Flexible Driveshafts with Bi-Directionally Balanced Torsional Stiffness Properties

This document provides flexible driveshaft devices. For example, this document provides flexible driveshaft devices that have bi-directionally balanced torsional stiffness properties. In some embodiments, the flexible driveshaft devices provided herein are utilized in medical device systems, such as endoluminal medical device systems. For example, in some embodiments the flexible driveshaft devices provided herein are utilized in endoluminal ultrasonic catheter systems.

DETAILED DESCRIPTION

This document provides flexible driveshaft devices that have bi-directionally balanced torsional stiffness properties. In some embodiments, the flexible driveshafts provided herein are applicable to catheter-based medical devices. Some such catheter-based medical devices are tubular devices that may be inserted in a bodily vessel, cavity, or duct and manipulated by a portion that extends outside the body. In some embodiments, the flexible driveshafts provided herein are applicable, without limitation, to medical devices used for neuro-surgical, gastrointestinal, urinary, intrauterine, intracardiac, ear-nose-throat, orthopedic, endoscopic, endoluminal, and intravascular procedures. In some embodiments, the flexible driveshaft devices provided herein can be utilized in devices that perform diagnostic or interventional procedures such as imaging, ablation, thrombectomy, atherectomy, cutting, abrading, biopsy, and delivery systems for implantable medical devices such as stents, stent grafts, and occluders to provide a few examples. The use of the flexible driveshaft devices provided herein is described in detail below in the context of an intravascular ultrasonic imaging catheter system as a non-limiting example implementation.

With reference toFIGS. 1A-1C, an example ultrasound catheter100includes an actuator control handle110, a flexible driveshaft120, and a distal tip130with an ultrasound array131. The driveshaft120exits the distal end of the control handle110and extends distally to the distal tip130that is coupled to the distal end of the driveshaft120. In some embodiments, the driveshaft120is within the lumen of a sheath, and the driveshaft120is free to turn within the lumen while the sheath is restrained from such turning. In some embodiments, no sheath with a lumen to house the driveshaft is used, and therefore the outer surface of the driveshaft120is exposed. In some embodiments, the outer surface of the driveshaft is a metallic or polymeric outer covering that is integral with the driveshaft. In some embodiments, no such covering is used. An image plane132is emitted from the ultrasound array131at the distal tip130. The image plane132is schematically represented.

The usage of catheter-based medical devices, such as the example ultrasound catheter100, is well known to those of skill in the art. In general, to use a catheter-based medical device the distal end of the catheter is inserted in a bodily vessel, cavity, or duct of a patient. The catheter is then maneuvered through the anatomy of the patient as needed to position a working portion of the catheter, such as the distal tip130with the ultrasound array131, at a target location. Guidewires are used in some cases. Additional guidance catheters are used in some cases. Radiographic visualization and/or other types of visualization techniques are used in some cases. Typically the catheter can be manipulated by a clinician at a proximal portion of the catheter that extends outside the patient's body. With the working portion of the catheter properly positioned within the bodily vessel, cavity, or duct of a patient, the procedure may then be performed.

It should be noted that the example ultrasound catheter100is not necessarily drawn to scale. For example, the length of the driveshaft120can be longer than depicted. In some embodiments, the length of the driveshaft120is about 20 cm to about 70 cm, about 60 cm to about 120 cm, about 100 cm to about 150 cm, about 130 cm to about 180 cm, about 160 cm to about 220 cm, or about 200 cm to about 260 cm or longer. Driveshafts having any practical length are envisioned within the scope of this document.

The ultrasound catheter100is provided as an example to illustrate an implementation of the flexible driveshaft devices having bi-directionally balanced torsional stiffness properties as provided herein. However, as mentioned above, many other implementations of the flexible driveshaft devices are also envisioned. Such implementations include other medical devices, as well as other devices and systems that are not directly medically related.

The actuator control handle110includes a panning knob112and a steering knob116. In some embodiments, the panning knob112and steering knob116are operated by a clinician to control the position and orientation of the distal portion of the driveshaft120and the distal tip130. In some embodiments, the movements of the panning knob112and steering knob116are powered manually. In some embodiments, the movements are powered using, for example, motors that are controlled by a clinician, an automation system, or a combination thereof. In some embodiments, the motors are coupled at a proximal end of the drive shaft120located outside the patient's body. In some embodiments, a micro-motor is coupled to the driveshaft120at a location that can be positioned internal to the patient's body during a medical procedure.

In some embodiments, endoluminal imaging systems use a controlled sweeping or rotary motion of the image plane132at the distal tip130to create 3D or 4D images. For example, in some embodiments a stepper motor drive system controlled by an automation system is used to controllably drive the rotation (e.g., the direction and speed) of the driveshaft120. In some such embodiments, by rotating the driveshaft120using the stepping motor, the image plane132can potentially be driven in a particular known, controllable, and repeatable sweep pattern. In some embodiments, various other types of motor driven systems are used.

In some embodiments, the image plane132is repeatedly swept back and forth through a prescribed angle of rotation and at a prescribed speed. For example, in some embodiments the image plane132is swept, or wobbled, back and forth at a specified rate of speed through a 90 degree arc of rotation. In such embodiments, the driveshaft120repeatedly reverses its direction of rotation. In some embodiments a 360° scan of a body lumen is performed. In some embodiments a 720° scan of a body lumen is performed. A variety of other prescribed patterns of sweeping and oscillating the distal tip130via the driveshaft120can be performed. As described further below, the flexible driveshaft devices provided herein facilitate improved accuracy, predictability, and repeatability of catheter-based medical devices, including endoluminal imaging systems that sweep and oscillate a distal tip130through a prescribed pattern of rotation at a prescribed speed.

The panning knob112is coupled, directly or indirectly, to the flexible driveshaft120. The panning knob112is used to effect a rotary or panning movement by the distal tip130. As the panning knob112is rotated, torque is transmitted via the driveshaft120from the panning knob112to the distal tip130. As a result, the rotation of the panning knob112tends to cause a rotation of the distal tip130and, consequently, the ultrasound array131. For example, the panning knob112can be rotated by the clinician in clockwise and/or counterclockwise directions as represented by arrow114. Such rotation will tend to cause a rotation of the distal tip130as represented by arrow134.FIGS. 1A and 1Brepresent, in comparison to each other, the rotation of the panning knob112and the resulting panning movement of the image plane132of the ultrasound array131at the distal tip130.

While a rotation of the panning knob112(manually and/or using a motor-driven system) tends to cause a panning movement of the distal tip130, in some instances the ratio of the rotation of the panning knob112to the rotation of the distal tip130is not 1:1. In other words, in some cases a rotation of the panning knob112by a certain number of degrees will result in a lesser or greater number of degrees of rotation of the distal tip130. That is the case because the flexible shaft120is likely to have some torsional elasticity. In general, and as described further below, as the torsional stiffness of the flexible shaft120is increased, the ratio between the rotation of the panning knob112and the rotation of the distal tip130trends toward a 1:1 ratio.

In some embodiments of catheter-based medical devices that use a flexible driveshaft, it is desirable to have a ratio between the rotation of the driveshaft at the actuator and the rotation at the tip that is about 0.5:1, about 0.6:1, about 0.7:1, about 0.8:1, about 0.9:1, about 1:0.5, about 1:0.6, about 1:0.7, about 1:0.8, about 1:0.9, or about 1:1. That is the case because such a ratio will allow a clinician or automation system to exert predictable and repeatable control over the rotatory movement and resulting position of the tip portion. For example, in the context of the example ultrasound catheter100, it is desirable for a rotation of the panning knob112by a certain amount to induce a predictable and repeatable amount of rotation of the ultrasound array131at the distal tip130. In some embodiments, the ideal ratio is 1:1, or substantially close to 1:1. In that case, the quality of the ultrasound images are enhanced, the ultrasound imaging procedure provides more accurate results, and the device is easier for clinicians to use—with accompanying shortened procedure times, lower costs, and less patient discomfort.

It is also desirable in some embodiments for the ratio between the rotation at the panning knob112in comparison to the rotation at the distal tip130to be approximately equal in both directions of rotation, i.e., clockwise and counterclockwise. That way, the clinician will be able to exert predictable and repeatable control over the rotatory movement and position of the distal tip130in both directions of panning rotation.

As depicted inFIG. 1C, the steering knob116is used to manipulate the distal portion of the driveshaft120. In some embodiments, a linear movement of the steering knob116in the proximal or distal direction as represented by arrow118induces the distal portion of the driveshaft120to bend transversely as represented by arrow124. This motion is facilitated by a driveshaft120that is flexible in bending. In this manner, a clinician can actively steer the ultrasound catheter100through the anatomy of a patient which may be tortuous in nature. In some embodiments, radiopaque markers122are included on the ultrasound catheter100to enable radiographic visualization while steering the ultrasound catheter100through the anatomy of the patient.

In some embodiments, it is beneficial for catheter-based medical devices to be laterally flexible along all or part of the catheter. For example, in some embodiments, steering of catheter-based medical devices is performed by transverse bending of the catheter. In some embodiments in which steering is not performed, laterally flexibility of the catheter is beneficial (e.g., during catheter insertion) so that the catheter can conform to the tortuous non-linear anatomy of the patient. Consequently, such catheters, or portions of such catheters, are benefitted by having driveshafts with lateral flexibility, i.e., being flexible in bending. That is, a driveshaft that is laterally stiff will be resistant to bending and steering, while a driveshaft that is laterally flexible will be more amenable to bending and steering.

In general, for at least the reasons described above, it is desirable in some embodiments for the driveshaft of catheter-based medical devices, such as the example ultrasound catheter100, to be a suitable combination of: (1) torsionally stiff, (2) equally torsionally stiff in both directions of rotation, and (3) laterally flexible.

A flexible driveshaft construction that may be used in attempt to meet the above properties consists of layers of helically wound wires (or “filars”) surrounding a core. The helically wound filars usually include at least two layers of filars, with the layers being wound in opposite directions. One set of filars is layered on top of another set of filars. The benefit of this construct is that it creates a driveshaft with a lower bending stiffness compared to a solid shaft of equivalent diameter, while providing a relatively high torsional stiffness. However, one shortcoming of this driveshaft construction is that it commonly has very different torsional stiffness properties in the clockwise versus the counterclockwise twist directions.

With reference toFIGS. 2 and 3, an example flexible driveshaft200with bi-directionally unbalanced torsional stiffness properties is depicted.FIG. 2provides a cut-away perspective view, andFIG. 3provides an orthogonal cross-sectional view at section3-3. The flexible driveshaft200includes a solid core210, a first helically wound layer of filars220, and a second helically wound layer of filars230. The first helically wound layer of filars220is wrapped around and overlays the core210, and the second helically wound layer of filars230is wrapped around overlays the first helically wound layer of filars220. The first helically wound layer of filars220and the second helically wound layer of filars230are wound in opposite directions. The first helically wound layer of filars220can be described as being left-wound (i.e., counterclockwise). The second helically wound layer of filars230can be described as being right-wound (i.e., clockwise). However, in some embodiments, the first helically wound layer of filars220are right-wound and the second helically wound layer of filars230are left-wound.

In the context ofFIG. 2, when a clockwise torque, as represented by arrow202, is applied to the driveshaft200the driveshaft200primarily transmits the torque via the core210and the second helically wound layer of filars230. The second helically wound layer of filars230transmits the clockwise torque202because its layer of right-wound filars tend to get tightened by the clockwise torque202. By contrast, the clockwise torque202tends to loosen the left-wound first helically wound layer of filars220. Therefore, relatively less clockwise torque202is transmitted by the first helically wound layer of filars220. On that basis, it can be said that in the context ofFIG. 2the right-wound filars are active when subjected to a clockwise torque, and that the left-wound filars are effectively inactive.

When a counterclockwise torque, as represented by arrow204, is applied to driveshaft200, the driveshaft200primarily transmits the torque via the core210and the first helically wound layer of filars220. The first helically wound layer of filars220transmits the counterclockwise torque204because its layer of left-wound filars tend to get tightened by the counterclockwise torque204. By contrast, the counterclockwise torque204tends to loosen the right-wound second helically wound layer of filars230. Therefore, relatively less counterclockwise torque204is transmitted by the second helically wound layer of filars230. On that basis, it can be said that in the context ofFIG. 2the left-wound filars are active when subjected to a counterclockwise torque, and that the right-wound filars are effectively inactive.

The torsional stiffness of a driveshaft can be expressed as:

where:S is the torsional stiffness;G is the shear modulus of the driveshaft material; andJ is the polar moment of inertia of the driveshaft.

Based on the above, the formula for torsional stiffness of a flexible driveshaft having a core and layers of filars is then:

where:Scoreis the torsional stiffness of the core; and(GJ)active filar layersis the shear modulus of the material of the active filar layer(s) multiplied by the polar moment of inertia of the active filar layer(s).

A driveshaft with bi-directionally balanced torsional stiffness properties has substantially equal torsional stiffness values in the clockwise and counterclockwise directions. This is stated as:

Scw=Sccw; or

As shown inFIG. 3, orthogonal cross-sections of helically wound cylindrical filars are approximately elliptical. The polar moment of inertia of an ellipse through its centroid is expressed by:

where:a is the radius of the major diameter of the ellipse; andb is the radius of the minor diameter of the ellipse.

The parallel axis theorem states that the polar moment of inertia about an axis parallel to an axis through the centroid, about which a moment was calculated is expressed by:

where:A is the cross sectional area; andd is the distance between the axes.

The area, A, of an ellipse equals nab, so the polar moment of inertia for a single filar can be described by:

where:R is the radius from the center of the driveshaft to the center of the filars for a given layer of filars.

The radius of the minor axis of the cross-section of a filar, b, is simply the radius of the filar. The radius of the major axis can be determined from the radius of the centerline of the layer of filars and the number of filars, N, used in that layer by:

Substituting (2πR)/N for a in the equation above for Jfilar, and multiplying by the number of filars in a layer, one obtains an expression for the polar moment of inertia for a layer having N filars, with filar diameters of 2b, at a centerline radius R:

As illustrated further below, this equation for the polar moment of inertia for a layer of filars can be used to calculate whether a flexible driveshaft design has bi-directionally balanced or bi-directionally unbalanced torsional stiffness properties.

Still referring toFIGS. 2 and 3, the example flexible driveshaft200has the following characteristics. The core210is 0.660 mm in diameter and is made of nitinol. The first helically wound layer of filars220has twelve (12) filars that are 0.178 mm in diameter and are made of stainless steel. The second helically wound layer of filars230has eleven (11) filars that are 0.254 mm in diameter and are also made of the same type of stainless steel.

When applying the equation above for the polar moment of inertia for a layer of filars, Jlayer, to the flexible driveshaft200, the ratio of the clockwise polar moment of inertia to the counterclockwise polar moment of inertia can be calculated to be about 5:1.

Since in this example all the filars are made of the same type of stainless steel, the shear modulus G is consistent. Therefore, the ratio of the clockwise torsional stiffness to the counterclockwise torsional stiffness is also about 5:1. In other words, the example flexible driveshaft200is five (5) times stiffer when rotated in the clockwise direction than it is when rotated in the counterclockwise direction. In the context of a flexible driveshaft used in a catheter-based medical device, such a discrepancy can hinder a clinician from being able to exert predictable and repeatable control over the rotatory movement and position of the distal tip of the catheter in both directions of rotation.

With reference toFIG. 4, a graph400of tip angle410versus actuator angle420exemplifies the differences between flexible driveshafts with bi-directionally unbalanced torsional stiffness properties (plot430) in comparison to flexible driveshafts with bi-directionally balanced torsional stiffness properties (plot440). Graph400relates to, for example, catheter-based medical devices, such as the example ultrasound catheter100, that use a flexible driveshaft. In general, graph400shows that control of the distal tip from the actuator is much less accurate using a driveshaft with bi-directionally unbalanced torsional stiffness properties (plot430) in comparison to the control of the bi-directionally balanced driveshafts (plot440) as provided herein.

Plot430illustrates that driveshafts with bi-directionally unbalanced torsional stiffness properties exhibit a number of performance deficiencies. First, portions432and434reflect instances where substantial rotations of the panning knob at the actuator result in virtually no rotation of the tip. Portion432, for example, indicates that the panning knob has been rotated from about 360 degrees to about 150 degrees, but that virtually no rotation of the tip has occurred. Similarly, portion434indicates that the panning knob has been rotated from about −360 degrees to about −50 degrees, but that virtually no rotation of the tip has occurred. Those instances provide examples of when the clinician does not have accurate control over the tip angle. As a result, the medical procedure is potentially more time-consuming, expensive, and the potential for patient discomfort is increased.

In addition, plot430includes portion436where, while the actuator is minimally rotated, the tip rotates from about −270 degrees to about −130 degrees. Further still, portions436and438are asymmetrical. These instances provide further examples of when the clinician does not have accurate control over the tip angle.

Unlike the driveshafts with bi-directionally unbalanced torsional stiffness properties, the bi-directionally balanced driveshafts as provided herein provide substantially better controllability of the tip angle, as illustrated by plot440. For example, plot440does not include instances where the panning knob of the actuator is rotated and no rotation of the tip takes place. Nor does plot440include instances where the panning knob of the actuator is minimally rotated but a large change in the tip angle results. Further, plot440is generally symmetrical. Therefore, the bi-directionally balanced driveshafts as provided herein provide substantially better controllability of the tip angle.

With reference toFIGS. 5 and 6, an example flexible driveshaft500with bi-directionally balanced torsional stiffness properties is depicted.FIG. 5provides a cut-away perspective view, andFIG. 6provides an orthogonal cross-sectional view at section6-6. The flexible driveshaft500is a three-layer design. The flexible driveshaft500includes a hollow core510, a first helically wound layer of filars520, a second helically wound layer of filars530, and a third helically wound layer of filars540. The first helically wound layer of filars520is wrapped around and overlays the core510; the second helically wound layer of filars530is wrapped around and overlays the first helically wound layer of filars520; and the third helically wound layer of filars540is wrapped around and overlays the second helically wound layers of filars530. The adjacent layers of helically wound filars are wound in opposite directions. That is, the first helically wound layer of filars520is left-wound, the second helically wound layer of filars530is right-wound, and the third helically wound layer of filars540is left-wound.

In some embodiments, the directions of the winds are reversed in comparison to the example flexible driveshaft500. For example, in some embodiments the first helically wound layer of filars520is right-wound, the second helically wound layer of filars530is left-wound, and the third helically wound layer of filars540is right-wound. In some embodiments, two or more adjacent layers of filars are wound in the same direction, rather than being wound in opposite directions. For example, in some such embodiments two or more adjacent layers of filars are both right-wound, or two or more adjacent layers of filars are both left-wound. All combinations and subcombinations of filar layer wind directions are envisioned within the scope of this disclosure.

The core510can have a variety of construction configurations. In some embodiments, the core510is a metallic material, e.g., nitinol, stainless steel, titanium, titanium alloys (e.g., titanium beta 3), chrome cobalt alloys, precipitation hardened stainless steels, ultra-high-strength steels (e.g., ferrium S53), or another suitable metal or metal alloy. In some embodiments, the core510is a polymeric material. For example, in some such embodiments the core is a thermoplastic polymer that is expanded above its glass transition temperature by pressure in order to bring the core510into direct contact with the first helically wound layer of filars520. In general, the material selection for the core can be based on the parameters desired for the flexible driveshaft. A metallic core may be a desirable core material in some embodiments of flexible driveshafts because of its generally high yield strength in bending, so as to resist plastic deformation in bending at greater than a minimum bend radius. A polymeric core may be a desirable core material in some embodiments of flexible driveshafts because of its ductility and relative low elastic modulus.

In some embodiments, the core510has different cross-sectional geometries (size or shape) at different portions along the axial length of the core510. In some embodiments, the core510has a cross-sectional geometry other than a circle, e.g., an ovular, square, triangular, or another suitable shape. In some embodiments, the cross-sectional geometry of the core510can be suited to transmitting torque via the flexible driveshaft500. For example, in some embodiments an ovular cross-section, or another noncircular cross-sectional shape, can be used to enhance the transmission of torque using the flexible driveshaft500.

In some embodiments, the core510is solid, rather than hollow as shown. In some embodiments of flexible driveshafts, no core is used in the finished flexible driveshaft device (e.g., refer toFIG. 8). In some such embodiments, the inside of the first helically wound layer of filars defines a lumen or hollow central area of the driveshaft.

The individual filars used to construct the layers of helically wound filars520,530, and540can have a variety of configurations. In some embodiments, the filars are a metallic material, e.g., nitinol, stainless steels (e.g., 316LVM), titanium, titanium alloys (e.g., titanium beta 3), or another suitable metal or metal alloy. In some embodiments, the filars are or include graphite, Kevlar, or a polymeric material. In some embodiments, the filars can be woven, rather than wound, layers. In some embodiments, individual filars can comprise multiple strands of material that are twisted, woven, or otherwise coupled together to form a filar. In some embodiments, the filars have different cross-sectional geometries (size or shape) at different portions along the axial length of the driveshaft500. In some embodiments, the filars have a cross-sectional geometry other than a circle, e.g., an ovular, square, triangular, or another suitable shape. In some embodiments, the filars of one layer have different configurations (e.g., their size, shape, construction, and material) than the filars of another layer. While in some embodiments the filars of a given layer each have the same configuration, in some embodiments, one or more filars of a given layer have different configurations than one or more of the other filars of the given layer. In some embodiments, a combination or subcombination of such factors are used in a single flexible driveshaft embodiment.

Referring toFIG. 6, the core510and layers of helically wound filars520,530, and540overlay on each other to define an overall outer diameter542of the flexible driveshaft500. In some embodiments, the overall outer diameter542is within a range of about 0.5 mm to about 2 mm, or about 1 mm to about 4 mm, or about 3 mm to about 8 mm, or about 7 mm to about 12 mm, or more than about 12 mm.

When a clockwise torque, as represented by arrow502, is applied to driveshaft500, the driveshaft200primarily transmits the torque via the core210and the second helically wound layer of filars530. The second helically wound layer of filars530transmits the clockwise torque502because its right-wound filars tend to get tightened by the clockwise torque502. By contrast, the clockwise torque502tends to loosen the first helically wound layer of filars520and the third helically wound layer of filars540. Therefore, relatively less clockwise torque502is transmitted by the left-wound first helically wound layer of filars520or the left-wound third helically wound layer of filars540. On that basis, it can be said that, in the context ofFIG. 5, right-wound filars are active when subjected to a clockwise torque, and that the left-wound filars are effectively inactive.

When a counterclockwise torque, as represented by arrow504, is applied to driveshaft500, the driveshaft500primarily transmits the torque via the core510, the first helically wound layer of filars520, and the third helically wound layer of filars540. The first helically wound layer of filars520and the third helically wound layer of filars540transmit the counterclockwise torque504because their left-wound filars tend to get tightened by the counterclockwise torque504. By contrast, the counterclockwise torque504tends to loosen the second helically wound layer of filars530. Therefore, relatively less counterclockwise torque504is transmitted by the right-wound second helically wound layer of filars530. On that basis, it can be said that, in the context ofFIG. 5, left-wound filars are active when subjected to a counterclockwise torque, and that the right-wound filars are effectively inactive.

Applying the formulas from above, it is known that for example flexible driveshaft500to have bi-directionally balanced torsional stiffness properties, the following relationship applies:

The example flexible driveshaft500can be designed to have substantially bi-directionally balanced torsional stiffness properties using several different filar configuration combinations. One example embodiment is configured as follows (this example is based on all filars being constructed of the same material, e.g., stainless steel (“SS”)):the first filar layer520has 12 filars that are 0.127 mm in diameter;the second filar layer530has 9 filars that are 0.254 mm in diameter; andthe third filar layer520has 18 filars that are 0.101 mm in diameter.

Alternatively, one could produce a three-layer flexible driveshaft with substantially bi-directionally balanced torsional stiffness properties using dissimilar types of filar materials (thereby affecting the shear modulus, G). One potential benefit of this would be to use a larger filar diameter for the outermost layer, which could deter the outermost filars from springing back, or unwrapping. One example embodiment using dissimilar types of filar materials is configured is as follows:the first filar layer520has 12 SS filars at 0.127 mm in diameter;the second filar layer530has 9 SS filars at 0.229 mm in diameter; andthe third filar layer540has 18 Titanium Beta 3 alloy filars at 0.152 mm in diameter.

While the example flexible driveshaft500is a three-layer design, a two-layer driveshaft design with substantially bi-directionally balanced torsional stiffness properties can also be constructed. One example embodiment is configured as follows:the first filar layer has 7 SS filars at 0.318 mm in diameter; andthe second filar layer has 18 SS filars at 0.114 mm in diameter.

The filar material can also be varied within a two-layer driveshaft. One example embodiment of a two-layer driveshaft with substantially bi-directionally balanced torsional stiffness properties, with dissimilar filar material is configured as follows:the first filar layer has 7 SS filars at 0.259 mm in diameter; andthe second filar layer has 18 Titanium Beta 3 alloy filars at 0.173 mm in diameter.

Several different flexible driveshafts with substantially bi-directionally balanced torsional stiffness properties can be configured, and the configurations described above are provided as non-limiting illustrative examples. Design parameters including but not limited to the filar diameters used for each filar layer, the number of filars in each filar layer, the filar wrap angle, the cross-sectional shape of the filars, the shear modulus of the material the filars are made from, and the number of filar layers can be selected as desired. When designing a flexible driveshaft with substantially bi-directionally balanced torsional stiffness properties any of those types of design parameters may be manipulated in order to design a flexible driveshaft with the desired properties for a particular implementation. In general, the design parameters with the greatest relative effect on torsional stiffness are the filar diameters and the shear modulus of the filar material.

FIGS. 7-9illustrate additional example flexible driveshafts700,800, and900that can be configured to have bi-directionally balanced torsional stiffness properties. Example flexible driveshaft700is a three-layer design like that ofFIG. 5, but with a solid core710. Example flexible driveshaft800is a three-layer design like that ofFIG. 5, but without a core. Example flexible driveshaft900is a four-layer design.

As example flexible driveshaft900illustrates, flexible driveshafts with substantially bi-directionally balanced torsional stiffness properties can be configured with four, or more, layers of filars. The flexible driveshaft900includes a solid core910, a first left-wound helically wound layer of filars920, a second right-wound helically wound layer of filars930, a third left-wound helically wound layer of filars940, and a fourth right-wound helically wound layer of filars950. The first helically wound layer of filars920is wrapped around and overlays the core910; the second helically wound layer of filars930is wrapped around and overlays the first helically wound layer of filars920; the third helically wound layer of filars940is wrapped around and overlays the second helically wound layers of filars930; and the fourth helically would layer of filars950is wrapped around and overlays the third helically wound layer of filars940. In other embodiments, the filars can be wound in several other directional configurations and combinations of configurations.

Applying the formulas from above, for driveshaft900to have bi-directionally balanced torsional stiffness properties the following relationship applies:

The example flexible driveshaft900can be designed to have substantially bi-directionally balanced torsional stiffness properties using several different filar configuration combinations. One example embodiment is configured as follows (this example is based on all filars being constructed of the same material, e.g., stainless steel (“SS”)):the first filar layer920has 14 filars that are 0.076 mm in diameter;the second filar layer930has 18 filars that are 0.076 mm in diameter;the third filar layer940has 10 filars that are 0.254 mm in diameter; andthe fourth filar layer950has 18 filars that are 0.102 mm in diameter.

Alternatively, one could produce a four-layer flexible driveshaft900with substantially bi-directionally balanced torsional stiffness properties where the filar material is varied. One such example embodiment is configured is as follows:the first filar layer920has 14 SS filars at 0.127 mm in diameter;the second filar layer930has 18 SS filars at 0.229 mm in diameter;the third filar layer940has 10 SS filars at 0.203 mm in diameter; andthe fourth filar layer950has 18 Titanium Beta 3 alloy filars at 0.152 mm in diameter.

The response of a shaft placed in torsion can be expressed as:

where:T is the applied Torque;□ is the angle of twist of the shaft;L is the length of the shaft; andS is the stiffness previously described.

FIG. 10shows a graph1000of torsion test data, where the applied torque is plotted versus □/L. The slopes of the plotted curves represent the torsional stiffness of the shafts. The data shown was generated from two different shaft constructions.

The first shaft construction tested, representing a commercially available design, (corresponding to plot1010) was an unbalanced two-layer construction configured as follows:the core was 0.660 mm in diameter and made of nitinol;the first filar layer had twelve filars that were 0.178 mm in diameter and were made of stainless steel; andthe second filar layer had eleven filars that were 0.254 mm in diameter and were also made of the same type of stainless steel.

The second shaft construction tested (corresponding to plot1020) was a balanced three-layer construction built to the following specifications:the core was 0.508 mm in diameter and made of nitinol;the first filar layer had twelve filars that were 0.127 mm in diameter and were made of stainless steel;the second filar layer had nine filars that were 0.254 mm in diameter and were also made of the same type of stainless steel; andthe third filar layer had eighteen filars that were 0.102 mm in diameter and were also made of the same type of stainless steel.

The plot for the two-layer shaft construction1010has a significant inflection at about 0.2 degrees/mm. The ratio of the slopes on either side of the inflection is about 4.7:1. This matches very well to the predictions one would make according to the calculations provided above.

In contrast, the plot for the three-layer shaft construction1020with the balanced design is approximately a straight line. The ratio of slopes on either side of zero is about 1.15:1, which matches very well with the predicted ratio of 1.10:1 as would be predicted according to the calculations provided above.

Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims.