Machine Design
The ball bearings are provided with a cage

To maintain the balls at a fixed distance apart
To facilitate slipping of balls
To prevent the lubricant from flowing out
To reduce friction

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Machine Design
Jam nut is a locking device in which

A slot is cut partly in 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 spitted and bent in reverse direction at other end
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

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