WO1986001565A1

WO1986001565A1 - Locking apparatus for blind fasteners

Info

Publication number: WO1986001565A1
Application number: PCT/US1985/001686
Authority: WO
Inventors: John August Louw; William Howard Owens
Original assignee: Allfast Fastening Systems, Inc.
Priority date: 1984-09-04
Filing date: 1985-09-04
Publication date: 1986-03-13
Also published as: EP0192758A1; EP0192758A4

Abstract

A locking apparatus for blind fasteners providing improved yield and fatigue performance of the blind fasteners. The locking apparatus utilizes a locking collar (28) which will slide over the fastener stem (16) and within the tubular rivet sleeve (10) of the fastener without preforming into the locking groove (24) on the stem. The locking collar (28) is formed so that upon the pulling of the stem (16) during installation of the blind fastener, specially formed tapered reliefs (38, 42) in the locking collar (28) then adjacent the locking groove on the stem (16) cause the region of the locking collar immediately thereabove to be encouraged tightly inward against the locking groove to provide a particular tight fit in the locking groove. At the same time, a specially tapered top end (48) of the locking collar (28) is flared outward into the recess in the fastener head, and the tubulars shank expands in compression to take up any clearances thereabout, all of the foregoing occurring before the breaking of the pulling portion of the stem occurs. The net result is that the flared portion of the locking collar (28) fits tightly in the recess, the other end of the locking collar fits tightly in the locking groove (24) and the cylindrical portion therebetween fits tightly on the adjacent portion of the stem and tightly within the adjacent portion of the rivet sleeve (10), independent of reasonable variations in the dimensions of the various components of the blind fastener within the dimensional tolerances of such component.

Description

LOCKING APPARATUS FOR BLIND FASTENERS

Background of the Invention

1. Field of the Invention.

The present invention relates to the field of blind fasteners, and sore particularly to blind fasteners of high strength and high fatigue life as ara used in aircraft manufacture and the like.

2. Prior Art.

The present invention comprises an improvement to the Blind Rivet Assembly With Locking Collar And Rivet Stem disclosed in U. S. Patent No. 4,012,984. That patent discloses a blind rivet assembly having a blind rivet stem extending through a hollow rivet so that it may be pulled, thereby to expand the tail of the hollow rivet. A collar on the stem has a head fitting in a locking groove within the grip length of the stem so that when the tail of the collar abuts a pressure element bearing against the head of the hollow rivet, it is bulged outwardly into a recess in the rivet head, thereby to interlock the rivet stem in the hollow rivet. A weakened portion, such as a break groove, is provided on the stem adjacent the bulged end of the collar whereby upon further pulling, the rivet stem breaks at the weakened portion, leaving the remaining part of the blind rivet assembly in the workpiece.

Such a blind rivet assembly has found substantial uses in aircraft construction and other applications. However, the design and operation of the locking collar of such blind rivets have certain characteristics which may result in loose stems upon installation or after some period of use, and which may result in the installed fastener exhibiting less than its full strength and fatigue life potential. In particular, the locking collar must fit within the locking groove and rivet sleeve for all combinations of tolerances of parts, with the worst combination being the largest allowable locking, groove and the smallest allowable locking collar. Consequently, the locking collar will fit slightly loosely within the locking groove for all other combinations of dimensions, even when the parts are right on the nominal dimensions. Thus on installation of the blind rivet, the locking collar is forced toward the tail of the rivet sleeve, to the extent it has clearance with respect to the locking groove, so that the stem of the installed rivet may move slightly toward the rivet tail under conditions of high stress or vibration, thereby reducing the strength and fatigue resistance of the rivet. Further, because, the skirt of the locking collar is effectively formed on installation by the buckling of the upper skirt portion of the locking collar, the folding or buckling action of the skirt necessitates a rather deep recess in the sleeve head. This results in an overlap between this recess and the locking groove of the mandrel. The

result is that the whole lock ring may get squeezed out of its pocket when a downward load is applied to the stem, so that after a very short travel, the stem is completely loose.

Brief Summary of the Invention

A locking apparatus for blind fasteners providing improved yield and fatigue performance of the blind fasteners is disclosed. The locking apparatus utilizes a locking collar which will slide over the fastener stem and within the tubular rivet, sleeve of the fastener without preforming into the locking groove on the stem. The locking collar is formed so that upon the pulling of the stem during installation of the blind fastener, specially formed tapered reliefs in the locking collar then adjacent the locking groove on the stem cause.the region of the locking collar immediately thereabove to be forced tightly inward against the locking groove to provide a particularly tight fit in the locking groove. At the same time, a specially tapered top end of the locking collar is flared outward into the recess in the fastener head, and the tubular shank is expanded in compression to take up any clearances thereabout, all of the foregoing occurring before the breaking of the pulling portion of the stem occurs. The net result is that the flared portion of the locking collar fits tightly in the recess, the other end of the locking collar fits tightly in the locking groove and the cylindrical portion therebetween fits tightly on the adjacent portion of the stem and tightly within the adjacent portion of the rivet sleeve, independent of reasonable variations in the dimensions of the various components of the blind fastener within the dimensional tolerances of such components. Alternate forms of the top end of the locking collar and in the recess in the fastener head particularly suited for use in fasteners wherein fastener head material will be removed after installation for surface smoothing purposes are disclosed.

Brief Description of the Drawings

Figure 1 is a cross section, partially cut away, of the rivet of the present invention as inserted but prior to pulling

Figure 2 is a view similar to Figure 1 after pulling and immediately prior to the breaking of the pulling portion of the stem from the stem in the installed rivet

Figure 3 is a partial cross section taken on an expanded scale illustrating the breaking away of the pulling portion of the rivet stem

Figure 4 is & side view taken in partial cross section of the locking collar of the present invention

Figure 5 is a partial cross section of the rivet of the present invention illustrating on the left side of the center line the rivet prior to pulling and the right side of the center line the rivet after pulling

Figure 6 is view taken on an expanded scale along line 6-6 of Figure 5

Figures 7 through 9 and 11 are views illustrating various characteristics of a prior art locking collar, and

Figure 10 is a view illustrating the flaring of the top of the locking collar of the present invention during pulling.

Figure 12 is a schematic cross section of the top of the locking collar of the embodiment of Figures 1 through 11 before and after pulling, taken on an expanded scale.

Figure 13 is a schematic cross section of the top of an alternate embodiment locking collar before and after pulling.

Figure 14 is a schematic cross section of the top of a still further alternate embodiment locking collar before and after pulling.

Detailed Description of the Invention

First referring to Figures 1 and 2, a blind rivet assembly inserted in an appropriate hole through two plates to be joined prior to pulling and during pulling, respectively, may be seen. The major parts of the rivet, for purposes of identification, comprise the tubular rivet sleeve 10 having an expandable sleeve tail 12 and preformed head 14, with a stem, generally indicated by the numeral 16, passing therethrough. The stem has a pulling portion 18, a plug portion 20 basically conforming to the inner diameter of the rivet sleeve 10, a smaller diameter portion 22 thereabove, a locking groove 24 between the plug portion 20 and the smaller diameter region 22, and a tail former 26 at the lower end thereof. These various parts of the blind rivet assembly in the present invention are generally in accordance with the corresponding parts of U.S. Patent No. 4,012,984, though in certain instances they will be of different proportions in accordance with the present invention. In addition to these various parts, the present invention includes a locking collar 28 of particularly unique design, which in cooperation with the various other elements of the blind rivet assembly, provides an improved locking action for the stem of the installed rivet, helping to eliminate the occurrence of any looseness in the installed rivet and improving the stress and fatigue capabilities of the rivet. Finally, in the embodiment disclosed, a pressure washer 30 is used which, as shall be subsequently seen, acts as an anvil during pulling, against which the head of the locking collar is formed. Other embodiments do not use the pressure washer, the same function being achieved by the abutting face of the pulling tool.

Figure 2 illustrates the rivet of Figure 1 during pulling. The pulling tool generally comprises a pressure element 32 through which the pulling portion 13 of the stem will pass so that the pressure element may rest flat against the pressure washer 30. The pulling tool also has a chuck-like portion, generally indicated by the numeral 34, which engages the pulling portion 13 of the stem, and through a mechanism not shown will pull the pulling portion of the stem along its axis until the sleeve tail 12 is expanded by the tail former 26 and the locking collar 28 is formed to lock the stem at the pulled position, after which the pulling portion 18 of the stem will break frpm the lower portion of the stem as shown in Figure 3 by the failure of the stem in tension at the break groove 36 (see Figure 1) in the stem provided to define a weakened region therein.

The locking collar 23 of the present invention may be seen in Figure 4, which is a side view, partially cut away to also illustrate a cross section thereof. As may be seen in the figure, the locking collar of the preferred embodiment is a generally tubular member specially formed in three regions. In particular, adjacent the lower end thereof is internal taper 33 just above a short tubular section 40. Cooperatively disposed with the internal tapered region 38 is an external tapered region 42, the upper portion 43 and the lower portion 50 of the locking collar being connected between the two tapered regions by section 44. Finally, the top of the locking collar 23 has an internal tapered region 46 extending outward at the top of the locking collar to at least a substantial part of the wall thickness of the basic tubular shape of the locking collar.

How referring to Figure 5, an illustration showing at one side of the center line the various parts of the blind rivet before pulling, and on the other side of the center line the rivet after pulling, may be seen. As may be seen at the left side of the figure, the locking collar 23 may be located anywhere along the rivet stem between the plug portion 20 and the pressure washer 30, as unlike the prior art, the locking collar is not preformed onto the stem and more particularly into the locking groove in any way. Thus, obviously though assembly of the various parts is required, a preforming step onto the stem is not.

Upon pulling of the rivet, a number of things happen to the locking collar to provide the improved locking action of the present invention. In particular, the locking collar 23 after pulling may be seen in the right hand portion of Figure 5, with the relevant portion of such figure being shown on an expanded scale in Figure 6. As is best shown in Figure 6, it may be seen that the lower portion 50 has been forced downward firmly against the sleeve head 14 so as tα eliminate any clearance therebetween. The region 44 joining the lower portion 50 and the upper portion 43 of the locking collar (Figure 4) has been greatly distorted, and in fact may partially or totally fracture without consequence. The tapered upper region of the lower portion 50 of the locking collar has in effect formed an inclined plane, cooperating with the outer tapered region 42 of the locking collar 23 to force the lower region 52 of the locking collar to deform inward to substantially totally fill the locking groove in the rivet stem, independent of the exact dimensions of the various parts within the allowed range of tolerances. Of particular importance is the fact that the locking collar wedges tightly against the upper region of the locking groove, thereby avoiding any clearance therebetween in the set rivet. This is to be compared with the prior art locking collar as illustrated in Figures 7, 8 and 9. In particular, as previously pointed out the head 56 of the prior art locking collar nominally will have some looseness in the locking groove 58 on the stem of the rivet. Even if the head of the locking collar is formed at the top of the locking groove as illustrated in Figure 7, the locking collar and head will move to the bottom of the locking groove as a result of the pressure on the top of the locking collar by the pressure washer, or by the pulling tool if no pressure washer is used. In general, during pulling the tubular portion 60 of the locking collar will yield in compression to take up any clearance between the smaller diameter 62 of the rivet stem and the inner diameter 64 of the rivet sleeve. However, because the head 56 of the locking collar is of greater cross sectional area than the tubular portion 60, the head will not yield in compression to entirely fill the locking groove, so that the clearance between head 56 and locking groove 58, wherever such clearance was originally located, will be positioned just above the head 56 as clearance 66 illustrated in Figure 9. This position of the clearance is the worst possible position of such clearance in an installed rivet, as some downward motion of the stem can occur without requiring any yielding or distortion of the set locking collar.

In the present invention, on the other hand, it may be seen from the various figures, particularly Figures 4 and 6, that the locking collar of the present invention does not have any regions of enlarged cross section such as the head 56 of Figures 7 through 9, but in fact because of the tapered regions 42 and 38, the regions of the locking collar which are formed into the locking groove are in effect the same or lesser thickness than the tubular region 63 of the locking collar. Accordingly, the portions of the locking collar forming in the regions of the locking groove readily deform and yield in compression tα assure very tight filling of the locking groove, independent of the particular dimensions of the various parts within the allowed tolerance range. In that regard, Figure 6 is a drawing accurately illustrating on an expanded scale, the cross section of the locking collar and adjacent regions of the rivet after pulling.

A second thing which occurs during pulling is that the top of the locking collar 23 flares outward as a result of the tapered region 46 (Figure 4) of the locking collar. In particular, the pressure washer 30 engages the top of the taper, which top is at or toward the outside diameter of the cylindrical region 68 of the locking collar. The result is that the compressive stresses adjacent the top of the locking collar during formation are highest adjacent the outside diameter of the locking collar, inducing compressive yielding in that region first to result in the flaring of the top of the locking collar as illustrated in Figure 10 to form the flared top 70 of the locking collar to lock the locking collar in the recess 72 of the preformed head 14 of the rivet. In comparison, in the prior art locking collar, the top 74 of the locking collar is forced to buckle tα serve the same function, thereby requiring a bigger and particularly a deeper recess in the head of the rivet, which frequently results in an overlap between that recess and the locking groove of the mandrel. Such an overlap is undesirable, the occurrence of such buckling outward tending to roll the top of the head of the locking collar out of the locking groove in the stem, further increasing any clearance in that region. Finally, the third thing which occurs during the setting of the rivet is that the tubular portion 68 (Fig. 6) of the locking collar yields in compression to fully fill any clearance between the smaller diameter portion 22 of the stem and the adjacent inside diameter of the rivet sleeve head 14. This is illustrated in Figure 6, wherein it may be seen that region 63 is tightly forced into such gap, the figure illustrating some degree of distortion in the sleeve head 14 caused by the very tight filling of the various gaps by the locking collar during setting. The degree of distortion is a function of the yield strengths of materials utilized. With the locking groove filled, the cylindrical portion of the locking collar expanded to remove all clearance between the stem and the sleeve in that region and to expand the sleeve in the hole in the material to be joined, and the top of the locking collar flared to fill the recess in the head, no further distortion of the locking collar can occur. This is to be compared with the prior art locking collar as illustrated in Figure 9. In particular, during setting the hoop strength of the head 56 of the locking collar prevents a corresponding yield in compression of the cylindrical region immediately adjacent thereto, so that a small underfill 76 occurs in that region. Finally, rivet stem movement ceases, resulting in the breaking of the pulling portion from the stem of the installed rivet. For a blind rivet assembly to have maximum strength and fatigue resistance when installed, it is essential that the rivet be tightly installed to force the two or more panels being joined in to tight face-to-face abutment, and that all clearances between the various internal parts of the rivet and the inner diameter of the sleeve be eliminated to the maximum extent possible. In particular, it is obvious that the rivet sleeve must be somewhat malleable for the sleeve to be somewhat expanded to fill the hole in the parts to be joined and for the tail of the sleeve to be properly expanded by the tail former. Consequently it is particularly important that the entire inner diameter of the rivet sleeve be supported internally by tight fitting parts and/or parts compressively distorted thereinto, as any clearance between the rivet sleeve and the stem would allow the rivet sleeve to distort in use under load, reducing the strength and fatigue life of the assembly. It is of course also important that the stem of the installed rivet be as positively locked in the set position as. possible, as failure to so lock the stem may allow the stem to move in the direction of the tail of the rivet upon the shock of the breaking of the stem, or subsequently in use of the rivet, reducing the support for and thus the strength of the sleeve and tail of the installed rivet.

Thus, it may be seen that the present invention locking collar provides optimum filling of the locking groove in the stem, independent of part dimensions, assures uniform compressive deformation of the locking collar in the tubular portion thereof to assure tight filling of the respective region between the stem and the sleeve (and to help expand the sleeve} and provides a definite and positive flaring of the top of the locking collar into a recess in the head, of the sleeve of minimum size, all to provide a blind rivet assembly of the highest strength and fatigue resistance.

Referring again to Figure 4 and Figure 6, it will be apparent that the upper portion 43 and the lower portion 50 of the locking collar 28 substantially separate into two pieces upon pulling of the rivet, and accordingly could be fabricated and assembled as twα pieces if desired. Such is not preferred however, as tα do so would require the handling of additional parts and would provide the opportunity to get one or both parts into the assembled rivet upside down so as not to properly function as described herein. Also, in theory the lower portion 50 of the locking collar, not distorting significantly during pulling, could be formed as the upper portion of the plug portion 22 of the stem, though again such is not preferred as to do so would in effect require the formation of a locking groove generally conforming to the lower part 52 of the installed collar (see Figure 6), a difficult locking groove tα both fabricate and inspect. Also, it should be noted that while the taper 46 at the top of the locking collar -of Figure 4 is shown as encompassing most of the diameter of the locking collar so as to define a relatively thin section ring at the top of the locking collar, the extent of this taper can be varied as desired in relation to the various other dimensions and proportions of the various parts making up the rivet, as the flaring will generally occur providing the taper is of at least some substantial extent, and preferably is of at least 50% of the thickness of the cylindrical section 63. Further, while it is preferred to have the diameter of the bottom of the tapered region 42 be smaller than the diameter at the top of tapered region 33 so that the tapered regions themselves will engage as inclined planes upon distortion of section 44, such is not an absolute requirement, as the desired action will occur so long as these two diameters are at least approximately equal.

In the previously described embodiment there may be either of two occurrences which are less than ideal. Both of these occurrences are illustrated in Figure 12, which is an enlarged view of the top of the locking collar and the recess in the head of the fastener, the left side of the figure illustrating the part3 before pulling and the right side illustrating the parts after pulling. As stated before, the tapered region 46 encourages the flaring of the top of the locking collar, with a very good tight filling of the recess 72 in the head of the rivet as a result thereof. Because the filling normally is so tight and complete, and probably for the further reason that filling occurs without that much cold working of the material adjacent the top inner diameter of the flare, some locking collar material 30 may tend to extrude into the region between the outer diameter of stem 16 and the inner diameter of the anvil washer 30 (see Figures 2 and 3) during pulling. This in turn may result in an otherwise unnecessary increase in the roughness across the head of the installed rivet.

The second condition which is sometimes encountered is a result of the shaving of the head of installed fasteners, sometimes done to provide an especially smooth aerodynamic surface across the top of the installed fasteners, and to assure that the surface of the installed fastener is flush with the surface of the adjacent material. In particular, though the amount of material removed from the head will in any event be relatively limited, in the upper limit the head may be shaved to a level approximately equal to the level indicated by the dashed line 32 in Figure 12. It may be seen therein that this extent of shaving will remove a substantial portion of the flare, leaving such a small portion of the flare as to appear to substantially weaken the effective retention of the stem by the locking collar. In practice it has been found that unshaved fasteners in accordance with the present invention are substantially stronger in this respect than prior art fasteners, and that while the shaving does slightly reduce the strength of the fastener, the strength of the shaved fastener still equals or exceeds that of the prior art devices. However, while one may reasonably expect some loss of strength to be incurred in the shaving process, it is of course highly desirable tα minimize that loss of strength, both in terms of actual physical measurements, and in terms of visual appearance of the installed and shaved fastener, or installed, pulled and sectioned test fasteners.

One approach to substantially eliminate the foregoing occurrences is shown in Figure 13. In this modified head design, the cylindrical recess 34 is approximately the same depth or perhaps even shallower than the depth of the cylindrical recess portion of recess 72 of the earlier design. Also, the cylindrical recess 84 is of generally smaller diameter than the diameter of the cylindrical portion of recess 72 of the earlier design, though the tapered region 86 is a much deeper taper than before. Further, the top of the locking collar 23 is generally symmetrically rounded so that the pressure thereon during forming of the head is essentially distributed over a ring or annulus substantially midway between the inner and outer diameter thereof. This in turn discourages flaring of the top of the locking collar but instead induces compressive yielding or upsetting of the locking collar on pulling, as in illustrated in the right hand portion of Figure 13.

The net result of this embodiment is that the relatively deep taper 36 of the tapered portion of the fastener head recess and the firm filling thereof by the upsetting of the top of the locking collar 23 assures that there will be a more than adequate amount of formed locking collar material in the recess for locking purposes even after any reasonable amount of material is shaved from the head of the rivet after pulling. Also, the relief 86 adjacent the inner diameter of the locking collar prior to pulling, formed by the radius on the top of the locking collar shown in Figure 13, a chamfer or otherwise, allows for substantial material flow in that region without extruding outward between the stem and the anvil washer, resulting in the substantial filling of the relieved area upon pulling, as shown as 33 in Figure 13, without any extrusion of material into the region between the inner diameter of the anvil washer and the stem.

The embodiment of Figure 13 achieves the desired result of eliminating the extrusion of any locking collar material between the anvil washer and the stem, and assures sufficient depth in the formed top of the locking collar after pulling so as to eliminate any appearance of significant weakening of the lock after shaving. However, the relatively long tapered region 36, together with the fact that the forming force on the top of the locking collar during pulling is approximately centered on the cross section thereof, may result in an undβsirsd buckling or bulging of the locking collar away from the stem in the region of the fastener head recass, as indicated by the dashed line 92 of Figure 13. This of course is also undesirable, and accordingly, there is a definite limit on the depth of the tapered region 36 which may be used without encountering this problem.

The preferred form of the recess in the head of the fastener and associated region of the locking collar may be seen in Figure 14. In particular, in this embodiment, the recess 94 in the head of the fastener is substantially deeper than the recess 72 (Figure 12) of the first disclosed embodiment, and is also of a substantially smaller diameter. Similarly, the tapered region 96 at the bottom of the recess 94 is a somewhat deeper taper than the corresponding region 98 of the embodiment of Figure 12, but not as deep as the embodiment of Figure 13. In that regard, the depth of the recess 72 in the embodiment of Figure 12 is approximately equal to the annular thickness of the locking collar 23 so that a simple flaring of the locking collar will substantially fill the recess. Obviously a thicker locking collar in the embodiment of Figure 12 would thereby provide a deeper formed lock so that much more of the formed head would remain after shaving. However, various other parameters such as sleeve diameters and thicknesses, stem diameter requirements, etc., limit the thickness of the locking collar and thus limit the depth of the formed head which may be obtained by the simple flare of Figure 12.

Making recess 94 of the embodiment of Figure 14 substantially deeper than recess 72 or the embodiment of Figure 12 is therefore not a mere matter of arbitrary choice or degree, as the deeper recess 94 substantially exceeds the thickness of the locking collar and accordingly, cannot be filled by a simple flare of the top of the locking collar thereinto. Instead, substantial upsetting of the top of the locking collar must occur for the filling of the recess 94 and tapered region 96. In addition, the embodiment in Figure 14 has the top of the locking collar shaped so that the upsetting force is not centered on the cross section of the locking collar but rather is disposed toward the outer diameter thereof, resulting in a relief 98 adjacent the stem 16. This distribution of load on the locking collar encourages a partial flare of the top thereof outward, with the combination of the flare and upsetting filling the recess 94 and the tapered region 96. As with the embodiment of Figure 13, the initial relief in the region 93 and of course the partial flaring of the top of the locking collar during pulling allows the required upsetting to tightly fill the recess in the head of the fastener without resulting in the extrusion of any locking collar material in region 100 into any space between the stem and the anvil washer, as was illustrated with respect to Figure 12. Thus the partial flaring, partial upsetting of the top of the locking collar during pulling which is achieved in Figure 14 results in a deeper formed locking collar top than the simple flaring of the embodiment of Figure 12, whereby an ample part of the formed locking collar top is retained after shaving of the installed fastener to retain a very high locking strength. At the same time the extrusion of locking collar material into any gap between the anvil washer and the stem is prevented primarily because of the extent of cold working of the locking collar in the recess material which would be required before such extrusion could start to take place, all without buckling of the locking collar in the recess which would tend to create gaps between the stem and the formed locking collar in the installed fastener. In that regard, buckling is avoided by the fact that the unsupported length of the locking collar within the recess in comparison to its thickness is still relatively short, and the partial flare of the top of the locking collar outward to engage the wall of the recess leaves both the inner diameter and outer diameter of the unsupported length of the locking collar unconstrained, whereby the locking collar will yield in compression to fill the recess as shown before buckling can occur.

Thus, it will be obvious to those skilled in the art that these and various other changes may readily be made without departing from the spirit in scope of the invention.

Claims

In the Claims

1. In a blind rivet assembly having a tubular rivet sleeve having a performed head at one end and an expandable sleeve tail on the other end thereof, a stem extended through said sleeve and said head, a tail former at the tail end of the stem adjacent said sleeve tail for expanding said tail, a pulling portion on the pulling end of the stem adapted to be engaged by a tool for pulling the stem, a plug portion of the stem adjacent said tail former, a locking groove in the stem adjacent said plug portion, the portion of the stem between said locking groove and said pulling portion being of smaller diameter than the interior diameter of said sleeve and head, the Improvement comprising a locking collar positioned over said stem between said plug portion and said pulling portion, said locking collar having an inner diameter to slide over the portion of said stem between said locking groove and said pulling portion, and an outer diameter to slide within said interior diameter of said sleeve, said locking collar having a first locking collar portion and a second locking collar portion, said first locking collar portion being βlideabla along said stem tα abut said plug portion of said stem, said locking collar being of such material, and the region of said first locking collar portion adjacent said second locking collar portion and the region of said second locking collar portion adjacent said first locking collar portion being cooperatively shaped to deform the region of said second locking collar portion adjacent said first locking collar portion tightly into said locking groove during pulling when said first locking collar portion is restricted from movement with said stem.

2. The improvement of Claim 1 wherein said first and second locking collar portions are integrally connected, whereby said locking collar is a one piece locking collar prior to pulling.

3. The improvement of Claim 1 wherein the and of said second locking collar portion opposite said first locking collar portion has an internal relief whereby forceful engagement of the end of said second locking collar portion having said internal relief with a surface substantially perpendicular with the axis of said blind rivet assembly will cause said second locking collar portion to flare in the region of said relief.

4. The improvement of Claim 3 wherein said internal relief comprises a tapered relief.

5. The improvement of Claim 1 wherein said region of said first locking collar portion adjacent said second lacking collar portion has an internal taper and said region of said second locking collar position adjacent said first locking collar portion has an external taper, said tapers cooperatively acting to deform the region of said second locking collar portion adjacent said first locking collar portion tightly into said locking groove during pulling.

6. The improvement of Claim 1 wherein said preformed head of some rivet sleeve has an annular recess in the face thereof of a diameter larger than the inner diameter of said tubular rivet sleeve and deeper than the radial thickness of said locking collar, and wherein the end of said second locking collar portion opposite said first locking collar portion has an internal relief whereby forceful engagement of the end of said second locking collar portion having said internal relief with a surface substantially perpendicular with the axis of said blind rivet assembly will cause, as pulling progresses, said second locking collar portion to partially flare outward in the region of said relief, and to thereafter yield in compression to substantially fill said annular recess in said preformed head.

7. The improvement of Claim 6 wherein the diameter of said annular recess limits the extent of the partial flare of said second locking collar portion during pulling.

8. A locking collar for use with a rivet of the type having a tubular sleeve with a preformed head at one end thereof, and a stem passing through the sleeve, the stem having a first diameter adjacent the preformed head end of the sleeve which is smaller than the internal diameter of the sleeve, a plug portion having a second diameter substantially equal to the internal diameter of the sleeve adjacent the opposite end of the sleeve, and a locking groove therebetween, the rivet being set by the pulling of the stem with respect to the sleeve along the axis of the rivet from the end of the stem extending through the preformed head on the sleeve said locking collar having a minimum inner diameter to fit over the first diameter of the stam and a maximum outer diameter to fit within the sleeve, and first and second ends perpendicular to the axis thereof, said first end for abutting the plug portion of the stem when pulling the rivet, said locking collar having a region of reduced compressive strength between said first and second ends thereof whereby a sufficient compressive load between said first and second end of said locking collar will cause yielding in said region of reduced compressive strength resulting in movement of the second locking collar portion adjacent said second and relative to and toward the first locking collar portion adjacent said first end, said region of reduced compressive strength being configured to deflect the region of said second locking collar portion adjacent said first locking collar portion inward with respect thereto, whereby said locking collar will substantially fill the locking groove in a rivet stem during pulling of the rivet.

9. The locking collar of Claim 3 wherein said second end of said locking collar has an internal relief, whereby said second end will flare when subjected to a sufficient compressive load applied substantially along the axis thereof.

10. The locking collar of Claim 9 wherein said internal relief comprises a tapered relief.

11. The locking collar of Claim 9 wherein said locking collar in regions other than the region of said internal relief and the region of reduced compressive strength is substantially tubular having an outer diameter corresponding to said maximum outer diameter and an inner diameter corresponding to said minimum inner diameter.

12. The locking collar of Claim 11 wherein said internal relief comprises a tapered relief of at least 50% of the thickness of said tubular section.

13. In a blind rivet assembly having a tubular rivet sleeve having a preformed head at one end and an expandable sleeve tail on the other end thereof, a stem extended through said sleeve and said head, a tail former at the tail end of the stem adjacent said sleeve tail for expanding said tail, a pulling portion on the pulling end of the stem adapted to be engaged by a tool for pulling the stem, a plug portion of the stem adjacent said tail former, a locking groove in the stem adjacent said plug portion, the portion of the stem between said locking groove and said pulling portion being of smaller diameter than the interior diameter of said sleeve and head, the improvement comprising a locking collar positioned on said stem, between said plug portion and said pulling portion for extending inward into said locking groove and outward into a recess in said preformed head to retain the stem in the pulled position upon pulling of the rivet, said locking collar having a tubular section with an internal relief at the and thereof facing said pulling portion of said stem, whereby compressive loads on said end of said locking collar facing said pulling portion of said stem during pulling will cause said and to flare into said recess in said preformed head.

14. The locking collar of Claim 13 wherein said internal relief is a tapered relief.

15. The locking collar of Claim 13 wherein said internal relief comprises a tapered relief of at least 50% of the thickness of said tubular section.

16. A blind rivet assembly comprising a tubular rivet sleeve having a performed head at one end and an expandable sleeve tail on the other end thereof, a stem extended through said sleeve and said head, a tail former at the tail end of the stem adjacent said sleeve tail for expanding said tail, a pulling portion on the pulling end of the stem adapted to be engaged by a tool for pulling the stem, a plug portion of the stem adjacent said tail former, a locking groove in the stem adjacent said plug portion, the portion of the stem between said locking groove and said pulling portion being of smaller diameter than the interior diameter of said sleeve and head, a locking collar positioned over said stem between said plug portion and said pulling portion, said locking collar having an inner diameter to slide over the portion of said stem between said locking groove and said pulling portion, and an outer diameter to slide within said interior diameter of said sleeve, said locking collar having a first locking collar portion and a second Idcking collar portion, said first locking collar portion being slideable along said stem to abut said plug portion of said stem, said locking collar being of such material, and the region of said first locking collar portion adjacent said second locking collar portion and the region of said second locking collar portion adjacent said first locking collar portion being cooperatively shaped to deform the region of said second locking collar portion adjacent said first locking collar portion tightly into said locking groove during pulling when said first locking collar portion is restricted from movement with said stem.

17. In a blind rivet assembly having a tubular rivet sleeve having a preformed head at one end and an expandable sleeve tail on the other end thereof, a stem extended through said sleeve and said head, a tail former at the tail and of the stem adjacent said sleeve tail for expanding said tail, a pulling portion on the pulling end of the stem adapted to be engaged by a tool for pulling the stem, a plug portion of the stem adjacent said tail former, a locking groove in the stem adjacent said plug portion, the portion of the stem between said locking groove and said pulling portion being of smaller diameter than the interior diameter of said sleeve and head, the improvement comprising a locking collar positioned on said stem between said plug portion and said pulling portion for extending inward into said locking groove and outward into a recess in said preformed head to retain the stem in the pulled position upon pulling of the rivet, said locking collar having a tubular section, with an internal relief at the end thereof facing said pulling portion of said stem, said recess having a diameter larger than the inner diameter of said tubular rivet sleeve and deeper than the radial thickness of said locking collar, whereby forceful engagement of the end of said second locking collar portion having said internal relief with a surface substantially perpendicular with the axis of said blind rivet assembly will cause, as pulling progresses, said second locking collar portion to partially flare outward in the region of said relief, and tα thereafter yield in compression to substantially fill said annular recess in said preformed head.

13. The improvement of Claim 17 wherein the diameter of said recess limits the extant of the partial flare of said second locking collar portion during pulling.