What is an acceptable safety device for a castle nut when used on secondary structures?

A cotter pin is an acceptable safety device for a castle nut when used on secondary structures because it provides a reliable method to secure the nut in place. The cotter pin fits through a hole drilled in the bolt's threaded end, and when bent over after insertion, it prevents the castle nut from loosening due to vibrations or movement. This is especially critical in assemblies where safety and integrity are essential, as it ensures that the fastener remains securely in place.

Locking washers, spring clips, and plastic caps might offer some degree of retention or locking, but they may not provide the same level of assurance against loosening as a cotter pin does. Consequently, cotter pins are widely recognized in various engineering applications as a standard method for securing fasteners like castle nuts, especially in scenarios where load and safety are a priority.