collar beam vs hammer beam

collar beam

noun
  • A horizontal piece of timber connecting and tying together two opposite rafters. 

hammer beam

noun
  • A member of a kind of roof truss (a hammer-beam truss), so framed as not to have a tiebeam at the top of the wall. Each principal has two hammer beams, which occupy the situation, and to some extent serve the purpose, of a tiebeam. 

How often have the words collar beam and hammer beam occurred in a corpus of books? (source: Google Ngram Viewer )