Bob, the question of the difference between a pulley and a sheave is a vexed one. To some extent it depends on the common terms used in the industry concerned: nautical people use sheaves, land-locked people mainly use pulleys. FWIW, which certainly isn't much, this is the way I personally use the terms:
1. The grooved wheel in a block (as in block-and-tackle) is usually but not always called a sheave. Pulley is probably technically acceptable, but less common.
2. If it is locked to a shaft - that is it has a central boss with a keyway or grubscrew for instance - it is, in my view, a pulley.
3. If it is an idler and therefore normally not locked to a shaft, either term would apply but I would call it an "idler pulley" because it is in a system of pulleys including a drive and at least one driven pulley. It would be odd to call the idlers sheaves and the drive and driven ones pulleys, in my opinion, because it would imply that the term pulley could not be correctly applied to grooved disks unless they are locked to shafts.
