I did some maths based on a thin shelled ring weighing in at 30 lb with a 20ft/s tip speed max comes to 250J no matter what size the weapon is. In reality it will be much less than this due the impossibility of all of the mass being on the edge.

The formula quite nicely simplifies to Max KE = (mk^2)/2 where m is the robots mass in kg and k is the maximum allowed tip speed in m/s

On a side note, I remember reading that the antweights DBA and Katjuscha have something on the order of 100J in their discs so these should be safe(ish)

Edit: Added units for the formula