Rotary type electrostatic spray painting device
BACKGROUND OF THE INVENTION 
     The present invention relates to a rotary type electrostatic spray painting device. 
     One known electrostatic spray painting device which has been used for painting, for example, bodies of motor cars, is of the rotary type and comprises a rotary shaft supported by ball bearings or roller bearings arranged within the housing of the painting device, and a cup shaped spray head fixed onto the front end of the rotary shaft. In this painting device, a negative high voltage is applied to the spray head, and paint is fed onto the inner circumferential wall of the spray head. Thus, fine paint particles charged with electrons are sprayed from the spray head and are attracted by an electrostatic force onto the surface of the body of a motor car, which is grounded. As a result of this, the surface of the body of a motor car is painted. In such a rotary type electrostatic spray painting device, about 90 percent of the paint sprayed from the spray head, can be efficiently used for painting the surface to be painted. Thus the amount of the paint which is wasted is small and, as a result, rotary type electrostatic spray painting devices are used in various industries. 
     In order to form a beautifully finished painted surface, it is necessary to prevent the spray paint from forming air bubbles. It is also necessary to reduce the size of the particles of paint as much as possible. However, in a conventional rotary type electrostatic spray painting device, since spray paint contains air bubbles therein, a large number of air bubbles are present within the paint layer formed on the surface to be painted and, as a result, it is difficult to form a beautifully finished surface. 
     In order to prevent the spray paint from developing air bubbles, another rotary type of electrostatic spray painting device has been proposed, in which a plurality of grooves is formed on the inner wall of the tip of the spray head. In this rotary type electrostatic spray painting device, since paint is spouted from the grooves of the spray head in the form of a filament and then divided into fine particles, it is possible to prevent the resulting fine particles from containing air bubbles. However, in this known rotary type electrostatic spray painting device, the filaments and particles of paint are relatively large size, and it is therefore difficult to form a beautifully finished surface. 
     As mentioned above, in order to form a beautiful finished surface when the surface is painted by using sprayed paint, it is necessary to reduce the size of the particles of paint as much as possible. In the case wherein the paint is divided into fine particles by using the centrifugal force caused by the rotation of a spray head in a rotary type spray painting device, the strength of the centrifugal force, which is related to the rotating speed of the spray head, has a great influence on the size of the particles of paint. In other words, the higher the rotating speed of the spray head, the smaller the size of the particles of paint becomes. Consequently, in order to form a beautifully finished surface by using a rotary type electrostatic spray painting device, it is desirable to increase the rotating speed of the spray head as much as possible. In conventional rotary type electrostatic spray painting devices, ball bearings or roller are used for supporting the rotary shaft of the electrostatic spary painting device and, in addition, a lubricant, such as grease, is confined within the bearings. However, when such bearings, which are lubricated by grease, are rotated at a high speed, the bearings instantaneously deteriorate. Therefore, in conventional rotary type electrostatic spray painting devices which use bearings which are lubricated by grease, the maximum rotating speed of the rotary shaft, and consequently of the spray head, is at most 20,000 r.p.m. However, in known arrangements where the rotating speed of the spray head is about 20,000 r.p.m., the size of the particles of paint is relatively large and, thus, it is difficult to form a beautifully finished painted surface. 
     The painting process for the bodies of motor cars comprises a primary spraying step, an undercoating step, and a finish painting step. However, since it is difficult to form a beautifully finished surface by using a conventional rotary type electrostatic spray painting device, such a device is used for carrying out the undercoating step, but not the finish painting step. 
     One known system for lubricating ball and roller bearings injects lubricating oil of a low viscosity into the region between the inner race and the outer race of the bearing. In this manner the friction between the balls or rollers and such races is greatly reduced, and the heat caused by the friction is absorbed by the lubricating oil. In situations where the above-mentioned jet lubricating system is applied to a rotary type electrostatic spray painting device, it is possible to increase the rotating speed of the rotary shaft of the electrostatic spray painting device over the speeds attainable when grease lubricated bearings are used. However, since the jet lubricating system requires a complicated and bulky lubricating oil feed arrangement, it is particularly difficult to apply such a jet lubricating system to a rotary type electrostatic spray painting device. In addition, if the lubricating oil mixes with the paint, the appearance of the painted surface is damaged. It is practically impossible to prevent the lubricating oil from leaking into the paint and, thus, it is inadvisable to apply the jet lubricating system to a rotary type electrostatic spray painting device. 
     A known electrostatic spray painting device which is capable of reducing the size of the particles of paint to a great extent while preventing the paint particles from containing air bubbles therein, divides the paint into fine particles by injecting a stream of air. In this air injection type electrostatic spray painting device, since the size of the particles of sprayed paint can be reduced to a great extent, as mentioned above, it is possible to form a beautifully finished surface. Consequently, the air injection type electrostatic spray painting device has been used for carrying out the finish painting step for the bodies of motor cars. It is a problem with the known air injection type electrostatic spray painting devices that the sprayed paint impinges upon the surface to be painted together with the stream of the injection air and therefore substantial portion of the sprayed paint escapes into the atmosphere with the stream of the injection air. The amount of the paint which adheres to the surface to be painted is only about 40 percent of the paint sprayed from the electrostatic spray painting device. Consequently, air injection type electrostatic spray painting devices suffer from the problem in that the consumption of the paint is inevitably increased. A further problem occurs in that the paint which escapes with the stream of the injection air, causes air pollution within factories. 
     It is therefore an object of the present invention to provide a rotary type electrostatic spray painting device which reduces the size of the particles of paint to be sprayed; reduces the quantity of paint used; and prevents paint particles from containing air bubbles. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a rotary type electrostatic spray painting device which comprises a metallic housing; a metallic rotary shaft rotatably arranged in said housing and having a front end and a rear end; a cup shaped metallic spray head fixed onto the front end of said rotary shaft and having a cup shaped inner wall which has a tip edge, said spray head having an annular tip wall and an annular step portion which radially extends outwardly from the tip edges of said cup shaped inner wall, said annular tip wall axially projecting from an outer periphery of said annular step portion. THe device further comprises feed means for feeding a paint onto said cup shaped inner wall; drive means cooperating with said rotary shaft for rotating said rotary shaft; non-contact type radial bearing means arranged in said housing and cooperating with said rotary shaft for radially supporting said rotary shaft under a non-contacting state; and non-contact type thrust bearing means arranged in said housing and cooperating with said rotary shaft for axially supporting said rotary shaft under a non-contacting state. A negative high voltage is received at a terminal connected to said housing. An electrode is arranged in said housing for electrically connecting said terminal to said spray head. 
    
    
     The present invention may be more fully understood from the description of a preferred embodiment of the invention set forth below, together with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-sectional side view of a rotary type electrostatic spray paint device according to the present invention: 
     FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1; 
     FIG. 3 is a cross-sectional view taken along the line III--III in FIG. 1; 
     FIG. 4 is a cross-sectional view taken along the line IV--IV in FIG. 1; 
     FIG. 5 is an enlarged cross-sectional side view of the spray head illustrated in FIG. 1; 
     FIG. 6 is an enlarged cross-sectional view illustrating the portion enclosed by the circle A in FIG. 5; 
     FIG. 7 is a side view of the tip of the spray head, taken along the arrow VII in FIG. 6; 
     FIG. 8 is a view of the tip of the spray head, taken along the arrow VIII in FIG. 6; 
     FIG. 9 is a view of the tip of a conventional spray head, and; 
     FIG. 10 is a graph showing the relationship between the size of paint particles and the rotating speed of the spray head. 
    
    
     DETAILED OF A PREFERRED EMBODIMENT 
     Referring to FIG. 1, a rotary type electrostatic spray painting device, generally designated by reference numeral 1, is comprised of a generally hollow cylindrical front housing 2 made of metallic material, and a generally hollow cylindrical rear housing 3 made of metallic material. The front housing 2 and the rear housing 3 are firmly joined to each other by bolts 4. A support rod 6, made of electrical insulating material, is fitted into a cylindrical hole 5 formed in the rear housing 3, and this rear housing 3 is fixed onto the support rod 6 by bolts 7. The support rod 6 is supported by a base (not shown). A rotary shaft 8 is inserted into the front housing 2. This rotary shaft 8 comprises a hollow cylindrical portion 8a located in the middle thereof, a shaft portion 8b formed in one piece on the front end of the hollow cylindrical portion 8a, and a shaft portion 8c fixed onto the rear end of the hollow cylindrical portion 8a. A spray head 9 made of metallic material is fixed onto the shaft portion 8b of the rotary shaft 8 by a nut 10. The spray head 9 comprises a spray head supporting member 12 forming therein an annular space 11, and a cup shaped spray head body 13 fixed onto the spray head supporting member 12. As illustrated in FIGS. 1 and 2, a plurality of paint outflow bores 16, each opening into the annular space 11 and smoothly connected to an inner wall 15 of the spray head body 13, is formed in an outer cylindrical portion 14 of the spray head supporting member 12. As illustrated in FIG. 1, an end plate 17 is fixed onto the front end of the front housing 2, and a paint injector 18 is mounted on the end plate 17. The paint injector 18 is connected to a paint reservoir 20 via a paint feed pump 19, and nozzle 21 of the paint injector 18 is directed to the cylindrical inner wall of the outer cylindrical portion 14 of the spray head supporting member 12. 
     A pair of non-contact type tilting pad radial air bearings 22 and 23 is arranged in the front housing 2, and the rotary shaft 8 is rotatably supported on the front housing 2 via a pair of the tilting pad radial air bearings 22 and 23. Both the tilting pad radial air bearings 22 and 23 have the same construction and, therefore, the construction of only the tilting pad radial air bearing 22 will be hereinafter described. Referring to FIGS. 1 and 3, the tilting pad radial air bearing 22 comprises three pads 24, 25, 26 arranged to be spaced from the outer circumferential wall of the hollow cylindrical portion 8a of the rotary shaft 8 by an extremely small distance, and three support pins 27, 28, 29 supporting the pads 24, 25, 26, respectively. Spherical tips 30, 31, 32 are formed in one piece on the inner ends of the support pins 27, 28, 29, and are in engagement with spherical recesses formed on the rear faces of the pads 24, 25, 26, respectively. Consequently, the pads 24, 25, 26 can swing about the corresponding spherical tips 30, 31, 32, each functioning as a fulcrum. A bearing support frame 33 is fixed onto the outer circumferential wall of the front housing 2 by means of, for example, bolts (not shown), and the support pins 28, 29 are fixed onto the bearing support frame 33 by means of nuts 34, 35, respectively. In addition, one end of a support arm 36 having a resilient plate shaped portion 36a is fixed onto the bearing support frame 33 by means of a bolt 37, and the other end of the support arm 36 is fixed onto the support pin 27 by means of a nut 38. Consequently, the pad 24 is urged onto the hollow cylindrical portion 8a of the rotary shaft 8 due to the resilient force of the support arm 36. 
     Turning to FIG. 1, a pair of disc shaped runners 39, 40 is inserted into the shaft portion 8c of the rotary shaft 8 and fixed onto the shaft portion 8c via a spacer 41 and a turbine wheel 42 by means of a nut 43. A stationary annular plate 44 is arranged between the runners 39 and 40, and the runners 39, 40 and the annular plate 44 construct a non-contact type thrust air bearing. As illustrated in FIG. 1, each of the runners 39, 40 is arranged to be spaced from the annular plate 44 by a slight distance. The annular plate 44 is fixed onto the front housing 2 via a pair of O rings 45, 46. As illustrated in FIGS. 1 and 4, an annular groove 47, extending along the outer circumferential wall of the annular plate 44, is formed on the inner wall of the front housing 2 and connected to an air feed pump 49 via a compressed air supply hole 48 which is formed in the front housing 2. A plurality of air passages 50, each extending radially inwardly from the annular groove 47, is formed in the annular plate 44. In addition, a plurality of air outflow bores 51, each extending towards the runner 40 from the inner end portion of the corresponding air passage 50, is formed in the annular plate 44, and a plurality of air outflow bores 52, each extending towards the runner 39 from the inner end portion of the corresponding air passage 50, is formed in the annular plate 44. 
     As illustrated in FIG. 1, a turbine nozzle holder 53 is fixed onto the front housing 2 at a position adjacent to the annular plate 44, and an annular air supply chamber 54 is formed between the turbine nozzle holder 53 and the front housing 2. The air supply chamber 54 is connected to a compressor 56 via a compressed air supply hole 55. The air supply chamber 54 comprises a compressed air injecting nozzle 57 having a plurality of guide vanes (not shown), and turbine blades 58 of the turbine wheel 42 are arranged to face the compressed air injecting nozzle 57. A housing interior chamber 59, in which the turbine wheel 42 is arranged, is connected to the atmosphere via a discharge hole 60 which is formed in the rear housing 3. The compressed air fed into the air supply chamber 54 from the compressor 56 is injected into the housing interior chamber 59 via the compressed air injecting nozzle 57. At this time, the compressed air injected from the injecting nozzle 57 provides the rotational force for the turbine wheel 42 and, thus, the rotary shaft 8 is rotated at a high speed. Then, the compressed air injected from the injecting nozzle 57 is discharged to the atmosphere via the discharge hole 60. 
     A through-hole 62 is formed on an end wall 61 of the rear housing 3, which defines the housing interior chamber 59, and an electrode holder 63 extending through the through hole 62 is fixed onto the end wall 61 by means of bolts 64. A cylindrical hole 65 is formed coaxially with the rotation axis of the rotary shaft 8 in the electrode holder 63, and a cylindrical electrode 66, made of wear resisting materials such as carbon, is inserted into the cylindrical hole 65 so as to be movable therein. In addition, a compression spring 67 is inserted between the electrode 66 and the electrode holder 63 so that the tip face 68 of the electrode 66 is urged onto the end face of the shaft portion 8c of the rotary shaft 8 due to the spring force of the compression spring 67. An external terminal 69 is fixed onto the outer wall of the rear housing 3 by means of bolts 70 and connected to a high voltage generator 71 used for generating a negative high voltage ranging from -60 kV to -90 kV. Consequently, the negative high voltage is applied to both the front housing 2 and the rear housing 3, and it is also applied to the spray head 9 via the electrode 66 and the rotary shaft 8. 
     In operation, paint is injected from the nozzle 21 of the paint injector 18 onto the circumferential inner wall of the outer cylindrical portion 14 of the spray head supporting member 12. Then, the paint, injected onto the circumferential inner wall of the outer cylindrical portion 14, flows out onto the inner wall 15 of the spray head body 13 via the paint overflow bores 16 due to the centrifugal force caused by the rotation of the spray head 9. After this, the paint spreads on the inner wall 15 of the spray head body 13 and flows on the inner wall 15 in the form of a thin film. Then, the paint reaches the tip 13a of the spray head body 13. As mentioned previously, a negative high voltage is applied to the spray head 9. Consequently, when the paint is sprayed from the tip 13a of the spray head body 13 in the form of fine particles, the particles of the sprayed paint are charged with electrons. Since the surface to be painted is normally grounded, the paint particles charged with electrons are attracted towards the surface to be painted due to electrical force and, thus, the surface to be painted is painted. 
     FIG. 9 illustrates the moment when spray particles are produced in a conventional spray head. In FIG. 9, reference numeral 100 designates the inner wall of a spray head, and 101 a tip edge of the inner wall of the spray head. As illustrated in FIG. 9, in a conventional spray head, paint 102 is spouted from the tip edge 101 in the form of a thin film. Then, the thin film of paint 102 is broken, and paint particles 103 are produced. However, in such a conventional spray head, when the paint particles 103 are formed from the thin film of paint 102, air is confined within the paint particles 103. This results in the paint particles 103 containing air bubbles therein. 
     Referring to FIGS. 5 and 6, an annular step portion 73, radially extending outwardly from the inner wall 15 of the spray head 9, is formed on the tip 72 of the spray head 9 and, in addition, a thin annular tip wall 74, extending in the axial direction of the rotary shaft 8, is formed in one piece on the outer periphery of the annular step portion 73. 
     As illustrated in FIG. 6, the annular tip wall 74 has an extremely thin thickness and has a function of charging paint particles with electrons. In addition, from FIG. 6, it will be understood that the annular step portion 73 has a width T which is considerably greater than the thickness S of the annular tip wall 74. As mentioned previously, the paint, injected from the paint injector 18, flows into the inner wall 15 of the spray head 9 via the paint outflow bores 16. Then, the paint spreads on the inner wall 15 of the spray head 9 and moves forward towards the tip 72 of the spray head 9 in the form of a thin film. When the thin film of paint reaches the annular step portion 73, the thin film of paint flows on the annular step portion 73 towards the annular tip wall 74 while being rapidly accelerated due to the centrifugal force. If the radius of the wall on which a paint flows is indicated by R, the velocity of the paint flowing on the wall is indicated by V and the thickness of the thin film of paint flowing on the wall is indicated by t, the amount Q of paint, flowing on the inner wall 15 of the spray head 9, is represented by the following equation. 
     
         Q=2πR·V·t 
    
     During the time the paint flows on the inner wall 15 of the spray head 9, the velocity V of the paint is relatively low. However, when the paint reaches the annular step portion 73 as mentioned above, the paint is subjected to an extremely great acceleration of 10 4  G (gravitational acceleration) to 10 5  G and, as a result, the paint is rapidly accelerated. Consequently, since the velocity V of the paint is rapidly increased, the thickness t of the paint film, flowing on the annular step portion 73, tends to become extremely thin. However, actually, the thickness t of the paint film cannot become extremely thin, and the paint film is broken and divided into a plurality of filament shaped streams 75, as illustrated in FIG. 7. Then, as illustrated in FIG. 8, the filament shaped streams 75 move forward on the annular tip wall 74 and are spouted from the edge of the annular tip wall 74. After this, the filament shaped streams 75 are broken, and paint particles 76 are produced. In the case wherein the paint particles 76 are formed from the filament shaped streams 75, since air is not confined in the paint particles 76, paint particles, containing no air bubble therein, are produced. In addition, the size of the filament shaped streams 75, formed on the annular step portion 73, is extremely small and, therefore, the size of the paint particles 75 is extremely small. 
     As mentioned previously, the rotary shaft 8 is supported by a pair of the tilting pad radial air bearings 22, 23 and a single thrust air bearing which is constructed by the runners 39, 40 and the stationary annular plate 44. In the tilting pad radial air bearings 22, 23, when the rotary shaft 8 is rotated, ambient air is sucked into the extremely small clearances formed between the hollow cylindrical portion 8a and the pads 24, 25, 26. Then, the air thus sucked is compressed between the hollow cylindrical portion 8a and the pads 24, 25, 26 due to a so-called &#34;wedge effect&#34; of air, and therefore, the pressure of the air between the hollow cylindrical portion 8a and the pads 24, 25, 26 is increased. As a result of this, the force radially supporting the rotary shaft 8 is generated between the hollow cylindrical portion 8a and the pads 24, 25, 26. On the other hand, in the above-mentioned thrust air bearing, compressed air is fed into the air passages 50 from the air feed pumps 49 via the annular groove 47. Then, the compressed air is injected from the air outflow bores 51 into the clearance between the annular plate 44 and the runner 40, and also, injected from the air outflow bores 52 into the clearance between the annular plate 44 and the runner 39. As a result of this, the pressure, which is necessary to maintain the above-mentioned clearances formed on each side of the annular plate 44, is generated between the annular plate 44 and the runners 39, 40. Consequently, the rotary shaft 8 is supported by the thrust air bearing and a pair of the radial air bearings under a non-contacting state via a thin air layer. As is known to those skilled in the art, the coefficient of viscosity of air is about one thousandth of that of the viscosity of lubricating oil. Consequently, the frictional loss of the air bearing, which uses air as a lubricant, is extremely small. Therefore, since the amount of heat caused by the occurrence of the frictional loss is extremely small, it is possible to increase the rotating speed of the rotary shaft 8 to a great extent. In the embodiment illustrated in FIG. 1, it is possible to rotate the rotary shaft 8 at a high speed of about 80,000 r.p.m. Consequently, as illustrated in FIGS. 7 and 8, since the paint, flowing on the annular step portion 73, is subjected to an extremely great acceleration, the size of the filament shaped streams 75 becomes extremely small and, as a result, the size of the paint particles becomes extremely small. 
     FIG. 10 illustrates the relationship between the size of the particles of sprayed paint and the rotating speed of the spray head in the case wherein the spray head 9 (FIG. 1) having a diameter of 75 mm is used. In FIG. 10, the ordinate S.M.D. indicates the mean diameter (μm) of paint particles, which is indicated in the form of a Sauter mean diameter, and the abscissa N indicates the number of revolutions per minute (r.p.m.) of the spray head 9. As mentioned previously, in a conventional rotary type electrostatic spray painting device, the maximum number of revolutions per minute N of the spray head is about 20,000 r.p.m. Consequently, from FIG. 10, it will be understood that, if the spray head having a diameter of 75 mm is used in a conventional rotary type electrostatic spray painting device, the minimum mean diameter S.M.D. of paint particles is in the range of 55 μm to 65 μm. Contrary to this, in the present invention, the maximum number of revolutions per minute N is about 80,000 r.p.m. Consequently, from FIG. 10, it will be understood that the paint can be divided into fine particles to such a degree that the mean diameter S.M.D. of paint particles is in the range of 15 μm to 20 μm. Therefore, it will be understood that, in a rotary type electrostatic spray painting device according to the present invention, the size of paint particles can be greatly reduced, as compared with that of paint particles in a conventional rotary type spray painting device. In addition, as mentioned previously, the same negative high voltage is applied to the housings 2, 3 and the rotary shaft 8. Consequently, there is no danger that an electric discharge will occur between the housings 2, 3 and the rotary shaft 8. 
     According to the present invention, by forming an annular step portion on the tip of a spray head, it is possible to prevent paint particles from containing air bubbles therein and, in addition, it is also possible to reduce the size of paint particles to a great extent. In addition, since the spray head can be rotated at a high speed of about 80,000 r.p.m., the size of the particles of sprayed paint can be reduced to a further extent. As a result of this, the size of paint particles becomes smaller than that of paint particles obtained by using a conventional air injection type electrostatic spray painting device. Consequently, in the present invention, it is possible to form an extremely beautiful finished surface and, therefore, a rotary type electrostatic spray painting device can be used for carrying out a finish painting step in the paint process, for example, for bodies of motor cars. In addition, in the present invention, since paint particles are created by rotating the spray head at a high speed, but are not created by air injection, the amount of the paint used to effectively paint the surface to be painted is about 90 percent of the amount of the paint sprayed from a rotary type electrostatic spray painting device. Consequently, since a large part of the sprayed paint is not dispersed within the factory, it is possible to prevent the problem previously mentioned, regarding air pollution from arising. In addition, the amount of paint used can be reduced. 
     While the invention has been described by reference to a specific embodiment chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the spirit and scope of the invention.