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Motion System Design

Direct driving controls axial forces

The continuous production of defect-free profiles with plastic extrusion requires precise technology. The extrusion process creates large axial forces on extruder screws that can damage components integral to the machine. Usually gearmotors with thrust bearings drive these systems and manage these axial forces.

However, according to Bernhard Krauss, application engineer of plastics at Baumüller Nürnberg GmbH, (with American offices in Bloomfield, Conn.), gears cause friction losses and require high maintenance and complex speed and torque calculations.

Direct-drive technology using high torque motors with integrated thrust bearings was developed for injection molding machine applications with Baumüller's hollow-shaft motors. The thrust bearings withstand forces up to 90,000 lbf. By replacing gearmotors with directly coupled high torque motors, critical process parameters can be easily changed through user programs.

To calculate the output of extrusion machines correctly, screw torque and speed parameters must be quantified. This calculation is colored by frictional losses in gears of conventional machines. Because high torque motors are synchronous, torque is derived almost directly from the drive output current to the motor, which in turn derives torque value directly from the drive current without inaccuracies. This also controls screw rotating speed.

High torque motors are easily integrated into extrusion machines. The hollow shaft of the DST series removes the screw from the rear of the machine without removing the motor. Reinstalling a screw is just as easy. The DST Baumüller motors reach torques up to 10,000 lbf-ft at a low speed of 100 to 300 rpm.

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