Clock mechanisms and gear trains — which gear arrangement is most commonly employed in mechanical clocks and timepieces to keep the hour and minute shafts coaxial while producing a large speed reduction?

Introduction / Context: Mechanical clocks and traditional timepieces require the minute hand shaft and the hour hand shaft to be coaxial, while the minute shaft must rotate 12 times faster than the hour shaft. The gear train that achieves this large, precise speed reduction and maintains coaxial shafts is central to clock design.

Given Data / Assumptions:

• Two output shafts (minute and hour) must be coaxial.
• Required speed ratio ≈ 12:1 between minute and hour hands.
• Compact layout, high accuracy, and ease of assembly are desired.

Concept / Approach: A reverted gear train is a compound train in which the input and output shafts lie on the same line (coaxial). The intermediate gear centers are arranged so the first and last gears share a common axis, enabling a large overall gear ratio in a compact envelope—ideal for clock mechanisms.

Step-by-Step Solution:

Verification / Alternative Check: Historical and modern clockworks commonly use reverted arrangements in the motion-work to deliver 12:1 while preserving a shared axis.

Why Other Options Are Wrong:
Simple gear train — Does not inherently ensure coaxial input and output for large reductions.
Sun and planet gear — More typical of Watt’s engine; not standard for coaxial clock motion-work.
Differential gear — Used to combine motions; unnecessary and overly complex here.

Common Pitfalls: Confusing any compound train with a reverted train; only reverted layouts guarantee coaxial input–output.

Final Answer: Reverted gear train.