Fastener 4 – April 2020

Topic: Helical Wire Thread Inserts

Answers Provided by: Scott Campbell

More commonly asked questions about problem-solving, insert materials, and installation.

Q:  How to prevent thread galling?
A:  This is a common problem when using screw-locking helical coil inserts and occurs when the mating fastener passes through the locking coils and binds.  The result is the insert threading out of the hole when the bolt is removed, or the insert is threaded too deep in the tapped hole.  
The most common cause of galling is the use of 303 series stainless steel bolts. Helical wire thread inserts are made of cold-rolled 304 stainless steel wire, work-hardened to a tensile strength greater than 200,000 psi and a hardness of Rc 43-50.  To minimize galling it is recommended to use heat-treated screws with a hardness of Rc36-43.
In applications where stainless steel screws are mandated, or when using heat-treated unplated bolts it is recommended to use an anti-seize compound such as moly-disulphide to minimize galling.  Specifying inserts with dry-film lubricant plating is another option.  Lastly, Nitronic 60® could also be specified.

Q:  What materials are helical wire thread inserts produced from?
A:  

• 304 Stainless Steel (standard), specification AS7245. Used for most general-purpose applications up to 425℃ (800℉).
• Inconel X-750, specification AS7246.  This is a nickel alloy for resistance to chemical corrosion and oxidation.  Also used for high temperature applications up to 750℃ (1300℉).
• Phosphor Bronze, specification AS7247.  Used for electrical bonding joints, copper parts requiring high resistance to some electrolytic couples, and saltwater applications up to 300℃ (570℉).
• Nitronic 60®, alloy number UNS S21800.  Specifically designed to minimize galling when used with stainless steel screws with excellent high temperature performance.

Q:  Should tapped holes be countersunk prior to insert installation?
A:  To facilitate insert installation and prevent a feather edge or burr build up on the tapped hole, countersinking on the drilled hole at 120° included angle is recommended and the average depth of the countersink is ½ pitch.

Q:  At what depth should the insert be seated?
A:  It is recommended that the inserts be installed ¾ to 1.5 pitches below the top surface of the tapped hole.