rack and pinion vs cogwheel

rack and pinion and cogwheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
rack and pinion	Yes	No	No	No
cogwheel	Yes	No	No	No

As nouns, cogwheel is a hypernym of rack and pinion; that is, cogwheel is a word with a broader meaning than rack and pinion:

rack and pinion: a wheel gear (the pinion) meshes with a toothed rack; converts rotary to reciprocating motion (and vice versa)
cogwheel: a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion

Other hypernyms of rack and pinion include gear, gear wheel, geared wheel.

rack and pinion (noun)	cogwheel (noun)
a wheel gear (the pinion) meshes with a toothed rack; converts rotary to reciprocating motion (and vice versa)	a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion