Machines – reversibility and efficiency criterion\nA machine whose efficiency is less than 50% is termed as:

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:

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)