Abstract:
A remote low voltage power source for electrostatic paint applicators used in hazardous locations. A low voltage generator and voltage conditioning and regulating circuitry are mounted in a small explosion proof housing. A generator shaft and a shaft on a voltage level adjusting potentiometer extend through and projects from the housing. The flamepath and the flame gap for the shafts meet explosion proof standards while permitting shaft rotation. An air driven turbine is mounted on the exterior of the housing and is connected to the projecting generator shaft to drive the generator. The low voltage is applied to a spray gun which includes circuitry for increasing the voltage to a high level for electrostatic charging sprayed paint.

Description:
BACKGROUND OF THE INVENTION 
     In one painting method which is frequently used for commercial operations, the atomized paint is electrostatically charged to a high voltage relative to the workpiece being coated. The electrostatic charge causes the atomized paint to be drawn to the workpiece. This significantly increases the paint transfer efficiency, thus reducing both the labor and material costs to paint the workpiece and reducing environmental problems. In a normal commercial painting operation, the workpiece is placed in a spray booth prior to painting. A low voltage power source, a compressed air hose and a paint hose are connected from exterior to the spray booth to the spray gun. The spray gun typically includes an oscillator and a voltage multiplying circuit for increasing the low voltage to a very high dc voltage for charging the paint as it is atomized. The high voltage may be, for example, up to 100 KV or more. 
     In some applications, it is not possible to use a conventional spray booth because of the size of the workpiece being sprayed. For example, an airplane will not fit into a conventional spray booth. In this case, the airplane hanger becomes the spray booth for confining the paint overspray and fumes. Since the paint may include flammable volatile organic compounds (VOC&#39;s) as solvent, the hanger is considered a hazardous location and extreme care must be taken to prevent sparks which possibly could ignite the VOC vapors. A conventional low voltage power source which is connected to an electrostatic spray gun does not meet certain safety standards for operation in hazardous locations. Conventional switches, potentiometers and other circuit components and connections present a risk of sparks. 
     One method for meeting the hazardous location safety standards has been to place the low voltage source directly in the spray gun. As shown in U.S. Pat. Nos. 4,219,865 and 4,290,091, the spray gun may be provided with an internal generator which is driven by an air turbine for generating a low voltage without any electrical connections to the spray gun. Conventional voltage multiplying and rectifying circuitry is used in the spray gun to convert the low voltage to a high dc voltage for charging the paint. In operation, the turbine shares air with the atomization air delivered to the spray gun. The turbine air must be exhausted from the spray gun after use. In some applications, this can cause problems with dust agitation. The internal turbine and generator also significantly increase the weight of the spray gun which the operator must hold during spraying. Since aircraft are very large, the painting time may be quite long and the added weight can quickly tire the operator. Because of the rotary motion of the turbine in the spray gun, vibrations are transmitted into the handle of the spray gun. These vibrations are felt by the operator when the gun is used. The cost of a spray gun with an internal turbine and generator is quite high. Consequently, it is quite expensive to maintain spare spray guns, since each gun must include a turbine and a generator. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, a low voltage power source including an electric generator and low voltage power regulating and control circuitry are mounted in a sealed, explosion proof housing which meets hazardous location safety standards. The low voltage power source is suitable for locating in a spray booth and in a hanger in which an airplane is being painted with paint having flammable solvents. The generator has a drive shaft which extends through and projects from the housing. An air driven turbine is attached to the housing and is connected to rotate the projecting turbine shaft. The low voltage power circuitry includes a voltage regulator and a potentiometer. The potentiometer has a shaft which extends through the housing for manually setting the output voltage level. The flamepath and the flame gap at the generator and potentiometer shafts are made to meet explosion proof standards. The turbine does not share the air source with the spray gun. The power source is located in an area where the exhaust air will not have an effect on the spray area. Since the rotating turbine and generator are not located in the spray gun, no vibrations are transmitted to the hand of the spray gun operator. Further, the spray gun will be much lighter than spray guns having an internal air turbine and generator and it will be less expensive for the user to maintain spare guns, since it is not necessary to purchase a turbine and a generator with each gun. 
     Accordingly, it is an object of the invention to provide an improved remote power generator for electrostatic paint applications which meets safety requirements for operation in hazardous locations. 
     Other objects and advantages of the invention will become apparent from the following detailed description of the invention and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic perspective view of a hand held electrostatic spray gun connected to a remote power source according to the invention; 
     FIG. 2 is a fragmentary cross sectional diagrammatic view through the generator and circuitry housing for the remote power source of FIG. 1; 
     FIG. 3 is an enlarged fragmentary view, in partial section, of a connector for attaching a power cable to the electrostatic spray gun; and 
     FIG. 4 is a block diagram for circuitry for rectifying and regulating the voltage applied to the electrostatic spray gun. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning first to FIG. 1, a low voltage power source  10  according to the invention is shown connected to an electrostatic hand held paint spray gun  11 . The spray gun  11  has three external connections: a low voltage insulated cable  12  connected to the power source  10 , a compressed air hose  13  and a paint hose  14 . The compressed air hose is connected to a conventional source (not shown) of pressurized air, such as an air compressor or a compressed air line. The paint hose  14  is connected to a pressurized paint tank. When an operator squeezes a trigger  15  on the spray gun  11 , an internal air valve (not shown) is opened to initiate a flow of atomization air to a nozzle assembly  16  and then an internal paint valve (not shown) is opened to initiate a flow of paint to the nozzle assembly  16 . 
     The cable  12  applies a low dc voltage to the spray gun  11 . The voltage may be, for example, at a regulated level between 0 and 10 volts dc. This voltage is applied to the input of a conventional high voltage power supply (not shown) internal to the spray gun  11 . The high voltage power supply includes an oscillator which converts the dc input voltage to a higher level ac voltage. The ac voltage is then applied to a capacitor and diode network which multiplies and converts the voltage to a very high level dc voltage, as is well known in the art. The actual level of the high voltage will depend on the level of the dc input voltage. By adjusting the level of the dc voltage applied on the cable  12 , the level of the high voltage is adjusted. 
     Referring to FIGS. 1 and 2, the power source  10  includes an air motor or turbine  17  which is mounted on an end cap  18  of an explosion proof housing  19  by means of a plurality of legs or brackets  20 . The air turbine  17  is connected through a hose  56  to a conventional source (not shown) of compressed air, such as an air compressor or a pressurized air line. Three spaced brackets  20  are shown in the illustrated power source  10 . The brackets  20  isolate the turbine  17  from the housing  19  to reduce noise. The housing  19  includes a tubular center portion  21  having opposed threaded ends  22  and  23 . The end cap  18  is screwed onto the threaded end  22  and an end cap  24  is screwed onto the threaded end  23 . The threaded ends  22  and  23  of the tubular housing portion  21  either may be internally threaded, as shown, or externally threaded for mounting the end caps  18  and  24 . The housing  19  defines a closed interior chamber  25  in which an electric generator  26  is located. The generator  26  is mounted on the end cap  18  and has a drive shaft  27  which extends through an opening  28  in the end cap  18  and has a projecting end  29 . The generator is illustrated as being mounted on the end cap  18  with a plurality of screws  30 . However, other techniques for mounting the generator  26  on the end cap  18  will be apparent to those skilled in the art. 
     Preferably, the generator shaft  27  is aligned on an axis of the housing  19 . The air turbine  17  has an output shaft  31 . The air turbine  17  is mounted on the end cap  18  so that its output shaft  31  is aligned with the projecting end  29  of the generator shaft  27 . The shafts  27  and  31  are connected together with a coupling  32  so that when the turbine shaft  31  rotates, it rotates the generator shaft  29  to cause the generator  26  to produce electrical power. 
     It will be appreciated that the electrical output from the generator  26  will vary with the speed at which it is driven by the turbine  17 . Preferably, the generator  26  produces ac power. Both the frequency and the output voltage will vary as a function of the turbine speed. The output from the generator  26  is applied on wires  33  to circuitry  34  which regulates and conditions the output to produce a constant low level dc voltage, for example, a constant dc voltage within the range of from 0 to 10 volts. A potentiometer  35  may be used to adjust the output voltage level within the range in order to adjust level of the high voltage generated in the spray gun  11 . The potentiometer  35  has a shaft  36  which extends through an opening  37  in the end cap  24 . A knob  38  is provided on the shaft  36  to facilitate voltage adjustment. 
     The low voltage electrical cable  12  has a threaded connector  39  which engages a threaded opening  40  in the end cap  24 . A flexible strain relief  41  is positioned on the cable  12  adjacent the connector  39 . A plurality of insulated wires  50  and  51  from the cable  12  are secured to the circuitry  34  with an end plug  43 . At the connector  39 , the cable is sealed with potting in order to form a gas tight connection and to meet explosion proof standards. 
     The housing  19  is of an explosion proof construction. The walls of the housing  19  are made sufficiently strong to withstand an internal explosion in the event that vapors or materials within the housing chamber  25  should be ignited. Preferably, the housing  19  is constructed to meet safety standards such as those developed by Factory Mutual Research. Equipment enclosed in a case has been defined as being “explosion proof” if the case is capable of: a) withstanding an internal explosion of a specified gas or vapor-in-air atmosphere; b) preventing the ignition of a specified gas or vapor-in-air atmosphere surrounding the enclosure due to internal sparks, flashes or explosion; and c) operating at temperatures which will not ignite the surrounding classified atmosphere. In order to meet these standards, the walls of the housing  19  must be of a sufficient strength. Where threaded connections are made, as where the end caps  18  and  24  are secured to the tubular portion  21 , and where the cable connector  39  engages the end cap  24 , a minimum number of threads must be engaged. 
     In addition to a minimum housing strength, there must be a minimum length of flamepath and no greater than a maximum gap of flamepath at all gaps, openings and joints in the housing. “Flamepath” is defined as the place where corresponding surfaces of two parts of an enclosure come together and prevent the transmission of an internal explosion to the atmosphere surrounding the enclosure. “Gap of flamepath” refers to the distance between the corresponding surfaces of a joint measured perpendicular to the surfaces. For circular surfaces, this gap is defined as the difference between the two diameters, or the diametrical clearance. “Length of flamepath” is defined as the shortest path along a joint surface from the inside to the outside of an enclosure. For a given internal volume for the enclosure and type of joint, the relationship between the length of flamepath and the gap of flamepath are defined for meeting explosion proof standards. For example, for an enclosure volume of between 6 and 120 cubic inches (between 98 and 1966 cc), if the length of flamepath along a shaft is between 1 inch (2.54 cm) and 1.57 inches (3.99 cm), then the gap of flamepath must be no greater than 0.008 inch (0.02 cm). Thus, a diametric clearance of no greater than 0.008 inch (0.02 cm) may be provided around the generator shaft  27  as it passes through the end cap  18 , provided the housing volume and minimum length of flamepath parameters are met. This permits passing the generator shaft  27  through the end cap opening  28  without a seal while avoiding the risk of a spark within the housing chamber  25  igniting the surrounding atmosphere. Similarly, suitable gap of flamepath and length of flamepath are provided between the potentiometer shaft  36  and the opening  37  in the end cap  24  to permit rotation of the shaft  36  while meeting explosion proof standards. 
     Referring to FIGS. 1 and 3, the low voltage cable  12  has an end connector  44  which plugs into a handle  45  on the spray gun  11 . The connector  44  has a cylindrical end  46  terminating at a plug  47  which engages a mating plug or socket (not shown) in the spray gun  11 . The cylindrical end  46  slides into an opening (not shown) in the spray gun handle  45  and is sealed to the handle with an o-ring seal  48 . A strain relief  49  surrounds the cable  12  where it joins the connector  44 . Within the cylindrical end  46 , two low voltage wires  50  from the cable  12  connect to the plug  47 . In addition, the cable  12  carries a third wire  51  which connects to one terminal on a small reed switch  52 . A second terminal on the switch  52  is connected to one of the low voltage wires  50 . A magnet (not shown) is positioned in the spray gun handle to operate the switch  52  when the connector  44  is fully seated in the spray gun handle  45 . Thus, the wire  51  carries a signal indicating whether or not the connector  44  is properly connected to the spray gun  11 . 
     FIG. 4 is an exemplary block diagram for the circuit  34 . The voltage produced by the generator  26  is applied over the wires  33  to a rectifier  53  which converts the ac voltage to a dc voltage. The dc voltage is then applied to a voltage regulator  54  which establishes a constant low level output voltage. The potentiometer  35  is connected to the voltage regulator  54  for setting the output voltage level to in turn select a high voltage level for charging paint discharged from the spray gun  11 . Preferably, a voltage clamp  55  is located between the voltage regulator  54  and the cable  12  to limit the maximum output from the circuit  34  for protecting the oscillator and voltage multiplying circuitry in the spray gun from any risk of excessive voltage in the event of a failure of the voltage regulator  54 . The wire  51  from the reed switch  52  in the spray gun connector  44  is connected to the voltage regulator  54  to inhibit an output voltage when the connector  44  is removed from or not properly seated in the spray gun handle  45 . Thus, any possible risk of a spark caused by a lose connection between the cable  12  and the spray gun  11  is eliminated. 
     The above described low voltage remote power source  10  has several advantages over an air operated power source located in a spray gun. First, the weight of the spray gun is significantly reduced, thus reducing possible strain on the operator&#39;s hand, wrist and arm. The cost of the spray gun, and of replacement spray guns, is reduced. Further, any vibrations produced by rotation of the air turbine  17  and the generator  26  are separate from the spray gun  11  and are not transmitted to the operator&#39;s hand. However, the remote power supply  10  is suitable for use in hazardous locations, such as in paint spray booths and aircraft hangers during spraying without an increased risk of an explosion. 
     It will be appreciated that various modifications and changes may be made to the above described preferred embodiment of remote power source for an electrostatic paint applicator without departing from the scope of the following claims.