linchpin vs pinball machine

linchpin

noun
  • A pin inserted through holes at the end of an axle or shaft, so as to secure a wheel or shaft-mounted device. 

  • A central cohesive source of stability and security; a person or thing that is critical to a system or organisation. 

verb
  • To adopt as, or serve as, a central cohesive source of stability and security. 

pinball machine

noun
  • A machine specially built to be used to play games of pinball. 

How often have the words linchpin and pinball machine occurred in a corpus of books? (source: Google Ngram Viewer )