Machine Design
The endurance limit of a material with finished surface in comparison to rough surface is

More
Less
Same
More or less depending on quantum of load

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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
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 slot is cut partly is middle of nut and then slot reduced by tightening a screw

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