Note: Descriptions are shown in the official language in which they were submitted.
PATENT
PD-9291
CEN ~IF'UG~,~LpLOOD PUMP AND MOTOR DRxVE
BACKGROUND OF THE 1NVENTxON
The'present invention relates to centrifugal
blood pumps intended for extracorporeal pumping of
blood.
Known blood pumps of this type have been found
to be less than totally reliable, due at least in part
to their mechanical complexity and to the use of
configurations which permit blood thrombus formation.
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PATENT
PD-9291
~~ OP TFiE INVENTION
It is a primary object of the present
invention to provide a novel centrifugal blood pump
which reduces the danger of blood hemolysis and thrombus
formation.
Another object of the invention is to provide
a novel centrifugal blood pump which is structurally
simpler than existing pumps of this type, and hence
operates more reliably.
The above and other objects are achieved,
3.9 according to the present invention, in a centrifugal
blood pump composed of an impeller housing having a
generally circular cross section and a longitudinal axis
and delimiting a blood pumping chamber having a blood
inlet port extending along the longitudinal axis and a
blood outlet port located at the periphery of the
chamber, an impeller .provided with a plurality of
radial.ly extending vanes disposed in the chamber, a
shaft supporting the impeller for rotation about the
longitudinal axis of the blood pumping chamber, and
magnetic drive means for rotating the impeller in a
sense to cause the vanes to propel blood from the inlet
port to the outlet port, by the improvement wherein the
vanes are configured to eacta have a blade~angle which
varies monotonicaily toward the periphery of the
chambers
The objects of the present invention are
further achieved by other novel features of the
impeller, by a novel magnetic drive system, and by a
novel sealing arrangement for the impeller shaft, all of
3a which will be described in detail below.
With regard to a primary aspect of the
invention, applicants have concluded that, in pumps of
the type under consideration, blood hemolysis is caused
by mechanical stresses imposed on the blood by the
pumping process and have conceived and developed a
PATENT
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novel impeller configuration which acts on the blood in
such a manner as to significantly reduce the occurrence
of hemolysis. Basically, impellers according to the
present invention are constructed to subject~blood as it
enters and flows through the pump to smooth velocity
transitions and to reduce cavitation in the pump,
particularly at the inlet.
Applicants have determined that this
objective can be achieved by giving the impeller vanes a
blade angle which varies from the end associated with
the pump inlet and to the end associated with the pump
outlet such that the tangent of the blade angle
increases, as a function of radial distance from the
impeller axis. It is presently believed that an optimum
1.5 result will be achieved if the tangent increases
linearly, or at least approximately linearly, from the
inlet to the outlet.
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PD-9291
~R~EF DESCRIPTION OF Ti~'~ DRAW7fj~~
Figure 1 is a cross-sectional, elevational
view of a preferred embodiment of a centrifugal blood
pump according to the present invention.
Figure 2 is a cross-sectional, elevational
view of one element provided in the pump of Figure 1.
Figure 3 is a top plan view of one impeller
component of the pump of Figure 1.
Figure 9 is a bottom plan view of the second
impeller element of the pump shown in Figure 1.
Figure 5 is a cutaway perspective view of a
portion of a magnetic drive system for the pump of
Figure. 1.
a
PATENT
PD-9291
D~SOR~PTION OF TIE PREFERRFIQ EMBODIMENTS
One embodiment of a centrifugal blood pump
according to the present invention is illustrated in
Figure 1, which is a cross-sectional view taken along a
plane passing through the axis of rotation of the pump
5 impell~r.
Ths illustrated pump includes a housing
composed of a forward housing part 2 and a rear housing
part 4,, parts 2 and 4 together enclosing a pump chamber
6. The pump housing further includes a bearing housing
8 and a bearing cap 10, with the rear end of housing 8
being closed by cap 10 and the forward end of housing 8
being closed by rear housing part 4.
Forward housing part 2 is formed to have an
inlet passage 12 which extends along the pump axis and
an outlet passage 14 which extends in a generally
tangential direction,at tae periphery of chamber 6.
Within chamber 6 there is mounted an impeller
which, according to the present invention, is formed of
a forward impeller part 16 and a rear impeller part 18,
parts 16 and 18 being joined and bonded together along a
plane perpendicular to the pump axis. Impeller 16, 18
is mounted on an impeller shaft 20 which is rotatably
supported by a pair of journal bearings 22~~secured in
bearing housing 8. The region within housing 8 between
bearings 22 is preferably filled witty a mass 24 of a
suitable grea.~e a
The interior surface of cap 10 is provided
with a aylindrical blind bore containing a steel ball 26
which constitutes a thrust bearing providing axial
support for shaft 20 and impeller 16, 18.
Between rear housing part ~ and the journal
bearing 22 adjacent thereto there is disposed a shaft
seal 36, which will be described in detail below.
Impeller parts 16 and 18 are formed to delimit
a plurality of circumferentially spaced, arcuate
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6 PD-9291
chambers, six such chambers being provided in one
practical embodiment of the invention. Each chamber
holds an arcuate drive plate 28 made of a.magnetically
permeable, but unmagnetized, material. Plates 28 can be
made relatively thin, a thickness of the order of .090°°
having been found to be suitable.
Forward impeller part 16 carries a plurality
of circumferentially spaced long vanes 30 and a
plurality of circumferentially spaced short vanes 32
interposed circumferentially between successive long
vanes 30. All vanes 30 and 32 project axially toward
inlet passage 12 and the edges of vanes 30 and 32 which
face toward inlet passage 12 conform generally to the
outline of forward housing part 2.
Rear impeller part 18 carries a plurality of
short vanes 34 each of which is aligned with and
corresponds in configuration to a respective one of
short vanes 32 and the outer portions of long vanes 30.
The portion of each long vane 30 which is radially
enclosed by a respective vane 34 extends axially toward
rear housing part 4 to the same level as the associated
vans 34 so that, at the side facing rear housing part 4,
the respective vane 34 forms a radial continuation of
the associated vane 30. '"
Impeller shaft 20 enters chamber 6 via a
passage provided in rear housing part 4, which passage
is dimensioned to provide the minimum permissible
clearance fob shaft 20. Preferably, a radial clearance
of no more than 0.001 to 0.002'° is provided.
Moreover, the edge of the shaft passage which
borders chamber 6 is formed to be sharp so as to
constitute a shear edge.
The passage for shaft 20 is isolated from the
interior of bearing chamber a by shaft seal 36.
As shown in Figure 2, shaft seal 36 is
composed of an annular flange portion 38 which will bear
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7 PD-9291
against the journal bearing 22 which is adjacent rear
housing part 4. Shaft seal 36 further includes two
concentric, radially spaced cylindrical portions 40, the
outer one of which bears against the surface of a
cylindrical relief opening provided in rear housing part
4. 7Lnner cylindrical portion 40 is dimensioned to
establish a close fit with shaft 20.
Hetween cylindrical portions 40 there is
interposed a pressure member 42 composed of a spiral
spring bent into a taroidal form and made of a suitable
material, such as stainless spring steel. Member 42 is
configured to apply a radial pressure to cylindrical
portions 40, thereby pressing those portions against
shaft 20 and the inner surface of the cylindrical relief
opening provided in rear housing part 4, respectively.
Thus, an effective seal is provided between chamber 6
and the interior of bearing housing g.
As a result of the close fit between shaft 20
and the opening in rear housing part 4, shaft.seal 36
is, in effect, "hidden" from chamber 6. This helps to
prevent thrombus formation on seal 36.
The supporting of shaft 20 by journal bearings
22, instead of ball-type bearings, and the elimination
of air from the bearing chamber by filling~it with mass
24 of grease, are major contributing factors to the
superior reliability of pumps according to the present
invention. With this arrangement, seal 36 is not
recyuired to effect a perfect sealing action but need
only prevent gross migration of blood and grease.
figure 3--is a flan view, looking in the
direction of fluid flow into the pump, of fortaard
impeller part 16, which is basically composed of an
inner hub portion 44 and an outer annular portion 46,
the two portions being secured together by means of long
vanes 30. Short vanes 32 are interposed between long
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8 PD-9291
vanes 30 so that vanes 30 and 32 are equispaced about
the circumference of upper impeller part 16.
Figure 4 is a plan view of rear.'impeller part
18, looking opposite to the direction of fluid flow
into the pump, i.e., opposite to the direction of the
view of Figure 3. Rear impeller part 18 is composed
essentially of an annular ring 48 carrying short vanes
34, each of which is aligned with an associated portion
of a respective one of vanes 30 or 32.
. According to preferred embodiments of the
invention, each vane 30 , 32, 34 is given a curvature
such that the variation of the tangent of the blade
angle as a function of impeller radius has a positive
value along the length of each blade.
Figures 3~and 4 additionally illustrate one of
the drive plates 28 which is installed between impeller
parts 16 and 18 and Which are spaced apart around the
circumference of the impeller.
As regards the axial spacing between vanes 30,
32, 34 and housing parts 2 and 4, these selected to be
small enough to achieve a satisfactory pumping force,
and yet large enough to minimize the shear forces
imposed on the blood. On, the basis of ttaese
considerations, in one exemplary embodiment of tDae
invention, th~ axial spacing betwee»'vanes 30 and 34 and
rear housing part 4 is ~f the order of 0:12 inch at the
outer diameter of the imphller. ~n this embodiment,
which is 111~strated in Figure l, the surface of rear
housing part 4 which delimits chambe~c 6 ~,as a slight
upward slope toward shaft 20 so that the axia-1 spacing
between the vanQS and that surface of lower housing part
4 exhibits a slight progressive decrease in the
direction toward shaft 20: This axial spacing dimension
was provided in a pump whose impeller vanes are
configured so that the inner end of each vane 30 is at a
distance of 0.3 inch from the axis of rotation of shaft
PATENT
9 PD-'9291
20 and th~ outer end of each vane is at a distance of
1.4 inches from the axis of rotation of shaft 20. Zn
fact, Figure Z is drawn to scale and represents a pump
having the above-stated dimensions.
Vanes 34 and the portions of vanes 30 which
project toward rear housing part 4 act to subject blood
which is present between the impeller and rear housing
part 4 to a radial outward force, and thereby prevent
blood from recirculating around the outer edge of the
impeller. Thus, the action of these vane portions
together with the sharp shear edge provided by rear
housing part 4 around shaft 20 at the side bordering
chamber 6 serve to sweep blood away from the region
where shaft 20 passes through rear housing part 4, which
is a potential area of stasis, and thus prevent thrombus
formation at that location.
As noted earli~r herein, vanes 30, 32, 34 are
configured with the goal of minimizing the acceleration '
and shocDc experienced by blood within the pump.
Preferably, the inlet blade angle of each vane, the
blade angle being, at any point along a blade, the angle
between the blade surface and a circle centered on the
axis of impeller rotation and passing through the point
in question, is selected so that, for a selected
impeller speed, the vel~aity produced by each vane
closely corresponds to the inlet fl~w Velocity of the
blood.
According to a preferred embodiment of the
invention, the configuratian of each vane 30, 32 , 34 was
determined on the basis of the following eguation:
R ~ R1 '!" Cl°~ '~' i:2°~c3 =1)
where:
R is the radial distance from each point along
the vane to the axis of rogation of shaft 20t
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PD-9292
R1 is the radial distance from the end of each
long vane 30 closest of the axis of shaft rotation to
that axis, i.e., at the inlet end of eaah.lQng vane 30;
a is the angle, in radians, about the axis of
5 rotation of shaft 20, between a line extending between
that axis and the inlet end of a long vane 30 and a line
extending between that axis and the point on the same
vane whose radial distance from the axis is R:
C1 = R1 ' tan ~A1, where ~1 is the blade angle
10 in radians, of a vane 30 at its inlet end;
C2 = (R2 tan ~2 - C1)/(C3~~2(C3 1))~
where R2 is the radial distance from the end of sash
vane furthest from the axis of rotation of shaft 20 to
that axis, B2 is hhe blade angle, in radians, of each
vane at the end furthest from the axis of rotation of
shaft 20, and ~2 is the value for 9 associated with the
radial distance R2; end
C3a(R2'tan ~2 - C1)'~2/(R2 - R1 - C1°~2)0
with the following selected parameters being used:
R1 = 0.3"; R2 = 1.4°'; fl1 = 0.1745 Rad = 10°;
~2 = 1.047 Rad = 60'; and ~2 = 2.094 Rad = 120'.
R1, R2, ~1, ~2 aid B2 are shown in Figure 3.
Blade angle, ~, is the angle, at a point along a vane,
between a line tangent to the blade surface and a line
tangent to a circle passing through that point end
centered on the axis of rotation of shaft 20.
Figure 5 illustrates the basic components of a
magnetic dri~~e according to the present invention. This
drive is composed of a plurality of permanent magnet
units 50 mounted on a plate 52 having a central opening
53 for connection to the shaft of a drive motor. a~ne-
half of the drive is shown in Figure 5. Each magnet
unit 50 is composed of two bar magnets 54 each having
its magnetic axis oriented parallel to the axis of
rotation of plate 52, with the magnets 54 of each unit
50 being oriented in polarity opposition to one another,
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as shown. Moreover, the magnetic poles of each unit 50
are oriented opposite td those of each adjacent unit 50.
Each unit 50 is further composed of an arcuate plate 56
of ferromagnetic material completing the magnetic
circuit at one end of the associated unit 50.
The magnetic drive is disposed directly
beneath rear housing part 4 so that magnet units 50
surround housing 8 and plate 52 is behind cap i0.
Thus, the end of each magnet unit 50 which is remote
ZO from plate 52 faces a respective one of plates 28. The
spacing between plates 28 and units 50 is made as small
as possible in order to minimize the air gap between
each plate 28 and its associated unit 50, and thus
maximize the magnetic. attraction exerted on each,
plate 28.
The arrangement of magnetic units 50 is such
that the magnetic fly~x path of each unit is completed
through a respective one of plates 28 and the magnetic ,
paths associated with adjacent ones of plates.28 are
maintained isolated from each other by the orientations
of the magnets associated with adjacent units 50. Thus,
as plate 52 is rotated, the magnetic attraction forces
exerted on plates 28 caaase impeller 16, 18 to rotate in
unison therewith.
In addition, the magnetic attraction exerted
by units 50 pulls impeller 16, l8 downwardly in order to
press shaft 20 against ball 26>
The drive arrangement shown in Figure 5
produces particularly strong magnetic forces, making
possible the use of thin, unmagnetized plates 28 and
permitting a sufficient drive force to be imparted to
. impeller 16, 18 even with a comparatively large air gap
between units 50 and pla~~s 28.
While the description above refers to
particular embodiments of the present invention, it will
be understoad that many modifications may be made
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without departing from the spirit thexeof. The
accompanying claims are intended to cover such
modifications as would fall within the trua.~cope and
spirit of the present invention.
The presently disclosed embodiments are
therefor~ to be considered in all respects as
illustrative and not restrictive, the scope of the
invention being indicated by the appended claims, rather
than the foregoing description, and all changes which
come within the meaning and range of equivalency of the
claims are therefore intended to be embraced therein.
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