Method and apparatus for sensing clogged nozzle

A method and apparatus for sensing and signaling a clogged nozzle condition in a spray gun of the type which includes a valve located adjacent the nozzle. The gun includes a restrictor located in the liquid flow path upstream of the valve and a pressure transducer located between the restrictor and the valve for measuring pressure drop when the valve of the gun is opened. A pressure drop of less than a predetermined amount is indicative of a clogged nozzle condition.

This invention relates to liquid spray apparatus and more particularly to a 
method and apparatus for detecting a clogged or partially clogged 
condition of the nozzle of such apparatus. 
There are many high speed coating applications wherein a liquid spray is 
applied to multiple discrete objects as the objects pass a spray gun. In 
most of these applications the spray gun is turned on and off at the 
frequency of objects passing the gun. One such application occurs in the 
coating of cans wherein either the can bodies or can ends are spray coated 
with a thin film of lacquer or other protective coating material as the 
can bodies or ends pass the gun. Quite commonly these can parts pass the 
gun at the rate of several hundred per minute and the gun is cycled, i.e., 
turned on and off, at that same frequency. 
One common problem in the coating of can bodies or can ends is insuring 
that the complete can interior surface is coated with the coating 
material. The purpose of the coating on the interior surface of the can is 
to prevent the can contents, as for example a food or beverage, from 
contacting the metal of the can body or end. Any such contact of a food or 
beverage results in contamination of the food or beverage and therefore 
the coating must be 100% complete and impervious to liquid. Any pin holes, 
cracks or imperfections of any kind cannot be tolerated. But, while 
complete surface coverage is critical, it is also important that no excess 
material be applied to the surface because of the very large number of 
cans being coated. Each spray applicator applies coating to literally 
millions of cans in the course of a year and therefore the spraying of 
excess material to insure complete surface coverage is very expensive over 
a long period of time. 
In the coating of can bodies, as in the coating of any surface which 
requires 100% surface coverage, there must be some excess material applied 
in order to provide some margin for error. However, in the can coating 
industry, as in many high speed coating applications, that margin for 
error is minimal, quite commonly 10 or 15 percent. A problem therefore 
arises if for any reason the spray emitted from the nozzle of the gun 
drops below that minimal safety margin, i.e., drops below that 10 or 15 
percent safety margin. 
Still another problem which occurs in high speed coating, but particularly 
in the coating of cans, is in determining when the spray has dropped below 
the safety margin and objects are being less than completely covered with 
spray material. This problem is particularly acute if the sprayed material 
is transparent, as for example a clear lacquer such as is commonly applied 
in the can industry. In that event, less than complete coverage of a 
surface cannot be detected visually and must be detected by some testing 
procedure, usually a random sampling test of the products. But that random 
sampling test may allow some partially coated products to pass before the 
sampling procedure detects or identifies the problem. 
It has therefore been one objective of this invention to provide a method 
and apparatus for determining whenever a high speed liquid coating 
apparatus is effecting less than 100% surface coverage of the objects 
being sprayed by the apparatus. 
Still another objective of this invention has been to provide a method and 
apparatus for determining when a spray gun is dispensing less than a 
predetermined quantity of material onto a sprayed object. Otherwise 
expressed, an objective of this invention has been to provide a method and 
apparatus for determining whenever less than a predetermined flow rate is 
being dispensed from a spray nozzle. 
Still another objective of this invention has been to provide a method and 
apparatus which is very sensitive to changes in flow from a liquid spray 
nozzle and therefore capable of determining a relatively small change in 
the flow rate from the nozzle. 
These objectives are accomplished and this invention is predicated upon the 
concept of measuring a pressure signal internally of the gun at a location 
between the valve of the gun and a restriction contained internally of the 
gun and utilizing that signal to determine the condition of the nozzle and 
particularly whether that nozzle is partially clogged. In practice, it has 
been found that by locating a restriction or restricted orifice upstream 
of the valve in the liquid flow stream to the valve and by measuring the 
pressure drop of the liquid in that flow stream when the valve is open, it 
is possible to determine the condition of the nozzle and whether that 
nozzle is partially clogged. If the restriction upstream of the valve has 
a flow rate approximately three times the flow rate of the nozzle orifice, 
there is a resulting pressure drop of approximately 10% of nozzle 
discharge pressure between the restriction and the nozzle orifice when the 
valve is opened so long as the nozzle orifice is unrestricted or 
unclogged. If the nozzle orifice becomes partially restricted or clogged, 
there is an additional pressure drop of less than 10% of the nozzle 
discharge pressure indicating the condition. Of course, if the nozzle 
orifice becomes completely clogged, there will be no pressure drop upon 
opening of the valve. 
The primary advantage of this invention is that it enables the condition of 
the nozzle, and whether it is partially clogged, to be measured at a 
location remote from the nozzle without either a visual inspection of the 
nozzle or of the products coated by the nozzle. This clogged condition or 
partially clogged condition can usually be detected by the practice of 
this invention long before the condition can be visually detected by 
inspection of the part or the nozzle. In the case of can coating 
applications this invention often enables a clogged condition of the 
nozzle to be detected even before it would otherwise be picked up by 
destructive or non-destructive tests of the coated product.

With reference to FIGS. 1 and 2 there is illustrated a dispensing gun 10 
incorporating the invention of this application. Generally, this gun 10 
comprises a body 11 through which liquid is supplied from an inlet 12 to a 
nozzle 13. Internally of the body there is a valve 14 and valve seat 15 
for controlling flow of the liquid from the inlet 12 to the nozzle 13. 
Opening and closing of the valve 14 is controlled by a solenoid 16 mounted 
atop the body 11. 
The body 11 comprises a ported body block 17 and a body extension 18 
secured to that block. The block has an axial throughbore 19 counterbored 
and threaded as indicated at 19a for the reception of a threaded sleeve 20 
of the solenoid. This axial throughbore 19 is intersected by a connecting 
passage 21 and a pressure take-off passage 22. The passage 21 
interconnects the inlet passage 12 with the axial throughbore 19 and 
comprises a first large diameter section 21a and a small diameter end 
section 23. As explained more fully hereinafter, a calibrated restriction 
25 is mounted within the small diameter section 23 of the connecting 
passage 21. At its outer end the passage 21 is threaded as indicated at 
26. A pipe threaded plug 27 is mounted within the threaded section 26 of 
the passage 21 so as to close that passage to all but the inlet passage 
12. 
The pressure take-off passage 22 is open to a transducer mounting passage 
30 within the body block 17. As explained more fully hereinafter, a 
transducer 31 is mounted within the passage 30. This transducer is 
operative to sense and transmit to a read-out device 32 a pressure signal 
indicative of pressure of liquid flowing through the gun. 
The gun body extension 18 comprises a tubular section 33 from which there 
extends a flange 34. This flange is bolted to the underside of the body 
block 17 by conventional threaded connectors. There is preferably an 
O-ring 35 sandwiched between the top surface of the flange 34 and the 
bottom surface of the block 17. 
The lower end of the body extension 18 is externally threaded as indicated 
at 37 for reception of a nozzle nut 38. This nozzle nut has an inwardly 
extending flange 39 engageable with the nozzle 13 for securing the nozzle 
to the outer end of the body extension. 
An axial bore 40 extends through the body extension 18 and communicates 
with the axial bore 19 of the body block 17. This bore 40 is counterbored 
at its lower end to receive the valve seat 15 which is fixedly secured 
therein. An axial passageway 42 extends through this valve seat for 
accommodating flow of liquid from the bore 40 through the passageway 42 
and out of the gun through the orifice 43 of the nozzle 13. 
Opening and closing of the valve 14 relative to the valve seat 15 is 
controlled by the solenoid 16. This solenoid includes an axially movable, 
tubular shaped armature 45 within which the upper end 46 of the valve stem 
47 of valve 14 is slideable. This armature 45 has an inwardly extending 
lip 48 engageable with an annular flange 49 of the valve stem 47 so that 
upon upward movement of the armature, the valve stem 47 of the valve 14 is 
lifted upwardly, thereby lifting the valve 14 from the seat 15 and 
permitting flow of liquid through the gun as explained more fully 
hereinafter. 
The coil 50 of the solenoid 16 is mounted within a housing 51 which 
includes a removable cap 52. The housing 51 is mounted over the hub 53 of 
the sleeve 20 and is secured thereon by a nut housing 54 and lock nut 55. 
The nut housing 54 and nut 55 are threaded over a plug 56 mounted in the 
upper end of the sleeve hub 53. 
There is a compression spring 59 located between a triangular shaped 
shoulder 57 on the upper end of the valve stem 47 and a recess 58 in the 
bottom of the plug 56. This spring 59 biases the valve 14 to a closed 
position. Additionally, there is a light compression spring 60 sandwiched 
between a shoulder 61 of the plug 56 and a shoulder 63 of the armature 45. 
This light compression spring 60 biases the armature 45 to a lower 
position in which the bottom surface of the lip 48 is engaged with the top 
surface of the body extension 18. In this lowered position of the 
armature, the lip 48 is located slightly below and out of engagement with 
the shoulder 49 of the valve stem so that upon energization of the 
solenoid coil 50, the armature moves approximately 0.030 inches upwardly 
before the lip 48 of the armature 45 contacts the shoulder 49 of the valve 
stem 47 and initiates opening of the valve 14. 
When electrical current is supplied to the coil 50 of solenoid 16, the 
armature 45 of the coil is caused to move upwardly. In the course of this 
upward movement the lower lip 48 of the armature engages the lower 
shoulder 49 of the valve stem 47, thereby causing the valve stem to move 
upwardly and lift valve 14 off of seat 15. When the valve 14 opens, 
pressurized liquid is free to flow from inlet 12 through the restrictor 25 
into a chamber 64 surrounding the armature 45. The liquid flows upwardly 
through this chamber 64 and through radial slots 65 in the top of the 
armature into the hollow interior 66 of the armature 45. The liquid then 
flows downwardly over the generally triangular shaped shoulder 57 of the 
valve stem and through radial ports 67 in the bottom of the armature into 
a chamber 68 in the interior of the body block 17. From the chamber 68 the 
liquid flows over the exterior of the valve stem 47 through the open valve 
14 and out of the gun through the nozzle orifice 43. 
The solenoid operated dispensing gun 10 heretofore described except for the 
restriction 25, the pressure take-off passage 22, the transducer passage 
30, and the transducer 31, are conventional and have long been available 
in the commercial market. Per se, this gun forms no part of the invention 
of this application. Rather, the invention of this application is 
concerned with the restriction 25, the pressure take-off passages 22, 30 
and transducer 31 which enable the condition of the nozzle orifice of the 
gun to be monitored. 
With reference now to FIGS. 3, 4 and 5 it will be seen that the restriction 
25 comprises a restrictor body 70 and a carbide insert 71. The insert 71 
is mounted within the body 70 and provides a restricted orifice 72 through 
which a controlled flow rate may be established. 
The restrictor body 70 comprises a large diameter cylindrical end section 
73 within which there is formed an annular groove 74. A smaller diameter 
cylinder section 75 extends axially from the larger end section 73. Both 
sections are provided with an axial bore 76. As may be most clearly seen 
in FIG. 4, the outer end of the passage 76 is counterbored as at 77. The 
carbide insert 71 is fixedly mounted within this counterbored section 77 
of the passage 76. Prior to the insert 71 being mounted within the 
counterbored section 77 of the passage 76, a V-shaped diametral cut 78 is 
machined into the inner surface of the insert. This V-shaped cut 
preferably defines an included angle of 60.degree.. It is ground to a 
depth of approximately one-half the thickness T of the insert 71. After 
machining of this cut 78 into the face of the insert, the insert is brazed 
into the counterbored section 77 of the passage 76. The insert is so 
oriented in the passage 76 that the diametral cut 78 extends at right 
angles to a trapezoidal shaped notch 79 formed on the end of the 
restrictor body 70. After having been brazed into the restrictor body, a 
second V-shaped notch 80 is machined at right angles to the notch 78. This 
second notch 80 is machined to a depth at which the two notches 78, 80 
intersect, resulting in the small restricted orifice 72 at the point of 
intersection of the two notches. By carefully grinding the notch 80 
progressively deeper into the insert 71, the equivalent diameter of the 
restricted orifice 72 may be accurately controlled. 
The outer end of the smaller diameter section 75 of the body is threaded as 
indicated at 82. This threading of the end section enables the restrictor 
25 to be attached to a tool (not shown) for insertion of the restrictor 
into the passage 21 of the gun body 17. To retain the restrictor 25 within 
that passage 21, an O-ring 83 is located within the annular groove 74 of 
the restrictor body. 
In one preferred embodiment of the invention, the orifice 72 of the 
restriction 25 is sized to have a flow rate 3.162 times the flow rate of 
the nozzle orifice 43. These relative orifice sizes effect approximately a 
10% pressure drop in the pressure of liquid contained within the liquid 
flow chambers 64, 68 of the gun when the valve 14 of the gun is opened. 
Otherwise expressed, this relative sizing of the orifices of the 
restriction 25 and nozzle 13 results in a 10% added pressure drop within 
the liquid flow chambers 64, 68 of the gun between closed and opened 
condition of the valve 14. In the absence of the restriction 25 between 
the inlet 12 of the gun and the valve 14, there would be very little if 
any appreciable reduction or change in pressure in chambers 64, 68 between 
closed and opened condition of the valve. Alternatively, if the orifice 72 
of the restriction 25 was sized so as to have a flow rate more closely 
matching that of the orifice 43, there would be a great pressure drop in 
chamber 68 between closed and opened condition of the valve 14, but there 
would also be a much greater pressure loss between the inlet 12 of the gun 
and the flow chambers 64, 68. Consequently, there would be a greater 
energy loss in liquid flow through the gun. The relative sizing of the 
orifices 72 and 43 of the restriction and nozzle respectively was chosen 
so as to generate an appreciable and measurable pressure drop between 
closed and open condition of the valve 14 while minimizing energy loss 
effected by the restriction 25. 
In the operation of the liquid dispensing gun 10, liquid is supplied to the 
inlet 12 and caused to flow through the passageways 21, 23 into the 
chambers 64, 68. When the valve 14 of the gun is opened by energization of 
the solenoid coil 50, liquid is permitted to flow through the valve seat 
15 and nozzle orifice 43 onto any substrate located beneath or in front of 
the gun nozzle. The pressure of fluid within the chamber 64 is measured by 
the transducer 31. This transducer transmits a signal via a lead 86 to the 
read-out device 32. In one preferred embodiment of the invention, the 
read-out is an oscilliscope upon which a pressure reading can be taken. 
With reference to FIGS. 6 and 7 there is an oscilliscope reading of two 
different nozzle conditions measured by the transducer 31 of the gun 10. 
FIG. 6 is a reading generated by the gun 10 when the nozzle 13 of the gun 
was fully opened and unclogged. As there illustrated, the liquid in 
chambers 64, 68 was at a pressure of approximately 500 psi when the valve 
14 was closed and when the valve 14 was opened, the pressure dropped 
approximately 56 psi and remained at that lower pressure until the valve 
14 was closed, at which time the pressure returned to 500 psi. With 
reference to FIG. 7 there is illustrated a reading generated by the 
oscilliscope 32 when the orifice 43 of the nozzle was restricted so as to 
have 10% less flow than did the nozzle employed in the gun to generate the 
reading of FIG. 6. All other conditions were substantially the same for 
obtaining the reading of FIG. 6 and FIG. 7. When the nozzle orifice was 
partially restricted or clogged so as to have 10% less flow, the 
transducer 31 of the gun 10 generated the reading of FIG. 7 wherein the 
pressure dropped 48 psi upon opening of the valve. This reduced pressure 
drop is indicative of a partially closed nozzle or clogged nozzle 
condition. In practice, this reduced pressure drop could be used by an 
operator at a location remote from the gun to indicate that less than full 
flow is being delivered through the orifice 43 of the nozzle 13 and to 
trigger stoppage of the gun until the nozzle can be removed and replaced 
or cleaned. 
It will be appreciated that the same transducer signal indicates either a 
completely clogged condition, in which event there would be no pressure 
drop between open and closed condition of the valve, or that the nozzle 
has blown out, in which event there is substantially greater pressure drop 
than 56 psi upon opening the valve. 
The primary advantage of this invention resides in its ability to enable a 
machine operator to detect a partially clogged nozzle condition. In the 
event of partial blockage of the nozzle, the reduced pressure drop seen on 
the oscilliscope 32 indicates immediately to the machine operator that the 
nozzle orifice is partially clogged and requires cleaning or to be 
replaced. In the absence of this invention within the gun, the operator 
can only determine such a condition by observing the spray results, but 
oftentimes, particularly in the application of clear spray materials it is 
impossible to observe such reduced flow with the naked eye. In that event 
reduced flow can only be detected by a lab testing technique. In many 
applications wherein the gun is spraying articles at the rate of several 
hundred per minute as is commonly the case in the can coating industry, 
many cans would receive less than a complete coating before the partially 
clogged condition could be determined. The invention of this application 
enables the nozzle condition to be monitored at all times and the usage 
stopped whenever less than a minimal flow rate is being dispensed from the 
nozzle orifice 
While I have described my invention as utilizing an oscilliscope as the 
pressure monitoring device 32, other devices could be substituted for this 
read-out device. For example, a control circuit could be substituted which 
would automatically stop gun operator pressure whenever a less than 
predetermined value was detected upon opening of the valve of the gun. 
That same signal could be responsive to a pressure drop in excess of a 
predetermined value (indicating nozzle blow-out) to terminate operation of 
the gun. Persons skilled in this art will appreciate other modifications 
and changes of this invention which may be made without departing from the 
spirit of my invention. Therefore, I do not intend to be limited except by 
the scope of the following appended claims.