BLIND FASTENER FOR COMPOSITE MATERIAL

Abstract: A blind fastener assembly (18) suitable for use in joint assemblies including advanced composite material workpieces (10,12). The blind fastener assembly (18) includes a core pin (20) having an enlarged head (28), an expander (22) having a head (50) and a tapered nose portion (52) and being mounted to the core pin (20), and a frangible sleeve (24) mounted to the core pin (20) between the enlarged head (28) and the expander (22). The blind fastener assembly (18) is insertable through aligned holes (14,16) in the workpieces (10,12) so that the head of the expander abuts (50) the accessible side of the workpiece. Relative longitudinal movement of the core pin (20) and the expander nut (22) causes the sleeve (24) to expand over the tapered nose (52) and to break into two pieces, one of the two pieces expanding over the other of the two pieces to provide a large bearing surface on the blind side of the workpieces.

Description: BACKGROUND OF THE INVENTION The present invention relates to improvements in blind fasteners and the joints produced therewith. Advanced composite materials have recently been gaining increasing usage in various aerospace applications because they offer potential weight savings and increased stiffness. The anisotropic properties of composite materials present a unique opportunity to optimize certain structural designs such as elongated aircraft stringers and ribs. In order to form acceptable joints of relatively thin sheet composite materials, conventional blind fasteners have not proven to be completely acceptable. One reason is the low allowable bearing stress of composite materials which means that the full tensile strength of the blind fastener may not be realized because of bearing failure of the composite material. Present blind fasteners have a relatively limited blind side bearing area and typically expand to approximately 1.3 times the original sleeve diameter. Another reason is that composite materials are weaker in shear, thus causing any holes to require greater edge distances and more area buildup to develop full efficiency of the composite material. Advanced composite materials are constructed of a resin matrix which has a tendency to crack when holes in the material are expanded, such as by cold working or by an interference fit fastener. Rivets which swell when upset may also crack the resin matrix. Finally, fasteners which form against the blind side composite material surface tend to damage the relatively soft material surface by digging or gouging out some of the material during the bearing surface formation process. Additionally, fasteners which form against the blind side composite material surface may cause stress concentrations during the formation process which exceed the compressive strength limit of such materials, thus causing damage. In a joint assembly as known from DE-A-28 21 356 the diameter reduced intermediate portion between the two portions of the sleeve means becomes folded outwardly at the end of the forming operation. It forms a circular fold which serves to compensate thickness variations of the workpiece. The fold does not bred because it is located with a longitudinal distance from the tapered nose portion of the expander means. The bearing surface of the secondary sleeve portion is made by another circular fold. The formation of the bearing surface might cause damage in the workpiece blind side surface. The present invention is directed to an improved blind fastener assembly as well as to an improved joint assembly comprising such fastener assembly which has a large bearing area, which does not expand the holes in the respective workpieces in which the fastener assembly is installed and in which the formation of the blind side bearing surface is not sensitive to variations in grip length. A prior art design for a blind fastener providing an enlarged bearing area is disclosed in US-A-4,579,491. Therein a sleeve and a coil washer expand over a tapered nose to form the bearing surface. While the fastener does provide a large bearing surface and is insensitive to variations in grip length, the core pin of this fastener is necessarily smaller in diameter than may be preferred, therefore exotic high strength materials are sometimes required to provide this fastener with a preselected strength. Therefore the present invention is also directed to providing a blind fastener of the type described by having an increased diameter core pin. SUMMARY OF THE PRESENT INVENTION The present invention provides an improved blind fastener assembly and an improved joint assembly using the blind fastener assembly. The blind fastener assembly has a core pin, an expander, and a sleeve, each having an initial diameter which permits it to pass through a hole in a workpiece. The core bolt has a head at one end and a shank extending therefrom. The sleeve has a primary portion and a secondary portion, the secondary portion being provided with a frustoconical region interconnected with the main portion by a circular web of material. The sleeve is fitted over the shank of the core pin with the primary portion abutting the head of the core pin. The expander is provided with a tapered nose portion at one end and a head at its other end. The expander is mounted to the shank with the nose portion abutting the secondary portion of the sleeve. In use, the blind fastener assembly is passed through a hole in a workpiece, or aligned holes in two or more workpieces, until the head of the expander abuts the access side of the joint. Relative longitudinal movement of the core pin and the expander forces the tapered nose into the sleeve and causes it to expand. As the sleeve expands, the web between the primary and secondary portions of the sleeve breaks. When the secondary portion of the sleeve abuts the blind side of the workpiece, the continued relative longitudinal movement of the core bolt and the expander causes the primary portion of the sleeve to expand over the frustoconical region of the secondary portion of the sleeve until the primary portion also abuts the blind side of the workpiece. The primary object of the present invention is to provide an improved blind fastener assembly providing a large diameter core bolt and a large workpiece blind side bearing area and to provide a joint assembly having a minimum danger of damages to the blind side surface of the workpiece. It is another object of the present invention to provide a blind fastener assembly in which the formation of the blind side bearing surface is not sensitive to variations in grip length. It is yet another object of the present invention to provide a blind fastener assembly in which the blind side bearing surface will conform to the contour of the blind side workpiece surface. It is still yet another object of the present invention to provide an improved blind fastener assembly for use with composite materials. It is another object of the present invention to provide an improved blind fastener assembly providing a large bearing surface to abut against the blind side workpiece surface using a small number of inexpensive components. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings is: Figure 1 a cross sectional of a joint assembly prior to any forming operation; Figure 2 the joint assembly of Figure 1 wherein the blind fastener assembly is in a first intermediate position during a forming operation; Figure 3 the joint assembly of Figure 1 wherein the blind fastener assembly is shown in a second intermediate position during a forming operation; Figure 4 the joint assembly of Figure 1 wherein the blind fastener assembly is depicted in a fully installed condition, and Figures 5 through 8 partial sectional views of alternate examples of blind fastener assemblies. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figures 1 through 4 work pieces 10 and 12 are provided with aligned holes 14 and 16 accepting a blind fastener assembly 18. The blind fastener assembly 18 includes a core bolt 20, an expander nut 22, a sleeve 24 and an expandable protective washer 26. The core pin or core bolt 20 has an enlarged head 28 at one end thereof and includes external threads 30 disposed on a shank 32 extending from the enlarged head 28. Sleeve 24 includes a primary sleeve portion 34 fitted onto the shank 32 of the core pin 20 such as to abut the enlarged head 28 thereof. The sleeve 24 also includes a secondary sleeve portion 36 interconnected with the primary sleeve portion 34 by a reduced diameter web 38. The secondary sleeve portion 36 has a cylindrical region 40 and a frustoconical region 42 extending between the web 38 and the cylindrical region 40. The protective washer 26 is also fitted on the core pin 20 and abuts the cylindrical region 40 of the secondary portion 36 of the sleeve 24. The protective washer 26 has a flat annular portion 44 and an axially oriented flange portion 46 extending longitudinally from the radially most portion thereof. The flange portion 46 is proportioned such as to fit over at least part of the secondary portion 36 of the sleeve 24. The expander nut 22 has an internal thread 48 engagable with the external thread 30 of the core bolt 20. The expander nut 22 is also provided with a head 50 disposed at one end thereof and a tapered nose 52 at the other end thereof. Intermediate the head 50 and the nose 52 of the expander nut 22, the expander nut is provided with a cylindrical surface 54 having the same outer diameter as each of the flange 46 of the washer 44, the main portion 34 of the sleeve 24, and the head 28 of the core bolt 20. Appropriate rotation of the core bolt 20 with respect to expander nut 22 will draw the enlarged head 28 of the pin 20 toward workpieces 10 and 12, also drawing the sleeve 24, which is carried on the shank of the pin adjacent to the head 28, toward the expander nut 22. Pin 20 may alternatively include a plurality of circumferential grooves (not shown) in place of external threads 30 and the mating threads 48 on the expander nut 22. During installation the core pin 20 is caused to move axially so as to draw the head 28 of the pin 20 toward the expander nut 22 and, hence, workpieces 10 and 12. Referring to Fig. 2 as the core pin 20 is advanced through the workpieces 10 and 12, the washer 26 is brought into contact with the end 45 of the expander's tapered nose portion 52. Continued advancement of core pin 20 causes the nose portion 52 of expander nut 22 to force the washer 26 and the secondary sleeve portion 36 to expand over the tapered nose portion 52. During the expansion of the sleeve 24, the web 38 breaks, separating the sleeve into a primary sleeve 34' and a secondary sleeve 36'. Continued movement of the head 28 and the sleeve 24 toward the workpieces 10 and 12 causes primary sleeve 34' to expand over the frustoconical region of the secondary sleeve 36' as is shown in Fig. 3. The washer 26 is opened by the leading edge 35 of the primary sleeve 36' and of workpiece 10 (Fig. 4). Advancement of the core pin 20 through the workpieces 10 and 12 is continued subsequent to the seating of washer 26 against workpiece 10, until the desired preload is reached. Thereupon, the shank 32 of the core pin 20 may fracture at a suitably dimensioned breakneck groove, not shown but well-known in the art. As shown in Fig. 5 through 8, minor alterations to the sleeve 24 and the washer 26 may be made to facilitate the expansion of the members and the opening of the washer. The washer 26 may be provided with a recess at inner diameter 56 as shown in Figures 5, 7 and 8 or a bevel 56, as shown in Figure 6 facilitating entry of the nose 52 into the center of the washer. The sleeve 24 may be provided with a bevel 58 as depicted in Figures 6, 7 and 8 facilitating entry of the nose into the center of the sleeve. A bevel 60, illustrated in Figure 5, or a beveled notch 60a illustrated in Figure 6 may be provided on the leading edge of the primary portion 34 of the sleeve 24 to cooperate with the flange 46 of the washer 26 such as to smoothly open the flange to form a flat bearing surface. Another embodiment of the washer 26a is shown in Fig. 7 wherein the washer is formed of a ring of resilient material. An alternated form of washer 26b is shown in Fig. 8 and is formed in the shape of a helical spring having two turns. While a washer having two turns is shown, it should be understood that more or less than two turns could be used. The spring is designed so that when it is fully expanded, there will be no overlap of its ends, thus presenting a flat bearing surface to the blind side of the workpiece 10. In Fig. 6 an annular recess 64 is formed on the inner wall 62 of the primary portion 34a of the sleeve 24 at a location adjacent the web 38a. During installation of the blind fastener assembly 18 using the sleeve 24 with the primary portion 34a the primary portion 34a will begin to advance over the secondary portion 36a due to the folding of the web 38a even before the web tears, thus facilitating the proper formation of the blind side bearing surface. For example the washer 26 may be completely eliminated for some types of installations.

Claims: A blind fastener assembly (18), adapted to be installed in a workpiece, said blind fastener assembly (18) comprising: a core pin (20) including a shank portion (32) and an enlarged head (28) at one end thereof; expander means (22) including an enlarged head (50) at one end thereof, a tapered nose portion (52) at the other end thereof, and a shank portion therebetween, said expander means (22) having a bore therethrough adapted to receive said shank portion (32) of said core pin (20); expandable sleeve means (24, 24a) carried on said core pin shank portion (32) between said nose portion (52) of said expander means (22) and the head (28) of said core pin (20), said expandable sleeve means (24, 24a) consisting of a primary sleeve portion (34, 34a) abutting said head (28) and of a secondary sleeve portion (36) facing said head (50) of the expander means (22) and having a reduced diameter intermediate portion between said primary and said secondary sleeve portions (34, 34a, 36), said secondary sleeve portion (36) having a cylindrical region (40), said expandable sleeve means (24, 24a) being operably engaged with said core pin (20) such that said core pin (20) causes said sleeve means (24, 24a) to advance in a direction towards the expander means head (50), said sleeve means (24) being adapted to expand over said expander means tapered nose portion (52), characterised in that said reduced diameter intermediate portion is an annular web (38, 38a) interconnecting said primary and secondary sleeve portions (34, 34a, 36), that said secondary sleeve portion (36) has a frusto-conical region (42) extending between said web (38, 38a) and said cylindrical region (40), so that said web (38, 38a) breaks as said sleeve means (24) expands and separates said sleeve means (24, 24a) into detached primary sleeve (34') and secondary sleeve (36'), and that under continued relative longitudinal movement of said core pin (20) and said expander means (22), said expander means (22) causes said primary portion (34, 34a) to expand over said frusto-conical region (42) of said secondary sleeve portion (36). The blind fastener assembly of claim 1, characterised by resilient washer means (26, 26a, 26b) carried on said core pin shank portion (32) between said expander means (22) and said expandable sleeve means (24, 24a), said expandable sleeve means (24, 24a) being adapted to force said washer means (26, 26a, 26b) to expand over said expander means (22) and over said tapered nose portion (50). The blind fastener assembly of claim 2, characterised in that said resilient washer means (26b) is a coil spring which unwinds as it is expanded. The blind fastener assembly of claim 2, characterised in that said resilient washer means (26) comprises an annular washer having a longitudinally extending outer flange (46), said outer flange (46) being opened to form a bearing surface by said primary sleeve portion (34) as said primary sleeve portion (34) is expanded over said secondary sleeve portion (36). The blind fastener assembly of claim 1, characterised in that said core pin (20) is threadably engaged with said expandable sleeve means (24, 24a), so that rotation of said core pin (20) causes advancement of said sleeve means (24, 24a) in a direction towards the enlarged head (50) of said expander means (22). The blind fastener assembly of claim 1, characterised in that the end of said sleeve means (24, 24a) facing said head (50) has a recessed region (58) proximate said core pin shank portion to facilitate engagement and expansion thereof by said expander means tapered nose portion (52). The blind fastener assembly of claim 2, characterised in that said washer means (26) has a chamfered interior edge (56a) adjacent the surface of said core member shank portion to facilitate expansion of said washer means (26) by said expander means nose portion (52). The blind fastener assembly of claim 1, characterised in that an internal annular recess (64) is formed in said primary sleeve portion (34a) of said sleeve means (24a) adjacent said reduced diameter web (30a). The blind fastener assembly of claim 8, characterised in that said internal annular recess (64) formed in said primary sleeve portion (34a) of said expandable sleeve means (24a) serves that said web portion (38a) folds as said sleeve means (24a) is expanded over said tapered nose portion (52) such as to facilitate the expansion of said first sleeve portion (34a) over said secondary sleeve portion (36) of said expandable sleeve means (24a). A joint assembly comprising two workpieces (10,12) firmly held together by a blind fastener assembly (18) according to claim 1, said blind fastener assembly being inserted through holes (14, 16) in said workpieces. The joint assembly of claim 10, characterised by resilient washer means (26, 26a, 26b) carried on said core pin shank portion between said expander means (22) and said expandable sleeve means (24, 24a). The joint assembly of claim 11, characterised in that said resilient washer means (26b) is a coil spring which unwinds as it is expanded. The joint assembly of claim 11, characterised in that said resilient washer means (26) comprises an annular washer having a longitudinally extending outer flange (46), said outer flange (46) being opened to form said bearing surface against said workpiece blind surface by said primary sleeve portion (34, 34a).

Inventors: STROBEL, BURTON C.,
Applicants: SPS TECHNOLOGIES, INC.
Application year: 1988
Main classification: F16B19
IPC codes: F16B19