idle wheel vs pulley-block

idle wheel and pulley-block both are nouns.

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

As nouns, pulley-block is a hypernym of idle wheel; that is, pulley-block 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-block: 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, pulley block.

idle wheel (noun)	pulley-block (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