Good Morning Gary,
I pulled up "wood chunker " on you tube and there are several videos and I think most are on the DOW site but easy to find on YouTube.
First we need a LOT of shearing force , there is no way around that. Next it needs to cycle at a comfortable speed for chunking.
Tom Collins shows how he design a chunker on his farm tractor. Patrick Johnson shows how he built his from scratch with a big shaft and flywheel.
If one has enough flywheel you can get by with very little horse power ( 5-10 maybe less)
I think Tom Collins approach is the less expensive and quickest to build because he is using the tractor that he already has. On the same note is Perter Coronis . Easy to build but using the equipment he already has.
In my case the truck axle was the best approach ( should be able to buy at a scrap yard for the weight.) Included are the cutting knifes with three spars ( rims) Flywheel and a spare ( brake drums) . Also the gear reduction is about a 6 to 1 so the flywheel is spinning about 400 rpm to make the cycles work out good.
I think the axle wood chunker could be powered by an electric or small gasoline motor . It works out nice for a small tractor just above an idle with the high speed pto. The tractors with the 540 rpm pto will have to run much faster and I don’t think they would be as efficient .