If α denotes the lead angle and φ denotes the angle of friction, then the efficiency of the screw is given by 

Explanation:

Power screw: It is a drive used in machinery to convert rotary motion into linear motion for transmission of power.

The efficiency of a power screw is given by:

\(\eta = \frac{{{\rm{work\;output}}}}{{{\rm{work\;input}}}}\)

\(\eta = \frac{{Fl}}{{P\pi {d_m}}}\)

where, F = force, l = length to which screw travels in the vertical direction 

\(\eta = \frac{{\tan \alpha }}{{\tan \left( {\alpha + \phi } \right)}}\)     

where ⍺ = helix angle or lead angle