Yet there are limits to such a system. The bevel stage in the gearbox, primarily used to redirect the rotating movement from the horizontal motor shaft into vertical direction of the grinding table, limits power capability. For design power of up to approximately 9,000kW, this can be overcome by increasing the gear ratio in the following planetary stage, which keeps the bevel stage size within feasible dimensions. However, this does not fulfil mill requirements and a further increase in drive power requires larger dimensions, especially the diameter of the bevel wheels. This decreases the overall reliability of the drive system. Conventional gear units cannot operate VRMs with higher design power. The drive system for these applications is based on two main principles: partition of power to several drive units and elimination of the weakest element in the drive train.