Installation system for securing workpieces of composite materials and the like and threaded fastener for such system

A fastener for securing together a plurality of workpieces of a composite type material and adapted to be located in aligned bores in such workpieces with a preselected interference fit and with the fastener adapted to secure such workpieces varying in total thickness from a preselected maximum to a preselected minimum total thickness, said fastener comprising:

• a pin member having an elongated shank and an enlarged head at one end,
• said pin member having an enlarged diameter smooth shank section adjacent said pin head,
• said pin member having a threaded section extending generally from said smooth shank section to the opposite end of said pin member,
• a breakneck groove section located within said threaded section to define a frangible portion at the free end of said threaded section,
• said breakneck groove section being the weakest section of said pin member and adapted to fracture at a preselected load of generally low magnitude,
• said threaded pin section having a maximum diameter less than the diameter of said smooth shank section,
• a thin gauged expansion sleeve having a tubular portion with an enlarged annular flange at one end,
• said sleeve having an inside diameter greater than said maximum diameter of said threaded pin section and less than the diameter of said smooth shank section,
• said tubular portion of said sleeve having a length no less than around the maximum total thickness of the workpieces, and having an outside diameter permitting assembly of said sleeve, with said pin member attached, into the workpiece bores,
• a nut member having a threaded nut section at one end and an enlarged head portion at its opposite end, said enlarged head portion having an enlarged counterbore having a diameter greater than the outside diameter of said tubular portion of said sleeve, said counterbore having an axial depth at least equal to the maximum extension of said sleeve tubular portion beyond the workpieces for workpieces of minimum total thickness, said threaded nut section having a preselected axial length, said frangible portion of said threaded section having an axial length no less than around one half of the axial length of said threaded nut section, said nut member having a preselected overall axial height,
• said pin member having the length of that portion of said pin shank from said pin head to said breakneck groove section being no greater than around the sum of the maximum total thickness of the workpieces, said preselected height of said nut member and the thickness of said sleeve flange,
• said threaded nut section of said nut member having threads formed for threaded engagement onto that portion of said threaded section on the pin head side of said breakneck groove section,
• said frangible portion having a thread form adapted to be threadably gripped for applying a relative axial, pulling force between said pin member and said sleeve,
• said breakneck groove section having an axial strength at least equal to the maximum axial load required to pull said pin member fully into said sleeve with said smooth pin shank section expanding said sleeve into the workpiece bores to provide the preselected interference fit, said breakneck groove section having an axial strength no greater than around twenty percent (20%) greater than said maximum axial load whereby said frangible portion of said shank can be severed from said shank at a minimum axial load, and
• a flat washer means having a clearance fit on the end of the sleeve for engagement with the exposed face of the workpiece assembly, whereby the workpiece assembly faces are captively retained between the washer means and sleeve flange,
• said sleeve flange and flat washer means being adapted to non-rotatably engage the exposed faces of the workpiece assembly while the pin is being drawn into the sleeve, whereby said flange and washer inhibit delamination of the workpiece composite materials,
• said nut member being adapted to be torqued onto said pin member to pull said pin member into said sleeve to cause said radial expansion and to be continued to be torqued to apply the final desired clamp load.

In association with a workpiece assembly comprising facially-engaged composite material workpieces having aligned holes therethrough defining an annular hole surface:

• the improvement comprising a sleeve-expansion pin having an enlarged head, a smooth-surfaced shank section, and threaded shank section;
• a freely-movable sleeve having a radial flange at one end, and a length that enables its other end to extend beyond an exposed face of the workpiece assembly when said sleeve is slidably inserted into the aligned holes in the workpiece assembly;
• an annular flat washer having a clearance fit on the end of the sleeve and being in engagement with the exposed face of the workpiece assembly, whereby the workpiece assembly faces are captively retained between the washer and sleeve flange, and
• a rotatable nut member threadable onto the threaded section of the pin to draw the pin into the sleeve;
• the diameter of the smooth-surfaced shank section of the pin being slightly greater than the freely-movable sleeve internal diameter, whereby when the pin is drawn rectilinearly into the sleeve said sleeve is expanded into an interference fit with the annular hole surface;
• said sleeve flange and said flat washer means being adapted to non-rotatably engage the exposed faces of the workpiece assembly while the pin is being drawn into the sleeve, whereby said flange and washer inhibit delamination of the workpiece composite material;
• and
• a breakneck groove formed in the threaded section of the pin to define a breakneck groove section, whereby an exposed section of the pin can be removed after the pin has been fully drawn into the sleeve, the axial strength of said breakneck groove section being no greater than around twenty percent (20%) greater than said maximum axial load twenty said frangible portion of said shank can be severed from said shank at a minimum axial load.