Power screws: condition for an overhauling screw A screw is said to be ‘‘overhauling’’ when the load tends to lower itself without external effort. Which efficiency range corresponds to this condition?

Concept
For a square-thread power screw, efficiency is η = tanα / tan(α + φ) where α is the helix angle and φ is the friction angle. Overhauling occurs when the load can drive the screw, i.e., roughly when α > φ, which leads to η > 0.5.

Check
If α = φ, η = tanα / tan(2α) < 0.5; thus to overrun (overhaul), α must exceed φ, making η > 0.5.

Final Answer
More than 50%