Crescent gear pump

A crescent pump including a housing having an inlet port and a discharge port, a driving gear, and a driven gear meshing with driving gear in a gap between the inlet and the discharge ports. External and internal troughs on the driving and the driven gears define pump chambers. A stationary crescent-body has a pair of arc-shaped walls which cooperate with the tips of the external and the internal gear teeth thereon in defining fluid seals. The edges of an inlet ramp at an upstream end of the crescent body and the edges of a discharge ramp at a downstream end of the crescent body define inner and outer upstream and downstream metering orifices which close and open in complementary fashion to maintain constant the rate of fluid leakage from the discharge port toward the inlet port. A pair of shaped metering grooves in the pump housing cooperate in defining a flow path between the discharge port and the inlet port through a succession of trapped volumes between the driving and the driven gears. The flow path has a pair of variable orifices therein calibrated to reduce the fluid pressure in the succession of trapped volumes at a controlled rate.

TECHNICAL FIELD 
This invention relates to a positive displacement fluid pump. 
BACKGROUND OF THE INVENTION 
A typical positive displacement fluid pump referred to generically as a 
"crescent gear pump" or "crescent pump" includes a housing having an inlet 
port and a discharge port, a driving gear rotatable on the housing about a 
first centerline, and a driven gear rotatable on the housing about a 
second centerline parallel to and laterally separated from the first 
centerline. External gear teeth on the driving gear mesh with internal 
gear teeth on the driven gear between the inlet and the discharge ports to 
isolate the discharge port from the inlet port in the direction of 
rotation of the driving and the driven gears. External and internal 
troughs on the driving and the driven gears between the gear teeth thereon 
define pump chambers in which fluid is transferred from the inlet port to 
the discharge port. A stationary crescent-body between the driving gear 
and the driven gear has a pair of arc-shaped walls which fit closely 
around the driving and the driven gears and cooperate with the tips of the 
external and the internal gear teeth thereon in defining fluid seals 
against fluid leakage from the discharge port to the inlet port. Pressure 
pulses attributable to succeeding ones of such pump chambers closing 
abruptly at an upstream end of the crescent body and opening abruptly at a 
downstream end thereof and to similarly abrupt closing and opening of 
trapped volumes between successive pairs of meshed external and internal 
gear teeth exert dynamic pressure forces on the driving gear which 
contribute to audible vibration of the crescent pump. 
SUMMARY OF THE INVENTION 
This invention is a new and improved crescent pump including a housing 
having an inlet port and a discharge port, a driving gear rotatable on the 
housing about a first centerline, and a driven gear rotatable on the 
housing about a second centerline parallel to and laterally separated from 
the first centerline. External gear teeth on the driving gear mesh with 
internal gear teeth on the driven gear in a gap between the inlet and the 
discharge ports. External and internal troughs on the driving and the 
driven gears between the gear teeth thereon define pump chambers in which 
fluid is transferred from the inlet port to the discharge port. A 
stationary crescent-body has a pair of arc-shaped walls which cooperate 
with the tips of the external and the internal gear teeth in defining 
fluid seals which minimize fluid leakage from the discharge port toward 
the inlet port around the crescent body. The edges of an inlet ramp at an 
upstream end of the crescent body and the edges of a discharge ramp at a 
downstream end of the crescent body define inner and outer upstream and 
downstream metering orifices which close and open in complementary fashion 
to maintain constant the rate of fluid leakage around the crescent-body. A 
pair of shaped metering grooves in the pump housing cooperate in defining 
a flow path across the gap between the discharge port and the inlet port 
through a succession of trapped volumes between the driving and the driven 
gears. The flow path has a pair of variable orifices therein which exhaust 
successive trapped volumes at a rate calculated to maintain substantially 
constant the net pressure force on the driving gear in the gap between the 
inlet port and the discharge port.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1-5, a schematically represented positive displacement 
crescent pump 10 according to this invention includes a housing 12, a 
first side plate 14, and a second side plate 16. The first and second side 
plates 14,16 close opposite ends of a circular bore 18 in the housing 12. 
A schematically represented arc-shaped slot 20 in the side plate 14 is 
connected to a fluid reservoir, not shown, through a passage 22 in the 
housing 12 and defines an inlet port 24 of the crescent pump. A second 
schematically represented arc-shaped slot 26 in the side plate 14 is 
connected to a fluid operated device, not shown, e.g. a fluid motor, 
through a second passage 28 in the housing 12 and defines a discharge port 
30 of the crescent pump. 
A driving gear 32 of the crescent pump in the circular bore 18 between the 
side plates 14,16 is supported on the housing 12 for rotation about a 
first centerline 34 with a pair of annular end walls 36A,36B of the 
driving gear closely facing the side plates 14,16, respectively. Torque 
for rotating the driving gear counterclockwise, FIG. 1, about the first 
centerline 34 is transferred from an external prime mover, not shown, to 
the driving gear through a drive shaft 38. A plurality of external gear 
teeth 40 around the periphery of the driving gear are separated by a 
corresponding plurality of external troughs 42. Each of the external gear 
teeth 40 has a tip 44 and a pair of flanks 46A,46B on opposite sides of 
the tip, FIGS. 1,5 and 8A-8C. 
A ring-shaped driven gear 48 of the crescent pump is journaled in the 
circular bore 18 between the side plates 14,16 around the driven gear for 
rotation about a second centerline 50 parallel to and laterally separated 
from the first centerline 34. A pair of annular end walls 52A,52B of the 
driven Rear closely face the side plates 14,16, respectively. A plurality 
of internal gear teeth 54 in an inside cylindrical wall of the driven gear 
48 are separated by a corresponding plurality of internal troughs 56. Each 
of the internal gear teeth 54 has a tip 58 and a pair of flanks 60A,60B on 
opposite sides of the tip. 
In the relative positions of the driving gear 32 and the driven gear 48 
depicted in FIG. 5, the flanks 46A of a plurality of external gear teeth 
40 on the driving gear bear against the flanks 60A of a corresponding 
plurality of internal gear teeth 54 on the driven gear at respective ones 
of a plurality of three seal points 62A,62B,62C. Driving torque is 
transferred from the driving gear to the driven gear at the seal points 
62A-62C which also cooperate in defining a first trapped volume 64 and a 
second trapped volume 66 each of which, in the relative positions of the 
driving and driven gears depicted in FIG. 5, spans about one-half of a gap 
"G", FIGS. 5 and 8A-8C, between a downstream end 68 of the discharge port 
30 and an upstream end 70 of the inlet port 24. 
As seen best in FIGS. 1-4, a crescent-shaped chamber between the driving 
gear and the driven gear overlaps the inlet port 24 and the discharge port 
30. A crescent-body 72 in the crescent-shaped chamber is rigidly secured 
to the housing and includes a pair of flat sides 74A,74B sealed against 
the side plates 14,16, an arc-shaped inner wall 76 facing the driving gear 
and separated radially from the tips 44 of the external gear teeth thereon 
by a clearance dimension "CL" and an arc-shaped outer wall 78 facing the 
driven gear and separated from the tips 58 of the internal gear teeth 
thereon by the clearance dimension CL. The clearance dimension CL is 
illustrated in exaggerated fashion in FIGS. 3-4 for clarity. 
The flat side 74B of the crescent body 72 has an inlet ramp 80 at an 
upstream end thereof facing the inlet port 24 and a discharge ramp 82 at a 
downstream end thereof facing the discharge port 30. A length dimension 
"L.sub.i " of the inner wall between the ramps 80,82 is a whole multiple 
of a circumferential pitch dimension "P.sub.i ", FIG. 1, between the tips 
44 of the external gear teeth 40 on the driving gear. The length of the 
edge of the inlet ramp 80 where it intersects the inner wall 76 and the 
length of the edge of the discharge ramp 82 where it intersects the inner 
wall are each equal to the pitch dimension P.sub.i. A length dimension 
"L.sub.o " of the outer wall 78 between the ramps 80,82 is a whole 
multiple of a circumferential pitch dimension "P.sub.o ", FIG. 1, between 
the tips 58 of the internal gear teeth 54 on the driven gear. The length 
of the edge of the inlet ramp 80 where it intersects the outer wall 78 and 
the length of the edge of the discharge ramp 82 where it intersects the 
outer wall are each equal to the pitch dimension P.sub.o. 
The external and internal troughs 42,56 define discrete pump chambers which 
transfer fluid around the crescent-body from the inlet port 24 to the 
discharge port 30. The edge of the inlet ramp 80 where it intersects the 
inner wall 76 of the crescent-body defines a wedge-shaped inner upstream 
metering orifice 84, FIG. 2, between the inlet port and the external 
troughs 42 which closes as the external troughs traverse the length of the 
inlet ramp. The edge of the discharge ramp 82 where it intersects the 
inner wall 76 defines a wedge-shaped inner downstream metering orifice 86, 
FIG. 2, between the discharge port and the external troughs 42 which opens 
as the external troughs traverse the length of the discharge ramp. 
Corresponding wedge-shaped outer upstream and outer downstream metering 
orifices, not shown, between the internal troughs 56 and the inlet and the 
discharge ports 24,30 are defined by the edges of the inlet and the 
discharge ramps where they intersect the outer wall 78 of the 
crescent-body. 
In an ordinary crescent pump of the prior art, not shown, the tips of the 
external and the internal gear teeth on the driving and driven gears 
cooperate with the inner and the outer walls of the crescent-body in 
defining multi-stage inner and outer seals against fluid backflow or 
leakage around the crescent-body from the discharge port toward the inlet 
port. The number of external and internal gear tooth tips fully sealed 
against the inner and the outer walls of the crescent-body varies 
cyclically by one gear tooth tip as the driving and driven gears rotate. 
The resulting cyclic variation of the fluid leakage rate around the 
crescent-body causes pressure pulses in the discharge port and 
corresponding dynamic pressure forces on the driving gear which have been 
observed to contribute to pump vibration. In the crescent pump 10 
according to this invention, the fluid leakage rate around the 
crescent-body is maintained substantially constant to minimize such 
pressure pulses and corresponding dynamic pressure forces. 
More particularly, when the crescent pump 10 is operating, a continuous 
succession of the pump chambers defined by the external troughs 42 
traverse the inner wall 76 of the crescent-body from left to right, FIG. 
2, to transfer fluid from the inlet port to the discharge port. The tips 
44 of the external gear teeth in the span L.sub.i of the inner wall are 
fully sealed and define a multi-stage inner fluid seal against leakage 
from the discharge port toward the inlet port. The combined restriction to 
fluid leakage afforded by these seals varies as the number of external 
gear tooth tips 44 fully sealed within the span L.sub.i varies cyclically 
between (n) and (n+1), where (n) is L.sub.i divided by P.sub.i. The tips 
58 of the internal gear teeth in the span L.sub.o of the outer wall are 
fully sealed and define a multi-stage outer fluid seal against leakage 
from the discharge port toward the inlet port. The combined restriction to 
fluid leakage afforded by these seals varies as the number of internal 
gear tooth tips 58 fully sealed within the span L.sub.o varies cyclically 
between (n') and (n'+1), where (n') is L.sub.o divided by P.sub.o. 
At the same time, a succession of external troughs 42 ahead of the span 
L.sub.i communicate with the inlet port through the inner upstream 
metering orifice 84 which closes as the succession of external troughs 
traverse length of the inlet ramp 80 until fully shrouded by the inner 
wall 76. Likewise, a corresponding succession of external troughs 42 fully 
shrouded by the inner wall 76 at the end of the span L.sub.i communicate 
with the discharge port through the inner downstream metering orifice 86 
which opens as the succession of external troughs traverse length of the 
discharge ramp 82. The rates at which the upstream and downstream metering 
orifices 84,86 close and open, respectively, are calculated to vary the 
combined restriction to leakage afforded by both metering orifices in 
complementary fashion relative to the combined restrictions afforded by 
the fully sealed gear tooth tips in the span L.sub.i so that the total 
restriction against leakage toward the inlet port between inner wall 76 
and the driving gear 32 remains substantially constant regardless of the 
position of the driving gear relative to the crescent-body. 
Similarly, a succession of the internal troughs 56 ahead of the span 
L.sub.o of the outer wall 78 communicate with the inlet port through the 
aforesaid outer upstream metering orifice which closes as the succession 
of internal troughs traverse the length of the inlet ramp 80 until fully 
shrouded by the outer wall. A corresponding succession of internal troughs 
56 fully shrouded by the outer wall 78 at the end of the span L.sub.o 
communicate with the discharge port through the aforesaid outer downstream 
metering orifice which opens as the succession of internal troughs 
traverse the length of the discharge ramp. The rates at which the outer 
upstream and outer downstream metering orifices close and open are 
calculated to vary the combined restriction to leakage afforded by both 
metering orifices in complementary fashion relative to the combined 
restrictions afforded by the fully sealed gear tooth tips in the span 
L.sub.o so that the total restriction against leakage toward the inlet 
port between outer wall and the driven gear remains substantially constant 
regardless of the position of the driven gear relative to the 
crescent-body. 
Referring to FIGS. 5-7 and 8A-8C, the crescent pump 10 further includes a 
first shaped metering groove 88 in the side plate 14 spanning a fraction 
of the gap G between the inlet and the discharge ports 24,30. The first 
metering groove 88 has a wedge-shaped bottom which defines a deep first 
end 90 of the groove exposed to the discharge port 30 and a second end 92 
of the groove where the bottom intersects the surface of the side plate 14 
facing the driving and the driven gears 32,48. The first metering groove 
cooperates with the end wall 36A of the driving gear in defining a passage 
open to the discharge port which becomes more restricted in the direction 
of rotation of the driving and the driven gears. 
A second metering groove 94 in the side plate 14 also spans a fraction of 
the gap G between the inlet and the discharge ports and has a wedge-shaped 
bottom defining a deep first end 96 of the groove exposed to the inlet 
port 24 and a second end 98 of the groove where the bottom intersects the 
surface of the side plate 14 facing the driving and the driven gears 
32,48. The second metering groove cooperates with the end wall 52A of the 
driven gear in defining a passage open to the inlet port which becomes 
less restricted in the direction of rotation of the driving and the driven 
gears. 
In the relative positions of the driving and driven gears depicted in FIG. 
5, the second trapped volume 66 is exposed to the inlet port 24 through 
the deep end 96 of the second metering groove 94 so that the fluid 
pressure in the second trapped volume is substantially equal to the low 
fluid pressure in the inlet port. Conversely, the first trapped volume 64 
is exposed to the discharge port 30 through the deep end 90 of the first 
metering groove 88 so that the fluid pressure in the first trapped volume 
is substantially equal to the high fluid pressure in the discharge port. 
In that circumstance, the net pressure force on the driving gear 32 in the 
gap G between the inlet and the discharge ports is attributable 
substantially completely to the fluid in the first trapped volume 64 and 
is thus proportional to the high fluid pressure in the first trapped 
volume. 
As the driving and the driven gears 32,48 rotate counterclockwise from 
their relative positions depicted in FIG. 5 to their relative positions 
depicted in FIG. 8A, the first trapped volume 64 overlaps the second end 
98 of the second metering groove 94 and thus becomes exposed to the inlet 
port 24 through a variable orifice 100 defined between the second metering 
groove and the end wall 52A of the driven gear which orifice is relatively 
small and, therefore, highly restricting. The first trapped volume 64 also 
remains exposed to the discharge port through a variable orifice 102 
defined between the first metering groove 88 and the end wall 36A of the 
driving gear which orifice is relatively large and, therefore, 
non-restricting. The metering grooves 88,94 thus define a flow path 
between the discharge and the inlet ports through the first trapped volume 
64 having the variable orifices 100,102 in series connection therein. 
As the first trapped volume traverses the gap G from its position depicted 
in FIG. 5 to its position depicted in FIG. 8A, an open volume 104 behind 
the first trapped volume communicates with the discharge port and overlaps 
the gap G. The fluid pressure in the open volume 104 equals the high fluid 
pressure in the discharge port 30 and exerts a pressure force on the 
driving gear 32 in the gap G. The series connected variable orifices 
100,102 in the aforesaid flow path through the first trapped volume 64 are 
calibrated to reduce the pressure force on the driving gear attributable 
to fluid in the first trapped volume at the same rate that the pressure 
force on the driving gear attributable to fluid in the open volume 104 
increases. Accordingly, the net pressure force on the driving gear in the 
gap G remains substantially constant. 
As the driving and the driven gears rotate further counterclockwise from 
their relative positions depicted in FIG. 8A to their relative positions 
depicted in FIGS. 8B-8C, the flow area of the variable orifice 100 
increases and the flow area of the variable orifice 102 decreases and the 
fluid pressure in the first trapped volume 64 decreases smoothly toward 
the low fluid pressure prevailing at the inlet port. At the same time, the 
open volume 104 further overlaps the gap G and the pressure force on the 
driving gear attributable to fluid therein at the high fluid pressure 
prevailing in the discharge increases. Again, the series connected 
orifices 100,102 in the aforesaid flow path through the first trapped 
volume 64 are calibrated to reduce the pressure force on the driving gear 
attributable to fluid in the first trapped volume at the same rate that 
the pressure force on the driving gear attributable to fluid in the open 
volume 104 increases. The result is that the net pressure force on the 
driving gear in the gap G remains substantially constant as a succession 
of trapped volumes traverse the gap G so that pressure pulses 
characteristic of prior crescent pumps and attributable to abrupt exposure 
of such trapped volumes to the inlet port do not occur. The crescent pump 
10 according to this invention, therefore, operates with less vibration 
than prior crescent pumps.