pulley vs idle pulley

pulley and idle pulley both are nouns.

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

As nouns, idle pulley is a hyponym of pulley; that is, idle pulley 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 pulley: 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 wheel, idler pulley.

pulley (noun)	idle pulley (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