When machining with very rigid and direct-driven rotary axes in ultra-precision milling, grinding and gear cutting machines, visible defects can occur on workpieces. This is known as the phenomenon of short-wave surface defects. They can often be recognized by optical effects on the surfaces, such as regular stripes or shadows. Grinding stars on the end faces are also indications of this type of defect. Such deviations in accuracy of less than one micrometer to a few micrometers lead to quality problems on high-precision workpieces in mould making, lens end machining or for top-quality gearing parts.

These deviations occur in rotary axes and rotary tables due to the slightest tilting of the axis or clamping surface. This is caused by pulsating axial and radial forces resulting from the magnetic fields of the torque drive. They act as interference forces and moments on the bearing via a lever arm. These loads, which are relatively small for the bearing, cause axis misalignments and tilting in the sub-micrometer range, which can leave the described marks on the workpiece surface. In the case of grinding stars in particular, the error period is usually due to the number of poles of the motor. The phenomenon is by no means new, but the surface qualities today are so high that the effects can become visible on the machining marks. In addition, the requirements for shape tolerances and surface qualities have increased so much in some industries that the cost of reworking is too high and the market demands a solution.

In order to avoid the resulting surface errors in ultra-precision manufacturing, the engineers at INA - Drives & Mechatronics (Idam), the direct drive specialist within the Schaeffler Group, have developed a "double" motor structure. This structure balances out the interference forces and torques that occur within the motor.

The new SRV series torque motors are based on this principle. Despite their more complex design, they can directly replace standard torque motors on the market. Air gap diameters from 89 to over 460 mm and magnet heights from 25 to 200 mm are available for the new series. Speeds of up to 14,000 ^min-1 can therefore be achieved.

According to Schaeffler, when using the new torque motors, there is no longer any measurable introduction of pulsating motor-related axial and radial forces into the machine structure. The machined surfaces show no optical defects and can be realized with very high dimensional and shape accuracy. am