Machines – reversibility and efficiency criterion A machine whose efficiency is less than 50% is termed as:
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Areversible machine
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Bnon-reversible machine (self-locking)
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Cneither reversible nor non-reversible machine
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Dideal machine
Answer
Correct Answer: non-reversible machine (self-locking)
Explanation
Introduction / Context:In machine theory (e.g., screw jacks, hoists), reversibility refers to whether a machine, when loaded, can back-drive in reverse. Efficiency plays a decisive role in this classification.
Given Data / Assumptions:
- Efficiency η = output / input (as a fraction or percentage).
- Reversible machine can be back-driven by load when input is removed.
- Self-locking (non-reversible) machine cannot be back-driven by the load.
Concept / Approach:For several classical machines (notably screw mechanisms), the condition for self-locking is η < 50%. Under this, frictional resistance is sufficiently large that the load does not cause reverse motion.
Step-by-Step Solution:
Identify the efficiency threshold: η = 50% is a key boundary for reversibility in screw-type mechanisms.If η < 0.5, friction losses exceed half the input, preventing back-driving; the machine is self-locking (non-reversible).Therefore, with efficiency less than 50%, the correct term is non-reversible machine.Verification / Alternative check:For a screw jack, self-locking condition is often derived using helix angle and friction angle relations. When friction angle exceeds helix angle, reverse motion under load is not possible; this aligns with low efficiency.
Why Other Options Are Wrong:
- Reversible machine: requires sufficiently high efficiency and low friction.
- Neither of the above / ideal machine: not aligned with accepted definitions; an ideal machine would have η = 100%.
Common Pitfalls:Assuming reversibility depends only on geometry; friction (hence efficiency) is crucial.
Final Answer:non-reversible machine (self-locking)