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FLEXO Magazine : August 2009
TECHNOLOGIES & TECHNIQUES feedback device and became a fully integrated part of the machine. This approach has the advantage of consuming the least amount of space. On the other hand, frameless motors are relatively expensive to fully integrate as they typically require substantial changes to the design of the underlying machine. Frameless motors are also more diffi cult to service because they are embedded into the machine. While the initial development cost burden is high, the benefi ts of higher performance, higher quality, and small space requirements justify this technology in some applications. FIGURE 3. A direct drive press confi guration. All rollers are driven independently by a direct-drive rotary motor, without the need for gears or gearboxes. considering a 10-color press, more than 100 parts can typically be removed from the BOM. When the anilox roller and plate cylinder are driven by a single motor via a gear system, it is diffi cult to separate the two axes for maintenance and exchange of printing sleeves or plates. In a direct-drive confi guration, the anilox, plate, and CI drum can be moved independently of each other for easy maintenance, cleaning, and change of plate blankets. This can also be viewed as a safety improvement since the rolls can be controlled independently. Finally, the direct-drive method also eliminates the need for alignment, lubrication and eventual replacement of the mechanical transmission system. SMALLER MOTORS Since the direct-drive motor is directly connected to the machine, inertia-matching is not required as it is on a conventional servo motor with gears. Stepper motors are typically sized to match the load in order to have enough torque to overcome disturbances when torque is low, which occurs as a result of nonlinearity caused by the torque roll-off or resonance at certain frequencies. However, closed-loop servo motors with controlled commutation are not prone to the same de-synchronization issues and torque losses. The servo system also maintains a linear and predictable speed torque curve without the need for special commutation sequences or anti-resonance control. For these reasons, DDR motor size can be based on the peak torque required for achieving the desired acceleration time specifi cations. With direct drives, inertia mismatch of 250 to 1 is common and mismatch of 800 to 1 has been implemented. In many fl exo presses, the size of the motor is dictated by the inertial matching requirements. The result is that a much smaller and more energy-effi cient DDR motor can be used in most applications. Today, many machine specifi cations, even in the industrial environment, list the maximum allowable audible noise levels. The audible noise level of a direct-drive system can be as much as 20dB lower than a geared system, as transmission components generate considerable noise levels. So, installing a direct-drive system can help achieve the required audible noise specifi cations. DDR ALTERNATIVES Direct-drive rotary technology has developed in an evolutionary manner. The original frameless direct-drive motors were designed into the machine architecture along with a 54 FLEXO AUGUST The next generation of DDR technology, sometimes referred to as full-frame systems, integrates all of the components of a complete motor including the rotor, stator, bearings and feedback device within a housing. The machine shaft slips through the bore in the motor and attaches to the rotor. This approach substantially reduces development costs since the motor no longer needs to be integrated with the printing press. The disadvantage of this approach is that the motor’s and the machine’s bearings must be precisely aligned, which is a complex and time-consuming task. The bearings in the motor and the load are directly coupled in a linear fashion making it nearly impossible to align the system components properly without causing premature bearing failure due to loading. The most recent approach to DDR systems, the cartridge DDR servo motor, is fully housed and ready for mounting to the machine. However it has no bearings and uses the host machine to support the motor’s rotor. This approach makes it easy to use direct-drive technology on machinery that already has bearings, particularly in applications such as printing where rollers already use heavy-duty, precision bearings. The motor has a hole in the middle which slips over the shaft of the anilox roller, plate cylinder, or central impression roll and the motor housing bolts to the machine frame. Installation typically takes less than fi ve minutes. The motor slides over the shaft until a motor pilot engages a machine pilot. The housing is secured with bolts. The motor rotor is then secured to the machine roller shaft by means of a compression coupling tightened to a specifi ed torque. The rotor is now rigidly connected to the machine shaft. The encoder alignment is pre-set so that no adjustments need to be made. Cables are connected and the motor is ready to run. A servo system equipped with a cartridge motor is expected to work for 10 years without any maintenance. Although the initial system cost might be higher compared to a conventional geared system, over a period of several years, eliminating the cost of repairs and periodic maintenance makes the overall cost of purchasing and operating a cartridge system lower. Even with the slightly higher initial cost, over a fi ve-year period, cartridge motors can reduce operating costs up to $10,000 per motion axis compared to conventional geared servo systems. It’s no secret why direct-drive technology is being increasingly used in new printing press designs. For printers, the higher quality and throughput associated with DDR systems translate into higher profi tability. For manufacturers of printing presses, direct-drive technology offers a substantial competitive advantage as well as easy integration with current and new machine designs. ■ ABOUT THE AUTHOR; Tom England is director of global product planning for Kollmorgen, A Danaher Motion company. www.danahermotion.com/kollmorgen; 540-633-3545. 2009 www.flexography.org