Machine Design
A screw will be self locking, if

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

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Machine Design
In an oil lubricated journal bearing, the coefficient of friction between the journal and the bearing

Remains constant at all speeds
Is minimum at zero speed and increases monotonically with increase in speed
Is maximum at zero speed and decreases monotonically with increase in speed
Becomes minimum at an optimum speed and then increases with further increase in speed

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