What is the difference between a coil thread insert and a Heli-Coil? Actually, there is no difference. Just as Kleenex is commonly used to refer to facial tissue and Xerox has become synonymous with photocopies, “helicoil” has become a commonly recognized term for screw thread inserts. Heli-Coil® is a registered trademark of Stanley Engineered Fastening, a division of Stanley Black & Decker. Its design has become the industry standard for coil thread inserts.
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Coil thread inserts, also known as helically wound inserts, are coiled wire inserts placed inside the drill hole to create a strong threaded surface to secure a bolt or other threaded fastener. When a drill hole has been stripped or the material is too soft to secure a drilled fastener, as can be the case with magnesium, wood, aluminum, zinc, or plastics, the coil provides an extremely strong and lasting bond by transferring the pressure exerted from the bolt to the sides of the parent material.
The helically wired coil is screwed into place and creates a threaded track for the screw threads of a bolt or fastener. The diameter of the coil is wider than the tapped hole, so as the bolt is screwed in, the coil compresses against the side of the tapped hole. This compression creates a safe and highly secure connection—much stronger and with more torque than similar assemblies. In fact, when used for repairs, coil thread inserts create a stronger bond than the original tapped hole.
With ordinary bolt-and-hole inserts, the top two screw threads carry 65-75 percent of the load. Helically wound inserts distribute the load so each coil shares part of the load across the length of the coil. The result is a stronger connection and longer thread life, even after assembling and disassembling multiple times.
Coil thread inserts are especially useful in applications that have to withstand stress, severe vibrations, and extreme temperatures. Even under extreme conditions, these inserts minimize wear of the threaded hole and increase the integrity and useful lifespan of the connection. That’s why coil thread inserts have become a popular fastener solution for critical applications such as automotive manufacturing, aerospace, railways, factory machinery, and transportation systems.
Coil thread inserts are quite versatile and available in a wide range of thread diameters and thread sizes. They also come in a variety of materials. Stainless steel coil thread inserts, for example, are resistant to corrosion and extend the useful life of the fastener. They also can withstand temperatures ranging from -320 degrees to +800 degrees Fahrenheit because the coils can expand and contract with temperature changes.
Tanged coil thread inserts have a straightened piece of coil at one end that extends across the radius of the coil. This piece is called a “tang” and is used as an anchor during installation. A special helical insert tool grips the tang and uses it to screw the coil into the tap hole. Once the coil is inserted and secured, the tang is broken off and removed.
CoilThread tanged inserts are extremely reliable and long-lasting. CoilThread is made from cold-rolled 304 stainless steel wire (AS), and work-hardened to a rating of Rc 43-50 and a tensile strength that exceeds 200,000 psi. More importantly, the finished surface is extremely smooth (8-16 microinches), which virtually eliminates thread erosion caused by friction. CoilThread is also available in other materials, such as phosphor bronze, Inconel X-750, Nitronic 60®, Nimonic 90®, and titanium, as well as stainless steel. They also come with special coatings for specific applications.
Tanged coil thread inserts fall into two basic subcategories:
Both free-running and screw-lock tanged helical inserts come in UNF sizes (measured in inches) and metric versions.
Tang-free coil thread inserts, as the name implies, do not have a tang at the bottom of the coil. Instead, there is a driving notch strategically located in the coil that serves as a grip point for the helical coil insert tool. Tang-free coils are identical to tanged coil thread inserts, other than lacking a tang, and are available in free-running and screw-lock configurations.
Increasingly, manufacturers are moving to tang-free coil thread inserts because they are just as easy to install and they eliminate the need to remove the tang. This removes the risk of loose tangs or pieces of metal getting into the machinery, and it speeds up installation and inspection time. Some helical coil makers estimate you can save between $0.15-$0.50 per hole by standardizing on tang-free coil thread inserts. These helically wound inserts are especially popular with electronics manufacturers and high-tech manufacturing—wherever smaller, high-precision inserts are used.
Tang-free coil thread inserts are available with the same free-running and screw-lock designs as tanged coil thread inserts, with all of the same advantages. They are made out of work-hardened steel and other materials for maximum strength with minimum friction. They also are available with custom coatings. All tang-free coil thread inserts are available in UNF (inches) and metric sizes.
Additionally, all coil thread inserts come in either bulk or strip-fed versions, where the inserts are positioned in a plastic ribbon which is passed through an assembly nozzle. Strip-fed coil thread inserts are used in high-volume production applications with tooling that accepts strip-feed inserts. All types can be inserted by hand or using a strip feed. With tangless coil thread inserts, ribbon-fed inserts can increase the number of installation cycles with less operator fatigue.
Kato Fastening Systems has made coil thread inserts since the s. Kato coil inserts are widely used in the aerospace industry and their ColiThread inserts are made of cold-rolled stainless steel with a tensile strength of 200,000 psi.
Kato inserts also are available in a variety of materials for different applications:
Arconic’s line of Recoil coil thread inserts are manufactured in Australia, although the company also has warehouses in North America, Asia, and Europe. Recoil fasteners are available tanged and tang-free in a wide range of metric and standard sizes and in free-running and screw-lock configurations.
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Recoil coil inserts also come in a variety of materials, including stainless steel (grades 304 and 316), Inconel X-750, phosphor bronze, Nimonic 90, and Nitronic 60. This versatility makes Recoil inserts useful for various applications and industries, such as automotive, ship building, power generation, manufacturing equipment, consumer electronics, aerospace, transportation, and industrial electronics, to name a few.
A variety of lubricants and coatings also are available:
In addition to helically wound inserts for manufacturing, Recoil also offers thread repair kits, complete with an STI (screw thread insert) tap, installation tool, tang removal tool, drill, stainless steel inserts, and instructions.
Here is an overview of some of the additional requirements you should understand when installing coil thread inserts:
Coil Thread Insert Tools – Helically wound coils can be inserted by hand or using any number of power tools. Coil thread insert tools are available in pneumatic, hydraulic, and electric-powered units from manufacturers such as Atlas Tools, Advel, Gesipa, and Recoil. Kato and Advel also offer manual tools, which are ideal for simple repair jobs.
Coil Thread Inserts Insert Lengths and Drill Sizes – In order to gain the full load value of the bolt, you have to choose the helically wound insert that is the right length for the job. A coil thread insert size calculator has been developed using the ASME B18.29.1– standard and the HASM standard to determine the right insert size for maximum efficiency.
Coil Thread Insert Sizes – In addition to coil length, you need to be sure you have the right coil thread insert width for the job. Determine whether you need tanged or tang-free inserts, the size of your tap holes, and the insert material that’s best for the job.
Coil Thread Insert Kits – For smaller applications and repair jobs, you may need a self-contained insert kit, which includes manual insert tools and a variety of inserts.
Once the hole is drilled, you need to tap the hole using an STI to create a screw thread. You can then use an STI Go or No Go gauge to ensure the pitch is correct through the entire tap hole.
The standard type of STI tap creates threads that match the coils of the insert. These taps are usually used on a CNC machine or high-precision equipment to ensure that the fit is exact. When preparing a hold for a screw thread insert, a drill bit is used first to open the hole to the correct minor diameter. The STI can then be inserted to create the proper threads to receive the insert.
There are different types of taps with various designs, including two-thread chamfers and four-thread chamfers, which are useful for drilling blind holes, and fluted taps, which remove chips of material as they turn.
Now you can install the insert itself. The coil thread insert is fitted on the end of the tool and screwed into the hole. If you are using tanged coil inserts, the tang will fit in a notch at the end of the bit to provide a secure grip to rotate the coil. With a tang-free coil, the bit should grip the notch in the coil. Once the insert is secured, another tool is used to grip and break the tang; to prevent having loose metal, it is important the tang be removed and not just broken off.
Now the coil is ready to accept the bolt or fastener. As the bolt is screwed into the tap hole, the coils or locking coils will flex outward, generating pressure on the bolt and the parent material for a secure seal. The spring-like design of the coil transfers the shear load into radial load across the entire length of the insert, which virtually eliminates failures caused by high stress.
Coil thread inserts are used to create especially strong connections for bolts or screws by reinforcing the tap hole. Helically wound inserts are used in manufacturing because they create more durable fastenings that are resistant to vibration and extreme temperatures. They also are used to repair stripped screw threads in drill holes.
A screw thread insert is an insert of coiled wire that is screwed into a tap hole to accept a bolt or threaded fastener. The insert is typically larger than the hole, so when the bolt or fastener is screwed in, pressure on the coils of the insert creates a stronger bond between the screw threads and the threads of the hole in the parent material.
A free-running coil thread insert consists of wound wire designed to fit into the threads of the tap hole so each coil has a unique fit. As the bolt or fastener is screwed into the hole, the coils act independently to form a more secure bond between the bolt and the parent material. Using a free-running coil thread insert also extends the load from the first few threads of the tap hole to the entire length of the coil.
A screw-locking insert is a coiled insert with one or more flat coils. When a bolt is inserted, the flat coils flex outward, placing additional pressure on both the bolt threads and the threads in the parent material to form a stronger, more stable connection.
Tang-free coil thread inserts use a notch in the coil itself rather than a tang extension for installation. This means fewer steps during installation because there is no tang to remove and less risk of loose metal interfering with other parts or machinery.
Coil thread inserts are made of a wide range of materials, including work-hardened steel, bronze, and titanium. The coil material should match the application. For example, steel coil thread inserts are most common because they are extremely durable; bronze inserts are less likely to corrode in saltwater; and titanium inserts are lightweight.
A time-sert is a solid bushing insert that is typically used to repair stripped threads. Unlike a coil thread insert, a time-sert is solid with threads already molted into the sleeve, and it has a flange at the top to ensure the time-sert doesn’t wind down into the repaired hole.
Wire thread inserts are a great way to reinforce or repair threads in a drilled hole. Sometimes, it's not cost-effective to remove the parts that have been damaged, but thread repair kits are an effective way to fix a thread without busting your budget.
In this article, we will take a look at what a thread insert repair kit is, when you need to use a thread repair kit and how to use an E-Z LOK coil thread repair kit.
Threads can become damaged in many different ways, which can determine if you need to use a thread repair kit. When a male thread becomes damaged, often it can be cleaned by running a die over it. When threads are only lightly damaged, sometimes you can reform the threads by running a nut up and down. However, there is a risk the nut will fuse to the thread. That's why it's safer to use a die.
When threads have been stripped from a hole, you have a few options to solve this problem. One solution is to drill out the hole and use a larger diameter thread to tap it. However, this newly tapped hole is just as prone to stripping as the original hole. The preferred solution is to repair and reinforce the tapped hole with a thread repair kit.