Patent Publication Number: US-11389811-B2

Title: Electrostatic coating device

Description:
TECHNICAL FIELD 
     The present invention relates to an electrostatic coating device that atomizes electrically charged coating material and blows the same to a target of coating (a workpiece). 
     BACKGROUND ART 
     An electrostatic coating device includes a voltage generating unit for generating a voltage to be applied to coating material supplied from a coating material source, a rotary atomizing head for ejecting electrically charged coating material, and an air motor, the voltage generating unit and the air motor being accommodated in a housing (see Japanese Patent No. 4726188, for instance). The air motor is driven with supply of driving air to a turbine, thereby rotating the rotary atomizing head. The coating material is accompanied by sprayed air discharged from the periphery of the rotary atomizing head and flies in the form of mist to the workpiece. 
     When driving air is introduced into a motor chamber in which the air motor is accommodated, the temperature of the driving air rapidly drops due to adiabatic expansion. This causes the wall surfaces of the motor chamber and the surrounding atmosphere to be cooled, possibly resulting in dew condensation. In the event of such a situation, water droplets attach to the workpiece with coating material, contributing to reduction in the coating quality. 
     Thus, Japanese Patent No. 4705100 proposes an arrangement for preventing dew condensation. 
     SUMMARY OF INVENTION 
     The conventional art described in Japanese Patent No. 4705100 requires formation of a flow passage for insulating air separately from a flow passage for driving air. Accordingly, it involves complicated structures of the air flow passages and hence of the electrostatic coating device, leading to increase in its size. 
     A general object of the present invention is to provide an electrostatic coating device having a housing of a simple structure. 
     A major object of the present invention is to provide an electrostatic coating device capable of preventing dew condensation by means of air circulating in the housing. 
     According to an embodiment of the present invention, provided is an electrostatic coating device including a voltage generating unit configured to generate a voltage to be applied to coating material, an air motor configured to rotate a rotary atomizing head configured to eject the coating material, and a housing configured to house the voltage generating unit and the air motor. The housing has a first air flow passage surrounding the voltage generating unit, a second air flow passage externally surrounding an air turbine which is a component of the air motor, and a third air flow passage through which the first air flow passage and the second air flow passage communicate with each other. 
     In this manner, the present invention causes air that purges around the voltage generating unit to flow into the second air flow passage so that the air turbine of the air motor is covered by the air flowing through the second air flow passage. In other words, an air curtain is formed around the air turbine. Due to the presence of this air curtain, transfer of the heat of wall surfaces of the motor chamber or of the atmosphere to driving air is avoided even if adiabatic expansion occurs when driving air is introduced into the motor chamber in the housing in order to drive the air motor. 
     Accordingly, dew condensation is prevented. Therefore, attachment of water droplets to the workpiece with coating material and resulting reduction in the coating quality or the like can be avoided. 
     Besides, in this case, the air curtain can be formed with air used for purging the voltage generating unit, thus eliminating the necessity to newly form a passage for directing air to the second air flow passage. Accordingly, complication of the air passages can be avoided. That is, the structure of the electrostatic coating device can be simplified and also increase in its size can be avoided. 
     In a case where the housing has a voltage generating unit housing section configured to house the voltage generating unit, and an air motor housing section configured to house the air motor, the third air flow passage is preferably configured to pass through a point of coupling between the voltage generating unit housing section and the air motor housing section. This is because occurrence of dew condensation or electrolytic corrosion in valves provided at the point of coupling can be avoided. 
     Further, the third air flow passage may be configured to pass around a valve provided in a coating material supply passage through which the coating material is supplied. 
     Further, the housing may have a discharge port from which air that has flowed through the second air flow passage is discharged. This can facilitate disposal of air. 
     According the present invention, an air curtain is formed around the air turbine with air that has purged the voltage generating unit during performing of electrostatic coating (during driving of the air motor). That is, an air curtain is present between the air turbine and the atmosphere. This avoids transfer of the heat of wall surfaces of the motor chamber and the surrounding atmosphere to driving air that has adiabatically expanded, and as a result, dew condensation is prevented. Therefore, attachment of water droplets to the workpiece and resulting reduction in the coating quality are avoided. 
     Besides, since air that has purged the voltage generating unit is introduced into the second air flow passage, it is not necessary to newly form a passage for directing air to the second air flow passage. Accordingly, complication of the air passages is avoided, so that the electrostatic coating device can be simplified in structure and increase in the size of the electrostatic coating device can be avoided as well. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of relevant portions of an electrostatic coating device according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view seen from the arrows at line II-II in  FIG. 1 ; 
         FIG. 3  is an illustration seen from the direction of arrow III in  FIG. 1  with a cover member removed; and 
         FIG. 4  is a schematic cross-sectional view of relevant portions of a first communication passage, a circular recess, and a second communication passage, which constitute a third air flow passage. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The electrostatic coating device according to the present invention is described in detail below by showing the preferred embodiment thereof and with reference to the accompanying drawings. 
       FIG. 1  is a schematic cross-sectional view of relevant portions of an electrostatic coating device  10  according to an embodiment. This electrostatic coating device  10  is provided at a tip arm of an articulated robot, not illustrated, and blows coating material to a workpiece, such as an automobile body not illustrated, after the articulated robot performed appropriate operations. 
     The electrostatic coating device  10  includes a housing  12  made of resin. More specifically, the housing  12  has a cascade housing section  14  (voltage generating unit housing section) extending substantially linearly, and a motor housing section  16  (air motor housing section) attached so as to be slightly inclined relative to the cascade housing section  14 , and the housing  12  is assembled by the coupling of the cascade housing section  14  and the motor housing section  16  via a coupling ring  18 . The cascade housing section  14  houses a cascade  20  serving as a voltage generating unit and the motor housing section  16  houses an air motor  22 . 
     The cascade housing section  14  is hollow and houses a purging air supply tube  24 . The purging air supply tube  24  is connected to an air source, not illustrated, via a fitting  28 . 
     The cascade housing section  14  has a long first housing hole  32 . The purging air supply tube  24  and the first housing hole  32  are in communication with each other via a communication hole  33 . On the other hand, the motor housing section  16  has a relatively short second housing hole  34 , and the second housing hole  34  is continuous with the first housing hole  32 . 
     The cascade  20  is housed in the first housing hole  32  and the second housing hole  34  thus being continuous. A given clearance is formed between the first housing hole  32  and the second housing hole  34 , and the cascade  20 . This clearance serves as a first air flow passage  40 . That is, the purging air supply tube  24  is connected to the first air flow passage  40  via the communication hole  33 . The cascade  20  is positioned and fixed in the first housing hole  32  and the second housing hole  34  via buffer materials  42 ,  44 . 
     The cascade  20  has a voltage generating section  52  with which a low-voltage cable  50  is connected, a voltage boosting section  54  containing a step-up transformer for increasing a voltage generated in the voltage generating section  52 , and an output terminal  56  for outputting the increased voltage (high voltage). That is, a relatively low voltage generated in the voltage generating section  52  is increased in the voltage boosting section  54 , after which the high voltage is applied to coating material via the output terminal  56 . 
     At a coupling portion of the cascade housing section  14  that is coupled with the motor housing section  16 , multiple docking valves  58  are provided as shown in  FIG. 2 , which is a cross-sectional view seen from the arrows at line II-II in  FIG. 1 . The docking valves  58  are valves that permit or block communication between various air channels and the like provided on the side of the cascade housing section  14 , including the purging air supply tube  24 , and various air channels (for example, an air discharge passage) and the like provided on the side of the motor housing section  16 . 
     A motor chamber  60  is formed inside the motor housing section  16 , with the air motor  22  housed in the motor chamber  60 . A portion of the inner wall of the motor chamber  60  is cut away in an annular shape such that a given annular clearance is formed between the cut-away portion and a wall  63  for forming an exhaust passage  62  in which driving air discharged from an air turbine  61  flows. This annular clearance serves as a second air flow passage  64 . Between the air motor  22  and the wall  63 , an O-ring  65  for sealing therebetween is provided. 
     The air motor  22  has a hollow shaft  66  provided with the air turbine  61 , and a feed tube  69  provided with a coating material supply passage  67  and a cleaning fluid supply passage  68  is passed through the hollow interior of the hollow shaft  66 . Coating material supplied from a coating material source flows through the coating material supply passage  67 , while cleaning fluid supplied from a cleaning fluid source flows through the cleaning fluid supply passage  68 . 
     A rotary atomizing head  70  is attached at the tip of the hollow shaft  66 . The air turbine  61  and the hollow shaft  66  rotate at high speed integrally with the rotary atomizing head  70  under the action of driving air supplied from a driving air supply tube, not illustrated. 
     A cover member  72  is positioned and fixed on an annular projection  16   a  of the motor housing section  16 . As shown in  FIG. 3 , which is an illustration seen from the direction of arrow III in  FIG. 1  with the cover member  72  removed, the cover member  72  covers and protects multiple gates  74  (valves) provided in the motor housing section  16 . Opening and closing of the respective gates  74  permit and block the communication between the coating material supply passage  67  and the coating material source and between the cleaning fluid supply passage  68  and the cleaning fluid source. Communication between the coating material supply passage  67  and the coating material source and communication between the cleaning fluid supply passage  68  and the cleaning fluid source never occur simultaneously. That is, either one of the coating material and the cleaning fluid is selectively ejected. 
     The cover member  72  is spaced from the motor housing section  16  by a given distance. That is, a clearance is formed between the cover member  72  and the motor housing section  16 . As described later, this clearance (a third communication passage  84 ) is where compressed air having passed through the first air flow passage  40  and moving toward the second air flow passage  64  flows. 
     Further, the housing  12  has a first communication passage  80  running from the vicinity of the output terminal  56  in the second housing hole  34  toward the docking valves  58 , a circular recess  81  formed around the docking valves  58  between the cascade housing section  14  and the motor housing section  16 , a second communication passage  82  running from the docking valves  58  toward the gates  74 , a third communication passage  84  running from the gates  74  toward the second air flow passage  64 , and a discharge passage  86  running from the second air flow passage  64  to a discharge port  85 . Although the housing  12  also has a passage for discharging the driving air in the exhaust passage  62 , this passage is not shown in the drawing. 
     The first air flow passage  40  and the second air flow passage  64  are in communication with each other via the first communication passage  80 , the circular recess  81 , the second communication passage  82 , and the third communication passage  84 . That is, the first communication passage  80 , the circular recess  81 , the second communication passage  82 , and the third communication passage  84  serve as a third air flow passage through which the first air flow passage  40  and the second air flow passage  64  communicate with each other. As will be understood from  FIGS. 2 and 3 , a certain phase difference is provided between the first communication passage  80  and the second communication passage  82  in the circular recess  81  and a certain phase difference is also provided between the openings of the second communication passage  82  and the second air flow passage  64  in the third communication passage  84 . 
     The electrostatic coating device  10  according to this embodiment is basically structured as described above, and the action and effects thereof are now described. 
     First, compressed air is supplied from the air source. The compressed air is introduced into the first housing hole  32  through the purging air supply tube  24 . The compressed air fills the insides of the first housing hole  32  and the second housing hole  34 , namely the first air flow passage  40 , and covers the entire cascade  20 . Thus, the output terminal  56  is also covered by the compressed air. 
     As compressed air is further supplied, excess compressed air enters the first communication passage  80  provided in the vicinity of the output terminal  56  from the second housing hole  34  (the first air flow passage  40 ), as shown in  FIG. 4 . The compressed air further moves from the first communication passage  80  toward the docking valves  58 , shown in  FIG. 2 , to enter the circular recess  81 . The compressed air then circulates to the opening of the second communication passage  82  having a phase difference. During this process, the surroundings of each docking valve  58  are purged. This prevents dew condensation or electrolytic corrosion from occurring on the docking valves  58 . 
     The compressed air further enters through the opening of the second communication passage  82 , flows through the second communication passage  82  formed in the vicinity of the second housing hole  34 , and then enters the third communication passage  84  formed between the motor housing section  16  and the cover member  72 . Then, while circulating into the opening of the second air flow passage  64  having a phase difference, the compressed air purges around each gate  74 . 
     Subsequently, the compressed air further enters through the opening of the second air flow passage  64  and enters the annular portion of the second air flow passage  64 , shown in  FIG. 1 . Since the second air flow passage  64  is specifically located so as to externally surround the wall  63  near the air turbine  61 , the wall  63  and the exhaust passage  62  are surrounded by the compressed air in the second air flow passage  64 . In other words, an air curtain is formed around the air turbine  61  and the exhaust passage  62  across the wall  63 . 
     The compressed air that has entered the second air flow passage  64  flows through the discharge passage  86  and is discharged out of the housing  12  from the discharge port  85  formed inside the cascade housing section  14 . 
     As described above, in this embodiment, the compressed air that purges the insides of the first housing hole  32  and the second housing hole  34 , in which the cascade  20  is housed, is also used for purging around the docking valves  58  and for purging around the gates  74 , and as an air curtain that surrounds the air turbine  61  and the exhaust passage  62 . This means that there is no need to separately form an air passage for guiding compressed air supplied from the compressed air source directly into the second air flow passage  64 . Besides, in this case, only the third communication passage  84  and the discharge passage  86  need to be provided in an existing electrostatic coating device  10 . Accordingly, the flow passage for compressed air is simplified. 
     For these reasons, the electrostatic coating device  10  can be simplified. That is, complication in the structure of the electrostatic coating device  10  or increase in its size can be avoided. 
     Meanwhile, driving air is supplied into the motor chamber  60  through the driving air supply tube. Under the action of this driving air, the air turbine  61 , which is a component of the air motor  22 , starts to rotate at high speed integrally with the hollow shaft  66  and the rotary atomizing head  70 . 
     Furthermore, the cascade  20  is energized. A voltage is generated in the voltage generating section  52  with which the low-voltage cable  50  is connected, is increased in the voltage boosting section  54 , and is taken from the output terminal  56  as a high voltage. 
     Further, pilot air is supplied to the gates  74  via a pilot air supply tube, not illustrated. Each of the gates  74  is formed by detachable insertion of a rod into a port, for example, such that when the rod detaches from the port upon supply of pilot air, coating material is supplied from the coating material source to the coating material supply passage  67 . The coating material is subjected to high voltage via the output terminal  56 , further atomized with the centrifugal force of the rotary atomizing head  70 , and then applied to the workpiece by electrostatic coating. 
     The driving air that has driven the air turbine  61  circulates to the exhaust passage  62  at relatively low temperature because the driving air has adiabatically expanded. Here, the exhaust passage  62  is surrounded by the air curtain flowing through the second air flow passage  64  across the wall  63 . This avoids transfer of the heat of the atmosphere around the motor housing section  16  to the low-temperature driving air in the exhaust passage  62 . 
     As a result, occurrence of dew condensation around the electrostatic coating device  10  and electrolytic corrosion of the air motor  22  is prevented. Therefore, attachment of water droplets to an automobile body with coating material and resulting reduction in the coating quality can be avoided. 
     During performing of the electrostatic coating, the circulation of compressed air as described above is continued in the first air flow passage  40 , the first communication passage  80 , the circular recess  81 , the second communication passage  82 , the third communication passage  84  (the third air flow passage), and the second air flow passage  64 . This can avoid degradation of resin-made members, including the cascade housing section  14 , electrolytic corrosion of the output terminal  56 , and the like. Also, by purging around the docking valves  58  and the gates  74 , it is possible to avoid the occurrence of dew condensation or electrolytic corrosion around the docking valves and the gates and remove coating material residue and the like. 
     When supply of pilot air is stopped, the rod re-enters the port to place the gate  74  in a closed state. This blocks the communication between the coating material source and the coating material supply passage  67 , whereby the ejection of coating material is stopped. 
     In this state, cleaning fluid is ejected into the feed tube  69  from the cleaning fluid source through the cleaning fluid supply passage  68 . With this cleaning fluid, the outer periphery of the feed tube  69  is cleaned. 
     The present invention is not specifically limited to the foregoing embodiment and various modifications are possible without departing from the scope of the present invention. 
     For example, the first air flow passage  40  and the second air flow passage  64  may be in communication via a single communication passage. In such a case, that communication passage serves as the third air flow passage. 
     REFERENCE SIGNS LIST 
     
         
           10  electrostatic coating device 
           12  housing 
           14  cascade housing section 
           16  motor housing section 
           18  coupling ring 
           20  cascade 
           22  air motor 
           32  first housing hole 
           33  communication hole 
           34  second housing hole 
           40  first air flow passage 
           56  output terminal 
           58  docking valve 
           60  motor chamber 
           61  air turbine 
           62  exhaust passage 
           63  wall 
           64  second air flow passage 
           67  coating material supply passage 
           68  cleaning fluid supply passage 
           69  feed tube 
           70  rotary atomizing head 
           72  cover member 
           74  gate 
           80  first communication passage 
           81  circular recess 
           82  second communication passage 
           84  third communication passage 
           85  discharge port 
           86  discharge passage