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Meaning of pitch circle

pitch' cir"cle

Pronunciation: [key]
— Mach. Mach.
  1. an imaginary circle within the profiles of the teeth of a gear, such that it rotates against a similar circle rotating at the same rate on a meshing gear. Also called
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.