Pulley vs. Sheave -- Definition please

I would like to find an official explanation of the difference between a "Pulley and a Sheave".

So far throughout the years I have heard several comparisons from back yard engineers and professionals:

1) There is no difference, the terms are inter-changeable. Personally don't think this one is correct. but don't really know. ??????

2) Sheaves drive on the flat inner surface of the belt, thus the sides of a sheave are perpendicular to the diameter (drive surface) between the sides. I Don't think this is accurate based on seening variable sheaves sold with tapered sides.


3) Pulleys are used to drive and be driven by v-belts and cog (synchronous) belts. (????) Maybe????

4) A sheave is used to transfer linear motion i.e a block and tackle; whereas a pulley is used to transfer rotational motion i.e. a belt driven fan unit. I like this one the best but am unsure if it is fact.

I think there is really a difference but in this period of history I think the exact definitions are truly lost.


Definitions or references to accurate info. is appreciated.