These tilts impair the surface quality of precision machining and lead to additional costs due to the need for reworking. For example, the phenomenon of short-wave surface defects can occur on workpieces when machining with rigid and direct-drive rotary axes in ultra-precision milling, grinding and gear cutting machines. Accuracy deviations of less than one micrometer up to a few micrometers lead to quality problems on high-precision workpieces in mould making, lens end machining or gear cutting parts of the highest quality. The deviations are caused by pulsating axial and radial forces from the magnetic fields of the torque drive, which act as interference forces and torques on the bearing via a lever arm. The error period is usually due to the number of poles, particularly in the case of sliding stars.

The phenomenon is not new, but surface qualities are now so high that these phenomena can become visible on the machining marks. In addition, the requirements for shape tolerances and surface qualities have increased in some industries to such an extent that the effort required for reworking is no longer tolerated. The engineers at INA - Drives and Mechatronics, a direct drive specialist within the Schaeffler Group, developed a "double" motor structure as a solution, in which the interference forces and torques occurring in the motor are balanced internally, thus effectively avoiding surface defects in ultra-precision manufacturing. The torque motors based on this principle are interchangeable with standard torque motors on the market despite their more complex design. Air gap diameters of 89 to 460 millimetres and magnet heights of 25 to 200 millimetres are planned for the new series. pb