Clock mechanism gear train — which gear arrangement is most commonly used in mechanical clocks and timepieces? Select the most appropriate option that correctly identifies the gear train used to keep hour and minute shafts coaxial while achieving large speed reduction.

Topic: Gear trains used in clocks and time-keeping mechanisms

Given data

• We need the gear train commonly used in clocks.
• Clock trains typically require large speed reduction and coaxial hour and minute shafts.

Concept / Approach
Mechanical clocks usually need (i) a very high overall speed reduction from the driving element to the hour hand, and (ii) coaxial input/output shafts so that the minute and hour hands share the same center. A reverted gear train provides both: the first and last gears are coaxial, enabling concentric shafts while permitting large reduction in multiple stages.

Step-by-step reasoning
1) A simple gear train typically does not keep the first and last gears on the same axis.2) A sun-and-planet gear is a special epicyclic arrangement used historically (e.g., Watt’s engine) for converting motion, not typical of clock trains.3) A differential gear combines two inputs to give a relative motion (automobile differentials), not for standard clock reductions.4) A reverted gear train aligns the first and last gear axes and allows large, compact reductions—ideal for clock hands sharing a common axis.

Verification / Alternative view
The hour and minute hands are coaxial in most clock faces; reverted trains are the canonical textbook solution to achieve coaxiality with significant reduction.

Common pitfalls

• Confusing epicyclic (planetary) sets with typical clock trains; while some modern mechanisms may use them, the standard textbook answer remains the reverted gear train.
• Assuming “simple” trains can easily give coaxial first/last gears—this is not generally the case.

Final Answer
Reverted gear train