gear box vs casing

gear box and casing both are nouns.

Word	Noun	Adjective	Verb	Adverb
gear box	Yes	No	No	No
casing	Yes	No	No	No

As nouns, casing is a hypernym of gear box; that is, casing is a word with a broader meaning than gear box:

Other hypernyms of gear box include case, shell.

gear box (noun)	casing (noun)
the shell (metal casing) in which a train of gears is sealed	the enclosing frame around a door or window opening
	the housing or outer covering of something
	the outermost covering of a pneumatic tire