Machine Design
An open belt drive is used when

Driven shaft is to be started or stopped whenever desired without interfering with the driving shaft
Shafts are arranged parallel and rotate in the same directions
Shafts are arranged at right angles and rotate in one definite direction
Shafts are arranged parallel and rotate in the opposite directions

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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
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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