Note: Descriptions are shown in the official language in which they were submitted.
'I`his illVClltiOIl relates to a hydraulic seal which
may be used in electLostatic spray coating guns for coating
materials which have a high or moderately high electrical
conductivity.
This is a division of copending Canadian Patent
Application Serial No. 291,431, filed November 22, 1977
as a division of copending Canadian Patent Application
Serial No. 281,163, filed on June 22, 1977.
Spray coating, both electrostatic and non-
electrostatic are established arts. In non-electrostatic
spray coating systems paint is atomized and directed toward
some article to be coated. In electrostatic spray coating
systems a high voltage electrical charge is applied to the
paint particles either before, during or after to the
atomization process. The high voltage electrical charge
applied to the paint improves the efficiency and coating
characteristics of a spray coating system used to coat
objects which are held at or near ground potential. There
are other applications for and general advantages of electro-
static spray coating systems, however, they need not be
discussed here, being well known in the art.
Electrostatic spray coating systems generally
employ an atomizing device or gun, a pump or other means to
supply paint to the gun, a source of high voltage electrical
power, and means connected to the high voltage power and
associated with the system to charge the paint. The subject
of the present invention deals with the spray coating gun,
including the means employed to charge the paint.
- 1 - ~ .
s~/~) c~
l l~ S ~
In (~e~ ral, eLectrostatic coating guns consist of
a barrel porti.on having a paint conduit. One end of the
paint conduit is connected to a source of coating material
under pressure, and the other end terminates in a spray
discharge device or nozzle. The nozzle, in the usual spray
coating situation, produces a flat-fan shaped cloud of paint
droplets. Many of the nozzles in the past could be rotated
:
- la -
Sb/J ~
SSlS
s~. thclt th~ fan l)a~:tern could bc oriellted horizontally, vertically or
at some intermediate position.
A valve is usually employed to control the discharge of paint.
It has been the general practice in various types of spray coating guns ,
to have the valving located in the conduit in the barrel very close to
the discharge orifice of the no~zle. Separating surface valves with
mating surfaces such as needle and seat or ball and sea~ type valves
have been common. A pull rod extending into the conduit has been used
to open and close the valve. Some type of seal between the opening
into the conduit and the pull rod itself prevented the gun from leaking
through the~opening. The s-eals in the past have in var~ous guns taken
the form of both packing type seals and packingless type seals. Packing
type seals are dynamic seals. That is, the pull rod slides inside the
packing material which is urged against the pexiphery of the opening
into the eonduit and is also urged against an out~ard surface of the
pull rod. These packing type seals are adequate for some systems but
had drawbacks in others, especially electrostatic systems. Packing
6eals of their very nature did not provide an electrical seal. Specif-
ically, in a system using paint having high to moderate electrical con-
ductivity, an electrical path could be established along the surface ofthe pull rod to the exterior of the conduit, since the paint would wet
the surfacç of the rod. This electric~l leaka~e path i~ undesira~le in
electrostatic spray coating systems since it cotlld present a path which
would short the high voltage electrical power to ground, or present a
safety problem of sparking or shock to the operator. Fllrther, the
sliding caused the packing material to l~ears especially when the coating
material in the conduit wns abrasive.
To overcome the disadvan~ages of the packed seal, various
electrostatic spray coating guns have employed packingless seals.
j 1/ ~r~ -- 2--
-`` 110~
These packingless seals generally took the form of a deformable
diaphragm, such as a bellows, surrounding the rod, In the bellows
type, one end of the bellows has a static seal to the periphery
of the opening into the conduit, and the other end of the bellows
has a static seal around the pull rod. The seals are termed
"static" because there is no sliding of the rod over the seal.
When the pull rod moves the bellows flexes while the seals remain
fixed with respect to the sealing surfaces.
The prior art bellows/static seal arrangements solved
some of the problems associated with sliding seals, the most
important being the friction wear and electrical insulation.
However, new problems arose in the prior art bellows seals. It
has become desirable to fabricate the bellows from a fluorinated
hyrdocarbon polytetrafluoroethylene and commonly known
as TFE "Teflon", "Teflon" being the trademark for a poly-
tetrafluoroethylene material, because of the superior electrical
and chemical properties of TFE"Teflon", Electrically> TFE"Teflon"
is a good insulation and does not arc-track. Chemically, TFE"Teflon"
is impermeable to almost all coating material; that is the coating
materials will not chemically attach the TFE"Teflon', nor will
these coating materials permeate the structure of the TFE"Teflon."
The prior art teflon bellows had heavy walled mechanical coupling
type ends~ For an example, see U.S. Patent No. 3,747,850. The
ends of such bellows, as well as the bellows itself, had been
machined parts. The heavy walled machined ends of these prior
art bellows were generally sealed to the rod and to the opening
by means of mechanical couplings similar to those used for some
types of pipes. The heavy walls did not readily deform greatly
, ~.'. .
when urged against another surface, Therefore, ei~her the sealing
- 30 surfaces re~uired close machining tolerances, or a gasket. Close
-3- -
cbr/)(
~ llOS~S
machine tolerances are expensive, and gaskets such as O-
rings do not exhibit the desirable characteristics of "Teflon".
Therefore, the seals were either expensive or alternatively,
if a gasket were used there was a weakness in the seal at
the gasket.
, According to the present invention there is provided
a hydraulic seal between an opening and a conduit and a rod
extending through ~he opening, the seal having a thin walled
bellows extending into the conduit and having an inside and
lG an outside surface, the bellows surrounding part of the rod.
The bellows has one end thereof as a generally thin walled
cylindrical shaped extension passing through the opening and
the other end thereof hydraulically sealed to the rod. The
one end of the bellows terminates in a thin walled flare
portion, and a deformable washer engages a peripheral surface
of the opening and surrounds the extension of the bellows.
Means sealing engages opposite surfaces of the flared portion
and urges the washer against the peripheral surface.
More specifically, the bellows may be made from
polytetrafluoroethylene, and the extension is a generally
thin walled cylindrically shaped extension of the bellows.
A "Teflon" jacketed elastomeric washer is provided around the
extension of the bellows and being outside of the conduit.
A hydraulic seal is formed by urged surface contact between
a first side of the "Teflon" jacketed washer and the periphery
of the opening into the conduit. A hydraulic seal is
provided between the second side of the "Teflon" jacketed
washer and the outside surface of the bellows at the flare
effected by urged surface contact to the second side and
to the outside surface of the bellows at the flare.
--4--
c_r/
SSlS
~n elastomeric washer is provided around the rod, and is
in urg~d surface contact to the inner surface of the
bel]ows at the flare. Means acting through the elastomeric
washer maintaining all of the urged surface contacts in
sealing relationship.
It is an object of this aspect of the invention
to provide seals and bellows which are easily constructed,
disassembled, and repairable.
llOSSl~
RIr;F DESC~II'llON O~''l'~lE__DRAWL~(,S
Figur~ 1 is a cross-sectional view of an air atomizing
electrostatic spray gun embodying a hydraulic seal of this in-
vention.
Figure 2 is an exploded cross-sectional view of the
bellows sealing arrangement of the present invention as provided
for the valve pull rod extending into the coating material conduit
in the barreI portion of the spray gun of Figure 1.
Figure 3 is a cross-sectional view of the spray gun of
Figure 1 through the plane defined by the dotted line 3 in
Figure 1, which shows the effect of angular displacement of the
electrode extension on the electrical path.
Although a speci~ic embodiment of the hydraulic seal
- according to the present invention is illustrated in combination
with an air atomizing electrostatic spray gun, it will be apparent
that it could be utilized in other hydraulic devices such as a
valve.
Figure 1 shows a cross-sectional view of an electro-
static spray gun. The spray gun generally consists of a metallic
2~ handle 1, a barrel 2 made of insulating material such as Delrin,
which is the trademark for a material formed of acetal resins,
a nozzle 3, and an electrode extension 4. One end of the barrel
2 is mounted to the handle 1, while the nozzle 3 is located at
the other end of the barrel 2. The electrode extension 4 is
mounted for annular displacement about the barrel 2.
The handle 1 is made of metal and is held at electri-
cally ground potential through a suitable electrical connection
(not sho~n). An air line 14 is connected to an air passage 5 in
the handle 1 through a suitable connector 8, The air passage 5
-- 6
cbr~J~,
ll(~SSIS
: .
extends through the halldle 1 and barrel 2 and eventually commun-
icates with a first air chamber 6 and a second air chamber 7 both
in the barrel 2 close to the nozzle 3. The air passage 5 extends
for part of its length through the handle 1 and barrel 2 in a
plane different than that through which the cross-section of
Figure 1 is taken, and therefore, phanton lines in the barrel
2 close to the nozzle 3 indicate the openings of the air passage
S to these first and second air chambers 6 and 7.
,.
- 7 - I
llQSS~
~ lso connected to tIIe butt end of the handle 1 is an insulated
electrical cable assembly 15. The cable assembly 15 is secured to the
butt end of the handle 1 by a suitablc retaining nut 10. An extension
20 of the cable assembly 15 is carried into an electrical conduit 9 in
the handle 1. The core o the cable assembly 15 can be any suitable
electrical conductor such as stranded wire or a cable core having dis-
tributed resistance in it such as described in U.S. Patent No. 3,348,186
issued to Rosen. A polyethylene sheath 21 surrounds the cable extension
20 to provide electrical insulation except for an electrical contact 45
at the end of the extension 20. The other end of cable 15 i5 connected
to a high ~oltage power ~upply (not shown). The specific novel details
of the electrical path through the spray gun will be described in
further detail below.
Still describing the gun generally and now referring to the
paint supply path of the gun, a palnt supply hose 16 carries paint
under pressure to a paint supply hose connection block 17, The
connection block 17 is metallic and is attached physically and elect-
rically to the butt end of the handle 1 of the gun. A passage (not
shown) through the block 17 communicates with one end of a nylon paint
supply link 18. The other end of the paint supply link 18 communicates
wlth a paint inlet opening 23 in the barrel 2 of the gun. The li~k
18 is attached betw~en the block 17 and the barrel 2 of the gun by
suitable pressure fluid connections.
The paint inlet opening 23 communicat:es with a paint conduit
22 in the barrel 2. The paint conduit 22 progresses to a discharge
orifice 24 of the nozzle 3. Needle and seat valving is provided
i~mediately upstream of the discharge orifice 24. The needle 25 of
the needle and seat valve assembly is attached to a pull rod 26 made -of
~ n acetal homopolymer commonly Icnown by the DuPon~ trade mark "Delrin"
;; - 8 -
, . ..
,~ ,
(shown in Figurc 2) "Delrin" being a trade mark for acetal resins.
The pull rod 26 extends into the paint conduit 22 through an
opening at the rear of the paint conduit 22. The paint conduit
22 is sealed closed around the pull rod 26 by means of a TFE
"Teflon" bellows 19 having a static seal to the rod at one end, and
a static seal to the periphery of the opening at the other end.
The details of this sealing arrangement will he described below.
The pull rod 26 is connected to a spring loaded trigger
27. When the trigger 27 is displaced in a rearward direction,
the needle 25 is retracted from the seat behind the discharge
orifice 24, and allows paint to be discharged.
When spraying abrasive coating materials, the needle and
seat valve assembly is preferably made of an abrasion resistant
material such as ceramic or carbide.
Referring now to the nozzle 3 portion of the gun,
generally it can be seen by those skilled in the art that it is
similar to prior art air atomizing nozzles in many respects. The
nozzle 3 consists of a fluid nozzle portion 28 with a ceramic
liner 30, air cap 29 and a retaining nut 35. All of these parts
other than the liner 30 are made of "Delrin". This nozzle asse~bly
- is similar to nozzles old in the art, save for the ceramic liner
30 in the fluid nozzle 28.
The fluid nozzle 28 has threads on the outward surface
of its rearward end for threadable attachment to the forward end
of the fluid passage 22 in the barrel 2. The fluid nozzle 28 is
J threaded into the barrel 3 until a rearward frusto-conical outer
surface on the liner 30 engages a mating surface surrounding
the flow passage 22. These two surfaces form a hydraulic seal so
that the fluid passa~ 22 extends only through the interior of the liner 30 to
the discharge orifice 24. The inside surface of the liner, immediately behind the
discharge orifice of 24 of the fluid nozzle 28, fo ~ the seat in the nee le and
: _ g _
sb/J~
,
110~
~t v~lvc.
An air ca~) 29 partlally surro-lnds ~he for~ard end of the fluid
no~zlc 28. Tl1c discharge orifice portion 24 of the fluid nozzle 28
ext~nds throug11 a centrally disposed hole in the air cap 29. A retain-
ing nut 35 threadably engages the barrel 3 and urges a rearward
frusto-conical surface o~ the air cap 29 against a mating surface on
the fluid nozzle 28 through the interaction of a circumferential
annular inward flange at the forward end of the retaining nut 35 with
circumferential outward flange on the air cap 29.
The first air chamber 6 in the noæzle portion is formed
between the surfaces of the barrel 3, retaining nut 35, air cap 29 and
fluid nozzle 28, Air passages in the air cap communicate with the
first air chamber 6 and terminate in air discharge openings 34.
Several air passages 31 are formed in the fluid nozzle 28.
These air passages are distributed uniformly around the axis of the
fluid flow passages and function to communicate pressurized air from the
second seàled air chamber 7 in the nozzle portion of a third ai~ chamber
32 close to the discharge orifice 24 of the fluid nozzle 28. Holes 33
in the air cap discharge air from the third air chamber 32. In oper-
ation, a~ is kno~n in the art, the interaction of air being dischargedfrom the air hole 33, 34, in tne air cap 29, interact to atomize and
shape the stream of fluid being discharged from the nozzle orifice 24~
The sealing surfaces of the air cap 29 are radially symmetrical,
and, therefore, the air cap 29 is rotatable about the axis of the fluid
discharge nozzle 24. That is, the air cap can be rotated 50 that the
flat fan spray of the noz~le can be oriented in th~ plane of the paper~
perpe~1dicular to the plane of the paper of any angle in between.
Referring again to the f3.uid path in genelal, it is notcd 1~ere
that the fluid conduit 22 is made large enough for most of its exten1:
-- 10 --
ll~551S
,
naintain fluid velocitics at a relatively low value. The only places
hcre the fluid velocity in the fluid conduit ~2 is at any rela~ively
high value is around the needle and seat valve and at the fluid dis-
charge orifice 24. However, because the needle and seat and the orifice
24 are formed ln tlle unitary abrasion resistant liner 30 the spraying
of highly abrasive materials will not rapidly deteriorate the surfaces
and components.
There are alternative approaches to construction a wear
resistant fluid nozzle. The approach taken here is a"Delrin"body with
a wear resistant liner 30. The fluid nozzle 28 could ha~e been made
totally out of wear resistant material, however, ~t has been found that
the liner approach offers distinct advantages. It is desirable to use
ceramic materials for the wear resistant surfaces in the fluid nozzle.
However, ceramic is brittle. The helrin body provides an added layer
of mechanical shock insulation for the ceramic material. If the whole
fluid nozzle were made of ceramic the chance of fracture would be in-
creased.
Even if a stronger material such as carbide were used for the
wear reslstant surfaces, problems would arise, It is desirable to make
the fluid nozzle in the shape depicted in Figure 1, so that the gun is
compatible with other fluid nozzles and air caps which are considered
as standard in the industry. The desirability of using "standard" fluid
nozzles and a~r caps is based upon the need for a versatile spray gun
which can use several different types of fluid nozzle.s and air caps.
~t is note~orthy that this fluid nozzle is topologically a rather com-
plex structure containing mating surfaces and small air passages. ~f
the fl~id nozzle ~ere a single piece of abrasion resistant material, the
abrication process for the fluid nozzle would be further compljcated;
namely, the very formation of the surfaces and maintenance of engineering
-- 11
~31
llV~
tolerances would bc dif~icult. With the "liner" approach used
in the preferred embodiment, the fabrication process is simplified.
The above-described electrostatic air atomizing spray
coating gun is also described and is claimed in above-identified
Canadian Patcnt Application Serial No. 281,163, co-pending Canadian
Divisional ~pplication Serial No. 291,429, filed No~ember 22,
1977, and co-pending Canadian Divisional Ap~lication Serial No.
291,430, also filed November 22, 1977.
Turning now to specific details of the present inven-
tion, and referring to Figure ~, the details of the bellows seal-
ing arrangement between the opening into the fluid conduit 22 and
the pull rod 26 which extends into the fluid conduit 22 can be
observed. As can be seen in Figure 2, the pull rod 26 extends
into the fluid conduit 22 from the rear of the spray gun. A
genexally cylindrical or tubular TFE ~Te~lon"bellows 19 surrounds
the rod 26. The convoluted section of the bellows 19 is thin
walled and has thin walled cylindrical extensions at each end.
At the rearward end of the bellows 19, the cylindrical extension
has been flared. At the forward end of the bellows, the cylin-
drical extension has been pushed over and encompasses a bulge ona pull rod 26. The bulge on the pull rod 26 is large enough to
slightly expand the thin walled extension o~ the bellows 19 but
is not large enough to permanently deform it. The cylindrical
extension must be at least moderately resilient so that upon
pushing the foxward end of the bellows 19 beyond the largest part
of the bulge the resiliency of the extension causes it to attempt
to return to its original size and, thereby, snugly con~orm to
- 12 -
rw/ ~
.
the shape of the bulge. The forward portion of the bulge is
a conical locking tapered surface. A bushing type member 40
has an internal locking tapered surface which mates to that
on the bulge of the rod 26. A nut 41 is threadably attached
to the pull rod 26 and is screwed down to such an extent that
the bushing type member 40 locks the end of the tubular
extension of the bellows 19 to the pull rod 26.
At the rearward end of the bellow 19 is a second
cylindrical extension of the convolutes with a flared rear-
ward end. A"~eflon"jacket 38, surrounds the tubular extensionof the bellows. The jacket 38 is made of"Teflon"and is
generally in the form of two thin walled
13 -
.,
sb/,l,l
llO55~S
deformable annular membranes which are spaced apart along a
common axis but which are continuous through their smaller or
inner annular diameter. The space between the membranes is
filled with rubber or some other elastomeric material 39.
One face of the jacket is urged against an annular face 37
of the barrel 2, which face 37 surrounds the opening into the
fluid conduit 22. The jacket is urged against the annular
face 37 around the fluid conduit 22 by means of a"Delrin"
second washer means 42. The flare of the rearward extension
of the bellows 19 is in urged engagement with the rearward
surface of the second washer means 42. A rubber washer 43 is
urged against the inside surface of the bellows at the
flare by a"Delrin"packing nut 36. The packing nut 36 forces
- the washer 43 against the flare which in turn is urged
against the second washer means 42 which in turn is urged
against the"Teflon"jacket 38 which in
turn is urged against the annular face surrounding the open-
ing into the fluid conduit 22. The sealing arrangement
between the bellow 19 and the opening is also described and
is claimed in co-pending Canadian Application Serial No.
291,431.
In this arrangement for the static seals at each
end of the bellows, fluid is only exposed to"Delrin"or
~eflon.~ These two substances exhibit excellent chemical
resistance to almost all spray coating fluids. There are no
rubber surfaces such as O-rings or packings which contact the
fluid in the fluid conduit 22. Further, these static seals
allow the use of a"Teflon"bellows which does not require
machining in its fabrication.
- 14 -
sb~,
Referring now again to Figure 1, the details of
the electrical path in the spray gun will be described. As
stated above, high voltage electrical power is supplied
to the gun through an insulated high voltage cable core 20
in high voltage cable assembly 15. The cable core 20
extends beyond the connecting nut 10 and is surrounded for
its entire length by a polyethylene sheath 21 which provides
electrical insulation.
- 14a -
sb~J;
T~lc h;lnclle 1 and b~lrlol ? of thc gUIl ar~ separable at a point
just forward of thc trigger 27. An electrical conduit 9 extends
through the handle 1 and into the barrel 2.
A polyethylene tu~e 44 extends from the point of separation 55
into both the electrical conduit 9 in the handle 1 and in the barrel 2
for a considerable distance in either direction. The electrical con-
duit 9 itself extends throu~h the handle 1, through the barrel 2, then
exits from the barrel into an extender support housing 51, and then
finally, through a passage in an electrode extender 52. The cable
extender support housing 51 is mounted for angular displace~ent and
is sealed from the exter~or of the electrical passage by 0~rlngs 58.
The details of the housing Sl, its mounting and the details of the
electrode extender 52 will be discussed below.
Continuing with the description of the electrical path itself,
a contact 45 at the end of the cable core 20 butts against one end of
a first electrically conductive spring 46. The second end of the firæt
spring 46 butts against an electrical contact on a cable extender 5~.
The cable extender 50 is flexible and of similar construction to that
of the cable core 20 and is sheathed by flexible polyethylene. The
cable extender 50 has electrical contacts 47, 48 at each end of its
length and extends in a continuous piece from electrical contact to
the first electrically conducting spring 46 at its rearward end to
electrical contact with a second electrically conductin~ ~pring 49 at
its forward end. The second electrically conducting spring 49 is
located at the forward end of the electrical conduit in the electrode
extension 52. The spring 49 also contacts one end of an elect:rode 54.
The electrode 54 is ~IQbedded in the extension 52 so ~hat ~ne end is e~:-
posed to the atmosphere and the other end is in electric~l conl:act ~ith
the second spring 49.
- 15 -
jl~ '
llV~Sl~
.
The electrode 54 cosnprises a tightly coiled filament of
electrically conductive spring steel, having the tip of the filament
which forms the spring directed generally along the length of the
spring st its exterior end, The tip pointed along the length of the
spring forms a needle like Corona point which effects the electrostatic
charging of the sprayed coating material.
The electrode 54 in the preferred embodiment has been made
uniformly flexible along its length so that it will resillently deform
regardless where a deforming force is applied.
The exterior support housing 51, wh~ch supports the electrode
extender 52, is"Delrin"and is mounted on the barrel 2 of the gun such
that a passage inside of the housing communicates with the electrical
passage 9 in the barrel 2 of the gun. The electrode extender 52 is
~ounted in an opening on the housing 51, An opening in the side of
electrode extender 52 provides communication between the passage in the
- housing 51 and a passage in the extender S2. O-rings 58 seal the
housing 51 closed around the barrel 2 and around the extensio~ 52.
This sealing is to prevent contaminants from reaching any surfaces in-
side o the electrical passage 9. Contaminants on these surfaces could
reduce the resistivity of the surfaces, and hence, give rise to a
possible electrical path which could short out the high voltage system
or present a danger of sparking.
Details of the housing 51 and construction of the electrical
passage 9 can be more ully appreciated by reference to Figure 3, which
is a cross-sectional view of the gun through the dotted line designate~
3 in ~igure 1. As can be seen in ~igure 3, the housing Sl surrounds
the barrel 2 of the gun. A nut 53 sealed by an 0--ring extends into a
recess 56 on the barrel 2. The nut 53 bears against the surface of
the recess S5 in order to ix the angular displacement o the hous;~s~
- 16 -
31/
110~5~
51. ~ e barrel 2 o the g~ has a flat surface which forms a cavity
or chambcr 57 bct~een the barrel 2 and the hottsing 51. This chamber
57 is to rcceive the cab1e extendcr 50 upon angular displacement of the
housing 51. The chamber 57 for the cable extender 50 could be in other
forms, or could extend further around the barrel 2 of the gun. It can
be appreciated, however, from observing the possible positions of the
housing under angular displacement (ind~cated by phantom lines) that
90 angular displacement will allow the electrode 54 to be properly
positioned with respect to the fan when spraying in virtually any
usable orientation. This is because, in virtually all commercial
applications, the fan is either oriented horizontally or vertically.
Referring now to both Figures 1 and 3, it is noted here that
upon angular displacement from a 45 orientation the cable extender 50
will have more of its length in the second chamber 57 around the barrel.
However, the first and second springs 46, 47, will lengthen or extend
themselves in order to maintain the electrical contact with the cable
extender 50.
The springs 46, 47, tend to relieve any longitudinal stress~s
in the cable extender 50 when more or less of the cable extender 50 is
wrapped around the barrel 2 in the second chamber S7. The contacts at
the springs are pivotable. Therefore, the pivotal contacts also
function to relieve torsional stresses in the electrical conductor 50
when the housing 51 is angularly displaced. Other pivotable contacts
and lengthening means could be substituted, however, the contactg used
in the preferred embodiment ha~e been ound acceptable.
Referring now to Figure 1, the placement of the electrode 54
will be considered. The extension S2 carrics t:11e electrode 54 exter-
nally forward of the ~pray nozzle orifice 24. The electrode 54 is
displaced from the axis of the nozzle opening 24. This displacement
- - 17 -
ill -
1 105~1~
~ the elcctrocle 5~ from tlle llo~lc orifice 24 is nccessitated by thefact that thc ~UI~ is desi~ned to operate witll hi~hly conductive mater-
ials. In electrostatic paint spray systems it is desirable to have the
paint supply for highly conductive materials maintained at ground
potcntial. If the electrode 54 would be positioned close to the nozzle
discharge orlfice 24, the electrical standoff through the air would not
be sufficient, since the fluid column of electrically conductive paint
would effectively represent an electrical ground potential at the
nozzle orifice 24. If the distance between the electrode 54 and the
nozzle orifice 24 is not sufficiently great, then the voltage at the
electrode 54 would be shorted out through the paint column or present
the possibility of sparking to this point of ground. The length of
the extension 52 is cl~osen so that it carries the electrode 5~ forward
of the nozzle by a distance sufficiently great to maintain a 20 kilovolt
per inch standoff hetween the electrode and the closest point of ground
and yet be close enough to the atomized particles of paint to effect-
ively charge them to a high voltage. The electrode 54 is displaced
from the axis of the spray so that it does not become covered with
coating material under operation.
Now considering other electrical isolation or standof~s be-
tween any point in the system which is at high voltage to a point which
is at ground potential, two different types of standoffs must be con-
sidered: the standoff through dielectrics and the standoff alon~ an
air path or along the surface of some component. The electrical ~tan~-
off through a dielectric can be controlled by selecting a material
whose dielectric constant and whose thickness maintains a sufficient
standoff. However, the 5tandoff along surfaces, or throu~ he air,
can on~y be maintained by displacement unless some type of an electrical
seal can be effected around the components, Flectrically insula~:in~
-- 18 --
~ 1 l~S~
~ dl8 bct-~cer~ colnl)ollcllts whic~l rem~in fixcd with respect to one another
can be achicved. For example, a nonconductive cement can be used.
Ho~ever, the nonconductive cementing process is itself an expensive
procedure. Further, when parts are to be movable with respect to one
another, cementing is incompatible with movability. Prior art high
voltage electrical sealing between movable parts in an electrostatic
spray coating gun have used an insulating grease such as described in
the above mentioned U.S. Patent No, 3,937,401. However, this approach
has been proven unacceptable for various reasons.
In the electrostatic spray coating gun, which is the subject
of the present in~ention, it is to be noted that the high voltage
standoff along air gaps or surfaces components is accomplished without
the necessity of electrically insulating seals. The standoff is main-
tained by means of physical displacement only and yet the structure
allows the mounting of the electrode extension on the barrel in such
a way that the electrode can be angularly displaced around the axis of
the spray pattern.
,, .
Because there is no discontinuity in the sheath around the
ca~le extension 50 right at the point of angular displacement of the
housing 51, there is no need for an electrically insulating seal at
this point.
It will be noted further that the electrical contacts between
cable 20 and the first spring 46 and between the first spring 46 and the
cable ex-tender 50 are removed from the point of angular displace~ent of
the housing Sl to a point proximate the junction of the barrel 2 and
the handle 1. Furtller, because the contacts to the first spring 46 are
made inside of the polyethylene tube 44, the standoff along surfaces and
air gaps (i.e., alon~ the discontinuity at the junction of the barrel
2 and the handlc 1 or to the halldle itself) is main~alned at a safe
jll ; - 19 -
llt3S~
level. In actual practice, safe or adequate standoffs or
iso].ati.on through air or a]ong noncontaminated surfaces
should be at ].east 0.0~ inches per kilovolt of electrical
power used; and along contaminated surfaces, at least 0.1
inches per kilovolt of electrical power used.
Having now described our invention, it can be seen
that many modifications can be made to the hydraulic seal as
described without departing from the scope and spirit o the
invention of which we claim.
- 20 -
s~/ . ,_,,
