Today we will compare 2 popular fastener products that are used to repair damaged threads or strengthen a weaker parent material like aluminum: Helical Wire Inserts and Key-locking Inserts. Both are commonly used in precision manufacturing and aerospace applications.
What are the main uses of these threaded inserts?
Both “Heli-coil” or Helical wire inserts and “Keensert” or Key-locking inserts fall under the “Threaded Insert” category of fasteners. A threaded insert, also known as a threaded bushing, is a fastener element that is inserted into an object to add a threaded hole. They may be used to repair a stripped threaded hole, provide a durable threaded hole in a soft material, place a thread on a material too thin to accept it, mold or cast threads into a work piece thereby eliminating a machining operation, or simplify changeover from unified to metric threads or vice versa. Although Helical Wire inserts and Key-locking externally threaded inserts are used in similar ways, there are 3 main distinctions that we detail below. But first, let’s take a closer look at each of these fasteners:
What is a Helical Insert Wire Insert? (aka Heli-coil)
Helical wire inserts are precision-formed continuous wire coils that provide permanent, wear-resistant threads which exceed the strength of the parent material. Often underestimated, the popular misconception is that Helical wire inserts are intended solely for thread repair, when, in fact, they have other significant uses. Not to be confused with other types of threaded inserts or rivnuts, Helical wire inserts resemble springs and are sometimes referred to as “Screw Thread Inserts (STI)” ,“Helicals” or by the popular brand “Heli-Coil” which is a registered trademark of Emhart® Teknologies, Inc.
Three typical applications for using Helical wire inserts are 1) to repair damaged threads in parts that would otherwise have to be scrapped; 2) to strengthen threads against failures due to stripping, seizing or corrosion; 3) to convert threads between inch and metric sizes.
What is a Key-locking insert? (aka Keensert or Keysert)
The Key-locking threaded insert is a solid bushing style insert that is threaded on both the inside and the outside and has wedges or “keys” attached to the top of the insert. These externally threaded inserts are used to distribute loads and repair or strengthen threads against failures due to stripping, seizing or corrosion. Key-locking inserts are commonly used in high torque and high temperature situations, and in applications where fasteners may be repeatedly removed and reassembled. Key-locking inserts are often referred to as Keenserts® and Keyserts, which are registered trademarks for the most popular brand names for this part. (Keensert® is a registered trademark of Huck Patents, Inc. & Keysert® is a registered trademark of Alcoa Fastening Systems).
Typical applications are transmission housings, electronic equipment and suspension units. Key-locking inserts are commonly found in aerospace assemblies.
How do they work?
Helical wire inserts work because the diamond shaped helically formed wire coil screws into a threaded hole to form a mating internal thread for a screw or stud.
Key-locking inserts are threaded into a pre-tapped hole, and then the keys are hammered into grooves through the threads, permanently locking the insert in place. Their “keys”, which are driven down into the tapped threads, provide a mechanical lock against rotation, especially when the mating stud or bolt will be removed frequently.
What materials, sizes and styles are available?
Key-locking inserts come in four main styles: Miniature, Thin Wall, Heavy Duty Wall (general purpose) and Extra Heavy Duty.
- Miniature Key-locking inserts are used in electronic and aerospace applications where size and strength are critical. These inserts may be installed in sheet material as thin as 1/16″.
- Thinwall inserts have smaller external threads than standard heavy-duty inserts and are ideal for tight spaces where less pull-out strength is acceptable.
- Heavy Duty inserts have a thick, heavy-duty thread wall, suitable for most applications.
- Extra-heavy duty inserts are used in oversized and overly worn holes.
Key-locking inserts can be internally self-locking or non-locking (the standard is non-locking and is sufficient for the most common applications).
Inserts with the internal self-locking feature are designed to securely lock a bolt when it is entered into the insert only a few turns. Even after repeated installations and removals of the bolt, the lock maintains sufficient locking torque to prevent the bolt from vibrating out.
Common Materials available include Carbon steel, Stainless Steel, Alloy 4140 and A286.
Internal Diameters available: American/Inch #2-56 to 1 ½-6 ” and Metric M2 to M24
Helical wire inserts
Helical wire inserts come in 2 main styles: tanged and tangless (currently, MF Supply spcializes in the tanged style).
The original and widely used tang style has a prong at one end to facilitate installation. The newer tangless style has no prongs to break off, retrieve, or lose. Tangless inserts are commonly used in electronic applications where loose tangs are might cause damage.
Helical wire inserts can be free-running or locking.
Free running is the most common style used for thread repair. The “free” diameter of the insert is larger than the installed diameter and this configuration generates balanced pressure distribution between the coils and threads.
Locking style inserts are recommended for applications that require constant torque, or are subject to stress or vibration. The locking style features a crimped turn that acts as a locking mechanism that grips the bolt or screw to prevent loosening from vibration, eliminating the need for lock wiring, lock-nuts, lockwashers, pellets/patches or other thread locking devices.
Common Materials available include Stainless Steel, Phosphorous Bronze, Inconel X-750 and Nitronic 60 and 90.
Internal Diameters available: American/Inch #2-56 to 1 ½-6” and and Metric M2 to M24. Standard inserts lengths measure from 1x to 3x the diameter. Note: larger sizes may be available by special order.
Why use a Key-locking insert vs. a Helical wire insert?
There are three main differences between Key-locking inserts and Helical wire inserts:
1) Strength: Key-locking inserts are stronger than Helical wire inserts
2) Ease of installation: Key-locking inserts are easier to install than Helical wire inserts
3) Price: Key-locking inserts are more expensive than Helical wire inserts!
So here is some direction on how to select the correct threaded insert for your application: In heavy wear, high vibration and high heat situations where saving space is not a concern and hole depth is limited, Key-locking inserts are the best bet.
When reducing costs and minimizing space are priorities, Helical wire inserts are the way to go. They are particularly useful for creating permanent strong threads in softer materials such as aluminum, titanium and magnesium alloys, and are best suited to lower heat and lower torque environments.
Stay tuned for our next blog, where we discuss, brands and mil-spec options, how to order and installation options.
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