Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROV13D DOUBLE-ACTING POSITIV~ DI~PLACEM~NT PUMP
Field of the Invention
The present invention relates to positive displacement pumps and, more
particularly to double-acting positive displacement pumps used to pump relatively
5 viscous liquids at a substantially constant pressure.
Background OI the Invention
Double-acting positive displacement pump~ have been utilized for years to
pump polyester resin o other viscous resin materials to an applicator gun. A
second double-acting positive displacement catalyst pump is normally slaved to
10 the piston rod of the resin pump~ which is driven by a single air motor. For
practical purposes, all resin and catalyst pumps are produced from standard stock
tubing, rods, packing and seals. In use, the resin and catalyst are pumped to the
applicator gun and are normally mixed within the gun. It is extremely important
that the delivery pressure to the applicator gun for both the resin and the catalyst
15 be substantially constant so as to obtain a proper admixture of the two
components prior to spraying onto a substrate. For this reason, accumulators have
been inserted in both the resin output and catalyst output lines downstream fromthe pump. In the better pumps of the prior art, the accumulators have been
precharged to provide sufficient back pressure during operation to assist in
20 smoothing out pressure fluctuations occurring on opposing strokes OI the piston
pump.
Despite the efforts of the manufacturers of prior pumps, a small pressure
differential exists between the downstroke and the upstroke of the resin pumps
used today. While many attempts have been made to compensate for this pressure5 differential, no satiseactory solutions have been found.
Summary of the Invention
The present invention comprises an improvement in double-acting positive
displacement pumps. These pumps include a cylinder having a first end and a
second end, a fluid intake and check valve at the first end and a fluid outlet at the
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second end. A piston is mounted for reciprocating movement between the first
and second ends o~ the cylinder. A piston rod is operably coupled to the piston and
extends throu~h an opening in one end Oe the cylinder. Seals are mounted on the
piston between the piston and the wall of the cylinder to prevent fluid elow past
5 the cylinder as the cylinder reciprocates. Seals are also provided between thepiston rod and the end of the cylinder to prevent fluid from escaping from the
cylinder other than through the fluid outlet. The improvernent evolved from the
recognition that the dimensions of the stock components and their arrangement
are for the large part responsible for the pressure differentials on the downstroke
lO and upstroke of prior pumps. Accordingly, the improvement comprises si~ing the
piston rod to a predetermined diameter relative to the diameter of the cylinder so
that the di-fferential pressures created on an upstroke and downstroke of the
cylinder counteract and balance the differential frictional forces generated by the
seals.
In an additional aspect of the invention, the diameter of the piston rod is
predetermined to also substantially compensate for the string weight of the pumpcomponents on the upstroke to substantially equalize the output pressure on boththe upstroke and the downstroke.
In still another aspect of the invention, the piston rod is driven by a double-
20 acting air motor. The piston rod diameter within the cylinder is further sized to apredetermined diameter to compensate for the differential forces generated by
the air motor on the upstroke and the downstroke to thereby again substantially
equalize the output pressure of the pump on the upstroke and the downstroke.
Brief Description of the Drawings
A better understanding of the present invention can be derived by reading
the ensuing specification in conjunction with the accompanying drawings wherein:~ IGURE 1 is a schematic diagram of a double-acting positive displacement
resin pump constructed in accordance with the present invention; and
FIGURE 2 is a comparison of the output pressures of a pump constructed in
30 accordance with prior art techniques and one constructed in accordance with the
present invention.
Detailed Description of the lnvention
Referring first to FIGURE 1, a resin pump generally designated 10 and a
catalyst pump generally designated 12 is mounted on a bracket 14 extending
35 transversely from the housing of the pump 10. The resin pump 10 includes a
cylinder 16 having a lower inlet end 18 and an upper outlet end 20. A piston 22 is
mounted for up-and-down reciprocating movement in the cylinder 16. The
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piston 22 houses a check valve 24 that allows resin to Elow from the lower portion
of the cylinder through the piston to the upper portion of the cylinder on the
downstroke of the piston. The lower end of the cylinder also carries a check
valve 26 that allows resin to flow into the cylinder 16 on the upstroke of the
5 piston 22 and prevents resin from flowing back towards the resin supply on the piston downstroke.
Resin flows outwardly through the outlet conduit 28 on both the downstroke
and the upstroke of the piston, through a filter 30, and thence into a resin
conduit 32 that carries the resin to a hand-held resin spray gun 34. An
lO accumulator 36 is coupled between the filter housing and the conduit 32. As the
system pressure builds up, resin flows into an accumulator reservoir 38 and
creates a fluid back pressure in the upper portion of the reservoir 38. As the
piston reverses, and consequently as check valves 24 and 26 momentarily float,
the back pressure in the accumulator will maintain a substantially constant linel 5 pressure in resin conduit 32 during reversal.
The piston 22 carries seals 40 that wipe the interior cylinder walls to prevent
resin from flowing past the piston as it reciprocates in an upward and downward
direction. The piston 22 is driven by a piston rod 42 affixed to the upper end of
the piston and extending through the upper end 20 of the cylinder 16. Seals 44
20 mounted in the upper end of the cylinder wipe the surface of the rod 42to prevent
resin from flowing past the rod to the exterior of the cylinder.
An air motor 50 is positioned at the upper end of the pump 10. The upper
end of the piston rod 42 is coupled to the cylinder 52 of the air motor. The airmotor is of standard construction. Pressurized air is supplied to the air motor to
25 drive the piston first in a downward direction and then in an upward direction. A
conventional shuttle valve 54 is employed to reverse the air~low.
The catalyst pump 12 is a similar double-acting positive-displacement
pump. Housed in a cylinder 60, the catalyst pump has substantially the same
components as the resin pump and operates in a manner very similar to that of the
30 resin pump. Catalyst is pumped from a supply through the cylinder 60 by the
piston 62, through an accumulator 64, and into a catalyst condult 66. Catalyst
conduit 66 is also coupled to the spray gun 34. The piston rod 68 of the catalyst
pump 12 is driven by a slave arm 70 that is coupled between the piston rod 42 ofthe resin pump and the bracket for the catalyst pump. As the resin piston rod 4235 reciprocates upwardly and downwardly, the slave arm similarly moves upwardly
and downwardly to reciprocate the piston rod 68 of the catalyst pump.
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Referring to FIGURE 2, it has been observed that the output pressure of the
resin will vary between the downstroke and the upstroke of the resin pump.
Output pressure of prior pumps is indicated by the prior art graph 90 of FIGURE 2
schematically dictating the differential output pressure on the downstroke "D" and
5 the upstroke "U". The graph shows two cycles Oe the pump. It is recognized that
this differential pressure will affect the amount of resin and catalyst entering the
spray gun 34 and resulting on the downstroke~ for example, with less catalyst
supplied to the gun because of the higher pressure, and on the upstroke with more
catalyst being supplied to the gun because of the lower resin pressure. As the
10 resulting mixture is sprayed onto a substrate, nonhomogeneity occurs, which can
lead to differential curing rates and other potential problems.
A careful analysis of the dynamic forces created in the operating resin pump
has lead to a discovery that allows the downstroke and upstroke output pressuresto be substantially equalized so that they will appear substantially constant as15 illustrated by graph 92 in FIGURE 2. More specifically, it is noted that a
different driving force is created on the downstroke of the air motor 50 relative
to the upstroke, different frictional forces are generated by both the piston rod
seals 44 and the piston seals 40, and additionally, a pressure differential is also
created by the nominal sizing of the piston rod 42 and the piston 40.
With regard to the air motor, air at a constant pressure is supplied to the
motor. T,~hen air is supplied above the cylinder 52, the air acts on the entire
surface of the cylinder. When, however, air is supplied to the lower surface of the
cylinder, the air acts on the area of the cylinder less the rod area. Thus, a slightly
lesser force is created on the upstroke than on the downstroke. Similarly, on the
25 downstroke of the piston 22 in the resin pump, substantially equal pressure is
generated on both sides of the seals, creating lîttle frictional resistance to
movement of the piston. On the upstroke, however, pressure on the upper side of
the seal is system pressure, sometimes on the order of l000 psi, while the pressure
below the seal is substantially zero. This differential pressure drives the seals
30 against the cylinder wall, generating substantial frictional forces and thus causing
resistance to upward movement of the piston. Similarly, the seals 44 generate
more frictional resistance to upward motion of the piston rod than downward
motion.
Lastly, it has been realized and understood that the actual sizing of the
35 piston rod and piston can contribute to creation of differential pressures on the
upstroke and downstroke. Prior art pumps have used components Oe nominal
sizes. For example, for a given pump, a piston rod having a nominal diameter of
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0.875 inch and a piston having a diameter on the order of 1.250 inch have been
employed. These dimensions provide a rod area of 0.601 square inch and a
cylinder area of 1.227 square inch. On a downstroke, the effective pressurizing
area is only that of the rod, that is, 0.601 square inch. On the upstroke, however,
5 the effective pressurizing area is the difference between the cylinder area and the
rod area that is 0.626 square inch. Thus, since the effective pressurizing area on
the downstroke is smaller than on the upstrokej the down pressure exerted on thepiston rod, assuming a constant driving force in both directions, is 4.2% greater on
the downstroke. For other standard pumps using the nominal piston rod and
l 0 cylinders, a similar result is found.
Thus, it has been realized that there are several factors contributing to the
creation of a greater system pressure on the downstroke than on the upstroke.
The long sought-after solution to this problem is to slightly increase the size of
the piston rod relative to the cylinder to reduce the pressurizing area on the
15 upstroke so as to compensate for pressure losses that are otherwise occurring. In
the standard pump specified above it has been found that increasing the diameterof the piston rod to approximately 0.900 inch will cause the system output
pressure on both the downstroke and the upstroke to be substantially equal. The
diameter of 0.900 inch calculates to a rod area of 0.635 inch when compared to
20 the cylin(ler area of 1.227 inch. It can be calculated that the up pressure
generated by the piston will be approximately 7.4% greater than the down
pressure. This increase in the up pressure will compensate for the losses that
otherwise occur. As a result, system pressure is substantially equalized on boththe upstroke and the downstroke.
A positive displacement pump modified in accordance with the present
invention has many advantages. The principal advantage is constant resin and
catalyst supply pressure to the resin gun 34 which results in a constant
homogeneous mixture being sprayed upon a substrate. More importantly, because
the improved pump substantially equalizes the output pressure, it is no longer
30 necessary to precharge the accumulators. Thus, only an atmospheric charge is
needed to compensate for the check valve slippage occurring on piston reversal.
This is a significant operating advantage because it is no longer necessary to
precharge the accumulators prior to initiating operation of the pump.
The present invention has been disclosed in conjunction with a preferred
35 embodiment. One of ordinary skill after reading the eoregoing specificatioll will
be able to effect various alterations and substitutions of equi~ralents without
departing from the broad concepts disclosed herein. It is therefore intended that
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the scope of protection granted to Letters Patent hereon be limited only by the
definition contained in the appended Claims and then equivalents thereof.
