Note: Descriptions are shown in the official language in which they were submitted.
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ELASTOMERIC CONNECTOR COUPLING MOTOR TO CAM ACTUATOR OF INFUSION PUMP
This application is a divisional application of Canadian patent application
Serial
No. 2,335,156, filed June 18, 1999.
Field of the Invention
This invention generally pertains to a connector for use in coupling a motor
drive
shaft to a driven shaft, and more specifically, in a medical drug infusion
pump, to an
elastomeric coupler for connecting a motor drive shaft to a cam shaft that
drives a plunger in
the pump, and to providing a restoring force for the plunger.
Background of the Invention
In many portable motor driven devices, small direct current (DC) motors are
connected to rotatably driven shafts using solid metal couplers. Such couplers
comprise a
short section of thick-walled tubing having two threaded orifices in the
tubing wall, adjacent
each end. A set screw is threaded into each orifice. The set screws are
tightened to engage a
drive shaft of the motor that is inserted into one end of the coupler, and to
secure a driven
shaft that is inserted into the other end of the coupler. Even if a fastener
locking substance is
1s applied, the set screws often loosen with use, causing scoring of the
shafts and possible
failure of the devices in which the couplers are installed.
Couplers are generally available from suppliers in only a limited range of
sizes. If
the coupler used to join two shafts is too large, it will not properly connect
the shafts and can
cause vibration, because its mass is not symmetrically distributed around the
center lines of
the two shafts. In addition, conventional couplers generally require that the
center lines of
the two shafts that are joined be relatively closely aligned. Thus, for
example, any
misalignrnent between a motor drive shaft and a driven shaft, even if slight,
is likely to cause
side loading of one or both shafts, producing greater bearing wear. Solid
couplers also
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transmit noise and vibration from the motor to other parts of the device in
which they are
used.
Ideally, it would be preferable to provide a more flexible coupling between a
motor
drive shaft and a driven shaft. Large motor couplers sometimes include fiber
reinforced
rubber assemblies clamped around the ends of two shafts to provide some degree
of
flexibility, but such couplers are too large for use in small devices.
It will therefore be apparent that a simple coupler, which addresses the
problems
noted above and is relatively low in cost, would be desirable for use in small
electric motor
powered devices. The prior art does not provide a suitable alternative to the
prior art solid
lo metal connector of the type described. or the too large and cumbersome
prior art fiber
reinforced rubber connector assemblies.
Summarv of the Invention
In accord with the present invention, a coupler is defined for connecting a
non-
cylindrical end of a drive shaft to a non-cylindrical end of a driven shaft.
The coupler
includes a generally cylindrical fitting having opposed first and second ends.
A first opening
is disposed at the first end of the fitting, and a second opening is disposed
at the second end
of the fitting. The first opening has a size and a shape generally
corresponding to a size and
a shape of the non-cylindrical end of the drive shaft. Likewise, the second
opening has a size
and a shape generally corresponding to a size and a shape of the non-
cylindrical end of the
driven shaft. The fitting is formed of an elastomeric material adapted to
elastically stretch
when the first opening is forced over the drive shaft and when the second
opening is forced
over the driven shaft, providing an interference fit in each case. The fitting
is thereby
adapted to drivingly couple the drive shaft to the driven shaft.
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Preferably, the fitting further comprises a web that is disposed transverse to
a
longitudinal axis of the fitting, between the first opening and the second
opening. This web
limits a depth to which the drive shaft and the driven shaft are advanced into
the first opening
and the second opening, respectively. Also, the web limits vibration of the
drive shaft
propagating into the driven shaft from the drive shaft.
A sleeve that is sized to snugly fit around an outer surface of the fitting in
an
interference fit provides a compression force that helps to keep the fitting
seated on the drive
shaft. An outer surface of the sleeve includes a cam surface adapted to act on
a plunger. As
the drive shaft rotates the driven shaft, the sleeve causes the plunger to
move along a
longitudinal axis of the plunger. Preferably, the cam surface is adapted to
only apply a force
against the plunger in one direction; the plunger is biased in an opposite
direction by an
elastomeric membrane which the plunger displaces.
An outer surface of the fitting is keyed to an inner surface of the sleeve.
Also, an
inner surface of the first opening includes a flat section adapted to seat on
a corresponding
flat section formed on the end of the drive shaft. In a similar manner, an
inner surface of the
second opening includes a flat section adapted to seat on a corresponding flat
section formed
on the end of the driven shaft.
The elastomeric fitting thus minimizes vibration transmission between the
drive shaft
and the driven shaft and helps to minimize audible noise. Because of its
elasticity, the fitting
can accommodate at least a limited degree of misalignment between the drive
shaft and the
driven shaft.
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In accordance with one aspect of the invention, there is provided a resilient
coupling assembly for transmitting a rotational drive force from a first shaft
to a
second shaft, comprising:
(a) a generally elongate elastomeric member having:
(i) a first orifice disposed at one end of the elastomeric member and
sized to provide an interference fit over the first shaft; and
(ii) a second orifice disposed at an opposite end of the elastomeric
member and sized to provide an interference fit over the second
shaft; and
(b) an elongate sleeve having a central opening that is sized to provide
an interference fit over an outer surface of the elastomeric member and
providing
a compressive force against the elastomeric member that retains the
elastomeric
member on at least one of the first and the second shafts, so that the shafts
are
coupled together through the elastomeric member, wherein the sleeve comprises
an elastomeric material that is overmolded onto a rigid material.
In accordance with another aspect of the invention, there is provided a drive
mechanism for a pump cassette, comprising:
(a) a shaft mounted on a chassis and adapted to be rotationally driven by
a prime mover, said shaft rotating a cam surface;
(b) a plunger having a first end coupled to the cam surface and an
opposed second end, said plunger being mounted to reciprocate back and forth
in
two opposed directions, said first end of the plunger being driven in a first
direction, towards the pump cassette, by a force applied to the plunger by the
cam
surface as the shaft rotates; and
(c) an elastomeric membrane exposed in an opening formed on a
surface of the pump cassette, said pump cassette being removably mounted to
the
chassis in a predefined position, so that the opening is disposed adjacent the
second end of the plunger, said second end of the plunger displacing the
elastomeric membrane into an interior of the pump cassette to force a fluid to
flow
through the pump cassette, said elastomeric membrane providing a restoring
force
that biases the plunger to move in a second direction, toward the cam surface,
so
that the cam surface does not apply a force to move the plunger in the second
direction.
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Brief Description of the Drawing Figures
The foregoing aspects and many of the attendant advantages of this invention
will
become more readilv appreciated as the same becomes better understood by
reference to the
following detailed description, when taken in conjunction with the
accompanying drawings,
wherein:
FIGURE 1 is an exploded isometric view of a motor and drive assembly in accord
with the present invention, and illustrates a pump cassette through which
medicinal fluid is
pumped by a plunger actuated by the drive assembly;
FIGURE 2 is an isometric view of an elastomeric coupler for coupling a drive
shaft
lo to a driven shaft;
FIGURE 3 is a cross section elevational view of the drive assembly;
FIGURE 4 is an exploded isometric view showing the motor and a portion of the
drive assembly;
FIGURE 5 is an isometric view of the motor, drive assembly, plunger, and pump
cassette; and
FIGURE 6 is an exploded isometric view of the motor, drive assembly, plunger,
and
pump chassis.
Description of the Preferred Embodiment
An exemplary application of the present invention is illustrated for use in
pumping a
medicinal fluid through a pump cassette 10, as shown in FIGURE 1. Pump
cassette 10 is
disposable, being intended for use with a single patient. Details of the pump
cassette are
disclosed in commonly assigned U.S. Patent No. 5,586,867.
Proximal tubing 14 and
distal tubing 16 are coupled to the proximal and distal ends of the pump
cassette. A reservoir
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(not shown) of medicinal fluid is connected to proximal tubing 14, while
distal tubing 16 is
connected into a patient's cardiovascular system to infuse the medicinal fluid
at a rate
determined by the speed with which pump cassette 10 is driven. Pump cassette
10 includes a
plastic housing 12 having an opening 18 formed in an upper surface thereof.
Opening 18
exposes an elastomeric membrane 20, which is sealed between the top and bottom
portions
of the plastic housing. The undersurface of elastomeric membrane 20 is exposed
to a fluid
path through pump cassette 10 and is operative to displace the medicinal fluid
when
elastomeric membrane 20 is forced inwardly into the interior of pump cassette
10. The pump
cassette is latched into a pump chassis 22, which is shown in FIGURE 6. The
pump chassis
is mounted in a pump housing (not shown) which includes a battery supply,
electronic
components for controlling the pumping action, and a user interface, none of
which are
shown. A pivotal member 25 on pump chassis 22 is spring biased to engage
housing 12 of
pump cassette 10 to latch it in a predefined position within the pump chassis.
Details of this
latching mechanism and other aspects of pump chassis are not pertinent to the
present
invention, and therefore are not disclosed herein.
A drive assembly 24 provides the force that deflects elastomeric membrane 20
into
the interior of pump cassette 10 to pump medicinal fluid into the patient's
body. Further
details of the drive assembly and its interaction with the pump cassette are
illustrated in
FIGURES 3, 4, and 5. Drive assembly 24 includes a DC motor 28 that is
energized with an
electrical current provided through a flex lead 30, which has a connector plug
32 adapted to
couple to a mating connector on a printed circuit board (neither shown) within
the interior of
the pump. A resilient 0-ring support 34 is provide around the outer surface of
motor 28 and
is seated within a pair of drive enclosure shells 26a and 26b, which are
fastened around the
drive assembly to enclose and support it, generally as illustrated in FIGURE
6. A second
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resilient 0-ring support 36 is disposed at the end of DC motor 28, providing
further support
to the motor and drive assembly within drive enclosure shells 26a and 26b.
Just beyond
0-ring 36, a drive shaft 38 extends from the end of DC motor 28 and is
connected with a
coupling 42 comprising an elastomeric material. Coupling 42 is also connected
to a driven
shaft 40 and is operative to transmit the rotational force from drive shaft 38
to driven
shaft 40.
Internal details of coupling 42 are illustrated in FIGURE 2. As shown therein
and in
FIGURES I and 4, a portion of drive shaft 38 that is connected to coupling 42
has a flat 76
on one side so that it is generallv "D"-shaped. This portion of drive shaft 38
extends into an
opening 74 in coupling 42. Opening 74 also has a flat 78 on a portion of its
circumference,
so that its shape is also "D"-shaped. but slightly smaller in size than the
end of drive shaft 38.
Thus, coupling 42 forms an interference fit with drive shaft 38 when the end
of the drive
shaft is inserted into opening 74 of the coupling. At the opposite end of
coupling 42 from
opening 74 is disposed an opening 80 having a flat 82 on one side and
therefore also being
generally "D"-shaped in cross section. A flat 84 is formed on the end of
driven shaft 40, so
that it is also "D"-shaped. but slightly larger in size than opening 80. Thus.
an interferencefit
is also provided when driven shaft 40 is inserted into opening 80 of coupling
42. A
transverse web 98 (shown in FIGURE 2) is disposed within coupling 42,
separating
opening 74 from opening 80 and limiting the extent to which drive shaft 38 and
driven
shaft 40 extend within coupling 42. Transverse web 98 also limits the
transmission of
vibration from drive shaft 38 to driven shaft 40, since it prevents the ends
of the drive shaft
and driven shaft from contacting each other.
The exterior surface of coupling 42 is generally cylindrical, but on one side,
a rib 72
extends generally parallel to the longitudinal axis of the coupling. Rib 72
has a semicircular
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cross section. Since the coupling is made of a elastomeric material, i.e.,
preferably from a
synthetic rubber, it is possible that either or both drive shaft 38 or driven
shaft 40 might
rotate within the coupling by distorting the coupling. To prevent such
distortion and
consequential slippage, a sleeve 86 is provided. The sleeve extends over the
outer surface of
coupling 42, in a snug fit, creating a compressive force that precludes either
of the shafts
distorting the coupling sufficiently to rotate within the "D"-shaped openings
formed in the
ends of the coupling. Sleeve 86 has an opening 88 with an internal diameter
approximately
equal to the external diameter of coupling 42. Further, a longitudinally
extending notch 90
having a cross-sectional profile and size corresponding to that of rib 72 is
formed on one side
of opening 88 to receive the rib when sleeve 86 is slipped over coupling 42.
It will be
apparent that coupling 42 could alternatively be provided with a notch to
receive a
correspondingly shaped and sized rib formed on the inner surface of the
opening into
sleeve 86, to key the coupling and sleeve. In a preferred embodiment. sleeve
86 is an
overmolded component that includes a rigid internal element (not shown) formed
of a hard
plastic or metal material. which is coated or overmolded with an elastomeric
material -
preferably synthetic rubber.
Just behind flat 84 on driven shaft 40 is disposed a circular groove 92 (see
FIGURE 3) that engages the internal elastomeric portion of sleeve 86 so that
the sleeve is
affixed to the end of driven shaft 40, which is connected to coupling 42.
Sleeve 86 is also
prevented from rotating about driven shaft 40 by its interference fit over
flat 84 on the driven
shaft. Use of sleeve 86 thus assists in transmitting the rotational force from
drive shaft 38 to
driven shaft 40.
Several significant advantages are derived from the use of coupling 42 to
connect
drive shafl 38 and driven shaft 40. Due to the elastomeric nature of coupling
42. the
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coupling allows for some misalignment between the center lines of drive shaft
38 and driven
shaft 40 and thus decreases side loading on either the driven shaft or the
drive shaft. The
decrease in side loading improves the operating life of the drive assembly, in
particular, the
life of bearings 46 and 50. Further, the decrease in side loading increases
the operating life
of the motor and enables a lower cost motor to be used, since less expensive
bearings are
required on the drive shaft of the motor than would be required to handle
higher side loading.
In addition, coupling 42 eliminates the need for mechanical fasteners to
attach drive shaft 38
to driven shaft 40 and decreases the noise level of the drive assembly, since
vibration in the
motor is at least partially isolated from components of the drive assembly
that are
downstream of coupling 42. Compared to prior art devices for coupling a drive
shaft to a
driven shaft, coupling 42 is relatively smaller and compact. Furthermore,
assembly of the
coupling and sleeve is relatively simple, so that a decrease in assembly time
and the number
of parts, and a corresponding resultant cost reduction in the drive assembly
is achieved by
using coupling 42 and sleeve 86 rather than a prior art type coupling.
Sleeve 86 serves yet a further purpose in this exemplary application of the
present
invention. Specifically, sleeve 86 includes a cam surface 96, which defines a
profile having
a locus of points at a varying radial distance from a center line of driven
shaft 40. As drive
shaft 38 rotates driven shaft 40 and sleeve 86, cam surface 96 is rotated. The
rotating cam
surface applies a force that is used to displace a plunger 58, causing it to
move toward the
pump cassette. A plunger foot 62 is formed on the end of plunger 58 and is in
contact with
elastomeric membrane 20 in pump cassette 10. Plunger 58 includes two O-rings
64 that seal
against the internal surface of an opening 66 in pump chassis 22 (FIGURE 6),
thereby
helping to prevent water and other liquids from entering the interior of the
pump housing.
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Cam surface 96 is formed on only a portion of the outer surface of sleeve 86.
The
portion of sleeve 86 that does not include cam surface 96 is radially
equidistant from the
center line of driven shaft 40. A bearing 46 is seated on this portion of
sleeve 86- and assists
in supporting the drive assembly during rotation of the drive shaft and driven
shaft.
As shown in FIGURES 1, 3, and 5, drive assembly 24 also includes a bearing 50
around driven shaft 40. Bearing 50 is held in place by a clip 52 that fits
within an annular
groove 94 forrned on driven shaft 40 (see FIGURE 3). Beyond clip 52 on drive
shaft 40 is
disposed a clutch assembly 54. Clutch assembly 54 includes a roller clutch 68.
At the end of
driven shaft 40 that is remote from sleeve 86 is disposed an encoder tab 56.
The encoder tab
is used for determining a home position of driven shaft 40 for purposes of
controlling the
drive assembly and for other purposes not related to the present invention.
A cam bearing 48 is disposed around cam surface 96 and comprises a radial ball
type
bearing; this cam bearing serves as a friction reducing interface between cam
surface 96 and
a loop 60 formed on the end of plunger 56 that is opposite the end on which
plunger foot 62
is disposed. Loop 60 is generally shaped like a square with rounded corners.
The radially
outer surface of cam bearing 48 does not contact an upper inner portion 100 of
loop 60, since
cam surface 96 does not apply any force to plunger 56 that would move the
plunger away
from elastomeric membrane 20. Instead, the elastomeric membrane provides a
restoring
force that lifts plunger 56 upwardly, away from the interior of pump cassette
10. Cam
bearing 48 contacts other inner portions of loop 60 and provides the force
that drives
plunger 56 toward pump cassette 10, to displace elastomeric membrane 20 into
the interior
of the pump cassette and thereby forces the medicinal liquid to flow through
distal line 16.
The clearance between cam bearing 48 and surface 100 of loop 60 is only a few
mils, but is
sufficient to prevent any scrubbing action between the outer surface of the
cam bearing and
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the inner surface of the loop that would reduce the efficiency with which
plunger 56 is driven
by drive assembly 24.
Although the present invention has been described in connection with the
preferred
form of practicing it, those of ordinary skill in the art will understand that
many
modifications can be made thereto within the scope of the claims that follow.
Accordingly,
it is not intended that the scope of the invention in any way be limited by
the above
description, but instead be determined entirely by reference to the claims
that follow.
