Dual acting piston pump having reduced back flow between strokes

A dual acting pump for dispensing pressurized liquids including a pump housing, a reciprocating pump shaft contained in a pumping chamber and at least one spring-biased check valve member. In the preferred embodiment, a spring-biased check valve member is connected to one end of the pump shaft for closing a flow passage in the pump shaft during movement in one direction and for opening the flow passage during movement in the opposite direction. Similar spring-biased check valves are connected to the pump inlet and pump outlet. The check valves comprise hollow balls and, in combination with the springs, this assures that liquid output from the pump is not reduced significantly when the pump shaft changes direction.

FIELD OF THE INVENTION 
The present invention generally relates to piston pumps and, more 
specifically, to dual acting piston pumps for dispensing liquids such as 
hot melt adhesive materials. 
BACKGROUND OF THE INVENTION 
Piston pumps generally have internal shafts that reciprocate back and forth 
to draw liquid into the pump and then force the liquid out of the pump. 
Dual acting piston pumps of this type dispense liquid in both directions 
of the reciprocating shaft movement. Examples of dual acting pumps are 
disclosed in U.S. Pat. Nos. 3,160,105; 3,995,966; and 5,067,882. 
Generally, these pumps include a pump body formed with a longitudinally 
extending passageway defining a pumping chamber divided into first and 
second pumping sections. The pumping chamber receives a portion of the 
pump shaft for reciprocating movement. The first section of the pumping 
chamber communicates with a discharge outlet formed in the pump body and 
the second section of the pumping chamber communicates with an inlet that 
receives a source of the liquid. The pump shaft carries a check valve and, 
in one direction of shaft movement, the check valve is moved to a closed 
position and material is forced out of the first section of the pumping 
chamber through the discharge outlet. At the same time, additional liquid 
is drawn in from the liquid source through the inlet into the second 
section of the pumping chamber. Reverse movement of the pump shaft into 
the second section of the pumping chamber opens the check valve associated 
with the shaft and forces liquid from the second section of the pumping 
chamber to the first section. This moves a corresponding amount of liquid 
through the discharge outlet. 
The SP Series and Series 3000 pumps of Nordson Corporation, Westlake, Ohio, 
include pump shafts with attached check valves as generally described 
above and further include a check valve at the pump inlet. Each of these 
check valves comprise free-floating solid balls. The ball at the inlet 
closes the inlet while the pump shaft moves from the first section of the 
pumping chamber to the second section as described above. The ball carried 
by the pump shaft alternately moves against and away from a valve seat as 
the pump shaft reciprocates to selectively prevent and allow liquid flow 
from the second section to the first section. 
Although existing piston pumps perform well in many applications, certain 
areas are still in need of improvement. One of these areas relates to the 
reduction in liquid output, measurable as flow rate and pressure, that 
occurs as the reciprocating pump shaft changes direction. In this regard, 
if liquid flows back toward the pump inlet as the shaft changes direction, 
this reduces flow rate and pressure at the outlet. These characteristics 
of typical dual acting piston pumps reduce liquid output from both the 
pump and any downstream dispensing device as the shaft changes direction. 
In hot melt adhesive dispensing operations, for example, many applications 
require uniform liquid discharge for purposes of obtaining an adequate 
adhesive bond. Reduced adhesive output from a pump can reduce adhesive 
bead widths or dot sizes to an extent that compromises bonding strength. 
Some applications further require the adhesive to be discharged laterally 
across a gap before reaching the substrate. In these applications, a 
reduced flow rate or pressure can also prevent the adhesive from hitting 
the substrate at the correct location or from hitting the substrate 
altogether. 
To address various problems such as those mentioned above, it would be 
desirable to provide a dual acting pump that minimizes irregular liquid 
discharge due to the change in direction of the pump shaft and, more 
specifically, due to back flow of liquid in the pump. 
SUMMARY OF THE INVENTION 
The present invention provides a pump for dispensing pressurized liquids 
generally including a pump housing having a liquid inlet passage and a 
liquid outlet passage. A pumping chamber disposed in the housing includes 
first and second sections, with the first section communicating with the 
liquid inlet passage and the second section communicating with the liquid 
outlet passage. The pumping chamber receives a reciprocating pump shaft 
having first and second ends. The second end of the pump shaft includes a 
flow passage that alternately moves liquid from the first and second 
sections of the chamber to the liquid outlet passage during reciprocating 
movement of the pump shaft. In accordance with one aspect of the 
invention, a spring-biased check valve member is operatively connected to 
the second end of the pump shaft and closes the flow passage during 
movement in the first direction. This spring-biased check valve opens the 
flow passage during movement in the second direction. The spring bias 
ensures that the ball quickly closes the flow passage as the pump shaft 
changes from the second direction of movement to the first direction. This 
significantly reduces back flow of liquid in the pump as the shaft changes 
direction. In accordance with a more specific aspect of the invention, the 
spring-biased check valve member is a hollow ball biased by a compression 
spring. Due to its hollow construction, the ball moves more quickly toward 
the closed position as the pump shaft changes direction. 
As an additional aspect of the invention, a spring-biased check valve 
member is connected with the liquid inlet passage and quickly closes the 
liquid inlet passage as the pump shaft changes from the first direction of 
movement to the second direction of movement. This spring-biased check 
valve member is preferably a hollow ball biased by a compression spring 
for the reasons discussed above. 
As another aspect of the invention, a spring-biased check valve member is 
connected with the liquid outlet passage and operates to quickly close and 
then reopen the liquid outlet passage during each change in the direction 
of shaft movement. As with the other spring-loaded check valves, this 
prevents back flow into the pump and assures that adverse losses of flow 
rate and pressure do not occur. This spring-biased check valve member is 
also preferably a hollow ball biased by a compression spring for the 
reasons discussed above. 
The invention further contemplates methods of dispensing pressurized 
liquids in manners that provide adequate liquid output from the pump 
during changes in direction of the reciprocating pump shaft. These methods 
can generally involve dispensing liquids, such as hot melt adhesive 
materials, using a dual acting pump having one or more features as 
generally described above and illustrated in detail below. 
Other objects, advantages and features of the invention will become more 
readily apparent to those of ordinary skill in the art upon further review 
of the following detailed description taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2, the preferred configuration of a dual acting 
pump 10 is illustrated in accordance with the principles of this 
invention. It should be noted that pump 10 is only one of many potential 
configurations that can benefit from the invention. Pump 10 may be 
disposed within a reservoir 12 containing liquid 14. Although many 
applications may benefit from the invention, pump 10 is particularly 
suited to dispense liquefied hot melt adhesive material from reservoir 12. 
Generally, pump 10 moves liquid 14 from an inlet 16 of pump 10 through an 
outlet 18 of reservoir 12. Pump 10 more specifically comprises a housing 
20 having an upper section 22 and a lower section 24. Lower section 24 
includes a liquid outlet 26 connected with outlet 18 of reservoir 12 by a 
suitable fluid fitting 28. 
A pump shaft 30 having a first end 30a and a second end 30b is mounted for 
reciprocating movement within upper and lower sections 22, 24 of housing 
20. First end 30a of pump shaft 30 carries an air-operated piston 32 
mounted for reciprocating movement within a piston cylinder 34 formed 
within upper housing section 22. Piston 32 separates upper and lower 
portions 36, 38 of piston cylinder 34. Upper portion 36 of cylinder 34 
communicates with a port 40 and, in a like manner, lower portion 38 of 
cylinder 34 communicates with a port 42. Suitable fittings 44, 46 connect 
with respective ports 40, 42 and ports 40, 42 may be connected in a 
typical manner to appropriate valving and pressurized air to reciprocate 
piston 32 and shaft 30 in opposite directions. 
A seal 48 prevents pressurized air from leaking out of cylinder 34 into a 
bore 50 that carries pump shaft 30. A second seal 52, mounted within lower 
housing section 24, prevents pressurized liquid from leaking out of 
housing section 24. Pump shaft 30 includes a flow passage 54 generally 
contained in second end 30b. Although flow passage 54 may take many 
alternative forms, passage 54 preferably includes an axially extending 
section 56 and radially extending sections 58, 60. An additional section 
62 of flow passage 54 may be contained in an increased diameter portion 64 
of pump shaft 30 at second end 30b. An outer surface 64a of shaft portion 
64 slides within a pumping chamber 66 with a close fit to chamber wall 
66a. As will be described further below, shaft portion 64 reciprocates 
between a first section 68 and a second section 70 of pumping chamber 66. 
Flow passage portion 62 contains a ball 72 having a interior 72a. Ball 72 
is biased by a compression spring 74 against a valve seat 76 to close an 
inlet 78 of passage 54. Spring 74 preferably is formed from stainless 
steel wire having a wire diameter of 0.026", an outer diameter of 0.360" 
and a free length of 0.620". Spring 74 has 4.74 total coils and a spring 
rate of 8.2 lb/in. Valve seat 76 is preferably disposed on a removable 
seat member 80 threaded into flow passage portion 62. A pin 82 is pressfit 
into pump shaft 30 and limits the movement of ball 72 away from valve seat 
76. As will be understood from the description to follow, this ensures 
that liquid can continuously flow past ball 72, through spring 74 and into 
flow passage portion 56 when ball 72 lifts off of valve seat 76. 
Another check valve 90 is mounted within a bore 92 communicating with 
pumping chamber 66. A washer 94 may separate check valve 90 from pumping 
chamber 66. A ball 96 having a hollow interior 96a is normally biased 
against a valve seat 98 by a compression spring 100. Spring 100 is similar 
to spring 74, except that it is formed from 0.038" stainless steel wire 
and has an outer diameter of 0.475"-0.505", a free length of 0.30", and a 
spring rate of 12 lb/in. Valve seat 98 may be an integral portion of check 
valve 90 or may be a separate member as shown. Compression spring 100 is 
disposed between washer 94 and ball 96 and allows ball 96 to raise off of 
valve seat 98 during the upward stroke of shaft 30 as shown in FIG. 1. 
A liquid outlet passage 110 extends from first section 68 of pumping 
chamber 66 generally to outlet 26. Another spring-biased check valve in 
the preferred form of a ball 112 having a hollow interior 112a is normally 
biased against a valve seat 114 by a compression spring 116. Ball 112 
prevents back flow of liquid into outlet passage 110 during each change in 
direction of pump shaft 30. As an alternative to the various check valve 
configurations detailed herein, other check valves may be utilized. 
In operation, pump shaft 30 reciprocates at a rate which may be about 30-60 
strokes/minute when dispensing many hot melt adhesives. Pump 10 will 
continuously pump liquid from inlet 16 through outlet 18 with reduced 
liquid back flow and increased liquid output during each directional 
change of shaft 30. Specifically, pump shaft 30 moves upward upon the 
introduction of pressurized air into cylinder section 38 and simultaneous 
exhaust of air from cylinder section 36. During this upward stroke, as 
shown in FIG. 1, ball 96 will raise from seat 98 and liquid 14 will flow 
through inlet 16 into pumping chamber section 70. During this same upward 
stroke, ball 72 connected with shaft 30 will be forced onto seat 76 by 
spring 74. Thus, liquid in pumping chamber section 68 will be forced into 
liquid outlet passage 110 by portion 64 of pump shaft 30. This liquid will 
travel through outlet passage 110 and push ball 112 off of valve seat 114 
against the bias of spring 116. The liquid will then exit through outlet 
18. 
During the downward stroke shown in FIG. 2, ball 96 engages valve seat 98 
and balls 72 and 112 are displaced from their respective valve seats 76, 
114. During the downward stroke, liquid will move from pumping chamber 
section 70 through inlet 78 and past ball 72 in shaft 30. The liquid will 
then travel through spring 74 and flow passage portions 56, 58 into 
pumping chamber section 68. This simultaneously forces liquid through 
liquid outlet passage 110. Liquid will exit past ball 112, just as 
described above with respect to the upward stroke. 
In accordance with the invention, when shaft 30 changes direction from the 
upward stroke to the downward stroke, balls 96, 112 will quickly engage 
seats 98, 114. Likewise, when shaft 30 changes from the downward stroke to 
the upward stroke, balls 72, 112 will quickly engage valve seats 76, 114. 
After each of these momentary changes in direction, ball 112 will reopen 
to allow flow through outlet 18. During each of these changes in 
direction, liquid will not flow back within pump 10 to an extent that 
adversely affects the end use. 
FIGS. 3 and 4 graphically illustrate the advantageous effects of the 
invention. The graph shown in FIG. 3 illustrates liquid pressure vs. scans 
(i.e., time) for a portion of a pump cycle including two changes in shaft 
direction. The pump comprised a Nordson SP Series pump as described in the 
background above having a check valve carried by the pump shaft and a 
check valve at the pump inlet. Each of these check valves comprised a 
solid, free-floating ball as described in the background. A bead loss line 
is drawn at the pressure which represents a decrease of more than 25%, 
from the set flow rate. The two significant dips in the pressure occur at 
the upshift and downshift of the pump shaft and, as shown in FIG. 3, a 
significant dip below the bead loss line occurs at one shift point. This 
pump was operated at approximately 21.5 cycles per minute and 30 psig 
inlet air pressure. In contrast, FIG. 4 illustrates a similar bead loss 
evaluation graph of a pump constructed in accordance with the invention. 
Specifically, compression springs and solid steel balls were used as check 
valves in the pump shaft and the pump inlet and a spring-loaded check 
valve was connected to the pump outlet. In comparison to the test 
illustrated in FIG. 3, the pump illustrated in FIG. 4 was operated with 
28.2 psig air pressure and at 21.18 cycles per minute. As shown in the 
test results, two dips in the pressure are visible at the changes in 
direction of the pump shaft. However, neither of these dips were below the 
bead loss line. One alternative manner of defining bead loss in absolute 
terms involves measuring the time periods during which a laterally 
directed adhesive bead falls more than 1/8" below an intended target line 
on a substrate. The invention also successfully passes these tests. 
Another manner of defining bead loss on a per cycle basis is in the form of 
a ratio between the lost bead time during a pump cycle relative to the 
total time of the cycle. FIGS. 5 and 6 graphically compare bead loss 
ratios between three different pump configurations. These configurations 
include a Nordson SP Series pump as described above, a modified SP Series 
pump having spring-loaded, solid balls used as check valves in the pump 
shaft and at the pump inlet, and a further modified SP Series pump, 
labeled "Modified SP Series II Pump", and having spring-loaded, solid 
balls used as check valves in the pump shaft and pump inlet and a separate 
spring-loaded check valve connected at the pump outlet as described above. 
FIG. 5 compares bead loss, as a ratio, between the three different pump 
configurations at 100 psi liquid pressure, while FIG. 6 illustrates a 
similar comparison at 200 psi liquid pressure. In each case, the graphs 
show that bead loss is significantly less using pumps configured according 
to the invention. This is particularly evident at higher flow rates. At 
the higher liquid pressure of 200 psi illustrated in FIG. 6, bead loss is 
negligible across all the tested flow rates for the Modified SP Series II 
pump. 
While the present invention has been illustrated by a description of the 
preferred embodiment and while this embodiment has been described in some 
detail, it is not the intention of the Applicant to restrict or in any way 
limit the scope of the appended claims to such detail. Additional 
advantages and modifications will readily appear to those skilled in the 
art. For example, various types of reciprocating pump configurations and 
spring-loaded check valve members may be substituted for those described 
specifically herein. This has been a description of the present invention, 
along with the preferred methods of practicing the present invention as 
currently known. However, the invention itself should only be defined by 
the appended claims, wherein I claim: