Definition
A shaft arrangement in which one shaft rotates inside another along a shared center axis, allowing two rotating assemblies to turn independently at different speeds on the same centerline. In a gas turbine engine, concentric shafts connect separate compressor and turbine stages so each spool can rotate at its own optimum speed.
Plain English
Two shafts mounted one inside the other, sharing the same center, so each can spin at its own speed without affecting the other.
Context Anchor
Seen in gas turbine engine descriptions, especially when the handbook explains how different compressor and turbine sections are connected inside the engine.
Derivation
From Latin 'con-' (together) and 'centrum' (center). 'Concentric' literally means 'sharing a common center.' That is exactly what these shafts do — they share one centerline, with one nested inside the other.
Why Pilots Care
The design improves efficiency and flexibility by allowing each compressor and turbine section to run at its own optimal RPM instead of being forced to a single speed.
Analogy
Think of two drinking straws, one inside the other, both lined up through the same middle. Each straw could rotate separately, but they still share the same centerline.
Intuition Check
A concentric shaft is not just any engine shaft. The important point is that two shafts share the same centerline, usually with one inside the other.
Example Sentence 1
The twin-spool turbine engine uses concentric shafts so the low-pressure and high-pressure sections can rotate at different speeds.
Example Sentence 2
During engine overhaul, technicians inspect the bearings on both the inner and outer concentric shafts for wear.