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?
Mechanical Engineering
Machine Design
Difficulty: Easy
Choose an option
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ALess than 50%
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BMore than 50%
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CEqual to 50%
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DNone of these
Answer
Correct Answer: More than 50%
Explanation
ConceptFor 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.
CheckIf α = φ, η = tanα / tan(2α) < 0.5; thus to overrun (overhaul), α must exceed φ, making η > 0.5.
Final AnswerMore than 50%