Self-Locking Machines – Efficiency Criterion For a machine to be self-locking (i.e., it will not run backward under load when the effort is removed), the efficiency must be:

Introduction / Context:
Self-locking (non-reversible) behavior is crucial in lifting machines like screw jacks and worm gears to prevent back-driving. Efficiency provides a quick diagnostic.

Given Data / Assumptions:

• Definitions: mechanical advantage MA, velocity ratio VR, efficiency η = MA / VR.
• Rule of thumb from machine theory relating reversibility to efficiency.

Concept / Approach:
A machine is reversible if it can be back-driven by the load when effort is removed. For many classical lifting mechanisms, reversibility occurs roughly when η > 50%. Conversely, self-locking generally requires η ≤ 50%, typically stated as less than 50%.

Step-by-Step Solution:
If η > 50% ⇒ losses are relatively small and back-driving is possible (reversible). If η < 50% ⇒ losses are large enough to prevent back-driving (self-locking). Hence, for self-locking: η must be less than 50%.

Verification / Alternative check:
Detailed derivations for specific mechanisms (e.g., screw with friction angle > lead angle) also imply non-reversibility, which correlates with low efficiency.

Why Other Options Are Wrong:
Equal to or greater than 50% would suggest potential reversibility. 100% efficiency is ideal and certainly reversible. “Independent” is not supported; the 50% criterion is standard.

Common Pitfalls:
Confusing self-locking with braking; self-locking is a passive geometric/frictional effect.

Final Answer:
less than 50%