US3426641A

US3426641A - Fastener unit with forming die

Info

Publication number: US3426641A
Application number: US688416A
Authority: US
Inventors: Irwin E Rosman
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-04-07
Filing date: 1967-12-06
Publication date: 1969-02-11
Anticipated expiration: 1986-02-11
Also published as: FR95724E

Description

Feb. 11, 1969 I; E. ROSMAN 3,426,641 FASTENER UNIT WITH FORMING DIE Filed Dec. 6, 1967 Sheet or 2 /6 0 a ZIz 5.1

AWV/A/ A? Ros/144W INVEN'IOR.

BY Ff F 1969 1. E. ROSMAN 3,426,641

FASTENER UNIT WITH FORMING DIE Filed Dec. 6, 1967 United States Patent 3,426,641 FASTENER UNIT WITH FORMING DIE Irwin E. Rosman, 23710 Clarendon St., Woodland Hills, Calif. 91364 Continuation-impart of application Ser. No. 633,331, Mar. 10, 1967. This application Dec. 6, 1967, Ser.

US. Cl. 85-37 Int. Cl. F16b 19/04; B2111 39/00; B23p 11/02 19 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of application Ser. No. 633,331 filed Mar. 10, 1967, now abandoned, which in turn was in continuation-in-part of now abandoned application Ser. No. 542,181 filed Apr. 7, 1966.

This invention relates to rivet type fasteners, wherein the shank is upset to form a head on the end of said shank. It pertains in particular to the art of joining structural members, referred herein generally as sheets, using rivet materials which are stronger than the sheet material. rRivet materials are defined to be stronger than the sheet when a rivet shank of such material set by conventional methods would swell and deform the hole in the sheet in a manner to conform to the shape of the upset head end of the shank. Such deformation can occur, regardless of the relative yield strength of the rivet and sheet material, when the compression strength of the rivet is stronger than the tensile loop strength of the sheets at the hole.

Higher strength rivets produce a more efiicient joint because the higher shear strength obtained more nearly matches the high bearing strength of the holes in the sheet. Such a joint, for example, would be of high strength titanium or steel alloy rivets used to join aluminum sheet, as opposed to the conventional material combinations of aluminum rivets in aluminum sheets. Higher shear strength rivets permit smaller holes to be used, subsequently resulting in lighter weight structure obtained by thinning the sheets an amount substantially equal to the net area gained in the sheet by the smaller hole size while maintaining the same pitch (hole spacing). Additional weight savings can be obtained with smaller hole diameters while maintaining the same acceptable edge distance to diameter ratio. This, then, results in narrowing the width of angle type stiffeners such as used in support of aircraft fuselage and wing skins.

This invention also pertains to high shear type riveted joints which are subject to dynamic, vibrational loads, where high fatigue life'is an important requirement. It has been demonstrated that high fatigue life is obtained in those riveted joints in which the rivet shank substantially swells the holes in each sheet during upset forming so that a high circumferential interference fit is obtained. Higher forming loads are therefore required to not only upset stronger rivet material but to sufiiciently swell the hole. It is also known that when higher strength rivets are upset in softer sheet materials under these higher forming loads. undesirable high local deformation takes place in the sheet adjacent to the upset head of the rivet causing distortion in the sheet making the structure around the hole vulnerable to premature cracking. It is therefore essential, that in order to achieve high fatigue life and avoid excessive sheet deformation, uniform Swelling of the holes in each sheet must be produced, while at the same time, high local deformations of the sheet must be prevented; it is a. major objective of this invention to provide a rivet type fastener system which can accomplish this and to provide a method of joining structure with such a fastener system.

It is also recognized that ductility of materials generally decreases as the strength increases. This characteristic is of particular concern in riveting high strength fasteners where forming of these lower ductility fasteners without cracking becomes more difficult. When the rivet is squeeze formed as in a typical riveting operation, as the head end is being upset, shank material is extruded from the head and into the hole. Thus a highly complex pattern of stresses are imposed on the head during the upset process, i.e., shear type flow stresses under extrusion, tension stresses under head expansion and compression stresses under the axial forming loads. It is therefore essential to control the configuration of the upset head in order to prevent head cracking of these less ductile, higher strength materials, This is another important objective of this invention.

It is another object to provide a rivet type fastener system which is of extremely light weight and high reliability under tensile loading, and in some embodiments of this invention, tensile capability is provided equal to or greater than that of a bolt of the same material and basic shank diameter, while still retaining high shear strength.

These objectives are achieved in this fasten-er and tool system which comprises a high strength rivet, a collar which is a variable orifice head forming and shank extrusion die, which controls the forming of the upset head as well as the extrusion of rivet material, a sleeve which controls expansion of the die and in addition provdes configuration control of the upset head, and a. piston which is guided by the sleeve and provides the upset forming loads to the rivet shank.

Other features and objectives will become apparent to those skilled in the art from the description to follow, with reference to the accompanying drawings:

FIGURE 1 is a cross section of the preferred embodiment of the invention before riveting, showing a rivet inserted in the mating holes of an assembly of sheets to be fastened together, a collar die, a collar restraining sleeve and a piston mounted in the sleeve;

FIGURE 2 is an enlarged view of the portion 2 of FIGURE 1;

FIGURE 3 is a fragmentary cross section of the embodiment of FIGURE 1 after the riveting operation;

FIGURE 3A is an enlarged fragmentary position of FIGURE 3;

FIGURE 4 is a fragmentary cross section showing an alternate configuration of the forming die;

FIGURE 5 is a fragmentary cross section showing another alternate configuration of the forming die;

FIGURE 6 is a fragmentary cross section showing an alternate arrangement of the forming die and the sleeve;

FIGURE 7 is a fragmentary cross section showing another alternate arrangement of the forming die and the sleeve; and

FIGURE 8 is a diagrammatic illustration of a hand riveting tool for upsetting the rivet.

FIGURE 1 shows a fastener unit comprised of a high strength rivet 10 and die 13 having an orifice 15. Also shown are structural members A and B and a rivet set tool assembly consisting of a sleeve 16 and piston 19.

Compression forces F and F (see FIGURE 8), transmitted by surface 21 of the preformed head 11 of rivet and end surface of sleeve 16, respectively, preload the sheets A and B together to prevent gaps therebetween before the upset load F is applied by the pistOn 19, said load being reacted by a riveting tool (see FIGURE 8) originally producing force F on the preformed head of the rivet. The sleeve 16 has a recess at its forward end comprised of surface 17 and annular shoulder 22 which locates the forward surface 7 of die 13 adjacent the sheet A. The sleeve has a central bore providing a cylindrical surface 23 that limits the upset head configuration of the rivet and also provides for concentric piloting of the piston. Orifice 15 of the die has an inner opening 18 (see FIGURE 2) which mates with the rivet shank diameter 24. The inner orifice surface of the die then continues as an arcuate portion 14 of increasing slope extending over a substantial portion of the inner surface of the die to insure minimum stress concentration; the orifice surface then continues outwardly and rearwardly as a tangent conical surface portion 9 of angle 11, illustrated as 45, extending to a back rearward surface 25 of the die. The orifice configuration is important to the forming of the upset head, it being desirable that a smooth, continuous Shape be used to prevent cracking of the driven head of the rivet and also to permit easier flow of the metal with lower forming loads. The forwardmost portion of the orifice at opening 18 is substantially cylindrical and the arcuate portion 14 is preferable a circular are, said circular are having a radius R whose ratio to the rivet shank diameter is approximately .20. The conical surface portion 9 preferably has an angle a measured from the longitudinal axis of the die which angle lies in the approximate range of 15 to 45. Outer surface 12 of the die 13 and mating surface 17 of the sleeve 16 can be cylindrical or slightly tapered as indicated by dashed line 12' in FIG- URE 2 to ease assembly and disassembly of the sleeve and die.

FIGURES 3 and 3A show the configuration of the riveted assembly after the shank has been upset. The dashed lines represent the before riveted condition of FIGURE 1. Uniform swelling of the holes 26 and 27 in sheets A and B, respectively, has occurred, and the die cylindrical opening 18 has expanded with the hole 26 so that there are no sharp steps or other discontinuities between the

holes

18 and 26 as expanded. This is an important feature of the invention. It is preferred that opening 18 of the die is equal to or smaller than hole 26 before upset; the dashed line 26' in FIGURE 2 indicates a condition wherein the hole in sheet A is initially larger than the die opening 18. The die 13 is restrained from radial expansion during rivet upset by the sleeve surface 17. The die inner opening 18 matches the expanded hole 26 after rivet upset in such a way as to maintain essentially cylindrical configuration of the holes 26 and 27 over their entire length. This matching is achieved principally by restraining the outer surface of the die and permitting the inner throat portion of the die to plastically deform under the loads transmitted from the rivet as it is progressively formed. In many instances, driven heads of non-ductile rivet alloys expand in a non-circular, oval type configuration in the fillet region of the upset head. Although the die is restrained by the sleeve, and is of minimum thickness to provide the required stiffness to match the hole expansion in the sheet, sufiicient plasticity of the die material is provided, so that the die orifice can conform to this non-circular driven head shape in the fillet region, thereby resulting in lower stresses during head formation and minimum head cracking. In addition, it has been found that the die material yield strength should preferably be equal to that of the sheet structure or the rivet, or lie between that of the sheet structure and the rivet in order to possess the desired plasticity and that the die outer diameter should be minimized to permit control of plastic deformation of the die by the restraining sleeve, and to effectively confine the die volume under the rivet head during upset.

For example:

(21) Using 2024-T3 aluminum sheet and a stronger rivet of 7075T73 aluminum alloy, the die can be either of the above 2024 or 7075 alloys.

(b) Using 7075T6 aluminum sheet and a stronger rivet of Titanium 6AL4V annealed material, the die can be either of the above 7075 or Ti 6AL4V alloys.

(0) Using Titanium 6AL4V annealed sheet material, and a stronger n'vet of Titanium 6AL4V solution treated and aged (S.T.A.) material, the die can be either of these titanium alloys or A-286 stainless steel alloy of comparable strength.

Since only the die remains with the sheet structure after the rivet head is formed, minimum weight associated with minimum volume is also achieved. It is to be noted that this minimum radial thickness die does not possess sufficient hoop strength to resist circumferential plastic expansion of its outer diameter without application of the sleeve external restraint during rivet upset forming. It has been found that a maximum ratio of the die outer diameter to the rivet shank diameter should preferably be not in excess of about 1.75 in order to permit effective control of the plastic adjustment of the die internal opening 18, to match the expansion of the adjacent structural hole. Depending on the softness of the structural material to resist axial bearing stresses from the die, ratios as low as 1.4 to 1.5 can be used. As the rivet shank is upset to form the head, the die orifice is permitted to plastically deform to match the fillet shape of the head, and the contact area between the forming head and the die orifice continually increases as the upset loads increase until substantially full containment of the orifice portion by the final head shape occurs. Containment of a die of small mass permits a broad selection of materials and strength properties. If a die of too large a diameter is used, excessive yielding of the die internal surface can occur (whether or not a sleeve is used to restrain the die outer diameter), thereby rendering the use of such die ineffective in the prevention of high, local deformation of the hole in the sheet. On the other hand, if either a small, restrained die or a large diameter unrestrained die of material yield strength substantially greater than the rivet is used in order to. prevent or limit the yielding of the orifice inner diameter, then discontinuities in the rivet shank can occur between the structure and the die as a result of greater expansion of the shank into the softer structural material directly adjacent the die orifice, thereby precluding the uniform cylindrical expansion of the rivet.

It can also be seen in FIGURES 3 and 3A that both the orifice and the upset head shapes have mutually adjusted essentially to the configuration shown. This important feature, along with the curvature at the arcuate portion of the die orifice, has been found to be of importance in preventing cracks from occurring in high strength rivet alloys of low ductility. It can therefore be seen that this orifice shape aids the extrustion process which occurs in the most common method of riveting, wherein the plastic flow of the material from the forming head during upset is caused to flow into the structural holes with the minimum stresses in the fillet region of the upset head. It is also important to note that during the upset of the head, the die expands axially so that surface 25 approaches and may engage shoulder 22 of the sleeve, and that the outer diameter of the upset head is limited by surface 23 of the sleeve under high upset loads. However, under low forming loads associated with softer sheet materials, the formed head need not fully expand to sleeve surface 23. Also forward surface 7 of the die bears against sheet surface 8, providing suflicient bearing area against the softer sheet during riveting compression forces as well as to satisfactorily resist the axial forces tending to separate the sheets under operating conditions.

The die orifice 15 provides for a smooth, continuous head fillet surface 28, and in association with surface 23, defines and controls the upset material so that head cracking is virtually prevented. Referring to FIGURE 2, it is noted that the diameter of the sleeve bore (surface 23) is substantially the same as the diameter of the outermost end 9 of the die orifice. This is provided to insure that there be no discontinuities between the head and fillet portion of the rivet and in fact, the inernal surface of the sleeve forms an additional die portion substantially coextensive with the outer, rear end of the extrusion die orifice. As illustrated in FIGURE 3A, the fillet surface 28 merges smoothly into the outer head surface of the rivet since it does not exactly conform to the slight disconinuity that may exist between the die orifice 9' and the sleeve surface 23.

It can be seen that the upset head is completely onclosed by piston surface 29 acting on the rear closure. This feature of complete enclosure precludes cracking of the upset head of a high strength rivet which might otherwise crack in a non-enclosed configuration. It is desirable that the upset head diameter 'be limited by the sleeve surface 23 to about 1.75 times the diameter of the rivet shank in order to preclude eccentric forming of the head, and in addition, to prevent external diametral cracking of the lower ductility alloys. This limitation also provides a minimum weight fastener while still retaining sufiicient bearing and shear strength area in the upset head.

Since the upset head diameter is essentially equal to that of the die outer diameter, a preferred embodiment such as shown in FIGURE 1 can be used wherein the die outer diameter at surface 12 is slightly larger than the upset head, thereby permitting a shoulder 22 of the sleeve to retain the die surface 7 against the adjacent sheet material.

FIGURE 4 shows a die 13a whose conical portion 9a of orifice 15a is of minimal slope angle a of approximately 15". Die 13a has an outside surface 12a which for a given rivet is essentially the same diameter cylindrical surface 12 of die 13 of FIGURE 1 and also has the same cylindrical opening 18a and arcuate curvature 14a as die 13. The shallow slope configuration of die 13a is used for extremely low ductility materials in order to permit head forming without cracking, to facilitate extrusion, and also to reduce the magnitude of the forming load.

FIGURE shows a die 32 to be used only for more easily formed rivet materials of higher ductility wherein slope angle a (reference FIGURE 2) can be increased to 90 as represented by surface 34 in FIGURE 5, i.e. surface 34 is perpendicular to the axis of the die. For a given diameter rivet, the diameter of cylindrical surface 12b is again essentially the same as diameter of surface 12 of die 13, and cylindrical opening 18b and arcuate curvature 14b are essentially the same as opening 18 and curvature 14 of die 13 in FIGURE 2. Since the upset head does essentially conform to this contour of maximum slope, maximum tensile capability of the fastener is obtained, essentially equal to or greater than that of a bolt of the same material and shank diameter.

FIGURE 6 shows an arrangement of the die 35 and sleeve 36 such that end surface 37 of the sleeve bears directly against the die shoulder surface 38 providing compressive preload to the sheet. Although the die orifice shape is similar to that in FIGURE 1, this shoulder arrangement can be used with other die orifice shapes. This alternate arrangement eliminates the internal recess at the forward end of the sleeve 16 in FIGURE 1.

FIGURE 7 shows another arrangement of a sleeve 40 and a die 39 having an orifice shape similar to that of die 13 of FIGURE 1. Surface 41 of sleeve 40 in this embodiment is spherically shaped to provide compressive preload to the sheets by hearing against like surface 42 of die 39. Surface 41 in this case also controls plastic deformation of the die under shank upset loads as in previous embodiments. Surfaces 41 and 42" are spherical to permit small angular variations between the tool axis and the rivet axis and a gap 43 is provided between sleeve 40 and the surface of sheet A to permit such variations.

A typical riveting operation is illustrated in FIGURE 8 wherein a piston is slidable in an outer sleeve 51, which is forced toward the member A by force F supplied by a suitable reversible power drive (not shown) connected to the extension 51a on the sleeve 51. The piston 50 is power driven by force F of a drive means to upset the end of the shank around the die 55 while the pre formed head is held stationary by means producing force F on tool 54. When the sleeve 51 is moved rearwardly by the reversible power drive with a force F.,,, the sleeve is removed from around the die while the piston exerts a force F;; on the rivet in the opposite direction. It is understood that the rivet can be impact. formed as well as squeeze formed. It is obvious that the magnitude of force F exerted by the piston must be reduced before the sleeve 51 is stripped from the die since otherwise, hoop tension failure of the die will result.

Although certain preferred embodiments have been shown and described, various modification will become apparent, in the light of this disclosure, and the invention should not therefore be deemed as limited.

What is claimed is:

1. A fastener for joining together structural members which have circular holes therein in aligment comprising:

a rivet of higher strength than said structural members and having a shank for insertion through said holes with the inserted end being adapted to be upset to form a head;

a die having an orifice therein terminating in an opening at its forward surface, said die being adapted to be placed around the shank of the inserted rivet with said forward surface adjacent one of said structural members; the diameter of said opening being substantially the same as that of said shank;

said orifice comprising a convexly arcuate surface extending rearwardly with increasing slope relative to the axis of the die and away from said opening, and a further surface of continually enlarging diameter ex tending rearwardly from said arcuate surface defining a smooth continuous surface contour;

said orifice acting to shape the upset end of said shank and provide a smooth continuous fillet surface for said head;

said die having an outer circumferential surface shaped for receiving an external circumferential restraint resisting radial expansion of said outer surface under shank upset loads, the hoop strength of said die being insutficient to prevent plastic radial expansion of said outer surface in the absence of said circumferential external restraint during upsetting of said head;

the material of said die adjacent said orifice experiencing plastic flow during upsetting of rivet material into said holes in the structural members so that after formation of said head, the opening of said orifice adjacent said one of said structural members matches the expanded hole in said structural member without substantial discontinuity therebetween;

said die thereby preventing high local deformation of the material of said one structural member on the side thereof adjacent said die.

2. A fastener as defined in claim 1 wherein the maxi mum outer surface diameter of said die is not in excess of about 1.75 times the diameter of the rivet shank.

3. A fastener as defined in claim 1 wherein the yield strength of the material from which said die is made lies in a range defined by the yield strength of the structural material of said members and the yield strength of the rivet material.

4. A fastener as defined in claim 1 wherein said further surface comprises a continuation of said arcuate surface until it becomes substantially perpendicular to the axis of the die so that no discontinuity exists which might cause high stress concentration areas to develop on the upset head of said rivet, said die and upset head being so proportioned to take an axial load of the same order as the tensile strength of the shank.

5. A fastener as defined in claim 1 in which the opening in said die is no larger than the hole in the said one structural member before upset of said shank.

6. A fastener system for joining together structural members which have circular holes therein an alignment comprising:

a rivet having a shank for insertion through said holes with the inserted end being adapted to be upset to form a head;

a die having an orifice therein terminating in an opening at its forward surface, said die being adapted to be placed around the shank of the inserted rivet with said forward surface adjacent one of said structural members, the diameter of said opening being substantially the same as that of said shank;

said orifice comprising an interior surface for shaping the inserted end during upset of said end to form said head;

said die having an outer circumferential surface shaped for receiving an external circumferential restraint resisting radial expansion of said outer surface under shank upset loads, the hoop strength of said die being insufiicient to prevent plastic radial expansion of said outer surface in the absence of said circumferential external restraint during upsetting of said head;

said die thereby preventing high local deformation of the material of said one structural member on the side thereof adjacent said die;

a sleeve having an interior surface portion at one end conforming in configuration with said outer circumferential surface of said die and adapted to be located adjacent said outer surface and around said die for exerting said circumferential restraint against said die and preventing radial enlargement of said outer surface under shank upset loads;

means in contact with the inserted end of said shank, and means in contact with the opposite end of said shank, said contacting means being adapted to produce axial compression forces on said shank to upset said shank.

7. A fastener device as defined in claim 6 wherein said one end of said sleeve bears against said one structural member to provide a preload compression force between said structural members, said sleeve having an internal surface portion for locating and holding said die adjacent said one structural member before and during upsetting of said shank.

S. A fastener device as defined in claim 6 wherein said one end of said sleeve bears against a bearing portion of said die to produce a preload compression force between said structural members during upset of said shank and to locate said die adjacent said one structural member before and during upsetting of said shank.

9. A fastening device as defined in claim 6 wherein said outer surface of said die comprises an outer spherical surface portion, said interior surface portion at said one end of said sleeve having an inner spherical surface conforming with said outer spherical surface portion of said die, said outer spherical portion bearing against said inner spherical portion to produce a preload compression force between said structural members during upset of said shank and to locate said die adjacent said one structural member before and during upsetting of said shank.

10. A fastening device as defined in claim 6 wherein said outer surface of said die comprises an outer conical surface portion, said interior surface portion of said sleeve comprising an inner conical surface conforming with the conical surface portion of said die.

11. A fastening device as defined in claim 8 wherein said bearing portion of said die comprises an outer flange on said die, said end of said sleeve having an end face conforming in shape with the surface of said flange.

12. A fastening system as defined in claim 6 wherein said sleeve has an internal surface extending from the axially rearwardmost surface of the die for limiting the outside diameter of the upset head and having substantially the same diameter as the rearwardmost portion of said die orifice so that said internal surface of the sleeve forms a die portion substantially co-extensive with the rearwardmost end of the die orifice.

13. A fastening system as defined in claim 12 wherein said internal surface of the sleeve is of a maximum diameter not exceeding about 1.75 times the shank diameter so as to thus limit the maximum diameter of the upset head.

14. A fastener device as defined in claim 12 wherein said inserted end contacting means comprises a piston guided by said internal surface and having a driving face for upsetting the end of the shank to produce said upset head, the upset end of the shank being continually located in the space defined by said driving face of said piston, said internal surface of the sleeve and the orifice of said die during upsetting of the shank to form said rivet head.

15. The method of forming a fastener for a structure consisting of a plurality of structural members having aligned circular holes therethrough for receiving a rivet comprising the steps of:

inserting through said holes the end of a rivet shank of higher strength than the structural members with the inserted end being adapted to be upset to form a head;

placing a die adjacent one of said members and around said shank to provide an orifice for shaping the end of said shank during upset thereof;

placing a sleeve in engagement with the outer circumference of said die to provide a circumferential restraint on said die and thereby prevent plastic expansion of the outer circumference of said die during upset of said shank;

applying an axial force to said sleeve in order to compressively preload said structural members against one another;

applying a compressive force to said shank to upset said shank and to expand the diameter of said holes and said orifice and to form an upset rivet head on said shank; and

removing said sleeve from said die by exerting a force on said sleeve in the opposite direction to said axial preload force after said compressive upset force has formed said head.

16. The method of forming a fastener as defined in claim 15 wherein said compressive force applying step comprises moving a piston coaxially with said die and against the inserted end of the shank.

17. A method of forming a fastener as defined in claim 16 wherein the piston is moved within the sleeve a sufficient distance to upset the inserted end of the shank into engagement with said sleeve which thereby limits the diameter of the upset rivet head.

18. A fastener as defined in claim 6 wherein said orifice surface comprises a convexly arcuate surface portion extending rearwardly in the direction away from said opening with increasing slope relative to the axis of said die and a further surface portion of continually enlarging diameter extending rearwardly from said arcuate surface portion.

19. A fastener system as defined in claim 6 having:

means for applying a force to said sleeve to provide a preload force in the direction of said structural References Cited members for forcing said structural members tovvard UNITED STATES PATENTS one another, said prcload force belng transmltted from said sleeve to the surface of said structural 21482391 9/1949 Webster 85-47 ge g i a q t 5 FOREIGN PATENTS sa1 mser e en con ac mg means COII'IPIlSllTg a p1s on and means for producing a force on said piston to 1 gel-many apply said axial force to said rivet shank; ermany' said opposite end contacting means comprising means for applying a reaction force on the end of said rivet 10 CARL TOMLIN Pnmary Examiner opposite the inserted end for opposing said preload R. S. BRITTS, Assistant Examiner. and piston forces during upset of the end of the Us CL shank. 29--509, 522