coupling vs mechanical device

coupling and mechanical device both are nouns.

Word	Noun	Adjective	Verb	Adverb
coupling	Yes	No	No	No
mechanical device	Yes	No	No	No

As nouns, mechanical device is a hypernym of coupling; that is, mechanical device is a word with a broader meaning than coupling:

coupling: a mechanical device that serves to connect the ends of adjacent objects
mechanical device: mechanism consisting of a device that works on mechanical principles

coupling (noun)	mechanical device (noun)
the act of pairing a male and female for reproductive purposes	mechanism consisting of a device that works on mechanical principles
a mechanical device that serves to connect the ends of adjacent objects
a connection (like a clamp or vise) between two things so they move together