P.O.,
Here is a hand powered one I made. Maybe you can use some of the ideas.
I used it to bend and roll 1/16-inch thick, 30-inch wide steel. I also rolled 1/8-inch thick, 12-inch wide steel. Could it roll thicker or wider? Maybe, but those dimensions were all I needed, hence the bottom roller distance.
The rollers are 32 inches long and are made from a heavy wall 3.5-inch pipe (I think it’s called steam pipe – but not sure). The ends are 3-inch long solid bar inserts with a one-inch hole bored in the center for the 1-inch “axles”. These are machined for a very slight press fit and are held in place with a pin. I think you can see it in one the pictures. The bearings are just a simple bronze bushing. The top roller bushing is a piece of round bar stock also fitted with a bronze bushing, with two groves milled to slide against the side edges.
The sides are 1/2-inch steel plate welded as shown. The axles ends have regular nuts (threads bored out) welded to them so I can turn them using a socket and a two-foot breaker with me as the power source. The top roller is made to come out if needed. For the fun of it, I machined each roller the entire length but that is not needed. All in all, very simple. This unit is made to bolt to a bench…a very stout bench!
I welded nuts on all axles in case I needed more “power” (translated: more people) but didn’t need that on the material I bent. To roll the steel I used, I only needed to “power” the top roller as that supplied enough friction to pull the metal through.
As you know, if one moves the bottom rollers farther apart the easier one can bend thicker material. So if you make those rollers adjustabe too, the more versatile your machine will be. Mine has sat in the storage shed for about 10 years now.
You can probably come up with better ideas but at least this may provided you with some starting concepts…