The present invention relates to fastener inserts and, more particularly, to helically coiled titanium wire fastener inserts which are useful in both substrates having tapped holes and nut assemblies.
Fastener assemblies come in a variety of shapes, sizes, designs and materials. Many fastening assemblies include not only a fastener such as a bolt, pin or screw, but also will include a fastener insert to be positioned within a tapped hole of a substrate or threaded nut barrel. One specific type of fastener insert useful in association with a threaded fastener is the helically coiled wire insert as described in U.S. Pat. No. 2,672,070 entitled xe2x80x9cWire Coil Screw Thread Insert for Molded Materialxe2x80x9d.
Generally speaking, tapped threads are strengthened due to the inherent flexibility of such helically coiled wire inserts since the insert provides a more balanced distribution of dynamic and static loads throughout the length of thread engagement. This flexibility also compensates for variation in lead and angle error.
Since helically coiled inserts generally do not exhibit crimping, staking, locking or swaging and do not require keying in place, helically coiled wire inserts greatly reduce stress which would otherwise be transferred to the receiving substrate. While such helically coiled wire inserts are generally useful as anchoring mechanisms for threaded fasteners, in order to be used in high strength applications, such inserts must be formed from high strength materials. Heretofore, 302/304 stainless steels have been used to manufacture fastener inserts.
The use of stainless steel inserts in fastener assemblies wherein the nut and/or fasteners are formed from other alloys leads to certain perceived problems such as corrosion, generally, and galvanic corrosion, in particular. By the phrase xe2x80x9cgalvanic corrosionxe2x80x9d, it is meant the electrochemical corrosion resulting from the current caused in a galvanic cell between two dissimilar metals in an electrolyte because of the difference in potential (emf) of the two metals.
Stainless steel fastener inserts have been coated with zinc chromate in an effort to prevent galvanic corrosion. However, application of the zinc chromate requires strict quantitative controls and is considered labor intensive. The application of too much zinc chromate can restrict movement. Additionally, the installation tools would require frequent cleaning to prevent build up of the zinc chromate on mandrels of the tool which is undesirable. The application of too little zinc chromate leads to certain other problems such as inadequate corrosion protection, for example.
Another recent approach to preventing galvanic corrosion in tapped holes is described in co-pending U.S. application Ser. No. 09/356,988. According to this document, improvements in preventing galvanic corrosion were seen as a result of coating fastener inserts made from stainless steel with a fluoropolymer composition. The additional step of applying a coating, sealant or plating to the insert can lead to added expense in the manufacturing process.
It is therefore a primary object of the present invention to provide titanium alloy fastener inserts which tend to limit, if not eliminate, galvanic corrosion in fastener assemblies.
Still another object of the present invention is to provide titanium alloy fastener inserts which are significantly lighter in weight than similarly sized and shaped stainless steel fastener inserts, e.g., improved strength to weight ratios.
To accomplish these objectives, among others, the present invention relates to a fastener inserts including a body formed from a titanium alloy comprising at least about 50.0 wt. % titanium, preferably 60.0 wt. % titanium, and, still more preferably, at least about 70.0 wt. % titanium. In addition to the titanium component, the alloy may include one or more of nitrogen, carbon, hydrogen, iron, oxygen, aluminum, vanadium, tin, ruthenium, palladium, cobalt, molybdenum, chromium, nickel, niobium, zirconium, silicon, hafnium, bismuth, yttrium, copper, tantalum, boron, manganese and tungsten.
As the alloy is being formed into a wire, the wire may be shaped to a desirable cross-sectional geometry and subsequently coiled. After allowing the wire to cool to approximately 23xc2x0 C., the wire may be heat treated as will be described in greater detail below to obtain specific tensile hardness and torque values. The resulting fastener inserts should have excellent corrosion resistance, be lightweight and provide superior strength.