Abstract:
A blind rivet comprising a rivet body with a hollow shank and a flange at one end has a mandrel extending through the shank of the rivet body and having a head with cutting edges that facilitate expansion of the end of the rivet body shank opposite to the flange when a pulling force is applied to the mandrel to break the mandrel and extract a portion of the mandrel from the rivet body shank. Cooperable portions of the rivet body shank and the mandrel engage one another and prevent rotation of the mandrel with respect to the rivet body. Other cooperable portions engage one another to ensure that the mandrel breaks without undue movement of the head into the rivet body shank.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of Japanese Patent Application No. 2004-038610 filed Feb. 16, 2004, incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a blind rivet and, more specifically, to a blind rivet of the type having a mandrel that cuts into the end of the rivet body shank while expanding its diameter. The invention is especially advantageous in a riveting operation involving a blind hole but is not limited to that application.  
         [0003]     Well known in the art is a blind rivet comprising a hollow metal rivet body and a metal mandrel longer than the rivet body, in which the mandrel passes through the rivet body and has a grip portion extending from a flange of the rivet body. This rivet has the advantage of being able to fasten a plurality of panels from one side.  
         [0004]     There are several different types of blind rivets. In the blind rivet disclosed in U.S. Pat. No. 4,620,825 (Patent Document No. 1), a mandrel head with a diameter greater than the rivet body shank is disposed adjacent to an end of the rivet body shank that is opposite to the flange. Cutting edges on the head protrude toward the adjacent end of the rivet body shank and extend lengthwise with respect to the shank. The cutting edges of the mandrel head cut into portions of the adjacent end of the rivet body shank and increase the diameter of adjacent end portions when a grip portion of the mandrel is gripped with a fastening tool and the mandrel is pulled from the flange. A plurality of members such as panels interposed between the expanded diameter portions of the shank and the flange are thus fastened together. When the end of the rivet body shank has been cut open to expand the diameter, it resembles an open flower. As a result, the rivet is sometimes called a petal-type blind rivet.  
         [0005]     In U.S. Pat. No. 4,897,003 (Patent Document No. 2), the end of the mandrel head facing the adjacent end of the rivet body shank has a conical shape with no cutting edges. Instead, slits are formed in the end of the rivet body shank. When the slitted portion is pushed open with the mandrel head, the diameter at the end of the rivet body shank expands.  
         [0006]     In U.S. Pat. No. 4,909,687 (Patent Document No. 3), as in Patent Document No. 1, cutting edges are formed on the mandrel head. Here again, the adjacent end of the rivet body shank is cut open by the cutting edges to expand the diameter of the end.  
         [0007]     A blind rivet unlike those in Patent Document No. 1 through No. 3 is disclosed in European Patent No. 691,479 (Patent Document No. 4). Here, an end of the rivet body shank expands when a middle portion of the shank buckles. In Patent Document No. 4, a portion at the end of the rivet body shank adjacent to the mandrel head has constricted diameter, and this portion is formed to buckle two-fold or three-fold. The result is a configuration with high fastening force.  
         [0008]     The blind rivets described in Patent Document No. 1 through No. 3 have rivet body shanks with ends that open up and expand in diameter. When there is a so-called blind hole to be closed, the expanded diameter portions of the rivet body shank can engage the inside wall of the hole to hold the blind rivet.  
         [0009]     For example, when a blind hole is formed as a mounting hole in one of two panels and a mounting hole in the other panel is a through-hole, the blind hole in the one panel is aligned with the through-hole in the other panel; the grip portion of the mandrel is gripped with a fastening tool; the mandrel head is inserted through the through-hole and into the blind holes, the flange is brought into contact with the other panel, and a strong force is applied to pull the mandrel from the flange until a breakable portion of the mandrel has been broken. During pulling, the diameter at an end of the rivet body shank is expanded by the mandrel head, and the rivet body shank end is secured in the blind hole in one of the panels. Because the flange remains in contact with the other panel, the one panel and the other panel are joined together. These blind rivets have the advantage of being able to fasten members when one of the members has a blind hole. Fastening is possible even when the member with the blind hole is made from a flexible material, and the flexible material can be kept from becoming warped during fastening.  
         [0010]     However, the blind rivets described in Patent Documents No. 1 through No. 3 have a problem in that the expanded end of the rivet body shank does not open stably or uniformly. This is believed to occur because the cutting force of the cutting edges is neither stable nor uniform when the plurality of cutting edges open up the adjacent end of the rivet body shank.  
         [0011]     The force applied to the slit portion in the blind rivet described in Patent Document No. 2 does not appear to be stable or uniform. Furthermore, in the blind rivets described in Patent Documents No. 1 through No. 3, the mandrel head is pulled into the rivet body shank farther than is necessary. Because such deep pulling of the mandrel head can actually result in closing an open end of a rivet body shank, the shank end can disengage from the inner wall of a mounting hole. A mandrel head that does not move more than necessary is therefore desired. Because the blind rivet described in Patent Document No. 4 is of a type in which a tip of the rivet body shank merely buckles, it is not suitable for engaging the inner wall of a blind hole. The blind rivet described in Patent Document No. 4 requires that a through-hole be formed as a mounting hole in each of the panels to be joined.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0012]     One purpose of the present invention is to stably and uniformly open an end of the rivet body shank in order to improve the fastening force, particularly when the fastening involves a blind hole. A second purpose of the present invention is to prevent the mandrel head from being pulled into the rivet body shank more than is necessary.  
         [0013]     As described in detail later, cutting edges on a mandrel head cut into portions of an adjacent end of a rivet body shank in expanding the diameter of the adjacent end of the rivet body shank when a grip portion of the mandrel is gripped and pulled, so as to break a breakable portion of the mandrel and to extract a portion of the mandrel through a flange of the rivet body shank. In a preferred embodiment, corner portions of a non-circular cross-section portion of the mandrel engage an inner diameter constricting portion of the rivet body shank. The cutting edges of the head of the mandrel are preferably formed so as to be continuous with the corner portions of the mandrel. By virtue of the invention, the cutting edges on the mandrel head stably and reliably cut into the adjacent end of the rivet body shank, allowing the rivet body shank to be cut open securely. The invention improves the fastening force when a blind rivet is used with a blind hole.  
         [0014]     In a preferred embodiment, the rivet body shank has a second inner diameter constricting portion which engages corner portions of the mandrel shank to resist the pulling force on the mandrel. In this way, the pulling force is increased; the breakable mandrel portion is quickly broken by the increase in the pulling force; and the mandrel head is kept from being pulled excessively into the rivet body shank. Consequently, the expanded end of the rivet body shank remains open and is less likely to disengage from the inner wall of a blind hole. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention will be further described in conjunction with the accompanying drawings, which illustrate a preferred (best mode) embodiment, and wherein:  
         [0016]      FIG. 1  is a semi-cross-sectional front view of the blind rivet in an embodiment of the invention;  
         [0017]      FIG. 2  is a cross-sectional view of the blind rivet in  FIG. 1  at line II-II;  
         [0018]      FIG. 3  is a cross-sectional view of the blind rivet in  FIG. 1  at line III-III;  
         [0019]      FIG. 4  is a cross-sectional view of the blind rivet in  FIG. 1  at line IV-IV;  
         [0020]      FIG. 5  is a perspective view of a main portion of the mandrel for the blind rivet in  FIG. 1 ;  
         [0021]      FIG. 6  is a semi-cross sectional front view of a blind rivet of the invention fastened so as to connect two members;  
         [0022]      FIG. 7  is a cross-sectional view of a blind rivet of the invention showing the fastening of two members;  
         [0023]      FIG. 8  is a graph showing the mandrel stroke versus extraction load curve (A) when using a blind rivet of the prior art, and the mandrel stroke versus extraction load curve (B) when using a blind rivet of the present invention;  
         [0024]      FIG. 9  is a diagram of a tensile strength test performed on two members fastened by a blind rivet of the invention;  
         [0025]      FIG. 10  is a table of the test result data for the test in  FIG. 9 ; and  
         [0026]      FIG. 11  is a graph of the test result data in  FIG. 10 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     One embodiment of a blind rivet  1  according to the invention is shown in  FIGS. 1-5 . The blind rivet  1  comprises a tubular metal rivet body  2 , and a metal mandrel  3  that is longer than the rivet body  2  and passes through the rivet body  2 . The mandrel  3  can be made of hard steel wire, for example, and the rivet body  2  can be made of a more flexible material than the mandrel (e.g., an aluminum alloy).  
         [0028]     As shown in  FIG. 1 , the rivet body  2  comprises a hollow shank  5  with a flange  6  at one end of the shank. The rivet body  2  shown in the illustrative embodiment has a round flange  6  and a round shank  5 . The mandrel  3  comprises a shank  9  somewhat longer than the rivet body shank  5  and a head  10  with a larger diameter than the adjacent end  7  of the rivet body shank  5 . The mandrel shank  9  is long enough to protrude from the rivet body flange  6 , and the protruding portion is a grip portion able to be gripped by a gripping member of a conventional fastening tool (not shown). The tip of shank  9  can be tapered to facilitate engagement with the fastening tool.  
         [0029]     A breakable portion  11  with a diameter smaller than other portions of the mandrel shank  9  is formed in a portion of the mandrel shank in the rivet body shank  5 . When the mandrel  3  is pulled using a fastening tool, the breakable portion  11  breaks once a predetermined pulling force is exceeded. Before the mandrel breaks, the pulling force is applied to the end  7  of the rivet body shank  5  from the head  10 , and this force deforms the shank  5 . The portion of the mandrel gripped by the fastening tool after the mandrel breaks is extracted from the rivet body  2  and can be recovered by the recovery box on the fastening tool. The position of the breakable portion can be varied, but in this embodiment it allows for the recovery of most of the mandrel  3  and for maintenance of a high recovery rate.  
         [0030]     The following is an explanation of the mandrel  3  with reference to  FIG. 2  through  FIG. 5 . The mandrel  3  is rod-shaped with a round cross-section shank  9  extending from the flange  6  to the breakable portion  11  (see  FIG. 4  and  FIG. 5 ), and is rod-shaped with square cross-section from the breakable portion  11  to the head  10 . As shown in  FIG. 5 , the shank portion  13  between the head  10  and the breakable portion  11  has a square cross-section somewhat larger than the breakable portion  11 , which in this embodiment has a square cross-section. The surface of the head  10  facing the end  7  of the rivet body shank  5  is an inclined surface, and cutting edges  14  are formed on the head  10 , protruding towards the adjacent end  7  of the rivet body shank  5  and the inner wall of the shank  5  and extending lengthwise with respect to the shank  5 . A plurality of cutting edges  14  are formed at predetermined intervals circumferentially on the inclined surface of the head  10 . In the embodiment shown in the figures, there are four cutting edges  14  formed at 90° intervals. The cutting edges are formed so as to extend as continuations of the corner portions of the square cross-section portion  13  of the mandrel shank  9 . The number of cutting edges can vary, but should be sufficient to provide cutting edges at corner portions of an adjacent polygonal cross-section portion.  
         [0031]     When the mandrel  3  is pulled from the rivet body  2  using the fastening tool, the cutting edges  14  cut into the end  7  and the inner wall of the rivet body shank  5  with a force corresponding to the mandrel pulling force, and the end  7  of the rivet body shank  5  is cut open. When there are four cutting edges  14  as shown in the figures, the end  7  of the rivet body shank  5  is cut open into four portions, and the diameter of the shank  5  is expanded.  
         [0032]     As shown in  FIG. 1 , the rivet body shank  5  has a first diameter constricting portion  15  near the end  7  adjacent to the cutting edges  14  of the mandrel head  10 . The inward constriction from the outer surface reduces the diameter compared to other portions of the shank  5 . As shown in  FIG. 3 , some of the inner wall of constricting portion  15  comes into close contact with the corner portions of the mandrel shank portion  13  with the square cross-section, which bite into the inner wall. In this way, the rivet body shank  5  can be secured with respect to the mandrel shank  9  without any rotating.  
         [0033]     Because the cutting edges  14  are continuous with corner portions of an adjacent portion with square cross-section, the surface of the end  7  of the rivet body shank  5  is positioned correctly, and the opening up of the shank  5  by the cutting edges  14  is stable. Because the shank  5  has a first diameter constricting portion  15  near the end  7  of shank  5  adjacent to the cutting edges  14  of the mandrel head  10 , some of the inner wall of the rivet body shank  5  (in the first diameter constricting portion  15 ) comes into contact with the outer surface of the shank portion  13  of the mandrel  3 ; the cutting edges  14  on the mandrel  10  stably and reliably cut into the end  7  of the rivet body shank  5 ; and the rivet body shank end  7  is reliably opened up stably and uniformly so as to improve the fastening force when fastening a blind rivet in a blind hole.  
         [0034]     The rivet body shank  5  also has a second diameter constricting portion  17 , between the flange  6  and the position corresponding to the breakable portion  11 , which is constricted inwardly from the outer surface and reduces the diameter of the shank  5 . As shown in  FIG. 4 , the inner wall of the second diameter constricting portion  17  comes into contact with an entire shank portion of the mandrel  3 . When the mandrel  3  is pulled, the breakable portion  11  moves into the second diameter constricting portion  17 , and the portion  13  with a cross-sectional dimension larger than the breakable portion moves towards the second diameter constricting portion  17 . However, because of its large dimension, it cannot move very far, and it resists the pulling of the mandrel  3 . As a result, the pulling force is increased, and the mandrel breaking force is quickly reached. This keeps the mandrel head  10  from being pulled into the rivet body shank  5  farther than necessary. Consequently, the end  7  of the rivet body shank remains open and is less likely to disengage from the inner wall of a mounting hole.  
         [0035]      FIG. 6  shows two members  18 ,  19  connected to one another using the blind rivet  1 . A through-hole  21  is formed as the mounting hole in the first member  18 , and a blind hole  22  closed at one end (the top end in  FIG. 6 ) is formed as the mounting hole in the second member  19 .  
         [0036]     The second member  19  can be a rigid material such as a metal panel or a flexible material such as plastic. The through-hole  21  and the blind hole  22  in the first and second members  18 ,  19  are aligned; the grip portion of the mandrel  3  is gripped using the fastening tool; the mandrel head  10  on the blind rivet  1  is inserted through the through-hole  21  and into the blind hole  22 ; and the blind rivet  1  is pushed into the through-hole  21  and the blind hole  22  until the flange  6  is brought into contact with the first member  18 . The mandrel  3  is then forcibly pulled from the flange  6  until the breakable portion  11  breaks. The strong pulling force causes the cutting edges  14  on the mandrel head  10  to cut open the end  7  of the rivet body shank  5  in the blind hole  22 , and the shank  5  is expanded in four directions outward radially, so the expanded diameter portion strongly engages the inner wall of the blind hole  22 . This engaging action strongly secures the rivet body shank  5  to the inner wall of the blind hole  22  in the second member  19 . Because the flange  6  is in contact with the first member  18 , the first member  18  and the second member  19  are connected to one another. The blind rivets of the invention have the advantage of being able to fasten members even with a blind hole. Fastening is possible even when the member in which the blind hole has been formed is made from a flexible material, and the flexible material can be kept from becoming warped during fastening.  
         [0037]      FIG. 7  is a diagram used to explain the action of the second diameter constricting portion  17  in the fastening operation of the blind rivet  1 . When a mandrel is pulled from a blind rivet of the prior art in which there is no second diameter constricting portion, the mandrel head may be pulled into the rivet body shank without breaking of the mandrel. This is especially possible when the second member  19 A fastened to the first member  18 A is a flexible material. Deep pulling of the mandrel head can act to close an open rivet body shank end inward in the radial direction, so that the shank end disengages from the inner wall of the hole.  
         [0038]     In contrast, the blind rivet  1  of the present invention shown in  FIG. 7  has a second diameter constricting portion  17  in the rivet body shank  5  between the flange  6  and the position corresponding to the breakable portion  11  of the mandrel  3 . When the mandrel  3  is pulled, the mandrel shank portion  13  meets the opening to the second diameter constricting portion  17  but is prevented from entering the second diameter constricting portion. Thus the force used to pull the mandrel  3  has to be increased, and the force required to break the mandrel is quickly reached. As a result, the head  10  of the mandrel  3  is kept from being pulled into the rivet body shank  5  farther than is necessary. Consequently, the expanded portion at an end of the rivet body shank remains open and is unlikely to disengage from the inner wall of a hole.  
         [0039]      FIG. 8  is a graph showing the mandrel stroke versus extraction load curve (A) when fastening uses a blind rivet of the prior art and the mandrel stroke versus extraction load curve (B) when fastening uses a blind rivet of the present invention. In curve (A), the mandrel does not reach breaking force, and the mandrel head is pulled into the rivet body without breaking the mandrel. In curve (B), the mandrel shank portion  13  engages the second diameter constricting portion  17  and applies extraction resistance to the pulling of the mandrel  3 . The mandrel  3  breaks properly (at P) when the maximum load y 1  is reached at stroke x 1 . In this way, the mandrel head can be kept from excessive movement even when the second member  19 A is made of a flexible material and even when it is hollow.  
         [0040]     A tensile (extraction) strength test was performed on the first member  18 A and second member  19 A fastened together using a blind rivet  1  of the present invention. In the test, as shown in  FIG. 9 , the tensile strength tester applied loads in the direction of arrows  33  and  34 , and the maximum load at which the fastened portion broke was measured. In this test, the first member  18 A was an aluminum (A5052) panel and the second member  19 A was a composite panel of aluminum and synthetic resin. Two first members  18 A were tested, one with a thickness (t) of 3.0 mm and another with a thickness (t) of 4.0 mm. The diameter of the mounting hole was 4.4 mm. The second member  19 A had a thickness (t) of 4.0 mm. The blind hole of the second member had a diameter of 4.4 mm and a depth of 3.0 mm.  
         [0041]      FIG. 10  is a table showing the extraction load data from the results of a tensile strength test performed three times, and  FIG. 11  is a graph of the result data. From the results, it is clear that the thinner (thickness t=3.0 mm) first member  18 A has higher extraction resistance or fastening force than the thicker one (thickness t=4.0 mm).  
         [0042]     While a preferred embodiment of the invention has been shown and described, it will be apparent that changes can be made without departing from the principles and spirit of the invention, the scope of which is defined in the following claims.