Machine Design
The shear plane in case of bolts should

Be normal to threaded portion of shank
Be parallel to axis of bolt
Never be across the threaded portion
Be across threaded portion of shank

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Machine Design
Castle nut is a locking device in which

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 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
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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