A castellated nut and cotter pin assembly is designed to prevent which problem?

• A. Rotation And Loosening
• B. Shearing
• C. Corrosion
• D. Fracture

Answer: (A)

Preventing rotation and loosening of a fastener under vibration is what this arrangement is designed to do. A castellated nut is tightened onto the bolt until the desired preload is achieved, and then the bolt hole is aligned with one of the nut’s slots. A cotter pin is threaded through that aligned hole and through the slot, and bent to secure. Because the pin passes through the bolt and into the slot, the nut cannot rotate freely. If the nut tried to turn, the pin would bind or shear, keeping the clamping force (preload) on the joint intact and preventing self‑loosening from vibration or dynamic loads.

This mechanism is not primarily about resisting shear forces, corrosion, or fracture. Those are addressed by other design considerations. The key purpose here is to lock the nut in place to maintain a constant joint tension.