Abstract:
A method of blind riveting to secure together a plurality of members ( 26, 27, 28 ) with aligned apertures, using a blind rivet comprising a tubular shell ( 11 ) with a head ( 13 ) and a stem ( 12 ) extending through the tubular shell. The method comprises the steps of: inserting the shell through the aligned apertures, from the near face of the near member, so that the remote end of the shell protrude beyond the remote face of the remote member and the nearest end of the shell is substantially level with the near face of the near member; supporting the near end of the shell while pulling the stem head to form a remote blind head; applying a force to the near member with respect to the stem, until any gap ( 28 ) between the members is taken up and deforming the now protruding portion of the shell to form a near-side head of the rivet. The invention also encompasses a blind riveting apparatus and blind rivets for carrying out the method described above.

Description:
BACKGROUND 
     Blind rivets (i.e. rivets which can be installed by access to one side only of the workpiece) are well known. Commonly a blind rivet comprises a tubular shell having an elongated shank with a preformed radially enlarged shell head at one end (the head end), in combination with a stem extending through the tubular shell and having a radially enlarged stem head at one end thereof (the head end) adjacent the other end (the tail end) of the shell shank. The other end portion of the stem protrudes from the head end of the shell. The shell shank is inserted through aligned apertures in the workpiece comprising the members to be riveted together so that the shell head abuts the near face of the workpiece and the tail end portion of the shell shank protrudes beyond the remote face (the blind face) of the workpiece. An increasing pulling force is then applied to the protruding portion of the stem relative to the shell, the reaction force being supported by the shell head, so that the stem head deforms the tail end portion of the shell shank radially outwards and axially towards the shell head, to form a blind head which abuts the blind face of the workpiece. The workpiece members are thus clamped together between the shell&#39;s preformed head and its blind head. Usually the stem is then broken off flush with, or slightly inside, the head of the shell, at a breakneck preformed at the appropriate position along the stem. The breakneck breaking load is at a tension load which is greater than the load needed to completely form the blind head. 
     Such blind rivets and the method of using them are well known. 
     Blind rivets which provide a high level of static and dynamic joint strength need to develop a high retained compressive force on the workpiece, between the preformed and blind heads, and to have a relatively large preformed head and also a blind side head which has a relatively large diameter in contact with the blind face of the workpiece, i.e. a relatively large blind side footprint. An example of such a blind rivet is described in GB 2 151 738 A, and is widely available under the registered trademark HEMLOK. 
     One problem with such high joint-strength rivets in the past is that they have been restricted in the amount of joint gap closure they can provide, i.e. the amount of gap initially present between the members to be joined, which the rivet can successfully close up during installation in the members, is limited. 
     SUMMARY 
     The present invention aims to overcome this problem, and aims to provide a blind rivet which develops a large blind-side head footprint, an enhanced sheet gapclosing ability and also produces a large compressive force on the completed joint. 
     GB 613882 discloses a blind rivet having a shell without a preformed head, and a method of riveting involving applying axial compression to the shell to form both the blind and near side heads. However the rivet is such that formation of the near side head is completed before formation of the blind side head begins. Furthermore the rivet comprises only a tubular shell without a stem, the placing tool being provided with a reusable mandrel which is removed from the rivet shell after the latter has been completely deformed. 
     GB 511588 (Chobert), a divisional of GB 511,531, describes a tubular riveting system for securing workpieces together. This earlier method employs a pull-through mandrel having an enlarged head. The riveting tool incorporates an inner sleeve around the mandrel, the sleeve having a smaller diameter than the hole in the workpieces and smaller diameter than the undeformed rivet. The workpieces are thus constrained against the force of the mandrel by the outer part of the tool. However, this prior art relates to pull-through riveting and is not directly applicable to breakstem riveting. Furthermore, the dimensions of the riveting tool do not allow space for a head to form on the rivet unless a countersink is provided in the workpiece nearest the tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Some embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings, in which: 
     FIGS. 1A,  1 B and  1 C show three successive stages in the deformation of the shell of a first example rivet to form a blind head; 
     FIGS. 2A to  2 E show five successive stages in the deformation of the shell of a second example rivet to form a blind head, to close the workpiece members together and to form a near side head; 
     FIGS. 3A to  3 F show six successive stages in the action of deforming the rivet of FIGS. 1A to  1 C or  2 A to  2 E by means of a hydraulically-powered riveting tool; 
     FIGS. 4A to  4 D show four successive stages in the deformation of a third example rivet, and FIG. 4E is an enlargement of part of FIG. 4B; 
     FIGS. 5A to  5 D show four successive stages (corresponding to FIGS. 4A to  4 D) in the deformation of a fourth example rivet; and 
     FIGS. 6A to  6 D show four successive stages in the deformation of a fifth example rivet. 
    
    
     DESCRIPTION 
     In the various FIGS. 1,  2 ,  3 ,  4 ,  5  and  6 , like or corresponding parts of the various rivets, and the placing tool, are indicated by like numerals for ease of understanding and comparison. Thus, all of the example rivets each comprises a tubular shell  11  of low carbon steel and a stem  12  of medium carbon steel. The stem has a radially enlarged head  13  at one end of slightly less diameter than the tubular shell. The stem and shell are assembled so that the stem head  13  is adjacent one face  21  (the tail end face) of the shell. The underhead face  14  is of slightly concavely dished, or part-conical, shape. The major portion  17  of the stem  12  is of uniform diameter, on which the inner wall of the shell is a close fit. However, the portion  15  of the stem immediately adjacent the stem head  13  is of reduced diameter, and this is joined to the remainder  17  of the stem by a transition portion  16  of a diameter intermediate the portion  15  and the major portion  17  of the stem. The stem portion  17  is provided with a breakneck  18  in the well understood way. The shell  11  is provided with an external circumferential groove  19 , which in this example rivet is about half-way between the ends of the shell. On assembly of the shell and stem, the tailmost end portion  20  of the shell, which overlies the stem portions  15  and  16 , is crimped or rolled inwardly into contact with those portions, as illustrated in FIG. 1A, so that its inside and outside diameter tapers inwardly. The tail end face  21  of the shell abuts the head underface  14  as shown in FIG.  1 A. The end  22  of the shell remote from the tail end face  21  is not preformed with a radially enlarged head, as is usual in blind riveting practice, but is of substantially uniform diameter and has a flat “head” end face  23 . 
     Accordingly the tool employed to place the rivet (which tool is illustrated in FIG. 3) is provided with a nosepiece  24  (illustrated schematically in FIGS. 1,  2 ,  4 ,  5  and  6 ) which has a flat annular anvil face  25 . This face  25  is of substantially larger external diameter than the rivet shell  11 . The tool is provided with jaws to grip the stem and pulling means, as is illustrated in FIG.  3 . The rivet stem  12  is inserted into the nosepiece and the stem is gripped by the jaws. The rivet is used to join together two metal sheets  26 ,  27 , there being a gap  28  between the near sheet  26  and the remote or blind side sheet  27 . The rivet is inserted into the aligned apertures  29 ,  29  in the sheets, in which the shell is a sliding fit, until the anvil face  25  abuts the near face  30  of the near sheet. Thus the “head” end face  23  of the rivet shell is substantially level with the near face  30 . The remote end of the shell including the shell circumferential groove  19  protrudes beyond the remote sheet  27 . 
     The tool is also provided with a sleeve  34  outside the nosepiece  24 , the purpose of which will be described later. 
     The tool is then actuated to apply a progressively increasing tension force F 1  to the stem  12  with respect to the nosepiece  24  which takes up the reaction force F 2  against the end face  23  of the shell  11 . The axial compression thus applied to the shell by the anvil face  25  and the underface  14  of the stem head  13 , causes the tailmost portion  20  of the shell to buckle outwardly as shown in FIG. 1B to form a bulb  31  between the groove  19  and the end  21  of the shell. Deformation in this way is promoted by weakening groove  19  in the shell, the tapered configuration of the portion  20  of the shell and the interengagement between the tail end face  21  of the shell and the underhead face  14  of the stem head. However it will be apparent to the man skilled in the art of blind rivet design that there are alternative and/or additional ways of promoting deformation of the tailmost portion of the shell. 
     As the opposing forces F 1  and F 2  are increased, the bulb  31  of FIG. 1B further collapses axially until it forms a blind head  32  on the shell in the form of a folded flange, as shown in FIG.  1 C. This blind head is of relatively large diameter and has a face  33  towards the near sheet  27  which is substantially flat and parallel to the face of the sheet, and is spaced apart from it. Note that formation of the blind head  32  does not rely upon its contact with the rear sheet  27  (although it may contact it). 
     FIGS. 1A to  1 C are intended to illustrate the construction and function of the rivet  11 ,  12  insofar as the formation of the blind head  32  is concerned. Further increase of the tension force F 1  will eventually cause further deformation of the rivet shell, in a manner similar to that which will now be described with reference to FIGS. 2 and 3. 
     The construction and function of the rivet and placing tool according to this invention, with respect to closing the gap between the sheets, and the formation of the near side head, will now be described with reference to other examples. 
     FIGS. 2A to  2 E illustrate a rivet which is substantially similar to that of FIG. 1, but is a modification thereof in that it has a physically longer shell  11  to provide a larger grip (i.e. the total thickness of sheets which the rivet can join). The rivet is used to join three sheets  26 ,  27  and  38 , with gaps  28  between adjacent sheets. The rivet shell is appropriately longer, so that when the end face  23  of the shell is level with face  30  of the near sheet  26 , the external groove  19  of the shell is also beyond the rear face of the rear sheet  27  (FIG. 2A corresponds to FIG.  1 A). Axial compression of the rivet shell forms a blind head  32  shown in FIG. 2B (which corresponds to FIG. 1C) in the same way as described with reference to FIG.  1 . 
     Up to the formation of the blind head  32 , the external sleeve  34  of the tool has played no part in the process. In FIG. 1 it is shown with its end face  35  remaining slightly retracted from the anvil face  25  and near face  30  of the sheets, whereas in FIG. 2 its end face  35  is level with the anvil face  25 . In both cases the sleeve  34  has so far not moved with respect to the nosepiece  24 . However, once the blind head  32  has been formed, the blind head can be used to pull the sheets  26 ,  34 ,  27  together. This is done by transferring the reaction force to the pull F 1  on the stem from the nosepiece  24  to the sleeve  34 . Preferably this transfer is progressive. The result is that, the rivet stem  12  is retracted with respect to the sleeve  34 , thus compressing the sheets between the shell blind head  32  and the sleeve end face  35  which abuts the near face  30  of the near sheet  26 . 
     If F 1  is the tension force on the stem  12 , F 2  is the reaction force applied by the nosetip anvil face  25  to the head end face  23  of the rivet shell, and F 3  is the reaction force applied by the sleeve end face  35  to the front sheet  26 , then at any position substantially F 1 =F 2 +F 3 , assuming that no resultant force is supported by the sheets. The “head” end portion of the rivet shell  11  progressively emerges from the front sheet  26 , with the nosetip  24  being retracted in unison with the rivet stem. Eventually the three sheets  26 ,  38 ,  27  are pulled into contact with each other so that the gaps  28 , 28  have disappeared, as in the position illustrated in FIG.  2 C. 
     It is now required to form a near side head on the rivet shell, i.e. to radially enlarge the “head” most end of the shell. 
     The placing tool is further actuated so that, whilst retaining the clamping force on the sheets between the blind head  32  and the sleeve  34 , the force F 2  on the nosepiece  24  is increased. In this example, the shell  11  is provided with a second external circumferential groove  36 , which is positioned so that it lies substantially level with the near surface  30  of the near sheet  26 , as illustrated in FIG.  2 C. This groove  36  has less depth than the shell tail end groove  19 , so that the head end groove  36  provides less weakening to the shell than the tail end groove  19 . Under the increasing axial compression on the sleeve, the “head” end portion of the sleeve, between the groove  36  and the end face  23 , buckles outwardly to form first a bulb and then a folded flange (like the blind head  32 ) which provides a near side head  37 , as illustrated in FIG.  2 D. Further increase in the tension force F 1  on the stem causes it to break at the break neck  18  (not shown in FIGS. 2A to  2 D), leaving the installed rivet to form a joint between the sheets  26 ,  38  and  27 , as illustrated in FIG.  2 E. 
     Note that the clamping or compression load on the sheets between the sleeve  34  and the already formed blind head  32 , whilst the near side head  37  is being formed, is not reduced by the force used in deforming the rivet shell to form the near side head. The near side head  37  is formed, clamping the sheets between it and the blind head  32 , whilst the sheets are already clamped together between the sleeve  34  and the blind head  32 . The result is that the riveted joint provides a higher retained clamping force on the sheets than if similar deforming forces were used to form the blind head on an equivalent blind rivet with a preformed near side head. Thus the riveted joint provided by the present invention is stronger. 
     One form of suitable riveting tool is shown schematically in FIGS. 3A to  3 F. Referring first to FIG. 3A, which shows the tool before a rivet is inserted in it, the tool  41  comprises a generally cylindrical main body  42  containing an upper hydraulic cylinder  43  and a lower pneumatic cylinder  44 , the upper cylinder  43  being approximately twice as long as the lower cylinder  44 . The two are separated by an annular wall  45  from which projects downwardly a cylindrical extension  46 , the lower end of which protrudes from the bottom of the body  42  to provide the tool nosepiece  24  with the flat annular anvil face  25 . 
     The tool sheet-contacting sleeve  34  surrounds the nosepiece  24 , for axial movement with respect to both the tool body  42  and the nosepiece  24 . The upper end of the sleeve  34  has an outward annular flange  47 , which reciprocates in the lower hydraulic cylinder  44  and is urged upwardly by a coil compression spring  48 . A stop (not shown) prevents the flange  47  from seating on the annular wall  45 , leaving a space between the flange  47  and wall  45  connected by means of a port  49  to a source of variable hydraulic pressure (not shown). 
     The tool body  42  also contains a pulling piston  51  which can reciprocate with respect to the tool body. The piston  51  comprises essentially a cylindrical piston, which at about the mid point of its length has an outward flange  52  which is a sliding fit in the upper hydraulic cylinder  43 . The flange is urged downwardly by a coil compression spring  53 , and is prevented from seating on the annular wall  45  by means of a stop (not shown), leaving a space between the flange  52  and wall  45  which is connected by means of a port  54  with a source of variable hydraulic pressure (not shown). The lower end part of the extension  46  forming the nosepiece  24  contains the usual jaw assembly  55  for gripping rivet stems and pulling them with respect to the anvil face  25 , and will not be described further. 
     Clearly increasing the hydraulic pressure supplied to the lower port  49  drives the sleeve  34  downwards against the urging of spring  48 , and increasing the hydraulic pressure supplied to the upper port  54  drives the piston  51  and jaw assembly  55  upwards against the urging of spring  53 . These hydraulic pressures are controlled in a conventional way by convenient known means, in order to move the sleeve  34  and jaw assembly  55  as required and to apply the required force to each of them in order to place a rivet in the way previously described. 
     In use, the stem  12  of a rivet is inserted into the nosepiece, where it is gripped by the jaw assembly  55  in the usual way, with “head” end of the rivet shell  11  in contact with the anvil face  25  as previously described. The tool is then moved to insert the rivet shell through the aligned holes  29  in the sheets to be riveted, until the anvil face contacts the near face  30  of the near sheet  26 . This is the portion illustrated in FIG.  3 B. FIGS. 3B to  3 F show a rivet similar to that shown in FIG. 1 being placed to rivet two sheets  26 ,  27  together, FIG. 3B corresponding to FIG.  1 A. FIGS. 3B to  3 F show the near sheet  26  as being in a fixed position, and the remote sheet  27  being pulled up towards it. 
     With no hydraulic pressure applied to the sleeve port  49 , a progressively increasing hydraulic pressure is applied to piston port  54 , thus pulling the rivet stem into the nosepiece whilst holding the rivet shell against the anvil face and forming the blind head  32  (FIG. 3C) as previously described. Whilst maintaining the hydraulic pressure at piston port  54 , hydraulic pressure to the sleeve port  49  is progressively increased, driving the sleeve  34  downwards to abut the part sheet  26  and then pulling on the blind head  32  to pull the sheets  27 ,  26  together (FIG. 3D) and apply clamping pressure to the sheets  26  and  27 . The nosepiece  24  and tool body  42  move upwards with the rivet stem  12  and rivet shell  11  (accommodating similar amounts of movement of the body of a conventional hand-held blind riveting tool is common practice). Whilst at least initially maintaining the hydraulic pressure to the sleeve port  49 , the hydraulic pressure to the piston port  54  is progressively further increased, thereby to drive the nosepiece  24  and anvil face  25  downwards, with respect to the rivet stem, thus forming the near side head  37  as previously described (FIG.  3 E). During the latter part of this process the hydraulic pressure supply to the sleeve port  49  may be progressively reduced, so as not to overstress the stem at the breakneck  18 . 
     The hydraulic pressure to the sleeve port  49  is then reduced sufficiently to allow the force of the spring  48  to push the sleeve  34  upwards and withdraw it from contact with the near sheet  26 , so that all the reaction to the pulling force exerted on the rivet stem  12  by the pulling jaw assembly  55  is taken up through the rivet head  37 , as illustrated in FIG.  3 F. The hydraulic pressure to the piston port  54  is then increased until the stem breaks at the breakneck, leaving the riveted joint. 
     FIGS. 4A to  4 E illustrate another example rivet and method of riveting incorporating two possible alternative features. Firstly, where rivets are likely to be used in oversized holes (i.e. at least some of the holes are likely to be of slightly larger diameter than the recommended size), the radial expansion of the shell to form the blind head  32  can be configured so that the part  56  of the shell immediately adjacent the blind head flange  32  is also somewhat radially expanded, as illustrated in FIG.  4 A. When the blind head is then pulled up against the remote sheet  27  to close the gap  28  and clamp the sheets  26 ,  27  together between the sleeve  34  and blind head  32 , as previously described, this radially enlarged part  56  is forced into the remote end of the hole  29  in the remote sheet  27 , to produce localised hole fill, as illustrated in FIG. 4B, providing enhanced sealing of the joint. As illustrated in enlarged FIG. 4E, the edge of the remote sheet  27  around the hole may bite into the part  56  of the shell. 
     Secondly, an alternative near side head form can be used. The anvil face of the nosepiece  24  is provided with a concavely curved profile as illustrated at  57  in FIGS. 4A to  4 C. When the uppermost part of the shell  11  is pulled against the concave anvil face  54  with sufficient force, it is rolled radially outwardly, as illustrated in FIG. 4C to form a near side head  58 . This is bent downward by the concave anvil face  57  until the outer periphery of the underside of the head  58  abuts the near face  30  of the near sheet  26 , as illustrated in FIGS. 4C and 4D. The uppermost part of the rivet shell is preferably suitably configured to co-operate with the concave anvil face  57  in this mode of deformation. 
     In certain applications of blind riveting, it is found more convenient first to insert the blind rivet in the hole in the sheets, and then to apply the tool to install the rivet. This method of operation is facilitated by the example rivet illustrated in FIGS. 5A to SD, in which the “head” end of the rivet shell is provided with a vestigial head  59  of minimal radial and axial extent, which is sufficient to engage the near sheet  26  and prevent the rivet from falling through the holes  29 ,  29  in the sheets, but would be ineffective to exert any substantial clamping force on the sheets  26 ,  27 . Installation of the rivet including formation of the near side head takes place in the same way as previously described. FIGS. 5A to  5 D illustrate the formation of a rolled-over near side head  57  as in FIGS. 4A to  4 D, but equally the vestigial head  59  could be used to produce the bulbed near side head form illustrated in FIG.  2 D. 
     Another example rivet and method is illustrated in FIGS. 6A to  6 D, for use in making joints between sheets  26 ,  27  which are substantially thinner and therefore weaker, than the sheets  26 ,  27  referred to previously in this case the force applied to the sheets  26 ,  27  between the blind head  32  and the tool sleeve  34  (which has a diameter much larger than the rivet shell  11  and approximately equal to that of the blind head  32 ) is sufficient to deform both the thin sheets in the annular region between the rivet shell  11  and the sleeve  34  into a part conical dished or dimpled form as illustrated at  61  in FIGS. 6B,  6 C to  6 D. The near side head  37  then abuts the top of this dimple. In order to facilitate deformation of the sheets in this way, the rivet is configured so that the blind head  32  has a convex shape on its side nearer the remote sheet  27 , as illustrated in FIG.  6 . 
     The methods of riveting, and the rivets, described in the foregoing examples are also advantageous in that it is simpler and less expensive to manufacture a blind rivet without a preformed near side head (or with only the vestigial head illustrated in FIGS. 5A to  5 D). 
     The invention is not restricted to the details of the foregoing examples.