Thread Inserts

Thread inserts are a means of providing a stronger anchorage for bolts, screws or studs, in the comparatively softer metal alloys (aluminium, magnesium and bronze), wood, plastics or composite materials. They may also be used when it is necessary to do a repair to a threaded hole that has suffered damage. There are two basic types of thread insert (Wire and Thin Wall), but the designs of each type will vary according to the many manufacturers or to the environment in which the fastener must operate.

Wire Thread Inserts

Wire thread inserts consist of a very accurately formed helical coil of wire, which has a diamond (rather than a round) cross-section and is usually made from corrosion-resistant steel or heat-resistant nickel alloy. Specifically sized drills, taps and thread gauges (provided by the insert manufacturer) are required to form the tapped holes for the inserts and another special tool is necessary to insert the wire coils correctly into their prepared holes.

Thin Wall Inserts

Thin wall inserts appear in a variety of designs, materials and surface finishes and consist of a thin tube, which is threaded internally and may/may not be threaded externally. Similarly, special tools are required from the manufacturer to prepare the holes for the inserts and various methods are adopted to secure each particular type of thin wall insert into its hole. Thin Wall inserts include:

• Key-Locked Inserts: Key-Locked inserts are threaded both internally and externally and after being screwed into the prepared hole, are (as their name implies) locked into their holes by tiny wedges or keys. The keys are then pressed (or hammered) into place between the insert and the wall of the hole.
• Swaged Inserts: Swaged inserts are also threaded internally and externally and are again screwed into the hole before a tool is used to deform (swage) the insert so that it is locked into the hole.
• Ring-Locked Inserts: Ring-Locked inserts with internal and external threads are screwed into holes which are counter bored, to allow a special lock-ring to be installed (after the insert) and yet another special tool is used to complete the locking action of the lock-ring.
• Bonded Inserts: Usually, bonded inserts are only internally threaded (to hold the bolt, screw, stud and so on) and are secured in the prepared hole by the use of adhesives.

From this information, it is obvious that great care must be taken to ensure that only the approved types of inserts are used in aerospace components and that the procedures for their installation and removal (laid out in the relevant Manuals) are carefully followed.