main frame

noun2

1. •
   The rigid frame upon which the moving parts of a machine are mounted; a chassis.

   • The main frames are of 1¼-in. thick plates spaced 3 ft. 2½ in. apart, the centre lines coinciding with the centre of the axlebox guide faces. — 1951
2. •
   The widest structural frame located near the middle of a ship.