US20160102724A1

US20160102724A1 - Concentric Arc Spline Rotational Spring

Info

Publication number: US20160102724A1
Application number: US14/792,882
Authority: US
Inventors: Elliott Potter; Aaron Hulse
Original assignee: Rethink Motion Inc
Current assignee: Rethink Motion Inc
Priority date: 2014-10-09
Filing date: 2015-07-07
Publication date: 2016-04-14
Also published as: EP3204745B1; EP3204745A1; EP3204745A4; US20160116353A1; US9772240B2

Abstract

A planar torsion spring where the spring comprises a first outer ring and a second inner ring. The inner ring is positioned within the first outer ring and possesses a same axis of rotation. The outer ring is connected to the inner ring with one or more splines. Each spline extends in one or more concentric arc segments to a maximum circumference relative to the position of the next concentric arc between the outer ring and the inner ring. The outer ring, the inner ring and the spline concentric arcs segments are positioned in the same plane. At least one spline connects the outer ring to the inner ring. The spline is positioned in a plurality of concentric arcs segments and sequentially positioning each arc to a maximum circumferential length relative to its position between the outer ring and inner ring.

Description

RELATED APPLICATIONS

This Disclosure claims priority to Provisional Application entitled “Elastic Torque Sensor for Planar Torsion Spring” filed Oct. 9, 2014 as application Ser. No. 62/061,815. This application also claims priority to Provisional application entitled “Concentric Arc Spline Rotational Spring” filed Jan. 1, 2015 as application Ser. No. 62/099,191. These provisional applications are incorporated by reference herein in their entirety. This application claims priority to and incorporates by reference herein provisional application Ser. No. 62/173,498 entitled “Elastic Torque Sensor for Planar Torsion Spring filed Jun. 10, 2015. This application claims priority to and incorporates by reference herein nonprovisional application Ser. No. 14/691,702 entitled Series Elastic Motorized Exercise Machine filed Apr. 21, 2015

FIELD OF USE

This disclosure pertains to the field of planar torsion springs.

BACKGROUND OF DISCLOSURE

Rotational torsion springs that work by storing energy with torsion or twisting; that is, a flexible elastic object that stores mechanical energy when it is twisted. When the rotational torsion spring (hereinafter “torsion spring”) is twisted, it exerts a force (torque) in the opposite direction, proportional to the amount (angle) the torsion spring is twisted. Torsion springs are known in the industry. However use is being adapted to new applications and optimal material shapes, dimensions and designs are frequently created on a trial and error basis.

SUMMARY OF DISCLOSURE

A torsion spring is a type of spring that stores mechanical energy when a twisting force (torsion) is applied. These include torsion bars where the torsion is resisted by shear stresses, and spiral torsion springs wherein the torsion is resisted by bending stresses about the axis of their curvature.
This Disclosure pertains to a rotational torsion spring comprising an inner ring positioned concentrically with an outer ring. The inner ring can be termed the input side of the torsion spring. The outer ring can be termed the output side of the torsion spring. The outer ring has a larger radius than the inner ring. Both rings share the same axis of rotation. The inner ring and the outer ring are connected with splines positioned between the inner ring and the outer ring. In a preferred embodiment, of the splines is configured with long arc segments. The long arc segments extend approximately parallel to the circumference of the inner and outer rings.
The disclosure illustrates a torsion spring comprising concentric arc splines connecting the inner ring and outer ring of the torsion spring. Each spline comprises a serpentine component that extends from the inner ring to an outer ring. The serpentine shape of each spline is preferably identical. Each spline is attached to the inner ring and outer ring by each spline forming an L shaped segments at its juncture with each ring.
Each of the splines has a depth dimension. For example, the depth is the dimension of a steel plate from which the spline may be cut. The depth is a dimension parallel to the center longitudinal axis of the inner and outer rings.
Each of the splines has a thickness. This is the dimension of the spline relative to the plane to the rotational spring. This can be also termed the spline thickness or width of the concentric arc. The geometry of the torsion spring subject of this disclosure allows the reduction of spline thickness for a specified torsion spring load. For example the spline subject of this disclosure may be less thick (thinner) than the spline disclosed in provisional application 62/061,815 which is incorporated by reference in its entirety. It will be appreciated that the spline thickness may vary depending upon its position relative the inner or outer rotational ring, etc.
The spline depth may also vary. The depth may be less than the depth of the concentric inner or outer ring.

SUMMARY OF DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention. These drawings, together with the general description of the invention given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIG. 1 illustrates a previous design of a planar torsional spring comprising two splines connecting the output side and the input side of the spring. Limited annular or concentric arc segments are illustrated.

FIG. 2a illustrates one design of a planar torsion spring taught by the disclosure comprising three splines with expanded concentric arc construction. FIG. 2a is a top view. The dimensions of spline thickness and spline depth are clearly illustrated. FIG. 2b illustrates a side view. The depth of the spline is clearly viable. It will be appreciated that each spline concentric arc has the same or similar axis of rotation. This is the axis of rotation of the inner ring and the outer ring or is based upon this axis of rotation.

FIG. 3 illustrates the locations and amounts of stress experienced by the rotational torsion spring subject of this disclosure. The Figure illustrates the stress analysis. The highly stressed portions of the structure are illustrated in red. The movement of the splines is exaggerated.

FIG. 4 illustrates the location and amounts of stress experienced by the rotational torsion spring of a two spline rotational torsion spring. Again, the areas of the torsion spring subject of high stress are illustrated in red. The red areas of FIGS. 3 and 4 have different amounts of stress associated with them. FIG. 3 has less stress magnitude than FIG. 4 even though they have the same load applied and design stiffness.

FIGS. 5a and 5b illustrate a comparison of an inner and outer ring where FIG. 5b shows the rings to be misaligned and do not share the longitudinal axis of orientation.

DETAILED DESCRIPTION OF DISCLOSURE

The planar torsion spring comprises an inner ring nested within a larger diameter outer ring. The rings are joined together by one or more splines. The splines can form elongated concentric arcs (hereinafter “concentric arc segments”) surrounding the inner ring. The design of the spline can be opposite the design of a wheel spoke radiating directly between an outer rim and inner hub. It will be appreciated the spoke will extend from the inner hub in a radially straight direction to the outer rim. It will be appreciated that the elongated concentric arc (serpentine) shape of the splines of the Applicant's design permits the greater deflection of the spline with lower stress. The Applicant's design achieves this improvement by the longer load path formed of the elongated design of the concentric arc segments of each spline. It will be further appreciated that the spline can be deflected or deformed by the rotation of one ring relative to the other ring. With fewer splines, each spline can be designed longer to achieve a wider range of stiffness, but a lower maximum achievable stiffness. With fewer splines, each spline can be designed to have a longer extended path between the inner ring and the outer ring. The thickness of the spline may be varied through the elongated length. The depth of the spline may also be varied
The Applicant discloses that incorporating more splines connecting the inner and outer rings allows a narrower range of achievable stiffness, but a higher maximum stiffness. There can be as few as 1 spline, and the practical upper limit of splines is dictated by the overall diameter of the spring. This disclosure teaches that fewer splines allows a broader range of stiffness, while a greater number of splines narrows the range of stiffness but increases the total stiffness of the spring. Further, this application claims priority to Nonprovisional application Ser. No. 14/691,702 entitled “Series Elastic Motorized Exercise Machine” filed Apr. 21, 2015 and which is incorporated in its entirety by reference herein.
FIG. 1 illustrates an example of a planar torsion spring 100. It will be appreciated that the flat spring lies in the plane of the page. The planar torsion spring has an axis of rotation 140. The axis of rotation is parallel to the longitudinal axis of the spring. The axis of rotation is parallel to the depth 135 of the planar torsion spring. This axis of rotation is shared with the outer ring 110 (the output side) and the inner ring 120 (the input side).
It will be appreciated that the axis of rotation of the torsion spring may be shared with the axis of rotation of other components of an apparatus utilizing the torsion spring. Such an apparatus can be the Series Elastic Motorized Exercise Machine.
The planar torsion spring illustrated in FIG. 1 illustrates a design utilizing two splines 130. As shown, the splines connect the input side 120 with the output side 110. Each spline comprises a single arc segment. The width of the arc segment has a distinct width apparently unrelated to the relationship of the arc to the varying circumference existing between the outer ring and inner ring. This design is distinct from the design illustrated in the Applicant's disclosure.
This disclosure pertains to a novel planar torsion spring. An example of the planar torsion spring of the Applicants' disclosure is shown in FIG. 2. The spring comprises a planar surface. This plane extends along the x and y axis. The spring has a radius in the x and y axis. The spring comprises an outer ring 110 (the output side) and an inner ring 120 (the input side). The output side is concentric about the input side. The input side and output side share the same axis of rotation 140. The axis of rotation is parallel to the longitudinal axis of the spring. The depth 135 of the spring is also parallel to the longitudinal axis. The axis of rotation and longitudinal axis are in the z direction.
The planar torsion spring can be described as an inner ring positioned within the concentric ring of an outer portion ring. The inner and outer rings are connected by one or more splines. The splines comprise multiple concentric arc segments positioned in a serpentine pattern between the inner circumference of the outer ring and the outer circumference of the inner ring. The Applicant's design illustrated in FIG. 2a achieves maximum extension of each spline relative to the circumference between the first outer ring (output side) and the circumference of the second inner ring (input side) and each spline has the maximum number of concentric arc segment splines between the inner circumference of the first outer ring and the outer circumference of the second inner ring. The maximum length of a spline and the number of concentric arc segments may be impacted by the number of spines.
Another definition of the disclosure would be a method for a planar torsion spring wherein the spring comprises fabricating a first outer ring, fabricating a second inner ring which is positioned within the first outer ring and possessing a same axis of orientation, further connecting the first outer ring with the second inner ring with one or more splines and extending the spline to a maximum length relative to the circumference between the first outer ring and second inner ring, fabricating the spline with the maximum number concentric arc segments between the inner circumference of the first outer ring and the outer circumference of the second inner ring and positioning the first outer ring, the second inner ring and the spline in the same plane.
The advantages of this construction include increased strength and flexure of the spring.
The above state definitions are now combined with the planar rotational spring diagramed in FIG. 2. Connecting the outer ring (output side) with the inner ring (input side) are splines 130. Three splines are illustrated. Each spline follows a serpentine path from the outer circumference of the inner ring to the inside circumference of the outer ring. The spline architecture includes a plurality of annular segments 131. In the illustration, each spline has three annular segments. Each segment comprises an arc of approximately 120°. Each annular segment is attached by a curved section of the spline 132. The curved sections reverse direction of the annular segments of the spline. Each spline attaches to the input and output sides by an L shaped protrusion 133 extending from the output or input side and immediately turning to form an annular segment.
It will be appreciated that there may be more than three annular segments. This could be achieved by making the thickness 134 of the spline narrower. This change in geometry may require the depth 135 of the spline of the torsion spring to be increased. It could also be achieved by making the diameter of the planar torsion spring larger.
The advantages of this construction, i.e., three splines constructed of three annular segments attached by curved segments and attached to the inner and outer rings by L shaped structures as illustrated in FIG. 2, increased strength and flexure of the spring. Each curved section 132 and L shaped structure 133 acts as a flexure point.
An additional advantage is that the input side can be deflected from the output side by an increased angle. It will be appreciated that the drawing illustrates the spring at an equilibrium state. If the output side is subjected to force, the spring will flex. When flexed, the output side of the spring may rotate while the input side stays in the same position. The increased flexure of the spring allows increased angle of rotation or deflection of the output side relative to the input side without permanent deformation of the spring.
In another embodiment, not shown, is one or more splines comprising a winding configuration about the other or winding about the input side.
Continuing the discussion/comparison of the novel 3 spline design described above and illustrated in FIG. 3 and the 2 spline configuration illustrated in FIG. 4, the stress analysis for both spring designs is illustrated. These two springs have the same stiffness and strength, but the novel spring design subject of this disclosure (FIG. 3) can be manufactured from more standard, less expensive alloy. The lower stress of the spring depicted in FIG. 3 is evidenced by the small portions of the spring that exhibit a red color and lower overall magnitude of force represented by the red color.
The geometry of the 3 spline torsion spring subject of FIG. 3 was made by the inventors. One advantage of the inventors' geometry is that each spline has a greater load path. It will be appreciated that the greater load path allows more deflection given a material and a lower amount of stress. concentration leading to a greater achievable stiffness range before overload.
Given a desired stiffness, the general geometry of the design reduces the stress in the material, resulting in a stronger spring than the design illustrated in FIG. 1.
The color drawings FIGS. 3 and 4 shows the areas experiencing stress (red). The magnitude of the area in red shows the amount of stress. It is important to note that the springs of FIGS. 3 and 4 were subjected to the same torque and stiffness but the splines of FIG. 3 shows greater strength due to geometry (less stress). FIG. 3 illustrates smaller areas of stress concentration and lower overall stress. It will be appreciated that the degree of distortion of the splines is exaggerated in FIG. 3 (and FIG. 4). Also the stress scale delineated on FIG. 3 and FIG. 4 is different. FIG. 4 has a higher scale of stress. FIG. 3 shows up to 6.454e8 N/m²In contrast; FIG. 4 illustrates 7.241 e8 N/m². It will be appreciated that if the spline is exposed to excessive force, the structure of the spline can break or be permanently deformed. The splines may also be subject of hysteresis wherein the shape of the spline is temporary deformed after bending due to torque.
It will be appreciated that the spring geometry includes the depth 135 of the splines, as well as the spline thickness 134 and spline load path (illustrated as 131, 132, 133 and 134). The geometry and material selection determine the spring stiffness. For example the planar torsion spring illustrated in FIG. 3 and comprised of standard steel alloys e.g., 17-4PH stainless steel can achieve the same stiffness and strength of the spring illustrated in FIG. 4 comprised of more expensive or more difficult to work with such as custom 465 stainless steel or maraging steel. Also, the spring illustrated in FIG. 3 can achieve a wider range of spring stiffness than the design in FIG. 4.
Further, the new spring geometry reduces stress concentration by distributing the load more predictably and evenly. This means that the peak stress in the material is less with the new design given a size and stiffness target. The new spring geometry (FIG. 3) illustrates a larger load path. It will be appreciated that the greater load path allows the forces created by spring deflection to be spread over a greater area, resulting in smaller and less consequential stress concentrations. Given that the overall dimensions between the two spring designs are the same, the new spring design allows the use of more standard alloys to get the same maximum load rating and stiffness.
The Applicants' design illustrated in FIG. 3 can also be modified using a parametric equation where strength and stiffness are input parameters and dimensions are the equation output. An example of such an equation is stated below. Utilization of such an equation allows the Applicants' design to be easily modified to determine stiffness. Note the equation can be used to select the spline thickness. It does not select the design pattern or number of arc segments. This allows designers to very quickly change the spring stiffness to match their intended application.
SplineThickness=−1e−4*DesiredStiffness²+0.0577*DesiredStiffness+3.4142
DesiredStiffness is in units of Nm/deg
SplineThickness is in units of mm
This equation is specifically for an inner ring diameter of 50 mm and an outer ring diameter of 210 mm with a 6.35 mm depth. The equation maintains the same form for different inner ring and outer ring diameters as well as different thicknesses, but it will have different coefficients.
The concentric serpentine nature of the splines helps to reduce stress due to radial misalignment of the input and output axes of rotation due to assembly tolerances. Radial misalignment is illustrated in FIG. 5a showing a simplified view of two planar torsion springs. The perspective of the illustration (showing two end views of torsion springs) is looking down the longitudinal axis of the torsion spring. The illustration on the left side of FIG. 5a shows the inner ring and outer ring in proper alignment. The right side of FIG. 5a illustrates the outer ring 501 of the torsion spring having a different axis of rotation relative to the inner ring 502. The left side of FIG. 5b again shows proper alignment. FIG. 5b shows axial misalignment, where both the outer 503 ring and inner ring 504 are concentric but in different positions along the longitudinal axis. In other words, the new design allows for greater misalignment axially or radially between the inner and outer rings before stress buildup causes problems. This allows for cheaper, lower precision manufacturing processes to be used in making the parts the spring attaches to. It will be appreciated that the selection of concentric arc segment splines enhances the tolerance of the torsion spring to misalignment. The tolerance for axial and radial misalignment allow for wider (easier) mounting tolerances, which makes manufacturing easier.
Another variable of the planar torsion spring of the Applicants' concentric arc segment spline design is that the length of each arc segment 131 (see FIG. 2) can be varied. Reducing or expanding the arc length will change the way the spring behaves, depending on the application. This variation is independent of the total number of splines. Given a number of splines, longer thinner concentric arc segments lead to a softer spring overall. The length of the arc segments is directly tied to the stiffness and strength of the spring.
A planar torsion spring comprising concentric arc segments tolerates radial or axial misalignment with reduced stress compared to other spring designs. Radial misalignment occurs when the axis of rotation of the inner ring and outer ring are not identical.
This specification is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the disclosure. It is to be understood that the forms of the disclosure herein shown and described are to be taken as the presently preferred embodiments. As already stated, various changes may be made in the shape, size and arrangement of components or adjustments made in the steps of the method without departing from the scope of this disclosure. For example, equivalent elements may be substituted for those illustrated and described herein and certain features of the disclosure maybe utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the disclosure.
While specific embodiments have been illustrated and described, numerous modifications are possible without departing from the spirit of the disclosure, and the scope of protection is only limited by the scope of the accompanying claims.

Claims

What we claim is:

1. A rotational torsion spring comprising:

a) an inner ring;

b) an outer ring;

c) one or more splines connecting the inner ring with the outer ring wherein each spline comprises a plurality of concentric arc segments which store energy when either the inner or outer ring is rotated in relation to the complementary inner or outer ring.

2. The rotational torsion spring of claim 1 comprising concentric arc segments that create an extended spline length wherein spline stress concentrations are decreased.

3. The rotational torsion spring of claim 2 wherein the extended spline length creates increased tolerance of the inner ring to radial misalignment of the outer ring.

4. A planar rotational torsion spring comprising an inner ring nested within an outer ring and the rotational torsion spring further comprises at least one spline connecting the outer ring to the inner ring and the spline forms concentric arc segments between the outer ring and the inner ring.

5. The planar rotational torsion spring of claim 4 further comprising the concentric arc segments each encircling at least 90 percent of the length of circumference existing along each arc segment.

6. The planar rotational torsion spring of claim 5 wherein at least one concentric arc segment is circumferentially oriented about an axis of rotation of the inner ring.

7. The planar rotational torsion spring of claim 5 wherein at least one concentric arc segment is substantially circumferentially oriented about an axis of rotation of the inner ring.

8. The planar rotational torsion spring of claim 4 comprising a single spline connecting the outer ring to the inner ring and the spline forms a plurality of concentric arc segments between the outer ring and the inner ring and the single spline achieves deflection of either ring relative to the other ring with higher stress at the same load of the spring than a planar rotational torsion spring comprising a plurality of splines.

9. A first planar rotational torsion spring comprising a number of n splines and achieving deflection of the arc segments with more stress than a second planar rotation torsion spring of the same load, material thickness and torsion spring diameter of the first torsion spring wherein the second torsion spring comprises n+1 splines.

10. A first planar rotational torsion spring comprising a number of n+1 splines and the first planar rotational torsion spring achieves a greater stiffness than a second rotational torsion spring comprising n splines and comprising the same material, material thickness and torsion spring diameter as the first planar rotational spring wherein more splines achieve greater stiffness than a spring with fewer splines.

11. The planar rotational torsion spring of claim 4 wherein the concentric arc segments form an extended load path.

12. The planar rotational torsion spring of claim 4 wherein the extended spline length created from the concentric shape decreases stress concentration in the spline.

13. A planar rotational torsion spring comprising an inner ring nested within an outer ring and the rotational torsion spring further comprises a plurality of splines wherein each spline comprises multiple arc segments wherein one arc segment of a first spline is fixed to the outer diameter of the inner ring and a different arc segment of the first spline is attached to the inner diameter of the outer ring.

14. The planar rotational torsion spring of claim 13 wherein each of the plurality of splines comprise a plurality of circumferential arc segments.

15. The planar rotational torsion spring of claim 14 wherein the arc segments are joined by curved segments and the arc segments are attached to the inner and outer rings by L shaped structures where the circumferential arc segments turn approximately 50 degrees into the inner or outer ring

16. The rotational torsion spring of claim 4 comprises a spring achieving a preselected stiffness with a computed spline thickness and known spring constant.

17. The rotational torsion spring of claim 4 wherein the design is selected based upon desired stiffness and strength and known spring constant.

18. A planar rotational torsion spring comprising an inner ring nested within an outer ring wherein the rotational torsion spring further comprises at least one spline connecting the outer ring to the inner ring and a spline forms concentric arc segments between the outer ring and the inner ring.

19. A planar torsion spring wherein the spring comprises a first outer ring, a second inner ring which is positioned within the first outer ring and possessing a same axis of rotation, the first outer ring connected to the second inner ring with one or more splines each extended in one or more concentric arcs to a maximum circumference relative to the position of the concentric arc between the first outer ring and the second inner ring and positioning the first outer ring, the second inner ring and the spline concentric arcs in the same plane.

20. A method of constructing a planar torsion spring wherein the spring comprises fabricating a first outer ring, fabricating a second inner ring which is positioned within the first outer ring and possessing a same axis of rotation, further connecting the first outer ring to the second inner ring with one or more splines, extending the spline in a plurality of concentric arcs, sequentially positioning each arc to a maximum circumferential length relative to its position between the first outer ring and second inner ring, fabricating the spline with the maximum number concentric arcs between the inner circumference of the first outer ring and the outer circumference of the second inner ring and positioning the first outer ring, the second inner ring and the concentric arcs of the spline in the same plane.