[sanddrag]

This year we used 1/2" keyed steel shaft. You can see the overall configuration in this picture. We used 1.5x2x1/8" wall box aluminum box tubing for the frame. We could have gotten away with 1x2x1/8. The wheels go on the outside of the tube and the sprockets go inside. Before it was welded we milled out a pockets (rectangular holes) on each side of the box tube. The pillow blocks fit in these pockets and can slide sideways to tension the chain. We have a small stationary block with a screw that is on the frame and the screw pushes against the pillow block to move it. It works really well. Extremely easy precision adjustment. Four 10-24 bolts clamp the pillow blocks to the box tube and keep it from moving. The pillow block on the outside has thru holes while the one on the inside has tapped holes so they pull together against the tube when tightened. Each end of the axle is tapped 10-24 and a bolt with a fender washer it used to "capture" the assembly.

The cutouts in the frame tube were made centerline. The bearing hole in the pillow block was made 3/64" off centerline so we flip the end two blocks up and the middle one down for a total difference of 3/16 lower for the center wheel.

You can see a picture of the pillow blocks here.
Picture showing sprockets
Good clean picture with tensioner screws
Up Close double sprocket with chain and tensioners