Q: What is the difference between a Duplex nylon insert and all-metal lock nut? I’ll be using this in an offshore marine application for salt air resistance and strength.
A: The main things to consider between an all metal and nylon insert lock nut is the temperature and the corrosive conditions of the application. Nylon insert lock nuts have a polymer insert on the threading to bite into the screw. They have a max usable temperature of about 350F. If you are using these Duplex lock nuts within this temperature range, this will be the most economic solution. However over time, UV and salt will degrade the polymer and this needs to be considered.
All metal lock nuts are for high temperature applications over 350F and are also excellent in highly corrosive applications as the corrosion limits are only from the metal and not the polymer. These are available in several styles including stover and flexloc and maybe a more robust solution for your application.
Both nylon insert and all metal lock nuts are considered “prevailing torque” lock nuts, where they are designed to deform elastically, which prevents them from freely spinning like a standard nut.