Machine Design
A screw will be self locking, if

Friction angle is equal to helix angle
Friction angle is more than helix angle
Efficiency of screw is 100%
Friction angle is less than helix angle

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Machine Design
In case of V belt drive

The belt should not touch the bottom of groove in the pulley
The belt should not touch the sides of groove in the pulley
The belt should touch the bottom of groove in the pulley
None of these

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