I am designing a Bale loading machine to load Small rectangular bales of 25kg. The machine will be driven by its wheels so no hydraulics and no power being drawn from the PTO. An example of one can be seen http://marongfarmmachinery.com/sales/view.php/16 I would like to put a clutch on the axle so the conveyor doesn't run unnecessarily. I have seen some old farming implements with clutches on the axles that activate the mechanism. I would like to know what these clutches are called so i can find information on them. Are they off the shelf items? Do i have to design my own? If they are off the shelf items who sells them? I read up a little on different types of clutches and the description of a mechanical lock-up clutch seems to suite what I am looking for. unfortunately All search results for this bring up stuff about automatic transmissions on cars which is of no use to me.