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Meaning of pitch surface

pitch' sur"face

Pronunciation: [key]
  1. (in a gear or rack) an imaginary surface forming a planea cylinderor a cone or frustrumthat moves tangentially to a similar surface in a meshing gear so that both surfaces travel at the same speed.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.