Almost all manufacturers have their "advertised" maximum workpiece weights and corresponding rpm's.  I would imagine that most machines could far exceed their advertised capacity if you were to calculate using bearing capacity alone, but remember, the instant a toolbit hits the workpiece, the calculation changes.  RPM is another story.  You can have a very heavy seemingly balanced workpiece turning just fine at, say , 350 rpm, but bump that up to 500 and the machine might start vibrating like crazy.  The weight and footprint of the machine have a lot to do with its practical capacity as well.