Internal
gears

Internal gears’ teeth on the inner cylindrical surface enable a wheel train where the spokes (radii) of the gears’ pitch lines turn in the same direction with respect to a given framework, so that rotation speeds will likewise have the same direction of opposite rotation.

They are applied in planetary gearings, where one or more of the cogwheels’ shafts are mobile.

We may conclude this transmission device is required whenever, within a given machine or system, a structural component needs to rotate with respect to another along a single axis, ensuring components are constrained.