In my opinion (based on my design engineering experience) anything between 10 and 25 degrees off vertical will work.
As you go closer to vertical, cleaning pressure increases, thrust load on the rotary joint bearings increase and speeds are reduced. As you go closer to 25 degrees, your cleaning pressures are reduced, the thrust load on the bearings are reduced and it's easier to get higher RPM's. After about 25-30 degrees everything diminishes drastically.
If you have radial bearings, they will fail faster as thrust loads increase (also the faster the RPM the sooner it will fail). With angular contact bearings, your thrust loads can go pretty high along with RPM.
Looks like a big difference between the Steel and HydroTwister set-up (7-1/2 to 22 degrees respectively). If you're going to ERR with the Steel Eagle set-up, do it towards 10 degrees rather than the other way. (Even with a gauge you're not going to set an exact angle.) I could tell you a lot more if I knew what brand, type and class bearings were in each one (if anyone cares).
Another item that was mentioned that I agree to be very critical for the rotary joint is keeping the arms straight as this will wear out the union very fast (you can usually feel it - it will wobble and shake more than normal when running).
Good info folks - "Surface Cleaner Engineering 102"
Does anyone know what brand rotary unions are in either of these units? I'd be curious to know. The brochures don't seem to give you the information on the most critical part of the system.
Regards,