Re: Five Frequently Asked Questions About Wire Thread & Key-Locking Inserts – Fastener Engineering

The global industrial fasteners market size is expected to grow, reaching $110.24 billion by 2025, according to a recent report by Grand View Research. Such a strong demand and diverse market mean numerous options are available for customers. This is a good thing for the industry, however, choosing the ideal components for projects can sometimes be challenging.

Fastener Engineer recently discussed options for wire thread and key-locking — two commonly used fasteners — with the experts at Crossroad Distributor Source. Crossroad is a master distributor, supplying industrial and fastener distributors throughout North America, Mexico, and the Caribbean.

Wire thread inserts are coiled fastening devices that are typically used to strengthen threads and protect tapped holes against failure. Key-locks are threaded inserts with locking keys, which secure them in place.

Crossroad’s president, Scott Campbell, and the company’s GM, Casey Campbell, shared five typical questions they receive about wire thread and key-lock inserts here:

1Question: For inspection purposes, how does one accurately measure the insert diameter and length of a wire thread insert before installation?
     Answer: The installed diameter and length of a wire insert can only be measured in its free state by first measuring the free coil diameter and then counting the number of coils from the notch of the tang to the top of the insert. The free coil diameter and the number of coils per a specific size insert can be found in the manufacturer’s literature, or generally on their website. Note that wire inserts come in five lengths, each a multiple of the diameter (i.e., 1.0, 1.5, 2.0, 2.5 & 3.0 X diameter) — and that length is the installed length of the insert.

2.  Question: Can a user use a standard drill and tap combination with wire thread or key-locking inserts?
     Answer: The answer is no for both inserts. Wire thread inserts require the use of screw thread insert or STI. Drill size recommendations will be shown in the manufacturer’s literature, typically, on their website. For key-locking inserts, standard taps may be used but the drill and tap combination is not standard. A slightly larger drill size must be used so that the keys of the insert can be driven into the parent material.

3.  Question: What are wire thread screw-lock inserts and why are they dyed red?
     Answer: Screw-lock inserts are used in applications where there may be vibration, allowing the mating fastener (screw) to loosen. The screw-lock insert “locks” the mating fastener into place where a certain amount of torque is required to release the fastener. This eliminates the need for a lock washer. Screw-lock inserts are dyed red for identification purposes only. Please note that metric screw-lock inserts may not be dyed red per applicable standards.

Power coil specifications.

4.  Question: Why use a wire thread insert instead of a key-locking insert?
     Answer: Cost is a major factor. Additionally, using a wire thread insert allows for a thread to be repaired maintaining the integrity of the original assembly, meaning that no additional material is removed from the parent material. When using a key-locking insert, additional material must be drilled out of the parent material to allow for the external threads of the key-locking insert’s outer wall. In cases where boss is critical, only a wire insert can be used.

5.  Question: Which system creates a stronger thread assembly, wire thread or key-locking inserts?
     Answer: There are varying schools of thought on this question, but many suggest that wire thread inserts have less pull-out and torque-out strength than key-locking inserts. Key-locking inserts are locked in place within the parent material by their locking keys, whereas wire thread inserts are locked in place by friction. However, each one has merits depending on the application and weight considerations.
Ultimately, it is typically the parent material or bolt in the assembly that will fail before the insert. There are many assembly strength tables available, which measure the shear strength of the parent material against the bolt material minimum ultimate tensile strength. For critical applications, manufacturers will suggest testing to determine the maximum torque that can be put on an assembly before failure.


Click here for Crossroad’s informational videos about the installation process for treaded inserts.

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