bumper vs mechanical device

bumper and mechanical device both are nouns.

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

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

bumper: a mechanical device consisting of bars at either end of a vehicle to absorb shock and prevent serious damage
mechanical device: mechanism consisting of a device that works on mechanical principles

bumper (noun)	mechanical device (noun)
a mechanical device consisting of bars at either end of a vehicle to absorb shock and prevent serious damage	mechanism consisting of a device that works on mechanical principles
a glass filled to the brim (especially as a toast)