Machine theory – classification by efficiency A lifting or transmitting machine whose overall efficiency exceeds 50% is commonly termed as which type of machine?

Introduction / Context:
In elementary machine theory, machines are classified by how readily they can work in reverse when the load is allowed to drive the mechanism. Efficiency thresholds are used to distinguish reversible from non-reversible (self-locking) machines. The question asks which term applies when efficiency exceeds 50%.

Given Data / Assumptions:

• Total efficiency η = output work / input work.
• No special geometry beyond the standard textbook conventions.
• Neglect dynamic effects; use quasi-static reasoning for reversibility.

Concept / Approach:
A machine is reversible if, when the effort is removed, the load can drive the machine in reverse and return energy through the mechanism. A common criterion taught in basic courses is that when η > 0.5 (50%), the machine tends to be reversible; when η < 0.5, it tends to be non-reversible (self-locking), although geometric friction angles can refine this in specific devices like screw jacks.

Step-by-Step Solution:

Verification / Alternative check:
Consider a screw jack: when friction angle φ is less than helix angle α (low friction, effectively higher efficiency), the jack can back-drive (reversible). When φ > α (low efficiency), it self-locks (non-reversible). This aligns qualitatively with the 50% heuristic taught in fundamentals.

Why Other Options Are Wrong:

• non-reversible machine: typically associated with low efficiency and self-locking behavior.
• ideal machine: implies η = 100%, frictionless; stricter than “greater than 50%”.
• self-locking machine: opposite of reversible; used where back-drive is undesirable.

Common Pitfalls:
Assuming “ideal” whenever efficiency is “high”. Ideal is a limiting case (η = 1), not merely above half.

Final Answer:
reversible machine