Definition of great wheel in US English:

great wheel

noun

  • 1An exceptionally large wheel, especially one in a piece of apparatus.

  • 2Watchmaking and Clockmaking the first wheel in the train; specifically a cogged wheel at the foot of a fusee, of slightly larger diameter than the adjacent largest step of the fusee.

Origin

Early 16th century; earliest use found in Stephen Hawes (b. c1474), poet.