Definition of "underrunner"

underrunner

noun
plural underrunners
1. The lower of a pair of grinding millstones.
  Quotations
  FOR SALE AT A BARGAIN. […] Four run stones, Munson underrunners, 3 ft diameter.
  1890, The Northwestern Miller, volume 29, page 563
  The arrangement is preferably applied to an underrunner mill.
  1896, Great Britain. Patent Office, Patents for Inventions. Abridgments of Specifications, page 11
2. An additional runner added to the bottom of a sledge to protect the main runners in harsh conditions.
  Quotations
  Less than half a mile to the south the party sighted an underrunner of a sledge, and assumed it to mark the location the Norwegians had assigned for 90°S.
  2002, Paul Simpson-Housley, Antarctica: Exploration, Perception and Metaphor, page 36
3. A runner (mechanical part intended to guide or aid something else to move) designed to be inserted underneath the item to be moved.
  Quotations
  A triple tandem tow using underrunners. This method has been used for long ocean tows by tugs with a single-drum winch.
  1989, John S. Blank, Modern Towing, page 302