#axis_of_rotation__axis  the center around which something rotates
  supertype:  mechanism  device consisting of a piece of machinery; has moving parts that perform some function
  subtype:  pivot.axis_of_rotation__pin  axis consisting of a short shaft that supports something that turns
     subtype:  fulcrum  the pivot about which a lever turns
     subtype:  pintle  a pin or bolt forming the pivot of a hinge
  subtype:  rotor_head__rotorhead__rotor_shaft__rotorshaft  the axis around which the major rotor of a helicopter turns

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