Which type of nut has a plain hex shape but can also come in self-locking form?

The type of nut that has a plain hex shape and can also come in a self-locking form is the plain hex nut. These nuts are commonly used in various applications due to their simple design and effectiveness in providing secure fastening. The plain hex shape allows for easy tightening with standard wrenches.

When it comes to self-locking variants, these typically incorporate features like nylon inserts or other locking mechanisms that prevent the nut from loosening due to vibration or dynamic loads, enhancing their reliability in applications where movement is present. This versatility is a significant advantage, making plain hex nuts a preferred choice in many mechanical contexts.

The other types of nuts mentioned have distinct shapes and functionalities that do not align with the criteria of having a plain hex shape while also offering self-locking features. For instance, check nuts are typically thinner and used to lock other nuts in place. Barrel nuts have a different shape and are usually utilized in furniture assembly or specific joining applications, whereas castle nuts feature slots for a cotter pin to secure them, which differs from the functionality of a hex nut.