Machine Design
Castle nut is a locking device in which

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
A slot is cut partly is middle of nut and then slot reduced by tightening a screw
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
One smaller nut is tightened over main nut and main nut tightened against smaller one by loosening, creating friction jamming

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Machine Design
The cracks in helical springs used in railway carriages usually start on the inner side of the coil because of the fact that

It is more stretched than the outer side during the manufacturing process
It is subjected to a higher stress than the outer side
It has a lower curvature than the outer side
It is subjected to a higher cyclic loading than the outer side

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