In the past, we have written about rivet nut threaded inserts as an elegant way to add a threaded fastener to soft substrates. However, there is more than one way to repair a stripped tap hole or add a strong, lasting threaded insert to soft materials. Coil thread inserts provide a strong threaded wall that can be used in virtually any type of hole to accommodate a bolt or threaded fastener.
As the name implies, a coil thread insert is a wound coil that is screwed into place to provide a strong footing for a bolt thread. The coil is inserted by screwing it into the hole. The width of the insert should be slightly larger than the hole to make a tight fit. When a right-sized threaded bolt is inserted in the hole, the pressure of the bolt pushes the coil against the walls of the hole to form a strong joint. In many applications, such as with wood or plastic, a coil insert is as strong or stronger than a rivet nut threaded insert.
Coil Thread Inserts Are Used Everywhere
Coil thread inserts are particularly useful for reinforcing threaded holes in softer materials. Aluminum, for example, is a great material for aerospace applications and boatbuilding because it is strong yet lightweight. However, aluminum is also soft, so it doesn’t handle steel fasteners well. Plastics also have become a universal manufacturing material, but most plastics are soft, so coil inserts are commonly used with plastics for applications such as electronics manufacturing. The same is true for fiberglass and softer woods such as pine or composites.
Coil thread inserts are also valuable when used with harder materials. For car repair, for example, coil inserts can be used to repair stripped cylinder heads or to repair exhaust systems. They also have become commonplace in construction and are used to host threaded fasteners in building frames and for concrete foundations because they won’t weaken with time.
A Coil Insert by Any Other Name …
The industry has other names for the coil thread insert. They are sometimes called wire inserts or helically wound inserts, but the most common nickname is “helicoil.” Heli-Coil® is actually a registered trademark of the STANLEY Engineering Fastener division and is specific to their wire threaded insert products, but the term helicoil has become generic, like Xerox for photocopies or Kleenex for facial tissues.
Of course, there are other quality coil thread insert manufacturers, such as Kato Fastening Systems and Arconic Fastening Systems. Each manufacturer offers inserts designed for different types of applications.
Kato is known for making coil thread inserts used in the aerospace industry. For example, they offer an Inconel X-750 insert made of nickel-chromium alloy hardened using aluminum and titanium so it can withstand temperatures up to 1,000 degrees Fahrenheit. They also make phosphor bronze inserts for saltwater applications, inserts with silicon and manganese added to reduce friction, and nickel-chromium-cobalt alloy coil thread inserts for extreme pressure and heat.
Arconic also makes coil thread inserts from Inconel X-750, phosphor bronze, stainless steel, and other materials. Their inserts are used in applications such as ships, power systems, transportation, and electronics, and some of their coil inserts feature lubricants and coatings as well for special applications.
Tanged or Tangless?
There are two basic types of coil thread inserts: tanged and tangless, which refers to how the insert is screwed into the hole.
The tang is a wire extension at the end of the coil thread that protrudes into the center and serves as a way for coil thread insert tools to grasp the insert to screw it into the hole. Once installation is complete, the tang is broken off and discarded. Tanged insert coils are typically made of cold-rolled 304 stainless steel wire (AS7345) which has a hardness of RC 43-50 and a tensile strength of 200,000 psi. They are also extremely smooth with a finished surface that eliminates thread erosion.
Tangless or tang-free coil inserts do not use a tang but instead have a driving notch that is used as a grip point by the insert tool. Tangless inserts are becoming increasingly popular, especially for manufacturing applications, because they can be inserted quickly and easily and they leave no loose tang pieces that can get into the machinery.
Both tanged and tangless wire inserts come in two designs. Free-running coil thread inserts are designed so each coil adjusts independently to the bolt threads. This allows for maximum surface contact between the bolt, the insert, and the hole for the strongest possible bond. Screw-lock coil thread inserts are designed with straight segments in the coil so when a threaded fastener is inserted, those straight segments bow out to form a stronger bond that secures it in the hole and prevents it from turning.
You can use coil thread inserts in a wide variety of applications, and there is a design and material that will suit most applications. If you want to learn more about coil thread inserts, you can contact one of our fastener experts or download The Essential Guide to Coil Thread Inserts.