I'm trying to model pressure inside of a cylinder.  I put an ourward pointing pressure on the ID of the cylinder and place roller fixtures at the end.  When I try and run it, I'm give a generic error complaining about boundary conditions.  Is the problem that I'm only restraining the cylinder in the z axis, but not the x and y?  How would I constrain the cylinder in the  x and y without distorting the analysis?  For example, I tried fixing the ends (and thus constraining in all dimensions), but the solutions balloons in the middle but not the edges.  I want to model this cylinder without any distortion due to fixtures.  Any ideas?  Thanks.