Patent Publication Number: US-2020291979-A1

Title: Bolt interface coating and thread transition geometry for sleeved fasteners used in composite applications

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/819,001 filed on Mar. 15, 2019, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to bolts having thread transition geometries for interference fasteners used in various substrates (e.g. metallic, plastic, and composite substrates). More particularly, the invention relates to bolts including stress reduction features configured to reduce damage to a sleeved fastener and to the bore of a substrate when installed in an expandable sleeved fastener. Such stress reduction features can include bolts having coatings and having a stress reducing transition region between a threaded portion and a non-threaded portion. 
     BACKGROUND OF THE INVENTION 
     Interference fit fastener systems generally include a sleeve for insertion into a bore of a substrate and a bolt that extends through the sleeve and bore. A force is applied to the bolt to draw the bolt into the sleeve thereby expanding the sleeve creating an interference fit in the bore. The sleeve is typically installed into the composite structure bore either with an interference fit or a slip fit. The bolt is then inserted into the sleeve. The bolt-sleeve interface is an interference fit. As the sleeve expands over the bolt, resulting from the interference fit, it comes into contact with the composite structure. This contact effectuates load transfer from the fastener to the structure, which reduces the possibility of arcing between the sleeve and the composite structure. 
     Prior art bolt and sleeve fasteners, such as those disclosed in U.S. Pat. No. 7,695,226 to March et al., have a variety of tapered head designs. The fasteners disclosed in U.S. Pat. No. 7,695,226 all have shanks that extend from flush or protruding heads of the fastener to a transition portion 55. “The transition portion 55 has a shallow lead-in angle that reduces the force that is needed for installation. Since less force is needed to install the fastener 10 into the interference condition, the fastener 10 allows for much longer grip lengths while diminishing sleeve stretch and premature sleeve failure.” U.S. Pat. No. 7,695,226, col. 5, lines 55-60. The disclosed sleeve members have “a generally uniform tubular portion 80 that terminates in an enlarged flanged shaped head 85 to receive the flush head 37 or protruding head 35 of the pin member  15 .” U.S. Pat. No. 7,695,226, col. 4, lines 56-59. Prior art sleeves, such as those disclosed in U.S. Pat. No. 7,695,226, do not conform to the entirety of the outer surface of the bolts and clearly do not conform to any transition areas on such bolts. 
     Prior art fasteners have been known to cause damage to the sleeve and may also cause irregular expansion of the sleeve, causing inefficient load transfer and damage to the composite structure. Thus, there is a need for improved sleeved fastener systems for use in various substrates. 
     SUMMARY 
     There is disclosed herein, a bolt for an interference fastener. The bolt includes a body extending axially between a first end and a second end. The body has a threaded area extending axially inward from the second end and a head configured on the first end. A cylindrical expansion area extends axially between the head and the threaded area. A transition area is disposed axially between the cylindrical expansion area and the threaded area. 
     In one embodiment, the transition area is tapered at a transition angle θ. 
     In one embodiment, the transition area includes a linear portion and a convex radiused portion. The linear portion is tapered at a transition angle θ and the convex radiused portion has a radius R. 
     In some embodiments, the transition area includes a continuous radiused portion. 
     In some embodiments, the transition area has a transition length L′. The transition length L′ is configured to include a surface defined by a logarithmic profile. 
     In some embodiments, the logarithmic profile is determined by the equation: 
     
       
         
           
             
               
                 
                   
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     In some embodiments, the transition area includes a first transition area and a second transition area. The first transition area has a first radius R 1  extending a first axial length L 1  between the cylindrical expansion area and the second transition area. The second transition area has a second radius R 2  extending a second axial length L 2  between the first transition area and the threaded area. 
     In some embodiments, the second radius R 2  is smaller in magnitude than the first radius R 1 . 
     In some embodiments, the transition geometry has three transition areas. The first transition area is linear and has a first transition angle θ′ between the cylindrical expansion area and the beginning of the transition area. The second transition area is a radius that is tangent to the first transition area. The third transition area is linear and has a second transition angle θ″ between the second transition area and the threaded region. The total transition angle θ=θ′+θ″. 
     In another aspect of the present invention, an interference fastener system is further disclosed. The fastener system includes an expandable sleeve having a hollow elongate stem extending axially between an insertion end and a head portion. The elongate stem has an inside surface and an outside surface. The expandable sleeve is sized to be installed in a bore of a substrate. A bolt has a body extending axially between a first end and a second end. The body has a threaded area extending axially inward from the second end and a head arranged on the first end. A cylindrical expansion area extends axially between the head and the threaded area. A transition area is disposed axially between the cylindrical expansion area and the threaded area. The bolt is pushed or pulled through the sleeve, expanding the sleeve. Then the nut is tightened to fix the bolt in place. In some embodiments, the nut is swaged over the threaded area of the bolt. 
     In one embodiment of the interference fastener system, the transition area is tapered at a transition angle θ. The transition angle θ is defined as the angle between the threaded area and the cylindrical expansion area. 
     In some embodiments, the transition angle θ is greater than about 20 degrees. 
     In certain embodiments, the transition angle θ is between about 25 degrees and about 60 degrees. 
     In some embodiments, the bolt is coated with a lubricant. 
     In a particular embodiment, the bolt has a first coating disposed on the body, and a second coating is disposed on the first coating. 
     In some embodiments, the first coating is a galvanic corrosion resistant coating and the second coating is a dry film lubricant. 
     There is disclosed herein a bolt for an interference fastener. The bolt includes a metallic body that extends axially along a longitudinal axis A between a first end and a second end, the body having a threaded area extending axially inward from the second end and a head configured on the first end. The bolt includes a cylindrical expansion area that extends axially between the head and the threaded area. The cylindrical expansion area has a first outside diameter that is greater than or equal to a second outside diameter of the threaded area. The bolt has a transition area located axially between the cylindrical expansion area and the threaded area. The transition area has one or more of: (a) one or more convex radiused portions; (b) is defined by a logarithmic profile according to the equation Y(x)=(A 1 /L 90 ) ln [1/(1−(x/L 90 ) 2 )]; and/or (c) a shape having more than one profile. 
     There is disclosed herein an interference fastener system. The interference fastener system includes an expandable sleeve that includes a hollow elongate stem that extends axially between an insertion end and a head portion. The elongate stem has an inside surface and an outside surface. The expandable sleeve is configured to be installed in a bore of a substrate. The stem is metallic. interference fastener system includes a bolt includes a metallic body that extends axially along a longitudinal axis A between a first end and a second end, the body having a threaded area extending axially inward from the second end and a head configured on the first end. The bolt includes a cylindrical expansion area that extends axially between the head and the threaded area. The cylindrical expansion area has a first outside diameter that is greater than or equal to a second outside diameter of the threaded area. The bolt has a transition area located axially between the cylindrical expansion area and the threaded area. The transition area has one or more of: (a) one or more convex radiused portions; (b) is defined by a logarithmic profile according to the equation Y(x)=(A 1 /L 90 ) ln [1/(1−(x/L 90 ) 2 )]; and/or (c) a shape having more than one profile. 
     In one embodiment, a coating is disposed on the transition area. 
     There is disclosed herein a bolt for an interference fastener. The bolt includes a body that extends axially along a longitudinal axis A between a first end and a second end. The body has a threaded area that extends axially inward from the second end and a head configured on the first end. The bolt has a cylindrical expansion area that extends axially between the head and the threaded area. The bolt includes a transition area that includes a first transition area that has a first profile, a second transition area that has a second profile, and a third transition area that has a third profile. The first transition area, the second transition area and the third transition area each are located axially between the cylindrical expansion area and the threaded area. The first profile, the second profile and the third profile have different configurations from one another. 
     In one embodiment, a coating is disposed on the transition area. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a perspective view of a bolt according to the present invention; 
         FIG. 2A  is a partial cross sectional view of one embodiment of the transition area of detail A as shown in  FIG. 1 ; 
         FIG. 2B  is an alternate embodiment of the transition area depicted in  FIG. 2A ; 
         FIG. 3  is a partial cross sectional view of an alternative embodiment of the transition area of detail A as shown in  FIG. 1 ; 
         FIG. 4  is a partial cross sectional view of another alternative embodiment of the transition area of detail A as shown in  FIG. 1 ; 
         FIG. 5  is a partial cross sectional view of yet another alternative embodiment of the transition area of detail A as shown in  FIG. 1 ; 
         FIG. 6  is a partial cross sectional view of another alternative embodiment of the transition area of detail A as shown in  FIG. 1 ; 
         FIG. 7  is a cross sectional view of an expandable sleeve according to the present invention; 
         FIG. 8  is an isometric view of an axial cross sectional view of a bolt of  FIG. 1  and the sleeve of  FIG. 7 ; 
         FIG. 9  is a partial axial cross sectional view of a bolt and sleeve of  FIG. 7  installed in a substrate; 
         FIG. 10A  is a perspective view of a bolt according to  FIG. 1  including a first and a second coating; and 
         FIG. 10B  is a perspective view of a bolt according to  FIG. 1  including an alternative arrangement of a first and a second coating. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in  FIG. 1 , a bolt  80  extends from a first end  80 A to a second end  80 B. The bolt  80  includes a threaded area  82  (e.g., male threads) that extends axially inward from the second end  80 B and terminates between the first end  80 A and the second end  80 B at a point P 2 . The bolt  80  includes a head  85  on the first end  80 A of the bolt  80 . The bolt  80  has a cylindrical expansion area  84  extending from the head  85  and terminating at a point P 1  at the transition area  180 T. Alternative bolts with different head geometries and various threaded ends (including but not limited to tapered heads, threaded ends with “pull type” ends adjacent to the threaded ends, etc.) do not depart significantly from the invention disclosed herein. 
     The transition area of the bolt  80  can be configured in a variety of stress relieving geometries. As shown in  FIG. 2A , a transition area  181 T of bolt  80 ′ is disposed between points P 1  and P 2  between the threaded area  82  and the cylindrical expansion area  84 . A linear transition area T extends between the transition area  181 T and the threaded area  82 . The transition angle θ is defined as the angle between the threaded area  82  and the cylindrical expansion area  84  as shown in  FIG. 2A . This transition angle θ is measured from a reference line, tangent to the transition area  181 T, at the intersection of the transition area  181 T and the threaded area  82 . The transition area  181 T includes a convex radiused portion with a radius R 1  from points P 1  to P 3  and a linear transition area T from points P 3  to P 2 . The depicted tangent reference line is colinear with the linear transition area T. As a result, the transition angle θ is the angle between the linear transition area T and the cylindrical expansion area  84 . In some embodiments, the ratio of the axial length of the radiused portion  181 T to axial length of the linear transition area T is between 0.5 and 1.5.  FIG. 2B  depicts an alternate embodiment of the bolt  80 ′, in which the linear transition area T′ is adjacent to the cylindrical expansion area  84  and the radiused transition surface  181 T′ is adjacent to the threaded area  82 . 
     Referring to  FIG. 3 , a transition area  182 T of bolt  80 ″ extends from a point P 1  at the cylindrical expansion area  84  to a point P 2  at the threaded area  82 . The transition area  182 T is defined by a continuous radiused portion having a radius R 1  from points P 1  to P 2 . The depicted reference line that defines the transition angle θ is tangent to the transition area  182 T at point P 2 . 
     Referring to  FIG. 4 , a transition area  183 T of bolt  80 ′″ is disposed between a first point P 1  at the cylindrical expansion area  84  and a second point P 2  at the threaded area  82 . In this embodiment, the transition area  183 T has logarithmic profile. A transition length L′ is defined as an axial distance, measured parallel to the A axis of the bolt  80 ′″, between the threaded area  82  and the cylindrical expansion area  84 . A length L 90  is defined as an axial distance at which the transition angle θ would be equal to 90 degrees. The transition angle θ is defined as the angle measured from a tangent reference line that intersects P 2  at a distance L′ as shown in  FIG. 4 . The depicted surface of the transition area  183 T is defined according to the equation: 
     
       
         
           
             
               
                 
                   
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     Where y is the ordinate, being perpendicular to the A axis of the bolt  80 ″′ directed radially inward from point P 1 . The values of L 90  and A 1  can be determined for any bolt geometry by measuring a first diameter D 1 , at point P 1  of the cylindrical expansion area  84 , and the second diameter D 2 , measured at point P 2  between the transition area  181 T- 188 T and the threaded area  82 , as depicted in  FIG. 1 . Once D 1  and D 2  are determined, the following equations are used to calculate A 1  and L 90 : 
     
       
         
           
             
               
                 
                   
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     In the depicted embodiment, P 4  is where a tangent reference line to the surface of the transition area  183 T would be a vertical line. 
     Referring to  FIG. 5 , a bolt  80 ″″ has a first transition area  184 T and a second transition area  185 T. The first transition area  184 T extends between the cylindrical expansion area  84  at point P 1  and the second transition area  185 T at point P 3 . The second transition area  185 T extends between the first transition area  184 T at point P 3  and the threaded area  82  at point P 2 . The first transition area  184 T is defined by a radius R 1  having a first axial length L 1 . The second transition area  185 T is defined by a radius R 2  having a second axial length L 2 . In one embodiment, the second radius R 2  is smaller than the first radius R 1 . In one embodiment, the ratio of the first axial length L 1  to the second axial length L 2  is between 1.0 and 4.0. The transition area  184 T ensures that there are no abrupt changes in the angle of the surface of the bolt. A first transition angle θ′ is defined between a reference line tangent to the second transition area  184 T at point P 3 . A second transition angle θ″ is defined between the reference line tangent to the first transition area  184 T at point P 3  and the cylindrical expansion area  84 . In the depicted embodiment, the cylindrical expansion area  84  is tangent to the first transition area  184 T at point P 1 . The total transition angle θ=θ′+θ″. In some embodiments, the angle θ″ varies over all the transition geometries while the angle θ varies between 20° and 60°, and is preferably between 30° and 35°. 
     Referring to  FIG. 6 , a bolt  80 ″″′ has a first transition area  186 T, a second transition area  187 T and a linear transition area T″. The first transition area  186 T extends from the cylindrical expansion area  84  from point P 1  to point P 3 . In the depicted embodiment, the first transition area  186 T defined by a line rotated about the longitudinal axis A at a first transition angle θ′ relative to the cylindrical expansion area  84 . The second transition area  187 T is defined by an arc having a radius R 1  rotated about the longitudinal axis from point P 3  to point P 4 . The linear transition area T″ extends from point P 2  at the threaded area  82  to point P 4 . The linear transition area T″ is defined by a line rotated about the longitudinal axis A at a second transition angle θ″ relative to the first transition area  186 T. The total transition angle θ=θ′+θ″. The first transition area  186 T has an axial length, measured parallel to the longitudinal axis A, of L 1 . The second transition area  187 T has an axial length L 2  and the linear transition area T″ has an axial length L 3 . In some embodiments, θ′ is between 0 degrees and 10 degrees and preferably between 2 degrees and 5 degrees. 
     As shown in  FIG. 7 , an expandable sleeve  410  for an interference fastener includes a hollow elongate stem  412  extending axially between an insertion end  410 A and a head portion  410 B. The head portion  410 B includes a flange  412 F that has a thickness T 10 . The elongate stem  412  has an inside surface  412 A, an outside surface  412 B and an overall axial length L. A portion of the stem  412  has a cylindrical shape along an axial length L 5  which is about 90 to 95 percent of the overall axial length L. The stem  412  includes a radially outward taper  412 R (e.g., truncated conical shape) extending axially toward the head portion  410 B. The taper  412 R has an axial length L 4 , which is about 5 to 10 percent of the overall axial length L of the stem. The taper  412 R forms an angle θ 20  relative to a line R 20  that is parallel to a longitudinal axis A. 
     As shown in  FIGS. 8-9 , the present invention includes a method for assembling the interference fastener system  99  in a substrate  50 . The interference fastener system includes the sleeve  410 , bolt  80  and a nut  90 , as shown in  FIG. 9 . In one embodiment, an initial step of the assembly involves sliding the sleeve  410  into a bore  52  of the substrate  50  without any lubrication thereon and without substantial frictional resistance. The insertion end  410 A of the sleeve  410  is slid into the entry end  52 A of the bore  52  until the flange  410 F abuts the substrate  50  proximate the entry end  52 A of the bore  52 . 
     Referring to  FIGS. 8 and 9 , the bolt  80  is positioned for entry into the sleeve  410  and is slid into the sleeve  410  until the threaded area  82  begins to protrude out of the second end  52 B of the bore  52 . As shown in  FIG. 9 , a nut  90  that has female threads  92  is threaded onto the threaded area  82  of the bolt  80 . The nut  90  is torqued onto the bolt  80  to further the expansion area  84  of the bolt  80  into the sleeve  410  and to radially expand the stem  412  into a cylindrical shape indicated by element number  410 ′ in  FIG. 9 . In some embodiments, the bolt  80  is pushed or pulled through the sleeve  410 , expanding the sleeve  410 . Then the nut  90  is tightened to fix the bolt  80  in place. In some embodiments, the nut is swaged over the threaded area of the bolt. In some embodiments, the bolt  80  is pushed all the way into the sleeve  410  and bore  52  before torqueing the nut  90  on the bolt  80 . The expansion of the sleeve  410  against the interior surface  54  of the bore  52  of the substrate  50  provides the electrical communication through the sleeve  410  to the substrate  50 . 
     The taper  412 R has utility in minimizing stresses applied to a substrate  50  when the sleeve  410  is radially expanded in the bore  52  against the interior surface  54 . Thus, sleeve  410  having the taper  412 R allows a cylindrical shaped stem  412  to be employed and installed in the bore  52  of the substrate  50  without lubrication and without damaging the interior surface  54  upon radial expansion of the sleeve  410 . This also prevents failure of the sleeve  410  during insertion in the bore  52 . 
     The stem  412  of the sleeve  410  is configured for uniformly distributing pressures when the sleeve  410  is expanded in a bore  52  of a substrate  50 . The stem  412  also minimizes the stress in the sleeve  410  to prevent sleeve failure during insertion. The stem  412  having a radially outward conical taper  412 R extending axially toward the head portion  410 B is an example of a stress minimizing feature. 
     The sleeve  410  is configured for insertion, insertion end  410 A first, into a hole or bore  52  in a substrate  50  (e.g., a substrate in an aircraft such as a panel made of a composite material), as shown in  FIG. 9 . The sleeve  410  is radially expanded in the bore  52  against an interior surface  54  that defines the bore  52 . The tightening of the nut  90  on the bolt  80  radially expands the sleeve  410 . In alternative embodiments, the sleeve  410  expands when the bolt  80  is pushed into the sleeve  410 , for example a force is applied to the bolt to draw the bolt into the sleeve thereby expanding the sleeve in an interference fit in the bore. 
     As shown in  FIG. 8 , the head portion  410 B of the sleeve  410  includes a flange  410 F extending radially outward from the stem  412 . In one embodiment, the head portion  410 B is configured in a flat shape, generally perpendicular to the stem  412 . While the head portion is shown and described as being a flange  410 F and/or a flat shape, the present invention is not limited in this regard, as other configurations may be employed, including but not limited to a sleeve having a head portion  410 B that has conical tapered shape or other shape suitable for use in any shape of countersunk hole in a substrate  50 . 
     In one embodiment, the sleeve  410  is manufactured from an electrically conductive material, such as a stainless steel, austenitic stainless steel, A286 CRES and AMS 5525. Employing an electrically conductive material for the sleeve  410  has utility in providing electrical communication through the sleeve  410  to the substrate that the sleeve is inserted in during instances of lightning surge flow through aircraft structure, thereby mitigating electrical arcing and protecting hardware. This also allows for the static electricity to dissipate through the sleeve  410  to the substrate without the need for a ground strap. 
     In some embodiments, the sleeve  410  can be coated on all exposed surfaces with a lubricant  102 , such as for example, cetyl alcohol or with a dry film lubricant such as graphite, molybdenum disulfide or PTFE. In some embodiments, the bolt  80  is coated with a galvanic corrosion resistant coating  104  such as an aluminum pigmented coating. 
     The transition areas  180 T- 188 T, T, T′, T″ and all or part of the cylindrical expansion region  84  are preferably coated with a solid dry film lubricant  102 . 
     In some embodiments, the entire bolt  80  is coated with a second lubricant  106  such as cetyl alcohol after an aluminum pigmented coating and a dry film lubricant  102  are applied. 
     In some embodiments, the outside of the sleeve  410  is coated with sealant before being inserted into the bore  52 . 
     Referring to  FIG. 10A , the bolt  80  is coated with a galvanic corrosion resistant coating  104 . The galvanic corrosion resistant coating  104  is then coated with a solid lubricant  102  coating thus allowing for elimination of the need for a solid lubricant coating on the inside of the sleeve  410 . The galvanic corrosion resistant coating  104  is optional, and may be used depending on the particular application. 
     Referring to  FIG. 10B , the axial length of the cylindrical area  82 ′ is defined as L and a solid lubricant  102 ′ is disposed on the cylindrical area  84 ′ a distance x. In the depicted embodiment x=L. In some embodiments the ratio of x to L is between 0.1 and 1.0. In detail B of  FIG. 10B , the solid lubricant  102 ′ is also disposed on the transition area  188 T a distance Y. The distance Y is measured along the surface of the transition area and the total length of the surface of the transition area  188 T is L′. In some embodiments, the ratio of the distance Y to the total length of the transition area  188 T of L′ is between 0.25 and 1.00. The lengths x and Y of the coating  102 ′ can be adjusted depending on the application. Both lengths are measured from point P 1  on the transition area  188 T. 
     In alternate embodiments, a sealant (not depicted) is substituted in place of one or both of the coatings  102 ,  104 ,  102 ′,  104 ′ as depicted in  FIGS. 10A and 10B . 
     Bolts having a coating on top of an aluminum pigment coating that reduces axial (frictional) installation forces on the sleeve during installation of the bolt reduces the likelihood that the sleeve will tear and eliminates the need for any lubrication or coating on the sleeve internal diameter. Such bolts having thread transition geometries having stress reduction features unexpectedly ensure uniform expansion of the sleeve and reduce the risk of structural damage to the substrate. 
     Although the present invention has been disclosed and described with reference to certain embodiments thereof, it should be noted that other variations and modifications may be made, and it is intended that the following claims cover the variations and modifications within the true scope of the invention.