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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Machine Design
In worm, gears, pitch lead angle is

None of these
The angle between the tangent to the pitch helix and an element of the cylinder
The angle between the tangent to the pitch helix and the plane of rotation
Half the angle between two inclined faces in axial plane

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