US2626589A - Electrostatic spray painting apparatus - Google Patents

Description

Jan. 27, 1953 J. LAMM 2,626,539

ELECTROSTATIC SPRAY PAINTING APPARATUS Filed Dec. 1, 1948 3 Sheets-Sheet l 3nvemor Jan. 27, 1953 I L. J. LAMM 2,626,589

ELECTROSTATIC SPRAY PAINTING APPARATUS Filed Dec. 1, 1948 3 Sheets-Sheet 2 Snuemor [6 2423 MW/7W Jan. 27, 1953 J, LAMM ELECTROSTATIC SPRAY PAINTING APPARATUS 3 Sheets-Sheet 3 Filed Dec. 1, 1948 a x w Patented Jan. 27, 1953 ELECTROSTATIC SPRAY PAINTING APPARATUS Lewis J. Lamm, Detroit, Mich., assignor to General Motors Corporation, Detroit, Micln, a corporation of Delaware Application December 1, 1948, Serial No. 62,913 3 Claims (01. 118-51) The present invention relates to spray coat-' ing apparatus and more particularly relates to apparatus for spray painting in which an electrostatic field is used to influence the deposition of coating materials. This invention further relates to electrostatic coating apparatus in which at least one of the articles being coated is maintained at high potential.

Due to the necessity of using a very high voltage to produce the necessary electrostatic field, certain difilculties have heretofore been experienced regarding fire hazards and personnel safety. Most of the prior apparatus has utilized a discharge electrode maintained at high potential, to permit electrostatic coating while maintaining the article to be coated at ground potential. Modern continuous conveyor type painting apparatus makes it desirable that the conveyor be maintained at ground potential in order that there will be no requirement of transferring articles from one conveyor to another during the painting and baking operations and in order that the personnel involved in these operations may be protected from contact with conductors carying high potential electricity. It is however quite desirable, for maximum efficiency in painting, that the articles being coated be maintained at high potential. Thus two articles may be painted simultaneously, one article acting as a discharge electrode for the other and vice -versa.

It is therefore an object of the present inventionv to provide an apparatus for maintaining articles to be coated at high electrical potential during the coating operation.

It is a further object of the present invention to provide means for removing the electrostatic charge from the articles being coated before the emergency of these articles from the spray booth.

It is a further object of the present invention to provide an economical and efilcient means of maintaining articles to be coated at high potentials of opposite polarity during the spray coating operation.

It is a further object of the present invention to provide means for disabling the high potential circuit under sparking conditions.

It is a further object of the present invention to anticipate sparking conditions and disable the high potential circuit in response to conditions which may cause sparking.

It is a further object of the present invention to provide means for automatically extinguishing any fire which may occur in the spray booth.

Other objects of this invention will become apparent upon reading the specification and in- I spection of the drawings and will be particularly pointed out in the claims.

Referring to the figures in the drawings, Figure 1 is a longitudinal sectional view of part of the apparatus utilized'in the present invention taken along the line l--l of Figure 2.

Figure 2 is a transverse partial sectional view taken along the line 2--2 of Figure 1.

Figure 3 shows means for charging the articles to be coated at a positive potential.

Figure 4 shows a circuit for charging the articles to be coated at a negative potential.

Figure 5 shows an alternate circuit for charging the articles at a negative potential.

' Figure 6 is a partial view of an alternate circuit for charging the articles at a positive potential.

Referring more particularly to Figures 1 and 2, an apparatus is illustrated for maintaining one group of articles carried by a conveyor at a high positive potential and a second group of articles carried in a parallel direction by the same conveyor at a negative potential, and means for discharging the electrostatic charge on 'these articles before they leave the spray booth. In these Figures, 1, 3 and 5 are articles to be coated, here illustrated as metal automobile doors. All of these articles are suspended from a common monorail conveyor 9 supported by rail 1 which is suspended from the ceiling H by'means of brackets l3 and It. This conveyor is of the standard monorail type which may be used throughout the manufacturing establishment and is maintained at ground potential to eliminate any difficulties which would be incurred by having such a conveyor carrying an electrical charge. The articles 1 and 3 are carried by supports I 5 and I1 respectively while the rticle 5 is carried by support 19. The articles 1, 3 and other articles carried on the left side of the conveyor are maintained at negative potential while the article 5 and those carried on the right side of the conveyor are maintained at positive potential. When this system is in operation a continuous double line of articles pass through the spray booth 2|, one line of articles being at positive potential and the other at negative poten-- spray guns 23, 25 and 21 which are supported on r a common stand 29. These spray guns are here shown as projecting a spray of coating material in a direction generally parallel to the path of travel of the two lines of articles to be coated.

terial.

It is however contemplated that under certain conditions of operation while painting articles of specific contours, it will be desirable and perhaps necessary to locate some of these spray guns so as to spray paint at an acute angle towards one or the other of the articles being painted with perhaps alternate guns spraying paint at acute angle to the articles on each of the cornveyors respectively. In order to permit the charging of these articles with high potential voltage without arcing while maintaining the overhead conveyor at ground potential, a novel system has been provided. The hangers I5, I! and I9 which support the articles I, 3 and 5 respectively are made of conducting material preferably copper or other metal of high conductivity. These metallic hangers are mechanically connected to the conveyor 9 by means of insulators l and I2, which insulators carry a hanger at each extremity thereof and each hanger is attached at its center to the monorail conveyor by suitable means. The purpose of these insulators is to mechanically support the hangers and the articles supported therefrom and to insulate these hangers and articles from the conveyor'- system. Rigidly attached to the support members l5, l1 and I9 and in electrical contact therewith are leaf spring resilient contact members 3|, 33 and 35 respectively of conducting ma- These contact members are rigidly attached to the conveyor system and a part of the electrical circuit including the individual articles to be coated and their hangers and proceed therewith through the spray booth.

It is desirable for obvious reasons that power supplies for producing the high voltage potentials be maintained in a stationary position.

' Therefore I have provided two stationary contact systems, one for introducing negative potential to the articles on one side of the conveyor and the other for introducing positive potential to the articles on the other side of the conveyor system.

It will be noted that these contact systems include troughs 31 and 43 respectively of insulating material extending parallel to the direction of travel of the conveyor. One of these troughs of insulating material 31 has rigidly supported therein a conducting contact member 4! which is electrically connected to the power source shown in either of Figures 3 or 6. This trough is filled with a spark inhibiting fiuid, of which oil may be considered an example. Under certain conditions of operation this trough may be filled with a gaseous fiuid of suitable mechanical and.

chemical properties; for example, carbon dioxide.

A second similar stationary contact member is shown arranged also parallel to the path of travel of the conveyor system and located adjacent the path of travel of the hangers i5 and [-1. This system includes a trough 43 supporting a contact member 45 immersed in a sparkinhibiting fluid 41. This contact member is so arranged that the contact member 3| slides over the surface thereof and completes an electrical circuit between the power supply which is shown in Figure 4 or 5, and the articles to be coated, here illustrated as l and 3.- The trough 43 also supports a contact member 49 maintained at ground potential and in spaced relationship to the contact member 45. The member 49 is also adapted to be contacted by the arms 3| and 33 to complete an electrical circuit between the articles I and 3 and thus discharging to ground any is of electrostatic attraction.

coated are maintained at a high potential by reason of this contact until the circuit is broken at the end of the stationary high potential rail. The charge remaining on the article being coated is then removed by member 3|, 33 or 35 contacting the stationary rail which is main-- tained at ground potential. It may thus be seen that as two parallel moving lines of articles enter the spray booth, each of the articles on these separate lines are raised to a high electrostatic potential (the articles on one line being raised to a positive potential and the articles on the other line being raised to a negative potential) to thus produce an intense electrostatic field therebetween. Coating material from the spray guns 23, 25 and 21 is projected into this electrostatic field and is influenced to anddeposited upon the articles to be coated by means Thus the loss of paint or other coating material due to over spray is maintained at a minimum. As these articles proceed through the booth and become completely coated, they have their charge removed by the completion of an electrical circuit between the article and ground.

Referring now to Figure 3, a circuit capable of maintaining the stationary rail 4| at high positive electrical potential and incorporating certain safety devices is illustrated. The spray booth is here generally shown as 2| and the overhead conveyor is 9 with its supporting channel I. This circuit and the associated equipment illustrated therewith show a means for disabling the circuit under arcing conditions or in case of fire, together with automatic means for extinguishing the fire. It is not intended however that it will always be necessary in particular installations to include both the circuit disabling and the fire extinguishing apparatus in a single installation. It is intended that these two systems may be used individually or in conjunction with each other.

Referring more particularly to Figure 3, a container for high pressure fire extinguishing gas is shown at 53. This gas may be carbon dioxide or any gas having suitable fire extinguishing properties. The container 53 is connected to conduits 55 and 51 having nozzles GI and 63 by means of a solenoid operated valve 59. This solenoid operated valve '59 is opened upon energization of the coil 55. This allows the fire extinguishing gas to be projected into the spray booth 2| through nozzles 5| and 63.

Referring now to the electrical circuit shown in this figure, 61 is a standard source of single phase alternating current. Ths source may be 110, 220 or 440 volts as is available for a particular installation, allowance being made in the transformers 69 and H for the potential of this voltage. This power source is connected to the low voltage primary of the transformer 69 through the main switch I3 and the relay 15. It

is connected to the primary of the transformer 'II by means of disabling switch 11. The transformer 69 has a high voltage secondary I9 which, together with rectifier 03 and condensers 85, 81 and 89, produces a D. C. voltage on the rail 4| through conductor 9| of sufficient magnitude for use in electrostatic deposition. I have found that a D. C. potential of 50 kilovolts on rail 4| and a like voltage on the rail 45 of opposite polarity is satisfactory. The condensers 85, 81 and 89 are so proportioned as to produce a D. C. potential of low voltage on conductor 93 suitable for negative grid bias of the gas filled tube 95.

Referring now to the transformer 'II a center tapped secondary 8| is shown which, together with rectifiers 91 and 99 and condenser |I, provide a full wave rectifier suitable to produce a D. C. voltage on the plate I03 of the gas filled tube\ 95 filtered by the circuit including coil 65, coil I and condenser I01. The coil I05, similar to the coil 65, has a second function, namely, that of a solenoid for operating the relay I5 which relay is operated together with valve 59 under conditions where the gas filled tube 95 is conducting plate current. maintained at ground potential while its cathode III is maintained at the positive potential impressed on the conductor 93. It may thus be seen that the grid I09 is thereby maintained at a sufliciently' negativ' bias with respect to cathode III to prevent the tube firing and conducting 7 current from the positive plate I03. Under nor- The grid I09 of this gas filled tube is mal operating conditions the stationary contact 4| is thereby maintained at very high D. C. potential to permit energization of the articles to be coated by electrostatic deposition. Under these normal conditions the plate I03 of the gas filled tube 95 is maintained at a positive potential with no current flowing through either the solenoid 95 or I05, the tube 95 under these conditions being biased beyond cutoif by the voltage produced across the condenser 89. If we assume that the article 5 which is in electrical contact with the rail 4| touches a grounded portion of the spray booth or spray guns so as to cause arcing, this arcing would immediately load the low capacity high resistance secondary l9 sufiiciently to reduce the D. C. voltage output of the system to a few hundred volts. This would immediately reduce the voltage across the low capacity condensers 85, 81 and 89 proportionate amounts and thus reduce the positive bias on the cathode III sufiiciently with respect to the grid |09 to allow firing of the tube 95. Under these conditions the tube fires permitting a high current to fiow from the plate I03 to ground. This causes a high current through the solenoids 65 and I05. Under these conditions the relay I5 is tripped causing deenergization of the power supply circuit for the contact member 4|. Simultaneous with the operation of this relay a solenoid 05 raises the valve 59 allowing carbon dioxide or other inert gas to be projected into the spraying booth through nozzles 6| and 63. In this manner a fire in the spray booth is prevented or extinguished immediately.

If rye assume that a fire has started in the spray booth by some means other than the grounding of an article being coated, so that this circuit is not loaded by said grounding, we must look to other means of causing actuation of the circuit than metallic contact between the high potential circuit and ground. It is well-known that burning gases are ionized and that ionized gases of this type are comparatively good conductors. Under these conditions current is permitted to flow between the high potential circuit and ground or between the two high potential circuits thus causing loading of the high resistance secondary I9 as described above. Under these conditions the voltage across the condenser 89 falls as explained above allowing the tube 95 to fire with resulting energization of the coils and I05, thus causing deenergization of the power supply circuit and the introduction of carbon dioxide gas through the nozzles GI and 63 to extinguish the fire as described above.

Due to the ion sheath around the individual grid wires, the grid of a gas filled tube is unable to regain control while the tube is conducting. The relay I5 will the.efore remain energized thus continuing its disabling action of the high voltage power supply and the solenoid 65 will remain energized thus continuing the spray of carbon dioxide gas into the spray chamber regardless of the fact that the contact within the spray chamber which caused the arcing condition has been broken so no current flows in the conductor 9|. This is necessary since under any other conditions the circuit breaker may continue to make and break thus causing a continuous arcing condition. The power supply circuit for the plate I03 of thegas filled tube may be disabled by opening either switch I3 or 11. This would allow the relay 15 to be reset and the grid I09 to regain control of conduction within the gas filled tube 95. Upon reclosing of the switches I3 and 11 the system is again in condition for operation.

Referring now to Figure 4, a circuit is shown capable of producing a potential of opposite polarity to that produced by the circuit shown in Figure 3. This circuit may be used in conjunc c tion with one of the type shown in Figure 3 for energizing the opposite line of articles to be spray coated. The circuit here shown is in general similar to that shown in Figure 3 but is modified as necessary to produce a negative potential on the stationary contact 45. Because of the similarity of these two figures, only those elements and circuits which are peculiar to Figure 4 will be described. In this circuit a rectifier I2I is provided of the same general type as the rectifier 83 in Figure 3. It is however reversed in Figure 4 so as to produce a negative D. C. potential on the conductor I23 and across the series condensers I25, I21 and I29. These condensers are so proportioned as to produce a low negative voltage across the condenser I29 of magnitude suitable for the negative grid bias of gas filled tube I3I.

This gas filled tube is similar to the gas filled tube 95 described in connection with Figure 3. The plate I 31 is provided with a positive plate potential by a circuit similar to that circuit providing plate voltage for the plate I03 in Figure 3. The cathode I33 is connected to ground and the grid I35 is provided with suitable negative grid bias under normal operating conditions by the voltage drop across the condenser I 29. Under normal operating conditions there is very small current drain on the conductor I23 similar to that on conductor 9| in Figure 3. The current drain on this conductor is however increased in a similar manner as that described in connection with 9| in Figure 3 under conditions where arcing is promoted, such as ionized conditions between the high voltage electrode and ground and under conditions of arcing when a high current is passed or under conditions of the area between the electrodes being ionized by flame from. a fire, the current drain is sufficient to cause a very great drop in voltage. Under these conditions the voltage across the condenser I29 approaches zero thus removing the negative grid bias from the grid I35. In a manner similar to that described in connection with Figure 3; the grid loses control and the tube I8I conducts plate current thus operating the relays and the solenoid valve to energize the circuit and cause the introduction of fire inhibiting gases into the spray booth.

The circuits of Figures 3 and 4 may be interconnected so that both these circuits operate a common solenoid valve 59 and/or a common relay I5 which relay controls the energization of both circuits. Thus these two circuits may be used in conjunction with each other or separately without departing from the scope of the present invention.

Referring now to Figure 5, a novel circuit incorporating certain features of the present in:

vention is illustrated. It is well-known in the electrical art that the loading of the secondary of a double tuned circuit reflects resistance back into the primary causing loading of that circuit. It is also well-known that in certain types of oscillators the loading of the plate circuit will cause the oscillator to go out of oscillation. These two phenomena are utilized in Figures 5 and 6 to provide a circuit which anticipates sparking conditions and operates to deenergize the circuit in response thereto. The secondary circuit will load under'arcing or' are anticipating conditions where there is a flame ionizing the electrostatic field sufficiently to load the primary circuit and the high voltage'high frequency current produced across the secondary tuned circuit in Figure 5, is rectified so that ahigh voltage D. C. is produced on the electrode 48.

.Under normal operating conditions, the D. C. drain on the electrode is very small and in the order of a few milliamperes. However, under short circuiting conditions, the current drain on this electrode is limited only by the power output of the power supply and may easily reach proportions sufficient to cause sparking. In the present invention the loading of the secondary tuned circuit by such short circuiting reflects a resistance back into the tuned circuit 90-92 thus causing the tank circuit to be heavily loaded and stop oscillation of the circuit. Under these conditions the tubes 86 and 88 have very high current drain thus causing suflicient current to pass through the coil I04 to actuate the manually reset relay I06. This deenergizes the oscillator circuit and thus quenches' the are formed at the short circuit.

Referring to Figure 6, the rectifier I I6 is identical to the rectifier H2. in Figure 5 except that its polarity is reversed. The rest of the circuit is identical with that shown in Figure 5. The circuits shown in Figures 5 and 6 may be substituted for those shown in Figures 3 and 4 respectively and used on opposite conveyors as mentioned above. The circuits shown in Figures 5 and 8 will of course be actuated in response to current flow caused by fire ionizing the gas in the electrostatic field very similar to the circuits shown in Figures 3 and 4 and may also be similarly used to control the introduction of fire extinguishing gas into the spray booth.

While I have described above a complete system showing a method of and apparatus for to produce a push-pull oscillator with a common output tank circuit consisting of condenser 90 and inductance 92. The inductance 92 is tapped to ground. One end of the inductance 92 is connected to the grids of the tubes 88 and 88 to produce self oscillation. The plates of these two tubes are energized respectively by alternate half cycles of the power supply voltage. The plates of the tubes 86 and 88 are both connected to the other end of the inductance 82 through condensers 94 and 96 respectively. It may thus be seen that the tuned circuit including the condenser 90 and the inductance 92 is thereby kept in continuous oscillation. The condenser 98 and resistance I00 provide aself-biasing circuit for the grids of these two tubes. The coils I02 and I04 provide a certain amount of filter action, while the coil I04 in addition to providing filter action, also provides actuation for the overload relay I06. The electrical characteristics of coils I02 and I04 are similar. The tank circuit, consisting of condenser 90 and inductance 92, has a high natural resonant frequency in the order of magnitude of several kilocycles. The inductance 92 is magnetically coupled to the coil I08, which together with the condenser IIO, has the same resonant frequency as the tank circuit previously referred to. wound that their mutual inductance provides approximately critical coupling. A rectifier H2 is connected to the high voltage side of the resonant circuit including the secondary inductance I08 and its tuning condenser II 0. This rectifier performs the same function as the rectifier I2I in Figure 4. It may thus be readily seen that The coils 92 and I08 are so electrostatic spray'coating of articles of manufacture, it is to be contemplated that the circuits shown in Figures 3 to 6 may be used in conjunction with the specific apparatus shown in Figures l and 2 or that these circuits may be used singularly and that the apparatus may be used in conjunction with other circuits for providing high voltage without departing from the intended scope of the present invention. While this description covers a specific embodiment of the present invention, it is not intended that the invention should be limited to this specific embodiment but that it should be construed as broad as possible in conformance with the hereto appended claims.

I claim:

1. In electrostatic spray coating apparatus for coating two articles of manufacture simultanesaid stationary contact members with inert fluid,

and means for bringing said electrical contact means into electrical contact with said stationary contact members during the time said articles are in said zone.

. 2. In electrostatic spray coating apparatus as accuse claimed in claim 1 in-which the coating material is projected in a direction generally parallel to the paths coated. v v

- 3. In electrostatic spray coating apparatus as claimed in claim 1 in which grounded stationary contacts are located longitudinally adjacent one end of the first mentioned stationary contacts,

said grounded stationary contacts being surrounded by an inert fluid and adapted to be engaged electrically by the movable contact carryof travel of the articles to be ing the articles to be coated subsequent to said movable contacts breaking electrical engagement with said first mentioned stationary con-- tact members. v

LEWIS J. name.

10 REFERENCES crrnn The following references are of record'in the file 'o t'thls patent: 4

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