U.S. patent number 3,747,850 [Application Number 05/199,114] was granted by the patent office on 1973-07-24 for electrostatic spray gun.
This patent grant is currently assigned to Nordson Corporation. Invention is credited to Donald R. Hastings, Don R. Scarbrough, Simon Z. Tamny, Frederick R. Wilhelm.
United States Patent |
3,747,850 |
Hastings , et al. |
July 24, 1973 |
ELECTROSTATIC SPRAY GUN
Abstract
An electrostatic spray system operable to spray either
electrically conductive materials or nonconductive materials. The
system includes apparatus for mechanically atomizing the coating
material and for electrostatically charging it prior to deposition
onto an article, which apparatus is characterized by an improved
spray gun having improved electrical isolation between the
electrically conductive portions of the gun and the nonconductive
portions. Specifically, it includes an improved connector for
connecting the gun to a supply hose without having to permanently
weld the two together to maintain an effective electrical
"standoff" between the hose and the gun. It also has an improved
cartridge for sealing the valve stem of the gun. The system of this
invention also includes an improved material storage tank and
insulative system for isolating that tank from ground when the
system is used to spray electrically conductive materials. It also
includes a grounding wire for connecting the tank to ground when
the tank is used to spray nonconductive materials.
Inventors: |
Hastings; Donald R. (Elyria,
OH), Scarbrough; Don R. (Elyria, OH), Tamny; Simon Z.
(Lorain, OH), Wilhelm; Frederick R. (Avon Lake, OH) |
Assignee: |
Nordson Corporation (Amherst,
OH)
|
Family
ID: |
22736284 |
Appl.
No.: |
05/199,114 |
Filed: |
November 16, 1971 |
Current U.S.
Class: |
239/3;
239/707 |
Current CPC
Class: |
B05B
5/1608 (20130101) |
Current International
Class: |
B05B
5/00 (20060101); B05B 5/16 (20060101); B05b
005/02 () |
Field of
Search: |
;239/3,15 ;118/629 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wood, Jr.; M. Henson
Assistant Examiner: Mar; Michael
Claims
Having described our invention, we claim:
1. An electrostatic spray gun having an electrically nonconductive
body,
a material passage in said body terminating in an outlet
orifice,
a valve for opening and closing said orifice, said valve comprising
a stationary element mounted in said body and a movable closure
element,
electrically conductive paint charging means adjacent said
orifice,
electrically conductive connector means adapted to connect said
paint charging means to a high voltage power supply,
an electrically nonconductive conduit adapted to connect said
reservoir to said body passage, and
the improvement which comprises
a removable valve control cartridge mounted on said body, said
cartridge including said movable valve closure element and a valve
stem extending rearwardly from said closure element,
an electrically nonconductive sleeve surrounding said stem, and
a pair of static seals located at opposite ends of said sleeve,
each of said seals being fixedly secured at one point against
movement relative to said stem and at another point against
movement relative to said body.
2. The spray gun of claim 1 in which said cartridge further
includes a compressible spring located between said movable valve
closure element and one of said static seals.
3. An electrostatic spray gun having an electrically nonconductive
body,
a material passage in said body terminating in an outlet
orifice,
a valve for opening and closing said orifice including a stem
having an electrically nonconductive section extending into said
passage for controlling opening and closing of said valve,
a first static sealing means located in such passage adjacent said
valve and surrounding said stem, said sealing means being fixedly
secured at one point against movement relative to said stem and at
another point against movement relative to said body so as to
prevent the flow of material rearwardly over said stem,
means for applying an electrical charge to material as it is
ejected from said orifice,
electrically conductive connector means adapted to connect said
antenna to a high voltage power supply, and
a second static sealing means located in said passage surrounding
said stem, said second sealing means being fixedly secured at one
point against movement relative to said stem and at another point
against movement relative to said body, said second sealing means
being spaced rearwardly from said first sealing means and operable
to prevent the ingress of contaminants forwardly along said
stem.
4. The spray gun of claim 3 in which said valve includes a
stationary valve seat and a movable valve element, said movable
valve element being secured to said valve stem and movable
therewith, and a spring located between said movable valve element
and one of said static seals for biasing said valve to a closed
position.
5. The spray gun of claim 4 in which said movable valve element,
said spring, said pair of static seals and at least a portion of
said movable valve stem are all interconnected in a separate
subassembly cartridge, which subassmebly cartridge may be removed
and reassembled as a unit in said body.
6. The spray gun of claim 5 in which said subassembly cartridge
further comprises a spacer sleeve located between and separating
said pair of static seals, and a guide element separating and
interconnecting two sections of said valve stem, said guide element
being slideable in said spacer sleeve.
7. The spray gun of claim 6 in which said spacer sleeve and guide
are both made from electrically nonconductive materials.
8. An electrostatic spray system including a spray gun having an
electrically nonconductive body,
a material passage in said body terminating in an outlet
orifice,
a valve for opening and closing said orifice,
electrically conductive paint charging means extending through said
orifice,
electrically conductive connector means connecting said paint
charging means to a high voltage power supply,
a material storage reservoir,
an electrically nonconductive conduit extending from said reservoir
to said body passage, and
the improvement which comprises
a nonconductive connector for connecting said conduit to said body
so as to establish communication between body passage and a passage
through said conduit, said connector comprising a pair of threaded
elements on said conduit and said body respectively, both of said
threaded elements having tapered pipe threads formed thereon so
that when secured together, said joined threaded elements have
approximately the same electrical conductivity characteristic of a
solid piece of said nonconductive material.
9. The spray system of claim 8 in which said valve comprises a
stationary valve seat, a movable valve element and a valve stem
connected to said valve element, said system further comprising a
pair of spaced static seals surrounding said stem, each of said
seals being fixedly secured at one point against movement relative
to said stem and at another point against movement relative to said
body.
10. The spray system of claim 9 which further includes a spring
located between said movable valve element and one of said static
seals for biasing said valve to a closed position.
11. The spray system of claim 10 in which said movable valve
element, said spring, said pair of static seals and at least a
portion of said movable valve stem are all inter-connected in a
separate subassembly cartridge, which subassembly cartridge may be
removed and reassembled as a unit in said body.
12. The spray system of claim 11 in which said subassembly
cartridge further comprises a spacer sleeve located between and
separating said pair of static seals, and a guide element
separating and interconnecting two sections of said valve stem,
said guide element being slideable in said spacer sleeve.
13. The spray system of claim 12 in which said spacer sleeve and
guide are both made from electrically nonconductive materials.
14. In an electrostatic spray system of the type which includes a
spray gun having an electrically nonconductive body, a material
passage in said body terminating in an outlet orifice, an
electrically conductive storage tank for supplying material to said
passage, a valve for opening and closing said outlet orifice,
electrical circuit means for applying an electrical charge to
material as it is ejected from the orifice, and an electrical
resistor in said circuit,
the method of retarding breakdown and failure of the electrical
resistor which method comprises
encasing said electrically conductive storage tank in an
electrically nonconductive housing to prevent electrical discharge
to atmosphere from the surface of said storage tank when said
system is utilized to spray electrically conductive materials.
15. An electrostatic spray system comprising a spray gun having an
electrically nonconductive body, a material passage in said body
terminating in an outlet orifice, an electrically conductive
storage tank for supplying material to said passage, a valve for
opening and closing said outlet orifice, electrical circuit means
for applying an electrical charge to material as it is ejected from
the orifice, and an electrical resistor in said circuit,
the improvement which comprises means to prevent breakdown and
failure of the electrical resistor when the system is utilized to
spray electrically conductive materials, which resistor breakdown
and failure preventing means comprises an electrically
nonconductive housing surrounding and encasing said electrically
conductive storage tank to prevent electrical discharge to
atmosphere from the surface of said storage tank.
16. An electrostatic spray system adapted for spraying either an
electrically conductive material or a nonconductive material
including
a spray gun having an electrically nonconductive body,
a material passage in said body terminating in an outlet
orifice,
a valve for opening and closing said orifice including a stem
having an electrically nonconductive section extending into said
passage for controlling opening and closing of said valve,
sealing means located in such passage and surrounding said
stem,
means for applying an electrical charge to material as it is
ejected from said orifice,
an electrical resistor,
first electrically conductive connector means connecting one end of
said resistor to said electrical charge applying means,
second electrically conductive ocnnector means connecting said
resistor to a high voltage power supply,
a material storage reservoir,
an electrically nonconductive conduit connecting said reservoir to
said body passage, and
means to prevent the failure of said resistor when said system is
used to spray electrically conductive materials, said resistor
failure preventing means comprising
electrically insulative mounting means for said reservoir,
electrically nonconductive means surrounding and encasing said
reservoir to prevent electrical discharge of said reservoir when
said system is utilized to spray electrically conductive materials,
and
means to ground said reservoir when said system is used to spray
electrically non-conductive materials.
Description
This invention relates to spray systems and particularly to
electrostatic spray systems.
Until recently very nearly all electrostatic spray equipment was
designed primarily if not exclusively for use with electrically
nonconductive paints or sprays. The spraying of electrically
conductive paints, as for example, water base paints, was generally
considered to be potentially too dangerous to permit its use in
electrostatic systems because such paints had to be used in
isolated systems which created shock hazards to the operating
personnel. Now, though, there is a great deal of interest in water
base paints and the electrostatic spraying of them because these
paints contain no atmosphere polluting solvents of the type toward
which a great deal of public criticism is now being directed.
The capability of spraying both nonconductive and conductive
materials in electrostatic spraying equipment has always been
highly desirable and has even in some instances been practiced.
However, when electrostatic systems have been used to spray
electrically conductive materials, such as water base paints, the
results have been less than optimal, primarily because of excessive
shunting of current to the grounded parts of the system, especially
to the grounded spray gun handle, with the consequent decrease in
voltage at a charging electrode at the forward end of the spray
gun.
Another objective of this invention has been to provide a better
sealed and better electrically insulated electrostatic spray gun
than has heretofore been available. Most hand manipulated or
manually operated electrostatic spray guns have a valve at the
front end of the gun which controls the passage of liquid through
the nozzle located adjacent the valve by actuation of a handle
located at the rear of the gun. A control rod extends from the
handle to the valve. In the case of electrostatic spray guns, this
rod, or at least a 3-inch section of it, is made of dielectric
material (as for example, one of the common plastics). The
dielectric portion of the rod is generally at least 3 inches in
length so that there is a 3-inch electrical standoff along the
surface of the rod from the electrically conductive portion of the
gun to the nonconductive or grounded handle portion of the gun. The
3-inch standoff is considered the minimum required for safe
utilization of equipment over which the electrical charge could
pass via dirt or other contaminants on the surface of the standoff
passage. One of the materials which commonly dirties or
contaminates the standoff surface is the liquid material being
applied by the spray equipment.
To protect the 3-inch standoff area of the rod against
contamination by the spray material, there is usually a static seal
located around the rod. One form of static seal which is commonly
used for this application is a bellows type seal which has one end
fixedly secured to the valve stem and an opposite end fixedly
secured to the dielectric body within which the stem is movable.
Exampes of patents which disclose the use of this type of seal are
Shaffer U.S. Pat. No. 3,583,632 and Beach U.S. Pat. No. 3,339,841.
While these seals have been found to be very adequate against the
ingress of paint or other liquid spray material into and over the
surface of the dielectric portion of the pull rod from the front of
the gun, they do not protect against the ingress of contaminants
from the rear of the gun. We have found and another aspect of this
invention is predicated upon the determination that one source of
loss of the 3-inch electrical standoff along the surface of the
pull rod occurs as a result of leakage of solvent material from the
rear or from the handle portion of the gun forwardly along the
valve stem. This leakage occurs when the gun is placed in a bucket
of solvent material (as commonly happens between shifts in a
production facility). To combat this reverse leakage of liquid
contaminants forwardly along the valve stem, we provide a second
static type bellows seal adjacent the rear end of the rod with the
two seals being separated by approximately a 3-inch electrical
standoff. This double bellows arrangement has the effect of
assuring that liquid contaminant cannot enter and dirty the
standoff surface from either end of the pull rod. For ease of
assembly the double bellows and the pull rod as well as the
surrounding and separating dielectric spacer or sleeve and a
portion of the valve and a spring for biasing the valve to a closed
position are all made as a cartridge assembly which may be easily
inserted and removed from the gun.
Another aspect of this invention is also predicated upon the
maintenance of a 3-inch electrical standoff or an equivalent seal
between an electrically conductive portion of the gun and an
exposed surface of the gun. One of the common areas where it is
difficult to maintain the 3-inch standoff when the gun is used to
spray electrically conductive materials is in the hose connection
between the dielectric or nonconductive paint or spray hose and the
dielectric barrel of the gun. This problem is compounded by the
necessity for using a double, coaxial paint conduit. This is due to
the fact that most materials available for the paint hose are
porous to some extent and become saturated by the water or solvent
component of the paint, and hence would provide another ready path
of shunt current to ground if made up by only a single layer.
Because of the difficulties in maintaining the 3-inch standoff at
this point, it is common to weld the hose to the gun, usually by a
spin welding technique. Such a connection is hydraulically and
electrically satisfactory but it is also extremely rigid and prone
to easy breakage. And when such a connection is damaged, since it
is welded and not removable, a major portion of the gun must be
removed and a time-consuming replacement made.
We have found that this premanent connection of the hose to the gun
is unnecessary and may be avoided while still maintaining the same
nonconductive electrical properties of the connection by forming
tapered pipe threads on the dielectric hose fitting and on the
nipple for receiving the hose fitting into the gun. When the
tapered pipe threads are joined, the two dielectric elements are
forced together and coupled so well that the two act as a single
solid piece of plastic or dielectric material insofar as the
electrical conductivity through the surface between the two. This
pipe threaded connection has the advantage of eliminating the
necessity for a 3-inch standoff and of avoiding the practice of
permanently welding the two parts together. We also provide a novel
means for securing the two members of the coaxial paint conduit to
the hose fitting while maintaining the necessary electrical
standoff at these junctions also.
It has also been discovered that there is excessively rapid
deterioration of resistors in the spray gun when electrostatic
spray equipment is used to spray water base or electrically
conductive sprays. It is well known that when electrostatic spray
equipment is used to spray nonconductive materials, the paint
reservoir or holding tank must be grounded so as to prevent an
electrical charge from slowly leaking back through the paint or
spray to the reservoir and building up to a dangerous potential.
When the equipment is used to spray electrically conductive paints
or sprays, the full electrical charge passes directly through the
paint back to the reservoir so that the reservoir is at full
electrical potential and must be maintained isolated from ground to
avoid short circuiting or shunting the current through the paint
conduit and reservoir. We have found that when the paint reservoir
is isolated from ground as it must be to spray conductive
materials, shunt current is caused to flow through a corona effect
at all of the corners of the reservoir when the tank is clean. As
soon as the tank becomes dirty there is an improved path over the
surface of the container via the contaminants for the current to
shunt to ground. This increased flow of shunt current from the
reservoir causes an increased current flow through the resistor
which is not sufficient to drop out a safety relay in the
electrical circuit but which is sufficiently high to slowly but
steadily deteriorate and destroy the resistor.
To correct this condition and eliminate this excessively rapid
deterioration of the resistor when the equipment is used to spray
electrically conductive materials, the invention of this
application incorporates a casing of dielectric material around the
tank or reservoir of spray material. This casing prevents the
current from shunting to atmosphere around the tank through a
corona discharge when the tank is clean and/or the greater current
flow to ground through contaminants when the tank is dirty. When
the tank and equipment are used to spray electrically nonconductive
materials, this dielectric cover around the tank is bypassed and
the tank is grounded by removing a section of the dielectric
covering and connecting the electrically conductive portion of the
tank directly to ground.
The primary advantage of this electrostatic spray system is that it
may be used for spraying both electrically conductive and
nonconductive materials with equal facility. This system has the
advantage of being safer in use either when spraying electrically
conductive or nonconductive materials than systems which have
preceded it because of the better protected standoff distance along
the surface of the pull rod. Additionally, the gun has the
capability of being disassembled from the spray hose for purposes
of cleaning and repairing it without sacrificing any safety or
creating a potential hazard to its use.
These and other objects and advantages of this invention will be
more readily apparent from the following description of the
drawings in which:
FIG. 1 is an exploded side elevational view of a manually operated
electrostatic air spray gun incorporating the invention of this
application;
FIG. 2 is an end elevational view of the gun of FIG. 1.
FIG. 3 is a cross sectional view taken on line 3--3 of FIG. 2;
FIG. 4 is a cross sectional view taken on line 4--4 of FIG. 2.
The gun 10 illustrated in the drawings is an air operated
electrostatic spray gun which relies upon the impact of an air
stream with a liquid stream to effect atomization of the liquid
stream. While the invention is described as applied to an air gun,
it should be understood, though, that the invention of this
application is equally applicable to all electrostatic spray guns
or spray systems.
The gun 10 comprises an electrically conductive metal handle
assembly 11, an electrically insulative barrel assembly 12, and an
insulative nozzle assembly 13. Paint or other spray material, which
may be in the nature of a coating or a varnish or a lacquer
(referred to in regard to this invention generically as paint) is
supplied to the gun from a reservoir or tank 14. A high voltage
source of electrical energy is supplied to the gun from an
electrical power pack 15.
The handle assembly 11 is generally made from a metal casting and
includes an air inlet 16, a trigger actuated air flow control valve
17, and a trigger 18 for controlling the flow of air through the
valve 17. There is also an adjustable air valve 20 in the gun
handle for controlling the shape or "fan" of the spray emitted from
the gun, as is explained more fully hereinafter.
The air inlet port 16 opens into a generally vertical air passage
21 which communicates with a transverse counterbored air valve
passage 22. The air passage 22 in turn communicates with a pair of
air lines 23, 24 which in turn communicate with passages 25, 26
through the barrel 27 of the gun.
The air valve 17 which controls air flow from the inlet passage 21
to the barrel passage 25, 26 comprises a trigger actuated pin 28, a
pin retainer 30, a plunger 31, and a plunger limiting adjustable
stop 32. One end of the pin 28 abuts the trigger 18 while the
opposite end of the pin is receivable in a central aperture of the
plunger 31. The retainer 30 is threaded into the handle and serves
as a guide for the pin. An O-ring seal (not shown) between the pin
28 and the retainer 30 precludes leakage of air from the air
passage 22. A compression spring 33 is located between the plunger
31 and the stop 32. This spring biases a resilient seal 34 on the
end of the plunger against a shoulder of the passage 22 so as to
maintain the air valve 17 in a normally closed position.
The "fan" control valve 20 comprises a single valve plunger which
is threaded into the counterbored threaded end of the passage 23.
Its end is tapered and is engageable with a tapered seat machined
into the air passage 23 to limit or close air flow through the
passage.
The barrel assembly 12 is made from an electrically insulative
material such as one of the common plastics and includes the main
body section 27 through which the pair of air lines or passages 25,
26 extend, as well as a material flow control passage 35 and an
electrical flow control passage 36. The material flow control
passage 35 is intersected by an inclined passage 37 through which
material is supplied to the passage 35 from a flow control pipe 38.
The opposite or lower end of this pipe 38 is supported from the
handle 11 by a bracket 40.
As may be seen most clearly in FIG. 4, air flow in the passage 26
of the barrel is controlled by the trigger actuated valve 17 while
air flow in the passage 25 is regulated by the valve 20 of the
handle. At the forward end of the barrel body 27, a passage (not
shown) through the nozzle assembly communicates between the air
flow passage 26 and the passage 41 of the nozzle assembly 13. This
latter passage 41 is located between a fluid nozzle 42 and an air
nozzle 43. It is open at the front so that it defines an annular
air passage 44 around the material orifice (not shown) of the fluid
nozzle 42. Air issuing from this air passage 44 impacts with the
stream of material issuing from the material orifice of the nozzle
42 and at least coarsely atomizes the stream. There may be
additional ports of the air nozzle 43 connected with the passage 26
to further atomize the stream. There are also a pair of fan shaping
ports (not shown) located in a pair of horns 45 of the air nozzle
43 which communicate through a passage of the air nozzle 43 with
the passage 25 of the barrel 27. Adjustment of the valve 20
controls the amount of flow of air issuing from the horns of the
nozzle and thus the degree of "fan" formed by the atomized
spray.
Flow of fluid material through the material or fluid nozzle 42 is
controlled by a valve 50 located at the forward end of the passage
35. This valve comprises a metal seat 47 located in a counterbored
recess 48 and a movable needle valve 50. At its forward end, the
needle valve 50 has a needle 51 receivable in an axial aperture of
the seat 47 to close the valve and preclude the passage of the
material into and through the nozzle assembly 13. The movable
needle valve 50 forms the end portion of a cartridge assembly 52.
At its opposite end this cartridge assembly is attached to the
trigger 18 so that in addition to controlling the flow of air
through the gun, the trigger controls the flow of material through
the gun. The connection of the trigger to the air valve 17 and a
retainer stem 53 of the cartridge assembly 52 is such that the air
valve 17 always opens and initiates air flow before the material
flow control valve 46 opens.
The valve control cartridge 52 comprises the movable needle valve
50, a stem assembly which consists of a pair of studs 54, 55
interconnected by a guide 56, and a surrounding seal which
comprises a pair of bellows seals 57 and 58 spaced apart by a
spacer sleeve 60. Additionally, the cartridge includes a
compression spring 61 located between the flange 60' on spacer
sleeve 60 and the flange 54' on stud 54. The stud 54, the bellows
seals 57 and 58, the guide 56, and the spacer sleeve 60 are all
made from electrically nonconductive materials.
The forward end of the stud 54 is threaded into a threaded aperture
in the rear of the needle valve block 46. Adjacent the threaded end
flange 54' cooperates with a shoulder 62 at the rear of the needle
valve 50 to lock or securely fix one end 63 of the bellows seal 57
to the valve stem. The opposite end of this bellows seal 57 is
permanently attached to the forward end of the sleeve 60 and sealed
with wall of passage 35 by an O-ring seal 64.
At its rearward end, the forwardmost stud 54 is threaded into the
slidable guide 56. This guide as well as the spacer sleeve 60
within which it is slidable are both made from electrically
nonconductive materials so as to provide an electrical standoff
between the two. At its rearward end this guide 56 has a threaded
aperture which receives the threaded forward end of the stud 55.
The rearward end of this same stud is connected to the trigger 18
by the trigger retainer 53 which has a necked-in section 66
received within a vertical slot of the trigger 18. Thus, rearward
movement of the trigger causes the retainer 53, the stud 55 and the
inner connective elements 54 and 56 of the valve stem to move
rearwardly with it and pull the needle valve 50 to an open position
against the bias of the spring 61.
The rearward end or shoulder 67 of the guide 56 cooperates with a
shoulder 68 on the forward end of the stud 55 to lock the forward
end 70 of the bellows seal 58 against movement relative to the
valve stem assembly. The rearward end 71 of this bellows seal 58 is
forced by a rear shoulder 72 of the spacer sleeve 60 against the
forwardmost flat surface or end surface 73 of the gun handle 11 so
that the rear end of the bellows seal 58 is fixed against movement
relative to the body 27.
All of the components of the valve control cartridge 52 are made
from insulative material except for the needle 51, the spring 61,
the rearwardmost stud 55, and the trigger retainer 53.
Consequently, the nonconductive components of the cartridge
maintain an electrical standoff between the electrically conductive
elements at the forward end of the gun and the metal conductive
elements, and particularly the handle, at the rear of the gun.
Also, reduction of conductive components in contact with the paint
reduces the capacity of the gun for build-up of capacitive energy
which could arc to ignite the volatile atmosphere or shock the
operator, thereby minimizing other safety hazards.
By fixing the forward end of both bellows seals to the valve stem
and the rearward end of the seals against movement relative to the
body 27, a static seal is formed by each of the bellows seals at
one end of the valve stem. The forwardmost bellows functions to
prevent paint or contaminants from moving rearwardly over the valve
stem assembly and, similarly, the rearwardmost bellows functions to
prevent contaminants from moving forwardly over the valve stem
assembly from the area in the vicinity of the trigger. Frequently,
the guns are placed in a bucket of solvent when not is use or
between work shifts so as to prevent the paint from hardening in
the gun and clogging the conduits. In the absence of a bellows seal
58 adjacent the rearward end of the valve stem, the solvent may
enter and flow forwardly along the stem and thereby provide an
electrical flow path which completely destroys the electrical
standoff between the electrically conductive and nonconductive
elements of the gun.
The material nozzle 42 is made from an electrically nonconductive
material which is threaded into a counterbore 74 in the forward end
of the body 27. It has an axial passage or bore (not shown) which
opens into the rear of the counterbore 74. The rear of the
counterbore 74 in turn communicates with the central aperture of
the valve seat 47 via a passage 75 such that material passing
through the aperture of the valve seat 47 may enter and pass
through the axial passage in the material nozzle 42. The axial
passage in the nozzle 42 terminates in a small diameter outlet for
discharge of a solid stream or jet of coating material.
The air nozzle 43 is also made from an electrically nonconductive
material. It is threaded over a threaded sector of the barrel and
has air passages which connect the ports in the horns 45 to the fan
air control passage 25 of the barrel.
Liquid or fluid is supplied to the material passage 35 of the gun
via the inclined passage 37 from the storage tank or reservoir 14.
This tank is connected via an electrically nonconductive fluid
conduit 76 to the electrically nonconductive coaxial flow control
pipe 38 which terminates into a non-conductive hose fitting 77. The
hose fitting 77 has tapered pipe threads 78 machined onto its
forward end which are received into tapered pipe threads formed on
the interior of the passage 37. Pipe threads, when viewed in cross
section are V-shaped as opposed to machine threads which generally
have the bottom of the V removed. Because the pipe threads are
tapered from one end to the other and are V-shaped in cross section
when joined, the two plastic nonconductive pipe threaded elements
of the fitting form a near perfect fluid seal which, insofar as
electrical conductivity through the seal is concerned, has
approximately the same dielectric characteristics as if the two
units of the coupling were made of a single piece of nonconductive
material. This pipe threaded coupling thus eliminates the necessity
to spin weld the hose fitting to the nonconductive body of the gun
as has heretofore been a conventional practice.
The inner paint hose 171 of coaxial conduit 38 is inserted through
a nonconductive lock fitting 80 into internal receiving opening
172, and rests with its forward surface against shoulder 173 of
hose fitting 77. By screwing lock fitting 80 tightly into threads
176 on the interior of fitting 70, ferrule 174 is wedged tightly
between fitting 70 and inner hose 171 to hydraulically seal the
junction. O-ring seal 175 seals the bottom end of fitting 77
against the entry of contaminants and the necessary standoff
distance to minimize the flow of shunt current between fitting 77
and lock fitting 80 is provided by the helical path around the
mating threads 176.
Outer hose 177 of coaxial conduit 38 enters the bottom of lock
fitting 80, seats with its forward surface against shoulder 178 and
is sealed against the entry of contaminants by O-ring seal 179.
Outer hose 177 is thrust forward into lock fitting 80 and held
tightly in place by the ferrule 180 of bracket fitting 181 which is
screw threaded to mating fitting 182 to also secure the coaxial
conduit to bracket 40. A clearance of at least 0.015 inch is
maintained between the inner and outer hoses 171 and 177
respectively to ensure that even if the inner conduit becomes
saturated by the water or solvent from the paint there will still
be sufficient dielectric strength in the air gap and the
unsaturated outer hose to prevent current from shunting through the
side wall of the coaxial conduit.
Electrical power is supplied to an antenna 81 which protrudes from
the orifice of the fluid nozzle 42. This power is supplied from the
electrical power pack 15 which is connected to the antenna via an
insulated cable 82, an aluminum washer 83, a spring 84, and washer
85, through a resistor 86 and an electrical lead 87, a washer 88
and spring 90. The end of the spring 90 terminates in the antenna
81.
The cable 82 is a conventional flexible coaxial cable within which
there is a central electrical conductor surrounded by an insulating
sheath, a conductive grounding sheath, and an encasing sheath of
insulation. This cable is secured to the gun by a conventional
locking plug 91. The electrically conductive portion of the cable
together with its electrically insulative sheath extends upwardly
from the plug into and through a nonconductive hose 92. This hose
fits within the handle of the gun and protrudes from the forward
end. It receives the aluminum washer 83 which makes contact with
the electrical conductor in the cable as well as the spring 84, the
washer 85 and the resistor 86. The electrical conductor 87 is
embedded in the nonconductive barrel 27 of the gun and carries the
electrical charge from the end of the resistor to the end of the
spring 90. The spring 90 is surrounded by a nonconductive antenna
holder 93 which fits into the fluid nozzle 42. At its forward end
the antenna portion 81 of the spring 90 extends from the antenna
holder 93 and out through the fluid orifice at the end of the
nozzle 42.
This system may be used to spray either electrically conductive or
nonconductive paints or sprays. When the system is used with
electrically conductive paints or sprays, as for example, water
base paints, the paint reservoir 14 is maintained isolated from
ground by insulative mountings 94, 95 and by the removal of a
grounding lead 96. When isolated in this manner the electrical
current is prevented from shunting back through the electrically
conductive material to the tank or reservoir and then dissipated to
ground through the corners of the tank. An electrically insulative
material or casing surrounds the tank and prevents corona discharge
from the tank as well as preventing the tank from becoming dirty
and then leaking current to ground through the dirt or contaminant.
This dielectric insulation thus prevents a slow but steady current
drain through the paint reservoir and consequent deterioration of
the resistor 86.
When the system is used to spray nonconductive materials a threaded
plug 98 in the casing 97 is removed and a new plug 100 inserted.
This new plug 100 has a spring-biased probe extending through it so
that it establishes good electrical contact with the electrically
conductive reservoir 14 contained within the casing 97. The
opposite end of the probe is secured to the grounding wire 96. Thus
when used to spray a nonconductive material, the grounding wire
prevents current from shunting rearwardly through the paint in the
paint conduit and building up capacitive charge on the reservoir.
This charge if allowed to build over a prolonged period of time
without a grounding wire on the reservoir can become very dangerous
because of the reservoir acting as a capacitor and storing a high
voltage charge.
One advantage of the system described hereinafter is that it is
readily interchangeable between spraying electrically conductive
and nonconductive sprays. Additionally, it has the advantage of
being easily assembled and disassembled for purposes of cleaning
and repairing it. The fluid control cartridge including the double
bellows arrangement and the insulative shield around it as a
separate subassembly facilitates assembly and disassembly.
Additionally, the use of non-welded components in the connection of
the electrically nonconductive coaxial hose to the nonconductive
body facilitates assembly and disassembly.
While we have described only a single preferred embodiment of our
invention, persons skilled in the art to which this invention
pertains will readily appreciate numerous changes and modifications
which may be made without departing from the spirit of our
invention. Therefore, we do not intend to be limited except by the
scope of the following appended claims.
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