Abstract:
An apparatus for fatigue testing an article includes a base having a platform for receiving an article to be tested, a shaft extending toward said platform for applying a force to said article, a coupling assembly connected to said shaft, said coupling assembly comprising a joint that pivots about an axis, and flex pivots disposed to be aligned with said axis about which said joint pivots.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/592,873 filed Aug. 23, 2012, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF DISCLOSURE 
     This disclosure relates to machines for fatigue testing of articles, and in particular, to machines for testing of endosseous dental implants. 
     BACKGROUND 
     One procedure for fatigue testing of dental implants includes having a loading shaft  10 , shown in  FIG. 1 , repeatedly apply a compressive load to an implant  12 . The direction of the force vector that applies the compressive load defines a load axis  14 , as shown in  FIG. 1 . 
     During testing, the implant  12  is implanted into a mounting surface  16 . A line that extends from the mounting surface to a point at which the force is applied defines a moment arm  18 . To the extent the loading shaft  10 , and hence the force vector, is not parallel to this moment arm  18 , there will be a torque that urges the implant  12  to rotate or bend. 
     The extent to which the implant  12  resists such rotation or bending is of considerable interest. In fact, ISO 14801 describes a testing procedure and a fixture to be used for such a procedure. According to section 5.2.1 of that standard, the loading force is to be applied such that no lateral constraint occurs. According to section 5.2.6, the loading device should be unconstrained in the transverse direction so as to avoid reducing the magnitude of the applied torque. The standard further specifies that this should be accomplished by either providing a universal joint or by point contact between whatever applies the loading force and the implant  12 . 
     The application of point contact is difficult, and when an implant  12  fails the test, the loading member falls away unless otherwise suspended. 
     Testing machines that rely on a universal joint to comply with the ISO standard are known. However, inherent in a conventional universal joint is a backlash that results from a delay between the time a drive motor applies a force and the time at which this force is actually transmitted down a shaft. This delay arises from slack in the bearings used in the pivots of a universal joint. In part as a result of this, universal joints require considerable maintenance. However, even with diligent maintenance, under the grueling test conditions of repeatedly applying a compressive load to an implant  12 , such joints have a tendency to fail prematurely. In addition, the wear on these bearings results in a constant dispersal of small metal particles or shavings during use. 
     SUMMARY 
     The invention is based on the recognition that flexural pivots can be used to both comply with the requirements of the ISO and to achieve a backlash and maintenance free joint with essentially infinite lifetime. 
     In one aspect, the invention features an apparatus for fatigue testing an article. Such an apparatus includes a base having a platform for receiving an article to be tested, a shaft extending toward the platform for applying a force to the article, and a coupling assembly connected to the shaft. The coupling assembly comprises a joint that pivots about an axis, and flex pivots disposed to be aligned with the axis about which the joint pivots. 
     In some embodiments, the flex pivot is configured to rigidly transmit an axial force. 
     In other embodiments, the base further includes a curved holder, and wherein the platform is mounted to slide along the curved holder thereby causing an angle between the shaft and the article to vary. 
     Also included are embodiments in which the shaft includes a distal tip having a contact face for loading the article, and wherein the contact face includes zirconia, those embodiments in which the shaft includes a distal tip having a contact face for loading the article, and wherein the contact face includes a material having a hardness between 1000 Hv and 8000 Hv, and those embodiments in which the shaft includes a distal tip having a contact face for loading the article, and wherein the contact face includes a material having a hardness between 1300 Hv and 8000 Hv. 
     In certain embodiments, the coupling assembly includes a first clevis, a second clevis, and a pivot block, and wherein the flex pivots pivotally couples the first clevis and the second clevis to the pivot block. 
     Also included among the many embodiments of the invention are those in which each of the flex pivots includes concentric cylinders having resilient webbing extending across the cylinders for applying a restoring force to urge the cylinders to maintain a relative position therebetween. 
     The apparatus is applicable to a variety of articles to be tested, such as articles that extend from an anchor point to a point where a load is to be applied. Among these are embodiments in which the platform is adapted to receive a dental implant, and those in which the platform is adapted to receive an orthopedic implant, such as, for example, a hip implant. 
     In yet other embodiments of the invention, the shaft is configured for backlash-free transmission of a compressive force against the article. 
     In another aspect of the invention, an apparatus for fatigue testing an article includes means for receiving an article to be tested, means for transmitting a compressive force to the article, means for coupling the means for transmitting a compressive force to an actuator, the means for coupling comprising flex pivots disposed to be aligned with an axis about which the means for coupling pivots. 
     In some embodiments, the means for coupling includes a first clevis, a second clevis, and a pivot block, and wherein the flex pivots pivotally couples the first clevis and the second clevis to the pivot block. 
     In another aspect of the invention, a method of making an apparatus for fatigue testing an article includes providing a base having a platform for receiving an article to be tested, providing a shaft extending toward the holder for applying a force to the article, connecting a coupling assembly to the shaft, the coupling assembly comprising a joint that pivots about an axis, and disposing flex pivots to be aligned with the axis about which the joint pivots. 
     Among the practices of the invention are those that also include providing the base with a curved holder, and mounting the platform to slide along the curved holder thereby causing an angle between the shaft and the article to vary. 
     Other practices also include providing the shaft with a distal tip having a contact face for applying a compressive load to the article, and selecting the contact face to comprise zirconia, or a material having a hardness between 8000 Hv and 1000 Hv, or a material having a hardness between 8000 Hv and 1000 Hv. 
     Yet other practices include providing the coupling assembly with a first clevis, a second clevis, and a pivot block, and wherein the flex pivots pivotally couples the first clevis and the second clevis to the pivot block. 
     In additional practices, the inventive method further includes providing each of the flex pivots with concentric cylinders having resilient webbing extending across the cylinders for applying a restoring force to urge the cylinders to maintain a relative position therebetween. 
     The manufacture of the apparatus can include customizing it for particular applications. For example, one practice includes adapting the platform to receive a dental implant, while another includes adapting the platform to receive an orthopedic implant, such as a hip implant. 
     In yet other practices, the shaft is configured for backlash-free transmission of a compressive force against the article. 
     These and other features of the invention will be apparent from the following description and its accompanying figures, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a dental implant to be tested by a loading fixture; 
         FIG. 2  shows an isometric view of a loading fixture for testing the dental implant shown in  FIG. 1 ; 
         FIG. 3  shows a dental implant to be tested using the loading fixture of  FIG. 2 ; 
         FIG. 4  shows the coupling assembly of the loading fixture of  FIG. 2 ; 
         FIG. 5  shows a cut-away view of a flex pivot to be used with the coupling assembly shown in  FIG. 4 ; and 
         FIG. 6  shows a hip implant to be tested by a loading fixture. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 , a loading fixture  20  for fatigue testing of a dental implant  12  features a base  22 , a shaft  24 , and a coupling assembly  26  that couples the shaft  24  to an actuator (not shown). 
     The base  22  encloses various load sensors and features a platform  28  for receiving the dental implant  12  that is implanted into and protrudes from a top surface  30  thereof. This platform  28  engages a track  32  formed on a curved holder  34  such that the platform  28  can slide along the curved holder  34  along a range of angles. In doing so, a vector normal to the top surface  30  of the platform  28  defines a variable angle relative to a load axis defined by the shaft  24 . 
     At a distal end  36  of the shaft  24  is a contact face  38  that contacts the dental implant  12  and transmits a load from the shaft  24  to the implant  12 . 
     As shown in  FIG. 3 , a dental implant  40  typically has a hemispherical top surface  41  that receives repeated loading during a typical fatigue test. As a result, the force is concentrated on a small portion of the contact face  38 . To avoid permanent deformation of the contact face  38  from repeated loading, it is useful to use a hard material for the contact face  38 . Examples of such materials include, but are not limited to, hardened steel, diamond, and zirconia. Particularly useful materials have a hardness between 1000 Hv and 8000 Hv. Also useful for use on the contact face  38  are materials having a hardness between 1300 Hv and 8000 Hv. 
     Referring now to  FIG. 4 , the coupling assembly  26  includes a first clevis  42  having first and second arms  44 ,  46  connected by a base  48 . The distal ends of the first and second arms  44 ,  46  have through holes  49 ,  50  that are aligned with each other. The coupling assembly  26  also includes a second clevis  52  having first and second arms  54 ,  56  connected by a base  58 , with through holes  60 ,  62  at the distal tips of the first and second arms  54 ,  56 . As was the case in the first clevis  42 , these through holes  60 ,  62  are also aligned with each other. The first and second devises  42 ,  52  are orthogonal to each other such that a line normal to the first and second arms  44 ,  46  of the first clevis  42  is orthogonal to a line normal to the first and second arms  54 ,  56  of the second clevis  52 . 
     The coupling assembly  26  also includes a pivot block  64  having a length and width chosen to fit between the first and second arms  44 ,  46  of the first clevis  42  and the first and second arms  54 ,  56  of the second clevis  52  respectively. A thickness of the pivot block  64  is chosen to accommodate recesses  66 A- 66 D that are the same size and shape as the through-holes  48 ,  50 ,  60 ,  62  on the arms  44 ,  46 ,  54 ,  56  of the devises  42 ,  52 . When assembled, these through-holes  49 ,  50 ,  60 ,  62  align with corresponding recesses  66 A- 66 D in the pivot block  64 . Four flex pivots  68 A- 68 D pass through corresponding through-holes  49 ,  50 ,  60 ,  62  and are secured into corresponding recesses in the pivot block  64 . 
     A side of the pivot block  64  has a groove  67 A for receiving a corresponding arm  46  of the first clevis  42 . to accommodate one of the arms. A similar groove  67 C is provided on an opposite side of the pivot block  64  to receive the opposite arm  44  of the first clevis  42 . These grooves  67 A,  67 C cooperate to restrain the corresponding flex pivots  68 A,  68 C from rotating beyond their fatigue rated rotation angles as the first clevis  42  pivots about the pivot block  64 . A similar pair of grooves  67 B,  67 D on the remaining two sides of the pivot block  64  cooperate to restrain the corresponding flex pivots  68 B,  68 D from rotating beyond their fatigue rated rotation angles as the second clevis  52  pivots about the pivot block  64 . 
     As a result, the shaft  24  can pivot about a first rotational axis defined by first and third flex pivots  68 A,  68 C and a second rotational axis defined by second and fourth flex pivots  68 B,  68 D. This enables the application of a force without any constraint to motion in a lateral direction. 
     Referring now to  FIG. 5 , a typical flex pivot  68 A includes two coaxial cylinders  70 A,  70 B adjacent to each other. A vertical web  72 A extending across the diameter of the cylinders  70 A,  70 B joins them together. Horizontal webs  72 B,  72 C extend across the cylinders along a direction orthogonal to that of the vertical web  72 A. The horizontal and vertical webs  72 A,  72 B,  72 C thus cooperate to urge the cylinders  70 A,  70 B to remain collinear in response to any radial force and to provide a restoring force when the cylinders  70 A,  70 B are rotated relative to each other. 
     Flex pivots  68 A- 68 D as described herein are essentially frictionless devices that allow pivoting for limited angles. Such pivots  68 A- 68 D do not require lubrication or maintenance and do not wear. As a result, such pivots  68 A- 68 D do not cause fine metal dust or shavings to be formed during use. 
     An additional advantage of such flex pivots  68 A- 68 D is the elimination of backlash. When flex pivots  68 A- 68 D are used, there is no slack to be taken up as is the case when, for example, a ball bearing is used. A flex pivot  68 A- 68 D eliminates the small but finite clearance in a ball bearing. Eliminating this clearance also eliminates the noise that results from backlash. As a result, when an actuator applies a force, the shaft  24  immediately and quietly transmits that force to the implant  12 . 
     Preferably, the implant  12  experiences no lateral forces during testing. However, as the shaft  24  swings from side to side, at least some lateral force is inevitable. To minimize this lateral force, it is preferable to make the shaft  24  as long as possible. 
     When a shaft  24  of length l experiences an excursion of distance d as a result of pivoting about an angle θ, the shaft  24  inevitably exerts a lateral force F lat , that is equal to the axial force F axial  weighted by the tangent of that angle θ, i.e. F lat= F axial  tan(θ). Since the angle θ is the arcsine of the ratio of the excursion distance d to the length l, for a fixed excursion d, the longer the shaft  24  the lower the lateral force will be. 
     As described herein, the loading fixture  20  is used to test dental implants  12 . However, the principles described herein are applicable to testing of any article subject to similar constraints. 
     In particular, the loading fixture  20  described herein can be used in a variety of applications in which one wishes to perform fatigue testing of a cantilevered article that is anchored at a point at some distance from where a compressive load is to be applied. For example,  FIG. 6  shows a hip implant  74  anchored through a mounting surface  16  on a base  22 . This hip implant  74  is to be fatigue tested by repeated application of a load from a contact face  38 . A loading fixture  20  as described above can be adapted to perform fatigue testing of the hip implant  12 . 
     Additional applications include testing of electronic components. For example, to test the integrity of solder joints connecting an integrated circuit to a circuit board, one might apply a loading force along a direction parallel to the circuit board. Or, in testing a composite material having an asymmetric distribution of fibers, it may be useful to apply a compressive force at some angle relative to the axis of the fibers to determine an extent to which bending or rotation may occur.