The KATO Tech Group receives inquiries daily on the proper design of bolted assemblies using locking inserts. In addition, we frequently receive complaints that the locking torque is too high, the inserts are damaging the screws, the inserts are moving with the screws, or the screws are breaking off, etc. etc. In 99.9% of the cases, the problems can be attributed to the tapped holes, or the screws, and not to the inserts themselves.
First, some background information to understand the design of the locking inserts. We’ll limit the discussion to UNC and UNF sizes. Locking inserts per MS21209 and NAS1130, are designed to produce locking torques in accordance with the parameters set forth in NASM8846 in Class 3B tapped holes per NASM33537, in 2024-T4 aluminum test blocks. In addition, the screws to be used for the testing are specified in NASM8846.
At KATO, every manufacturing lot of locking inserts, tanged, or Tangless®, are torque tested per NASM8846. The screws used are cadmium plated socket head cap screws with a hardness of Rc 36 to 42. Since the inserts have a hardness range of Rc 43 to 50, the heat treated screws are close enough to the insert hardness to withstand the frictional forces imposed by the insert locking coils. And, (This is very important, are you paying attention?) the cadmium plating on the screws acts as a lubricant to prevent galling.
If you are still awake, we will proceed. In previous issues of KATO Fastening News we have addressed the proper methods for drilling, countersinking, and tapping STI (screw thread insert) tapped holes. This article assumes that the tapped holes are prepared properly, will accept a GO thread plug gage, the minor diameters are within specification, and the insert is not installed into the chamfered thread at the bottom of a blind hole.
If any of these factors are out of specification, or if thread forming taps were used instead of thread cutting taps, the insert locking torque can be adversely affected. Other practices that can affect locking torque are the use of thread locking compounds (a big no-no!), and excessive zinc chromate primer getting behind the insert locking coil, preventing it from expanding outward.
Galling is a very common problem experienced by customers using locking helical coil inserts. Typically galling and seizing occur when the bolt passes through the locking coil and binds. This is particularly evident with like materials (inserts and screws) and it can cause the problems noted at the beginning of this article. The most common cause of galling is the use of dead soft male threaded fasteners, e.g., 300 series stainless steel screws. KFS recommends the use of 300 series stainless steel bolts be avoided if possible. Stainless steel requirements should use A286 aircraft quality screws or equivalent. In applications where stainless steel screws are mandatory, or when using heat-treated unplated bolts, an anti-seize compound, e.g., molybdenum disulfide, must be used in order to minimize galling and maximize cycle life. Having the lubricant or plating on the screw is actually better than on the insert alone. Here is why: After the lubricant/plating wears off of the two locking threads of the insert, the lubricity between the bolt and insert is eliminated. However, the lubricity is maximized if it is on every thread of the bolt or screw. Specifying CoilThread inserts with dry film lubricant or cadmium plating is another option available to the customer.
In summary, locking inserts work best when the bolts or screws are heat treated to be in the range of the insert hardness, and when galling is minimized, and cycle life is maximized by the use of an anti seize compound (plating or lubricant) And, after reading this, we hope you will agree that problems with locking inserts are never our fault – Right?
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