Abstract:
An improved flexural pivot design simplifies the structure, fabrication, assembly, and integration into a device. A set of flexures  200  is affixed to two surfaces of a stationary member  100  and a single surface of a rotational member  300 . The set of flexures follow opposing centers of curvature  411  and  412  to provide a stable center of rotation  400  and allow repeatable limited-angle rotational motion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS: This application claims the benefit of Provisional Patent Application Ser. No. 60/428373 filed Nov. 22, 2000. 
    
    
     
       FEDERALLY SPONSORED RESEARCH  
         [0001]    Not Applicable  
         SEQUENCE LISTING  
         [0002]    Not Applicable  
         BACKGROUND  
         [0003]    1. Field of Invention  
           [0004]    This invention advances previous flexural pivot designs by simplifying the manufacture, assembly, and integration into a device.  
           [0005]    2. Description of Prior Art  
           [0006]    A typical pivot consists of a combination of bearings which provide both radial and axial stiffness, while allowing a low resistance to rotation about an axis. Radial and thrust journal bearings can accomplish this with simple components, but the continual or intermittent sliding contact generates significant resistance to rotation, and material wear limits the service life. Radial and thrust roller bearings can provide the axial and radial stiffness under heavy loads, but require precision roller elements, raceway structures, and lubrication methods. A pre-loaded set of two ball bearings can provide axial and radial stiffness for light loads, but again require precision balls, raceway structures, and lubrication.  
           [0007]    Journal, ball, and roller bearings allow a simple co-axial configuration of the fixed and rotational portions. A shaft is attached to the inner diameter of the bearings and the outer diameter of the bearings are attached to a sleeve or housing. The shaft can rotate or the sleeve can rotate. With a rotating sleeve, the shaft can be fixed at one end (cantilevered) or both ends (doubly-supported). Fixing the shaft at both ends (doubly-supported) offers a significantly more rigid structure over a cantilevered attachment, and is required in many applications. The co-axial placement of the shaft and sleeve allow a straight-forward centering of the rotational mass between the axial spacing of the bearings. The centering of the rotational mass between the radial bearing elements is usually desired to uniformly distribute radial loads.  
           [0008]    However, journal, ball, and roller bearings have several disadvantages. They require oil or grease lubrication and the associated oil replenishment or grease seals for ensuring a long service life in harsh environments. Lubricants can migrate, decompose, or give off gasses over their working life, and degrade the performance of the bearing. Migration or off-gassing of lubricants may also contaminate surrounding parts of the device in which the bearing is mounted. These contaminants can cause failure of sensitive components such as those found on satellites or inside magnetic data storage drives. In satellites, the vacuum of space will draw-out chemical compounds from the lubricants, which can foul sensors. In magnetic data storage drives, off-gassed chemical compounds can foul critical air bearing surfaces which are designed to operate at air gaps of less than a millionth of an inch. Journal, ball, and roller bearings are also sensitive to contamination; if particles are introduced between the moving surfaces, an increased force is required to roll over the particle. Hard particles will damage the bearing surfaces and hasten bearing wear. Given the sliding wear or rolling resistance, these bearings exhibit a hysteresis-effect; frictional forces oppose motion in both rotational directions. Additionally, the properties of lubricants vary with temperature; at low temperatures an increased force is required to displace lubricants. Further, lubricants are typically non-conductive, which electrically isolates the rotational portion from the fixed portion, allowing a generally un-desirable voltage potential to develop between the two portions.  
           [0009]    Advantages of Flexural Pivots:  
           [0010]    For applications requiring rotational motion within a limited angular range, +/−30 degrees for example, Lucas produces and markets a line of flexural pivots as described in U.S. Pat. No. 3,811,665. FIG. 1 depicts a Lucas flexural pivot  10 . A pair of flexures  13  are attached to the inner diameter of a stationary member  11 . The flexures are also attached to the inner diameter of a rotational member  12 .  
           [0011]    Flexural pivots such as the Lucas pivot provide many advantages over journal bearings, ball, or roller bearings. They require neither lubrication nor the associated seals and oil replenishment systems. Without the temperature-sensitive greases or oils, their performance varies little over a wide range of temperatures. They are not sensitive to contaminants. If fabricated out of metal components, the Lucas flexural pivot can provide a continuous, low-resistance electrical path to eliminate voltage potentials between the moving and stationary portions of the pivot. With no rolling or sliding interactions, the life of a flexural pivot can be many times that of a journal, ball, or roller bearing pivot. With no friction forces to oppose rotational motion regardless of rotational direction, there is no hysteresis-effect exhibited by a flexural pivot.  
           [0012]    Further, the flexural members of a flexural pivot provide a restoring force to the pivot, such that it will return to a repeatable angular position when external forces are removed. This is beneficial during assembly to control the relative positioning of the stationary and rotate-able portions. It is also beneficial to the application, when it is desired for the rotate-able portion to return to a pre-determined rotational location.  
           [0013]    While the Lucas flexural pivot solved many problems, the design limited its applications. The rotating and fixed housings are both tubular forms, which allow for limited attachment methods. Attachment is necessarily made on the outer diameter of both the fixed and rotating housings, which are the same size and closely spaced along the axis of the pivot. Hence, the mating parts must be closely placed along the axis of the pivot, but not at the same axial location. The simplest of the Lucas pivots offers two separate tubular forms, one fixed and one rotational. The fixed portion is held at one end and the rotational portion is at the other end of the pivot axis. The fixed portion cannot be held at both ends of the pivot&#39;s axis. Hence, the rotational portion must be cantilevered. This cantilevered attachment offers significantly lower stiffness than a doubly-attached shaft, as journal, ball, and roller bearing pivots allow.  
           [0014]    Further, the rotating and fixed housings of the Lucas pivot are complex forms, generally requiring wire electron-discharge machining, are well as conventional lathe and milling machine operations. Assembly of the flexures into the inner diameters of these tubular housings is difficult, especially for small pivots and small tubular diameters.  
           [0015]    Objects and Advantages:  
           [0016]    The current invention simplifies the fabrication of flexural pivot components, uncomplicates the assembly, improves upon methods of attachment, and allows scaling to smaller sizes. These features allow integration of this flexural pivot into a wider range of applications. These applications can now take advantage of the benefits of flexural pivots in applications where journal, ball, or roller bearing pivots have been the only choice.  
         SUMMARY OF INVENTION  
         [0017]    The present invention offers a simplified flexural pivot construction and allows improved attachment methods for the fixed and rotational portions. A set of flexural members are preloaded in opposite directions to provide a stable rotational axis. These flexural members can be formed as a unitary set, as compared to the two independent flexures of the Lucas pivot. Affixing features on the stationary and rotate-able portions are simplified to external surfaces, as compared to complex internal features of the tubular forms in the Lucas pivot. Assembly is thereby simplified to external operations, as compared to the intermeshing components assembled into the tubular forms for the Lucas pivot. Fabrication of the stationary and rotate-able members can be simple metal stampings, as compared to the milled and electron-discharge machining required of the Lucas pivot components.  
           [0018]    Integration into an application is greatly simplified. Since the stationary and rotational portions are placed on either side of the rotational axis, the rotational load can be centered between the flexural elements, along the axis of rotation. The stationary portion can be affixed at both ends (doubly-supported), providing maximum mechanical stiffness. Further, with simple attachment features, the rotate-able and stationary portions can easily be integrated into other components of the application. The stationary member can easily be integrated into the application chassis. The rotate-able portion can easily be integrated into the rotational component of the application.  
           [0019]    All of these simplifications are accomplished, while maintaining the advantages of a flexural pivot: the absence of a hysteresis-effect, lubrication requirement, tolerance to contaminants, stable performance over temperature, continuous low-electrical resistance, long life, and a restorative force to maintain a zero-load angular position. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS:  
       [0020]    [0020]FIG. 1 is an isometric view of the prior art Lucas flexural pivot.  
         [0021]    [0021]FIG. 2 is an isometric view of the current invention with a mounting flange parallel to the axis of rotation.  
         [0022]    [0022]FIG. 3 shows a top or axial view of the current invention, showing centers of curvature.  
         [0023]    [0023]FIG. 4 shows the current invention, but from the opposite viewing direction as FIG. 2.  
         [0024]    [0024]FIG. 5 is an isometric view of an alternate configuration of the current invention with a mounting flange perpendicular to the axis of rotation.  
         [0025]    [0025]FIG. 6 shows the alternate configuration, but from the opposite viewing direction as FIG. 5. 
     
    
     REFERENCE NUMERALS  
       [0026]    [0026] 10  Lucas pivot (prior art)  
         [0027]    [0027] 11  Stationary member (prior art)  
         [0028]    [0028] 12  Rotational member (prior art)  
         [0029]    [0029] 13  Flexures (prior art)  
         [0030]    [0030] 100  Stationary post  
         [0031]    [0031] 101  Backside stationary post surface  
         [0032]    [0032] 102  Frontside stationary post surface  
         [0033]    [0033] 103  Spot welds  
         [0034]    [0034] 104  Mounting holes  
         [0035]    [0035] 200  Flexure system  
         [0036]    [0036] 201  Upper backside flexure  
         [0037]    [0037] 202  Upper frontside flexure  
         [0038]    [0038] 203  Lower frontside flexure  
         [0039]    [0039] 204  Lower backside flexure  
         [0040]    [0040] 300  Movable post  
         [0041]    [0041] 301  Movable post surface  
         [0042]    [0042] 303  Spot welds  
         [0043]    [0043] 304  Mounting holes  
         [0044]    [0044] 400  Axis of rotation  
         [0045]    [0045] 401  Angular direction (clock-wise)  
         [0046]    [0046] 402  Angular direction (counter clock-wise)  
         [0047]    [0047] 403  Axial direction  
         [0048]    [0048] 405  Lateral direction  
         [0049]    [0049] 406  Roll direction  
         [0050]    [0050] 407  Pitch direction  
         [0051]    [0051] 411  Approximate backside center of curvature  
         [0052]    [0052] 412  Approximate frontside center of curvature  
         [0053]    [0053] 500  Movable post  
         [0054]    [0054] 504  Axial mounting holes  
       DETAILED DESCRIPTION  
       [0055]    Description—Preferred Embodiment:  
         [0056]    The preferred embodiment of the current invention is depicted in FIG. 2. A flexure system  200  is comprised of an upper backside flexure  201 , a upper frontside flexure  202 , a lower frontside flexure  203 , and a lower backside flexure  204 . The flexure system  200  is affixed between a stationary post  100  and a movable post  300 . The stationary post  100  has two surfaces onto which the flexure system  200  is affixed: a backside stationary post surface  101  and a frontside stationary post surface  102 . The upper backside flexure  201  and the lower backside flexure  204  are affixed tangent to the backside post surface  101 . The upper frontside flexure  202  and the lower frontside flexure  203  are affixed tangent to the frontside stationary post surface  102 . The movable post  300  has a movable post surface  301  for affixing the flexure system  200 . The flexure system  200  is affixed tangent to the movable post surface  301 .  
         [0057]    Before assembly, the flexure system  200  is flat. During assembly, the upper backside flexure  201  and lower backside flexure  204  are preloaded against the backside stationary post surface  101 . This preload causes the backside flexures  201  and  204  to deform and assume a shape with variable curvature. As shown in FIG. 3, the approximate backside center of curvature  411  of backside flexures  201  and  204  is spaced away from the stationary post  100 , in a direction normal to the backside stationary post surface  101 . Similarly, during assembly, the upper frontside flexure  202  and lower frontside flexure  203  are preloaded against the frontside stationary post surface  102 . The preload causes the frontside flexures  202  and  203  to deform and assume a shape with variable curvature. The approximate frontside center of curvature  412  of frontside flexures  202  and  203  is spaced away from the stationary post  100 , in a direction normal to the frontside stationary post surface  102 . After affixing, the preload force of the backside flexures  201  and  204  will equal in magnitude the preload force of the frontside flexures  202  and  203 . The preload forces are in opposite directions and hence, provide a stable resting position of the movable post  300 . Preload forces are chosen to ensure no yielding during the assembly, full limit rotation of the movable post  300 , or during static and dynamic loading.  
         [0058]    As shown in FIG. 2, spot welds  103 , produced by such methods as resistance, laser, or ultrasonic, are the preferred method of affixing the flexure system  200  to the stationary post  100 . Spot welds  303  are the preferred method of affixing the flexure system  200  to the movable post  300 . Alternatively, adhesives, such as epoxies, glues, contact cement, and pressure-sensitive-adhesive, or mechanical fasteners, such as screws and spring-clips, could be used to affix the flexure system  200  to the stationary post  100  or to the movable mount  300 .  
         [0059]    [0059]FIG. 2 depicts threaded mounting holes  104 , which are located at both ends of the stationary post  100 , allowing either doubly-supported attachment of the stationary post  100  for highest mechanical stiffness or cantilever attachment of the stationary post  100  for the simplest attachment. The movable post  300  has mounting holes  304 , aligned with the lateral direction  405 , which allow the attachment of the device to be rotated. FIG. 5 and FIG. 6 depict an alternate configuration of the movable post  500  with axial mounting holes  504 .  
         [0060]    Preferably the flexure system  200  is fabricated out of a high tensile strength spring steel alloy which will exhibit a fatigue-limit stress, allowing a nearly infinite flexural cycle life. A steel stationary post  100  and steel movable post  300  can allow assembly with the preferred spot welds  103  and  303 .  
         [0061]    Operation—Preferred Embodiment:  
         [0062]    As shown in FIG. 3, the movable post  300  will rotate about an axis of rotation  400 , which is approximately parallel to the backside flexure center of curvature  411  and the frontside flexure center of curvature  412 . The axis of rotation  400  lies approximately on a line drawn between the backside flexure center of curvature  411  and the frontside flexure center of curvature  412 .  
         [0063]    Rotation of the movable post  300  about the axis of rotation  400  in an angular direction  401  (clockwise from above) will generally reduce the radius of curvature of the backside flexures  201  and  204 , while increasing the radius of curvature of the frontside flexures  202  and  203 . Similarly, rotation of the movable post  300  about the axis  400  in a direction  402  (counter-clockwise from above) will generally increase the radius of curvature of the backside flexures  201  and  204 , while reducing the radius of curvature of the frontside flexures  202  and  203 . During rotation in either direction  401  or  402 , the axis of rotation  400  will move slightly.  
         [0064]    As seen in FIG. 2, the tangency of the system of flexures  200  at the affixing movable post surface  301  provides a high mechanical stiffness in a longitudinal direction  404 , the axial direction  403  and the pitch direction  407 . The placement of the center of curvatures  411  and  412  on either side of the flexural system  200  provides a moderate mechanical stiffness in the lateral direction  405 . The spacing of the flexural system along the axial direction  400  provides a moderate stiffness in the roll direction  406 .  
         [0065]    Description—Additional Embodiment:  
         [0066]    While the preferred embodiment demonstrates one design of the current invention, many variations exist which may be chosen to optimize integration into different applications. For instance, the stationary post  100  of the preferred embodiment could be allowed to rotate, while the movable mount  300  could be fixed.  
         [0067]    The backside flexures  201  and  204  could be located at the same positions along the axial direction as the frontside flexures  202  and  203 . This may necessitate that the flexure system is comprised of two parts, the backside flexures  201  and  204  could be combined into a single backside flexure and the frontside flexures  202  and  203  could be combined into a single frontside flexure. The backside flexures  201  and  204  could alternatively be affixed to a different surface of the movable post  300 , perhaps on the surface opposite the movable post surface  301 . As a further variation, a single part comprised of frontside flexures  202  and  203  could be affixed to a single part comprised of backside flexures  201  and  204 , which in-turn is affixed to the movable post surface  301 .  
         [0068]    The number of backside flexures and frontside flexures is a design variable. A single backside flexure and single frontside flexure can be used for the simplest design. Multiple backside flexures and frontside flexures may be employed to meet certain design requirements. Multiple backside flexures and frontside flexures allows the rotate-able structure to be interleaved between flexures.  
         [0069]    Backside flexure stiffness may be matched to the frontside flexure stiffness to provide a balanced mechanical rotational stiffness. Or the backside flexure stiffness may be unequal to frontside flexure stiffness to allow different restoring forces in the clockwise and counter-clockwise rotational directions.  
         [0070]    The flexures of the preferred embodiment are shown as rectangular forms, but different geometries could be devised for optimizing mechanical stiffnesses, load stresses, pivot life, or variation of the axis of rotation  400 .  
         [0071]    Given the simple features required on the stationary post  100 , it could be integrated into the chassis of the application, instead of fabricated as a separate part. Similarly, the movable post  300  could be integrated into the rotate-able component of the application, eliminating attachment features and attachment parts.  
         [0072]    With the appropriate fabrication technology, two or more of three parts of the current invention, the flexure system  200 , stationary post  100 , and movable post  300 , could be fabricated as a single part. For example, all three parts could be molded in a plastic resin simultaneously.  
         [0073]    Alternate materials can be used in the construction of the flexure system  200 , stationary post  100 , and movable post  300 . Many ferrous and non-ferrous alloys could be used for typical high-strength constructions. Plastic resins could be employed for simplified fabrication. Ceramics or semi-conductors could be used for MEMS (Micro Electro-Mechanical Systems) applications, where devices are fabricated and assembled using semiconductor processes.  
         [0074]    Conclusion:  
         [0075]    The present invention simplifies the construction of a flexural pivot. The simple shapes of the stationary and rotational members simplify the fabrication of these components. The flexures can be made from flat raw materials. The attachment of the flexures is made on flat or simply-curved, external features of the stationary and rotational members. With this simple construction, the present invention allows flexural pivots to be scaled to fit into extremely small devices. Further, the simplified methods of attachment to the stationary and rotational members allows integration of these components into other parts of a device.