idle wheel vs pulley

idle wheel and pulley both are nouns.

Word	Noun	Adjective	Verb	Adverb
idle wheel	Yes	No	No	No
pulley	Yes	No	No	No

As nouns, pulley is a hypernym of idle wheel; that is, pulley is a word with a broader meaning than idle wheel:

idle wheel: a pulley on a shaft that presses against a guide belt to guide or tighten it
pulley: a simple machine consisting of a wheel with a groove in which a rope can run to change the direction or point of application of a force applied to the rope

Other hypernyms of idle wheel include block, pulley-block, pulley block.

idle wheel (noun)	pulley (noun)
a pulley on a shaft that presses against a guide belt to guide or tighten it	a simple machine consisting of a wheel with a groove in which a rope can run to change the direction or point of application of a force applied to the rope