U.S. patent number 3,929,286 [Application Number 05/545,152] was granted by the patent office on 1975-12-30 for apparatus and method for electrostatically spraying highly electrically conductive water-based coating material.
This patent grant is currently assigned to Nordson Corporation. Invention is credited to Donald R. Hastings, Jeffrey S. Noss.
United States Patent |
3,929,286 |
Hastings , et al. |
December 30, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Apparatus and method for electrostatically spraying highly
electrically conductive water-based coating material
Abstract
An electrostatic spray apparatus and method for spraying highly
electrically conductive water-based coating material in which
safety hazards are minimized. Included is a spray gun from which
electrostatically charged water-based coating material is emitted
toward an electrically grounded object to be coated; an enclosure
of insulative material having upper and lower compartments
electrically isolated from each other; a metallic coating supply
container or tank located in the upper compartment for supplying
coating material to the gun; an insulative hose interconnecting the
tank and gun having a section thereof which passes through the
lower compartment; a charging electrode located in the lower
compartment in communication with the bore of the hose; a high
voltage source connected to the charging electrode for charging the
coating as it passes through the section of hose in the lower
compartment; an opening in the upper compartment normally closed by
a cover to facilitate access to the tank for replenishing the
coating material; and a high voltage switch located in the lower
compartment which is automatically actuated concomitantly with
removal of the enclosure cover to electrically ground the charging
electrode, and hence the system, thereby eliminating shock hazards
when the tank is accessed for replenishment purposes and the like.
Other features are also disclosed.
Inventors: |
Hastings; Donald R. (Elyria,
OH), Noss; Jeffrey S. (Bay Village, OH) |
Assignee: |
Nordson Corporation (Amherst,
OH)
|
Family
ID: |
24175073 |
Appl.
No.: |
05/545,152 |
Filed: |
January 29, 1975 |
Current U.S.
Class: |
239/3; 361/1;
239/708 |
Current CPC
Class: |
B05B
5/1608 (20130101) |
Current International
Class: |
B05B
5/00 (20060101); B05B 5/16 (20060101); B05B
005/08 () |
Field of
Search: |
;239/15,274,288,3,302
;317/3,9R ;307/94 ;118/12,621 ;427/13,27 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Love; John J.
Attorney, Agent or Firm: Wood, Herron & Evans
Claims
Having described the invention, it is claimed:
1. A system for electrostatically spraying highly electrically
conductive water-based paint with improved safety, comprising:
a spray device for directing electrostatically charged water-based
coating material toward an object to be coated which is maintained
at a potential different from that of the charged coating
material,
a container for highly electrically conductive waterbased coating
material,
a fluid conduit interconnecting said container and said spray
device for transporting water-based coating material from said
container to said spray device,
a source of electrostatic charging potential for charging
water-based coating material in said container, conduit and spray
device,
an electrically insulative enclosure surrounding said container,
said enclosure having an opening therein provided with a removable
cover to selectively expose and facilitate access to said coating
container, and
means to electrically ground said charged coating material
concomitantly with removal of said enclosure cover, thereby
dissipating electrical energy stored in capacitive form in said
system and in turn minimize electrical shock hazards upon removal
of said cover to expose said coating container for access
thereto.
2. The system of claim 1 wherein said electrical grounding means
includes:
an electrically grounded movable electrical conductor, and
means to move said conductor concomitantly with removal of said
cover from a first position physically displaced from said charged
coating material to a second position in electrically conductive
relation to said charged coating material.
3. The system of claim 2 wherein said enclosure includes a
compartment electrically insulated from said container, said
movable conductor being located within said compartment, and
wherein said conductor moving means moves said conductor into
electrically conductive relationship with said coating material
concomitantly with removal of said cover to thereby electrically
ground said system.
4. The system of claim 3 wherein said movable conductor includes a
pivotally mounted electrically conductive rod within said
compartment, and wherein said conductor moving means includes a
movably mounted pin biased to project outwardly from said
compartment through said enclosure proximate said opening, said pin
moving outwardly to move said conductor into electrically
conductive relation with said coating material only upon removal of
said cover.
5. A system for electrostatically spraying highly electrically
conductive water-based paint with improved safety, comprising:
a spray device for directing electrostatically charged water-based
coating material toward an object to be coated which is maintained
at a potential different from that of the charged coating
material,
a container for highly electrically conductive water-based coating
material,
a fluid conduit interconnecting said container and said spray
device for transporting water-based coating material from said
container to said spray device,
a source of electrostatic charging potential for charging
water-based coating material located in said container, conduit and
spray device,
an electrically insulative enclosure surrounding said container,
said enclosure having an opening therein provided with a removable
cover to selectively expose and facilitate access to said coating
container, said enclosure further having a compartment electrically
isolated from said container,
a high voltage switch located in said compartment having one
terminal connected to ground and another terminal connected in
electrically conductive relation to said coating material, said
switch being selectively operable between a first condition in
which said terminals are electrically connected to ground said
coating and a second condition in which said terminals are
electrically disconnected, and
an actuator for switching said switch to its first condition
concomitantly with removal of said cover to ground said coating
material and thereby dissipate energy stored in said system.
6. A system for electrostatically spraying highly electrically
conductive water-based paint with improved safety, comprising:
a spray device for directing electrostatically charged water-based
coating material toward an object to be coated which is maintained
at a potential different from that of the charged coating
material,
a container for highly electrically conductive water-based coating
material,
a fluid conduit interconnecting said container and said spray
device for transporting water-based coating material from said
container to said spray device,
a source of electrostatic charging potential for charging
water-based coating material located in said container, conduit and
spray device,
an electrically insulative enclosure surrounding said container,
said enclosure having an opening therein provided with a removable
cover to selectively expose and facilitate access to said coating
container, said enclosure further having a compartment electrically
insulated from said container through which a section of said fluid
conduit passes,
an electrode within said compartment, said electrode connected in
said conduit section to physically contact waterbased coating
material flowing therethrough from said container to said spray
device,
a high voltage switch located in said compartment selectively
operable between a first condition in which said electrode is
grounded and a second condition in which said electrode is
ungrounded, and
an actuator for switching said switch to its first condition
concomitantly with removal of said cover to ground said electrode
and thereby dissipate energy stored in said system.
7. The system of claim 6 wherein said high voltage switch includes
a movably mounted electrically grounded conductor, said conductor
being movable, when said switch is in its first condition, to a
first position in electrically conductive relation to said
electrode to ground said system, and movable, when said switch is
in its second condition, to a second position in electrically
nonconductive relation to said electrode.
8. The system of claim 6 wherein said electrostatic charging source
has a high voltage output electrically connected to said electrode
for charging said water-based coating material as it flows through
said conduit section in said compartment, said high voltage
terminal being electrically connected to said container only
through the highly conductive coating material located in said
conduit section between said electrode and said container.
9. The system of claim 5 wherein a section of said fluid conduit
passes through said switch compartment, wherein said system further
includes:
an electrode within said compartment, said electrode connected in
said conduit section to physically contact water-based coating
material flowing therethrough from said container to said spray
device, and wherein said electrostatic charging source has a high
voltage output electrically connected to said electrode for
charging said water-based coating material as it flows through said
conduit in said compartment, said high voltage terminal being
electrically connected to said container via only one conductive
path, said path including the highly conductive coating material
located in said conduit section between said electrode and said
container.
10. The apparatus of claim 1 further including:
an electrical switch connected in electrical circuit with said
charging source and mounted to said enclosure, said switch having
first and second conditions in which said power supply is enabled
and disabled, respectively, and
an actuator responsive to movement of said cover to operate said
switch between said first and second conditions in response to
movement of said cover between closed and open positions,
respectively.
11. The apparatus of claim 1 further including:
a manually operated actuator mounted to said spray device for
controlling the flow of coating material emitted from said spray
device,
an electrical switch connected in electrical circuit with said
charging source, said switch having first and second conditions in
which said electrostatic source is enabled and disabled,
respectively, and
an operator responsive to said manually operated actuator for
operating said electrical switch between said first and second
conditions for enabling and disabling said electrostatic source,
respectively, in response to manual operation of said actuator to
initiate and terminate coating emission from said spray device,
respectively.
12. The apparatus of claim 1 further including:
a first electrical switch connected in electrical circuit with said
charging source and mounted to said enclosure, said switch having
first and second conditions in which said power supply is enabled
and disabled, respectively,
a first actuator responsive to movement of said cover to operate
said first switch between said first and second conditions in
response to movement of said cover between closed and open
positions, respectively,
a manually operated second actuator mounted to said spray device
for controlling the flow of coating material emitted from said
spray device,
a second electrical switch connected in electrical circuit with
said charging source, said second switch having first and second
conditions in which said electrostatic source is enabled and
disabled, respectively, and
an operator responsive to said manually operated second actuator
for operating said second electrical switch between its first and
second conditions for enabling and disabling said electrostatic
source, respectively, in response to manual operation of said
second actuator to initiate and terminate coating emission from
said spray device, respectively.
13. A system for electrostatically spraying highly electrically
conductive water-based paint with improved safety, comprising:
a spray device for directing electrostatically charged water-based
coating material toward an object to be coated which is maintained
at a potential different from that of the charged coating
material,
a container for highly electrically conductive water-based coating
material,
a fluid conduit interconnecting said container and said spray
device for transporting water-based coating material from said
container to said spray device,
a source of electrostatic charging potential for charging
water-based coating material in said container, conduit and spray
device,
an electrically insulative enclosure surrounding said container,
said enclosure having an opening therein provided with a removable
cover to selectively expose and facilitate access to said coating
container,
an electrical switch connected in electrical circuit with said
charging source and mounted to said enclosure, said switch having
first and second conditions in which said power supply is enabled
and disabled, respectively, and
an actuator responsive to movement of said cover to operate said
switch between said first and second conditions in response to
movement of said cover between closed and open positions,
respectively.
14. A system for electrostatically spraying highly electrically
conductive water-based paint with improved safety, comprising:
a spray device for directing electrostatically charged water-based
coating material toward an object to be coated which is maintained
at a potential different from that of the charged coating
material,
a container for highly electrically conductive water-based coating
material,
a fluid conduit interconnecting said container and said spray
device for transporting water-based coating material from said
container to said spray device,
a source of electrostatic charging potential for charging
water-based coating material in said container, conduit and spray
device,
an electrically insulative enclosure surrounding said container,
said enclosure having an opening therein provided with a removable
cover to selectively expose and facilitate access to said coating
container,
a first electrical switch connected in electrical circuit with said
charging source and mounted to said enclosure, said switch having
first and second conditions in which said power supply is enabled
and disabled, respectively,
a first actuator responsive to movement of said cover to operate
said first switch between said first and second conditions in
response to movement of said cover between closed and open
positions, respectively,
a manually operated second actuator mounted to said spray device
for controlling the flow of coating material emitted from said
spray device,
a second electrical switch connected in electrical circuit with
said charging source, said switch having first and second
conditions in which said electrostatic source is enabled and
disabled, respectively, and
an operator responsive to said manually operated second actuator
for operating said second electrical switch between its first and
second conditions for enabling and disabling said electrostatic
source, respectively, in response to manual operation of said
second actuator to initiate and terminate coating emission from
said spray device, respectively.
15. A method of electrostatic spraying in which safety hazards are
minimized when replenishing a container with highly electrically
conductive water-based coating material following electrostatic
spraying of charged coating material from a spray device supplied
from said container via a hose, said method comprising the steps
of:
enclosing the container with an insulative enclosure while spraying
charged water-based coating material supplied to the spray device
from said container via an electrically insulative hose,
removing a cover from an opening in the enclosure to facilitate
access to said container subsequent to termination of spraying,
and
electrically grounding the conductive coating material
concomitantly with removal of the cover.
16. The method of claim 15 wherein said grounding step
includes:
moving a grounded electrical conductor located within the enclosure
into electrically conductive relationship with the coating material
concomitantly with removing the cover when accessing the
container.
17. The method of claim 15 further including the steps of:
actuating an electrostatic power supply to charge the water-based
coating material when spraying, and
de-actuating the power supply concomitantly with removing the cover
when accessing the container.
18. The method of claim 17 wherein said de-actuating step includes
operating a switch mounted to the enclosure and in electrical
circuit with the power supply, concomitantly with removing the
cover when accessing the container via the enclosure opening.
19. The method of claim 15 further including the steps of:
actuating an electrostatic power supply to charge the water-based
coating material when spraying, and
de-actuating the power supply concomitantly with terminating flow
of the coating material to the spray device.
20. The method of claim 19 wherein said de-actuating step includes
operating a first switch in electrical circuit with the power
supply concomitantly with terminating flow of coating material to
the spray device.
21. The method of claim 17 further including the step of:
de-actuating the power supply concomitantly with terminating flow
of the water-based coating material to the spray device.
22. The method of claim 17 wherein said de-actuating step
includes:
operating a first switch in electrical circuit with the power
supply concomitantly with terminating flow of water-based coating
material to the spray device, and
operating a second switch mounted to the enclosure and in
electrical circuit with the power supply concomitantly with
removing the cover when accessing the container via the enclosure
opening.
23. A method of electrostatic spraying in which safety hazards are
minimized when replenishing a container with highly electrically
conductive water-based coating material following electrostatic
spraying of charged coating material from a spray device supplied
from said container via a hose, said method comprising the steps
of:
enclosing a container with an insulative enclosure while spraying
charged water-based coating material supplied to the spray device
from said container via an electrically insulative hose,
actuating an electrostatic power supply to charge the water-based
coating material when spraying,
removing a cover from an opening in the enclosure to facilitate
access to said container subsequent to termination of spraying,
and
de-actuating the power supply concomitantly with removing the cover
when accessing the container.
24. The method of claim 23 wherein said de-actuating step includes
operating a switch mounted to the enclosure and in electrical
circuit with the power supply concomitantly with removing the cover
when accessing the container via the enclosure opening.
25. The method of claim 23 further including the step of:
de-actuating the power supply concomitantly with terminating flow
of the coating material to the spray device.
26. The method of claim 25 wherein said de-actuating step
includes:
operating a first switch in electrical circuit with the power
supply concomitantly with terminating flow of coating material to
the spray device, and
operating a second switch mounted to the enclosure and in
electrical circuit with the power supply concomitantly with
removing the cover when accessing the container via the enclosure
opening.
27. The method of claim 15 wherein said grounding step
includes:
closing a high voltage switch which is located within a compartment
electrically isolated from said container enclosure, concomitantly
with removing the cover, to thereby electrically connect the
conductive coating material to ground potential and discharge
electrical energy stored in the system.
28. The method of claim 27 further including the step of:
transporting the water-based coating material from the container to
the spray device via the hose along a path passing through said
high voltage switch compartment, and
said switch-closing step includes connecting to ground potential
the conductive coating material flowing through the hose as the
material passes through the high voltage switch compartment,
thereby grounding the system through a switch in a compartment
which is electrically isolated from the container enclosure.
29. The method of claim 28 further including the step of:
energizing an electrode which is located in the switch compartment
in physical contact with the material flowing through the section
of the hose located in the switch compartment, said energization
being with an electrostatic charging potential to electrostatically
charge the coating material flowing through the hose as the
material passes through the high voltage switch compartment.
30. The method of claim 27 further including the steps of:
transporting the water-based coating material from the container to
the spray device via the hose along a path passing through said
high voltage switch compartment, and
energizing an electrode which is located in the switch compartment
in physical contact with the material flowing through the section
of the hose located in the switch compartment, said energization
being with an electrostatic charging potential to electrostatically
charge the coating material flowing through the hose as the
material passes through the high voltage switch compartment.
31. The system of claim 1 wherein said fluid conduit has a bore
through which coating material is transported from said container
to said spray device, said conduit including:
a. an inner zone surrounding said bore, said inner zone including
material which is substantially chemically inert and impermeable to
said water-based coating material, and
b. an outer zone of dielectric material surrounding said inner
zone, said outer zone having an outer surface substantially free of
electrically conductive material to minimize the capacitance of the
conduit, and hence the electrical energy stored in the conduit in
capacitive form, when the conduit is physically spaced from a
grounded object,
said inner and outer zones having dielectric constants and radial
thicknesses to avoid dielectric breakdown therein when subjected to
a potential difference equal to the difference in potential between
ground potential and the potential existing at the interface of the
transported coating and the inner zone.
32. The system of claim 1 wherein said fluid conduit is hollow,
said conduit having a dielectric wall with a zone in contact with
the water-based coating which is substantially chemically inert and
impermeable to the coating, said wall constructed to withstand
dielectric breakdown when subjected to an electrical potential by
electrostatically charged coating material in said conduit, said
conduit having an exterior surface which is substantially free of
electrically conductive material to minimize the capacitance of the
conduit, and hence the electrical energy stored in the conduit in
capacitive form, when the conduit is physically spaced from a
grounded object.
33. The system of claim 5 wherein said fluid conduit has a bore
through which coating material is transported from said container
to said spray device, said conduit including:
a. an inner zone surrounding said bore, said inner zone including
material which is substantially chemically inert and impermeable to
said water-based coating material, and
b. an outer zone of dielectric material surrounding said inner
zone, said outer zone having an exterior surface which is
substantially free of electrically conductive material to minimize
the capacitance of the conduit, and hence the electrical energy
stored in the conduit in capacitive form, when the conduit is
physically spaced from a grounded object,
said inner and outer zones having dielectric constants and radial
thicknesses to avoid dielectric breakdown therein when subjected to
a potential difference equal to the difference in potential between
ground potential and the potential existing at the interface of the
transported coating and the inner zone.
34. The system of claim 5 wherein said fluid conduit is hollow,
said conduit having a dielectric wall with a zone in contact with
the coating which is substantially chemically inert and impermeable
to the coating, said wall constructed to withstand dielectric
breakdown when subjected to an electrical potential by
electrostatically charged coating material in said conduit, said
conduit having an exterior surface which is substantially free of
electrically conductive material to minimize the capacitance of the
conduit, and hence the electrical energy stored in the conduit in
capacitive form, when the conduit is physically spaced from a
grounded object.
35. The system of claim 9 wherein said fluid conduit has a bore
through which coating material is transported from said container
to said spray device, said conduit including:
a. an inner zone surrounding said bore, said inner zone including
material which is substantially chemically inert and impermeable to
said coating material,
b. an outer zone of dielectric material surrounding said inner
zone, said outer zone having an exterior surface which is
substantially free of electrically conductive material to minimize
the capacitance of the conduit, and hence the electrical energy
stored in the conduit in capacitive form, when the conduit is
physically spaced from a grounded object,
said inner and outer zones having dielectric constants and radial
thicknesses to avoid dielectric breakdown therein when subjected to
a potential difference equal to the difference in potential between
ground potential and the potential existing at the interface of the
transported coating and the inner zone.
36. The system of claim 9 wherein said fluid conduit is hollow,
said conduit having a dielectric wall with a zone in contact with
the coating which is substantially chemically inert and impermeable
to the coating, said wall constructed to withstand dielectric
breakdown when subjected to an electrical potential by electrically
charged coating material in said conduit, said conduit having an
exterior surface which is substantially free of electrically
conductive material to minimize the capacitance of the conduit, and
hence the electrical energy stored in the conduit in capacitive
form, when the conduit is physically spaced from a grounded
object.
37. The system of claim 1 wherein said spray device further
includes means for controlling the flow of coating material to said
spray device, and wherein said electrical grounding means is
operative to ground said system when said coating material flow is
continuously stopped for a time period exceeding a predetermined
interval.
38. The system of claim 37 wherein said predetermined interval is
in the approximate range of 1-6 seconds.
39. The system of claim 5 further including:
manually operated means for controlling the flow of coating
material to said spray device, and
a second actuator for switching said switch to its first condition,
to thereby ground said electrode and dissipate energy stored in
said system, when said coating material flow is continuously
stopped for a time period exceeding a predetermined interval.
40. The system of claim 8 further including:
manually operated means for controlling the flow of coating
material to said spray device, and
a second actuator for switching said switch to its first condition,
to thereby ground said electrode and dissipate energy stored in
said system, when said coating material flow is continuously
stopped for a time period exceeding a predetermined interval.
Description
This invention relates to electrostatic spray coating and more
particularly to an apparatus and method for minimizing safety
hazards when electrostatically spraying highly electrically
conductive water-based coating material.
In the recent past there has been a considerable increase in the
emphasis placed on providing employees, particularly in the
industrial field, with working conditions which are free from
recognized safety and health hazards. This trend has, in part, been
the result of the Occupational Safety and Health Act which requires
employers to provide their employees with safe places of
employment. In the field of electrostatic paint spraying, in an
effort to comply with various safety and health regulations issued
pursuant to the Occupational Safety and Health Act, there has been
a marked trend among employers to switch from solvent-based
coatings to water-based coatings. Coating materials of the
solvent-based variety, such as varnishes, lacquers and the like
create an atmosphere which is both explosive and toxic. The
explosive nature of the environment presents a safety hazard should
a spark inadvertently be generated, such as by accidentally
electrically grounding the nozzle of the spray gun, which can then
ignite the solvent in the atmosphere, causing an explosion. The
toxic nature of the workplace atmosphere creates a health hazard
should the employee inhale the solvent vapors which are
present.
Because both of the foregoing hazards are eliminated by switching
to water-based coatings, the increased emphasis on providing
employees with safe and healthful workplaces has resulted in a
shift by industry to electrostatic spray coating products of the
water-based type. Unfortunately, the switch from electrostatically
spraying solvent-based coatings to those of the water-based type
has sharply increased the risk of electrical shock, which risk was
relatively minor with solvent-based coatings. Specifically, the
increased risk of electrical shock accompanying electrostatic
spraying of water-based paints is occasioned by the fact that
water-based paints are extremely electrically conductive, with
resistivities often falling in the range of 100-10,000
ohm-centimeters. This is in contrast to resistivities of
200,000-1,000,000 ohm-centimeters for moderately electrically
conductive coatings such as metallic paint, and resistivities
exceeding 1,000,000 ohm-centimeters for solventbased lacquers,
varnishes, enamels, and the like.
By way of background, with coating materials which are either not
electrically conductive or only moderately electrically conductive,
as is the case with metallic and solvent-based paints, the charging
electrode, which is maintained at a high electrostatic potential,
is usually placed near the nozzle of the spray device thereby
electrostatically charging coating material as it is emitted. Due
to the relatively high resistivity of the coating material, when
the electrode is so placed, the column of coating material in the
hose, which connects the spray device to the coating supply tank,
has sufficient electrical resistance to prevent any significant
electrostatic charging of the material in the tank or the tank
itself. However, when the coating material is highly electrically
conductive, as is the case with waterbased paints, the resistance
of the coating column in the hose is very low, with the result that
a high voltage charging electrode located in the vicinity of the
spray device nozzle, electrostatically charges not only the emitted
particles, but the coating material in the hose and supply tank, as
well as the supply tank itself. Under such circumstances, operating
personnel inadvertently coming into contact with an exposed supply
tank risk serious electrical shock.
To avoid the aforementioned electrical shock problem associated
with a system in which highly conductive water-based coating
material in the hose and tank becomes charged as an incident to
spraying, it has been proposed in the past to enclose the supply
tank in an electrically insulative housing. However, even when this
is done electrical shock hazards still exist, particularly when the
insulative enclosure is opened to expose the tank and gain access
thereto for replenishing the coating supply. Such hazards exist,
notwithstanding that the electrostatic charging source may have
been de-energized prior to accessing the coating tank, due to the
fact that the electrostatic charge on the coating material in the
tank and hose does not immediately dissipate, or discharge,
following de-energization of the electrostatic charging source, but
rather requires a finite time, dependent on existing current
leakage paths, to discharge to a nonhazardous level. Thus, a
residual charge of a hazardous level, albeit gradually decreasing,
remains on the tank for a finite time, often as long as 40 seconds,
after the electrostatic charging source is de-energized. Hence,
operating personnel contacting the coating container after
de-energization of the charging source, but prior to reduction of
the residual electrostatic charge to a safe level, are exposed to
shock hazards.
Accordingly, it has been an objective of this invention to provide
an apparatus and method for electrostatically spraying highly
conductive water-based coating material which minimizes shock
hazards heretofore existing when a normally enclosed tank of
conductive coating material is accessed following deenergization of
the charging source, but prior to discharge of the residual
electrostatic charge on the coating material through the normal
current leakage process. This objective has been accomplished in
accordance with certain principles of this invention by providing,
in combination with an insulative enclosure surrounding a tank
containing electrostatically charged waterbased coating material,
means for automatically electrically grounding the system as a
concomitant to removal of the enclosure cover to access the tank.
As a consequence, residual charge in the system is automatically
and immediately dissipated to ground as an incident to accessing
the paint tank, eliminating shock hazards which heretofore existed
when a tank was accessed shortly after de-energizing the charging
source, but prior to dissipation of the stored residual charge.
In a preferred form of the invention the automatic electrostatic
grounding arrangement includes a high voltage grounding switch
located in a compartment of the insulative enclosure separate and
apart from the compartment in which the paint tank is housed. Upon
removal of the cover of the tank compartment, the high voltage
switch, which is connected between the tank and ground,
automatically discharges the system to ground potential. Placement
of the high voltage switch in a separate compartment, vis-a-vis in
the same compartment as the tank, provides a number of advantages.
For example, by reason of the high voltage switch being absent from
the tank compartment, the switch is protected against possible
damage due to paint spillage, jarring or the like, as an incident
to tank replenishment and/or removal. In addition, the high voltage
switch compartment, by reason of being separate from the tank
compartment, can be sealed against unauthorized tampering or the
like without inconveniencing personnel needing access to the tank
compartment for replenishment purposes, etc. Finally, the size of
the tank compartment can be made smaller since the requisite
electrical standoff between the tank and compartment walls can be
provided by measuring directly from the tank to the compartment
walls, rather than from the compartment walls to the high voltage
electrical switch which, if located in the tank compartment, would
be mounted on the tank, thereby increasing the actual distance
required between tank and compartment walls to provide a
predetermined electrical standoff.
In accordance with another aspect of the invention, the hose which
supplies paint from the tank to the spray device passes through the
high voltage switch compartment whereat an electrode communicating
with the bore of the hose carrying the conductive water-based paint
is alternatively selectively connected to either (a) the high
voltage source for charging the paint as it flows through the hose
or (b) ground potential via the high voltage switch for discharging
the system. By virtue of this arrangement, the paint hose serves
multiple purposes. Specifically, it transports paint from the tank
to the gun, as well as establishes a conductive path for both
charging the paint and grounding the system. Thus, separate
electrical conductors interconnecting the tank to the charging
source and high voltage grounding switch are unnecessary. Not only
does this reduce cost, but it makes tank removal for maintenance or
like purposes easier since once the tank compartment cover is
removed and the tank accessible, the only fitting or connection to
the tank requiring disconnection is the fitting interconnecting the
tank and paint hose. A second hose, which supplies air to the tank
for pressurizing the paint, is typically connected to the tank
cover. However, the tank can be removed without disconnecting this
hose by merely removing the cover from the tank before removing the
tank itself.
A further advantage of this particular aspect of the invention is
that separate openings between the tank and high voltage switch
compartments need not be provided for electrical conductors which
might otherwise be needed were separate conductors designed to
charge the paint and discharge the system provided. The only
openings in the wall separating the compartments which are needed
are those necessary to accommodate the paint and air hoses.
Accordingly, only two seals need be provided in the wall between
the compartments, reducing initial equipment manufacturing cost as
well as operating problems occasioned by paint leakage from the
tank compartment into the high voltage switching compartment.
In the preferred form of the invention, the high voltage switch
includes an electrically grounded rod pivotally mounted within the
switch compartment. The rod, upon removal of the tank compartment
cover, is moved into electrically conductive contact with the paint
hose electrode by a linkage which interconnects the cover and rod,
thereby grounding the system through the paint column in the hose
and dissipating the residual charge on the coating material in the
tank hose and gun.
Another aspect of this invention contemplates electrically
grounding the system by means of the high voltage grounding switch
under remote control from the gun trigger, whenever the user of the
gun releases the trigger for an abnormal period of time exceeding
the normal, momentary-type release typically encountered in the
course of spraying an article. A typical example of such an
abnormal, long-duration trigger release is when the user must
interrupt spraying for a protracted period to replenish the tank.
Since a user is more likely to inadvertently touch the gun nozzle
and receive a shock when he has finished spraying and, for example,
contemplates replenishing the tank, than he is when engaged in
spraying an article, by grounding the system automatically each
time spraying is concluded, shock hazards are minimized. Of course,
momentary trigger releases, since they do not result in discharging
the system, do not interfere with normal spraying, as might
otherwise occur were the system discharged every trigger release,
due to the lag time necessary for the charge to build up to the
desired operating level following turn-off.
In accordance with a further aspect of the invention, also designed
to minimize shock hazards, an electrical switch in the energization
circuit of the high voltage power supply is provided which, upon
removal of the insulative cover from the enclosure surrounding the
tank, disables the high voltage charging supply. Thus, when the
cover of the insulative enclosure is removed, thereby exposing the
tank, the supply of electrostatic charging potential to the coating
material is automatically terminated.
In accordance with a still further aspect of the invention, also
designed to minimize safety hazards, a second electrical switch is
connected in the energization circuit of the high voltage power
supply. The second switch is designed to be placed in an open
circuit condition, thereby automatically deenergizing the high
voltage supply, when the spray gun trigger is released to stop the
flow of coating material to the gun. Should the operator, after
concluding spraying, accidentally cause the nozzle of the gun,
which is typically charged due to the conductive nature of the
coating material, to come into contact with a grounded object, such
as in the course of placing the gun on a support, the likelihood of
electrical shock due to inadvertent grounding is minimized.
In accordance with another aspect of the invention, also designed
to minimize electrical shock hazards and extend the life of the
hose used to interconnect the gun and tank, the hose is constructed
in a very unique manner never before used in water-based coating
spray systems. Specifically, the hose is provided with a dielectric
wall which has a zone in contact with the water-based coating which
is substantially chemically inert and impermeable to the coating,
as well as constructed to withstand dielectric breakdown when
subjected to the electrostatic charging potential by the charged
paint flowing through the hose. In addition, the hose has an
exterior surface free of electrically conductive material. The
impermeability and inertness of the dielectric wall to the
water-based paint prevents it from physically deteriorating and
becoming water-logged which, if permitted to occur, would
eventually lead to arcing through the wall and ultimately failure
of the hose. The absence of an outer cladding of conductive
material minimizes the capacitance of the hose, and hence the
electrical energy stored therein in capacitive form, particularly
when the hose is physically spaced from a grounded object, such as
a factory floor. Reduction in stored energy reduces shock hazards
should an operator inadvertently contact the gun nozzle before the
system has been discharged, such as while spraying. Even when the
conductively unclad hose of this invention is in contact with a
grounded surface, such as a factory floor, its capacitance, and
hence energy storing capability, is still significantly less, e.g.
50 percent, than conductively clad hoses. Thus, shock hazards are
significantly reduced with the conductively unclad hose of this
invention even when the hose rests on a grounded factory floor or
like grounded surface.
These and other advantages and objectives of the invention will
become more readily apparent from a detailed description of the
preferred embodiment taken in conjunction with the drawings in
which:
FIG. 1 is an elevational view, partly in crosssection, of the spray
device, coating container and interconnecting hose 1 showing the
electrical interlock and automatic system grounding features of
this invention;
FIG. 1a is an enlarged view of the connection between the
electrically conductive plate and hose shown in FIG. 1.
FIG. 2 is a bottom view taken along line 2--2 of FIG. 1;
FIG. 3 is an elevational view of a modified form of grounding
mechanism; and
FIG. 4 is a schematic view of a portion of a second embodiment of
the invention which is operative to ground the system when either
the cover of the insulative tank enclosure is removed or the flow
of coating material to the spray device terminates for longer than
a predetermined interval.
Electrostatic spray coating systems of the general type to which
this invention relates are designed to spray highly electrically
conductive water-based coating materials, i.e., having
resistivities in the range of 100-1,000,000 ohm-centimeters. By way
of illustration only, one such water-based coating material
frequently used is water-soluble bake enamel, manufactured by
Muller Industries, having a resistivity of 318 ohm-centimeters.
Typically these systems include, as a principle component thereof,
an electrostatic spray device such as an electrostatic spray gun
10. The gun 10 has an electrically grounded handle 11 of
electrically conductive material designed to be manually grasped in
use by the operator and an electrically insulative barrel 12 which
at its forward end terminates in a nozzle 13. A spray 14 of finely
divided, or atomized, particles of highly conductive coating
material flows from the gun nozzle toward an object 15 being coated
when a manually operated actuator on the handle, such as a trigger
18, is actuated by the operator. A source of coating material 21 is
connected to a coating inlet 16 of the gun 10 via a flexible
conduit, hose or supply line 24. The coating inlet 16 communicates
with the nozzle 13 via fluid passages 19 and 20 in the barrel 13
and handle 11, respectively. Actuation of the gun trigger 18 opens
a normally closed flow valve 17 in the gun 10 via an
interconnecting plunger 18a to permit the flow of conductive
coating material to the nozzle 13 whereat it is atomized and
emitted as the spray 14.
Electrostatic spray systems also include an electrical power pack,
or booster supply, for transforming commercially available low
voltage AC power, e.g., 60 Hertz - 115 volts, to high DC voltage,
e.g., 50KV - 100KV, and an electrode connected to the high voltage
power supply and in contact with the conductive coating material
for electrostatically charging the coating material such that as
coating particles are emitted from the gun nozzle their attraction
to the article 15 being coated, which is typically maintained at a
potential different than that of the charging potential, for
example, at zero or ground potential, will be enhanced.
Depending upon whether or not the gun is of the "air" type, wherein
atomization of the coating material is effected by impact of an air
stream with the liquid coating material, a source of air may or may
not be connected to the gun via an air line for impinging air on
the coating stream in the region of the nozzle. If the spray gun is
of the "airless" type, wherein atomization of the coating particles
in the region of the nozzle is effected hydraulically, the air line
may be omitted. The electrostatic spray gun 10 which is illustrated
is of the airless type. However, it should be understood that the
invention is equally applicable to other types of electrostatic,
manual and automatic, spray guns and systems.
The fluid conduit or hose 24 interconnecting the coating inlet 16
of the gun 10 and the source of coating material 21 preferably
includes an inner dielectric layer 24a and an outer dielectric
layer 24b.
The inner layer or zone 24a is preferably chemically inert with
respect to the water-based coating being transported within the
central bore 24c defined thereby such that the surface of layer 24a
will not be significantly corroded, dissolved, eroded, or otherwise
physically or chemically deteriorated by chemical interaction with
the coating being conveyed through the bore. The inner layer 24a is
also preferably essentially impermeable with respect to the coating
conveyed through bore 24c, effectively establishing a fluid-tight
barrier between the coating-transporting bore 24c and the remaining
layer or zone 24b of the conduit 24. Tetrafluroethylene has been
found to be both chemically inert and impermeable to water-based
paints and therefore a good material from which to construct layer
24a. When tetrafluroethylene is used, a wall thickness of 40 mils
is preferable.
The establishment of a barrier between the bore 24c and zone 24b
which, with respect to the coating being transported, is
substantially fluid-tight limits possible permeation of the
dielectric zone 24b by the coating should the latter zone be
permeable, which is often the case where the zone 24b is fabricated
of flexible dielectric material since many flexible dielectric
materials are permeable to water-based coatings. Were significant
permeation of zone 24a permitted to occur, an electrically
conductive path through the zone 24b could be established, assuming
the latter is permeable, leading to undesirably high electrical
current leakage in a radial direction through the wall of conduit
24.
The layer or zone 24b functions, in combination with the inner zone
24a, to establish a dielectric breakdown resistant barrier in the
radial direction which withstands dielectric breakdown when the
interface between the inner zone 24a and the coating in bore 24c is
subjected to a high voltage as necessarily occurs when the
electrostatic charging voltage is applied to the interface via the
column of highly conductive water-based coating in the bore 24c
which is in electrical contact with the electrostatic charging
source. In a preferred form of the invention, the dielectric
breakdown resistant zone 24b is fabricated of extruded hollow low
density polyethylene tubing having a thickness of approximately 60
mils. Such a construction is relatively flexible, in addition to
having the desired electrical properties of low radial current
leakage and high dielectric breakdown resistance. Specifically,
polyethylene has a resistivity of 10.sup.15 - 10.sup.16
ohmcentimeters and a dielectric breakdown resistance of
approximately 700 volts per mil and, like zone 24a, resists
substantial radial electrical current leakage flow and dielectric
breakdown when subjected to voltages thereacross on the order of
the charging potential. Other dielectric materials could be
utilized for layer 24b depending upon the degree to which it is
desired that the zone material be chemically inert and impermeable
to the water-based coating. For example, polypropylene and vinyl
plastics may be used.
It has been found desirable to fabricate the zones 24a and 24b of
material which provides a combined, or average, dielectric strength
of approximately 800 volts per mil, although average, or combined,
dielectric strengths ranging between 250 volts per mil and 1,000
volts per mil are satisfactory for specific applications. If
composite dielectric strengths of lesser values are used, the
thickness of the conduit wall measured in the radial direction may
become undesirably large, increasing the bulk and stiffness of the
coating conduit.
If desired, the hose layer 24b may be provided with a tough outer
layer or skin of polyurethane (not shown) for abrasionresistance
purposes. A polyurethane skin thickness of 25 mils has been found
to afford satisfactory resistance to abrasion.
The outer surface of the hose 24, whether it be layer 24b or an
abrasion-resistant polyurethane skin (not shown), is free of
electrically conductive material, i.e., does not contain a layer or
cladding of conductive material. It has been discovered, when
conductive cladding is omitted in construction of a hose used to
spray highly conductive water-based coatings, that the capacitance
of the hose is significantly reduced, particularly when the hose is
spaced from a ground surface such as a factory floor, in comparison
to the capacitance of a conductively clad hose. By reducing the
capacitance of the hose, its ability to store electrical energy in
capacitive form is reduced, in turn reducing shock hazards should
an operator inadvertently contact the nozzle of the spray device
when the system is in charged state, such as when the operator is
spraying with the power supply energized. Even when the
conductively unclad hose of this invention is in contact with a
grounded surface, such as a factory floor, its capacitance, and
hence energy-storing capability, is still significantly less, e.g.,
50 percent, than a conductively clad hose. Thus, shock hazards are
reduced with the conductively unclad hose of this invention even
when the hose rests on a grounded factory floor or like grounded
surface.
The coating material source 21 includes an inner metallic
container, preferably aluminum, of conventional design capable of
holding anywhere from one gallon to 50 gallons of highly
electrically conductive coating material 29. The coating material
container or tank 28 is provided with a removable aluminum cover 30
which in use is held in sealed engagement with the
coatingcontaining tank via circumferentially spaced clamps 31. An
air conduit 32, preferably of electrically insulative material, is
connected between the tank cover 30 and a pressurized air supply 34
to subject the interior 35 of the tank 28 above the level of the
coating 29 to pressurized air for maintaining the coating material
under pressure. With the coating material 29 in the tank 28
pressurized, the coating is pressure fed to the gun 10 via the hose
24 and a suitable fitting 28a in the container wall. If desired, an
air-operated agitating mechanism (not shown) having an impeller
immersed in the coating material 29 can be provided for the purpose
of insuring that the coating material in container 28 is maintained
in a homogenous state.
As noted, the coating material 29, with respect to which this
invention possesses a particularly high degree of utility,
typically has a resistivity in the range of 100-1,000,000
ohmcentimeters. While a specific resistivity range has been used to
define high conductivity water-based coating, it is understood that
such a resistivity value is arbitrary and relative, and employed
only for the purpose of illustration. Accordingly, a coating
material having a resistivity above 1,000,000 ohm-centimeters could
conceivably be considered as highly conductive notwithstanding that
it falls near, although without, the specific numerical value
given.
Connected in the hose 24 is an electrically conductive fitting 36
which communicates with the hose bore 24c and hence with the
coating material pressure fed from the container 28 to the spray
gun 10. Electrically connected to the fitting 36 is an electrically
conductive plate 38. The plate 38 is connected via a current
limiting resistor 40 to an electrically insulated high voltage line
42 output from a source of high voltage DC electrostatic charging
potential 44. Conductive coating material pressure fed from the
container 28 to the gun 10 via the hose 24 is electrostatically
charged to the desired high voltage potential, e.g., 50KV - 100KV,
as it passes through the electrically conductive fitting 36. Since
the coating material is highly electrically conductive, the column
of coating material within the hose bore 24c and gun passages 19
and 20 is also at the electrostatic charging potential output from
the voltage supply 44 on line 42. Thus, the atomized coating
material 14 emitted from the gun nozzle 13 is electrostatically
charged at the potential of line 42, and the charged coating
particles will be attracted to the grounded article 15 being
coated. In addition, the coating material 29 within the metallic
container 28, as well as the metallic container itself, are at the
potential of high voltage line 42 due to the conductive nature of
the coating material in the container and in the section of hose
between the electrostatic charging fitting 36 and the tank outlet
fitting 28a.
An electrically insulative enclosure 45, including a removable
electrically insulative cover 46 and an electrically insulative
base 48, is provided to completely enclose, and hence electrically
insulate from the surrounding environment, the container 28.
Specifically, when the cover 46 is located with its lower rim 46a
seated on a cooperating peripheral ledge or lip 48a of the base 48,
a chamber 50 is provided within which the metallic container 28 is
located in electrical isolation from the environment. Preferably,
the insulative base 48 is configured to provide a well 48b within
which the lower portion of the container 28 nests. Obviously, when
the enclosure cover 46 is removed from seating engagement with the
base 48 to expose the container 28, access to the container
replenished with coating material. The enclosure 45, and
particularly cover 46, are sized such that the point whereat the
tank 28 comes closest to the enclosure is spaced from the enclosure
by a distance sufficient to provide an "electrical standoff"
between the tank and the enclosure.
The enclosure 45, particularly the region enclosed by the base 48
located below the well 48b, defines a second chamber 54. Located
within the chamber 54 is the electrically conductive plate 38 and
fitting 36. Preferably fitting 36 and plate 38 are spaced from the
floor or the like support 56 on which the base 48 sits by a
distance X sufficient to provide an "electrical standoff" between
the floor 56 which is typically at ground potential and the
conductive elements 36 and 38. This prevents arcing between ground
and the conductive elements 36 and 38 when the latter are
electrostatically charged from the high voltage power supply 44 via
electrically insulative cable 42 which passes through a suitably
located aperture 49 in the base.
Also located within the chamber 54 is a system grounding assembly
58 which, in a manner to be described, functions as a high voltage
switch to electrically ground the system when the enclosure cover
46 is removed from the base 48, i.e., as a concomitant to cover
removal. The system ground assembly or high voltage switch 58, in a
preferred form, includes an electrical conductor 60, preferably in
the form of an elongated conductive rod, which is connected to
ground potential via an electrical line 61. The electrically
conductive rod 60 is mounted for pivotal movement by a bracket 62
secured to the inner wall 63 of the base 48. One end 60a of the
pivotally mounted conductive rod 60 extends vertically upwardly
through an aperture or hole 65 in the lip 48a of the base 48 at a
point which underlies the cover rim 46a, while the other end 60b of
the conductive rod 60 extends vertically downwardly. The rod 60 is
biased by suitable spring means (not shown) such that the rod tends
to pivot in a clockwise direction about bracket 62 as viewed in
FIG. 1.
When cover 46 is in its closed position with respect to the base
48, such that cover rim 46a seats on base lip 48a, the rod end 60a
is urged downwardly by the cover rim to the solid line position
shown in FIG. 1. With rod end 60a in the solid line position due to
depression thereof via the cover rim 46a seating on base lip 48a,
the rod 60 is located at its counterclockwise limit of travel
(shown in solid lines) in which the rod end 60b is spaced from the
conductive plate 38. Thus, with the cover 46 in place on the base
48 and the conductive rod 60 in its solid line position shown in
FIG. 1, the electrically grounded conductive rod end 60b is
displaced from the electrically conductive plate 38, with the
result that the system is not grounded.
When the cover 46 is removed and its lower rim 46a no longer seats
on base lip 48a, the rod end 60a is free to move upwardly through
aperture 65 to the dotted line position shown in FIG. 1 due to the
spring bias action, with the result that grounded rod end 60b makes
electrical contact with the electrically conductive plate 38. Since
the electrically conductive plate 38 also makes electrical contact
with the conductive coating material in the tube bore 24c via the
conductive fitting 36, and in turn with the coating material 29 in
the container 28, the system is electrically grounded when cover 46
is removed.
While in the preferred embodiment the system is grounded when the
cover 46 is removed by electrically grounding the conductive plate
38, it will be apparent to those skilled in the art that electrical
grounding of the system could be accomplished in other suitable
manners. For example, the grounded conductive rod 60 could make
contact directly with the metallic container 28 by relocating the
pivot 62 and rod element 60b as shown in FIG. 3, such that element
60b moves through an aperture 66' in the well 48b underlying
container 28' in response to removal of the cover 46' which permits
rod end 60a' to rise through aperture 65' in lip 48a' underlying
cover rim 46a'. A still further variant comprehends providing a
microswitch in the base 48 which, upon removal of the cover 46, is
actuated to energize a relay, solid state switch or the like. The
relay, in turn, when energized, would complete an electrical
circuit between the metallic container 28 or conductive plate 38 to
a source of electrical ground potential.
By virtue of providing automatic system grounding, safety hazards,
such as electrical shock, are minimized when the cover 46 is
removed by operating personnel to gain access to the container 28
for refilling it with coating material or the like. As previously
noted, since the coating material is highly conductive, the
contents of the metallic container, as well as the metallic
container itself, are electrostatically charged as an incident to
electrostatically charging the coating material during spraying,
whether the electrostatic charging electrode is incorporated in the
nozzle assembly 13, the hose 24 (as shown), or connected directly
to the metallic tank 28. Were the cover 46 to be removed without
provision for automatically grounding the system, a serious safety
hazard would be presented by virtue of the charged nature of the
container 28 were personnel to inadvertently contact the exposed
container after the cover 46 has been removed.
As described, in the preferred form of the invention the automatic
electrostatic grounding arrangement includes a high voltage
grounding switch 58 which is located in a compartment 54 of the
insulative enclosure 45 which is separate and apart from the
compartment 50 in which the paint tank 28 is housed. Placement of
the high voltage switch assembly 58 in a separate compartment,
vis-a-vis in the same compartment as the tank 28, provides a number
of unobvious advantages. For example, with the high voltage switch
58 absent from the tank compartment 50, the switch is protected
against possible damage due to paint spillage, jarring or the like,
as an incident to tank replenishment and/or removal. In addition,
the high voltage switch compartment 54, by reason of being separate
from the tank compartment 50, can be sealed against unauthorized
tampering or the like without inconveniencing personnel needing
access to the tank compartment for replenishment purposes, etc.
Finally, the size of the tank compartment 50 can be made smaller
since the requisite electrical standoff between the tank 28 and
tank compartment walls can be provided by measuring directly from
the tank walls to the compartment walls, rather than from the
compartment walls to the high voltage electrical switch which, if
located in the tank compartment, would be mounted on the tank,
thereby increasing the actual distance required between tank and
compartment walls to provide a predetermined electrical
standoff.
As also described in connection with the preferred embodiment of
the invention, the hose 24 which supplies the highly conductive
paint from the tank 28 to the gun 10 passes through the high
voltage switch compartment 54. In the compartment 54 an electrode
or plate 38, which communicates with the hose bore 24c carrying the
highly conductive water-based paint, is alternately selectively
connected to either the high voltage source 44 via line 42 for
charging the paint or to ground via the high voltage switch
arrangement 58 and line 61 for discharging the system. This
structural aspect of the preferred embodiment of the invention also
provides a number of unobvious advantages. For example, by virtue
of this arrangement the paint hose 24 serves the multifold purpose
of transporting paint from the tank 28 to the gun 10, as well as
establishing a conductive path for both charging the paint and
grounding the system. As a consequence, removal of the tank 28 for
maintenance or the like is easier since once the tank compartment
cover 46 is removed and the cover 30 of the tank unclamped and
taken off, the only fitting to the tank 28 requiring disconnection
is the fitting 28a interconnecting the tank 28 and paint hose 24.
Thus, by reason of the multiple functions performed by hose 24,
tank removal can be made without need for disconnecting separate
electrical conductors between the tank and sources of charging
potential and ground potential which might otherwise be required
for charging the paint and discharging the system.
A further advantage of this particular aspect of the invention is
that the number of openings between the tank and high voltage
switch compartments 50 and 54 is minimized by reason of the fact
that separate openings are not needed for connecting charging and
discharging conductors to the tank. The only openings needed are
those to accommodate the paint hose 24 and air hose 32.
Accordingly, only two seals S.sub.1 and S.sub.2 need be provided in
the wall between the compartments 50 and 54, reducing initial
equipment manufacturing cost as well as operating problems
occasioned by paint spillage from the tank compartment 50 into the
high voltage switching compartment 54.
To further minimize safety hazards to operating personnel, a
cover-operated electrical interlock switch assembly 64 and a
trigger-operated electrical interlock switch assembly 67 are
provided. The cover-operated electrical interlock switch assembly
64 includes a pair of electrical contacts 64a and 64b which are
adapted to be electrically connected by a movable electrically
conductive switch element 64c when the cover 46 is in the closed
position with the rim 46a thereof seating on base lip 48a. An
insulative switch actuator in the form of a vertically shiftable
plunger 64d slidably interfits in a hole 68 formed in the base rim
48a. The plunger 64d at its lower end is secured to the conductive
contactbridging element 64c such that when the cover 46 seats on
the base 48 the conductive element 64c bridges contacts 64a and
64b. When the cover 46 is removed from the base 48, the plunger 64d
is free to rise under the action of a compression spring 69 located
between a stationary portion 70 of the base 48 and the
contactbridging element 64c, disconnecting contacts 64a and
64b.
The switch contacts 64a and 64b are connected between a low voltage
source 71, such as a conventional 60 Hertz, 120 volt supply, and
the high voltage power supply 44. Whenever the cover 46 is removed
from the base 48, open-circuiting switch contacts 64a and 64b, the
electrical circuit between the low voltage source 71 and the high
voltage supply 44 is interrupted, disabling the high voltage supply
44. Thus, whenever the cover 46 is removed to expose the metallic
container 28, which as noted contains electrically conductive
coating material, the supply of electrostatic charging potential to
the coating material via line 42, plate 38 and fitting 36, and
hence to the container 28, is terminated.
The trigger-operated electrical interlock switch assembly 67
includes stationary switch contacts 67a and 67b which are adapted
to be electrically connected by a conductive bridging element 67c
when the trigger 18 is placed in its actuated position shown in
solid lines in FIG. 1. The bridging element 67c is mounted for
movement with the trigger 18 via an insulative stud 73. A
compression spring 72 normally places the trigger 18 in its
de-actuated position shown in dotted lines in FIG. 1. In the
de-actuated trigger position the valve 17 is closed, terminating
the flow of electrically conductive coating material to the gun and
the switch contacts 67a and 67b are open-circuited.
Switch contacts 67a and 67b are connected between the low voltage
supply 71 and the high voltage supply 44. Thus, when the trigger 18
is placed in its de-actuated condition shown in dotted lines in
FIG. 1, the circuit between the low voltage supply 71 and the high
voltage supply is interrupted, disabling the high voltage supply.
Hence, any time the trigger 18 is released to terminate spraying of
conductive coating, the application of high voltage from the supply
44 to the conductive coating material via line 42, plate 38 and
fitting 36, is terminated. This reduces safety hazards should the
operator, after releasing the trigger, bring the nozzle of the gun
into contact with a grounded object such as when placing the gun on
a grounded support.
In a preferred embodiment of the invention the trigger-operated
interlock switch contacts 67a and 67b and the cover-operated
interlock switch contacts 64a and 64b are connected in electrical
series circuit relation between the low voltage supply 71 and the
high voltage supply 44. As such, the supply of electrostatic
charging potential to the coating material terminates when either
the cover 46 is removed to expose the metallic container 28
containing the coating material, or the trigger 18 is released upon
conclusion of spraying.
While the cover-operated interlock switch assembly 64 has been
described as being operated by a cover-actuated plunger 64d, other
switching arrangements operated in response, or as an incident or
requisite, to removal of the cover could be utilized. For example,
the switch contacts 64a and 64b could be incorporated in a magnetic
reed switch located proximate the lip 48a of the base 48 to be
operated by a permanent magnet mounted to the rim 46a of the cover.
In accordance with such an arrangement, when the cover is seated on
the base the permanent magnet associated with the cover rim 46a
actuates the reed switch to place the contacts 64a and 64b in
electrical contact, which condition continues until the cover 46 is
removed from the base and the permanent magnet associated with the
rim 46a thereof displaced from the reed switch associated with base
lip 48a. Similarly, other forms of cover-operated interlock
switches could be utilized including those operating on optical
principles, other forms of proximity switches, etc. The
trigger-operated interlock switch assembly 67 may, like the
cover-operated interlock switch 64, take forms other than that
shown. In fact, the switch 67 could be located remote from the
trigger 18, but operated in response to gun trigger movements in
accordance with well known pneumatic switch-operating techniques.
In addition, disablement of the high voltage power supply 44 by
switches 64 and 67 could be accomplished by placement of the
switches other than in series between the power supply 71 and the
high voltage supply 44.
In accordance with the embodiment depicted in FIG. 4, the system is
automatically grounded when either or both of two events occur,
namely, when either or both the cover 46' of the insulative tank
enclosure is removed, or the trigger 18' of the spray gun 10' is
released, terminating the flow of coating material, for a time
period exceeding a predetermined interval. The apparatus includes a
first air valve 80 responsive to the trigger 18' of the gun 10' and
a second air valve 82 responsive to the position of the cover 46'
of the insulative tank enclosure. The air valves 80 and 82 are
connected in a logical OR configuration between an air supply 34'
and an air operated piston/cylinder actuator 86 such that an
actuating rod 88 is returned by a spring 90 to its inner position
(shown in FIG. 4) whenever the tank enclosure cover 46' is removed
and/or the gun trigger 18' is released. Return of the actuating rod
88 to its inner position (shown in FIG. 4) under either or both of
the foregoing circumstances pivots an electrically conductive rod
92 about a stationary pivot point 94 to the position shown in FIG.
4 wherein it contacts electrode 38' communicating with the bore 24c
of the coating supply hose 24'. With rod 92 contacting electrode
38', the electrode 38' is connected to ground potential via rod 92
and line 61'. Since the grounded electrode 38' contacts the
electrically conductive water-based paint flowing in hose bore 24c,
the system is grounded, dissipating electrical energy stored
therein in capacitive form. In addition to grounding the system
when the conductive rod 92 is in the position shown in FIG. 4, the
rod 92 also places a switch 96 in its normally open circuit
position to electrically disconnect wires 98 and 99. With wires 98
and 99 disconnected, the high voltage power supply 44' is
de-energized, terminating the supply of high voltage charging
potential to the electrode 38' via line 42', in turn removing
charging potential from the coating material.
The valve 80, considered in greater detail, has an inlet port S
connected to the air supply 34', an outlet port A, and an exhaust
port E. Valve 80 also includes a movable actuator 80' responsive to
the gun trigger 18'. When the gun trigger 18' is in its actuated
position (shown in solid lines in FIG. 4) to permit the flow of
coating material to the gun 10' via hose 24', inlet port S of valve
80 is connected to outlet port A of valve 80, providing pressurized
air on line 81. The valve 80 is constructed such that when the gun
trigger 18' is released for a predetermined interval or longer to
terminate the flow of coating material to the gun 10', that is, is
moved to the dotted line position shown in FIG. 4 and maintained in
such position for a predetermined interval or longer, port S and
port A are disconnected and port A vents to atmosphere via exhaust
port E. If the trigger 18' is released for a duration less than or
equal to the predetermined interval, and thereafter reactuated and
returned to its solid line position, ports S and A do not become
disconnected.
The predetermined interval of trigger release required before ports
S and A of valve 80 become disconnected is preferably adjustable,
for example, selectable in the approximate range of 1-6 seconds.
Trigger releases equal or less than such predetermined interval are
associated with normal momentary trigger releases encountered
during spraying when de-energization of the charging source and
grounding of the system are not desired due to the time lag
occurring after trigger actuation before the system charges to its
operating level, while trigger releases greater than the
predetermined interval are associated with the conclusion of
spraying, such as when the tank is to be replenished, and system
discharge and charging source de-energization are desired.
When the gun trigger 18' is moved to its release position and
remains released for a continuous duration exceeding the
predetermined interval for which the valve 80 is set, the ports S
and A are disconnected, with port A venting to atmosphere via
exhaust port E, and remain so until the trigger is reactuated. In
this condition pressure is removed from line 81. Upon reactuation
of the trigger 18', the ports S and A are immediately connected to
place pressurized air on line 81. Valve 80 may be of any
conventional type, such as a three-way normally closed time delay
valve commercially available from Clippard, Inc., designated Model
No. R-331.
Valve 82 has input port S, an output port A and exhaust port E. The
valve 82 has an actuator 82' which is responsive to the position of
the cover 46' of the tank enclosure. When the cover 46' is in its
closed position preventing access to the tank, the cover lip 46a'
places actuator 82' in the actuated position shown in FIG. 4,
connecting port S to port A. Assuming the gun trigger 18' is
actuated placing pressure on line 81, when the cover 46' of the
tank enclosure is in its closed position shown in FIG. 4, actuating
actuator 82' which in turn causes ports S and A to be connected,
pressure will be present on line 83. With line 83 pressurized, the
piston 95 is driven leftwardly in stationarily mounted cylinder 86
to drive the actuator 88 toward the left, in turn pivoting rod 92
clockwise about point 94. Pivotal movement of rod 92 in the manner
indicated removes ground potential from the electrode 38', as well
as actuates switch 96 to cause high voltage from the supply 44' to
be input via line 42' to the electrode 38' to charge the coating
material flowing in the hose 24'. Should the cover 46' of the tank
enclosure be removed, the valve actuator 82' immediately moves
upwardly, disconnecting ports S and A of valve 82 and instead
connecting port A to exhaust port E. With ports A and E of valve 82
connected, even if pressure exists in line 81 due to actuation of
the gun trigger 18', there is no pressure in line 83 and the piston
95 and actuator 88 return to their rightmost position under the
action of spring 90. Movement of the actuator 88 to the right
grounds electrode 38' to discharge the system, as well as places
switch 96 in an open-circuit condition to de-energize the power
supply 44' and remove high voltage charging potential from
electrode 38'. Valve 82 may be of any conventional type, such as a
three-way poppit valve commercially available from Clippart, Inc.,
designated Model No. MAV-3P.
In the embodiment of FIG. 4, valve 80 is responsive to the gun
trigger 18' which regulates flow of coating material to the gun 10'
of an "airless" spray system, i.e., a system in which the coating
is atomized hydraulically. If an "air" system is used in which the
coating is atomized in the nozzle by impingement with atomizing air
supplied to the gun, the valve 80 could be responsive to the flow
of the atomizing air. That is, the valve 80 could be actuated to
immediately connect its ports S and A when a flow of atomizing air
is present, and could be deactuated to connect port A to port E
when the atomizing air flow has been terminated for a duration
exceeding the predetermined interval. Alternatively, valves 80 and
82 could be replaced by electrical switches responding to the
trigger 18' and enclosure cover 46' similar to switches 64 and 67
of FIG. 1.
* * * * *