Gear trains: Identify the correct classification when gear axes move relative to a fixed reference axis. In such trains, the planet (or carrier-mounted) gears have centers that revolve about a fixed axis, unlike simple/compound/reverted trains whose gear axes remain fixed. Choose the correct option.

Given data

• A gear train in which the axes of gears themselves move relative to a fixed reference axis.

Concept / Approach
In simple and compound gear trains, all gear axes are fixed in space.In a reverted train, the first and last gear are co-axial, but the axes are still fixed.In an epicyclic (planetary) train, planet gears rotate about their own axes and revolve about a fixed axis carried by the arm/carrier; hence their axes move.

Reasoning
The only classification that matches “gear axes move relative to a fixed axis” is the epicyclic (planetary) gear train.

Common pitfalls
Confusing “reverted” (co-axial input/output) with “epicyclic” (moving axes).

Final Answer
Epicyclic (planetary) gear train