CA2024755A1 - Automatic grounding device of an electostatic spray coating system - Google Patents

Abstract

Abstract Automatic Grounding Device of an Electrostatic Spray Coating System The grounding device contains a cylinder (24) from electrically insulating material in which a piston (16) is movable between two different axial positions by compressed air, against the force of a mechanically pre-stressed compression spring (34). In the one position, the piston (16) is spaced from the grounding contact (26) a distance such that the high voltage cannot arc over in between. The voltage can be turned on only in this position of the piston (16). As the compressed air is turned off or fails due to a defect, the compression spring (34) abruptly moves the piston (16) into the other position, in which it electrically connects the compression spring (34) with the grounding contact (26). The compression spring (34) 19 electrically connected with the system components (6, 8, 10) that are arranged insulated, so that these now are grounded. In this position of the piston (16), the voltage cannot be turned on. The cylinder (24) is preferably the leg of a table on which the electrically insulated system components (6, 8, 10) are arranged.

(Figure 1)

Description

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TRANSLATION

Ransburg-Gema AG

Our C-se P 588 DE June 29, 1989 ..

Autom-tic Groundin~ Device of an Electrostatic S~rav Coatine Svstem Th- invention concerns an autooatic grounding device of an electrostatic spr-y co-tlng syste1~ according to the preamble of claim l In prior pray coating sy~tems, the grounding device comprises at lesst one ;. ~ , grounding witch Such grounding device is known from the German patent docunont 35 26 013 Bec-use of the risk of isparking arc overs between the oont-ot~ of the grounding witch, theise re cont-ined preferably in an oil-fill-t ~witch chamber In l-ctro~tatic spr-y co-ting, the coating m-teri-l iis electrost-tically ch-rget with high volt-ge by the spray org-n, i9 ~-to~ized and el-ctric-lly ttr-ct-d by th- obJ-ct to be coated, which i8 connect-d to ground potenti-l The volt-g- mounts up to 140,000 volts In the c-~e of electric-lly conductive p-ints, the voltage transfers from the -spray organ through the electrically conductive paint, and also through the palnt f--d line, to the entiro paint supply -ystem To avoid hazardous volt-ge differences between eh- various ~y~tem parts, all of the system parts which during the ~pray coating proc-ss re passed by the p-int re oonnect-d to the sa~e voltage as the spray organ or as high-voltage elec-.:
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trodes arranged in the spray organ These system parts must be insulated from ground To avoid connecting at least the paint container from which the spray organ is supplied with paint to high voltage, and to instead enable its grounding, there are already systems in which in the feed line between contalner and spray organ there is a device contained which subdi-vide~ the paint flow But al50 these oystems require the insulated instal-lation of all system parto that are arranged after the p-int dividing device and are electrically connected with the spray organ, through the ' paint flow, and for that reason must be connected to the same high-voltage potential These oystem parts consist mostly of electrically conductive material and poosess a large material mass Thus, they also form a large c-p-city which upon shutoff of the voltage otores a high electrical energy This electrical energy needs to be dissipated to ground potential before an -`~
operator can touch these system parts after the shutoff of the electrical volt-g- The initially cited grounding switcheo serve to diosipate this hlgh lcctrlc-l ener W after the voltage shutoff, and also to protect the op-rator from th- pos-lbility that the volt-ge io turned on inadvertently or through a tochnic-l eff-ct whil- the op-rator io still in the spray co-elng yJt-m. , . - :':
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Th- problem to be solved by the invention i8 to provide an automatic grounding device which abruptly connects components of spray coating systems that are arran8ed insulated against ground and represent large -el-ctrical capacitieo with ground potential, thoreby diooipating abruptly fast l-ctrlc-l encrgy from these componento to ground potential without ~ny occurrence of hazardous electrical oparks or arc-overs and without ,~ .., ::

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requiring oil-filled switch chambers. The grounding device according to the invention is to be of a simple design and should be able to be actuated in a simple manner through few commercially available components.

This problem is inventionally solved through the characterizing features of claim 1.

In the inventional device, a piston designed as a contact element is abruptly moved into a "grounding position" by a comp~ession spring as the compressed air, which against the compression spring holds the piston in a "high voltage on" position, fails or is shut off. The compressed air ~tros~es the compression spring when moving the piston from the "grounding position~ axially to the "voltage on position," so that upon failure or shutoff of the compressed air, the feed line of which is vented thereby, tho compres~ion ~pring will abruptly relax. This creates by way of electri-cally conductive surfaces of the piston, on its end faces which through the pl~ton re connected with each other, very quickly an electrical connection from tho trounding contact to the electrical surfaces of the piston and from these, through the compression spring, to the system components which aro oloctrically connected wlth the compresslon spring. This quick electri-c-l connectlon between the plston and the grounding contact suppresses the creatlon of hazardous electrically sparking arc-overs between the piston and the grounding contact. The spark occurs within the cylinder from oloctrically insulating material and, therefore, cannot result in an lgnltion or explosion of paint, even when the pslnt is explosive or forms as paint mi~t an explosive mixture with air and such paint or paint mist is ~:- 2 ~ 3 ~
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contained in the vicLnity of the grounding device In the spray coating operation, a larger cloud of mist is normally contained in the vicinity of the spray organ snd of the obJect being coated Therefore, the coating of tho obJect is normally performed within a booth through which the ob~ects to be coated are passed ~ .
The invention will be described hereafter with reference to the drawing, in which a pre~erred embodiment of the invention is schematically illustrated as an example Figure l shows schematically an automatic grounding device according to the invention, for an electrostatic spray coating system The invontional automatic grounding device 2 illustrated in the drawing is n int-gr-l p-rt of an electrostatic spr y coating system 4 The spray oo-ting y~te~ 4 comprlses a p-int supply syste~ 6 for feeding paint to a ~pr-y organ 8, which in the illu~trated embodiment i9 a rotary atomizer but ~ay ln another mbodl~ent lso b- ~pr-y nozzle The palnt ~upply ~ystem 6 ~nd th- spr-y org-n 8 re rr-n8ed on t-bl-top lO from electrlc-lly conductlvo ~aterl-l nd connected to an electrlcal voltago between 20,000 ;~
nd 140,000 volts of a voltage generator circuit 14, by way of an electri- -cal llne 12 always only when a plston 16 of the automatic grounding device , ~ I i .~ .
2 i9 in a "voltage on position ~ The tabletop lO ls ~upported by ~everal legs fro~ electrically insulating material and ls thereby ln~ul-ted from the ground potenti-l 20 The drawlng shows only the leg 22 of the tabletop lO, whlch ls an lntegral part of the grounding device 2 The leg 22 con- ~ ;
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sists of a cylinder 24 from electrically insulating material that supports the tabletop 10 and of a grounding contact 26 from electrleally conductive material whlch features a collar 27 which supports the cylinder 24, and foatures a peg type proJection 28 which protrudes into the bottom end of the eyllnder 24 sealing lt gastight The grounding contact 26 i9 at ground potontlal 20 and, additionally, electrically well connected with the ground potentlal 20 through line 29 Axially movable in the cylinder 24, the piston 16 i8 in gastight eontaet wlth the inside cylinder wall 30, through Jeals 31 ant 32 The piston 16 eonsists of eleetrically conductive materi-al A comprossion spring 34 froo electrieally conductive material 19 mounted under mechanical prestress between the piston 16 and the housing 36 from l-etrieally conduetive material, of a meehanieally operable pneumatie switeh 38 The upper end 39 of the eompression spring 34 is in electrical eonn-etion with the housing 36 of tho pneumatie switeh 38, and the bottom and 40 of the eompression spring 34 is in eleetrieal eonneetion with the upper nd f-eo 41 of the piston 16, the bottom end face 42 of which bears on the grounding eont-et 26 nd is thu~ eleetrieally conneeted with it The ~loetrleally eonduetive housing 36 of tho pneu~atle ~wlteh 38 19 eleetri-eally eonnoetod with the tabletop 10 through an electrically conductive eonn-etlng eloment 44 Thus, ln the ~grounding position" of the piston 16 lllu~trated ln the drawing, tho paint supply system 6 and the spray organ 8 sre eonneeted to ground potential by way of the tabletop 10, the electrical eonneeting element 44, the houslng 36 of the pneumatie switch 38, the eompr-~lon ~prlng 34 nd tho pi~ton 16 as well a~ the grounding contaet 26 And th- grounding line 29 2a~475~

In this "grounding position," the pneumatic switch 38 interrupts the pneumatic connection between a compressed air feed line 48 and a compressed sir connecting line 50, both of which consist of electrically insulating materlal. The compressed alr connection line 50 i9 hooked with its one end 52 to the pneumatic switch 38 and with its other end 54 to a pneumatically oper-ble electrical ~witch 56, which in the "grounding positionn causes an interruption between an electricA1 line 58 snd an electrical line 60, so that no electrical signal in the line 58 can proceed to the line 60 and from there to the voltage generating c~rcuit 14. In the absence of such a signal, the voltage line 12 is not connected to the voltage of the voltage generator 14 but only when the electrical switch 56 is changed over to a po~ition in which the two electrical lines 58 and 60 are connected with each other. The electrical switch 56 is a pneumatic/electrical converter, since it converts a pneumatic signal on the compressed air connection line 50 to an electrical signal on the electrlcal line 60. The electrical switch S6 is through the compressed air connection line 50 spatially 90 far removed from the said electrlcally conductive parts that no voltage can .lump over to the electrical switch 56 from these~

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The lower end section 62 of the piston 18 tapers in the form of a truncated cone loaving between it, the grounding contact 26 and the cylinder 24 an annular pressure space 64 whoss air pressure acts axially, from below, on the piston 16 against the force of the compression spring 34, even while the piston 16 still bears with lts bottom end face 42 on the grounding contact 26, a9 illustrated in the drawlng. On the slde of the plston 16 facing toward the groundlng contact 26, a second compressed alr llne 66 7 ' ~02~7~j extends through the cylinder 24 into the pressure space 64 The two com-pressed air feed lines 48 and 66 are connected to a common supply line 70 which ln the illustrated valve position, through a changeover valve 72, vonts into the surrounding atmosphere by way of an air outlet 74 The changeover valve 72 is in the illustrated embodiment a pneumatic limit ~witch whlch is ctu-ted by a gate 76, for instance a door, which denies persons access to the palnt supply system 6 and the spray organ 8 while these are connected to the voltage of the voltage generator 14 The drawing shows the gate 76 in its open position in which it permits persons to pass A~ the Bate 76 closes, the changeover valve 72 moves into a valve position ln whlch lt connects compressed air supply 78 with the supply line 70 and thus l-o wlth the compressed air feed lines 48 and 68 The compressed air cts ln the pressure chamber 64 on the underslde of the piston 16, thereby moving it ag-inst the forco of the compression spring 34 axially in the cyllnd-r 24 ~cert-ln dl~tance~ 80 toward the pneumatic switch 38, in a ~volt-g -on posltion~ in which the piston 16, through a switching component 82, puJh-s down on n actu-ting element 84 of the pneumatic switch 38, thereby closlng the pneu~-tic ~witch 38 nd connecting the compressed ir fo-d line 48 by w-y of the pnou~atlc ~witch 38 with the compre~sed alr conn ctlon line 50 The compressed air actuates thereby the electrical witch 56 moving it in a po~ition in which it connects the electrical lines 58 and 60 with e-ch othar permitting n electric-l signal to proceed to the voltage generator circuit 14~ Only with this electrical signal present on the voltago g-nor-tor circuit 14 will lt be possible to connect the voltage with tho volt-ge line 12, elther utomatically or by h-nd The "certain dlst-nco~ 19 90 large th-t no voltage arc-over can occur between the piston 20247~5 . . .

16 and the grounding contact 26, which now are separated from each other by the "certain distance" 80. The switching component 82 is oblong in shape, is eontsined in the compression spring 34, and is supported by said com-pre~sion spring 34 approximately in the eenter 86 of the eompression spring. This shows that the eompression spring 34 could eonsist also of two eompression spring halves whieh in the spring eenter 86 are eleetrieally connected with each other through the switching component 82. Upon a eompressed air pressure drop, for instanee when the gate 76 is opened, the eompression spring 34 moves the piston 16 abruptly fast onto the grounding contact 26, thereby opening the pneumatic switch 38 and the eleetrieal switeh 50 and turning the voltage on the line 12 off, while at the same tlme eonneetlng the palnt supply system 6 and the spray organ 8 via the tabletop 10, the eleetrieal eonneeting element 44, the housing 36 of the pneumatie switeh 38, the eompression spring 34 and the piston 16 as well as the grounding eontaet 26 and the grounding line 29 with ground potential 20. Due to the abruptly fsst movement of the piston 16 onto the grounding eontaet 26, only a small spark ean be ereated as the eleetrieal eapaeity of the system is electrically discharged. This spark, sdditionally, occurring within the electrically insulating cylinder 24 in a limited spaee, there is no explosion hazard given, not even when eoating material, for instanee mi~ty paint, is eontained on the outside of the eylinder.

~ ' Through an opening which, not illustrated in the drawing, extends through the eylinder 24 and aets as a flow restrictor, the pressure space 64 can be permanently vented, even while the piston 16 is in the "voltage-on posi-eion~" This makes it possible to continuously remove from the pressure 2~24~

~pace 64 electrically charged air particles wbich through the sparking are created between the piston 16 and the grounding contact 26 as the piston 16 is belng moved downward fro~ the voltage-on position to the grounding po~ition Function-l d-~crlption The lnaul-ted table with the tabletop 10 and leg 22 -rvos the insulated in~tall~tion of the paint supply system 6, specifical-ly of paint containers and solvent containers of the spray coating syste~
These cont-iners aust during the spray coating operation have the saoe leetrieal voltago potontial as the spray organ 8 Due to the high voltage of up to 140,000 volts requirod for the olectrostatic coating, the insulat-ing tabl- n--d~ to be inst-llod in a locked room, a spray booth or a paint roo~ which c-nnot be ontered when the spray coating system is turned on R plenishing the containers and performing service work, however, requires ~hutting th- volt-go off and oloctric-lly di-sipatlng the capacitive charge on~r W Of the ~y-te~ co~pononts To that nd, the table is equipped with th- grounding l-g 22 A~ ~oon as the cc-~s gato 76 to th- insulating table i- elo--d, eompre-sed air to tho grounding log 22 is role-~ed via the pnou~atle ~witeh 72 Thi~ pro~ur- r-i~e~ tho pi~ton 16 e-u-ing it to aetuat- tho pneu~atie li~it witeh 38 which t the top is installed in the 8 22 This cr--te~ pnou~-tic signal on the co~pressed air connecting lino 50, which through the pneu~atic/electric converter 56 releases the ~pray coating systom, thus per~itting the voltage to be turnod on in the voltago g-nor-tor circuit 14 As soon 9 tho gato 76 opens, which also may ba ~ort of a barrier, tho co~prossod air ~ust be ~hut off As the eo~-pr-ssed air is shut off, tho pressure of the eo~pression spring 34 abruptly " `

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forces the piston 16 downward onto the grounding contact 26, thereby establishing a ground connection.

The pi~ton 16 consists of electrically eonductive material. According to a modifiet embodiment, the piston 16 may consist also of electrically insu-lating material and may be provided, on its end faces, with electrically eonduetive material whieh eleetrieally is eonnected with eaeh other.
, In another leg of the tabletop lO, not illustrated, a high-ohm electrical ;
resistor may be inst:alled which eonstantly conneets the tabletop lO with the ground potential 20, thereby dissipating eleetrical charges from the paint ~upply system 6 to ground potential also when the voltage ls turned on. Thls makes it possible to reduee the size of the spark which is gener-at-d between the piston 16 and the grounding eontaet 26 as the piston 16 move~ toward the grounding eontaet 26.

Aeeording to ~pecifie embodiment of the invention, the eylinder 24 may foature, near the grounding eontaet 26, an opening 90 through whieh air from tho pressure spaee 74 ean in ehoked fashion eseape into the surround-lng tmosphere also when the piston 16 bears on the grounting eontaet. This avolds the ereation of an explosive ion eoncentration in the pressure spaee 64. -~

The switehing eomponent 84 serves to bridge the distanee between the piston 16 and th- aetuating element 84 of the pneumatie switeh 38. In a modified embodiment, the switehing eomponent 84 may eonsist of eleetrically insulat-- 2 0 2 ~

lng material and may be screwed into the compression spring 34 consisting of one piece. According to another embodiDent of the invention, the switch-ing component 82 may be omitted, namely when the piston 16 and/or the ~ctuating element 84 extends into the compression spring 34 sufficiently f-r for the piston 16 to be able to actuate the element 84.

Claims (3)

1 Automatic grounding device (2) of an electrostatic spray coating system (4) for the automatic grounding of system components (6, 8, 10) which are arranged insulated from ground potential (20), in contingence on the operating condition of the spray coating system (4), characterized by 1.1. a cylinder (24) from electrically insulating material, 1.2. a piston (16) which is axially movable in the cylinder (24), 1.3. a compression spring (34) which forces the piston (16) on the grounding contact (26), is in electrical connection with the system components (6, 8, 10) that are arranged electrically insulated, and which, additionally, is electrically connected with the grounding contact (26) through the piston (16) only as long as the compression spring (34) holds the piston (16) on the grounding contact (26), so that in this "grounding posi-tion" the system components (6 8, 10) of insulated arrangement will be grounded, 1.4. a compressed air inlet (64, 66) to the cylinder (24), upon the end face of the piston (16) facing toward the grounding contact (26), through which inlet compressed air can be admitted on this end face of the piston (16) moving the piston (16) against the mechanically prestressed compression spring (34) a "certain distance" away from the grounding contact (26), axially, in a "voltage-on position,"
1.5. a mechanically actuable pneumatic switch 938) which in a pneumatic line (70, 48, 50) controls the supply of compressed air from a compressed air supply (78) to a pneumatically ac-tuatable electrical switch (56) and which features, arranged in the cylinder (24), an actuating element (84) that is actuated by the piston (16) as it proceeds to its "voltage-on position"
or is moved away from it, with the electrical switch (56) enabling in the "voltage-on position" of the piston (16) the actuation of the voltage of the spray coating system (4) that is required for the electrostatic coating, but not in the "grounding position" of the piston (16), 1.6. with the "certain distance" being sufficiently large to prevent the occurrence of an electrical arc-over of the electrical voltage from the compression spring (34) and the piston (16) to the grounding contact (26) while the piston (16) is contained in the "voltage-on position" and the compression spring (34) is connected to high voltage.

2. Grounding device according to claim 1, characterized in that an oblong switching component (82) is arranged in the compression spring (34) and held by it, which component has in the "grounding position"
an axial spacing from both the piston (16) and the actuating element (84) of the pneumatic switch (38) and which component while the piston moves in the area of its "voltage on position " transfers this movement of the piston to the actuating element (84) of the pneumatic switch.

3. Grounding device according to claim 1 or 2 characterized in that the cylinder (24) forms a leg or at least partial length of a leg of a table on the tabletop (10) of which the system components (6 8 10) insulated from ground are arranged.