Machine Design
A screw will be self locking, if

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

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Machine Design
The plasticity is the property of a material which enables it to

Resist fracture due to high impact loads
Regain its original shape after deformation when the external forces are removed
Retain deformation produced under load permanently
Draw into wires by the application of a tensile force

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