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FLEXO Magazine : August 2010
Technologies & Techniques The Case for Hollow Shaft Torque Motors significant Advantages on High energy Consuming and High Dynamic Applications By Harald Poesch Today ’s machine designer must evaluate more factors than ever in approaching a new project. Likewise, the integrator and retrofit engineer has expanded options, not only as a result of new technologies, but also because of critical areas of focus, such as reduced energy consumption, faster assembly time, vendor reduction and smaller footprint achievement. In the realm of motion control, one type of motor with a relatively short history has made significant advancements that necessitate a new look at its potential in many applica- tion areas. These applications range from machine tool rotary tables to various packaging, printing, converting, extruding, papermaking, plastic film and materials handling machinery, anywhere direction must be reversed with a very high degree of accuracy, no backlash (hysteresis) and the maintaining of motion control, contrasting the necessary decoupling of a conventional motor and gearbox. Enter the often-overlooked permanent magnet, synchro- nous torque motor. Torque motors are direct drives built for rotary axes where high torque and high precision are required at relatively low speeds. With significantly lower installation time, mainte- nance requirements, component part count and space allow- ance, these motor types are frequently viable alternatives to geared motors. Two popular varieties of torque motors exist today. They are the complete torque motor, requiring only direct flange- mounting to the machine and connection of the rotor to the machine shaft, and the built-in torque motor, where the stator and rotor are supplied as individual components that are directly integrated into the machine mechanics. Complete torque motors are often found on extruder main drives, feed heads on injection molding machines, roller drives on papermaking machines, wire drawing devices, textile machine web stretching and winders/cross cutters on packaging equipment. Built-in torque motors are typically used on machine tool rotary tables, swivel axes, dynamic tool turrets and turning spindles, as well as printing press cylinders, chill rolls in cast film and foil stretching machines, indexing tables in metal- forming presses and all other types of high-dynamic, high- precision path and speed control applications. Both types of torque motors feature a hollow shaft, which al- lows media or mechanical components to be guided through the rotor cavity. Torque motors are multiple pair pole synchronous motors, similar in operation to rotary synchronous servo motors. The rotor is equipped with permanent magnets, while the stator contains the motor windings. The high number of pole pairs leads to a design generating high maximum torque at low speeds. In the past, since eddy current losses increase with the number of pole pairs and the running speed of the motor, torque motors were considered applicable only at relatively low speeds. New water-cooled designs have countered this principle, allowing a high power density. Today ’s torque mo- tors can accommodate speeds of 1,000rpm or higher. As a result of these higher pole pair designs and because many mechanical power transmission components that gen- erate backlash, heat, friction and noise are eliminated, torque motors offer these benefits to designers: • Small footprint due to high torque density • Excellent rotational accuracy and repeatability due to direct load control • Space-saving machine designs, due to the elimination of gear units and belt transmissions • Low maintenance a direct result of having fewer mechan- ical parts in the drive train • High energy efficiency because mechanical losses in the drive train are eliminated. While higher in cost than geared motors, it is currently estimated that torque motors offer the designer a payback of three to four years in energy savings alone, which does not include the immediate increased performance and upfront cost savings, both in installation and maintenance. Obvious vendor reduction and inventory advantages are also realized through the use of these motors. On a typical multi-layer blown film line, for example, the use of torque motors can reduce the footprint of the extrusion section by half and the consumed production space decreas- es accordingly, leading to an increase in production rates per square foot. Torque motors were also long thought to be susceptible to chemical and other ambient atmospheric contamination, but new designs have been adapted to withstand corrosive atmo- spheres, such as a paper mill dry hood and, being water-cooled, are performing satisfactorily in many harsh environments with- out heat damage. Enclosures are available up to an IP54 rating with overload capability up to 2.5 times the rated torque. • Torque motors are direct drives built for rotary axes where high torque and high precision are required at relatively low speeds. • New water-cooled designs allow a high power den- sity, accommodating speeds of 1,000rpm or higher. • Current technology torque motors have been adapt- ed to withstand corrosive atmospheres such as a paper mill dry hood. 80 FLeXO August 2010 www.flexography.org August2010_mech.indd 80 8/13/10 7:46 AM
Sustainable Summer 2010