Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02229056 1998-02-09
WO 97/07337 PCT/CA95/00481
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Description
OUTLET PRESSURE CONTROL FOR INTERNAL GEAR PUMP
Field of Invention
'1'llie irncsor~t~~,~ ~olnten t~ .~ _ r__ _ _~_m_
la4J 111Y\r11L1V11 W.1CLLGJ lV gerotor puiiips iclr pumping incompressure
fluids, and in particular relates to pumps having a feedback sensing device
and
more than one outlet port. The feedback sensing device is used to reduce
outlet flow as outlet back pressure increases, facilitating a reduction of
input
power requirements at non-critical operating conditions.
Background of the Invention
Gerotor pumps have been known for many years. In general a
machined lobate, eccentrically mounted rotor element interacts with a mating
machined, lobate driven member and a chamber having a circular cross
section. In a typical constant displacement liquid pump, the eccentrically
mounted rotor element having n lobes cooperates with a surrounding lobate
ring gear having n+ 1 lobes, itself contained within a close fitting
cylindrical
enclosure. Such constant displacement pumps are often used with non-
compressible fluids, such as water or hydraulic oil.
In automotive use, it may be desirable to operate a fluid pump with a
drive motor whose speed varies independently of the output flow requirement.
The outlet pressure such a pump creates may be excessive depending on the
nature of the flow demanded. For example, if motor speed, and therefore flow
output is high, and the downstream requirement is low, it is desirable to
divert
flow back to the pump inlet to avoid excessive pressure in the system.
The design point for these pumps is usually determined by the flow
' 25 rate and pressure developed at an idle speed under maximum temperature
conditions. The pump may be driven directly from the main drive shaft, and
will have an operating speed the same as, or directly proportional to, the
engine generally. In these circumstances idle may correspond to a speed in the
range of 1000 r.p.m. or less, and average speed may be in the 3000 to 4000
SUBSTITUTE SHEET (RULE 26)
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r.p.m. range. The same pump operating at very high speed, perhaps 7000 to
8000 r.p.m., will pump far more oil than is required, and may be capable of
producing far greater pressures than necessary. In that case the majority of
the ,
oil will be directed back to the inlet. Traditionally a pressure relief valve
is
used to 'dump' the excess back to a sump.
Another difficulty with such pumps is that the outlet pressure against
which such a pump operates may vary depending on the nature of the flow
demanded, and on the viscosity of the oil. For example, if the downstream
load is closed, it is desirable to divert flow back to the pump inlet. As
noted
above, pressure relief valves are often used for this purpose. In a second
example, under cold starting conditions hydraulic fluid may need to circulate
for some time before reaching a steady operating temperature and moderate
viscosity. A pump of sufficient size and power to run at full flow and
pressure
under these higher viscosity conditions may be significantly undersized
relative
to the normal operating conditions to which it will be exposed. In either case
a pump which continues to work at full flow at the relief pressure is wasting
a maximum amount of energy.
Whether the excess pressure is produced because the engine is running
faster or because the fluid is cold, in all cases, rather than pressurizing
fluid
merely to force it through a pressure relief valve, it would be desirable to
direct that fluid without pressurization to a bypass. If resistance in the
bypass
is small relative to the pressure relief, the potential energy saving is
roughly
equal to the flow re-directed multiplied by the difference between relief
pressure and intake pressure.
A number of earlier inventors have described devices in the general
field of this invention. U.S. 3,175,800 to Donner et al., discloses a pressure
responsive spool multistage spool valve, but does not alter the fluid supplied
to the system.
U.S. 2,446,730 to Wemp discloses a gerotor pump which works in co-
operation with a spool valve to provide pressure relief on a pressure schedule
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varJ~i.n5 with ambient temperature, but the relieF valve otk~erwise operates
in
the conventional manner.
U.S. 3,?24,662 to Oldenburg presents a two phase, or vapour cycle air
conditioning system in which a radially sliding vane puzrap is used to
compress
gas emanating from a law pressure evaporator. In~plicity, Oldenburg prevents
liquid phase refrigerant from entering the compressor, and thereby causing
damage, by providing an input pressure sensing si~.al to a reciprocating spool
valve, causing the compressor to unluad, or idle, when full cooling demand
is not present.
U'.S. >,33$,161 to Eley discuses a tivo lobed pump with slidibg spool
valve which may alterxxately direct hydraulic fluid to a load or to the pump
inlet through a.rl internal bypass passage, but the use of the spool valve is
controlled by an operator and is inu:nded to operate as an 'C?n'-'Off'
control,
in effect.
1~ De-~-3 913 :~14 illusu-dtes a pump having a rotor, stator and folL4wer
while DF-r1-3 8?4 398 shows a aerotor pump.
Furthermore, U.S. Patent No. 3,057,689 and x.,597,718 illustrate other
variations of pumps.
None of these eaxlier inventions pxovides a cnnstant d15p1aCemerit pump
which is self regulating in response to discharge pressure as in the presezrt
j~vcntl0~-
Sumnlar~ of the Invention
The pzesent invention concerns a positive displacemczzt pump for
pumping a fluid from an intake condition to a disehar;e condition, tktat pump
comprising a rotor, stator, and follower set haviua at least one inJ.et and at
le:~st two outlets; a valve controlling at least ox~e of those outlets, that
valve
sequetltially movable among at least (a) a full flow position, (b) x partial
flow
positions and fc) a pressure relief position; and that valve bei~ responsive
to
the discharge conditiozt.
3p In another aspect of the invention, the pump outlets include at least a
first ouClet az~d a last outlet; the pump comprises an inta.i~e, a discharge
and
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a bypass passage; the bypass passage, intake, and inlet are in mutual fluid
communication; the last outlet is in fluid communication with the discharge;
the first outlet gives onto the valve; the valve comprises at least two
exhaust ,
ports, a first of them communicating with the discharge and a second one
communicating with the bypass passage, and the bypass passage being in fluid '
communication with the intake; in a full flow position of the valve the first
exhaust port being open and the second exhaust port being closed; and in
partial flow and pressure relief positions of the valve the firsf exhaust port
being closed and the second exhaust port being open.
In another aspect of the invention the valve comprises a pressure relief
valve having a pressure relief port in fluid communication with the bypass
passage; the last outlet is in fluid communication with the pressure relief
valve
and with the discharge; the pressure relief port is closed in the full and
partial
flow positions, and open in the pressure relief position, whereby in the
pressure relief position the pressure relief port permits fluid to flow from
the
last outlet to the bypass passage.
In a further aspect of the invention the stator comprises a cavity having
a cylindrical wall for containing the rotor and follower; the rotor is mounted
eccentrically relative to the cylindrical wall; and the outlets are radial
outlets
disposed in the cylindrical wall whereby fluid departing the rotor, stator and
follower set traverses that wall.
In one embodiment of the present invention there is disclosed a gerotor
pump in which the rotor is an inner gerotor; the follower is an outer gerotor
having a number of lobate teeth that is one greater than the number of lobate
teeth of the inner gerotor, the follower having an equal number of tooth roots
therebetween; those inner and outer gerotors engage to create a series of
variable geometry chambers therebetween; the stator comprises a cylindrical
surface for containing the rotor and follower, the cylindrical surface
comprising at least two outlets; the follower has a mating cylindrical surface
for sliding engagement within the cylindrical surface of the stator; the
follower
comprises radial ports, one disposed in each root, whereby during rotation of
the follower within the stator the radial ports periodically and sequentially
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communicate with the outlets; the pump has an operating cycle comprising an
intake cycle and an exhaust cycle; in the exhaust cycle fluid is expelled from
each of the chambers in succession through those radial ports; the exhaust
cycle comprises a pressurizing portion; in the full flow position the
5 pressurizing portion comprises that portion of the exhaust cycle in which
each
radial port is in fluid communication with any outlet; in the partial flow
position the exhaust cycle comprises a bypass portion in which any one radial
port is in fluid communication with the first outlet; and in that partial flow
position the pressurizing portion comprises that portion of the exhaust cycle
in which each radial port is in fluid communication with the balance of the
outlet ports.
Alternatively, the gerotor pump may be constructed in an embodiment
in which the rotor, stator, and follower set has at least three outlets; the
valve
controls at least two outlets; and the valve is movable among (a) a first,
fully
open position (b) a second, high reduced flow position (c) a third, low
reduced
flow position and (d) a fourth, pressure relief position.
It is another aspect of this invention to provide a positive displacement
pump for pumping a fluid from an intake condition to a discharge condition,
said pump comprising: a rotor, stator, and follower set having at least one
inlet and at least two outlets; a valve controlling at least one of said
outlets,
said valve sequentially movable among at least (a) a full flow position, (b) a
partial flow position, and (c) a pressure relief position; an intake, a
discharge,
and a bypass passage; said valve sensible to pressure at said discharge; said
valve tending to move from said full flow position as said discharge pressure
increases; said outlets include at least a first outlet and a last outlet;
said
bypass passage, said intake, and said inlet are in mutual fluid communication;
said last outlet is in fluid communication with said discharge; said first
outlet
communicates with said valve; said valve comprises at least two exhaust ports,
a first of said exhaust ports communicating with said discharge and a second
of said exhaust ports communicating with said bypass passage, and said bypass
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passage in fluid communication with said intake; in said full flow position of
said valve said first exhaust port being open and said second exhaust port
being closed; and in said partial flow and pressure relief positions of said
valve said first exhaust port being closed and said second exhaust port being
open.
Another aspect of this invention resides in a positive displacement
pump for pumping a fluid from an intake condition to a discharge condition,
said pump comprising: a rotor, stator, and follower set having at least one
inlet and at least two outlets; a valve controlling at least one of said
outlets,
said valve sequentially movable among at least (a) a full flow position, (b) a
partial flow position, and (c) a pressure relief position; said valve
responsive
to said discharge condition, said stator comprises a cavity having a
cylindrical
wall for containing said rotor and said follower; said rotor is mounted
eccentrically relative to said cylindrical wall; and said outlets are disposed
in
said cylindrical wall whereby fluid departing said rotor, stator and follower
set traverses said wall.
A further aspect of this invention relates to a positive displacement
pump for pumping a fluid from an intake condition to a discharge condition,
said pump comprising: a rotor, stator, and follower set having at least one
inlet and at least three outlets; a valve controlling at least two of said
outlets,
said valve is movable among (a) first fully full flow position, (b) a second
high reduced flow position, (c) a third, low reduced flow position; and (d) a
fourth pressure relief position; said valve responsive to said discharge
condition.
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Brief Description of Drawings
Figure 1 is an horizontal cross section of the gerotor pump of the
present invention, and comprises four sequential Figures la, 1b, lc, and 1d.
5 Figure 2 is an horizontal cross section of the gerotor pump of the
present invention taken in a plane parallel to and above that of Figure 1
showing the geometry of the intake port and internal bypass flow passages.
Figure 3 is a partial vertical cross section showing the relationship of
the cross sections shown in Figures 1 and 2. Figure 1 is taken on section 'X-
X' and Figure 2 is taken on section 'Y-Y'
Figure 4 shows a longitudinal section of a spool valve of the gerotor
pump of Figure 2 taken on section 'Z-Z' ald includes four sequential Figures
4a, 4b, 4c, and 4d corresponding to Figures la, 1b, lc, and 1d.
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Detailed Description of the Best Mode for Carrvin~ Out the Invention
Commencing with Figure 1, a gerotor pump is shown generally as 2. ,
This gerotor pump is one example, or species, of constant displacement,
rotating pump having variable geometry chambers. The cross section of Figure
1 is taken in a plane perpendicular to the axis of a drive shaft 3 by which
the
pump is driven. Drive shaft 3 transmits torque by a keyway or any mechanical
equivalent to a keyway, and might include flats, as shown in Figure 1, or
splines. Pump 2 comprises a main inlet 4, a stator, or casing 6, an inner
gerotor, or keyed lobate rotor 8, a correspondingly lobate outer gerotor or
follower ring 10, a bypass passage, or return passage 12 cast into casing 6,
a spool valve 14, a discharge I6 and an intake 18.
In the illustrated embodiment rotor 8 comprises eight lobate teeth 20
disposed for co-operation with the nine inwardly oriented lobate teeth 22 of
follower ring 10, as is well known in the art. Casing 6 comprises a circular
cylindrical surface 24 for close tolerance, sliding engagement with a mating
external cylindrical face 26 of follower ring 10, and a perpendicularly planar
face 28 upon which follower ring 10 may slide and rotate. For clarity the
matching, upper perpendicular, opposed face 29 has not been shown in Figure
1. Cylindrical surface 24 is eccentrically disposed relative to shaft 3 to
which
rotor 8 is mounted. Those skilled in the art will recognize that although a
rotor, stator, and follower set of eight and nine teeth has been described
mechanisms of this kind generally comprise a first gear of a number of teeth
'n', and a second gear of one more teeth, 'n+1', and which maintain line
contact between the lobes of the rotor and follower. The minimum number of
teeth will be determined by the number of outlets chosen. Due to the eccentric
nature of the mounting a series of chambers 30 is formed between the opposed
faces, . the lobate surfaces of rotor 8 and the lobate surfaces of follower
ring
10. These chambers approach zero volume at the closest point, or perihelion,
of cylindrical surface 24 to shaft 3, and reach their maximum volume at the
farthest point, or aphelion, therefrom.
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As illustrated, there is a radial port 34 radially traversing follower ring
at each root section intermediate two adjacent lobate teeth 22. It is more
common in gerotor pumps for such ports to be located in the out-of-plane
direction, that is to say for example, in planar face 28, or 29. As shown in
5 Figure 2, lower and upper perpendicular faces 28 and 29 comprise just such
an intake port 35, which has a bifurcated arcuate shape subtending roughly
165 degrees of arc to permit inflow into chambers 30 over roughly 180
degrees of rotation. Since, as shown in figure 3, that portion of the depth of
intake port 35 below planar face 28 is greater than that portion above face
29,
10 the majority of oil will enter chambers 30 from below. Casing 6 also
comprises an inlet 36 and exhaust outlets 37, 38, and 39. Each of these
outlets
is disposed to align periodically with radial ports 34 such that fluid may
exit
corresponding chambers 30. Outlets 37 and 38 are separated by a first land 40
and outlets 38 and 39 are separated by a second land 41. Radial ports 34 will
be blocked during that period of each cycle when sweeping past closed portion
42 of surface 24 between outlet 39 and inlet 36 and again when sweeping past
portion 43 between inlet 36 and outlet 37.
Gerotor pumps, or rotating vane pumps with variable geometry
chambers have an operating cycle that may be divided into an intake cycle and
an exhaust cycle. Considering one particular chamber 30, for example, the
intake cycle commences when chamber 30 passes the aphelia) point of the
eccentric, at which chamber 30 has its minimum volume, approaching nil. As
the pump continues to turn, counter-clockwise in the figures, chamber 30
expands, drawing in fluid through intake port 35. At the perihelia) extremity,
the intake cycle ends when the trailing edge of chamber 30 loses contact with
intake port 35. Chamber 30 is at its maximum volume.
The exhaust cycle commences just as the leading edge of radial port
34 exposes the first edge of outlet 37, and continues until the trailing edge
of
radial port 34 clears the last edge of outlet 39, at which time chamber 30 is
again reduced to its minimum, nearly nil, volume at the aphelion.
In the present invention the exhaust cycle may variously include both
a first, bypass portion, and a second, pressurizing portion. If valve 14 is in
.w... w . ;.; . . , , ._r; .m -. , .,. . m ..... . _ m.r ,a:1 _:>;r:1! 4v..~.
.: I I
CA 02229056 1998-02-09
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the full flow position thsre will be no bypass portion and the pressurizing
portion will occupy the entire o:~haust cycle. In that case any ditninutian in
the
size of chamber 30 will e~cpel the full flow of working fluid against the
prevailing discharge pressure.
By contrast, iri partial flow positions such as the high reduced r~low
position and the low reduced flow positions described below, the first portion
of, the exhaust cycle will expel fluid frotxt chamber 30 through radial port
34,
then through an outlet, such as outlat 37, to valve I4, manifold 92 and
passas~e ~2 whence it returns to intake 1.8. This bypass portiozz is followed
by
a pr~ssLwiziztg portion corresponding to that part of the exhaust cycle in
which
chamber 30 expels fluid through the balaxice of outlets, such as outlet 39,
which are i~a fluid commuztication with discharge 16, and hence sensible to
that higher discharge pressure.
E.~thaust outlets 3'7, 38, and 39 each converge toward a corresponding
13 throat, 45, 46, and 47, the first two of which give onto or communicates
with
spt~ol valve 14. As is best seen iii Figures 1 and 4, spool valve 14 comprises
a hollow cylinder 48 machined into casizlg 6 and a mufti-chamber bobbin 54
disposed for close fitting slidable motion tberealong. Iz~ the embodiment
described herein bobbin 50 h.as two waists, S2 and 54, although the present
invention could be practised with a larger number of waists as may be found
convenient. In general spool v aloes of this kind have square shouldered
waists,
or rebates, although they need not have, provided a flow passageway is
created between the cylinder wall and the hollowed out waist portion of. the
bobbin. In the illustrated euzbodiment t~ohhin Sq has three pistons, indicated
?S in figure 4a as 60a, 60b, and 60c. Bobbin 5(9 is hollow. A retu.rzi. spring
6~
has a first end disposed within bobbin 50 and a second end captured by end
cap b4, which also serves to close off and seal the otherwise ope>!i end of
cylinder 48. A hollow abutting shoulder 66 limits travel of bobbix~ SO away
fromi end cap 64 and ensures that face 6$ of piston title is exposed to the
static
pressure prevailing in that portion of casi~; 6 contig,.~ous with a passage
'70
leading fYOm throat 47. Face 68 is thus sensible to the prevailing discharge
pressure, in this way face 6$ performs the functions of both a position
control sensing
AMENDED SHEET
1\L v 1 w . 1 1 \ .11 : . w1 . . r .. . . . .. . - ,
r I .l . ).1 _ , n:1': ~ 1 n. o . . _1 n
1 CA 02229056 1998-02-09
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device responsive to the discharge condition of the fluid, in this case
responsive to the discharge pressure, and as transducer Gvhich converts that
sensed pressure into a mechanical signal, or mechanical motion to move the
spool valve 14 away from its fully opened position as pressure increases, Any
number of electromechanical or hydraulic devices aa~d linl~ages would serve
this purpose. In the preferred embodiment use of the last piston 60e in this
way permits the sensing and transducing functions to he performed with a
very szz~all number of parts - a piston and piston facE - which are directly
connected, are directly in line with, and form an inseparable Bart o;f, bobbin
54 of spool valve 14.
As seen in Figures 1 and 2, return passage 12 has been formed in
casing 6 for carrying fluid from a passage, or manifold 9'2, generally
disposed
above cylinder 4S, to the intake side of the pump generally, and to the
vicinity
of inlet 36 in particuiaz. As seetz iz~ Fro xes 1 and ~. cylinder 48
intersects, and
is in fluid communication with throat .t5 a.nd throat =16. cylinder 4$ also
intersects apertures 82, and 84 through which fluid may under certain
conditions flow to discharge i5. Finally, cylinder 48 intersects three by pass
pons 86, $i3. and 90, which give unto or communicates ~cvith manifold 92.
Antechamber $0 of valve 14 adjacent cap 64 is vented to passage 92 as well
to prevent oil ~roztx beiz~D trapped behind bobbin 5Q.
The series of drawings of Figures la, Ib. lc, and 1d, showing
sequential positions of bobbin SO further helps to exglain the action of spool
valve 14. Figure la, illustrates a first, full flow position in which the
pressure
at discharge l.fi is reiativeiy Io.v, either because the motor driving the
pump
2~ is only turning slowly, or because there is little downstream flow
resistance.
Under these conditions the fall flow of fluid expelled from. any chamber 30
is directed to discharge 1b and none is directed to passage x2. For example,
this may be any condition up to a given discharge pt~essure, perhaps 50 psig.
Piston 60c remains seated agairxst hcxilow shoulder C6. Throats 45 and 4b are
open to cylirLdcr ~i8 and ports $~ and !34 permit fluid to flow across the
spate
provided by avaists 52 and 54 and exit to discharge 16. Passage 70 is is
AMENDED SNEET
CA 02229056 1998-02-09
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unimpeded fluid comzx~unication with discharge 16. Ports $6, $$ axtd 9fl are
closed off by patoz~s 60a, 6flb and 6dc.
As the pump speed, outflow, and resistance in the Load increases the
st~'TLTC pressuze sensed at face 6$ of piston 60c also increases, eventually
lifting
face 6$ off shoulder 66 azxd moving piston 60b to occlude exit port $2 as
shown in fib re 1b. This rttay be desigztated as a partial flout. or high
reduced
tow position. This is a partial flow position because only part of the flow is
directed to di.scbarge x6 while another part is directed to passage 1Z. Just
as
r:.~ciL port $? is closed, first bypass zetuzn port 86 opens, permitting fluid
to
t~Iow upwardly into, and along manifold 9~ in fluid communication with
bypass or return passage 1z. The pzessnre of the fluid disclaazged along this
path is only greater than the pump inlet pressure, or relative vacuum, by an
amount determined by the fluid resistanrre in those passages. This amount is
small relative to load pressures. The world required to move fluid through
return passage 1. is correspondingly small. Land 40 serves to segregate this
u.nprossurized llow front the higher pressure required to force hydraulic
fluid
out discharge 16. For example, if the first discharge pressure were aS above,
the exit port 82 would close at 50 prig.
As the static pressure sensed at discharge ~.6, and :uence in a cavity 69,
increases yet further, bobbin 50 will be displaced further toward end cap 64.
Eventually, as shown in Figure lc corresponding to another partial flow, or
low reduced flow pOSltioll, piston 6(lC will occlude exit port 84 and open
return port S8, causing more flow to be directed to return arid less to flow
to
the load. In this instance land 41 segregates unpressurizcd fluid from
2S pressurized discharge through port ~~ and throat 43. For example, if the
first
discharge pressure is arbitrarily set at 50 psiff, this condition may be
reached
when the discharge pressure is perhaps app~roximat~ly 6D psig.
Finally, as shown in fiatlre 1 d, there is a pressure relief position in
which the dis::harge pressure is so high that ~bobi~in Sfl has been displaced
far
3d enough far piston 6flc to uncover port 40, which is. in effect, a high
pressure
relief valve, fort 90 is ozzly partially uncovered Le.~.ving a slit, or
orifice, such
that the pz-essure drop across the orifice concesponds to soixze pressure
greater
AMENDED SHEET
.,m. ". . .., w,n . .wm ~~~ _., ,. .~. . __z ,.i - _
ri:~ ,;:! _:~:l:U 1~,...., t=
q CA 02229056 1998-02-09
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than the relief pressure. Consistently with the above example, this pressure
relief might occur at 70 prig. These example values are arbitrarily chosen,
and
are merely intended to illustrate that at each stage the disch:~rge pressure
is
increasing. In the pressure relief mode the worlt done to compress the fluid
to the relief pressure is lost, but this amount is less than the full flow by
the
amount vented through ports 86 and 88. The continued circulating flow
tl~x-ough pump 2 also discourages ove-cheating when the dischaitge is shut
off.
In general the present invention may be extended to a variable
geometry charnber. constant displacement pump having a plurality of outlets
giving or communicates sequentially onto a suitable valve. Qf those outlets at
least one, the last, corresponding to outlet 39, is in fluid communication
with
discharge J.6. For tech outlet giving anw or communicates with the valve, in
rhis case spon3 valve 14, there are two exhaust pons. For example, in the
preferred embodiment outlet 37 corresponds to a first exhaust part, aperture
$2, which leads to discharge Ifi, anti a second eRhaust port, bypass port
lift,
which leads to the bypass, or return passage 13. Similarly outlet 38
cox~cspQnds to aperture 84 and bypsss port 88. The valve 14, and more
particularly bobbin 5~, reciprocates sequentially between the Full flow and
pressure relieT positrons as pressure ixrcreases or dec;zeases, traversing
intermediate positions in order.
As noted, spool valve 14 is self actuating, responding to load
conditions at the outlet_ although flow t)arough the pu~onp may increase in
absolute terms as the mntor driving the puna.p turns more cptickly, the flow
cii.splaced per revolution decreases. In dlis sense the flow is reduced.
relative
2S to the fully open flow that would otherwise occur at that rate of
revolution.
The spool valve cuts back;. the flow as the outlet pressure increases, that.
is to
Say, as the pump becomes more heavily loaded or as it is driven more rapidly
by, for cxarn,ple, an acr..eleraling motor. It permits ma:zimum flew per
revolution when the pump is unloaded, or if the rotor is being drivetx at a
lower speed, such as idle.
The principles of the present invention may be Practised, with suitable
modifications, with a gerotor pump of any chosen number or lobes which
satisfy the condition that the spool have at least three .regimes, the first
being
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a fully open flow, the second a partially open flow, and the third a pressure
relief flow. Similarly the principles of the present invention may also be
practised with reciprocating vane pumps, the efficacy thereof depending on the
quality of the seals.
Although the illustrative embodiment of the present invention herein
is described with reference to the accompanying drawings, it is understood
that the invention is not limited to that precise embodiment and that various
changes and modifications may be effected therein by those skilled in the art
without departing from the scope, substance, or spirit of the invention.
