Alberto also started negotiations for the acquisition of ultracentrifuges from West Germany. He held many patents in magnetic bearings and ultracentrifuges. The division went on to design and manufacture a range of preparative and analytical ultracentrifuges. The elimination of air friction is useful for flywheel energy storage and ultracentrifuges. There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge. Modern ultracentrifuges are typically classified as allowing greater than 100,000 g . This occurs because ultracentrifuges can reach maximum angular velocities in excess of 70,000 rpm. Ultracentrifuges can also be used in the study of membrane fractionation. Ultracentrifuges spin the rotors under vacuum, eliminating air resistance and enabling exact temperature control.