This ZIP file contains: One (1) custom material of a conveyor belt (using one of the ribbed door textures) that animates at exactly 32 units per second; one (1) uncompiled map demonstrating how this can be used in conjunction with path_track and func_tracktrain entities.
Do be advised that this is going to wreak havoc on your edict count—there's three dynamic entities for each item, at least—so probably don't try to build an entire working milk factory using this method. The ideal solution for a game this complex would be to just have a single animated model of everything. But assuming you lack either the skill or software to make such a thing, this will work in a pinch as a proof of concept to show someone who can, if nothing else.
Also, this does not work on players. Conveyor belts are not rides. Please exit the conveyor belt.