A castellated nut is a fastener with several slots cut through the top which resemble the traditional top of a castle's towers. The slots provide a method of securing the nut in place by installing a cotter key through a pre-drilled hole in the bolt and locking it through the castelations in the nut. Commonly found on automobile axle shafts and spindles, the castellated nut prevents the nut from backing off of the bolt or stud and allowing the components to become separated. While common tools can be used to install and remove a castellated nut, special care should be given to avoid breaking off the castelations as they leave the threaded area of the bolt and become fragile.
In many manufacturing operations, machinery is used that requires a particular setting to be locked in. Using a castellated nut is a very easy method of assuring the adjustment or setting remains in place. Functioning much like safety wiring on a nut or bolt, the castellated nut allows for quick changes and adjustments without requiring the cutting and reapplication of the safety wire. A cotter key placed through a hole in the threaded stud or bolt, running through the castelations of the nut and folded over will keep the nut from turning. When needed, removal of the cotter key will allow the castellated nut to be adjusted or removed completely.
In some manufacturing situations where the adjustment of the castellated nut occurs frequently, a quick-release clip will be used to lock the nut in place instead of a cotter key. The clip is easily pushed through and pulled out of the nut's locking notches and saves time when compared to the effort required to straighten the cotter key, pull it out of the nut and then reinsert it and bend it over once the adjustment has been made. The cotter keys are also subject to breaking if bent and straightened enough times. A small piece of broken cotter key can also potentially damage a machine if it would fall into the machine.
Variations of the castellated nut have been found on ancient Roman chariots and they are used on jet aircraft, making them a very tried-and-true locking fastener. In an age where locking nuts come in many different forms and thread locking liquids occupy much of the shelf space at the corner hardware store, the castellated nut remains a key player when it comes to locking a fastener in place.