Abstract:
Fastening systems for parts that endure high vibration shear loads have traditionally been difficult or expensive to produce. This application describes a fastening system with multiple conical surfaces and eccentric offsets. The novel conical fastener system allows parts to be assembled with reduced tolerance controls at interface features while improving alignment precision. The eccentric conical fastening system is particularly well suited for assemblies with high shear loads in high vibration/shock environments, and/or for systems that have extremely precise pointing requirements.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. §119(e) to provisional application No. 60/572,916 filed on May 19, 2004 titled “Nested Off-Center Conical Bushings for Alignment of Any Number of Non-Aligned Features.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     The present invention was supported in part by contract DE-AC02-76SF00515 from the Department of Energy. The U.S. Government has certain rights in the invention. 
     
    
     FIELD  
       [0003]     The invention relates to eccentric mechanical fastening systems, and, more particularly, to eccentric fastening systems with conical surfaces.  
       BACKGROUND  
       [0004]     When fastening two or more items together with greater than or equal to two fasteners, issues of non-alignment arise. Typically, adjustment of tolerances can yield a set of parts that can be assembled. In high vibration environments (i.e., launch vehicles, high speed trains, racecars, hammer mills, steel recyclers, etc) shear joints are required and are typically designed with very tight tolerances to prevent relative motion between the parts of an assembly. In most cases, these parts must be match machined to achieve desired tolerance control. Tolerance control processes can be very expensive, particularly when match machining of large parts too very tight tolerances is involved.  
         [0005]      FIG. 1  shows a prior art eccentric cylinder. The cylinder  102  has an outer cylindrical surface  106  and an inner cylindrical hole  104 . The axes of the hole  104  and the outer surface  106  are parallel but not aligned. Therefore, the hole  104  is eccentric relative to the outer cylindrical surface  106 . Rotation of the type of bushing shown in  FIG. 1  does not allow alignment to a desired center.  
         [0006]      FIG. 2  shows the eccentric elements shown in  FIG. 1  as part of a prior art fastening system. The eccentric cylinder  102  allows misalignments between fastening features in a first part  202  and a second part  204  to be compensated for. Unfortunately, the range of adjustment of the position of the hole one of four relative to the parts  202 ,  204  is extremely limited. It should be fairly obvious that unless the misalignments between the parts  202 ,  204  is exactly the amount of the eccentric offset off the eccentric cylinder  102 , then the axis of the hole  104  will be misaligned relative to a fastening feature of the second part  204 . Thus, when a bolt is securely fastened with a nut through the assembly, it is highly likely that the bolt will experience bending stresses. Cyclic bending stress can cause fatigue, which can significantly reduce material strength properties.  
         [0007]      FIG. 3  shows a prior art double eccentric cylindrical system. The two piece double eccentric system overcomes the misalignments issues found with a single eccentric cylindrical fastening system shown in  FIGS. 1 and 2 . The eccentric cylinder  102  with offset hole  104  is shown. Also shown is a second eccentric cylinder  302 . The second eccentric cylinder  302  has an inner cylindrical surface  304  and an outer cylindrical surface  306 . The axis of the inner cylindrical surface  304  and the axis of the outer cylindrical surface  306  are parallel, but offset. Thus, the inner and outer cylindrical surfaces  304 ,  306  are eccentric. The outer cylindrical surface  106  of the first eccentric cylinder  102  fits inside of the inner cylindrical surface  304  of the second eccentric cylinder  302 .  
         [0008]     The two piece double eccentric cylindrical system shown in  FIG. 3  allows the location of the hole  104  to be positioned anywhere from the axis of the outer cylindrical surface  306  to a combination of the eccentric offsets away from that axis. Thus, problems with fastener bending stresses and other issues with misalignments between parts and fastening features can be greatly reduced.  
         [0009]     Unfortunately, the prior art double eccentric cylindrical fastening system shown in  FIG. 3  has several disadvantages. First of all, the machining tolerances required for both the first and second eccentric cylinders  102 ,  302  and that of the parts to be fastened together are quite high. It is very difficult to machine all of the features required to allow parts to be assembled while achieving a zero clearance joint.  
         [0010]     When the assembled parts are used in a high vibration environment, the combination of excess clearances in the prior art fastening schemes can allow the parts to move relative to each other. In many instances this movement is unacceptable.  
         [0011]     Prior art cylindrical bushings with offset axes for their outside versus inside diameters allows for rotational adjustment of the bushings to find a desired center (i.e., axis of fastener or vault hinge pin). To prevent the bushings from rotating, it is understood that the prior art relies on friction at the perpendicular faces of the bushings/fixed parts or on tack welding the bushings together and to the fixed part after completion of the alignment process. This may be tolerable for a lightly loaded static joint (i.e., a joint that does not experience cyclic or fluctuating loads).  
         [0012]     Experience with high vibration environments has shown that friction alone can not be relied upon to hold joints together. Under high vibration/shock loads, effective fastener preload is reduced with a corresponding reduction in friction forces. As friction forces reduce, the joint begins slipping back and forth in a cyclic fashion; this can lead to fatigue and subsequent catastrophic failure (i.e., broken parts). Tack welding prevents motion under light cyclic loads. Load management can be increased with deeper weld penetrations, but it is impractical to achieve a full penetration welds with current practice. In addition, welding induces internal stresses that cause dimensional changes to the parts being welded and can alter the desired alignment of the joint. If tack welding is done incorrectly, rework of the joint can be impractical.  
         [0013]     Thus, there is a need for a fastening system that does not require super high tolerance machining of large parts, that works well in high vibration and/or shock environments and that can provide a means of adjustment in the field.  
       OBJECTS AND ADVANTAGES  
       [0014]     In view of the above, it is a primary object of the present invention to provide a fastening system that is adjustable, that does preferably not allow clearance toward joint slippage, that does not induce bending stress into joint fasteners, that can be pre-loaded to benefit from energy storage in the joint, and that is affordable to produce.  
         [0015]     One advantage to the system is that it greatly reduces the need for high specification tolerance control and machining of large parts, reduces the need for high specification tolerance control and machining to small parts, and, therefore reduces the overall cost of the system.  
         [0016]     Another advantage to the system is that the system may be designed such that the conical elements lock in place after tightening the fastener to a final specification that results in an almost ideally perfect shear joint.  
         [0017]     Another advantage to the system is that the system may be designed such that the conical elements can easily come apart with relaxation of the fastener tension (for example, an adjustable, separable joint that can be used to deploy items or that can be used to secure and/or release doors such as for landing gear openings for aircraft and/or openings for underwater submersible vehicles).  
       SUMMARY  
       [0018]     This application describes a fastening system with two eccentric elements. One element includes two conical surfaces, while the other element includes one conical surface. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]      FIG. 1  shows a prior art single eccentric cylinder.  
         [0020]      FIG. 2  shows a prior art single eccentric fastening.  
         [0021]      FIG. 3  shows a prior art double eccentric cylindrical fastening system.  
         [0022]      FIG. 4  shows an example of a double eccentric conical fastening system.  
         [0023]      FIGS. 5   a - c  show conical and eccentric features.  
         [0024]      FIG. 6  shows an example of a cross-sectional view of and eccentric conical adjustment system without fastener.  
         [0025]      FIG. 7  shows an example of a cross-sectional view of and eccentric fastening system.  
         [0026]      FIG. 8  shows an example of a cross-sectional view of and eccentric conical fastening system with a stud in one of the parts.  
         [0027]      FIG. 9  shows an example of a cross-sectional view of and eccentric conical fastening system with the fastener an as integral part of the Single Surface Conical Element.  
         [0028]      FIGS. 10   a - d  show examples of Rotation Drive features.  
         [0029]      FIG. 11  shows an example of a Single Conical Surface element with an integral shaft.  
         [0030]      FIGS. 12    a - d  show examples of Torque Reaction/Limiting features.  
         [0031]      FIG. 13  shows an example of a fastener assembly.  
         [0032]      FIG. 14  shows an example of a fastening method. 
     
    
     DESCRIPTION  
       [0033]     Compared to the prior art, the use of conical surfaces in a fastening system has several advantages. The addition of the conical surfaces allows the bushings to translate along the fastener axis until the surfaces contact. As the fastener is further tightened, the conical elements wedge into their respective seats and the joint becomes radially and axially preloaded (much like storing energy in a spring). The radial and axial preload increases the joints ability to manage vibration and/or shock loads. Additionally, since the conical surfaces seat, there is no clearance to allow joint slippage at those interfaces. The conical features allow the joint to easily come apart, depending on the half cone angle, in the event of needed rework. (Note: A specially designed expanding collet puller may be required for separation of a lockable joint.) The described fastening system includes two axis eccentricities and several conical mating surfaces. The conical surfaces have several advantages over prior art cylindrical surfaces, including 1) elimination of joint clearances, 2) the ability to preload (i.e., pre-stress) the joint and 3) the ability to manage high shear loads in high vibration and/or shock environments.  
         [0034]      FIG. 4  shows an example of the double eccentric conical fastening system. A first conical element  402  has an outer conical surface  404 , an inner conical surface  406 , and a top surface  408 . The outer and inner conical surfaces  404 ,  406  have axes that are parallel but offset. Thus the first conical element  402  has an eccentric nature.  
         [0035]     A second conical element  412  has an outer conical surface  414 , a hole  416 , and a top surface  418 . The axes of the outer conical surface  414  and the hole  416  are parallel but offset. Thus the second conical element has an eccentric nature. The outer conical surface  414  of the second conical element  412  has substantially the same half cone angle (defined below) as the inner conical surface  406  of the first element  402 .  
         [0036]     The conical nature of the first and second conical elements  402 ,  412  allow the first and second conical elements  402 ,  412  to fit together with no radial clearance. The double eccentric nature of the elements  402 ,  412  allows the center of hole  416  to be positioned to anywhere from the axis of the outer conical surface  404  to the combination of the eccentric offsets of the first and second conical elements  402 ,  412  away from the axis of the outer eccentric surface  404  (i.e., anywhere within a circle that has a radius that is equal to the sum of the eccentric offsets).  
         [0037]      FIG. 5   a  shows an example of half cone angle. The first element  402  has two associated conical angles. The outer conical surface  404  has a half cone angle relative to its axis  504 . This angle is shown by φ (phi). The inner conical surface  406  has a half cone angle relative to its axis  502 . This angle is shown by θ (theta). The eccentric offset is the distance between the two axes  502 ,  504 . The top surface  408  and the bottom surface of the first conical element  402  may be perpendicular to the axis  504  or at an angle, depending on the design considerations of the system.  
         [0038]      FIG. 5   b  shows another example of half cone angle. In this case, the outer conical surface  414  of the second conical element  412  is shown with a half cone angle, β (beta), relative to the axis  506  of the outer conical surface  414 . There is an eccentric offset between the axis  506  of the outer conical surface  414  and the axis  508  of the hole  416 . The top surface  418  and the bottom surface of the second conical element  412  may be perpendicular to the axis  506  or at an angle, depending on the design considerations of the system.  
         [0039]      FIG. 5   c  shows another example of half cone angle. In this case, a different embodiment of the second conical element  412  has a fastener shaft  510  extending from its bottom surface (instead of a hole  416  as shown in  FIG. 5   b ). In this case, there is also an eccentric offset between the single surface conical axis  506  and the fastener shaft axis  512 .  
         [0040]     The half cone angles φ, θ, β (phi, theta, beta) are preferably greater than 0 degrees. A designer may want a system were the joint locks together after being torqued to specification. Half cone angles in a range of 7 to 8 degrees work well for this purpose. Angles from greater than 0 to 7 degrees may also be useful when even more locking is desired. If a designer prefers that the assembly be easily disassembled, then half cone angles greater than 8 degrees can be used. One example is a range of 15 to 60 degrees. An example of an adjustable center and separable joint is where the interior conical surface cone angle of the first element,  402 , is 8° corresponding to the exterior surface half cone angle of the second element,  412 , providing adjustability and a lockable section of the joint.  
         [0041]     The exterior conical surface half cone angle of the first element may be non-locking for larger half cone angles (greater than 8 degrees for example, and even more so for angles greater than 15 degrees) and is similar to the half cone angle of the second part,  602 . In this case, the joint is adjusted and torqued to specification, locking it in place, removal of the axial force via some actuator allows the joint to separate at the non-locking portion of the joint. Another benefit of this example is that the actuator can be cyclic and would allow the joint to act as an adjustable latch-on, latch off mechanism. The foregoing example demonstrates that it is not essential that all half cone angles be the same. Other examples of usable half cone angles are 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, and 75 degrees.  
         [0042]     It is preferred that the half cone angle of the conical surfaces that mate be substantially the same. Thus, it is preferred that half cone angle of the inner conical surface  406  of the first conical element  402  be substantially the same as the half cone angle of the outer conical surface  414  of the second conical element  412 . Likewise, it is preferred that half cone angle of the outer conical surface  404  of the first conical element  402  be substantially the same as the half cone angle of the outer conical surface of the part that the outer conical surface  404  mates with.  
         [0043]     The following should be considered when viewing FIGS.  6 - 9 : (1) clearances shown are greatly expanded to assist conceptual understanding of joint (in real practice the clearances would be minimized), and (2) bottom surfaces of conical elements  402  and  412  desirably do not contact fixed part  604  (i.e., the conical elements desirably seat properly so that joint can be preloaded).  
         [0044]      FIG. 6  shows an example of a double eccentric conical fastening system. This view is a cross-section. A first fixed part  602  and a second fixed part  604  are shown. The first and second parts  602 ,  604  are fastened together with the double eccentric conical fastening parts. The first and second parts  602 ,  604  are fixed relative to each other with the system is fastened, but the parts  602 ,  604  may be moved relative each other with the system is unfastened. The first element  402  has an inner conical surface and an outer conical surface. The second conical element  412  has an outer conical surface that mates with the inner conical surface of the first element  402 . Distances between the various elements shown in  FIG. 6  may not be to scale. For example the first element  402  may be much closer to the second part  604  than is shown. Various methods of attaching the parts are shown in the following figures and described below.  
         [0045]      FIG. 7  shows an example where the assembly shown in  FIG. 6  is fastened together with a bolt  702  and a nut  704 . An optional torque reaction/limiting element  708  goes in between the second element  412  and the nut. For the purposes of this application, a torque reaction/limiting element may also be referred to as a torque limiting element. The nut  704  (or the optional torque reaction/limiting element  708 , if used) contacts the second conical element  412 , but does not contact the first element  402 . This is desired so that any tightening force applied by the nut  704  translates to the second element  412 , which translates the force into the first element  402 . The first element  402  translates the force into first part  602  (i.e., force from the nut causes second element  412  to seat into the first element  402  and first element  402  to seat into the first part  602 ). The optional torque reaction/limiting element  708  is advantageous to prevent either of the conical elements  402 ,  412  from rotating. Rotating can not only ruin the adjustment, but worse than that, it can induce almost pure shear pre-stress into the fastener that can be cut via a scissor action with application of cyclic shear forces (e.g., vibration and/or shock environments). Placement of a torque reaction/limiting element  708  between the second conical element  412  and the nut  704  allows the nut  704  to be tightened without applying a moment to the second conical element  412 .  
         [0046]      FIG. 8  shows another example of the assembly shown in  FIG. 6 . In this case, a stud  706  is threaded into the second part  604 . A nut  704  threads on to the stud  706  to fasten the assembly. In contrast to what is shown in  FIG. 7  where the second part  604  has a through hole to accommodate the bolt  702 , the second part  604  in  FIG. 8  is threaded to receive the stud  706 . This approach further reduces clearances between cylindrical features of the joint by doing away with one interface between two cylindrical surfaces.  
         [0047]      FIG. 9  shows an example of the fastening system with a second conical element that has an integral shaft that can be used for fastening, or to connect an actuator. In this example, the second conical element  412  has an integral shaft that receives a nut  904 . Also shown is an optional torque reaction/limiting element  708 . Optional rotation drive enabling features  908  allow the second conical element  412  to be easily rotated relative to the parts  602 ,  604  and the first conical element  402 . The rotation drive enabling feature  908  may be a protrusion or a depression in the top surface (left side in this figure) of the second conical element  412 . Other types of rotation drive enabling features might exist on the sides of the first conical element  402  (which would be radially about first element  402  at the left most side of first element  402  as shown in  FIG. 9 ).  
         [0048]      FIG. 10   a  shows an example of rotation drive enabling features on the second element  412 . The rotation drive enabling features may be for either the first conical element  402  (which has two conical surfaces) or the second conical element  412  (which has one conical surface). For  FIGS. 10   a - d,  the generic element  1002  shows how the rotation drive enabling features may be applied to either the first conical element  402  or the second conical element  412 . In  FIG. 10   a  one or more depressions  1004  are in the top surface of the element  1002 . The depressions  1004  may be of any shape, but holes are very easy and inexpensive to machine. The interior surface  1006  of the element  1002  is minimally affected by the depressions  1004 .  
         [0049]      FIG. 10   b  shows the element  1002  with one or more rotation enabling protrusions  1008 . The protrusions  1008  may be cylindrical or faceted in nature.  
         [0050]      FIG. 10   c  shows the element  1002  with machined slots  1010 . The slot  1010  allows the element  1002  to be easily rotated with a flat instrument, such as a screwdriver or with a specially designed, or standard, spanner wrench. The slot  1010  is also inexpensive to manufacture.  
         [0051]      FIG. 10   d  shows the element  1002  with one or more rotation drive enabling features  1012  on the side of the element  1002  at either end. The features  1012  may be flat or curved. One example is to machine a six sided set of features  1012 . In this case, where the faces of the features are substantially parallel to the axis of outer conical surface, the top of the element  1002  would appear and act like a nut. In another example, the set of features  1012  may be machined with one or more spline faces.  
         [0052]      FIG. 11  shows another embodiment of the second conical element  412 . In this case, the second conical element  412  does not have a hole through it. Instead, the second conical element  412  has an integral shaft  1106 . The shaft  1106  is desirably “ended” for receiving a nut, or for attachment to an actuator system. An optional feature  1104 , which can be a torque reaction/limiting and/or a rotation drive enabling feature, is shown on the top of the second conical element  412 . If the feature  1104  is used as a torque reaction/limiting feature, then a separate torque reaction element  708  may not be needed. Of course, it is possible to use a torque reaction/limiting feature as a rotation drive enabling feature as well.  
         [0053]      FIG. 12   a  shows an example of a torque reaction/limiting element  1202 . The torque reaction/limiting element  1202  has a hole  1204  for passing through a shaft or bolt. The torque reaction/limiting element  1202  has one or more faces that allow a moment to be applied to the element  1202 . The faces may be flat or curved.  
         [0054]      FIG. 12   b  shows a side view partial assembly. The assembly has a nut  704 , a torque reaction/limiting element  1202 , and a second conical element  412 . The second conical element  412  has a single conical exterior surface.  
         [0055]      FIG. 12   c  shows another example of a torque reaction/limiting element  1210 . The torque reaction/limiting element  1210  has a hole  1204  for passing through a shaft or bolt. The torque reaction/limiting element  1210  has one or more faces that allow a moment to be applied to the element  1210 . The faces may be flat or curved. The faces may extend for the entire length of the torque reaction/limiting element  1210 , or they may extend for a smaller portion as shown in  FIG. 12   c.    
         [0056]      FIG. 12   d  shows another side view partial assembly example. The assembly has a nut  704 , a torque reaction/limiting element  1210 , and a second conical element  412 .  
         [0057]      FIG. 13  shows an exploded assembly example of a double eccentric conical fastening system. The assembly includes a first eccentric conical element  402 , a second eccentric conical element  412 , a torque reaction/limiting element  1210 , a nut  704 , a shaft  1310 , a first part  602 , and a second part  604 . The first eccentric conical element  402  is shown with optional rotation drive enabling features  1316 . Likewise, the second eccentric conical element  412  is shown with optional rotation drive enabling features  1318 . In this example, the shaft  1310  threads into the second part  604 . The shaft goes through the first part  602 , the eccentric conical elements  402 ,  412 , the torque reaction/limiting element  1210 , and receives the nut  704 . The first part  602  has an inner conical surface to receive the first eccentric conical element  402 .  
         [0058]      FIG. 14  shows an example of a method for assembling a double eccentric conical fastening system. For assembly, the first and second parts are brought together and aligned to desired relationship with respect to one another. The technician then identifies the two conical fasteners that are the greatest distance apart. The technician then validates that the centers of those two furthest apart fasteners are within the adjustment range of the joint (i.e., within a circle whose radius is equal to the sum of the off-center distances). Preferably without changing the relationship between the first and second parts, the technician installs the first element and then rotates and/or counter-rotates the conical elements until alignment is achieved with respect to the specific fastener center. At this point the technician installs the torque reaction limiting feature, as/if needed, installs the nut and tightens that specific fastener. This process is repeated for the second conical fastener set. At this point the first part is held securely to the second part and the remaining conical fasteners can be adjusted into position quite quickly and subsequently tightened. After all conical fasteners have been adjusted, seated, and snugly tightened, they can be brought up to final torque specification using a criss-cross pattern working from the inner most fasteners outward.  
         [0059]     Suitable materials for the first and second conical elements  402 ,  412  include high strength plastic, steel, aluminum, brass, and titanium. Other materials may also be appropriate. The conical elements  402 ,  412  may be made of similar or dissimilar materials.  
         [0060]     Another embodiment is an actuated separable joint with nested eccentric conical elements.  
         [0061]     It will be apparent to one skilled in the art that the described embodiments may be altered in many ways without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their equivalents.