle body  11  does not interfere with the third parts shelf (left parts shelf)  85 . Consequently, it is possible to carry the vehicle body  11  into the constricted cart  30  without significantly lifting the vehicle body  11 . The level of the ceiling rail  76  can be lowered, and reduction of the lifting amount of the lift hanger  75  can be reduced. Accordingly, size reduction of the first workpiece transfer apparatus  72  is achieved. 
         [0085]    After loading the vehicle body  11  into the constricted cart  30 , at or around the timing of switching from high speed to low speed, as shown in  FIG. 16( b ) , the left sub-cart  90  is moved forward relatively to the original position (arrow ( 6 )), and the lock pin  97  is attached to the left sub-cart  90 . 
         [0086]    In the work area  73  of  FIG. 7 , parts are assembled to the vehicle body  11 . That is, as shown in  FIG. 17 , parts stored in the first parts shelf  111  are attached to the front portion of the vehicle body  11 , parts stored in the second parts shelf  112  are attached to the rear portion of the vehicle body  11 , parts stored in the third parts shelf (left parts shelf)  85  are attached to the left portion of the vehicle body  11 , and parts stored in the right parts shelf  80  is assembled to the right portion of the vehicle body  11 . 
         [0087]    In the second high speed area  69  shown in  FIG. 7 , the vehicle body  11  is unloaded from the cart  10 . This operation of unloading the vehicle body  11  from the cart  10  is performed smoothly in the same manner as in the case of  FIGS. 16( a ), 16( b ) . 
         [0088]    Next, a modified example of the left sub-cart  90  will be described. As shown in  FIG. 18 , the front slider  94  has a right triangular shape. A hook  114  extends from the center of the front side of the left sub-cart  90 . This hook  114  is coupled to the front slider  94  using a pin  115 . Since the left sub-cart  90  is towed at its central point in the vehicle width, even in the case where the fitting pin  99  is detached from the rear slider  95 , the left sub-cart  90  is not swayed to the left or right significantly. Therefore, the left sub-cart  90  can move straight with a higher degree of stability. 
         [0089]    In the embodiment, since the left sub-cart is positioned outside the closed transportation path, the left sub-cart is provided movably for the constricted cart. In the case where the left sub-cart is positioned outside the closed transportation path, the right sub-cart may be movable. Further, both of the left and right sub-carts may be provided movably for the constricted cart. 
         [0090]    In the embodiment, the constricted cart is used as the cart on which the workpiece is placed. However, the cart may take any form. Further, the workpiece may not be the vehicle body as long as the workpiece is a casing before assembling of parts to the workpiece. The workpiece may be of any type, or may have any structure. It should be noted that the present invention is suitably applicable to vehicle production lines where there is a strong need to improve the productivity. 
         [0091]    Further, in the embodiment, as shown in  FIG. 1 , the cart  10  is formed by one cart apparatus  20  and one circular cart  110 . Alternatively, the cart  10  may be formed by a plurality of cart apparatuses  20  and a plurality of circular carts  110 . In the case where a plurality of carts  110  are used, it is possible to handle workpieces using a large number of parts. 
         [0092]    Further, in the work area  73  shown in  FIG. 7 , in addition to or instead of operation of attaching parts, other types of operation such as machining operation may be performed. 
       INDUSTRIAL APPLICABILITY 
       [0093]    The present invention is suitably applicable to vehicle production lines. 
       DESCRIPTION OF REFERENCE NUMERALS 
       [0000]    
       
           11 : workpiece (vehicle body) 
           15 : drive source (pinch roll) 
           17 : control unit 
           20 : cart apparatus 
           30 : cart (constricted cart) 
           40 : guide rail 
           60 : workpiece transportation apparatus 
           61 : first low speed movement path 
           62 : first high speed movement path 
           63 : second low speed movement path 
           64 : second high speed movement path 
           65 : closed transportation path 
           66 : first low speed area 
           67 : first high speed area 
           68 : second low speed area 
           69 : second high speed area 
           71 : parts supply area 
           72 : first workpiece transfer apparatus 
           73 : work area 
           74 : second workpiece transfer apparatus 
           78 : outside of closed transportation path 
           79 : inside of closed transportation path 
           80 : right parts shelf (fourth parts shelf) 
           81 : caster 
           85 : left parts shelf (third parts shelf) 
           90 : left sub-cart 
           93 : elongated rail 
           94 : front slider 
           95 : rear slider 
           100 : right sub-cart