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Shaft Torsional Stiffness

Valid for Cartridge DDR

A primary reason machines utilize direct drive motors like Cartridge DDR is for the purpose of increased servo performance. A high performance servo system is one where the shaft closely follows the command input and rejects any disturbance torque that might be applied. Examples of applications that benefit from high servo performance are electronically registered print rolls, coating rolls where speed variation reduces the quality of the product, packaging machines where an accurate servo facilitates increased product throughput, indexing tables that require precise, rapid positioning, and any application where servo accuracy affects product quality or throughput.

To optimize the servo performance, consider the machine design, especially between the motor and the load. The machine design must provide for the highest possible torsional stiffness between the motor and the load. Low torsional stiffness between the motor and the load can introduce resonant oscillations and greatly reduce the ability of the motor/drive to accurately respond to either a changing command input or to a changing disturbance torque.

The best ways to maximize torsional stiffness are:

  • Keep shaft length short by locating the motor as close as possible to the load.
  • Keep shaft diameter as large as possible. Step up shaft size when exiting the Cartridge DDR Motor when possible, especially for shafts longer than 4 inches (10 cm).
  • Avoid compliant couplings between shaft and load.

The two illustrations below show examples of a shaft with high torsional stiffness and a shaft with low torsional stiffness.

ddc_torsional_stiffness.jpg

A long and/or small diameter shaft between the Cartridge DDR Motor and the load acts like a spring between the inertia of the motor and the load and set up a resonance between the two.

For example, on the Cartridge DDR Motor, there is a specified diameter of the shaft to interface with the compression coupling. The machine designer should insure that the diameter of the shaft increases as much as possible when it leaves the motor. Large bearing races and a short axial length between the motor and the load are examples of design approaches that help maximize the servo performance.