Syringe pump abstract
A lead screw mounting system for a syringe pump in which a second
end of a lead screw is located within a hollow connection tube that
connects a drive device engaged with the lead screw to a plunger
drive head. A bearing mount is formed at the second end of the lead
screw and a split bearing mounted to the bearing mount. The bearing
mount has diverging bearing surfaces in an hour glass shape and
the bearing has a complementary inner shape. The bearing is larger
than the inner diameter of the connection tube and is in constant
contact with the tube. During operation, the bearing tends to move
up one or the other of the diverging bearing surfaces of the bearing
mount depending on which direction the hollow connection tube is
moving over the bearing creating a wedge action. The split bearing
functions to keep the lead screw concentrically located within the
hollow connection tube thereby increasing flow uniformity of the
syringe pump.
Syringe pump claims
What is claimed is:
1. A syringe pump having a lead screw with a drive device in threaded
engagement with the lead screw, the drive device moving along the
lead screw in response to rotation of the lead screw, and a drive
head adapted to drive a syringe plunger into the barrel of a syringe
to expel fluid contents of the syringe, the syringe pump comprising:
a lead screw having a first end and a second end with the first
end mounted to a frame; a connection tube located over the lead
screw and connected between the drive device and the drive head
to communicate movement of the drive device along the lead screw
to the drive head, the connection tube having a hollow portion with
an inner surface; wherein the second end of the lead screw is located
within the hollow portion of the connection tube but is otherwise
unmounted; and a bearing located at the second end of the lead screw
in contact with the inner surface of the hollow portion of the connection
tube thereby creating a guided cantilever mount of the second end
of the lead screw within the connection tube.
2. The syringe pump of claim 1 wherein: the second end of the lead
screw comprises a bearing mount having outwardly diverging bearing
surfaces; and the bearing is located at the bearing mount, the bearing
engaging one or the other of the diverging bearing surfaces in response
to movement of the connection tube over the bearing in a particular
direction.
3. The syringe pump of claim 2 wherein the outwardly diverging
mounting surfaces of the bearing mount diverge outwardly in axial
directions from a bearing surface center location, the bearing engaging
one or the other of the diverging mounting surfaces depending upon
the direction of movement of the connection tube over the bearing.
4. The syringe pump of claim 2 wherein the bearing mount has a
generally hourglass shape.
5. The syringe pump of claim 2 wherein the bearing has inner tapered
surfaces for engaging the diverging bearing surfaces of the bearing
mount.
6. The syringe pump of claim 1 wherein the bearing is biased outwardly
into continuous contact with the inner surface of the connection
tube.
7. The syringe pump of claim 6 wherein the second end of the lead
screw comprises a bearing mount having outwardly diverging bearing
surfaces; the bearing is located at the bearing mount, the bearing
engaging one or the other of the diverging surfaces in response
to movement of the connection tube over the bearing in a particular
direction; and the bearing has an inner surface that is shaped in
an approximate complementary shape to the diverging bearing surfaces;
whereby the bearing locates the second end of the lead screw in
the approximate center of the connection tube.
8. The syringe pump of claim 1 wherein the bearing has a larger
outer diameter than the inner diameter of the hollow portion of
the connection tube within which it is mounted, the bearing having
a discontinuity that permits the bearing to compress to thereby
be mounted within the connection tube.
9. The syringe pump of claim 8 wherein the bearing has a notch
formed in an outer surface so that the bearing will bend at a desired
location.
10. The syringe pump of claim 8 wherein the bearing has a plurality
of notches formed in the outer surface of the bearing so that the
bearing will bend at desired locations to provide a more uniform
force against both the bearing surfaces and the inner surface of
the connection tube to more accurately locate the second end of
the lead screw within the connection tube.
11. The syringe pump of claim 2 wherein: the bearing and bearing
surfaces of the bearing mount are made of materials selected such
that the bearing easily slides along the bearing surfaces; whereby
the bearing moves up one or the other bearing surfaces to maintain
contact between the inner surface of the connection tube and the
respective bearing surface to more accurately center the second
end of the lead screw in the connection tube.
12. The syringe pump of claim 11 wherein the bearing mount has
a generally hourglass shape.
13. The syringe pump of claim 11 wherein the bearing has inner
tapered surfaces for engaging the diverging bearing surfaces of
the bearing mount.
14. A syringe pump having a lead screw, a nut device engaged with
the lead screw that moves along the lead screw in response to rotation
of the lead screw, and a drive head adapted to drive a syringe plunger
into a syringe barrel, the syringe pump comprising: a hollow connection
tube located over the lead screw and connected between the nut device
and the drive head such that movement of the nut device along the
lead screw is communicated by the connection tube to the drive head,
the connection tube having an inner surface; wherein the lead screw
is mounted at a first end to a frame; wherein a second end of the
lead screw is continuously located within the hollow connection
tube and comprises a bearing mount having outwardly diverging bearing
surfaces; and a bearing located at the bearing mount, the bearing
configured to engage the bearing surfaces of the bearing mount and
the inner surface of the connection tube so as to centrally locate
the second end of the lead screw in the connection tube during movement
of the connection tube along the lead screw.
15. The syringe pump of claim 14 wherein the bearing is biased
outwardly into contact with the inner surface of the connection
tube and wherein the bearing engages one or the other bearing surface
depending on which direction the connection tube is moving over
the lead screw.
16. The syringe pump of claim 14 wherein the bearing has a larger
outer diameter than an inner diameter of the connection tube within
which it is mounted, the bearing having a discontinuity that permits
the bearing to compress to thereby be mounted within the connection
tube.
17. The syringe pump of claim 14 wherein the bearing has a notch
formed in an outer surface so that the bearing will flex at a desired
location.
18. The syringe pump of claim 14 wherein the bearing has a plurality
of notches formed in the outer surface of the bearing so that the
bearing will flex at desired locations.
19. The syringe pump of claim 14 wherein the bearing has inner
tapered surfaces for engaging the diverging bearing surfaces of
the bearing mount.
20. A lead screw mounting system for use in a syringe pump having
a drive head adapted to drive a syringe plunger into the barrel
of a syringe to expel fluid from the barrel, the lead screw mounting
system comprising: a lead screw mounted at a first end to a frame;
a nut device engaged with the lead screw that moves along the lead
screw in response to rotation of the lead screw; a hollow connection
tube located over the lead screw and connected between the nut device
and the drive head such that movement of the nut device causes movement
of the connection tube over the lead screw and causes movement of
the drive head, the connection tube having an inner surface; wherein
a second end of the lead screw is continuously located within the
hollow of the connection tube; wherein the second end of the lead
screw comprises a bearing mount; and a bearing located at the bearing
mount, the bearing engaging both the bearing mount and the inner
surface of the connection tube so as to centrally locate the second
end of the lead screw in the connection tube during movement of
the connection tube along the lead screw.
21. The lead screw mounting system of claim 20 wherein the bearing
is biased outwardly into contact with the inner surface of the connection
tube.
22. The lead screw mounting system of claim 20 wherein the bearing
mount comprises two diverging mounting surfaces, the bearing engaging
one or the other depending upon the direction of movement of the
connection tube over the bearing.
Syringe pump description
BACKGROUND
[0001] The invention is related generally to lead screw drive mechanisms
for medical infusion pumps, and more particularly, to a bearing
system for mounting a lead screw in a syringe pump.
[0002] The infusion of fluids such as parenteral fluids into the
human body is accomplished in many cases by means of a syringe pump
having a lead screw and a screw drive device comprising a mechanism
that translates the rotational motion of the lead screw into linear
motion. The screw drive device is mounted to a syringe plunger driver
head that typically grasps the plunger flange of a syringe and that
applies the linear motion of the screw drive device to the syringe
plunger to empty the syringe at a controlled rate.
[0003] Because syringes are of different lengths and are filled
with different amounts of infusion fluids, the extension of the
syringe plunger from the syringe barrel can differ from syringe
to syringe. Many screw drive devices therefore include a disengagement
mechanism that the operator uses to disengage the lead screw drive
device from the lead screw threads. A disengagement mechanism control
is typically located at the plunger driver head and can take the
form of a lever or levers. Once disengaged, the operator may move
the plunger driver head, and therefore the screw drive device to
the position of the newly-mounted syringe plunger flange. The plunger
driver head may then engage the syringe plunger flange and once
engaged, the disengagement control may be released at which time
the plunger driver head will grasp the plunger flange and the lead
screw drive device will engage the threads of the lead screw at
the new position. It is desirable that this disengagement mechanism
and this plunger driver head be easy to use to facilitate operator
usage of the pump.
[0004] Such a lead screw drive device with its integrated disengagement
control and connected plunger driver head, although necessary, can
impart substantial forces on almost any part of a lead screw. The
screw drive device may be located at any position along the lead
screw depending on the length of the syringe mounted for use and
depending on the level of medical fluid remaining in the syringe.
Additionally, certain medical fluids are more difficult to pump
due to their viscosity or for other reasons, further placing an
increased load on the lead screw. For these reasons, it is desirable
to provide substantial mounting stability to the lead screw so that
efficiency is maintained in the development of rotational movement,
in the translation of that rotational movement to linear movement,
and in the application of that linear movement to the syringe plunger
head.
[0005] It is also the goal of syringe pump manufacturers to produce
pumps having increased flow uniformity. That is, manufacturers strive
to produce pumps that will pump exactly the selected flow rate throughout
the infusion and not vary from that selected flow rate, until the
syringe is exhausted or the rate is changed by the operator. However,
mechanical tolerances of the syringe pump parts, interactions with
the syringe, or other reasons can cause the flow rate of a syringe
pump to vary from the selected rate. A variance from the prescribed
and selected flow rate can be undesirable, especially if significant,
in that the patient may not receive the desired level of the infusion
fluid when needed. Manufacturers continue to refine their pump designs
to reduce these variances in flow rate as much as possible.
[0006] In one lead screw arrangement, one end of the lead screw,
i.e., a first end, is mounted through a transfer plate and has a
pulley mounted to its end. The transfer plate forms a part of the
inner frame of the syringe pump and consequently provides a stable
and rigid mounting point for the lead screw. The lead screw pulley
is directly engaged to the drive pulley of a motor through a drive
belt. A bearing may surround the lead screw at the portion located
through the transfer plate to lessen the effects of friction. In
another arrangement, both the first end of the lead screw and the
drive shaft of the motor may have gears and may be interconnected
through an intermediate gear or gears, although this arrangement
can result in less efficiency. In one design, the second end of
the lead screw may also be mounted to a rigid plate with a bearing
thus providing firm mounting to both ends of the lead screw. However,
mounting the second end of the lead screw to a rigid mounting plate
is not always an available option, especially when an extension
tube must be used between the screw drive device and the plunger
driver head.
[0007] The disengagement mechanism is typically formed as part
of the drive device and permits selective engagement and disengagement
of the drive device with the lead screw so that the drive device
may be selectively positioned on the lead screw to accommodate different
lengths of the syringes. A typical disengagement mechanism includes
half-nuts that are spring loaded into contact with the threads of
the lead screw. Through a series of levers and cams, the half-nuts
may be moved outwards from engagement with the lead screw threads
so that the drive device may be slid along the lead screw to the
desired position. The length of the lead screw and the disengagement
mechanism are designed to easily move the drive device along a substantial
portion of the lead screw so that the smallest syringes and the
largest syringes for which the pump is designed can be used with
the pump.
[0008] In one particular design, the second end of the lead screw
is located within a hollow connection tube that connects the screw
drive device with the plunger driver head. The second end of the
lead screw is not rigidly mounted but instead "floats"
within the connection tube. The length of the lead screw is selected
to exceed the travel of the syringe plunger within the syringe barrel
so that syringes of various sizes may be accommodated. When the
syringe barrel is full, the syringe plunger will be at the proximal
end of the barrel with the plunger stem extended almost its entire
length outside the syringe barrel. This configuration results in
the overall syringe being almost twice the length of its barrel.
Because some syringes are relatively long, the lead screw may be
located at one end of the pump housing, for example the distal end,
with the connection tube extending from the lead screw to a point
near the other end of the housing, for example the proximal end,
to engage the syringe plunger stem flange. However, the second end
of the lead screw will always be located within the connection tube
regardless of where the syringe plunger driver head is located.
[0009] In the approach described above where the second end of
the lead screw is located within the hollow connection tube and
is allowed to "float" in the tube, rigid mounting of that
second end is not possible. Because there is a size difference between
the outer diameter of the lead screw second end and the inner diameter
of the hollow interior of the connection tube, the angle of the
lead screw within the connection tube can change. Even a slight
change in the angle between the two has been found to lessen the
flow uniformity of the pump. The second lead screw end tends to
move within the connection tube depending on the forces exerted
on the lead screw thus adding inefficiency to the translation of
the rotational motion of the lead screw to the linear motion of
the screw drive device. The lead screw threads can change their
angle of engagement with the screw drive device threads resulting
in greater or lesser friction between the two and consequently resulting
in lowered flow uniformity or flow accuracy of the pump.
[0010] A further undesirable effect of the floating second end
of the lead screw is that it interacts with the interior of the
connection tube scoring or gouging out the tube thereby imparting
increased wear, and causing a larger difference in size between
the lead screw and the connection tube thereby allowing for even
more movement of the second end of the lead screw in the future.
[0011] In an effort to reduce the undesirable effects caused by
movement of the floating lead screw, the floating second end of
the lead screw has been hollowed to reduce its weight. This has
been found to lessen the damage it does to the connection tube and
can lower the amount of movement of the second end resulting in
greater flow uniformity. However, the manufacturing process of hollowing
a lead screw increases the cost of the screw as well as increases
the rate of lead screw waste due to errors made during the hollowing
process. This waste also increases manufacturing costs.
[0012] Hence, those skilled in the art have recognized a need for
a stabilizing mechanism to be used with the second end of the lead
screw so that the end is held in axial alignment with the connection
tube and the drive device during operation. Further, those skilled
in the art have recognized a need for reducing the costs of manufacturing
a lead screw. The invention satisfies these needs and others.
SUMMARY OF THE INVENTION
[0013] The present invention is directed to a system and method
for mounting the second end of a lead screw within an connection
tube such that the lead screw is stabilized and aligned with the
connection tube is maintained during the entire range of movement
of the connection tube over the lead screw. The present invention
also provides for changing the second end of a lead screw such that
those lead screws previously found to be defective due to poor workmanship
are now usable. Further, the invention provides for a lead screw
that may be solid in configuration thus foregoing a manufacturing
step of hollowing the screw thus reducing the cost of the lead screw.
[0014] In one aspect, the invention is directed to a syringe pump
having a lead screw with a drive device in threaded engagement with
the lead screw, the drive device moving along the lead screw in
response to rotation of the lead screw, and a drive head adapted
to drive a syringe plunger into the barrel of a syringe to expel
fluid contents of the syringe, the syringe pump comprising a lead
screw having a first end and a second end with the first end mounted
to a frame, a connection tube located over the lead screw and connected
between the drive device and the drive head to communicate movement
of the drive device along the lead screw to the drive head, the
connection tube having a hollow portion with an inner surface, wherein
the second end of the lead screw is located within the hollow portion
of the connection tube but is otherwise unmounted, and a bearing
located at the second end of the lead screw in contact with the
inner surface of the hollow portion of the connection tube thereby
creating a guided cantilever mount of the second end of the lead
screw within the connection tube.
[0015] In more detailed aspects, the second end of the lead screw
comprises a bearing mount having outwardly diverging bearing surfaces
with the bearing located at the bearing mount, the bearing engaging
one or the other of the diverging bearing surfaces in response to
movement of the connection tube over the bearing in a particular
direction. In further aspects, the outwardly diverging mounting
surfaces of the bearing mount diverge in axial directions from a
bearing surface center location, the bearing engaging one or the
other of the diverging mounting surfaces depending upon the direction
of movement of the connection tube over the bearing. Further, the
bearing mount has a generally hourglass shape.
[0016] In yet other aspects, the bearing has inner tapered surfaces
for engaging the diverging bearing surfaces of the bearing mount
and the bearing is biased outwardly into continuous contact with
the inner surface of the connection tube. Further, the bearing has
an inner surface that is shaped in an approximate complementary
shape to the diverging bearing surfaces, whereby the bearing locates
the second end of the lead screw in the approximate center of the
connection tube.
[0017] Other detailed aspects include the bearing having a larger
outer diameter than the inner diameter of the hollow portion of
the connection tube within which it is mounted, the bearing having
a discontinuity that permits the bearing to compress to thereby
be mounted within the connection tube. Further, the bearing has
a notch formed in an outer surface so that the bearing will bend
at a desired location. In more detail, the bearing has a plurality
of notches formed in the outer surface of the bearing so that the
bearing will bend at desired locations to provide a more uniform
force against both the bearing surfaces and the inner surface of
the connection tube to more accurately locate the second end of
the lead screw within the connection tube.
[0018] In yet other detailed aspects, the bearing and bearing surfaces
of the bearing mount are made of materials selected such that the
bearing easily slides along the bearing surfaces, whereby the bearing
moves up one or the other bearing surfaces to maintain contact between
the inner surface of the connection tube and the respective bearing
surface to more accurately center the second end of the lead screw
in the connection tube.
[0019] Other aspects and advantages of the invention will become
apparent from the following detailed description and the accompanying
drawings, illustrating by way of example the features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a perspective view of a syringe pump showing a
syringe mounted on the pump for use in infusing its contents to
a patient through a medical fluid administration set, a drive head
in contact with the syringe plunger flange to drive the syringe
plunger into the syringe barrel to expel the contents of the syringe
into the fluid administration set that is connected to the vascular
system of a patient;
[0021] FIG. 2 is a simplified perspective view of the drive device
of the syringe pump of FIG. 1 in which a lead screw is mounted at
a first end through a transfer plate and has a pulley for drive
connection with a motor, the lead screw having a floating second
end located within an connection tube that connects a drive device
engaged with the threads of the lead screw with the drive head shown
in FIG. 1;
[0022] FIG. 3 presents a side view of an embodiment of a lead screw
in accordance with aspects of the invention and as used in FIGS.
1 and 2 showing the first end mounted through a cross-sectioned
transfer plate via a bearing with a drive pin, with the cross-sectioned
drive pulley shown mounted to the first end of the lead screw via
a mounting nut, and the second end of the lead screw having an "hour
glass" shaped bearing mount, or bearing race, for mounting
a bearing;
[0023] FIG. 4 is a side view of a bearing in accordance with aspects
of the invention usable with the bearing mount located at the second
end of the lead screw of FIG. 3 to stabilize the second end of
the lead screw within the connection tube of FIG. 2 showing the
split of the bearing, a plurality of slots in the external surface
of the bearing, and the inner bearing tapers for engagement with
the bearing mount;
[0024] FIG. 5 is a perspective view of a cutaway portion of the
bearing of FIG. 4 along lines 5-5 showing the inner profile that
mates with the hourglass shape of the bearing mount (race) located
at the second end of the lead screw, and the external bearing surface
that mates with the inner surface of the connection tube;
[0025] FIG. 6 is an assembled view of the bearing at the second
end of the lead screw of FIG. 3 in accordance with aspects of the
invention, with the bearing of FIGS. 4 and 5 in place on the bearing
mount, showing the bearing in cross section;
[0026] FIG. 7 shows the bearing in contact with the inner surface
of the connection tube, the bearing also in contact with the proximal
diverging surface of the bearing mount due to movement of the connection
tube in the proximal direction; and
[0027] FIG. 8 again shows the bearing in contact with the inner
surface of the connection tube, the bearing in this case being in
contact with the distal diverging surface of the bearing mount due
to movement of the connection tube in the distal direction.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] Referring now to the drawings with more particularity, wherein
like reference numerals designate like or corresponding elements
among the several views, there is shown in FIG. 1 a view of a syringe
pump 10 having a plunger driver system in accordance with the principles
of the invention. A syringe 12 is shown mounted in the pump with
certain mounting and sensing details removed for clarity of illustration.
The syringe pump includes a syringe cradle 14 in which the syringe
barrel 16 will rest. The syringe barrel flange 18 will be located
in a barrel flange groove 20 in the pump 10 to immobilize the syringe
barrel from longitudinal movement during movement of the syringe
plunger 22 within the barrel.
[0029] Details of the barrel flange groove are not shown in FIG.
1 nor are they described here since such a feature is well known
to those skilled in the art. Additionally, the function of the groove
in holding a syringe barrel longitudinally immobile may be provided
by different structure than that shown. Further a barrel clamp typically
is also used to retain the syringe barrel at its position in the
syringe pump, but is not shown here in order to preserve the clarity
of other features.
[0030] The syringe plunger flange 24 having an inner side 26
is interconnected with the syringe plunger 22 by a syringe plunger
stem 28. When mounted in the syringe pump 10 properly, the plunger
flange 24 is held at a plunger drive head 30 with a pair of pivotally
mounted plunger retaining arms 32 one of which is shown in the
closed position in FIG. 1. The second pivotally mounted arm has
been removed for clarity purposes. A disengagement lever 34 is used
to disengage the plunger drive head 30 from the threads of a lead
screw (not shown) as well as control the positions of the retaining
arms 32 to allow removal and insertion of a syringe plunger flange
24. Disengaging the plunger drive head 30 from the threads of the
lead screw permits the operator to move the plunger drive head 30
along the lead screw to the correct position to capture the plunger
flange of a syringe 12. As is well known and as is described in
the BACKGROUND section, syringes 12 having different quantities
of fluid in them may be provided for use with a syringe pump 10
and the plunger 22 may be located at different positions in relation
to the barrel 16. Additionally, syringes of different sizes may
be usable in the syringe pump 10 which also results in the plunger
flange 24 being at different locations, depending on the size of
the syringe and the level to which it is filled. The ability to
manually move the drive head 30 permits the accommodation of syringes
of different sizes with different beginning plunger positions.
[0031] From the foregoing, it will be appreciated that the plunger
driver system as shown and described provides a versatile system
to accept various sizes of syringes and results in easier pump operation
as well as resists siphoning. For further details on a drive head
with a disengagement lever and flange-grasping arms, refer to U.S.
Pat. No. 6428509 to Fielder, issued on Aug. 6 2002 which is
hereby incorporated by reference.
[0032] Returning to FIG. 1 the syringe 12 is connected to a patient
36 through a fluid administration set 38 comprising a length of
tubing 40. The tubing 40 is mounted to a sharpened cannula 42 that
has been introduced to a blood vessel of the patient 36. As the
drive head 30 moves in the distal direction, fluid residing in the
syringe barrel will be expelled into the tubing 40 of the fluid
administration set 38 and flow into the patient's vein through the
cannula 42. It should be noted that the administration set 38 may
have additional features, such as a pressure sensor disk, flow controllers,
ports, or other devices mounted to it or formed as part of it. Such
devices have not been shown in this figure so as to maintain clarity
of illustration of the basic administration set.
[0033] FIG. 2 presents further details of a syringe pump drive
assembly 50 showing a threaded lead screw 52 mounted to a transfer
plate 54. The transfer plate forms part of the internal frame of
the syringe pump, its anchored nature providing a rigid mounting
point for the first end of the lead screw. A corner of the transfer
plate 54 has been cut away so that a drive pulley 56 mounted to
a first end 58 of the lead screw can be more clearly seen. The first
end 58 is mounted within a first bearing 60 for providing a lower
friction mounting arrangement with the transfer plate 54. In this
case, the first bearing has been welded to the transfer plate to
achieve a permanent mounting. A drive motor 62 is also mounted to
the transfer plate 54 and includes a motor pulley (not shown) on
the other side of the transfer plate. A drive belt 64 interconnects
the motor pulley with the drive pulley 56 and transfers rotary movement
developed by the motor to the drive pulley 56 and thereby, to the
lead screw 52. Different arrangements of coupling the rotary motion
of the motor to the lead screw may be used, including a direct drive
arrangement or a series of gears. Efficiencies may vary depending
on the particular drive arrangement chosen.
[0034] A screw drive device 66 is mounted to the lead screw 52
and, although not shown, is prevented from rotating due to its mounting
arrangement in the syringe pump. Such mounting arrangements are
well known to those skilled in the art and hence, no further details
are provided here. It therefore will translate the rotation of the
lead screw to linear motion. Because the lead screw is firmly anchored
in the syringe pump and can only rotate, the drive device will move
along the lead screw. An optical position determination system 68
is shown comprising a set or markers 70 with an optical reading
device 72. The optical position determination system may also be
capable of determining the speed of movement of the drive device
66 based on the time between sensing various markers. Such a position-determination
and speed-of-movement system is known to those skilled in the art
and no further details are provided here. U.S. Pat. No. 5236416
to McDaniel, issued on Aug. 17 1993 describes such a system and
is incorporated herein by reference. Other designs and mechanisms
may be used to accomplish the same result.
[0035] The screw drive device 66 is connected to the plunger drive
head 30 that was shown in FIG. 1 with a hollow connection tube 74.
The connection tube is firmly mounted to the screw drive device
66 and to the drive head 30. Thus, linear movement of the screw
drive device along the lead screw causes the drive head to move
commensurately. Such a system is known to those skilled in the art
and no further details are provided herein. U.K. Patent No. GB 2
224 444 to Welmed Limited, inventor B. Lim, published May 9 1990
describes and shows such a drive arrangement and is incorporated
herein by reference.
[0036] It will be noted by reference to FIG. 2 that the lead screw
52 resides partially within the connection tube 74 and has a second
end 76 located fully within the connection tube. In accordance with
the previous description, the drive device 66 moves along the lead
screw as the lead screw rotates. Further, the drive device can be
manually located anywhere along the lead screw so that the drive
head 30 may engage a mounted syringe plunger flange (seen in FIG.
1). For example, when a large syringe is mounted to the syringe
pump and that syringe is full, the drive head 30 may need to be
moved much farther to the right in the figure (proximal direction),
in which case, less of the lead screw will reside within the connection
tube. As the lead screw is rotated by the motor 62 to move the drive
head 30 to the left direction (distal) in the figure to empty the
syringe, more of the lead screw will be located within the connection
tube. In all cases, the second end 76 of the lead screw will always
be located within the connection tube.
[0037] To overcome some of the problems facing prior syringe pump
drive devices as previously reviewed, the second end 76 of the lead
screw 52 in the embodiments shown includes a connection tube bearing
78 in accordance with aspects of the invention. The bearing is configured
to make contact with the inner surface of the connection tube to
keep the second end 76 of the lead screw better centered within
the connection tube 74. Other than the bearing, the second end of
the lead screw is unmounted. The lead screw is thus generally in
a cantilever arrangement; however, the addition of the bearing that
contacts both the lead screw second end and the inner surface of
the connection tube results in a "guided cantilever mounting"
of the second end of the lead screw. This "guided" mounting
has been found to result in the lead screw constantly being concentric
with the connection tube which greatly improves flow uniformity
of the syringe pump.
[0038] Referring to FIG. 3 a side view of the lead screw 52 is
shown. The transfer plate 54 lead screw pulley 56 and the first
bearing 60 are shown. As mentioned above, in a preferred embodiment
the first bearing 60 is welded to the transfer plate; however, it
may be press fit or held in place through other means. The pulley
56 may be mounted to the lead screw in various well known ways.
In this case, the pulley has a drive pin 80 that assures rotation
of the pulley with the rotation of the lead screw 52 and a retaining
nut 82 that affixes the pulley to the first end 58 of the lead screw.
Other mounting techniques for both the bearing and the pulley are
possible.
[0039] At the second end, or proximal end 76 of the lead screw
52 a bearing mount 84 or bearing race is formed. Formed at a location
proximal to the threads 85 and between the threads and the bearing
mount is a thread undercut 86. The bearing mount has a land 88 at
each end with outer chamfers 90. The outer chamfers facilitate sliding
the bearing mount into the connection tube. The bearing mount also
comprises two outwardly diverging bearing surfaces 92 separated
by a bearing mount undercut 94. The bearing mount 84 has the general
appearance of the shape of an hourglass. The bearing mount undercut
94 is a production feature providing a convenient place in which
the bearing may be compressed during the process of sliding the
second end 76 of the lead screw into the connection tube 74. The
crest of the bearing, as is shown and described below, will be located
at least initially in the undercut 94 and because of the depth of
the undercut, bearings of differing sizes may function well in the
bearing mount 84. This permits the bearings to be made with looser
tolerances which reduces manufacturing costs.
[0040] A suitable lead screw bearing 100 in accordance with aspects
of the invention is shown in FIGS. 4 and 5. The bearing comprises
an outer bearing surface 102 an inner bearing crest 104 two tapered
surfaces 106 and chamfers 108 on either side of the outer bearing
surface. The bearing is split 110 as shown in FIG. 4 to allow the
bearing to be more easily mounted over the bearing mount 84. Three
slots 112 are also formed in the outer bearing surface to control
the points of bending of the bearing. Because the slots are spaced
at ninety degrees from each other, including the split 110 the
bearing more uniformly applies outward pressure to the connection
tube, which provides better centering action of the lead screw within
the connection tube. The three notches 112 and the split 110 have
the effect of producing four segments with a more controlled spring
effect as well as making the bearing easier to compress when inserting
it into the connection tube 74. Although three notches are shown
in this embodiment, more or fewer notches may be used as desired.
[0041] In FIG. 6 the bearing 100 is shown mounted to the bearing
mount 84 of the lead screw 52. The crest 104 of the bearing is located
within the bearing mount undercut 94 of the bearing mount which
provides a good starting position for the bearing. It will be noted
from the figure that when centered on the bearing mount, the bearing
inner diameter is larger than the outer diameter of the corresponding
points of the bearing mount 84 and the outer diameter of the bearing
is larger than the outer diameter of the threads of the lead screw
52. This is so that the outer surface of the bearing will always
make contact with the inner surface 114 of the connection tube 74
(FIG. 2). The connection tube is typically formed by injection molding
and because of that process, the inner channel 114 through the connection
tube is tapered from one end to the other. Consequently the lead
screw bearing 78 must be able to accommodate the tapering inner
diameter of the connection tube. Hence, the outer diameter of the
lead screw bearing is set so that it will make contact with the
largest inner diameter of the connection tube. The chamfers 108
of the bearing 78 shown in FIG. 4 assist in mounting the second
end 76 of the lead screw within the connection tube 74. Whether
the second end of the lead screw is pushed forward into the connection
tube or drawn rearward into the connection tube, the outer chamfers
provide an angle of contact between the bearing and the ends of
the connection tube that tend to compress the bearing so that insertion
into the connection tube is facilitated. Additionally, the chamfers
remove any sharp edges of the bearing that may have otherwise provided
greater friction with the inner surface of the connection tube once
the bearing is mounted within the connection tube.
[0042] Additionally, the bearing 78 tapered surfaces 106 are complementary
to the diverging surfaces 92 of the bearing mount 84. Because the
bearing 100 is formed of a low friction substance, it will easily
slide along the diverging surfaces 92 of the bearing mount as needed
to maintain the lead screw second end 78 centered within the connection
tube 74. The diverging surfaces 92 of the bearing mount give the
mount a general hourglass appearance. It can also be noted that
the bearing mount 84 is symmetric about the bearing mount under
cut 94 and this shape provides a distinct advantage, as is demonstrated
in FIGS. 7 and 8.
[0043] Referring now to FIG. 7 the bearing 100 and bearing mount
84 located at the second end 76 of the lead screw 52 are shown mounted
within the connection tube 74 which is partially shown. The bearing
is in contact with the inner surface 114 of the connection tube.
Because the connection tube is being drawn in the proximal direction,
possibly because the drive head 30 is being attached to a new syringe
(FIG. 1), the friction between the bearing outer surface 102 and
the inner surface 114 of the connection tube draws the bearing up
the proximal diverging surface 116 of the bearing mount in a "wedging"
action. Because the tapered bearing surfaces 106 have the same angle
as the diverging bearing surfaces 92 of the bearing mount 84 an
inclined plane type of arrangement results. This wedging action
has the effect of providing even more stabilizing force against
the lead screw to keep it centered within the connection tube.
[0044] FIG. 8 presents another condition in which the connection
tube 74 is being moved over the second end 76 of the lead screw
52 this time in the distal direction. This is probably due to the
syringe pump 10 being operated to expel the contents of the syringe
12 into the administration set 38 (FIG. 1). Consequently, the drive
head 30 and connection tube 74 are moving toward the syringe barrel
16 in the distal direction. Because the connection tube is being
drawn in the distal direction, the friction between the bearing
outer surface 102 and the inner surface 114 of the connection tube
draws the bearing up the most distal diverging surface 118 of the
bearing mount in the wedging action described above. As also mentioned,
this wedging action has the effect of providing even more stabilizing
force against the lead screw to keep it centered within the connection
tube, and in this case where medical fluid is being infused into
the patient, this centering action has been found to result in better
flow uniformity from the syringe pump.
[0045] This wedging action results because the bearing 100 is larger
than the inner diameter of the connection tube 74 and consequently
exerts a continual force against the interior of the connection
tube, and because of the friction between the inner surface 114
of the connection tube and the outer surface 102 of the bearing.
Additionally, the diverging surfaces 92 of the bearing mount 84
and the complementary tapered surfaces 106 of the bearing 100 facilitate
movement of the bearing into the wedging action.
[0046] The lead screw may be formed of stainless steel with an
electroless nickel plate. The bearing may be formed of Delrin.TM.
material, an acetal resin available from E.I. DuPont de Nemours
and Company. The connection tube may be formed of glass-filled nylon.
Other materials may be used for these components as well.
[0047] While in the present embodiment, the bearing mount is cut
from the same piece of material as the threaded portion of the lead
screw, other approaches are possible. For example, in the case where
hollowed lead screws exist, the bearing mount may be made separately
and attached to the proximal end of the lead screw, such as by adhesive
or welding thereby salvaging lead screws or possibly providing a
lower cost lead screw.
[0048] Although specific embodiments of the invention have been
described and illustrated it is clear that the invention is susceptible
to numerous modifications and embodiments within the ability of
those skilled in the art, and without the exercise of the inventive
faculty. Thus, it should be understood that various changes in form,
detail and application of the present invention may be made without
departing from the spirit and scope of the invention. |