Transfer system

Abstract: A transfer system of this invention includes an air table (26), and a linear motor (27) cooperating with the air table (26) to float and move a pallet (32). The transfer system is designed to eliminate the need to increase a radius of curvature for each turning section (G,H) of the air table (26). A lighter pallet of this invention is employed in the transfer system. A transfer system includes ball valves (22) openable upon contact with a pallet (32) to apply air under pressure to a pallet (32), a linear motor (27) adapted to move the floating pallet (27), and guides (33,35) adapted to guide the floating pallet (32). A pallet (301) includes a pallet body (302) made of a light material, and an external load part (303) made of metal. The external load part (303) has a lower surface (303b) projecting slightly from the lower surface of the pallet body (302). Guides (307) are mounted to the outer periphery of the pallet body (302). A linear motor secondary conductor (310) is mounted to the lower surface of the pallet body (302).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transfer system as defined in the preamble of claim 1 for transferring a load such as a heavy object to be processed. 2. Description of the Related Art Figs. 27 to 31 show transfer systems known in the art. Referring first to Figs. 27 to 29, a conventional transfer system includes a truck 1. 2 is a table connected to a truck body 3 by a vertical support 2'. 6 are wheels rotatable about a horizontal shaft which is mounted to the truck body 3. 4 is a guide of a U-shaped cross section. 5 are a pair of rails fixed to the inner walls of the guide 4. The wheels 6 of the truck 1 are supported on the rails 5. 7 are wheels each rotatable about a vertical shaft which is connected to the truck body 3. The wheels 7 are in contact with the inner side walls of the guide 4. 8 is a secondary conductor of a linear motor. 9 are primary coils of the linear motor which are fixedly mounted on the bottom of the guide 4 and spaced apart from one another in the direction in which the truck is advanced. 10 are stations. 11 is a turning section of the guide 4 constructed to change the direction of advancement of the truck. With this arrangement, the wheels 6 are placed on the rails 5, and the wheels 7 are brought into contact with the side walls of the guide 4. A load (a heavy object to be processed) is placed on the table 2. The truck 1 is moved from one station 10 to another station 10 along the guide 4 when the linear motor primary coils 9 are energized. Figs 30 and 31 show another transfer system known in the art. 14 is a base. 15 is an air duct seated on the base 14. 12 is a top plate of the air duct 15. 13 are a number of nozzle holes formed in the entire surface of the top plate 12. 16 are linear motor primary coils mounted within the air duct 15. 17 is a pallet including a linear motor secondary conductor on its lower surface. 18 is a pressurized air supply source. The base 14 and the air duct together form an air table. Air under pressure is supplied from the pressurized air supply source to the air duct 15 and then ejected from the nozzles 13 to float the pallet 17 together with a load above the top plate 12 of the air duct 15. The pallet 17 together with the load is transferred from one station to another station when the linear motor primary coils 16 are energized. Conventionally, a transfer system employs a pallet to carry a light load of several kilograms to several hundred kilograms. The pallet together with the light load is floated on an air table and moved by a linear motor as shown, for example, in Figs. 30 and 31. In the prior art transfer system shown in Figs. 27 to 29, the table 2 is attached to the truck 1 by a single post or support 2'. This causes the table 2 to swing and move out of alignment with the guide 4. Thus, a load on the table can not be moved in parallel to the guide 4. Also, if the section 11 of the guide 4 has a small radius of curvature, a pair of front wheels 6, the four wheels 7 and a pair of rear wheels are oriented as shown in Fig. 28. This inhibits a change in the direction of movement of the truck 1. To this end, the guide 4 is constructed in a factory so that the section 11 may have a large radius of curvature. This deteriorates systematic arrangement of the transfer system. In addition, the truck 1 can not be stopped in a predetermined position. This deteriorates the operability and safety in a production line. In the prior art transfer system shown in Figs. 30 and 31, a large pressurized air supply source is required to inject air under pressure from a number of nozzle holes 13 of the air duct (or air table) not only before each pallet 17 passes, but also after the pallet has passed. This results in an increase not only in the cost of the system, but also consumption of the pressurized air and thus, running cost of the system. In the prior art transfer system incorporating the air table and the linear motor, a pallet must be rigid and heavy to transfer a heavy load, for example, of several tons (about two tons) at a high speed (2 m/sec). To transfer such a heavy pallet at a high speed, it is necessary to increase the size of the linear motor and the flow rate of pressurized air. However, this results in an increase in the overall size of the transfer system. In order to improve production efficiency, a pallet is transferred to a press machine in a production or press line while being floated by air under pressure. A force is then applied to press a load on the pallet. The pallet must be rigid to accommodate this force. This results in an increase in the weight of the pallet. To transfer such a heavy pallet, it is necessary to provide a larger linear motor and increase the flow rate of air under pressure. This results in a corresponding increase in the overall size of the press system. A prior art transfer system comprising an air table, a linear motor, and a pallet guide for smoothly guiding a floating pallet along the table, is described in US-A-4 718 539. In this prior art system detecting means are provided to start and stop an air supply to valves by sections of the table upon the presence of the pallet. From US-A-3 889 602 there is known a passenger vehicle running on a track in which ball valves are opened by the vevicle in order to provide lift and propulsion to the vehicle. From GB-A-1 297 727 there is known an air flotation table with ball air valves fitted in the top surface and adapted to be opened by the weight of an article being conveyed. SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide a transfer system which enables accurate movement of a pallet with a load on an air table, which can systematically be arranged, and which reduces its equipment and running costs. Another aspect of the present invention is to provide a pallet which is lightweight and which can reduce the size of a linear motor and the flow rate of air under pressure and thus, the overall size of a transfer system designed to float the pallet above an air table and move the pallet by the linear motor. According to the present invention, there is provided a transfer system as defined in claim 1. Preferred embodiments of the invention are described in the subclaims. In the transfer system of the present invetion, the pallet is placed on the air table, and a load is placed on the pallet. The ball valve, located immediately below the pallet, is opened to apply air under pressure to the pallet. The acceleration primary coils are then energized to accelerate the floating pallet. The pallet is moved to a station while being guided by the guide. When the pallet reaches the station, the deceleration primary coils are energized to reduce the speed of the pallet, and the pallet stoppers are operable to stop the pallet. The pallet is thereafter moved to a next station after the work on this station is completed. The guide rollers and the grooved guide cooperate to guide the pallet. When the pallet reaches the turning section of the air table, the deceleration primary coils are energized to reduce the speed of the pallet. The pallet stoppers are then operable to stop the pallet. The guide roller mounted to the inner front of the pallet is moved into the grooved guide through a recess. Thereafter, the acceleration primary coils mounted to the turning section of the air table are energized to accelerate the pallet. At this time, the guide roller mounted to the inner rear of the pallet is moved out of the grooved guide through a recess. When the pallet reaches a next station, the direction of conveyance of the pallet is changed in a similar manner. In the transfer system of the present invention, the pallet is placed on the air table, and a load is also placed on the external load part of the pallet. The pallet together with the load is then lifted by air under pressure. The linear motor and the guides cooperate to move the pallet to a predetermined station. When the pallet reaches the station, application of the air under pressure is prevented. This causes the pallet to be placed on the air table. The lower surface of the external load part projects slightly from the lower surface of the pallet body. Thus, the pallet body made from a light material is in no way contacted with the air table. Instead, the external load part made of iron or other metals is brought into contact with the air table to support the load. As a result, no external force (for example, pressing force) is applied to the pallet body. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, reference may be made to the following description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a plan view of a transfer system according to a first embodiment of the present invention; Fig. 2 is a sectional view of a ball valve; Fig. 3 is a sectional view showing the manner in which the ball valves are assembled in the transfer system; Fig. 4 is a front view, in vertical section, of a guide; Fig. 5 is a plan view of pallet stoppers; Fig. 6 is a side view of the pallet stoppers; Fig. 7 is a side view of the pallet stopper arranged in the intermediate section of an air table; Fig. 8 is a plan view of the pallet stopper arranged in the 90 ° turning corner or section of the air table; Fig. 9 is a plan view of the pallet stopper arranged in the intermediate section of the air table; Fig. 10 is a plan view of a grooved guide according to a second embodiment of the present invention; Fig. 11 is a plan view showing acceleration primary coils and deceleration primary coils of a linear motor; Fig. 12 is a side view, in vertical section, of an air table employed in the second embodiment; Fig. 13 is an enlarged view, in vertical section, of the air table; Fig. 14 is a plan view of a transfer system according to a third embodiment of the present invention; Fig. 15 is a plan view of a pallet stopper mounted to the 90 ° turning section; Fig. 16 is a side view of the pallet stopper shown in Fig. 15; Fig. 17 is a side view of a pallet stopper arranged in the intermediate section; Fig. 18 is a side view of a portion of the air table to which the pallet stopper is mounted; Fig. 19 is a front view, in vertical section, of the air table; Fig. 20 is a plan view showing a fourth embodiment of the present invention; Fig. 21 is a side view of the fourth embodiment; Fig. 22 is a bottom plan view as seen in the direction of the arrow I in Fig. 21; Fig. 23 illustrates a transfer pallet for the transfer system of the present invention; Fig. 23(I) is a side view as seen in the direction of the arrow II in Fig. 20, and Fig. 23(II) is an enlarged side view showing a bevel portion of the pallet body; Fig. 24 is a plan view of a transfer system including an air table on which the transfer pallet is moved, and a linear motor by which the transfer pallet is lifted above the air table; Fig. 25 is a bottom plan view illustrating a fifth embodiment; Fig. 26 is a sectional view taken on the line III-III of Fig. 25; Fig. 27 is a front view, in vertical section, of a conventional transfer system; Fig. 28 is a plan view showing a turning section in which the direction of conveyance of a pallet is changed; Fig. 29 is a plan view showing the layout of the transfer system shown in Fig, 27; and Fig. 30 is a plan view of another transfer system known in the art; and Fig. 31 is a side view of a conventional transfer system. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 1 to 9 illustrate a first embodiment of the present invention. Referring to Fig. 1 to 6, 26 is an air table or frame of a rectangular shape as viewed in plan. 22 are a number of ball valves 22 formed in the entire air table 26. Referring to Figs. 2 and 3, 23 is a ball in each ball valve 22. 22a is a spring disposed to urge the ball 23 toward a valve seat 22b to close the ball valve. In Figs. 1, 3, 4 and 5, 32 is a pallet. Referring to Fig. 4, 24 are linear motor secondary conductors attached to the lower surface of the pallet 32. 25 is a base of the air table 26. In Fig. 1, (a) denotes an outer turning section of the air table 26 in a face-to-face relation to the pallet as conveyed. 27 are linear motor primary coils mounted to the air table 26. In Figs. 4 and 5, 35 is a grooved guide provided on the outer periphery of the air table 26. 36 is a mounting plate adapted to fix the grooved guide 35 to the base 25. A guide roller 33 is mounted to each corner of the pallet 32 and rotated within the grooved guide 35. In Figs. 1, 6 and 9, 30 are pallet stoppers each provided at the front side of the 90° turning section of the air table 26 in the direction of conveyance of the pallet. Each pallet stopper 30 comprises a stopper plate 30a, guides 30b adapted to support retractable guide rods integral with the stopper plate 30a, and a shock absorber 30c disposed between the guides 30b and the stopper plate 30a. The pallet stoppers 30 are mounted to the base 25. In Figs. 1 and 5 to 7, 28 are pallet stoppers centrally located in the air table 26. Each pallet stopper 28 comprises a retractable stopper plate 28a, guides 28a' adapted to support the stopper plate 28a, and a shock absorber 28b disposed between the stopper plate 28a and the guides 28a', a support table 28e adapted to support the guides 28a', a link mechanism 28c adapted to vertically move the support table 28e, and a drive cylinder 28d connected between the link mechanism 28c and the base 25. In Figs. 1, 5, 6 and 9, 29 are pallet stoppers cooperating with the pallet stoppers 30 and 28 so as to stop the pallet 32 in the 90 ° turning section and the intermediate section of the air table 26. Each pallet stopper 29 comprises an air motor 29a, and a wedge-shaped stopper arm 29b. When the pallet 32 comes into contact with the pallet stoppers 30 and 28, the air motor 29a is energized to swing the wedge-shaped stopper arm 29b in the direction of the arrow shown in Fig. 9. This causes the wedge-shaped stopper plate to come into contact with the rear surface of the pallet 32 and cooperate with the pallet stoppers 30 and 28 to stop the pallet in the 90 ° turning section and the intermediate section of the air table. As shown in Figs. 1 and 5, the air table has stations 31 provided at the four turning sections as indicated by C, D, G and H, and the intermediate sections as indicated by A, B, E and F. The pallet stoppers 30 and 29 are located at the front and rear sides (or upstream and downstream sides in the direction of conveyance of the pallet) of each of the turning sections C, D, G and H of the air table 26. Also, the pallet stoppers 28 and 29 are located at the front and rear sides of each of the intermediate sections A, B, E and F of the air table 26. A high pressure air is fed to each ball valve 22 from its bottom by a source of air supply (not shown). As shown in Fig. 3, the ball 22, when the ball valve is closed, is brought into contact with the bottom of the pallet 32 against the action of the spring 22a and air. Air under pressure is introduced into the interior of each ball valve as shown by the arrow in Fig. 3 to cause the ball 22 to close the valve. A mechanism may be incorporated to prevent the ball 22 from moving down within the valve. This eliminates the need for the springs 22a. Each pair of linear motor primary coils is mounted on the air table 26 and located at opposite sides of each of the stations A to H in the direction in which the pallet is conveyed. As shown in Fig. 5, a pair of notches 37a and 37b are formed in the inner side of the 90 ° turning section of the grooved guide 35 to allow the guide rollers 33 to pass. Operation of the transfer system according to the first embodiment shown in Figs. 1 to 9 is as follows. The pallet 32 is placed on, for example, the station H. A load is then loaded on the pallet 32. The pallet 32 is pushed down against the action of the spring 22a. Air under pressure is supplied to float the pallet 32 together with the load. The linear motor primary coils 27 are energized to move the pallet toward the station A while the guide rollers 33 of the pallet 32 is rotated within the grooved guide 35 of the air table 26. When the pallet 32 together with the load has reached the station A of the air table 26, the speed of the pallet 32 is decreased by the linear motor primary coils 27. The pallet 32 is then brought into contact with the pallet stopper 28 and pushed toward the pallet 28 by the pallet stopper 29. After the pallet 32 is stopped, the pallet stoppers 28 and 29 are moved down to their original positions. The pallet 32 together with the load is then moved toward the station B. The pallet 32 is stopped in a similar manner. The pallet 32 is next moved toward the station C or 90 ° turning section of the air table until it comes into contact with the pallet stopper 29. Pallet stopper 29 is then operable to push the pallet 32 from its back. This results in stoppage of the pallet 32. The pallet 32 is repeatedly moved and stopped in a similar manner when it passes the stations D to H. When the pallet 32 passes the stations or turning sections of the air table H, C, D and G, the guide rollers 33 are moved through the notches 37a and 37b to allow the pallet 32 to change its direction. A transfer system according to a second embodiment of the present invention will now be described with reference to Figs. 10 to 13. In Figs. 10 to 13, 21 is a transfer system. In Figs. 12 and 13, 124 is a rectangular air frame or table. 122 are pressurized air ball valves assembled in the air table 124. 123 is a ball located within each ball valve 122. 123a is a spring seat mounted within each ball valve 122. 123b is a spring disposed between the spring seat 123a and the ball 123 and adapted to normally urge the ball 123 against a spring seat 122a so as to close the ball valve 122. Air under pressure is introduced into the interior of the ball valve 122 to push the ball 123 against the valve seat 122a. A mechanism may be incorporated to prevent the ball 123 from falling down from the valve seat 122a. This eliminates the need for the spring 123b. 128 is a base on which the air table 124 is mounted. In Figs. 10 and 11, a denotes a 90 ° turning section provided at each corner of the air table 124. Fig. 11, 125a are acceleration linear motor primary coils. 125b are deceleration linear motor primary coils of the type known in the art. Each pair of acceleration primary coils 125 are mounted to the air table 124 in a spaced relation. Each pair of deceleration primary coils 125b are mounted to the turning section of the air table 124 in a spaced relation. In each turning section, the longitudinal axis of each acceleration primary coil 125a extends at right angles to the longitudinal axis of each deceleration primary coil 125b. In Figs. 10 and 12, 126 is a grooved guide provided on the outer peripheral edge of the air table 124. 129 is a mounting plate by which the grooved guide 126 is secured to the base 128. 131 is a transfer pallet. 132 is a linear motor secondary conductor mounted to the lower surface of the pallet 131. 130 are four guide rollers mounted on each corner of the pallet 131 and adapted to rotate within the grooved guide 126. 127 and 127d are notches formed in the grooved guide 126 to pass the guide rollers. These notches are formed in the inner turning section of the air table. In Fig. 10, 133 are pallet stoppers which are similar in structure to the pallet stoppers 30 in the first embodiment. The pallet stoppers 133 are mounted to the grooved guide 126. In operation, the pallet 131 is placed on the air table 124, and a load is loaded on the pallet 131. The ball 123 of the ball valve 122 is then pushed down against the action of the spring 134. Air under pressure is applied to the pallet 131. This causes the pallet 131 together with the load to float. The linear motor primary coils 125a are operable to move the pallet 131 in the direction of the arrow A shown in Fig. 10. At this time, the guide rollers 130 are rotated within the grooved guide 126 so as to guide the pallet in that direction. When the pallet 131 reaches the turning section of the air table 124, the deceleration primary coils 125b are operable to reduce the speed of the pallet 131. The pallet 131 is stopped when it comes into contact with the pallet stopper 133. At this time, the guide roller, located at the inner front of the pallet 131, is moved into the grooved guide 126 through the notch 127c. The acceleration primary coils 125a are then operable to accelerate the pallet 131. The pallet 131 is moved toward the next turning section of the air table 124. At this time, the guide roller, located at the inner rear of the pallet 131, is moved out of the grooved guide 126 through the notch 27d. When the pallet 131 reaches the next turning section, the pallet is turned by 90 ° in a similar manner. A tranfer system according to a third embodiment of the present invention will now be described with reference to Figs. 14 to 19. 221 is a rectangular frame or air table. 222 are pressurized air ball valves assembled in the air table 221. In Figs. 16, 18 and 19, 224 is a base on which the air table 221 is mounted. In Fig. 13, a denotes an outer turning section located at each corner of the air tbale 221 in a face-to-face relation to the pallet as conveyed and identical in structure to those employed in the first and second embodiments. In Fig. 14, 223a are acceleration linear motor primary coils. 223b are deceleration linear motor primary coils. Each pair of acceleration primary coils 223a are mounted to the air table 221 in a spaced relation to one another. A pair of deceleration primary coils 125b are mounted to each 90 ° turning section of the air table 124 in a spaced relation to one another. In each turning section, the longitudinal axis of each acceleration primary coil 223a extends at right angles to the longitudinal axis of each deceleration primary coil 223b. In Figs. 18 and 19, 225 is a grooved guide provided above the outer peripheral edge of the air table 221. 226 is a mounting plate by which the grooved guide 225 is secured to the base 224. 227 is a transfer pallet. 229 is a linear motor secondary conductor mounted to the lower surface of the pallet 227. 228 are four guide rollers mounted on each corner of the pallet 227 and adapted to rotate within the grooved guide 225. In Figs. 14 to 16 and 18, 230 are pallet stoppers. As shown in Figs. 15 and 16, each pallet stopper 230 comprises a stopper plate 232, a pair of guides 234 adapted to support a corresponding pair of retractable guide rods integral with the stopper plate 232, and a shock absorber 233 disposed between the guides 234 and the stopper plate 232. Each pallet stopper 230 is attached to the outer turning section a of the air table 24. 231a and 231b are pallet stoppers located at the intermediate sections, rather than the turning sections, of the air table. As shown in Fig. 17, the pallet stopper 231a (231b) comprises a retractable stopper plate 235, a guide 237 adapted to support the stopper plate 235, a shock absorber 236 disposed between the stopper plate 235 and the guide 237, a support table 238 on which the guide 237 is supported, a link mechanism 239 by which the support table 238 is vertically movable relative to the base 224, and a drive cylinder 240 mounted between the link mechanism 239 and the base 224. The pallet stopper 231a is located at the intermediate sections, rather than the 90 ° turning sections, of the air table 224. Operation of the transfer system according to the third embodiment of the present invention will now be described with reference to Figs. 14 and 19. The pallet 227 is first placed on the air table 221, and a load is mounted on the pallet 227. The ball of the ball valve 222 is pushed down against the action of the spring. Air under pressure is applied to the pallet 227 to float the pallet 227 together with the load. The acceleration primary coils 223a are operable to accelerate the pallet 227. The pallet 227 is moved in the direction of arrow X shown in Fig. 14. At this time, the guide rollers 228 are rotated within the grooved guide 225 to guide the pallet 227 toward the station A, for example. Immediately before the pallet 227 reaches the intermediate section of the air table, the drive cylinder 240 is operable to lift the support table 238, the guides 237 and the shock absorber 236 and the stopper plate 235 through the link mechanism 239. This causes the stopper plate 235 to contact the pallet 227. The resulting shock is absorbed by the shock absorber 236 so that the pallet 227 may be stopped in the intermediate section of the air table without any shocks. After the pallet 227 is stopped, the drive cylinder 240 of the pallet stopper 231b is rendered operative to lift up the support table 238, the guides 237, the shock absorber 236 and the stopper plate 235 through the link mechanism 239. This causes the stopper plate 235 to position in the rear of the pallet 227. When processing of the load in the intermediate section is completed, the drive cylinders 240 of the pallet stoppers 231a and 231b are rendered operative to lower the stopper plates 235, 235. The pallet together with the load is then moved toward the next station or 90 ° turning section of the air table in a manner as will hereinafter be described. When the pallet 227 together with the load reaches the 90 ° turning section of the air table 221, the deceleration primary coils 225b are energized to reduce the speed of the pallet 227. The pallet 227 is stopped when it contacts with the pallet stopper 230. Thereafter, the pallet 227 together with the load is moved toward the next station. The pallets of the first to third embodiments may be used in a press process in which glass products such as a cathod-ray tube or funnel are made, or in a transfer process of pressed products. Also, the pallets may be used to transfer other loads in a press or transfer process. It is not essential that the shape of the pallet be square. The pallet may be rectangular, hexagonal or polygonal. The direction of conveyance of the pallet may be changed by 120 ° when a hexagonal pallet is employed. In the first to third embodiments thus described, the direction of conveyance of the pallet is changed by 90 ° , but may be changed by any degrees. Each turning section of the air table may be in the form of T or cross or may have a plurality of branches. In the latter case, it is preferable that a retractable stopper is provided at each turning section. A transfer pallet for the transfer system of the present invention will now be described with reference to Figs. 20 to 24. 301 is a transfer pallet. 302 is a pallet body. The pallet body 302 has a rigid lower surface. The pallet body 302 is light and is made, for example, of aluminum. 303 is an external load. The external load part 303 has an upper circular loading section 303a and a lower contact section. The external load part 303 is made of metal such as iron so that it may not be deformed or broken. The circular loading section 303a of the external load part 303 is received in a circular recess centrally formed in the pallet body 302. The contact section 303b of the external load part 303 is fit in a recess formed in the lower surface of the pallet body 302. The external load part 303 is thus integral with the pallet body 302. 304 are bolts by which the external load part 303 (contact section 303b) is secured to the pallet body 302. As shown in Fig. 21, the external load part 303 (contact section 303b) projects downwardly from the lower surface of the pallet body 302 by about 1 mm. 306 is an air table. 305 is a clearance left between the pallet body 302 and the air table 306 when the pallet 301 is placed on the air table 306. 307 are guide rollers mounted on each corner of the pallet body 302. 308 are a number of ball valves formed in the entire air table 306. 309 is a bevel portion formed in the lower corner of the pallet 301 for contact with each ball of the ball valve 308 (When the ball is brought into contact with the pallet body 302, it is pushed down to open each air inlet). 310 is a secondary linear motor conductor received in a recess formed in the lower surface of the pallet body 302. The secondary conductor 310 is an iron plate welded to the lower surface of the pallet body 302 and has a lower end which does not project from the lower surface of the pallet body 302. 311 is a grooved guide mounted to the air table 306 to receive the guide rollers 307. 312 is a mounting plate by which the grooved guide 311 is mounted to the air table 306. As shown in Figs. 20 and 24, it is sufficient to attach the mounting plate 312 to only one side of the air table 306 to guide the pallet. 314a are acceleration primary coils of a linear motor mounted to the air table 306. 314b are deceleration primary coils mounted to the air table 306. The ball valve 308 and the associated ball are the same as those used in the first to third embodiments. The air table is a rectangular frame as in the first to third embodiments. The acceleration primary coils 314a and the deceleration primary coils 314b are arranged in the same manner as those in the third embodiment. The longitudinal central axis of one station extends at right angles to that of another station. Operation of the transfer pallet in the fourth embodiment shown in Figs. 20 to 24 is as follows. The pallet 301 is placed in the air table 306, and a load is also placed on the external load part 303 (circular loading station 303a). Air under pressure is then applied through the ball valves 308 to float the pallet 301 together with the load above the air table 306. The acceleration primary coils 314a are energized to accelrate the pallet 301. The guide rollers 307 of the pallet 301 and the grooved guide 311 cooperate to guide the pallet 301. This prevents irregular movement of the pallet 301 until the pallet 301 reaches a station. At this time, the ball valves 308 are sequentially rendered ON (opened) by the rigid lower surface of the pallet 301 to keep the pallet 301 lifted above the air table. When the pallet body 302 is moved away from the air table, the balls 308 are pushed up by air under pressure and by the spring so as to render the ball valves OFF (or to close the ball valves). When the pallet reaches a certain station, the deceleration primary coils are operable to reduce the speed of the pallet 301. The pallet 301 is thereafter stopped by stoppers. These stoppers may include the pallet stoppers 28 to 30 of the first embodiment, the pallets stoppers 133 of the second embodiment, and the pallet stoppers 230, 231a, and 231b of the third embodiment. When the pallet 301 is thus stopped, the ball valve 308, located immediately below the pallet 301, is closed by means not shown. The external load part 303 made of iron (contact section 303b) is placed on the air table 306. Since the lower surface of the external load part (contact section 303b) projects slightly from the lower surface of the pallet body 302, the pallet body 302 made of a light material is in no way contacted with the air table 306. The external load part (contact section 303b) is in contact with the air table 306. By this arrangement, no external force (for example, pressing force) is applied to the pallet body 302. Figs. 25 and 26 illustrate a fifth embodiment of the present invention wherein a secondary conductor 310 includes an aluminum plate 316 and a steel plate 317 attached to the aluminum plate 316 and is fit in the recess formed in the lower surface of the pallet body as in the fourth embodiment. In the secondary conductor or composite material 310, a portion of the aluminum plate 316 which is not engaged with the ball valves 308 is recessed as at 318a to provide an air pocket 318. Air under pressure is applied from the ball valves 308 to the air pocket 318 to more positively float the pallet 301. The operation of the rest of the fifth embodiment is identical to that of the embodiment shown in Figs. 20 to 24. The pallets of the fourth and fifth embodiments may be used in a press process in which glass products such as a cathod-ray tube or funnel are made, or in a transfer process of pressed products. Also, the pallets may be used to transfer other loads in a press or transfer process. It is not essential that the shape of the pallets be square. The pallets may be rectangular, hexagonal or polygonal. The direction of conveyance of the pallet may be changed by 120 ° when a hexagonal pallet is employed. In the fourth and fifth embodiments thus described, the direction of conveyance of the pallet is changed by 90 °, but may be changed by any degrees. Each turning section of the air table may be in the form of T or cross or may have a plurality of branches. In the latter case, it is preferable that a retractable stopper is provided at each turning section. Advantages of the present invention are as follows. According to the invention, the pallet is placed on the air table, and a load is placed on the pallet. The ball valve, located immediately below the pallet, is opened to apply air under pressure to float the pallet. The primary coil of the linear motor is then energized to accelerate the pallet. The pallet is moved toward a station while the guide of the pallet is engaged with the guides of the air table. When the pallet reaches the station, the primary coil of the linear motor is energized to reduced the speed of the pallet, and the pallet stoppers are operable to stop the pallet. The pallet is thereafter moved to a next station. Thus, the pallet together with the load can be moved exactly on and along the air table. This arrangement eliminates the need for a turning section of an increased radius of curvature. Also, the overall transfer system can be well organized or arranged. Only the ball valve which is located immediately below the pallet is opened. This allows the use of a smaller source of pressurized air supply so as to reduce the equipment cost of the system. Also, it enables a reduction in the consumption of air under pressure and thus, the running cost of the transfer system. According to the invention, when the pallet together with a load reaches the turning station of the air table, the deceleration primary coil of the linear motor is energized to reduce the speed of the pallet. The pallet is stopped when it comes into contact with the pallet stoppers. At this time, the guide roller at the inner front of the pallet is moved into the grooved guide through the recess. Next, the acceleration primary coil of the linear motor is energized to accelerate the pallet. The pallet is moved to the turning section of the air table. At this time, the guide roller at the inner rear of the pallet is moved out of the grooved guide through the recess. When the pallet reaches the next turning station, the direction of conveyance of the pallet is changed in a similar manner. According to the invention thus explained, the pallet can smoothly be transferred. The pallet can smoothly be stopped in each turning station. Also, the direction of conveyance of the pallet can smoothly be changed. According to the invention, the pallet is placed on the air table, and a load is placed on the pallet. Air under pressure is applied from the ball valves to the pallet. The acceleration primary coil of the linear motor is energized to accelerate the floating pallet. The pallet is moved to the intermediate section while the guide of the pallet is engaged with the guides of the air table. When the pallet reaches the intermediate section, the deceleration primary coil of the linear motor is energized to reduce the speed of the pallet. The pallet is stopped when it comes into contact with the pallet stoppers. After the load has been processed, the pallet is moved in a similar manner to the next station while being guided by the guides. In each turning section of the air table, the pallet stopper mounted to the outer turning section in a face-to-face relation to the pallet as conveyed is operable to stop the pallet. The pallet is then moved in an intended different direction. This arrangement eliminates the need for a turning section of an increased radius of curvature and enables neet arrangement of the overall transfer system. Also, the pallet together with a load can accurately be stopped in a predetermined position. This improves the operability as well as safety in a production line. According to the invention, the transfer pallet comprises a pallet body made from a light material, and an external load part made of metal. This results in a decrease in the weight of the overall pallet and enable a corresponding reduction in the size of the linear motor and the flow of air under pressure, and thus, the size of the transfer system wherein the air table and the linear motor cooperate to float and tranfer the pallet. The lower surface of the external load part projects slightly downwardly from the lower surface of the pallet body. This allows the external load part to accommodate any external forces. Thus, no external force is exerted on the pallet body. The pallet can thus has a required strength.

Claims: A transfer system comprising an air table (26, 124, 221, 306), a linear motor adapted to move a floating pallet (32, 131, 227, 301), and a pallet-guide (35; 126, 225, 311) provided on said air-table, characterized in that ball valves (22, 122, 222, 308) are mounted on the air table (26, 124, 221, 306) and are opened upon contact with the pallet (32, 131, 227, 301) so as to apply air under pressure to float the pallet (32, 131, 227, 301), said pallet (32, 131, 227, 301) comprises a pallet body (302) made of a light material and an external load part (303) made of metal, said external load part (303) having a lower surface projecting slightly from the lower surface of the pallet body (302), and said pallet body (302) including a plurality of guide rollers (33, 130, 228, 307) on the outer periphery of its upper surface and a secondary conductor (24, 132, 229, 310) of the linear motor on its lower surface. The transfer system according to claim 1, characterized in that acceleration primary coils (125a, 223a, 314a) and deceleration primary coils (125b, 223b, 314b) of said linear motor are mounted to turning sections of said air table (26, 124, 221, 306), pallet stoppers (30, 133, 230) are mounted to the outsides of the turning sections of the air table (26, 124, 221, 306) in a face-to-face relation to the pallet (32, 131, 227, 301) as conveyed, and the guides (35, 126, 225, 311) on the side of the air table (26, 124, 221, 306) adapted to engage with the guide rollers (33, 130, 228, 307) of the pallet (32, 131, 227, 301) are mounted to the turning sections, and include notches (37a, 37b, 127c, 127d) through which the guide rollers of the pallet (32, 131, 227, 301) are allowed to pass. The transfer system according to claim 2, characterized in that said pallet stoppers (28, 29, 231a, 231b) are retractably mounted to intermediate sections, other than the turning sections, of the air table (26, 124, 221, 306).

Inventors: HIDAKA HARETARO C O MIHARA MAC, OOSHIMA TADAYOSHI C O MIHARA M, SHIMIZU NAOYA, TSUCHIMOTO YOSHIHIRO, HIDAKA, HARETARO, C/O MIHARA MACHINERY WORKS, OOSHIMA, TADAYOSHI, C/O MIHARA MACHINERY WORKS, SHIMIZU, NAOYA, TSUCHIMOTO, YOSHIHIRO
Applicants: ASAHI GLASS CO LTD, MITSUBISHI HEAVY IND LTD, ASAHI GLASS COMPANY LTD., MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1991
Main classification: B65G35
IPC codes: B65G35, B65G54, B65G51