**5.5 Castellated nut**

Castellated nut uses tension to mechanically "grip" the bolt or threads. Cotter pin is also sometimes used in conjunction with this device to prevent nut from falling off. However, cotter pin will not prevent loss of clamp load after initial loosening. It fails when vibrational load or impact causes the fastener to lose clamp load. This device does not seal the threaded assembly, and the high cost of castellated nut is required, too (see **Figure 3**) [1].
