Machine Design
Castle nut is a locking device in which

A slot is cut partly is middle of nut and then slot reduced by tightening a screw
One smaller nut is tightened over main nut and main nut tightened against smaller one by loosening, creating friction jamming
Through slots are made at top and a cotter pin is passed through these and a hole in the bolt, and cotter pin spitted and bent in reverse direction at other end
A hard fibre or nylon cotter is recessed in the nut and becomes threaded as the nut is screwed on the bolt causing a tight grip

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Machine Design
When a machine member is subjected to torsion, the torsional shear stress set up in the member is

Maximum at both the centroidal axis and outer surface of the member
Maximum at the centroidal axis and zero at the outer surface of the member
Zero at the centroidal axis and maximum at the outer surface of the member
Zero at both the centroidal axis and outer surface of the member

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