Thanks! In my opinion your answer is very theoretic, because for inertia of e.g. axis 5 the robot has to hold axis 6 and the mass of the tool and so on. Let´s assume I have all the jointforces for axis 5 and 6 during a trajectory with and without load. Is this enough to calc the inertia?

Thanks for your answer. Isn't there a length Missing at the last formula?