U.S. patent number 3,781,145 [Application Number 05/252,006] was granted by the patent office on 1973-12-25 for vane pump with pressure ramp tracking assist.
This patent grant is currently assigned to Abex Corporation. Invention is credited to Jack W. Wilcox.
United States Patent |
3,781,145 |
Wilcox |
December 25, 1973 |
VANE PUMP WITH PRESSURE RAMP TRACKING ASSIST
Abstract
Means for preventing vane "skip" or seperation from the cam
surface of a vane pump, at the inner end of the pressure ramp on
the cam surface. Flow restricting orifice means are provided for
the respective vanes, such that the inward movement of the vane in
its rotor slot in the pressure zone causes fluid to be displaced
from the inner end of the vane slot through the orifice means,
thereby establishing a pressure differential force beneath the vane
which resists such inward vane movement. The increased pressure at
the inner end of the vane slot is limited by partial release to the
pressure port through an opening in the cheek plate, which
communicates with the inner ends of the vane slots as the vanes are
passing through the pressure zone at those vane positions at which
the tracking assist is not advantageous.
Inventors: |
Wilcox; Jack W. (Columbus,
OH) |
Assignee: |
Abex Corporation (New York,
NY)
|
Family
ID: |
22954248 |
Appl.
No.: |
05/252,006 |
Filed: |
May 10, 1972 |
Current U.S.
Class: |
418/1; 418/82;
418/268 |
Current CPC
Class: |
F01C
21/0863 (20130101) |
Current International
Class: |
F01C
21/08 (20060101); F01C 21/00 (20060101); F04c
017/00 () |
Field of
Search: |
;418/79,81,268,1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Freeh; William L.
Claims
I claim:
1. A method of peventing vane skip in the operation of a vane pump
where the vanes are moving across the inner end of a pressure ramp
therein, said method comprising,
displacing fluid from the inner end of the vane slot by inward
movement of the vane on the pressure ramp,
directing the displaced fluid through a flow restricting orifice
thereby to establish a pressure differential across said
orifice,
applying the higher pressure across said orifice to an inwardly
facing surface of the vane thereby to oppose the inward vane
movement,
and limiting the higher pressure on said inwardly facing surface
prior to the position at which the respective vane comes onto the
inner end of the pressure ramp.
2. The method of claim 1 wherein the said limiting of pressure is
timed to occur while the respective vane is traversing the central
portion of the pressure ramp.
3. The method of claim 1 wherein said higher pressure is applied to
the inner end of the respective vane.
4. In a vane type hydraulic pump having rotor and stator members,
vanes mounted in vane slots in one member for engaging a cam
surface presented by the other member, the cam surface including a
pressure ramp in a pressure zone of the pump which ramp cams the
vanes into their respective slots as they move relatively
thereover,
the improvement comprising,
flow restricting means for each said vane through which fluid in
the inner end of the slot is displaced when the vane is moved into
its slot, such displacement establishing a pressure differential
force on an inwardly facing surface of the vane opposing the inward
vane movement,
and a relief port communicating with said inwardly facing surface
for controlling the pressure thereon during movement of the vane
along the pressure ramp,
said relief port positioned to communicate with the inner end of a
vane slot only when the respective vane is traversing a central
portion of the pressure ramp.
5. The improvement of claim 1 wherein said relief port communicates
with a pressure port of the pump.
6. The improvement of claim 5 wherein said relief port is in the
form of an aperture in a cheek plate and has a diameter smaller
than that of the inner end of a vane slot.
7. The improvement of claim 1 wherein said flow restricting means
comprises a passage extending through the vane between the inner
and outer ends thereof.
8. The improvement of claim 7 wherein said passage includes a sharp
orifice which is defined by a bore that meets an end surface of the
vane at substantially a right angle.
9. In a hydraulic vane pump having a rotor carrying vanes that are
slidable in vane slots therein, a stator presenting a cam surface
engaged by the outer ends of the vanes as the rotor rotates, the
cam surface including a pressure ramp in a pressure zone of the
pump which ramp cams the vanes inwardly in their respective slots
as they traverse it, and a cheek plate presenting a pressure port
opening to the space between the rotor and cam surface in the
pressure zone,
the improvement comprising,
flow restricting means in each vane providing a path for restricted
flow of fluid in an outward direction to the outer end of the vane
from an inwardly facing surface thereof, said flow restricting
means being sized so that flow therethrough resulting from inward
vane movement on the pressure ramp will establish a pessure
differential force on said inwardly facing surface,
and a relief port in said cheek plate, said relief port connected
to said pressure port and positioned to communicate with the said
inwardly facing surface where the inward velocity of the vane is at
its highest value, but not when the vane is at the inner end of the
pressure ramp.
10. The improvement of claim 9 where said relief port communicates
with said inwardly facing surfae where the respective vane is
moving on the central part of the pressure ramp.
11. The improvement of claim 9 wherein said inwardly facing surface
is the inner end of the vane.
12. The improvement of claim 11 wherein said relief port is
positioned to communicate with the inner end of a vane slot.
13. The improvement of claim 9 wherein the flow restricting means
is at least one restricted passage extending radially through the
vane, between the inner and outer ends thereof.
14. The improvement of claim 13 wherein the passage includes a
sharp, short orifice.
15. A vane for use in a hydraulic pump of the type having a rotor
carrying vanes that are slidable in vane slots therein, a stator
presenting a cam surface engaged by the outer ends of the vanes as
the rotor turns, the cam surface including a ramp in a pressure
zone of the pump which ramp cams each vane inwardly in its
respective slot as it traverses the ramp, and a cheek plate
presenting a pressure port opening to the space between the rotor
and cam surface in the pressure zone.
said vane having flow restricting means associated with it
providing a path for restricted flow of fluid between the inner and
outer ends of the vane, said flow restricting means being sized so
that flow therethrough resulting from inward vane movement on the
said ramp will establish a pressure differential force beneath the
vane of magnitude sufficient to prevent skip at the inner end of
the pressure ramp,
said flow restricting means being provided in the form of a passage
including a sharp, short orifice, flow across which establishes the
pressure drop, and a larger diameter passage portion connecting
said sharp orifice to an opposite edge of the vane.
16. The vane of claim 15 wherein said passage is a radial passage
in the vane, and said orifice is adjacent the inner edge of the
vane, and meets said inner edge at a right angle.
Description
This invention relates to an improvement in hydraulic pumps of the
type having rotor and stator members, one of which mounts vanes
that engage a cam surface presented by the other member. More
particularly, the invention is directed to means for maintaining
contact of the vanes with the cam surface in the pressure zone of
the pump.
In the most common type of commercial vane pump, the vanes are
mounted by the rotor for inward and outward movement in vane slots
relative to the axis of the rotor rotation. The tips of the vanes
slide over or "track" on the cam surface of the stator that
encircles the rotor. The cam surface is contoured, or shaped, so
that the spacing between it and the rotor periphery varies around
the rotor. This so-called pumping space, bounded by the rotor
surface, the cam surface, and cheek or port plates on each side of
the rotor, is generally regarded as comprising four zones: the
pressure zone, the suction zone, a transfer zone (which lies
between the suction zone and the pressure zone in the direction of
rotation), and a sealing zone (which lies between the pressure zone
and the suction zone in the direction of rotation). The variations
in cam surface-rotor spacing in these zones cause radial vane
movement and consequent changes in the volume of the intervane
space or transfer pocket between each pair of vanes.
Each pocket increases in volume in the suction zone and diminishes
in volume in the pressure zone. The pocket receives fluid through
the suction port as it traverses the suction zone, and transfers
the fluid by moving it from the suction zone across the transfer
zone to the pressure zone. As the volume of the pocket is reduced
in the pressure zone, fluid is expelled from the pocket to a
pressure port. The pocket volume decrease in the pressure zone
occurs because the cam surface approaches the rotor surface more
closely there, and thereby pushes the vane back into its slot. The
projection of the vane from the rotor is thus diminished, and the
corresponding reduction of pocket volume results in the positive
displacement of fluid therein to the outlet or pressure port.
The approach of the cam surface toward the rotor surface across the
pressure zone is called the pressure ramp, the term "ramp" being
used to designate an inclined surface, as opposed to an arc of
fixed or constant diameter about the rotor axis.
By reason of the fact that the pressure ramp is an inward ramp, it
positively displaces the vanes inwardly. At the start of the
pressure ramp where the inward vane movement begins, a good contact
of the vane tip with the cam surface is obtained, and the vane does
not separate from the cam surface. However, vane separation does
sometimes occur adjacent the inner end of the pressure ramp, near
the area where the ramp blends into the so-called minor diameter
portion of the cam surface in the sealing zone. This separation, or
"skip," can be detected as a "bare" space on the cam surface,
followed by "chop marks" which are wear or impact dents on the cam
surface. These can be seen by the naked eye in severe cases, and
under magnification in other instances. The chop marks occur where
the vanes, after separation from the cam surface, hit the surface
as they regain contact with it, where the pressure ramp flattens
out. Such vane tip-cam surface separation presents opportunity for
escape of high pressure fluid if the vane skips beyond the sealing
point on the minor diameter portion. Moreover, when the vane
regains contact with the cam surface there is an impact with
resulting wear and noise. The phenomenon occurs at virtually all
speeds of operation and is not pressure dependent.
It is believed that pressure ramp skip occurs as a result of inward
vane momentum that causes the vane to continue to move inwardly,
even where the angulation of the ramp is diminishing. In other
words, the skip occurs because the rate of inward vane movement
exceeds the apparent rate at which the cam surface approaches the
rotor, so that the vane momentarily moves away from the ramp.
This invention is predicated upon means for assisting and improving
vane tracking on the pressure ramp, and in particular for ovecoming
the skip at the inner end of the pressure ramp. The invention is
based upon the provision of means for supplying an outwardly
directed extra hydraulic force on the vane at the position where
skipping tends to occur, adjacent the inner end of the pressure
ramp. This force resists and retards the skipping movement, by
blocking or opposing the inward momentum of the vane, so that the
vane does not move inwardly beyond the position to which the cam
surface actually cams it.
The invention also calls for means for controlling the extra
hydraulic force beneath the inner end of the vane, so that the
force acts only transiently beneath each vane in the pressure zone
where it is needed and desired. It can be limited elsewhere, so
that unnecessary vane to cam loading is minimal.
The extra hydraulic force which assists in vane tracking is
developed by the displacement of fluid through a flow restricting
orifice associated with, and preferably in, each vane. The flow is
toward the tip or outer end of the vane, and this flow through the
restrictor establishes a higher pressure on an inwardly facing
surface of the vane, e.g., below the vane in the vane slot, than at
the outer end of the vane. A net outward force is thereby
established. The flow restricting orifice is preferably in the form
of one or more narrow openings extending between the inner and
outer ends of the vane.
Fluid is caused to flow through the restrictor, thereby to
establish the force, by the inward movement of the vane into the
vane slot. When the pressure ramp pushes the vane inwardly in the
slot, trapped fluid in the slot below the vane is displaced
upwardly through the orifice, and thereby creates a higher pressure
at the inner end of the vane than at the outer end. It is this
pressure differential which gives rise to the outward force on the
vane that prevents it from skipping, by slowing the rate of inward
vane travel where the slope on the pressure ramp becomes less
steep.
The orifice can be of various shapes, but it is preferred that it
be a sharp orifice formed by a bore portion which is relatively
short (in axial dimension) and relatively narrow (in diameter) that
connects with a counterbore portion, longer and larger in diameter,
that extends the remainder of the distance through the bore. A
sharp orifice configuration is highly responsive to vane velocity,
and is almost free of the viscosity-related effects that accompany
use of a long bore wherein a major part of the pressure drop is
caused by surface drag.
In an orifice of the preferred type, the pressure drop across the
orifice is proportional to the square of the inward velocity of the
vane, divided by the square of the cross-sectional area of the
orifice: P = k (V/A) .sup.2. The velocity V in this equation is
determined by the shape of the pressure ramp and the rate of rotor
rotation. The force varies as the square of the speed, as shown by
the following illustrative examples:
RPM 2400 1200 600 Force, lbs. 60 15 3.75
The particular area on the cam surface at which vane skip is most
pronounced will depend upon its contour. It usually will be at the
inner end of the pressure ramp. It is at the area where vane
lift-off would otherwise start to occur, that the undervane
hydraulic assist is needed.
Since the tracking assist force is dependent upon vane radial
velocity, the force will be established wherever fluid is displaced
through the orifice by radial vane movement. Such vane movement
occurs across the entire pressure ramp, and hence this force could
act on the vane across the entire pressure ramp, including areas
where it is not needed and would result in extra vane load and
wear.
As previously indicated, the invention contemplates, in addition to
the restriction for creating the differential force, means for
controlling and limiting the excess force at those positions of the
vane on the pressure ramp where the force is not needed for
tracking assist, and where it would be undesirable.
An orifice sized to provide the desired level of force at the inner
end of the pressure ramp, would establish a greater than necessary
force at other parts of the pressure ramp where the vane velocity
is higher but at which the assist is not desired. If not relieved,
this force would peak at a value which would constitute an
excessive, or at least unnecessarily high, loading on the vane, and
would tend unduly to increase vane tip wear.
In order to limit the peak undervane pressure and the resulting
assisting force acting on the vane elsewhere than at the portion of
the ramp where it is needed, the invention provides relief port
means whereby excess undervane pressure is spilled or relieved
where it is not desired, so that an excessively high pressure peak
is avoided. The port is closed at the inner end of the ramp so that
the necessary pressure force is available to act on the vane at
that point at which ramp skipping would otherwise occur.
The relief port is preferably formed in the cheek plate which is at
the front face of the rotor, and it communicates with the inner end
of the vane slot in the region in which excessive or unnecessary
pressure would otherwise exist. The port is connected to the main
pressure or outlet port of the pump. The inner end of each vane
slot is only transiently in communication with the relief port, as
it sweeps by the latter, but this brief communication is sufficient
to limit the excess pressure. It is most desirable that the relief
port be positioned at that angular location (with respect to the
cam surface) where the inward velocity of the vane is the greatest,
since no assist is needed at that point and it is at that point
that the pressure build-up beneath the vane by flow through the
restricting orifice would be the greatest. Since the point of
highest vane radial velocity is generally at the middle of the
pressure ramp, the relief port is desirably centered there. This
central or symmetrical location is especially useful in reversible
pumps, wherein the cam ring can be reversed in the body to
establish an opposite direction of rotor movement.
The invention can best be further described by reference to the
accompanying drawings, which show a preferred embodiment of the
invention as incorporated in a radially balanced vane pump having
vanes of the two-lip type. In the drawings:
FIG. 1 is an axial section of the pump, and is taken on a line
through the opposed pressure zones thereof;
FIG. 2 is a plan view of the front cheek plate or port plate, taken
along line 2--2 of FIG. 1, illustrating the undervane pressure
relief ports, and showing superimposed thereon in dot-dash lines
the momentary positions of rotor vane slots;
FIG. 3 is a side elevation, partly broken away, of a vane of the
type used in the pump shown in FIG. 1;
FIG. 4 is an end elevation of the vane shown in FIG. 3;
FIG. 5 is an enlarged view of a portion of the cam ring and rotor
assembly of the pump, taken along line 5--5 of FIG. 1; and
FIG. 6 is a diagrammatic illustration showing how the undervane
pressure established in accordance with the invention is related to
the angular position of the vane on the pressure ramp, and also
shows in dashed lines the peak or overpressure which could result
in the absence of the undervane pressure relief port.
The invention described and claimed herein is broadly applicable to
vane pumps in which both the inner and outer ends of the vanes are
exposed to fluid pressure, including pumps which have single lip
vanes as well as those which have double lip vanes. The vanes may
be biased by hydraulically operated pistons, or by springs. The
invention is described hereinafter in relation to a balanced pump
having hydraulically operated vanes of the two-lip type, but it
should be understood that this is by way of illustration and not
limitation.
Referring to FIGS. 1-5, the pump shown there as one type of
environment for the invention includes a housing or casing formed
by a body casting 1 having a generally cylindrical internal
chamber, and an end cap 2 having a recessed shoulder 3, which
telescopes into one end of the body and is sealed thereto by an
O-ring 4. The body and end cap are connected by bolts, not shown.
The end wall 5 of cap 2 has an opening through which the pump
operating shaft 6 extends. In cap 2, shaft 6 is supported for
rotation by a bearing 7 which is secured against axial movement in
the opening. A seal 8 prevents the leakage of fluid along shaft 6.
The shaft extends into body 1 from end cap 2, and at its rear or
inner end is carried for rotation by a roller bearing 9 mounted
within a central bore in the body 1.
The end cap 2 supports and is sealed around a front cheek plate 10,
sometimes called a port plate, which has a smooth, flat inner
surface 11 that bears against a side or radial face 13 of an
annular cam ring or stator 14. On its opposite side surface 17, cam
ring 14 bears against a smooth, flat surface 18 of a rear cheek or
port plate 19, and holds the latter against an internal shoulder
(not shown in FIG. 1) in body 1. The cam ring itself, as well as
the housing and cam ring together, are sometimes referred to as a
stator. Cam ring 14 and the front and rear cheek plates 10, 19
respectively, are clamped together by bolts, not shown. Both cheek
plates have central openings through which shaft 6 extends.
A fluid intake passage (not shown in FIG. 1) extends into body 1
and communicates with a pair of annular channels 23, 24 which
encircle the internal cavity within the body. These annular
channels 23, 24 distribute fluid from the intake passageway to the
suction ports in the cheek plates. The cam ring 14 is supported
radially by an annular rib 26 formed in body 1 between the annular
channels 23, 24. The cam ring encircles a rotor 28 which is
connected to and driven by shaft 6 through splines 29. The spline
joint permits proper running alignment of the rotor between the
opposed flat surfaces 11 and 18 of the front and rear cheek plates
10 and 19 respectively. The rotor has a plurality of radial vane
slots 31 (see FIG. 5) in each of which a vane 32 is mounted.
The cam ring 14 has an inwardly facing cam surface 34 that is
contoured to provide a balanced or symmetrical pump construction in
which there are pairs of diametrically opposite low pressure, inlet
or suction zones 37 (see FIG. 5) and high pressure outlets or
exhaust zones, one of which is shown at 38 in FIG. 5. Each vane
engages the cam surface 34 of the cam ring 14, and the side edges
of the vane slide over the smooth flat surfaces 11 and 18 of the
front and back cheek plates on opposite sides of the rotor. The
pairs of adjacent vanes divide the annular pumping space between
the rotor, cam surface, and cheek plates into a series of transfer
pockets or intervane spaces, several of which are designated at
40a, 40b, 40c, and 40d in FIG. 5. The intake passageway
communicates via the annular channels 23, 24 around cam ring 14,
through passages cored in the cheek plates 10 and 19 to paired main
suction ports spaced 180.degree. apart in surfaces 11 and 18
thereof. As seen in FIG. 2, two main suction ports 43 and 44 are
formed in front cheek plate 10, and they are fed through channel
24. Similar main suction ports (not shown in the drawings) are
formed in rear cheek plate 19 and are fed through channel 23. The
main suction ports are aligned with the corresponding suction zones
37 in the pumping space between the rotor periphery 36 and the cam
surface 34. Each main suction port is connected by a branch passage
(not shown), with an undervane suction port in its cheek plate,
such as the port 45 in the front cheek plate 10. The undervane
suction ports 45 are radially positioned so that the inner ends 46
of the vane slots 31 will pass across them as the rotor turns. (In
FIG. 2 the inner ends 46 of the vane slots are superimposed in
phantom on cheek plate surface 11.) The use of undervane suction
ports is conventional in the art, and they are shown in
conventional form in the drawings; however, it should be understood
that the undervane suction ports may be of the specially shaped
type shown in the co-pending patent application of Swain and Adams
titled "Vane Pump Having Extended Undervane Suction Ports," Ser.
No. 255,580, filed May 22, 1972. The undervane suction ports do not
comprise a part of the invention. A shallow drain slot such as that
designated at 47 in FIG. 2 extends radially in the faces 11 and 18
of the respective cheek plates 10 and 19, from the undervane ports
45 thereof, to the central shaft openings. Slot 47 drains fluid
from the cavity around shaft 6.
As shown in FIGS. 1 and 2, the front cheek plate 10 includes two
diametrically opposed main pressure ports 51 and 52. These ports
are generally T-shaped in plan, and they are centered substantially
90.degree. from the main suction ports 43 and 44. They open to the
pressure zones 38 between the rotor and the cam surface, and may be
provided with conventional bleed slots as indicated at 53. The main
pressure ports 51, 52 are connected through internal passages 55,
56 in cheek plate 10 to a fluid outlet or delivery passage 57 in
end cap 2, which in turn leads to a fluid outlet or delivery
coupling (not shown) that in use is connected to an external
hydraulic circuit.
There is associated with each main port 51, 52 in cheek plate 10 a
second or relief pressure port 58. Each port 58 is connected
through a passageway 59 with the respective pressure passage 55 or
56 in the cheek plate. As seen in FIG. 2, the undervane relief
ports 58 may be circular in configuration, and preferably they are
substantially centered with respect to the main pressure ports 51,
52 and the pressure zone 38 of the pump. They are positioned
radially so that their inner edges are in line with the bottoms of
the vane slot inner ends 31. The diameter of each port 56 is
preferably less than that of the rounded inner end of the vane
slot.
The cam ring may be aligned with respect to the two cheek plates by
dowel pins (not shown in the drawings) projecting from its faces 13
and 17. The dowel pins are registrable in holes, one of which is
designated at 62 in FIG. 2, in the respective cheek plates 11 and
18, as is known in the art.
In the embodiment shown in the drawings, each vane 32 has a medial
groove along its outer edge, as designated at 63. Two lips are
defined on the outer end of each vane, on opposite sides of groove
63. With respect to the direction of rotor rotation indicated by
the arrow in FIG. 5, the leading lip of each vane is designated 64
and the trailing lip 65. The inward lead or so-called pressure ramp
67 on cam surface 34 in the pressure zone 38, is evident in FIG. 5.
By reason of the inward lead of pressure ramp 67, only the front or
leading lip 64 of a vane will engage the ramp in the pressure zone,
and the rear or trailing lip 65 of the vane will be spaced inwardly
from the pressure ramp in that zone. Conversely, in the suction
zone 37, where the cam surface 34 has an outward lead, only the
rear or trailing lip 65 of a vane engages the suction ramp 68.
It is known to use either springs or hydraulically operated means
to provide an actuating force on each vane that will bias the vane
toward the cam surface. The illustrated pump incorporates hydraulic
piston biasing means. Use of such pistons for this purpose is known
in the art, and they are not part of the invention. Specifically,
in rotor 28, a radial bore or cylinder 69 extends inwardly from the
inner end 46 of each vane slot 31. The bores 69 are interconnected
at their inner ends by an annular pressure chamber 71. Fluid can
flow into and out of pressure chamber 71 only through the radial
bores 69, the chamber 71 being closed inwardly, as by a sleeve
member 72 secured to the rotor. A generally cylindrical piston
valve element 73 slides in each radial bore 69. Each piston has an
axial bore 74 and its outer end is conically tapered and forms a
valve with the inwardly facing surface or inner end 75 of the
respective vane. The operation of such hydraulic piston vane
biasing means is described in U.S. Pat. No. 3,223,044, to which
reference may be made. The remainder of the inner end surface 75 of
each vane (i.e., except at the piston) is exposed to the pressure
of fluid in slot end 46, and the resulting pressure force, together
with the piston force, urges the vane outwardly.
At least one, and preferably two, flow restricting means designated
generally at 77 are provided in each vane and extend from the
inwardly facing surface or inner end 75 thereof, radially outwardly
to groove 63. The restricting means 77 establish a pressure drop or
a differential pressure when fluid flows through it. The
restricting means 77 preferably comprises a flow restricting
orifice or inner end portion 78 which is relatively short (in the
axial direction) and relatively narrow in cross-section, and a
connecting passage 79 that has a larger diameter than the
restrictor portion 78 and which connects the latter to the top of
the vane. By way of example, orifice 78 may be formed by a No. 44
drill, and may have a length of about 0.050 inch, but these values
are illustrative and one will depend on the volume of displaced
fluid and on the desired force.
The restrictor portion 78 preferably forms a sharp or right angled
corner 80 with the inner end 75 of the vane, and has the
characteristics of a "sharp orifice" as that term is understood in
the art. As will be described in greater detail hereinafter, fluid
flow through the orifice 78 in the direction from the inner end 75
of the vane toward groove 63, creates a pressure differential, the
pressure beneath the vane (in the vane slot) being greater than the
pressure above the vane (in groove 63).
OPERATION
When shaft 6 is driven by a prime mover (not shown), fluid is
received into the intervane pockets 40 as they sweep sequentially
through suction zone 37. At the same time, fluid is supplied
through the undervane suction port 45, into the inner end 46 of the
respective vane slot. Thus, in the suction zone the pressures on
the inner end outer ends of the vanes are equal and opposed, and
are substantially offsetting. The biasing force supplied by the
piston 73 holds the vane outwardly and maintains contact with the
suction ramp. As the rotor turns (counterclockwise in FIG. 5) and
carries the vanes through the transfer zone 41 that lies between
the suction and pressure zones, the volume of the pocket does not
change greatly, if at all; as a result there is little or no flow
through the restrictor 77, and they maintain static balance between
the pressures on the inner and outer vane ends. When the vane comes
onto the pressure ramp 67, the inward angulation of the ramp exerts
a camming force on the leading lip 64 of the vane, and displaces
the vane inwardly in its slot. Since the inner end of the vane slot
is full of fluid (having been filled in the suction zone), this
inward motion must displace fluid from the slot. Prior to the
position at which the inner end of the vane slot comes into
communication with the undervane relief port 58 in the pressure
zone, the only path (excepting minor leakage) for fluid release is
through the flow restrictor means 77. Therefore, the inward
movement of the vane in its slot causes fluid to flow outwardly
through restrictor 77. This creates a higher pressure in the vane
slot than at the outer end of the vane in groove 63. This pressure
differential acts over the inwardly facing vane surface (except at
piston 73), and supplies an extra vane biasing force that
supplements the force of the piston.
At the initial portion of the pressure ramp, where the ramp
angulation is increasing, the vane is positively cammed inwardly
and is being accelerated, so that a pressure assist beneath the
vane is not required to maintain vane contact with the cam surface.
In order to minimize the vane loading on the pressure ramp at those
areas thereof where skipping does not occur, the relief port 58
opens to the vane slot inner end when the vane is moving on the
center part of the pressure ramp. This is indicated
diagrammatically in FIG. 6, by the legend "relief port opens."
Absent the control of pressure the relief port provides, the
undervane pressure would approach a peak pressure P where the
inward vane velocity was the highest (in the middle of the pressure
ramp) as indicated by the broken line in FIG. 6. This would lead to
unnecessary wear. Provision of the relief port prevents this. A
pressure force assist is maintained during the critical portion of
the vane travel on the inner end of the pressure ramp, but the peak
is "cut off" by relief through the undervane port. (The relief port
is sized small enought that it does not spill the entire excess
pressure beneath the vane.) When the vane appoaches the inner end
of the pressure ramp, where the angulation of the latter starts to
diminish, the inward momentum of the vane would cause it to skip on
the ramp, were it not for the tracking assisting force.
As illustrated diagrammatically in FIG. 6, the trailing edge of
pressure port 58 is preferably positioned angularly or "timed" to
close at the inner end of the ramp. The extra pressure force decays
as vane velocity drops, but nonetheless is sufficient to prevent
the skip.
Positioning of relief port 58 in the middle of the ramp is
especially useful in a reversible pump, since the timing of the
port opening and closing will be the same for either direction of
rotation. Where the pump is not reversible, the port may open even
earlier, to release the pressure load on the initial part of the
ramp as well.
It is recognized that undervane ports in the pressure zone are
shown in U.S. Pat. No. 2,919,651, issued to D. B. Gardiner. There,
however, no flow restricting means are provided in the vanes. Since
fluid is displaced from the vane slot across the entirety of the
pressure ramp, and must have somewhere to go, the cheek plate
restrictor consequently must communicate with each slot across the
entirety of its travel on the ramp, to accommodate the displaced
fluid. This subjects each vane to an extra outward force across the
entirety of the ramp even where no tracking assist is needed. The
pressure at the center of the ramp is not limited or partially
released, and higher wear results.
In the embodiment described, the restrictor means comprises one or
more orifices contained wholly within each vane. It will be
appreciated by those skilled in the art that restrictor means may
alternatively be provided in the rotor, or at the edge of the
vanes, as by a narrow slot or groove formed in the vane which is
closed by the cheek plate against which the vane slides. Such means
are comprehended by the term "flow restricting means" used in the
claims.
The short orifice configuration illustrated is desirable and is
preferred (in contrast to a long restrictor) because it is freer
from oil acceleration effects.
While the invention has been described in relation to a
particularly preferred embodiment, those skilled in the art will
understand from the foregoing specification that it comprehends
alternative embodiments, within the scope of the claims which
follow.
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