Patent Publication Number: US-7210638-B2

Title: Electric arc spraying system

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the improvements of electric arc spraying systems for performing effective thermal spraying. 
   2. Description of the Related Art 
   In electric arc spraying, use is made of two consumable metal wires (target wires) each of which is supplied to the corresponding one of two contact chips provided in a spraying gun. In operation, an arc is generated between the target wires, and the heat from the arc melts the tips of the target wires. In accordance with the melting speed, the wires are fed to keep the arc generation. The melted metal is atomized into droplets by compressed gas, and these droplets are injected to the surface being coated. 
     FIG. 13  shows the configuration of a typical arc spraying system. Specifically, a system power source  1 , designed to operate on the commercial power, supplies electric power to a spraying gun  2  under constant-voltage control provided by an inverter control circuit, for example. A compressor  3  generates a jet of compressed gas. The compressed gas from the compressor  3  is supplied via a solenoid valve (not illustrated) in the power source  1 , and into the spraying gun  2 . Meanwhile, the two target wires are unwound from two wire reels  5   a  and  5   b , respectively, and then sent forward by the “push-side” wire feeders  4   a ,  4   b . These target wires are guided through two guide tubes  6   a ,  6   b  to the spray gun  2 , which is located away from the wire feeders  4   a ,  4   b.    
   The spraying gun  2  is provided with two “pull-side” wire feeders (not illustrated) for moving the target wires, and with two contact chips (not illustrated) to which the target wires are brought for receiving electrical power. The thermal spray voltage and the target wire feeding speed are adjusted by a remote control unit  7 . 
   Referring now to  FIG. 2 , a recent cylinder block (formed with four bores  8   a – 8   d ) used for an automobile engine is made of an aluminum alloy for weight reduction. Each of the bores  8   a – 8   d  accommodates a reciprocating piston and is therefore susceptible to abrasion. To protect the bores from such abrasion, an iron sleeve may be inserted into each bore. Alternatively, the inner walls of the bores may be coated with an iron-based material by thermal spraying. This method is more advantageous than the iron sleeve protection since the number of parts is reduced, thereby contributing to the weight and size reduction of the cylinder block. 
   Thermal spraying to a bore may be performed by inserting a spraying gun into the bore, and then causing the gun to spray in a direction perpendicular to the bore&#39;s longitudinal axis. At this time, the gun needs to be rotated about the bore&#39;s longitudinal axis so that the spraying is conducted equally to the entire inner wall of the bore that surrounds the gun. However, this thermal spray method is not achievable by the arc spraying system shown in  FIG. 13 , because the rotation of the spraying gun will unduly twist the guide tubes  6   a ,  6   b  since the two push-side wire feeders  4   a ,  4   b  are stationary. 
   In light of the above, plasma spraying or flame spraying is utilized as an alternative to the electric arc spraying because in these methods the spraying gun can be rotated easily. As known in the art, the plasma spraying is a method in which plasma jet is utilized to melt and blast powdery spray material to form a coating on an object. The flame spraying is a method in which flammable gas is burned to melt a spray material and the melted metal is blasted by compressed air onto an object to form a coating. (See JP-A-2004-225101 for example.) 
   However, the plasma spraying and the flame spraying suffer high running costs due to the use of expensive materials such as the working gas, the combustion gas and the melting substances. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an electric arc spraying system that is capable of performing efficient thermal spraying at low costs and contributing to improvement of the productivity. 
   According to the present invention, there is provided an electric arc spraying system comprising: a spraying gun for thermally spraying an inner surface of an object by blasting compressed gas substantially perpendicularly to a supplying direction of target wires; a spraying gun rotation mechanism for rotating the spraying gun; wire supplying sources loaded with the target wires; a wire feeder rotation mechanism for rotating the wire supplying sources synchronously with the spraying gun in rotation; wire feeders provided on a side of the spraying gun or the wire supplying sources for feeding the target wires; and wire support cables for guiding the target wires from the wire supplying sources to the spraying gun. 
   Preferably, the system of the present invention may further comprise a cable support mechanism for supporting two wire support cables and causing the two wire support cables to cross with each other. In this case, the exiting direction of the target wires from the wire supplying sources may be opposite to the entering direction of the target wires into the spraying gun. The two wire support cables may be arranged to extend in parallel to each other between the wire supplying sources and the cable support mechanism. The two wire support cables may be inserted into the cable support mechanism in a mutually crossing manner. The two wire support cables may be arranged to extend in parallel to each other between the cable support mechanism and the spraying gun. 
   Preferably, the cable support mechanism may include a support main body and a rotation member which is rotatably supported by the support main body. The rotation member may be formed with two cable insertion holes crossing with each other. 
   Preferably, the cable support mechanism may comprise a first cable support and a second cable support. The first cable support may include a first support main body and a first rotation member which is rotatably supported by the first support main body and formed with two cable insertion holes parallel to each other. The second cable support may include a second support main body and a second rotation member which is rotatably supported by the second support main body and formed with two cable insertion holes parallel to each other. The two wire support cables may be crossed with each other between the first cable support and the second cable support. 
   With the above arrangements, the rotation of the wire supplying sources can be synchronized with the rotation of the spraying gun, from the beginning to the end of the thermal coating procedure. Thus, it is possible to reduce the occurrence of twisting in the wire support cables. Further, according to the present invention, the rotation radius of the spraying gun can be reduced to e.g. 70 mm. Therefore, the spraying gun in use does not interfere with jigs or the object being coated. This contributes to the realization of an arrangement as shown in  FIG. 1 , in which use is made of two arc spraying systems. The two spraying guns may be disposed at an interval corresponding to the pitch of bores so that two inner surfaces of the bores can be simultaneously coated by thermal spraying. In this way, the efficiency and productivity in thermal spraying are significantly improved. 
   According to the present invention, the wire supplying source may be a pail pack in which a target wire is stored. This increases the amount of loadable target wire up to three times over the possible loading amount by a conventional wire reel. Accordingly, it is possible to conduct a long-time continuous operation without changing the wire reels. That leads to a remarkable increase in productivity. 
   Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an electric arc spraying system according to a first embodiment of the present invention. 
       FIG. 2  illustrates how thermal spraying is performed to the inner surface of a bore formed in a cylinder block for a 4-cylinder engine. 
       FIG. 3  is an enlarged view showing a tip portion of a spraying gun. 
       FIG. 4  shows an electric arc spraying system according to a second embodiment of the present invention. 
       FIG. 5  shows an electric arc spraying system according to a third embodiment of the present invention. 
       FIG. 6  shows an electric arc spraying system according to a fourth embodiment of the present invention. 
       FIG. 7  illustrates the rotation of two parallel wire support cables. 
       FIG. 8  illustrates the rotation of two crossing wire support cables. 
       FIG. 9  shows an electric arc spraying system according to a fifth embodiment of the present invention. 
       FIG. 10  shows a cable support mechanism for the fifth embodiment. 
       FIG. 11  shows an electric arc spraying system according to a sixth embodiment of the present invention. 
       FIG. 12  shows first and second cable supports for the sixth embodiment. 
       FIG. 13  shows the configuration of a typical arc spraying system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the invention will be described below with reference to the accompanying drawings. 
   Reference is first made to  FIGS. 1–3  which illustrate an electric arc spraying system according to a first embodiment of the present invention. Specifically,  FIG. 1  illustrates two arc spraying units used for performing thermal spraying,  FIG. 2  four bores of a cylinder block subject to the thermal spraying, and  FIG. 3  the tip or lower end of a spraying gun of the arc spraying unit. Of these figures,  FIGS. 2 and 3  will also be referred to for describing the second through the fourth embodiments. 
   As shown in  FIG. 1 , the first electric arc spraying unit  30  is provided with two pail packs  32   a ,  32   b  that are arranged side-by-side on a wire feeder rotation mechanism  33 . Each pail pack contains an appropriate length of a target wire  31   a  or  31   b  which is spirally stacked in the pail pack. The pail packs  32   a ,  32   b  are rotated by the rotation mechanism  33 . This rotation is synchronized with the rotation of a spraying gun  37  to be described later. The rotation axis  33   a  of the mechanism  33  is parallel to the spraying gun&#39;s rotation axis  37   a.    
   Two push-side wire feeders  35   a ,  35   b  send forward the target wires  31   a ,  31   b  pulled out of the pail packs  32   a ,  32   b . The target wires  31   a ,  31   b  are guided by two flexible wire support cables  36   a ,  36   b  to be brought to the spraying gun  37 . The wire support cables  36   a ,  36   b  curve gently, with their apex supported by e.g. a bearing (not shown). 
   The spraying gun  37  is provided with a pull-side wire feeder  38 , which forwards the two target wires  31   a ,  31   b  (which have reached the spraying gun  37 ) to contact chips  39   a ,  39   b , respectively (see  FIG. 3 ) provided at a front or lower portion of the spraying gun  37 . A power supply slip ring  40  receives electric power from the power source  1 , and this power is supplied to the two contact chips  39   a ,  39   b . A rotary coupling  41  for supplying compressed gas receives compressed gas from a compressor  3  and supplies the compressed gas to a nozzle  42  (See  FIG. 3 ). This nozzle is formed with a compressed gas blasting hole  42   a , from which the compressed gas is blasted substantially perpendicularly to the feeding direction of the target wires  31   a ,  31   b  (the blasted gas is indicated by reference numeral  43  in  FIG. 3 ). The spraying gun  37  is mounted on a spraying gun rotation mechanism  34 , and is rotated about the rotation axis  37   a  by a motor  34   a.    
   The second arc spraying unit  50  functions in the same manner as the first arc spraying unit  30  described above. To this end, the second unit  50  is provided with components such as target wires  51   a – 51   b , pail packs  52   a – 52   b , a wire feeder rotation mechanism  53  (rotation axis  53   a ), a spraying gun  57  (rotation axis  57   a ), push-side wire feeders  55   a – 55   b , wire support cables  56   a – 56   b , a pull-side wire feeder  58 , contact chips  59   a – 59   b , a power supply slip ring  60 , a compressed gas supply rotary coupling  61 , a nozzle  62  (with a compressed gas blasting hole  62   a , from which compressed gas  63  is blasted), a spraying gun rotation mechanism  54  and a motor  54   a  of the rotation mechanism  54 . The function of these components is the same as that of the counterparts of the first arc spraying unit  30 . 
   In the first and the second arc spraying units  30 ,  50 , the spraying gun rotation mechanisms  34 ,  54  are associated with a spraying gun lift mechanism  65  (which raises and lowers the rotation mechanisms  34 ,  54 ) and with a spraying gun rotation axis positioning mechanism  66  (which shifts the spraying guns&#39; rotation axes sideways). 
   The spraying system according to the first embodiment is operated in the following manner. As shown in  FIGS. 1 and 2 , the lift mechanism  65  and the rotation axis positioning mechanism  66  bring the spraying gun  37  of the first unit  30  and the spraying gun  57  of the second unit  50  to a position above the cylinder block  8  so that the rotation axes  37   a ,  57   a  of the respective spraying guns align with the center lines of a first bore  8   a  and a third bore  8   c . Then, the lift mechanism  65  lowers the spray guns  37 ,  57  in an arrow-indicated direction X 2  into the bores  8   a ,  8   c , respectively. In the first arc spraying unit  30 , the two push-side wire feeders  35   a ,  35   b  send two target wires  31   a ,  31   b  from the pail packs  32   a ,  32   b . The wires  31   a ,  31   b  are guided by the wire support cables  36   a ,  36   b  until they reach the spraying gun  37 . 
   Upon input of a start signal to the power source  1  (see  FIG. 13 ), the compressor  3  begins to supply compressed gas, through a solenoid valve (not illustrated) in the power source  1  and via the rotary coupling  41  of the spraying gun  37 , to the nozzle  42 . Meanwhile, the pull-side wire feeder  38  in the spraying gun forwards the target wires  31   a ,  31   b  (which come from the pail packs  32   a ,  32   b ) to the contact chips  39   a ,  39   b  (see  FIG. 3 ). 
   Electric power supplied from the power source  1  is transmitted, via the slip ring  40  and the contact chips  39   a ,  39   b , to target wires  31   a ,  31   b . Then, the target wires  31   a ,  31   b  are short-circuited, and an arc is generated at an arc generation position between the tips of the target wires  31   a ,  31   b.    
   The tips of the two target wires  31   a ,  31   b  are continuously melted by the arc heat. By selecting an appropriate thermal spray voltage and the target wire feeding speed, it is possible to keep the arc. Meanwhile, the compressed gas is blasted substantially perpendicularly to the feeding direction of the target wires  31   a ,  31   b , from the compressed gas blasting hole  42   a  of the nozzle  42 . The metal, melted by the arc heat, is atomized and blasted by the jet of the compressed gas, forming a thermal spray blast  43  to be sprayed onto the inner surface of the first bore  8   a . Simultaneously, the spraying gun  37  is rotated by the spraying gun rotation mechanism  34 , and the two pail packs  32   a ,  32   b  are rotated by the rotation mechanism  33  in synchronization with the rotation of the spraying gun  37 . 
   The operation of the second arc spraying unit  50  is the same as that of the first arc spraying unit  30  described above. Specifically, the compressed gas from the compressor  3  is supplied to the nozzle  62  via the rotary coupling  61  of the spraying gun  57 . Also, two target wires  51   a ,  51   b  from the pail packs  52   a ,  52   b  are moved by the push-side wire feeders  55   a ,  55   b . The wires are then sent by the pull-side wire feeder  58  to the contact chips  59   a ,  59   b  (See  FIG. 3 ) which are provided at a lower portion of the spraying gun  57 . Electric power is supplied from the power source  1 , via the slip ring  60 , to the contact chips  59   a ,  59   b . Then, the target wires  51   a ,  51   b  are short-circuited at an arc generation position, thereby generating an arc between the tips of the two wires. 
   Meanwhile, the compressed gas is blasted substantially perpendicularly to the feeding direction of the target wires  51   a ,  51   b , from the compressed gas blasting hole  62   a  of the nozzle  62 . The metal, melted by the arc heat, is atomized and blasted by the jet of compressed gas, forming a thermal spray blast  63  to be sprayed onto the inner surface of the third bore  8   c . Simultaneously, the spraying gun  57  is rotated by the spraying gun rotation mechanism  54 , and the two pail packs  52   a ,  52   b  are rotated by the rotation mechanism  53  in synchronization with the rotation of the spraying gun  57 . 
   Upon rotation of the two spraying guns  37 ,  57 , the lift mechanism  65  lowers the spraying guns  37 ,  57  in the arrow-indicated direction X 2 . In this way, the inner surfaces of the first bore and the third bore are thermally coated. Thereafter, when a stop signal is inputted to the power source  1 , the blasting of the compressed gas is stopped. At the same time, the feeding of the target wires  31   a – 31   b  and  51   a – 51   b  is stopped, and the supply of the thermal spray current is stopped. Thus, the thermal spraying is terminated. 
   Then, the lift mechanism  65  lifts the two spraying guns  37 ,  57  out of the cylinder block  8  in an arrow-indicated direction X 1 . Next, the rotation axis positioning mechanism  66  moves the spraying guns  37 ,  57  horizontally so that the spraying guns&#39; rotation axis  37   a  and the spraying guns&#39; rotation axis  57   a  align with the center lines of the second bore  8   b  and the fourth bore  8   d , respectively. Thereafter, the same operation as described above is repeated to thermally coat the inner surface of the second bore  8   b  and the inner surface of the fourth bore  8   d.    
   In the first embodiment described above, use is made of two kinds of wire feeders, i.e., the push-side and the pull-side wire feeders, for ensuring stable supply of the target wires. According to the present invention, however, either the push-side feeders or the pull-side feeders may suffice. Further, the synchronized rotation between the rotation mechanism and the spraying gun rotation mechanism may be achieved by providing each of these rotation mechanisms with a servomotor configured to be controlled by a servo-controller. 
   With the above-described arrangement, a perfect synchronization is possible between the rotation of the wire supplying sources (the pail packs in the illustrated embodiment) and the rotation of the spraying guns through the entire thermal spraying procedure, so that the wire support cables are not twisted. Further, it is possible to make compact the spraying guns, whose rotation radius is reduced to e.g. 70 mm, whereby the spraying guns do not interfere with jigs or the object being coated. Thus, the arrangement as shown in  FIG. 1  is possible, in which two arc spraying units are disposed at an interval corresponding to the bores for performing simultaneous thermal spraying to the internal surfaces of the bores. Advantageously, this contributes to enabling efficient and low-cost thermal spraying and improving the productivity significantly. 
   Further, in the arc spraying system according to the first embodiment of the present invention, target wires are stored in the pail packs. This makes it possible to increase the amount of loadable target wires up to three times over the amount possible in the conventional spraying systems. Therefore, a long-time continuous operation is possible, which serves to remarkably improve the productivity. 
     FIG. 4  shows an electric arc spraying system according to the second embodiment of the present invention. Like  FIG. 1 ,  FIG. 4  illustrates how the inner surfaces of bores formed in a cylinder block of a 4-cylinder engine is thermally coated with the use of two arc spraying units. In the second embodiment, the first arc spraying unit  47  is provided with two pail packs  32   a ,  32   b  that are disposed in tiers, i.e. one above the other, with the rotation axes of the two pail packs  32   a ,  32   b  aligned with the rotation axis  44   a  of a wire feeder rotation mechanism  44 . 
   Likewise, in the second arc spraying unit  67 , two pail packs  52   a ,  52   b  are disposed in tiers, with their rotation axes aligned with the rotation axis  64   a  of a wire feeder rotation mechanism  64 . The other components, having the same function as the counterparts of the first embodiment, are indicated by the same signs used as in  FIG. 1 , and no separate description thereof is given below. Further, the arc spraying system of the second embodiment operates in essentially the same manner as the system of the first embodiment, and no separate description is given. 
   In addition to the advantages of the first embodiment, the second embodiment enjoys the following advantages. As noted above, the rotation axes of the pail packs  52   a – 52   b  of the second embodiment is aligned with the rotation axis of the rotation mechanism  64 . As a result, the centrifugal force occurring upon rotation of the pail packs  52   a – 52   b  does not collapse but preserve the neat piles of the accommodated target wires. Therefore, the supply of the target wires is performed properly. Further, it is possible to reduce both the size of the components of the driving source for the rotation mechanism  64  and the size the relevant mechanical structure, since the pail packs and the rotation mechanism have a smaller moment of inertia and therefore requires smaller driving force. 
     FIG. 5  shows an electric arc spraying system according to the third embodiment of the present invention. Like  FIG. 1 ,  FIG. 5  illustrates an instance in which two arc spraying units are used for thermal spraying. It should be noted that in the figure, elements such as a cylinder block, a spraying gun lift mechanism and a spraying gun rotation axis positioning mechanism, which are actually used, are not shown since these are the same as those shown in  FIG. 1 . 
   As shown in  FIG. 5 , two wire reels  71   a ,  71   b  hold two coils of target wires  31   a ,  31   b  respectively. The push-side wire feeders  73   a ,  73   b  send the target wires  31   a ,  31   b . These two wire reels  71   a ,  71   b  and two push-side wire feeders  73   a ,  73   b  are mounted on a wire feeder rotation mechanism  74  and rotated by a motor  74   a  in synchronization with a spraying gun rotation mechanism  80  to be described later. The rotation mechanism has its rotation axis  74   b  extending in parallel to a spraying gun&#39;s rotation axis  76   a . Wire support cables  75   a ,  75   b  are flexible, and guide the target wires  31   a ,  31   b  which come out of the two push-side wire feeders  73   a ,  73   b  until they reach a spraying gun  76 . 
   The spraying gun  76  is provided with a pull-side wire feeder  77 , which further sends the two target wires  31   a ,  31   b  from the wire reels  71   a ,  71   b . The target wires  31   a ,  31   b  are thus sent respectively to two contact chips  39   a ,  39   b  (See  FIG. 3 ) provided at a lower portion of the spraying gun  76 . A power supply slip ring  78  receives electric power from the power source  1 , and supplies the power to the two contact chips  39   a ,  39   b.    
   The compressed gas supply rotary coupling  79  receives compressed gas from the compressor  3 . The compressed gas is then supplied to the nozzle  42  (See  FIG. 3 ) at the tip of the spraying gun  76 . The nozzle  42  has a compressed gas blasting hole  42   a , from which the compressed gas is blasted substantially perpendicularly to the feeding direction of the target wires  31   a ,  31   b . The spraying gun  76  is mounted on a spraying gun rotation mechanism  80 , and is rotated by a motor  80   a.    
   The second arc spraying unit  90  has essentially the same function as of the first arc spraying unit  70 , and is provided with wire reels  91   a – 91   b , target wires  51   a – 51   b , push-side wire feeders  93   a – 93   b , a wire feeder rotation mechanism  94 , a motor  94   a  of the rotation mechanism (its rotation axis  94   b ), a spraying gun  96  (its rotation axis  96   a ), wire support cables  95   a – 95   b , a pull-side wire feeder  97 , contact chips  59   a – 59   b , a power supply slip ring  98 , a compressed gas supply rotary coupling  99 , a nozzle  62  (with a compressed gas blasting hole  62   a ), a spraying gun rotation mechanism  100  and a motor  100   a . These components function in the same manner as the counterparts of the first arc spraying unit  70 . 
     FIG. 5  does not illustrate elements such as a cylinder block, a spraying gun lift mechanism or a spraying gun rotation axis positioning mechanism, which are actually provided. The arc spraying system of the third embodiment operates in the same way as that of the first embodiment in  FIG. 1 . The difference in arrangement between the third and the first embodiments is that the third embodiment utilizes wire reels  71   a – 71   b  in place of the pail packs of the first embodiment. 
   As a result of the above-described arrangement, it is possible to reduce the size of the spraying guns so that the guns do not interfere with jigs or the object being coated. Thus, in the third embodiment again, the two arc spraying units  70 ,  90  can be disposed at an interval corresponding to two bores whose internal walls are subjected to simultaneous thermal spraying. Advantageously, this contributes to enabling efficient and low-cost thermal spraying and also to improving the productivity significantly. 
   It should be noted here that in the arc spraying unit  70  according to the third embodiment, the distance between the wire reels  71   a ,  71   b  and the spraying gun  76  can be short enough to dispose of the push-side wire feeders  73   a – 73   b . On the other hand, when the pull-side wire feeder  77  is not provided to attain further size reduction of the spraying gun  76 , the push-side wire feeders  73   a ,  73   b  need to be provided. 
   The spraying gun rotation mechanism  80  may be configured to vertically move independently of the rotation mechanism  74 . For more stable supply of the target wires  31   a – 31   b , however, it may be preferable to cause the spraying gun rotation mechanism  80  and the rotation mechanism  74  to simultaneously move upward or downward. 
     FIG. 6  shows an electric arc spraying system according to a fourth embodiment of the present invention. Like  FIG. 5 ,  FIG. 6  illustrates an instance in which two arc spraying units are used for performing thermal spraying. It should be noted that the figure does not show a cylinder block, a spraying gun lift mechanism and a spraying gun rotation axis positioning mechanism, which are actually used, since these are the same as those shown in  FIG. 1 . 
   As shown in  FIG. 6 , the rotation mechanism&#39;s axes  74   b ,  94   b  are not parallel to the rotation axes  76   a ,  96   a  of the spraying gun rotation mechanism. Instead, the axes  74   b ,  94   b  are slanted to the rotation axes  76   a ,  96   a  at an angle θ 1 , which ensures more stable supply of the target wires from the reel to the gun. The other arrangements and functions of the fourth embodiment are the same as those of the third embodiment shown in  FIG. 5 , and the same reference characters are used for indicating the same or similar elements. 
   In the first through fourth embodiments described above, the bores&#39; inner surfaces are thermally coated by using two arc spraying units. According to the present invention, three or more electric arc spraying units may be used simultaneously, so that the thermal coating can be more efficiently. 
   In the first embodiment illustrated in  FIG. 1  and the second embodiment illustrated in  FIG. 4 , the wire support cables  36   a – 36   b  have their front ends connected to the pull-side wire feeder  38 , and their base ends connected to the push-side wire feeders  35   a – 35   b . In this arrangement, the direction in which the target wires are sent out from the push-side wire feeders  35   a ,  35   b  is opposite to the direction in which the target wires go into the pull-side wire feeder  38 . With such a configuration, an inconvenience may occur when two parallel wire support cables are rotated in the manner to be described below. 
   In the situation shown in  FIG. 7 , the pail packs  32   a ,  32   b  are placed on the rotation mechanism  33 , and the target wires  31   a ,  31   b  from the pail packs are sent by the push-side wire feeders  35   a ,  35   b  respectively. The target wires  31   a ,  31   b  are guided by the flexible wire support cables  36   a ,  36   b  until they reach the pull-side wire feeder  38 . 
   As shown in  FIG. 7(A) , initially, two wire support cables  36   a ,  36   b  are arranged in parallel to each other. Then, the pull-side wire feeder  38  turns in a predetermined direction (anticlockwise in the figure), and in synchronization with this rotation, the rotation mechanism  33  turns in the opposite direction (clockwise). Correspondingly, the wire support cables  36   a ,  36   b  are caused to rotate in the arrow-indicated direction. Since the cables are flexible and their ends are fixed, the wire support cable  36   a  is compressed, whereas the other wire support cable  36   b  is stretched, as shown in  FIG. 7(B)  through  FIG. 7(D) . Then, as the cables  36   a ,  36   b  take the parallel position shown in  FIG. 7(E) , their lengths return to the initial one. Thereafter (not shown in the figure), the wire support cables  36   a  is stretched and the wire support cables  36   b  is compressed. 
   In the above-described process, the target wires  31   a – 31   b  in the cables are not subjected to the compressing nor stretching force because they are not fixed at their ends. Thus, the frictional resistance between the wires  31   a – 31   b  and the cables  36   a – 36   b  varies as the cables  36   a ,  36   b  rotate. As a result, the target wires  31   a ,  31   b  may undulate, which hinders a proper wire feeding operation. Specifically, the length of the target wires  31   a ,  31   b  protruding from the contact chips  39   a ,  39   b  (see  FIG. 3 ) may fail to remain constant (that is, becomes too long or too short). This can lead to drawbacks such as occurrence of short-circuiting between the target wires, occurrence of sputters or unexpected variation of the arc-generating position with respect to the compressed gas blasting hole  42   a . Consequently, it may become difficult to make a uniform thermal coating layer. 
   In order to cope with the above, the two wire support cables  36   a ,  36   b  may be arranged to cross with each other, as shown in  FIG. 8 . This figure illustrates the behavior of the crossed wire support cables  36   a ,  36   b  as they are rotated. Specifically, as shown in  FIG. 8(A) , two wire support cables  36   a ,  36   b  take an initial position in which they are crossed with each other. Then, as show in  FIG. 8(B)  through  FIG. 8(E) , the pull-side wire feeder  38  turns in a predetermined direction (anticlockwise in the figure), while the rotation mechanism  33  turns in the opposite direction (clockwise) synchronously with the wire feeder  38 . In this process, the wire support cables  36   a ,  36   b  also turn in the arrow-indicated direction. With such a cable-crossing arrangement, as seen from the figure, it is possible to prevent the wire support cables  36   a – 36   b  from being compressed or stretched as they are rotated (in other words, their original lengths are unchanged). Therefore, the frictional resistance between target wires  31   a – 31   b  and the wire support cables  36   a – 36   b  does not vary, so that the feeding of the target wire  31   a ,  31   b  is performed stably, and a uniform thermal coating is formed. 
     FIG. 9  shows an electric arc spraying system according to a fifth embodiment of the present invention, illustrating an instance where the thermal spray is performed with the use of only one arc spraying unit. As shown in the figure, the pail packs  32   a ,  32   b  are on a wire feeder rotation mechanism  33 . Target wires  31   a ,  31   b  in the pail packs are sent by push-side wire feeders  35   a ,  35   b  respectively. Two wire support cables  36   a – 36   b  are arranged in parallel to each other from the push-side wire feeders  35   a ,  35   b  to a cable support mechanism  110 . The wire support cables  36   a ,  36   b  are then crossed with each other by the cable support mechanism  110 . Thereafter, the wire support cables  36   a ,  36   b  are parallel to each other from the cable support mechanism  110  to a pull-side wire feeder  38  mounted on the spraying gun  37 . The cable support mechanism  110  is positioned at or near the apex of the cable-extending curve. 
   Referring to  FIGS. 10A and 10B  together with  FIG. 9 , the cable support mechanism  110  is described.  FIG. 10A  is a sectional front or plan view and  FIG. 10B  is a right side view of the support mechanism  110 . As shown in these figures, the cable support mechanism  110  includes a support main body  111 , and a rotation member  112  that is rotatably supported by the main body  111 . The rotation member  112  is formed with two cable insertion holes  112   a – 112   b  crossing with each other. The main body  111  is held by a support post  114  (see  FIG. 9 ). A bearing  113  is provided between the rotation member  112  and the support main body  111  to minimize the time-lag in rotation between the end portion and apex portion of the cables  36   a – 36   b.    
   The spraying system of the fifth embodiment operates in the following manner. The push-side wire feeders  35   a ,  35   b  send the target wires  31   a ,  31   b  from the pail packs  32   a ,  32   b . Since the wire support cables  36   a ,  36   b  are crossed with each other by the cable support mechanism  110 , the target wires  31   a ,  31   b  guided by the wire support cables  36   a ,  36   b  are crossed with each other and sent to the pull-side wire feeder  38  mounted on the spraying gun  37 . 
   As the spraying gun  37  rotates in the arrow-indicated direction as in  FIG. 9  and the rotation mechanism  33  rotates in the opposite direction synchronously with the gun  37 , the wire support cables  36   a ,  36   b  also rotate in the arrow-indicated direction in the figure. Then, the rotation member  112  in the cable support mechanism  110  also rotates in the arrow-indicated direction. In this process, the wire support cables  36   a ,  36   b  are not be contracted or stretched since there is no compressing or pulling force acting on the cables as described with reference to  FIG. 8 . Consequently, there is no change in the frictional resistance between the target wires  31   a ,  31   b  and the wire support cables  36   a ,  36   b . Thus, it is possible to supply the target wires  31   a ,  31   b  stably, and to form a uniform thermal coating layer. 
     FIG. 11  shows an electric arc spraying system according to a sixth embodiment of the present invention. In this embodiment again, the thermal spraying is performed with the use of only one arc spraying unit. As shown in the figure, a cable support mechanism  119  includes a first cable support  120  and a second cable support  130 . In  FIG. 11 , the elements which are the same as or similar to those shown in  FIG. 9  are indicated by the same reference characters, and their functions are not described below. 
   Referring to  FIGS. 12A and 12B  together with  FIG. 11 , the first cable support  120  and the second cable support  130  are described.  FIG. 12A  is a front view, and  FIG. 12B  is a side view of the first cable support  120  and the second cable support  130 . 
   As shown in  FIG. 11  or  FIG. 12A , the first cable support  120  includes a first support main body  121  and a first rotation member  122  which is held rotatably by the first support main body  121 . The rotation member  122  is formed with two parallel cable insertion holes  122   a ,  122   b . The first support main body  121  is supported by a first support post  124  ( FIG. 11 ). A bearing  123  is provided between the first rotation member  122  and the first support main body  121  to minimize the time-lag in rotation between the end portion and apex portion of the cables  36   a – 36   b.    
   Likewise, the second cable support  130  includes a second support main body  131  and a second rotation member  132  which is held rotatably by the second support main body  131 . The rotation member  132  is formed with two parallel cable insertion holes  132   a ,  132   b . The second support main body  131  is supported by a second support post  134 . A bearing  133  is provided between the second rotation member  132  and the second support main body  131  to minimize the time-lag in rotation between the end portion and apex portion of the cables  36   a – 36   b.    
   With the above-described arrangement, two wire support cables  36   a ,  36   b  run in parallel to each other from the push-side wire feeders  35   a ,  35   b  to the first cable support  120 , at which the wire support cables  36   a ,  36   b  go into the first cable support  120 . Then, the wire support cables  36   a ,  36   b  cross with each other between the first cable support  120  and the second cable support  130 , and then go into the second cable support  130 . Thereafter, the wire support cables  36   a ,  36   b  run in parallel to each other from the second cable support  130  to the pull-side wire feeder  38  mounted on the spraying gun  37 . 
   Preferably, the first cable support  120  and the second cable support  130  are attached at an angle to the respective support post  124 ,  134  as shown in  FIG. 11 , allowing the wire support cables  36   a ,  36   b  to move smoothly through the holes in the rotation members. 
   The operation of the sixth embodiment is substantially the same as that of the fifth embodiment. Further, due to the twin cable supports  120 ,  130 , the target wires  31   a ,  31   b  are supplied more stably, which contributes to forming of a more uniform thermal coating layer. 
   In the fifth embodiment shown in  FIG. 9  and the sixth embodiment shown in  FIG. 11 , the cable support mechanisms are supported by a support post. Alternatively, these cable support mechanisms may be suspended from the ceiling, or may be fixed to a wall.