Background c_f £j__£ invention

1. Field of the invention.

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

2. Prior Art.

U. S. Patent No. 4,012,984, entitled "Blind Rivet Assembly with Locking Collar on Rivet Stem" 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 aga-inst 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, and to some extent is displacing the market for blind rivet assemblies of earlier widely used designs, probably at least in part because of its repeatability of installation, thereby minimizing the number of rivets which must be drilled out and replaced. Because of the large number of such rivets currently being used and the use of

such rivets in applications requiring maximum strength, cost and performance of rivets of this general type is of prime importance.

In the rivets disclosed in the foregoing patent, the stem of the rivet has a shear ring thereon normally disposed adjacent the inner end of the tubular sleeve before pulling, with a tail former on the end of the stem adjacent the shear ring comprised of a conical section followed by a cylindrical section followed ultimately by a small flange-like shoulder collar to retain shear rings which inadvertently otherwise come loose. During pulling the inner end of the tubular sleeve of the rivet flares as a result of compressive yield. The shear ring shears from the rest of the stem and slides up the conical portion of the tail-former to further flare the tubular sleeve, finally coming to rest at some position along the cylindrical portion of the tail-former of the sleeve dependent primarily upon the thickness of material being joined in relation to the grip range of the rivet used.

In the prior art rivets having a stem in accordance with the disclosure of U. S. Patent No. 4,012,984, the entire tail-former portion of the rivet is formed by machining, so that the conical portion and the cylindrical portions of the tail-former are surfaces of revolution and, in the embodiment specifically disclosed in the patent, smooth surfaces of revolution. This results in two characteristics of the rivet which are less than ideal. In particular, if one pulls a rivet having no lock ring thereon, it is noted that the pulling force varies substantially with stem position, thereby indicating that the separation strength of the inner end of the pulled rivet will vary substantially,

depending on 'where the actual grip is within the allowed grip range of the particular rivet being pulled. Further, if the actual grip is at or near the maximum grip for the particular rivet, the tail-former portion of the stem is pulled well up into the portion of the sleeve within the hole through the parts being joined. Since the tail-former portion of the stem is larger in diameter than the inner diameter of the sleeve, and the sleeve initially substantially fills the hole in the parts and is otherwise constrained by the pulling tool, the movement of the tail-former into this region of the sleeve causes the extrusion of some sleeve material back out along the stem, thereby lifting the flared inner end of the sleeve away from the adjacent surface of the parts being joined, allowing the parts being joined to separate slightly under even nominal loads.

Brief Summary nf ϋiε Invention

Blind rivet stems and method of making the same resulting in reduced cost in the fabrication of the rivet and increased strength in the installed rivet. In accordance with the method, after the shear ring is formed on the stem, the relief adjacent the shear ring on the tail-former end of the stem is formed by a rolling operation, with the cylindrical portion of the tail of the stem being knurled in the same or in a separate rolling operation. If desired, a shoulder collar may also be formed at the tail end of the stem prior to or subsequent to the rolling operation or operations. The knurled stem tail provides better gripping by the installed rivet and -increases the strength thereof by providing a relatively high and constant pull relatively independent of grip, and by tending to prevent the extrusion ^* o.f the tubular sleeve out of the hole in the parts being joined on the passage of the stem tail into the portion of the tubular sleeve within the hole, both due to the tendency of the knurled region to carry the sleeve material with it during drawing of the rivet, and for the further reason that the knurling provides a good grip on the shear ring and the end of the sleeve, yet has sufficient open spaces within which the sleeve material may extrude if required.

Brief Description c_f. the Drawings

Figure 1 is an illustration of a stem blank for a blind rivet assembly.

Figure 2 is an illustration of the stem blank of Figure 1 illustrating the roll forming of the tail-former region immediately adjacent to the shear ring.

Figure 3 is an illustration of the stem blank of Figure 2 after the roll forming of the knurled region 34 thereon.

Figure 4 is an illustration of an alternate embodiment roll formed stem blank.

Figure 5 is a view illustrating a rivet in accordance with the present invention prior to and after setting.

Figure 6 is a view similar to Figure 5 illustrating a prior art rivet prior to and after setting.

Figure 7 is a diagram illustrating the pulling force versus time for a rivet in accordance with the present invention with the locking collar thereof removed.

Figure 8 is a diagram similar to Figure 7 for a prior art rive .

Detailed Descrip ion _2_£ £___£ Invention

First referring to Figure 1, a forired stem blank having a shear ring 20 separating the tail-former end 22 from the remainder of the stem, generally indicated by the numeral 24, may be seen. The formation of the portion 24 of the stem is generally in accordance with prior art forming methods for such stems and accordingly, will not be discussed in detail herein. In that regard in Figure 1 and in various following figures, this portion of the stem will be illustrated as a blank (in an unfinished condition) though it is to be understood that -this portion of the stem may in fact be completed before the tail-former portion.

The first step in forming the tail-former end 22 of the blank is to form a tapered relief.26 of controlled depth by a rolling operation, using a roll forming tool shown. schematically in Figure 2 as roll 28. The depth of the tapered region 26 is preferably such that the diameter immediately adjacent the shear ring 20 is less than the diameter of the plug portion 30 adjacent the opposite side of the shear ring. If desired, the tapered region 26 may have a short cylindrical portion at the bottom of the taper immediately adjacent the shear ring 20 to better facilitate guaging of the finished parts, though it has been found that such a cylindrical portion is not required.

The next step in the formation of the tail-former portion of the stem in accordance with the present invention method is to knurl the cylindrical portion 32 of the tail-former to provide the knurled surface 34, as shown in Figure 3. Thus, in this embodiment, the tail-former portion of the stem is formed by two

rolling operations, one to provide the taper immediately adjacent the shear ring 20, and the second to knurl surface 34 to provide the desired surface texture, the purposes of which shall be subsequently described. Obviously, these two rolling operations may be done in either order, or alternatively could be done simultaneously, depending upon the equipment and tooling used, though the successive rolling operations as described are preferred to first accurately form the tapered region and then provide the knurling without any significant effect on the dimensional accuracy of the taper.

The function and advantage of the knurled region 34 may be illustrated with the aid of Figures 5 through 8. Figure 5 illustrates the present invention, the left half of the figure illustrating a complete rivet assembly inserted through pieces 36 and 38 to be joined prior to pulling, and the right side of the figure illustrating the same rivet after pulling. The specific rivet with which the invention is preferably used also utilizes a unique unitary locking collar generally indicated by the numeral 40, comprised of an upper portion 42 and a lower portion 44, the portions 42 and 44 being joined by a relatively thin intermediate section 46. The portion 42 has an internally tapered upper region 48 to encourage the flaring thereof when the rivet is being set. As with prior art rivets, the rivet is set by pulling the upper pulling portion 50 of the stem with respect to a pressure washer 52, or alternately directly against the rivet sleeve by an appropriate pulling tool. As the stem moves upward with respect to the rivet sleeve 54, the lower end 56 of the rivet sleeve first yields and bulges. As the stem continues to

move upward, shear ring 20 tends to move into the bulged rivet sleeve, then shears off of the stem and slides along the tapered region 26 to somewhat expand the shear ring and further flare the sleeve end. During the pulling, the lower end of portion 44 of the locking collar bottoms against the lower surface 58 of the locking groove 60 while the top of portion 48 engages the pressure washer 50, (or the anvil of the pulling tool if no pressure washer is used). As motion of the stem continues, region 46 of the locking collar 40 yields, allowing the lower portion 44 to force the lower end of upper portion 48 into the locking groove 60 in the stem. At the same time, the upper end of portion 48 is flared into groove 62 in the head 64 of the sleeve 54, thereby resulting in a very strong and positive lock of the. stem to the sleeve by the time the pulling portion of the stem fractures away from the remainder of the stem in the set rivet.

The final set rivet is as shown on the right hand side of Figure 5. In general, any particular rivet will have a useful grip range dependent upon the general proportions and size of the rivet, the grip range being the range of total thickness of the parts being joined which may be adequately accommodated by the particular rivet. Accordingly, the nature of the bulge and the relative location of the various parts at the inner end of the set rivet will vary depending upon the thickness of the parts being joined in comparison to the limits of the grip range. If the total thickness of parts 36 and 38 is approaching the upper or larger limit of the grip range, the knurled region 34 and particularly the tapered region 26 of the stem typically will

begin to pull well into the hole through the parts being joined before the pulling portion of the stem fractures at the end of the pull. In the prior art rivet shown in 'Figure 6, this same condition will more than fill the hole through the parts by expansion of region 66a, whereafter some extrusion of the sleeve material will result. In particular, in the prior art rivet, the extrusion will tend to occur in the region of the taper because of the radially outward pressure caused thereby resulting in some extrusion of the sleeve material down through the bottom of the hole in member 38a, thereby moving the bulged inner end of the sleeve away from the surface of member 38a. This of course is highly undesirable as the rivet will now allow some movement between the two parts being joined with only minimal restraint.

In the present invention, as shown in Figure 5, the same need for movement or displacement of the region 66 of the sleeve is encountered. However, the extrusion of the sleeve material downward so as to lift the bulged lower end of the sleeve away from member 38 is inhibited by two factors. In particular, the knurled region 34 on the tail-former provides a greater upward force on the bulged lower end of the sleeve to encourage whatever extrusion does occur to result further up the sleeve to provide better hole filling of the sleeve. Further, the portion of the knurled region 34, because of the open spaces in the knurl, provides some space for the sleeve to extrude into without downward movement of the bulged or formed lower end of the sleeve against the motion of the knurled surface. Thus the present invention, while resulting in a stem of substantially reduced

cost, further results in an improved rivet, particularly when used at or near its maximum grip.

It is believed that the rivet stem and method of making the same in accordance with the present invention results in improved holding strength of the inner or blind end of the rivet, as is suggested by Figure 7 and 8. In particular, the locking collars on rivets in accordance with the present invention and corresponding prior art rivets have been removed and then the rivets set, with the setting force being measured and recorded throughout the setting operation. The resulting force versus time curves for a rivet in accordance with the present invention and for a co responding prior art rivet are shown in Figures 7 and 8 respectively. It should be noted that the X axis of these curves are labeled time rather than position .as the equipment used did not facilitate the measurement of position per se, though as shall subsequently be seen, the plots versus time are readily correlatable to position.

The force versus time plot for the setting of a rivet in accordance with the present invention, but not having a locking collar thereon, may be seen in Figure 7. Initially the force increases in region 68, representing an increasing load exerted on the sleeve 56 (see Figure 5) by the shear ring 20. In region 70, the load or pulling force tends to level off or even decrease as yield in the sleeve forms the lower end thereof. Thereafter, the pulling .force steadily increases in region 72, until at point 74 the shear ring shears from the rest of the ste , resulting in a substantial drop in the pulling force required as the shear ring slides up the tapered region 26 and along the knurled area

34 of the tail-former. The force measured during this occurrence is represented by region 76, beginning in the area generally indicated by the numeral 78 and ending in the area generally indicated by the numeral 80 when the stem is stopped by the pulling tool, after which the pulling force steadily increases in region 82 until the stem breaks (point 84) at the breaking groove 86 (see Figure 5) .

The same form of diagram for a prior art rivet in accordance with Figure 6 is shown in Figure 8, the parts thereof which are similar to corresponding parts of Figure 7 being identified with the same numerals followed by the suffix a. It will be noted that while the shape of the pulling force versus time curve of Figure 8 is different in some respects from the curve of Figure 7, generally speaking, the same areas as identified in Figure 7 are readily identifiable in Figure 8. Of particular importance to this discussion is the region 76 of Figure 7 for a rivet in accordance with the present invention and the corresponding region 76a for the prior art rivet. In particular, it will be noted that the region 76 of Figure 7 is approximately horizontal and of a higher average value than the corresponding region 76a of the prior art rivet. In essence, this region is the force required for further pulling of the stem after the shear ring shears rom the rest of the ste at point 78 and prior to the engagement of the stem with the pressure washer and/or the anvil of the pulling tool at point 80. As such, the force at any point along regions 76 and 76a is at least somewhat indicative of the holding force of the formed inner end of the rivet, as failure of the inner end of the rivet by separation of the materials being

joined will cause a stretching or deformation of the sleeve, and further motion of the shear ring along the tail-former in accordance with the force curves. As may be seen by comparing regions 76 and 76a of Figures 7 and 8 respectively, this force in general is higher and more uniform for rivets in accordance with the present invention than it is for prior art rivets, thereby indicating a stronger formed inner end of the rivet throughout the grip range for rivets in accordance with the present invention over those of the prior art.

It will be noted that the force required for pulling in region 76 of Figure 7 has a rather cyclical variation around a rather uniform average, which variation appears relatively regular in terms of time period. This suggests that the regularity of a conventional knurl for region 34 as shown in Figure 3 provides some regular steps in accordance with the size of the knurl, giving rise to this regular variation in force. Accordingly, as an alternate embodiment of the invention, the knurled region 34 may be formed with a knurl characterized by a different leads in the clockwise and counterclockwise directions, as shown by the knurled region 34a in the embodiment of Figure 4. This has the effect of breaking up the regularity of the knurled pattern which, it is believed, will tend to reduce the variation of pulling force along region 76 of Figure 7, providing a further improvement to the holding power of the set rivet.

One variation of the prior art rivet of Figure 6 utilizes circular grooves in the surface of region 86 of the stem tail formed by a machining operation, apparently intended to better

restrict the sliding of the shear ring thereon. The corresponding pulling force for such rivets, when plotted as in Figures 7 and 8, shows a very high amplitude variation in pulling force along the equivalent of region 76a as each new step is encountered. The problem with such a machined surface, of course, is that depending upon the specific grip on which the rivet is being used, there can be no assurance as to where the shear ring will stop with respect to the tail-former, thereby providing no assurance as to what the true force will be within the relatively high variation encountered in this region for such a tail design. In the present invention, of course, the roll forming eliminates the restriction of a generated surface as the surface of the tail-former, allowing the knurling and even the knurling with different leads in the two directions to provide the desired random surface, giving rise to all the advantages hereinbefore stated and at the same time substantially reducing the manufacturing cost of the stems.

Finally, of course, the embodiment of the present invention shown in Figures 1 through 3 and 5 do not have a shoulder collar thereon. A shoulder collar such as shoulder collar 88 (Figure 4) may be formed in the end of the stem such as by upsetting the end of the stem preferably prior to roll forming the tapered region 26 and knurled region 34. This shoulder collar normally would be a circular collar to ultimately retain a shear ring which otherwise came loose, though the knurling of the present invention provides sufficient shear ring retention so as to generally render a shoulder collar unnecessary. Thus while certain preferred embodiments of the present invention have been

disclosed and described in detail herein, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope thereof.