Which type of nut is designed to prevent the screw or bolt from loosening?

The correct answer is a lock nut. A lock nut is specifically designed to keep a screw or bolt from loosening over time due to vibrations or other forces. It achieves this through several mechanisms, such as using nylon inserts or being made with a split design that creates tension against the threads of the bolt when tightened. This construction allows the lock nut to resist movement, ensuring that the connection remains secure even under stress.

In contrast, while cap nuts, wing nuts, and stop nuts serve distinct purposes, they do not have the same locking capabilities. Cap nuts are primarily used to cover and protect the end of a bolt, wing nuts allow for hand-tightening and quick adjustments, and stop nuts are typically used to limit the travel of a bolt or screw. These functions do not include the feature of preventing loosening, which is central to the functionality of a lock nut.