pulley vs idle wheel

pulley and idle wheel both are nouns.

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

As nouns, idle wheel is a hyponym of pulley; that is, idle wheel is a word with a more specific, narrower meaning than pulley:

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
idle wheel: a pulley on a shaft that presses against a guide belt to guide or tighten it

Other hyponyms of pulley include bollock, bullock block, fairlead, idle pulley, idler pulley.

pulley (noun)	idle wheel (noun)
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	a pulley on a shaft that presses against a guide belt to guide or tighten it