Solenoid pump adapted for noiseless operation

A solenoid-actuated fluid pump having reciprocable armature operable by a magnetic circuit and with structure in the flux path at the upper and lower air gaps to substantially change the reluctance across the air gaps as the armature moves beyond a predetermined location. The armature has slots for passage of fluid therethrough and which have a C-shaped cross-section to minimize the air gaps.

TECHNICAL FIELD 
The present invention relates generally to solenoid-actuated hydraulic 
pumps such as fuel oil pumps energized by half-wave rectified alternating 
current. 
BACKGROUND ART 
U.S. Pat. Nos. 3,874,822 and 3,877,841 disclose one form of prior art 
solenoid-actuated fuel oil pump in which an electromagnetic plunger or 
armature surrounded by an electromagnetic coil is supported within a main 
pumping chamber by coil springs abutting opposite ends of the armature. 
Connected to the lower end of the armature is a pressure plunger or piston 
whose lower end telescopes into an intake pressure chamber. The latter is 
separated from the main pumping chamber by a check valve on the discharge 
side of the intake pressure chamber. Another check valve on the suction or 
inlet side of the intake pressure chamber keeps fluid from being pumped 
back through an inlet side of the intake pressure chamber. A half-wave 
rectified alternating current applied to the coil provides for 
intermittent energization of the coil because current flows only in one 
direction through the coil in a cycle. When current is flowing through the 
coil, the armature is driven upwardly to compress the upper one of the 
coil springs by the electromagnetic forces resulting from energization of 
the coil. Then, when current flow stops, the stored forces in the upper 
spring push the armature downwardly, its momentum propelling it past a 
neutral spring force position in the main pumping chamber thereby 
compressing the lower coil spring. As current is reapplied to the coils, 
the armature again is forced upwardly against the upper spring. 
In the present solenoid pump as well as the prior art pump described above, 
the power stroke of the piston is with the upstroke of the armature so 
that, as the piston enters the main pumping chamber from the intake 
chamber, fuel oil is forced through a longitudinal passage in the armature 
and out of the main pumping chamber through a hole in a magnetic force 
adjusting rod or plug which also serves as the upper reaction member for 
the upper armature spring. 
DISCLOSURE OF THE INVENTION 
Under normal ideal operating conditions, the travel of the armature within 
the main pumping chamber is limited by the hydraulic load imposed on the 
pump. Accordingly, the armature springs are kept from being compressed 
excessively. It is desirable to avoid excessive compression of the 
armature springs because excessive compression can cause the springs to 
fail prematurely and thereby render the pump inoperative. Under actual 
operating conditions, however, air bubbles may pass through the pump or 
the pump may be subjected to a period of dry operation, resulting in a 
momentary or extended loss of hydraulic load on the pump. Such loss of 
hydraulic load can cause the armature to travel beyond its normal range of 
movement and, should the armature move upwardly to strike against a member 
such as the magnetic force adjusting plug, the operation of the pump 
becomes undesirably noisy. 
The present invention aims to eliminate the foregoing cause of noise from 
the operation of the solenoid pump by keeping the upper end of the 
armature from striking the plug yet without causing excessive compression 
of the armature springs by limiting upward movement of the armature to an 
overtravel position spaced below the lower end of the plug. To these ends 
in one form of the invention, the pump is uniquely constructed to reverse 
the directional effect of the application of magnetic forces to the 
armature during the upstroke of the armature under conditions absent 
hydraulic load so that over travel of the armature beyond its normal upper 
range limit is limited to a position below the magnetic force adjusting 
plug. Advantageously, this is achieved by constructing the components of 
the pump providing the magnetic circuit around the coil to produce an 
increase in the lower gap reluctance which exceeds a simultaneous decrease 
in the upper gap reluctance when the armature moves upwardly within the 
main pumping chamber beyond a selected position spaced above the neutral 
spring position and below the end plug. Because the magnetic force driving 
the armature results from a decrease in the over-all reluctance of the 
magnetic flux circuit, if the rate of decrease of the reluctance of the 
upper air gap is less than the rate of increase of the reluctance of the 
lower air gap, the magnetic force will oppose the upward movement of the 
armature in the event that the other portions of the flux circuit maintain 
substantially the same reluctance. As a result, when the armature reaches 
that predetermined point where the rate of decrease in the reluctance of 
the upper air gap is exceeded by the rate of increase in the reluctance of 
the lower air gap, the magnetic forces on the armature will reverse. More 
particularly herein, the armature is of a preselected axial length less 
than the axial distance between the opposite ends of portions of the 
magnetic flux path through upper and lower magnetic gaps. Specifically, 
herein, the length of the armature is approximately equal to the axial 
distance between the aforementioned predetermined point of magnetic force 
reversal in the pumping chamber and the opposite end of the flux path 
portion through the lower magnetic gap. 
Additionally, by virture of the configuration of the magnetic pump 
component associated with the upper magnetic gap, advantage is taken of 
magnetic flux saturation of a section of such component to further reduce 
the rate of decrease in the upper air gap reluctance relative to the 
simultaneous rate of increase of the lower gap reluctance thereby lowering 
the magnitude of the magnetic force applied to the armature. Prior to 
saturation, this component section advantageously limits the reluctance 
across the upper magnetic gap to a low value and thus provides for a 
relatively high magnetic flux density across the upper gap so that, for 
instance in starting the pump, a high magnetic force is generated to move 
the armature from its neutral position. Preferably, herein the saturable 
component section is in the form of an annular lip projecting downwardly 
from the magnetic force adjusting plug. 
In another form of the present invention, dashpot means are provided in the 
upper end of the main pumping chamber to coact between the upper end of 
the armature and the end plug to limit upward movement of the armature in 
the chamber to the aforementioned overtravel position. Advantageously, 
herein, the dashpot is defined by a recess in the armature which retains 
both the lower end of the upper armature spring and a quantity of fuel 
oil. The plug lip also is sized to telescope with the recess in the 
overtravel position and thereby limit upward movement of the plug by 
damping flow of oil from the recess. 
The provision of uniquely shaped flow passages in the periphery of the 
armature is a further advantageous effect of the present invention in 
providing for straight flow through of fuel from one end of the armature 
to the other while avoiding a significant increase in the area of the 
magnetic gaps, particularly the lower magnetic gap. Herein, the 
circumferential width of each of such flow passages at the periphery of 
the armature is less than the maximum circumferential of the passage's 
width in the interior of the armature. 
The foregoing and other advantageous effects of the present invention will 
become more apparent from the following description of the best mode of 
the invention when read in conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
As shown in the drawings for purposes of illustration, the present 
invention is embodied in a solenoid-actuated hydraulic pump 10 such as may 
be used in pumping a low volume of fuel oil from a tank (not shown) to a 
burner nozzle (not shown) of a household furnace. Herein, the pump 
includes a body 11 with a magnetic housing 13 attached thereto and 
containing an electromagnetic coil 14. The coil is connectable through 
lead wires 15 to a source (not shown) of half-wave rectified alternating 
current which provides the power for the pump. More particularly, the coil 
is supported within the housing 13 on a nonmagnetic spool 16 which is 
attached to the inside surfaces of the housing. The central aperture of 
the spool as well as a recess 18 in the pump body define a pumping chamber 
17 and a non-magnetic sleeve 22 telescoped within the spool aperture and 
recess receives movable means including a generally cylindrical armature 
19 adapted for reciprocation in the chamber with the intermittent 
energization of the coil 14 by the half-wave rectified current. A magnetic 
circuit (see FIG. 2) in the pump is completed through the armature 19, an 
end plug 34 in the upper end of the main pumping chamber 17, the housing 
13 and a sealing ring 12 connected to the housing and telescoped into 
sealing engagement with the recess 18. 
In operation, hydraulic fluid in the form of fuel oil is drawn from the 
fuel tank (not shown) through an inlet bore 20 in the pump body with each 
upstroke of the armature 19. To provide the suction force for drawing in 
fuel oil through the bore 20, a smaller diameter piston or plunger 21 
reciprocates with the armature within an intake chamber 23 that is 
separated from the main pumping chamber 17 by a cylinder 24. The lower end 
of the intake chamber 23 communicates with a passage 25 in turn 
communicable with bore 20 for delivering fuel oil to the intake chamber by 
way of an intake check valve 27. The piston 21 is formed separate from the 
armature 19 but is urged continuously against the lower end of the 
armature by a spring 28. A strainer 26 between the passage and the bore 20 
provides for filtration of the incoming fuel oil and the intake check 
valve 27 which is spring loaded closed and is located upstream of the 
intake chamber in the passage 25 keeps fuel oil from flowing out of the 
intake chamber toward the strainer with each downstroke of the piston 21. 
Immediately downstream of the intake chamber 23 within the passage 25 is 
located a discharge check valve 29 spring loaded closed and through which 
fuel is ported to a bore 30 communicating with the lower end of the main 
pumping chamber 17. Accordingly, with each downstroke of the piston 21 the 
volume of fluid previously drawn into the intake chamber 23 by upward 
movement of the piston between the upper and lower phantom line positions 
shown in FIG. 1, is pumped across the discharge check valve 29 and into 
the main pumping chamber. Then, with the next upstroke of the piston 21, 
the same volume of oil is driven from the main pumping chamber by the 
piston 21 as a portion of the piston enters the main pumping chamber from 
the intake chamber 23. 
In flowing from the inlet end of the main pumping chamber 17 torward the 
upper outlet end of the chamber 17, the fuel oil passes through a series 
of peripheral slots or passages 31 formed in the armature 19 and extending 
longitudinally therethrough to open at opposite ends of the armature. As 
seen in FIG. 4, the slots also open radially of the armature. At the 
outlet end of the pumping chamber 17, the fuel oil passes through a hole 
33 in the magnetic end plug 34 to a discharge chamber 35 defined by the 
discharge fitting 36 which threadably captivates the end plug 34 within 
the outlet end of the main pumping chamber 17. A discharge port 37 in the 
fitting 36 provides communication between the chamber 35 and an outlet 
conduit (not shown) leading to the fuel oil burner (not shown). 
Reciprocal within the discharge chamber 35 is a magnetic valve 39 having 
peripheral slots 38 for fuel oil to pass by the sides of the valve within 
the chamber during pumping. But, as shown in FIG. 1, the valve is urged 
into a position closing the port 37 by a spring 40 acting between the 
valve 39 and the end plug 34. When the coil 14 initially is energized for 
normal pumping operation of the pump, the valve 39 is drawn downwardly 
against the end plug 34 by magnetic forces overcoming the spring 40. The 
magnetic hysteresis characteristics of the material forming the body of 
the valve 39 is such that the valve remains continuously open during the 
application of half-wave rectified current to the coil but, when the pump 
is turned off, the valve 39, of course, is urged into the closed position 
by the spring 40 to stop the flow of fuel out of the chamber 35. 
In order for the armature 19 to reciprocate when half-wave rectified 
alternating current is applied to the coils 14 of the pump 10, the 
armature is urged downwardly by an upper spring 41 toward a neutral spring 
force position (see FIG. 2) with spring 28. In this position, the 
reluctance across an air gap (represented by the double cross-hatched area 
43) at the upper end of the armature is much greater than the reluctance 
in the portion of the flux path across the annular air gap (represented by 
the double cross-hatched area 44 shown only in FIG. 2) adjacent the lower 
end of the armature. Accordingly, with the initial application of the 
half-wave rectified current to the coil 14, the armature is drawn upwardly 
(as seen in FIG. 2) by magnetic force as long as current flows through the 
coil due to the fact that the initial upward movement of the armature will 
tend to reduce the over-all reluctance of the magnetic circuit. Because 
current flows through the coil in only one direction during a cycle, the 
spring 41 causes the armature to downstroke once with each current cycle. 
Thus, for instance, with the sixty-cycle current, the armature and 
accompanying piston 21 reciprocate sixty times within each second. 
However, in part because of the momentum generated by the moving parts of 
the pump, reversal in the direction of movement of the armature at the 
upper end of its stroke does not occur instantaneously with the loss of 
current flow through the coil 14. But normally, owing to the damping 
effect of the oil flowing through the armature passages 31 and the work 
being performed by the pump in pressurizing the oil, normal upward 
movement of the armature under hydraulic load is limited to avoid striking 
the upper end of the armature against the lower end of the plug 34. 
However, from time to time during operation of the fuel pump of the 
present character, an air bubble may be transmitted through the pump or 
the pump may be subjected to dry operation such that the armature does not 
stroke against a significant, if any, hydraulic load. Under such 
conditions, the armature will travel upwardly beyond from its normal 
limits of reciprocation. Repeated, excessive compression of the operating 
springs of the pump can lead to spring fatigue and pump failure and, 
should the upper end of the armature strike against the plug 40 
undesirable operating noise is generated. 
The present invention contemplates utilizing the magnetic force generated 
by the magnetic circuit to keep the armature 19 from traveling upwardly an 
excessive distance beyond the normal upper limit of movement of the 
armature by reversing the directional effect of the magnetic force on the 
armature so as to keep the upper end of the armature from striking the 
plug 34 and creating noise and to keep the upper spring 41 from being 
excessively compressed and failing prematurely. For these purposes, the 
magnetic circuit (partially illustrated by a general magnetic flux line 
45) within the pump includes a first portion 46 through the upper air gap 
43 and a second portion 47 through the lower air gap 44 wherein the rate 
of increase in reluctance of the flux path in the lower air gap at a 
preselected position of the armature spaced upwardly of its neutral spring 
force position in the main pumping chamber 17 and below the end plug 34 
exceeds the rate of decrease in reluctance across the upper air gap. By 
virtue of the foregoing structure, even during dry operation of the pump, 
the armature 19 is kept from striking the plug 34 (see armature overtravel 
limit FIG. 2) and the spring 41 is kept from being excessively compressed. 
In the present instance, the representative general flux line 45 of the 
magnetic circuit for the exemplary pump is shown in FIG. 2 only and, upon 
progressing in a counter clockwise direction as viewed in beginning with 
the end plug 34, the circuit is completed along the line 45 from the upper 
surface of the plug to the magnetic valve 39 by face to face contact 
between the valve 39 and the plug during the application of half-wave 
rectified alternating current of the coil 14. From the valve 39, the 
circuit spans a fixed distance air gap 48 (double cross-hatched in FIG. 2) 
extending to and through the magnetic housing 13 and the magnetic sealing 
ring 12 From the sealing ring, the magnetic circuit is completed along the 
portion 47 of the flux line 45 across the lower air gap 44 to and through 
the armature 19. At the upper end of the armature, the magnetic circuit 
finally is closed along the portion 46 of the flux line 45 through the 
upper air gap 43 to the end plug 34. 
While the magnetic circuit herein is represented by a dashed line, it will 
be appreciated that the total magnetic flux within the circuit occupies 
three dimensional space thus giving the flux path within the fixed gap 48, 
a constant, generally sleeve-like configuration. The flux path in the air 
gap 43, however varies in size and density with movement of the armature 
19 in the pumping chamber 17. Herein, reversal in the directional effect 
of the magnetic force is obtained by also varying the dimensions of the 
lower air gap 44 with armature movement. Specifically, the length of the 
lower air gap is varied such that at a selected armature position spaced 
upwardly of the neutral spring force position and below the end plug 34, 
the rate of change in the total reluctance of the magnetic curcuit 
reverses direction, thus reversing the directional effect of the magnetic 
force on the armature 19. 
The foregoing may be understood more readily with the following explanation 
and knowledge that the magnetic force (F.sub.m) which is applied to the 
armature 19 is a function of the square of the flux .phi. through the 
circuit times the rate of change in the total reluctance (R.sub.t) of the 
circuit relative to armature movement (X), dR.sub.t /dx. In addition, the 
magnetic flux .phi. is a function of the magnetomotive force (MMF) of the 
coil and the inverse total reluctance (R.sub.t). The general magnetic 
force equation is expressed as follow: 
EQU F.sub.m =-1/2.phi..sup.2 (dR.sub.t /dx)=-1/2(MMF/R.sub.t).sup.2 (dR.sub.t 
/dx) (1) 
Additionally, the total reluctance (R.sub.t) of the circuit at any position 
of the armature 19 is equal to the sum of the component reluctances and 
may be expressed by the following formula: 
EQU R.sub.t =R.sub.f +R.sub.l +R.sub.u +R.sub.c (2) 
in which 
R.sub.f represents the reluctance of the fixed gap 48, 
R.sub.l represents the reluctance of the lower gap 44, 
R.sub.u represents the reluctance of the upper gap 43, and 
R.sub.c represents the reluctance of the magnetic components of the 
circuit. 
Moreover, the reluctance (R) for any section of the flux path is a function 
of its length (w), as measured longitudinally of the flux path, divided by 
its cross-sectional area (A) and may be obtained from the following 
equation: 
EQU R=w/Auu.sub.o (3) 
where 
u is the permeability of the medium through which the flux is flowing 
relative to free space; and 
u.sub.o is the permeability of free space, a constant. 
In considering the total reluctance formula (2) as applied to the present 
magnetic circuit, the reluctances R.sub.f and R.sub.c can be considered to 
be substantially constant because the flux path length and cross-sectional 
area (refer to formula (3) above) within these portions of the circuit 
remain essentially unchanged even when the coil 14 is energized and the 
armature moves. Accordingly, the rate of change of the total reluctance 
(R.sub.t) depends directly upon rate of change in the reluctance (R.sub.u) 
and (R.sub.l) of the upper and lower air gaps 43 and 44. 
Herein, in order to provide a rate of change in the reluctance of the lower 
gap 44 which is greater than the rate of change in the reluctance of the 
upper gap 43 at a preselected upper position of the armature 19, the 
effective axial length of the armature is selected to be less than the 
axially measured length of the main gap as defined by the members in the 
flux path forming the two poles whereby the effective air gap at the upper 
end becomes less than the effective air gap at the lower end. The main gap 
in association with the armature provides the upper and lower air gaps. 
The armature is in a magnetically neutral position when movement in either 
direction will not produce a reduction in the reluctance of the magnetic 
circuit, taken as a whole, resulting in no magnetic force being applied to 
the armature. Preferably, the magnetic neutral position of the armature is 
located above the normal upper limit of travel of the armature. The effect 
of this structural feature in the operation of the present pump may be 
more readily understood in considering the graph shown in FIG. 3. 
In the graph of FIG. 3, the reluctance curves for upper gap 43, the lower 
gap 44 and for the total reluctance are shown as straight line curves in 
order to aid in simplifying this description. However, it is to be noted 
that a true representation of these three reluctance curves would show the 
curves to be curvilinear rather than linear. With this in mind and 
considering the initial energization of the solenoid coil 14, the armature 
19 is drawn upwardly from its neutral spring position by the magnetic 
force (F.sub.m) applied to the armature in opposition to spring 41. As the 
armature moves upwardly, the length of the upper air gap 43 becomes 
shorter through the air. In accordance with the reluctance formula (3), 
the flux path shortens in relation to its cross-sectional area, the 
reluctance decreases in magnitude. The graph curve Ru represents generally 
the reluctance of the upper gap with respect to armature position (X). The 
vertical line O through the center of the curve represents the neutral 
spring force position with armature position above the neutral spring 
force position being represented as positive (+X) to the right and 
negative (-X) to the left. From the curve Ru it is seen that for some 
distance (X) if armature travel from the neutral spring force position of 
the armature, the rate of decrease in the reluctance associated with the 
upper gap is greater than the rate of increase in the reluctance of the 
lower air gap 44 as shown by the graph curve R.sub.1. Accordingly, in the 
summation curve of the reluctances R.sub.t, the slope of the summation 
curve decreases at a fairly steep rate. However, at a point S in the 
reluctance curve Ru, the slope of the curve flattens significantly in 
dictating a change in the rate of decrease in the reluctance of the upper 
gap 43 with respect to (X) to a lesser rate of decrease which is desirable 
in reducing the force (F.sub.m) and hence the acceleration of the armature 
upwardly prior to reversing its direction of travel. Herein, this 
desirable change in dR.sub.u /dx is achieved through the provision of a 
downwardly projecting magnetic lip 49 which is integrally formed with the 
lower end of the plug 34. While the lip serves as a retainer for the upper 
end of the spring 41, it also provides for a shorter upper gap 43 upon 
initial energization of the coil 14 thereby limiting the reluctance across 
that gap to a low value so that a high magnetic force (F.sub.m) is 
generated for moving the armature 19 upon energizing the coil 14. When the 
lip 49 becomes magnetically saturated, however, as is represented by the 
point S in the curve Ru, the lip no longer produces a reduction in 
reluctance with further upward movement of the armature. Therefore, any 
further decrease in the reluctance across the upper gap is primarily due 
to a reduction in the length of the air gap 43 which does not bear any 
substantial relation to the proximity of the upper end of the armature to 
the lip. 
Because of the selected length of the armature 19, when the armature moves 
upwardly, the reluctance associated with the lower gap 44 also changes but 
it increases in magnitude due to the decreasing of the area A of the gap 
44 with upward movement in conformity with the reluctance formula (3). 
Accordingly, as shown in FIG. 3, the reluctance (R.sub.l) of the lower air 
gap 44 increases in magnitude so the slope of the curve R.sub.l is 
positive. At a predetermined distance upwardly from the neutral spring 
force position, the rate of increase of reluctance of the lower air gap 
increases sharply and to the extent that it exceeds the rate of decrease 
in the reluctance of the upper air gap for the same position of the 
armature 19. Herein, this increase in the rate of change of the reluctance 
of the lower air gap is due primarily to the passage of the lower end of 
the armature upwardly beyond a shoulder 50 defined by a notch 51 formed in 
the magnetic sealing ring 12 on the housing 13. Upon the passing upwardly 
of the shoulder 50, the length of the lower air gap 44 dramatically 
increases so that the rate of change in the reluctance of the flux path in 
the lower air gap with further upward movement of the armature is based 
upon an air gap length W significantly longer than the air gap length 
associated with lower positions of the armature relative to the shoulder. 
The present invention additionally contemplates limiting upward movement of 
the armature 19 to the overtravel position (see FIG. 5) through the 
provision of dashpot means in the upper end of the main pumping chamber 
17. For this purpose, the armature is formed with an elongated, generally 
cylindrical recess or reservoir 53 (see FIG. 5) of the same general 
cross-sectional configuration but slightly larger than the lateral 
cross-sectional area bounded by the radially outward surface of the lip 
49. The lower end of the recess is closed so that during normal operation 
of the pump, oil collects in the reservoir 50. However, should an air 
bubble pass through the pumping chamber 17 causing the armature to 
overtravel, the armature is limited in upward movement to an overtravel 
position spaced below a lower end surface 54 of the plug 34 by the damping 
effect produced by the oil as it is forced between the lip and the inside 
walls of the reservoir. 
In accordance with another advantageous feature of the present invention, 
the armature 19 passages 31 are of a unique C-shaped cross-sectional 
configuration so as to provide for non-tortuous flow of fuel oil past the 
armature without a significant increase in the cross-sectional area and/or 
length of the magnetic gaps 43 and 44. For this purpose, the 
circumferential width of each of the passages 31 at the periphery of the 
armature is less than the maximum circumferential width of such passage in 
the interior of the armature (see FIG. 4). 
In view of the foregoing, it is seen that the present invention brings to 
the art a new and improved solenoid pump 10 particularly constructed to 
keep from generating noise or excessively compressing the upper spring 41 
in the absence of a hydraulic load. Advantageously, this is accomplished 
by reversing the directional effect of the magnetic force generated by the 
magnetic circuit in the pump to urge the armature 19 downwardly instead of 
upwardly once the armature passes upwardly of a selected position in the 
pumping chamber 17.