How CoilThread® Screw Locking Inserts Work:
One of the biggest misconceptions regarding locking type helically-coiled screw thread inserts is that the insert has a feature which locks it into place in the parent material. This is not the case. Locking inserts apply a locking, or prevailing, torque on the male fastener.
Locking inserts contain one or more coils that have a series of straight segments. Upon entry of the bolt these straight segments are flexed outwardly, creating pressure on the bolt. This pressure is applied between the flanks of the bolt thread so that contact area is maximised.
CoilThread® screw locking inserts are manufactured to meet the locking torque requirment specified in NASM8846. NASM8846 requires the inserts to pass a 15 cycle torque test; it is often possible to realise a cycle life far in excess of specification requirements.
Location of Locking Coil(s):
For Tanged inserts 1, 1.5, and 2 diameter lengths: The centre of the locking coil(s) equals 1/2 the number of free coils.
For Tanged inserts 2.5 and 3 diameter lengths: The locking coil is located the same distance from the tang as 2 diameter length inserts.
For Tangless® inserts the centre of the locking coil equals 1/2 the number of free coils.
Locking inserts conform to NAS1130 (unified Tangless®), NA0276 (metric Tangless®), NASM21209 (unified tanged), MA3329 (metric tanged), and produce locking torques in accordance with NASM8846 (unified) and MA1565 (metric) when used with heat treated cadmium-plated bolts. When using unplated heat treated or stainless steel bolts, an anti-seize compound, e.g. molybdenum disulfide, must be used in order to minimise galling and assure maximum cycle life. Wear life of the male fastener can also be improved by specifying dry film lubricant or cadmium plating. The use of 300 series stainless steel bolts should be avoided when using locking inserts. Use A286 aircraft quality screws or equivalent (Rc 36-43).
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