The downside is all the moving parts are on the outside (and therefore a hazard). With an archimedes screw - all the movement is safely locked away inside the casing.<br><br>Still a great bit thinking though.,(reply>), I'm sure the moving parts on the outside is less of a hazard.<br>For one, if a malfunction occurs, the product inside won't get contaminated. Another reason is that it's much easier to maintain with everything on the outside, readily accessible. You don't need to essentially break the thing apart when you want to fix it.