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These components are used in combination with a screw that has a pre-drilled radial hole. The nut is attached and a pin is passed through the notches and the hole in the screw, preventing the nut from turning.

Several types of pins may also be used for this purpose. These include:

A cotter pin, also known as a split pin — a fastener with twin tines, which after insertion are bent apart to prevent removal.

An R-clip, also known as a hairpin cotter pin or hitch pin — a sprung metal fastener with one straight leg inserted into the hole and one profiled leg that grips the outside of the nut.

Safety or locking-wire — a wire that’s passed through the notches and the hole, then twisted, and anchored to secure the nut.

With six notches spaced at 60-degree intervals, the castellated nut can only be locked where a notch corresponds to the hole. After correct torquing, it’s necessary to turn the nut again up to 30 degrees (in either direction) to locate the hole.

As fine-tuning the torque is not possible, castellated nuts are better suited to low-torque applications. They’re unsuited to applications that require a specific preload.

Castellated nuts are often threaded with Unified inch fine (UNF) or Unified inch coarse series (UNC) with thread diameter — typically from 1/4 to 1-1/2-inch in varying nut widths and heights.

A castellated nut has a cylindrical top of a smaller diameter where the notches are, with a higher profile than a typical nut of its size. It’s similar to a slotted nut but the rounded section featured on a castellated nut allows the pin to be secured tighter to the nut than is possible with a slotted nut.