Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Rotational atherectomy systems and methods with shock absorbing element
INVENTORS
Joseph Higgins, a citizen of United States of America, resident at Minnetonka,
MN
Jeffrey Allen McBroom, a citizen of United States of America, resident at
Champlin, MN
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. Patent
Application 14/208478
filed March 13, 2014, which claims the benefit of U.S. Provisional Patent
Application 61/932409
filed January 28, 2014 entitled devices, systems and methods for a shock
absorbing drive system
for medical devices and U.S. Provisional Patent Application 61/787027 entitled
rotational
atherectomy device with biasing clutch, filed March 15, 2013, the contents of
which are hereby
incorporated by reference herein in their entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] FIELD OF THE INVENTION
[0004] The invention relates to devices and methods for removing tissue
from body
passageways, such as removal of atherosclerotic plaque from arteries,
utilizing a rotational
atherectomy device. In particular, the invention relates to improvements in a
rotational
atherectomy device having a biasing clutch and/or a shock absorbing element.
[0005] DESCRIPTION OF THE RELATED ART
[0006] Atherectomy is a non-surgical procedure to open blocked coronary
arteries or
vein grafts by using a device on the end of a catheter to cut or shave away
atherosclerotic plaque
(a deposit of fat and other substances that accumulate in the lining of the
artery wall). For the
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purposes of this application, the term "abrading" is used to describe the
grinding and/or scraping
action of such an atherectomy head.
[0007] Atherectomy is performed to restore the flow of oxygen-rich blood
to the heart, to
relieve chest pain, and to prevent heart attacks. It may be done on patients
with chest pain who
have not responded to other medical therapy and on certain of those who are
candidates for
balloon angioplasty (a surgical procedure in which a balloon catheter is used
to flatten plaque
against an artery wall) or coronary artery bypass graft surgery. It is
sometimes performed to
remove plaque that has built up after a coronary artery bypass graft surgery.
[0008] Atherectomy uses a rotating shaver or other device placed on the
distal end of a
catheter to slice away or destroy plaque. At the beginning of the procedure,
medications to
control blood pressure, dilate the coronary arteries, and prevent blood clots
are administered.
The patient is awake but sedated. The catheter is inserted into an artery in
the groin, leg, or arm,
and threaded through the blood vessels into the blocked coronary artery. The
cutting head is
positioned against the plaque and activated, and the plaque is ground up or
suctioned out.
[0009] The types of atherectomy are rotational, directional, and
transluminal extraction.
Rotational atherectomy uses a high speed rotating shaver to grind up plaque.
Directional
atherectomy was the first type approved, but is no longer commonly used; it
scrapes plaque into
an opening in one side of the catheter. Transluminal extraction coronary
atherectomy uses a
device that cuts plaque off vessel walls and vacuums it into a bottle. It is
used to clear bypass
grafts.
[0010] Performed in a cardiac catheterization lab, atherectomy is also
called removal of
plaque from the coronary arteries. It can be used instead of, or along with,
balloon angioplasty.
[0011] Several devices have been disclosed that perform rotational
atherectomy. For
instance, U.S. Patent No. 5,360,432, issued on November 1, 1994 to Leonid
Shturman, and titled
"Abrasive drive shaft device for directional rotational atherectomy" discloses
an abrasive drive
shaft atherectomy device for removing stenotic tissue from an artery, and is
incorporated by
reference herein in its entirety. The device includes a rotational atherectomy
apparatus having a
flexible, elongated drive shaft having a central lumen and a segment, near its
distal end, coated
with an abrasive material to define an abrasive segment. At sufficiently high
rotational speeds,
the abrasive segment expands radially, and can sweep out an abrading diameter
that is larger than
its rest diameter. In this manner, the atherectomy device may remove a
blockage that is larger
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than the catheter itself. Use of an expandable head is an improvement over
atherectomy devices
that use non-expandable heads; such non-expandable devices typically require
removal of
particular blockages in stages, with each stage using a differently-sized
head.
[0012] U.S. Pat. No. 5,314,438 (Shtunnan) shows another atherectomy device
having a
rotatable drive shaft with a section of the drive shaft having an enlarged
diameter, at least a
segment of this enlarged diameter section being covered with an abrasive
material to define an
abrasive segment of the drive shaft. When rotated at high speeds, the abrasive
segment is
capable of removing stenotic tissue from an artery.
[0013] A typical atherectomy device includes a single-use disposable
portion, which can
be attached and detached from a non-disposable control unit (also referred to
as a controller).
The disposable portion includes elements that are exposed to saline and to the
bodily fluids of the
patient, such as a handle, a catheter, a rotatable drive shaft, and an
abrasive head. The handle
includes a turbine that rotates the drive shaft, and a knob that can
longitudinally advance and
retract the drive shaft along the catheter. Often, the device has a foot
switch that activates the
handle.
[0014] Typical known atherectomy devices use pneumatic power to drive the
drive shaft,
with the controller managing the amount of compressed air that is delivered to
the turbine in the
handle. The compressed air spins the turbine that, in turn, spins the drive
shaft, and spins an
abrasive crown attached to the drive shaft. Orbiting motion of the crown
enlarges and widens
the channel opening of a restricted or blocked vascular vessel.
[0015] There is currently a great deal of effort devoted to incorporating
other types of
rotational actuators into the atherectomy devices, primarily to replace the
need for a source of
compressed air. A motor requires a way limit the torque delivered to the drive
shaft. For
instance, if the distal end of the drive shaft encounters an obstacle and gets
stuck (i.e., stops
rotating), it is preferable that the torque delivered to the drive shaft be
limited, so that the drive
shaft does not wind up excessively and abruptly release. Such a sudden release
of energy may
result in damage to the patient or the device, and should be avoided.
[0016] Accordingly, there exists a need for a clutch between the motor and
the drive
shaft in a rotational atherectomy device.
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BRIEF SUMMARY OF THE INVENTION
[0017] In some embodiments, a rotational atherectomy system may include an
elongated,
rotatable, flexible drive shaft having a distal end for insertion into a
vasculature of a patient. The
drive shaft may include a proximal end opposite the distal end that remains
outside the
vasculature of the patient. The system may include a motor for rotating the
drive shaft and a
shock absorbing element may be provided for coupling the motor to the drive
shaft. During
steady state conditions, the shock absorbing element may transfer the full
torque from the motor
to the drive shaft. However, during abrupt increases in the differential
torque between the motor
and the drive shaft, the shock absorbing element may absorb a portion of the
increasing torque
and, at the same time, may maintain a mechanical coupling between the drive
shaft and the
motor preventing slippage.
[0018) In other embodiments, a rotational atherectomy system may include a
clutch
having a characteristic threshold torque, comprising a motor plate
rotationally connected to the
motor, a drive shaft plate rotationally connected to the drive shaft, the
motor plate and the drive
shaft plate being parallel and coaxial, being disposed directly longitudinally
adjacent to each
other, and being held proximate one another longitudinally with a space
therebetween, and a
biasing clutch configured to rotationally engage the motor plate and the drive
shaft plate.
[0019] While multiple embodiments are disclosed, still other embodiments
of the present
disclosure will become apparent to those skilled in the art from the following
detailed
description, which shows and describes illustrative embodiments of the
invention. As will be
realized, the various embodiments of the present disclosure are capable of
modifications in
various obvious aspects, all without departing from the spirit and scope of
the present disclosure.
Accordingly, the drawings and detailed description are to be regarded as
illustrative in nature and
not restrictive.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0020] While the specification concludes with claims particularly pointing
out and
distinctly claiming the subject matter that is regarded as forming the various
embodiments of the
present disclosure, it is believed that the invention will be better
understood from the following
description taken in conjunction with the accompanying Figures, in which:
[0021] Figure 1 is a perspective view of a known rotational atherectomy
device.
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[0022] Figure 2 is a block diagram of the motor, the drive shaft and the
clutch that
mechanically couples them together.
[0023] Figure 3 is a schematic drawing of the clutch of Figure 2.
[0024] Figure 4 is a plot of rotational speed of the drive shaft and
torque at the distal end
of the drive shaft, for a typical procedure.
[0025] Figure 5 is a plot of the torque transmitted to the proximal end of
the drive shaft,
versus the torque of the motor.
[0026] Figure 6 is a plot of torque at the distal end of the drive shaft
versus time for a
distal-end-stopping event, for a known gas turbine system.
[0027] Figure 7 is a plot of torque at the distal end of the drive shaft
versus time for a
distal-end-stopping event, for the present motor-driven system with the clutch
of Figure 3.
[0028] Figure 8A.' shows a biasing clutch according to some embodiments.
[0029] Figure 8B shows a schematic diagram of one embodiment of the
biasing clutch of
Figure 8A.
[0030] Figure 8C shows a schematic diagram of another embodiment of the
biasing
clutch of Figure 8A.
[0031] Figure 8D shows a schematic diagram of another embodiment of the
biasing
clutch of Figure 8A.
[0032] Figure 9A shows a block diagram of a motor, a drive shaft, and a
shock absorbing
element that mechanically couples them together.
[0033] Figure 9B shows a plot of rotational speed of the drive shaft
versus time for a
distal-end-stopping event, for a motor driven system with the shock absorbing
element of Figure
9A.
[0034] Figure 10A shows a motor diagram of an atherectomy device having a
shock
absorbing drive gear, according to some embodiments.
[0035] Figure 10B shows a close-up view of the drive gear of Figure 9B,
according to
some embodiments.
[0036] Figure 11A shows a motor diagram of an atherectomy device having a
shock
absorbing take-off element, according to some embodiments.
[0037] Figure 11B shows a close-up view of the take-off element of Figure
10A,
according to some embodiments.
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[0038] Figure 12A shows a motor diagram of an atherectomy device having a
shock
absorbing drive belt, according to some embodiments.
[0039] Figure 12B shows a close-up view of the drive belt of Figure 11A,
according to
some embodiments.
[0040] Figure 13A shows a motor diagram of an atherectomy device having a
shock
absorbing drive belt and idler system, according to some embodiments.
[0041] Figure 13B shows a close-up view of the drive belt of Figure 12A,
according to
some embodiments.
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DETAILED DESCRIPTION OF THE INVENTION
[0042] An atherectomy device is disclosed, with a clutch between the motor
and the drive
shaft. The clutch may include two plates that rely on friction to transmit
torque from one plate to
the other. The clutch may have an attractive magnetic normal force that holds
the plates together
or, in another embodiment, a biasing mechanism may hold the plates together.
For relatively low
torques, as is the case during normal use, a static frictional torque may hold
the plates together,
and the plates spin together without slipping. For relatively high torques, as
occurs when the
distal end of the drive shaft encounters an obstacle and stops abruptly, the
high torque exceeds
the maximum possible static frictional torque, and the plates slip. When
slipping, the plates
transmit a kinetic frictional torque that is low enough to avoid damage to the
patient or to the
atherectomy device. In some cases, the torque levels associated with a
stoppage of the drive
shaft distal end are chosen to mimic those of a known atherectomy device, in
which a gas-driven
turbine is clutchlessly attached to the drive shaft. In other embodiments,
rather than a frictional
force being used to transmit the torque, a biasing mechanism having variable
torsional strength
may be used to transfer the torque. For example, a spring may connect the
clutch plates and may
transfer the torque based on a torsional stiffness of the spring. When an
obstruction is
encountered, the variable torsional stiffness may allow the spring to wind and
may allow for
delay in an excessive torque at the distal end being applied. As such, the
biasing mechanism in
this embodiment may allow for a speed or current interrupter to cause the
drive shaft to stop, for
example.
[0043] The preceding paragraph is merely a summary, and should not be
construed as
limiting in any way. A more detailed description of the several embodiments
follows.
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[0044] Figure 1 is a schematic drawing of a typical rotational atherectomy
device. The
device includes a handle portion 10, an elongated, flexible drive shaft 20
having an eccentric
enlarged abrading head 28, and an elongated catheter 13 extending distally
from the handle
portion 10. The drive shaft 20 is constructed from helically coiled wire as is
known in the art
and the abrading head 28 is fixedly attached thereto. The catheter 13 has a
lumen in which most
of the length of the drive .shaft 20 is disposed, except for the enlarged
abrading head 28 and a
short section distal to the enlarged abrading head 28. The drive shaft 20 also
contains an inner
lumen, permitting the drive shaft 20 to be advanced and rotated over a guide
wire 15. A fluid
supply line 17 may be provided for introducing a cooling and lubricating
solution (typically
saline or another biocompatible fluid) into the catheter 13.
[0045] The handle 10 desirably contains a turbine (or similar rotational
drive mechanism)
for rotating the drive shaft 20 at high speeds. The handle 10 typically may be
connected to a
power source, such as compressed air delivered through a tube 16. A pair of
fiber optic cables
25, alternatively a single fiber optic cable may be used, may also be provided
for monitoring the
speed of rotation of the turbine and drive shaft 20. Details regarding such
handles and associated
instrumentation are well known in the industry, and are described, e.g., in
U.S. Pat. No.
5,314,407, issued to Auth, and incorporated by reference herein in its
entirety. The handle 10
also desirably includes a control knob 11 for advancing and retracting the
turbine and drive shaft
20 with respect to the catheter 13 and the body of the handle.
[0046] The abrasive element 28 in Figure 1 is an eccentric solid crown,
attached to the
drive shaft 20 near the distal end of the drive shaft 20. The term "eccentric"
is used herein to
denote that the center of mass of the crown is laterally displaced away from
the rotational axis of
the drive shaft 20. As the drive shaft rotates rapidly, the displaced center
of mass of the crown
causes the drive shaft to flex radially outward in the vicinity of the crown
as it spins, so that the
crown may abrade over a larger diameter than its own rest diameter. Eccentric
solid crowns are
disclosed in detail in, for example, U.S. Patent Application No. 11/761,128,
filed on June 11,
2007 to Thatcher et al. under the title, "Eccentric abrading head for high-
speed rotational
atherectomy devices", published on December 11, 2008 as U.S. Patent
Application Publication
No. US2008/0306498, and incorporated by reference herein in its entirety.
[0047] There is currently an effort to replace the gas-driven turbine of
the known
atherectomy device with an electric motor. Such a motor has different
mechanical characteristics
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than the turbine, such as an increased rotational inertia. The present
application is directed
mainly to a clutch that connects a motor to the drive shaft. Such a clutch can
limit the torque
delivered by the motor, so that if the distal end of the drive shaft
encounters an obstacle and
suddenly stops rotating, the clutch will prevent a damaging amount of torque
from being
delivered to the drive shaft. Aside from the motor, many or all of the other
elements of the
known atherectomy device of Figure 1 may be used with the present disclosed
head design,
including the catheter 13, the guide wire 15, the control knob 11 on the
handle 10, the helically
coiled drive shaft 20 and the eccentric solid crown 28.
[0048] Figure 2 is a block diagram of the motor 30, the drive shaft 20 and
the clutch 40
that mechanically couples them together. In this figure and those that follow,
the "motor" may
be an electric motor, a gas-driven turbine, or any suitable device that
generates a controllable
amount of rotation. During normal use, the clutch 40 is engaged, and the
rotation produced by
the motor 30 is passed directly on to the drive shaft 20. In the event that
the distal end of the
drive shaft 20 becomes caught or encounters a blockage that suddenly stops its
rotation, the
clutch disengages, so that the motor 30 does not continue to rotate the
proximal end of the drive
shaft. Such a continued rotation would excessively wind up the drive shaft,
and the torques
associated with such a winding could potentially damage the blood vessel of
the patient or the
atherectomy device itself, which are both undesirable outcomes.
[0049] Additionally, the clutch may provide a convenient interface between
the drive
shaft, which is typically a replaceable or disposable element, and the motor,
which is typically
used repeatedly.
[0050] Figure 3 is a schematic drawing of the clutch 40 of Figure 2. The
clutch 40
includes two plates, 41 and 42, held together by an attractive magnetic force.
The plates 41, 42
are attached to spindles that rotationally couple them to the motor 30 and
drive shaft 20,
respectively.
[0051] During normal operation, including spin-up, constant rotational
speeds, and spin-
down, the difference in torque between the motor and the proximal end of the
drive shaft is
relatively small. For these small torque differences, the magnetic attractive
force is sufficient to
hold the plates 41 and 42. together, and the proximal end of the drive shaft
is spun along with the
motor.
[0052] If the distal end of the drive shaft encounters an obstacle and is
suddenly stopped
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from rotating, the torque difference between the motor and the proximal end of
the drive shaft
increases rapidly and eventually exceeds the static frictional torque that
holds the plates together.
When this happens, the plates slip rotationally with respect to each other,
and transmit a kinetic
frictional torque from one to the other as they slip. A detailed discussion of
these frictional
effects follows below.
[0053] Note that torque is the rotational analog of the quantity, force.
Torque produces a
change in angular momentum, much like linear force produces a change in linear
momentum.
Because the rotational inertia of the device components remains roughly
constant throughout
their operation, a non-zero torque therefore produces a change in rotational
speed.
[0054] Note also that the two plates 41 and 42, which are held together
magnetically,
may provide a convenient interface for replacement. For instance, after a
procedure has been
performed, the drive shaft and associated mechanical parts may be removed by
detaching the
magnetically-attracted plates 41 and 42. Plate 42 is disposed of, along with
the drive shaft, while
plate 41 remains with the motor unit and may be used repeatedly.
[0055] Figure 4 is a plot of rotational speed of the drive shaft and
torque at the distal end
of the drive shaft, for a typical procedure. Initially, the drive shaft is at
rest and there are no net
torques present. During the "spin-up" phase, the motor applies a non-zero
torque to the proximal
end of the drive shaft, and the rotational speed of the drive shaft increases.
Once a desired
rotational speed is reached, the torque of the motor is reduced to keep the
drive shaft at a
constant rotational speed: Note that the actual torque applied by the motor to
the proximal end of
the drive shaft may be small but non-zero, in order to overcome the effects of
friction between
the proximal and distal ends of the drive shaft. The plot shows the torque at
the distal end of the
drive shaft, which is truly zero when the distal end of the drive shaft
rotates at a constant
rotational speed. During the "spin-down" phase, the motor applies a non-zero
torque in the
opposite direction to reduce the rotational speed of the drive shaft to zero.
[0056] The typical torque levels shown in Figure 4, which commonly occur
during use,
are usually below a threshold at which the plates 41, 42 in the clutch 40
begin to slip. During
normal use, the clutch remains engaged, and the static frictional force
between the plates holds
the plates together. It is desired that the plates slip, and the clutch
disengages, only during an
atypical event, such as when the distal end of the drive shaft becomes stuck
and stops rotating.
However, it is possible that the plates may slip during spin-up and/or spin-
down, due to the spin-
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up and/or spin-down torques exceeding the threshold.
[0057] At this point, it is instructive to review the physics of
frictional forces, in order to
better understand when the clutch plates hold together, and when they slip.
[0058] Consider for a moment two linear plates, rather than two rotating
plates as in the
true clutch of Figure 3. The linear plates are held together by a normal force
that can be
generated magnetically, as is the case of the clutch of Figure 3, or can be
generated externally.
For relatively small forces parallel to the contact surfaces, the plates hold
together. In other
words, if one pushes gently on one plate, parallel to the contact surfaces,
the other plate holds
with it and there is no slippage. For relatively large forces parallel to the
contact surface, such as
a strike with a hammer, the plates no longer hold together, and slip past each
other along the
contact surface.
[0059] The threshold at which slippage begins to occur is given by the
product of the
normal force (i.e., the force holding the plates together, generated
magnetically or otherwise) and
a coefficient of static friction. The coefficient of static friction is a
dimensionless quantity that is
typically less than one. For forces less than this threshold, the plates hold
together. For forces
greater than this threshold, the plates slip.
[0060] As an example, consider the interface between a rubber tire and a
road surface.
For a small normal force, as is the case when the tire is simply resting on
the road under the
effects of its own gravitational weight, it is easy to drag the tire along the
road surface. For a
large normal force, as is the case when the tire supports the weight of a car,
it is quite difficult to
overcome the frictional forces that keep the tire in contact with the road. In
practice, skidding
only occurs for large forces, such as slamming on the brakes during driving
conditions.
[0061] From this example, we may state a first general principle for our
clutch: the
normal force (i.e., the magnetically-generated force that attracts the plates
to each other)
determines the threshold at which slipping between the plates begins to occur.
[0062] Such a normal force is controllable at the design phase of the
clutch, and may be
controlled by the lateral distribution of magnetic materials in the plates, as
well as the
longitudinal distribution of those materials. For instance, the normal force
decreases as the
longitudinal spacing between the magnetic particles increases; such spacing
can be achieved in
many ways, such as by coating the magnetic particles with a non-magnetic
layer.
[0063] Returning to the example of the two linear plates, consider now the
case when the
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plates are already slipping past each other. There is a resisting force
generated at the contact
surfaces, which would slow down and eventually stop the slipping motion, if no
other forces
were at work. Likewise, if one were to push of the sliding plates parallel to
the surfaces with a
force equal to the resisting force, there would be no net forces on the plates
and the plates would
maintain a constant velocity between them.
[0064] The resisting force is equal to the product of the normal force and
a coefficient of
kinetic friction. The coefficient of kinetic friction is also a 'dimensionless
quantity, also typically
less than one. Furthermore, the coefficient of kinetic friction is usually
less than the coefficient
of static friction; this is the reason behind the effectiveness of automotive
anti-lock brakes, which
can impart a greater stopping force if there is no skidding involved.
[0065] Importantly, the resisting force does not depend on the velocity
between the
plates; as long as there is slipping between the plates, the resisting force
depends only on the
normal force between the plates.
[0066] We may state a second general principle for our clutch: the normal
force (i.e., the
magnetically-generated force that attracts the plates to each other)
determines the torque
transmitted from one plate to the other when the plates are slipping.
[0067] These two general principles are summarized in Figure 5, which is a
plot of the
torque transmitted to the proximal end of the drive shaft (vertical axis),
versus the torque of the
motor (horizontal axis).
[0068] If there were no clutch present, and the drive shaft were
rotationally attached
directly to the motor, the ."no slipping" curve in Figure 5 would increase
from the origin to the
upper right edge of the plot in a 1:1 relationship. In other words, for a
clutchless attachment, all
of the motor torque is always transmitted to the drive shaft.
[0069] At relatively low torques, at which the clutch is engaged and the
plates are in
contact and do not slip with respect to each other, the 1:1 relationship is
seen. In normal use,
such as during the spin-up and spin-down portions of the atherectomy cycle,
the torques
produces by the motor are considered relatively low, so that the clutch
remains engaged
throughout the procedure. On the plot in Figure 5, this corresponds to the 45-
degree branch
extending to the right and upward from the origin (labeled "no slipping").
[0070] At some particular torque threshold, we want slipping to start, in
order to prevent
damage to the patient and to the device itself This threshold occurs at the
top-right point of the
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"no slipping" curve, and is proportional to the normal force. Slipping occurs
when the torque of
the motor equals or exceeds this threshold value.
[0071] When there is slipping between the plates in the clutch, the torque
that is
transmitted to the drive shaft cannot exceed a particular "slipping" value,
regardless of how large
the actual torque of the motor is. This limits the maximum torque that can be
transmitted to the
drive shaft, which also prevents damage to the patient and to the device
itself. This "slipping"
torque value is also proportional to the normal force, and may be referred to
herein as a
"residual" torque.
[00721 Note that because the kinetic coefficient of friction is generally
less than the static
coefficient of friction, the two curves intersect as shown in Figure 5, with
the "no slipping"
portion extending upward at to the right, beyond the intersection point.
[0073] In general; the curves in Figure 5 are scalable in proportion to
the normal force. If
the normal force is doubled, for example, the "no slipping" curve extends
twice as far to the top-
right, and the "slipping" torque value is doubled. The normal force is
controllable during the
design phase of the clutch, through the choice of magnetic materials in the
plates and the lateral
and longitudinal placement of those materials.
[0074] The curves of Figure 5 are plotted as torque versus torque. In
order to see how
these torques evolve in time when the distal end of the drive shaft is
abruptly stopped, two
examples are presented in Figure 6 and 7. Figure 6 pertains to a known system,
in which the
drive shaft is connected to a gas turbine, and does not use a clutch. The
rotational inertia of the
gas turbine is small enough so that the associated torques do not cause any
damage to the patient
or to the device. Figure 7 pertains to a system that uses a higher-rotational-
inertia motor, such as
an electric motor, which uses the clutch to prevent damage. In particular, the
peak and steady-
state torque values in Figure 7 are chosen to mimic those in Figure 6, which
have been
determined to be acceptable in practice.
[0075] We first turn to Figure 6, which is a plot of torque at the distal
end of the drive
shaft versus time for a distal-end-stopping event, for a known gas turbine
system. The known
gas turbine system does not have a clutch.
[0076] Initially, both the motor and drive shaft are spinning together.
The rotation is
assumed to be at a constant rotational speed, so there is no net torque on the
distal end of the
drive shaft. =
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[0077] Next, the distal end of the drive shaft is stopped abruptly, as
would happen if it
got stuck or encountered an obstacle in the blood vessel.
[0078] Following the abrupt stop, the drive shaft begins to wind up, or
rotationally
compress. Such a compression is analogous to a linear spring; the more it is
compressed, the
harder it becomes to impart additional compression. In this phase, the draft
shaft essentially
"pushes back" rotationally on the motor, and the motor slows down.
[0079] There comes a point when all the rotational energy has gone into
rotationally
compressing the spring, and the spring and motor are stopped at the spring's
maximum
compression point. At this point, the distal end of the drive shaft
experiences its maximum
torque.
[0080] Following the maximum compression, the drive shaft "springs back"
and unwinds
a bit. During this unwinding, the motor and the proximal end of the drive
shaft run in reverse.
In practice, there may be some "ringing" to this curve, as the energy in the
system oscillates
between kinetic (movement) and potential (rotational compression of the drive
shaft). Much of
the "ringing" is damped due to friction, and the oscillations become
increasingly small as system
settles to a stationary steady state. The "ringing" is omitted from Figure 6.
[0081] At this steady state, the motor is stopped but is still applying a
torque. The drive
shaft is also stationary, but is stationary in a rotationally compressed
position due to the motor
torque.
[0082] The entire horizontal axis of Figure 6 may last on the order of
milliseconds. The
known gas turbine may have a control system that detects when its rotational
speed falls below a
threshold value or falls to zero and subsequently shuts off the motor. Such a
control system may
require a particular length of time to react, typically on the order of
several seconds. These
control systems cannot react directly to portions of the curve of Figure 6,
though, because the
spike and settling to steady-state typically occurs much more rapidly than the
control system can
react.
[0083] There are two torque values to note on the curve of Figure 6. The
first value is
the peak value, which occurs when the drive shaft is most tightly wound and
the motor is
stopped. The second value is the steady-state value. Both of these torque
values have been
deemed safe for use in the known, gas turbine-driven atherectomy system. As a
result, the clutch
40 may be designed to mimic one or both of these safe torque values.
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[0084] Figure 7 is a plot of torque at the distal end of the drive shaft
versus time for a
distal-end-stopping event, for the present motor-driven system with the clutch
of Figure 3. One
difference between Figure 6 and 7 is that for the present clutch design, the
motor continues to
turn throughout the clutch disengagement; for the known gas turbine of Figure
6, the turbine
stops along with the drive shaft. Such a stopping of the present motor is not
feasible because of
the relatively large rotational inertia of the motor.
[0085] Initially, both the motor and drive shaft are spinning together.
The rotation is
assumed to be at a constant rotational speed, so there is no net torque on the
distal end of the
drive shaft. The clutch is engaged, and there is no slipping between the
plates of the clutch.
[0086] Next, the distal end of the drive shaft is stopped abruptly. As
with Figure 6, the
spiked torque associated with stopping the distal end is omitted from Figure
7.
[0087] Following the abrupt stop, the drive shaft begins to wind up, or
rotationally
compress. In this phase, the draft shaft essentially "pushes back"
rotationally on the motor, and
the motor may slow down. In practice, this slowing down of the motor may be
very slight,
because the rotational inertia of the motor may be quite large, especially
compared with that of
the gas turbine discussed above.
[0088] Eventually, as the distal end of the drive shaft remains fixed and
the proximal end
of the drive shaft continues to wind, there will reach a point when the torque
difference between
the motor and the proximal end of the drive shaft equals the threshold torque,
beyond which the
clutch plates start to slip. This threshold point corresponds to the peak of
the curve in Figure 7.
[0089] One may trace the progress thus far in Figure 5. Initially, while
the motor and
drive shaft are spinning together, the system as at the origin. After the
distal end is stopped, the
system rises upward and to the right along the "no slipping" curve. The
threshold point, which is
the peak of the curve in Figure 7, is at the top-right-most edge of the "no
slipping" curve in
Figure 5.
[0090] Once the plates begin to slip, the clutch becomes disengaged. The
motor
continues to rotate, along with plate 41 of the clutch 40. The other plate 42,
however, rotates
more slowly than the plate 41, and eventually stops and unwinds, along with
the proximal end of
the drive shaft. Once any ringing effects have died off and steady state is
reached, the drive shaft
is stationary and slightly wound, the proximal end of the drive shaft is
stationary, the plate 42 is
stationary, the plate 41 remains rotating along with the motor, and rotating
plate 42 transmits
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enough torque to stationary plate 41 to keep the drive shaft slightly wound.
[0091] Essentially, the torque transmitted by the clutch 40 in its
slipping mode is
analogous to the torque of the gas turbine of Figure 6 when the gas turbine is
stationary. In fact,
during the design phase of the clutch 40, the attractive magnetic normal force
between the plates
can be set so that the steady-state torque of Figure 7 matches that of Figure
6, since the steady-
state torque of the gas turbine has been deemed safe for use. Alternatively,
the attractive
magnetic normal force between the plates can be set so that the peak torque,
i.e., the threshold
torque value at which the plates begin to slip (the peak in Figure 7), matches
that of Figure 6. As
a further alternative, both the peak and steady-state torque values can be met
by texturing one or
both surfaces of the clutch, adjusting the diameter of the contact surfaces,
and/or adjusting the
materials on the opposing faces in the clutch.
[0092] Although the plates 41 and 42 are drawn in Figure 3 as being
coaxial and circular,
other suitable shapes and orientations may be used. One or both surfaces may
optionally be
textured, which can adjust the surface area in contact and may affect the
frictional performance
of the interface. In addition, the plates 41 and 42 may optionally be curved,
and may have
mating curvatures that fit together. For instance, one plate may be convex
with a particular
radius of curvature, and the other plate may be concave with the same radius
of curvature.
[0093] Referring now to Figure 8A, an additional embodiment of a clutch
140 is shown.
In this embodiment, a clutch 140 is formed by a boundary element 141 formed by
a pair of loft
flanges secured to one another and rotationally coupled (e.g., keyed) to the
motor. The clutch
may also include a boundary element 142 formed by another loft flange secured
to a tube 139,
such as a hypotube that is used to rotate the drive shaft. As shown, a biasing
mechanism 144
may be arranged between the boundary elements 141, 142 to transfer the torque
from the motor
to the drive shaft via the boundary elements 141, 142. It is to be appreciated
that, while the
boundary elements 141, 142 have been described as loft flanges, these elements
may take other
forms such as disc-shaped plates, square plates, hollow or solid cylindrical
cylinders, or other
shaped boundary elements 141, 142 may be provided.
[0094] Referring now to Figure 8B, a schematic diagram of a first biasing
mechanism
144A is shown. In this embodiment, the function of the clutch 140 may be very
similar to that of
the magnetic clutch 40 previously described. That is, a spring 148A or other
biasing mechanism
144A may extend from one of the faces of one of the loft flanges 142A and a
plate 146A may be
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provided on one end of the spring 148A opposite the flange 142A that the
spring 148A is
attached to. The plate 146A may frictionally engage the faces of the opposing
loft flange 141A.
The spring 148A may have a relaxed length longer than the distance between the
loft flanges
141A, 142A and, as such, the spring 148A may be compressed to fit within the
loft flanges
141A, 142A thereby creating a longitudinal normal force that presses the plate
146A against the
loft flange 141A. Like the magnetic clutch described, the friction between the
plate 146A and
the loft flange 141A may be sufficient to support transfer of a threshold
torque between the
plates, but when the threshold torque is exceeded (e.g., when the drive shaft
hits an obstruction)
the plate 146A and the loft flange 141A may slip thereby reducing the torque
transfer to a torque
based on kinetic friction in lieu of static friction. It is noted, however,
that in addition to the
slippage of the plate 146A and the loft flange 141A the use of a spring 148A
to bias the plate
may provide some amount of additional torsional play in the system. As such,
when the drive
shaft hits an obstruction, in some embodiments, the torsional stiffness of the
spring 148A may
result in some relative rotation of the hypotube and motor prior to the
slippage of the frictional
surface. It is to be appreciated that while the spring 148A is shown as being
attached to the loft
flange 142A on the hypotube 139A and the plate 146A is shown to frictionally
engage the loft
flange 141A coupled to the motor, the reverse may also be provided.
[0095] Referring now to Figure 8C, a schematic diagram of a second biasing
mechanism
144B is shown. In this embodiment, the function of the clutch 140 may remain
similar to that of
the magnetic clutch. In this embodiment, rather than placing the spring 148B
and plate 146B
between the loft flanges 141B, 142B, the plate 146B may be positioned on the
tube 139B and
secured to prevent longitudinal movement of the plate 146B relative to the
tube 139B. The loft
flange 142B on the tube 139B may be longitudinally slidable, but rotationally
coupled to the
hypotube 139B through a key or square drive fitting or other relative rotation
resisting
connection. Like the embodiment of Figure 8B, a spring 148B may be provided
that has a
relaxed length longer than the space available in the system. As such, the
spring 148B may be
compressed between the plate 146B and a loft flange on the hypotube 139B
thereby biasing the
loft flange 142B on the tube 139B against the loft flange 141B coupled to the
motor and creating
a friction based torque transferring connection. This embodiment may also be
reversed by
placing the plate 146B and spring 148B on the motor side of the clutch, for
example. Like the
embodiment of Figure 8B a threshold torque may be supported, but may be
overcome if an
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obstruction is encountered. However, unlike the embodiment of Figure 8B, since
the spring
148B is not carrying torque in this embodiment, play provided in Figure 8B may
not be
available. However, it is appreciate that the plate 146B and spring 148B may
be rotationally
coupled to the tube 139B and the loft flange 142B on the tube 139B may be
positioned on the
tube 139B to allow for both rotational and longitudinal movement such that the
spring 138B
transfers the torque to the loft flange 142B. In this latter situation the
play from the spring 148B
may be provided.
[00961 Referring now to Figure 8D, a schematic diagram of yet another
biasing
mechanism 144C is shown. In this embodiment, the two boundary elements 141C,
142C of the
clutch may be directly secured to one another with a biasing mechanism 144C
such as a spring
148C, for example. The spring 148C may have a relaxed length equal to, larger,
or smaller than
the space provided between the loft flanges 141C, 142C. When in place, the
spring 144C may
have a torsional resistance that may change based on how tightly wound the
spring 148C is. As
such, upon actuation of the drive shaft, the spring 148C may wind up tighter
until equilibrium is
found between the torque required to rotate the drive shaft and the torsional
resistance supplied
by the spring 148C. In this embodiment, when an obstruction is encountered,
the added torque
applied to the spring 148C may cause the spring 148C to further wind allowing
the loft flanges
141C, 142C of the clutch to rotate relative to one another and, thus, not
fully transfer the torque
from the motor. In this embodiment, since no slippage is provided by the
clutch, the system may
be further equipped with a speed or current limiting switch for switching off
the motor when an
obstruction is encountered. However, at initial impact between the distal end
of the drive shaft
and the obstruction, the clutch may allow for some play in the system and
avoid relatively high
torques from being realized. Systems for releasing the torque provided by the
motor based on
torque, current, voltage, speed reduction and the like are discussed in U.S.
Patent Application
No.: 12/713,558, the contents of which are hereby incorporated by reference
herein in their
entireties.
[0097] Referring now to Figure 9A, a block diagram and torque graph are
shown that
may reflect the embodiment of Figure 8D. That is, in the embodiment of Figure
8D, for
example, a slipping type clutch is not provided and, instead, a shock
absorbing spring 144C is
provided to elastically couple the motor 30 to the drive shaft 20. The block
diagram of Figure
9A shows this in a block form indicating that as the motor 30 spins a first
direction, the shock
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absorbing element 240 may directly transfer the torque to the drive shaft 20.
That is, at initial
startup, for example, the shock absorbing element 240 may elastically stretch
or compress or
deform in shear by an initial amount until the elasticity of the shock
absorber finds equilibrium
with the torsional friction/resistance being encountered by the drive shaft
20. Beyond that point,
aside from situations where the torsional resistance witnessed by the drive
shaft 20 varies, the
motor 30, the shock absorber 240, and the drive shaft 20 may rotate at a state
of equilibrium
where the rotational speeds differ by any gearing changes, but otherwise
remain consistent.
[0098] Referring now to FIG. 9B, a time versus distal end torque diagram
may be
reviewed. As shown, at the left side of the graph, the drive shaft 20 may be
spinning and the
torque at the distal end of the drive shaft 20 may be negligible. That is,
while some resistance to
tip spinning may be present, for our purposes, we may assume that the steady
state condition
provides a tip resistance that is approximately zero.
[0099] When the tip of the drive shaft 20 comes to an abrupt stop the
torque applied to
the distal end of the drive shaft 20 may begin to increase. That is, assuming
that an interrupter is
not immediately activated to interrupt the motor rotation, the motor 30 may
continue to rotate
when the distal end of the drive shaft 20 stops. This additional torque
created due to the non-
rotating distal tip and the rotating motor 30 may cause the torque at the
distal end of the drive
shaft to increase. That is, the rotating motor 30 may continue to act on the
shock absorbing
element 240, which, although it may give slightly, will also transfer some of
the additional
torque through to the drive shaft 20. As the motor 30 continues to rotate,
additional torque may
be applied to the shock absorbing element 240, which may absorb some of the
torque, but may
also transfer some additional torque to the drive shaft 20. The result is that
the shock absorbed
torque at the distal end of the drive shaft 20 may be lower and take longer to
reach than the
inertial torque shown in dashed lines in FIG. 9B. That is, in the non-shock
absorbed condition,
the motor 30 may rotate until the drive shaft 20 reaches its fully wound
unresilient condition
causing the motor 30 to come to a stop in a short amount of time and causing
the full torque of
the motor in addition to its loss of momentum to be transferred through the
drive shaft 20. In
contrast, the shock absorbing embodiments (see FIGS 8D, 10-13) provide more
time for the
motor 30 to come to a stop because the shock absorbers 240 allow the motor 30
to rotate through
a larger amount of rotation before coming to a stop. Since the torque due to
the momentum of
the motor 30 is dependent on how abruptly the motor 30 stops, this shock
absorbing effect
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reduces the peak torque. As such, if the motor is uninterrupted, the final
torque at the distal tip
may be as high as the torque applied by the motor 30, but the torque at the
peak may be only
slightly higher than the applied torque.
[0100] Referring now to Figure 10A, a first embodiment of a shock
absorbing system
may be shown. As shown, the motor 30 may include a rotating drive 242 that is
rotationally
coupled to a drive gear 244. The drive gear 244 may be rotationally engaged
with a take-off gear
246 that may be keyed or otherwise coupled to the drive shaft 20 such that
rotation of the take-
off gear 246 imparts rotation on the drive shaft 20 of the atherectomy device.
Accordingly, the
motor 30 may be directly geared to the drive shaft 20 and a slip clutch may be
omitted.
However, as shown, the drive gear 244 may be a shock absorbing drive gear 244
such that
resistance to rotation at the drive shaft 20 may be absorbed slightly by the
drive gear 244 before
increasing the torque in the drive shaft 20 due to motor rotation.
[0101] As shown in more detail in Figure 10B, the drive gear 244 may
include an internal
hub 248 for securing to the rotating drive 242 of the motor 30. The gear 244
may also include a
peripheral ring 250 having a plurality of teeth 252 selected together with the
tooth count on the
take-off gear 246 to provide a suitable gear ratio and allowing for an
efficient motor speed
relative to drive shaft speed. The hub 248 of the drive gear 244 may be
secured to the peripheral
ring 250 of the drive gear 244 with a resilient or shock absorbing system.
That is, for example,
as shown, the shock absorbing system may include a plurality of radially
extending elements 254
extending generally radially outward from the hub 248 to an inner surface of
the peripheral ring
250. In the present embodiment, the radially extending elements 254 may be
substantially S-
shaped and there may be four of these shapes. Still other shapes and numbers
of radially
extending elements 254 may be provided. For example, a series of spokes,
struts, or a
substantially flat diaphragm may be provided. Still other resilient shock
absorbing elements may
be provided for resiliently connecting the hub 248 to the peripheral ring 250.
[0102] In some embodiments, the drive 244 gear may be a molded product
that may be
tuned by adjusting and/or changing the molded geometry. In other embodiments,
the shock
absorbing portion of the gear 244 may be a separate component and tuning or
adjusting the
resiliency of the system may involve removing and replacing the shock
absorbing portion with
one of higher or lower resiliency.
[0103] The S-shaped portion of the radially extending elements 254 may
have a base 256
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having an axis extending substantially directly radially outward from the hub
248. The S-shaped
portion may include a bent portion 258 turning approximately 90 degrees from
the base 256 and
then an approximately 180 degree U-turn portion 260 may be provided. A central
crossing
member 202 may return across the shape slightly past the base 256 to another U-
turn portion
264. An additional 90 degree portion 266 may be provided and an opposing base
268 on the
inside surface of the peripheral ring 250 may be provided that is
substantially radially aligned
with the corresponding base 256 on the hub 248. It is to be appreciated that
while a particular
shape for the radially extending portion 254 has been described, still other
geometries for the
radially extending portion 254 may be provided.
[0104] As can be appreciated, the resilient radially extending portions
254 of the drive
244 gear may deflect under load allowing the hub 248 to rotate relative to the
peripheral ring 250
thereby increasing the rotation allowed for the motor 30 in a stoppage
condition. This increased
rotation may extend the time and distance over which the motor 30 is drawn to
a stop thereby
reducing the amount of inertial torque transmitted to the drive shaft 20 due
to the stoppage.
[0105] Referring now to Figure 11A, another embodiment of a shock
absorbing system is
shown. In this embodiment, similar to that of Figure 10A, a motor 30 includes
a rotating drive
242 coupled to a drive gear 270. The drive gear is engaged with a take-off
gear 272. However,
unlike the embodiment of Figure 10A, the present take-off gear 242 might not
be keyed to the
drive shaft 20, but may, instead be coupled to the drive shaft 20 with a
resilient or shock
absorbing element 274. As such, resistance to rotation experienced by the
drive shaft 20 may be
absorbed slightly by the shock absorbing element 274 when the rotation of the
motor 30 attempts
to increase the torque in the drive shaft 20.
[0106] As shown in more detail in Figure 11B, the take-off gear 272 may be
arranged
concentrically on the drive shaft 20, for example, but may be free to rotate
relative to the drive
shaft 20. However, one face of the take-off gear 272 may be engaged with one
end of a resilient
member 274 such as a coil, spring, or other biasing mechanism. The transition
between the take-
off gear 272 and the resilient member 272 may include a washer plate, for
example. The
resilient member 274 may be position around the drive shaft 20 and may extend
away from the
take-off gear 272 to a free end. At the free end of the resilient member
opposite the take-off gear
272 a coupling element such as another washer plate may secure the drive shaft
to this opposite
end. The two ends of the resilient member 274 may be welded, keyed, pinned, or
threaded
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through for example, or otherwise fixed to the take-off gear 272 and the drive
shaft 20. As such,
rotation of the take-off gear 272 due to the drive gear 270 may cause rotation
of the resilient
member 274 thereby causing rotation of the drive shaft 20. However, where the
drive shaft 20
experiences resistance to rotation, the resilient member 274 may absorb some
of the torque of the
motor 30 before transferring such torque to the drive shaft 20. As such, the
impact of the inertial
force on the drive shaft 20 due to the rotating motor 30 may be reduced. It is
to be appreciated
that while a resilient member 274 in this embodiment has been described as a
coil or spring, the
resilient member 274 may include a resilient cylindrical bushing made of a
resilient material, a
mesh material or another material allowing for the torque transfer between the
take-off gear 272
and the drive shaft 20 to be controlled.
[0107] Referring now to Figure 12A, yet another shock absorbing device may
be
provided. In this embodiment, unlike the embodiments of Figures 10A and 11A,
the present
embodiment may be a belt-drive system. As such, the system may include a motor
30 having a
rotating drive 242. The rotating drive 242 may have a drive pulley 276
arranged thereon and
keyed thereto to or otherwise coupled to transfer rotational motion between
the rotating drive
242 and the pulley 276. The system may also include a take-off pulley 278
arranged generally in
plane with the drive pulley 276 and rotationally coupled to the drive shaft
20. In this
embodiment, the drive pulley 276 may be rotationally coupled to the take-off
pulley 278 with a
resilient belt 280.
[0108] In more detail in Figure 12B, the belt 280 may be arranged to
extend around the
drive pulley 276 and the take-off pulley 278. The belt 280 may be arranged
relatively tightly on
the two pulleys 276, 278 thereby transferring torques from the motor 30 to the
drive shaft 20
based on the friction of the belt 280 on the surface of each pulley. It is to
be appreciated that
when the drive pulley 276 is rotating, for example, clockwise in figure 12B,
the left side of the
belt 280 may be in a higher level of tension than the right side stemming from
any resistance to
rotation that may be present in the drive shaft 20 or other downstream portion
of the system.
When the drive shaft 20 encounters resistance to rotation, the resilient belt
280 may stretch on
the higher tension side of the system and tension on the other side of the
system may be slightly
relieved due to the increase in differential torque. As such, the belt 280 may
be effective to
absorb some of the torque from the motor 30 when the drive shaft 20 encounters
resistance to
rotation. It is to be appreciated that several different belt profiles or
cross-sections may be
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provided. In some embodiments, a round belt, a triangular belt, a rectangular
belt, a trapezoidal
belt, or other shaped cross-section may be provided. In some embodiments, the
cross-sectional
shape of the belt 280 may be particularly selected due to its ability to
absorb differential toque by
deforming based on internal shear forces.
[0109] Referring now to Figure 13A, yet another embodiment of a shock
absorbing
device may be provided. In this embodiment, like the embodiment of Figure 12A,
the system
may be a belt-driven system. However, it will be appreciated that this
particular system could be
chain driven or otherwise driven with a more rigid-type belt. In this
embodiment, the rotating
drive 242 of the motor 30 may include a drive pulley 276 that is rotationally
coupled to a take-off
pulley 278 on the drive shaft 20 with a belt 280, for example. However, an
additional idler
pulley may be provided and, as shown, two idler pulleys 282A, 28213 may be
provided. Like the
system of Figure 12A/12B, the present system may rely on the belt to absorb
some of the
differential torque, however, the idler pulleys 282A, 282B may also be
resilient allowing for
more control over the level of shock absorption in the system.
[0110] Referring to the more detailed view of Figure 13B, the several
pulleys and a belt
are shown. As shown, the idler pulleys 272A, 2728 may be arranged along and
generally close
to the tangent line connecting the outer belt surface of the broader diameter
drive pulley 276 to
the outer belt surface of the smaller take-off pulley 278. The belt 280 may be
routed around
respective inside surfaces of the idler pulleys 282A, 282B creating a
substantially inverted tear
shaped belt route. The idler pulleys 282A, 282B may be positioned on
resiliently secured center
shafts 284A, 284B such that increased tension on one side of the system may
draw one of the
idler pulleys 282A out of position while a decrease in tension on the opposing
side may cause the
respective idler pulley 282B to take up any slack occurring in the belt 280.
For example, where
the belt in Figure 13B is rotating clockwise about the drive pulley 276 a
particular amount of
tension may be present in the left portion of the belt 280, while a slightly
lesser amount of
tension may be present in the right portion of the belt 280. When the drive
shaft 20 encounters
an obstruction, the resistance to rotation of the drive shaft 20 may increase
causing the tension in
the left portion of the belt 280 to increase drawing the left idler pulley
282A outward. Similarly,
the tension in the right portion of the belt 280 may decrease allowing the
right pulley 282B to
move inward to take-up any slack. Since the amount of increased tension on the
left may be
similar to the amount of decreased tension on the right, the two idler pulleys
282A, 282B, in
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some embodiments, may be arranged on a common frame such that the right pulley
moves
inward by an amount equal to or similar to the amount that the left pulley
moves outward.
[0111] The present shock absorbing systems may be advantageous for
absorbing
relatively high torsional loading without damaging the medical device drive
shaft. The device
may provide a means for absorbing the loads allowing the shaft and crown to
stop rotating
without stopping the whole drive system stopping. In the absence of such a
system, the drive
shaft may otherwise be the shock absorbing element and may need to be
considerably stronger
than those using the shock absorbing technology. With the shock absorbing
technology, the
drive shaft may be designed to perform the function of rotating the sanding
member or crown,
without the added strength due to shock loading. As described, the shock
absorbing member
may be arranged on the motor side (i.e., like Figures 10A/10B), on the drive
side of the drive line
(i.e., like Figures 11A/11B), or the member may be placed between the drive
side and the driven
side of the drive line (i.e., like Figures 12A/12B and 13A/13B).
[0112] The description of the invention and its applications as set forth
herein is
illustrative and is not intended to limit the scope of the invention.
Variations and modifications
of the embodiments disclosed herein are possible, and practical alternatives
to and equivalents of
the various elements of the embodiments would be understood to those of
ordinary skill in the art
upon study of this patent document. These and other variations and
modifications of the
embodiments disclosed herein may be made without departing from the scope and
spirit of the
invention.
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