What This Thumb Screw Does in a D-Sub Assembly
The Molex 1731120256 is a thumb screw from the FRS, FCT series, designed as a jackscrew or thumb screw accessory for D-sub backshells. It carries a 4-40 UNC thread and measures 27.3 mm in length. This is the hardware that secures the backshell to the D-sub connector body, pulling the cable shield termination tight against the connector shell and providing strain relief for the cable bundle. Without it, the backshell floats on the connector and the shield connection relies on the cable gland alone.
Thread Size and Fit — the 4-40 UNC Standard
The 4-40 UNC thread is the standard size for D-subminiature connectors with UNC 4-40 threaded inserts in the connector body. This matches the thread on the mating jack-screw posts.
Compatibility — Which Backshells It Works With
Listed for use with D-sub backshells from the FRS, FCT series. In practice, any D-sub backshell with a 4-40 UNC threaded hole in the backshell body will accept this screw. The thumb-screw head allows tool-less tightening in the field, which is useful for cable assemblies that get swapped or re-terminated during maintenance. The screw is RoHS3 compliant, so it passes the material declaration for equipment sold into the EU market.