Pannigen, The Netherlands —When machining large components, it is usually more convenient to move the cutter, not the workpiece. Maintaining the cutter's positioning accuracy over long travel lengths, however, can be difficult. Drive components must be rugged enough to withstand shock loads during the machining operation, yet maintain high precision—preferentially with zero backlash.
For machine tool designers at Unisign, the Dutch company's Uniport machine tools presented yet another challenge: finding a drive small enough to fit inside the well space below the machine's traveling gantry. Their solution? Two Sumitomo FCA-45 Fine Cyclo gearboxes, each one driven by an ac servo motor. Positioned on either side of the gantry, the cyclo drives move the gantry at speeds to 80 ft per minute via a rack-and-pinion system. Despite such a high rate of travel, the machine maintains a 0.0004 inch x-axis repetition accuracy.
Compact gearboxes and servomotors fit in the well space below the traveling gantry.
Originally invented in Germany, the cyclo drive features three major moving parts: 1) a high-speed shaft assembly with integrally mounted eccentric cam and roller bearing assembly, 2) cycloid disks, and 3) a slow-speed input shaft. During operation, a rotating eccentric rolls one or more cycloid disks around the internal circumference of a ring gear housing. The number of teeth on the disk and the number of rollers on the gear housing determine the gearbox reduction ratio.
Cyclo drives for standard applications incorporate two cycloid disks, but the Fine Cyclo model used in the Unisign machine tool has three disks arranged at 120°.
Apart from contributing to the zero-backlash feature, three disks provide increased performance under vibration and shock loads. To put a figure on performance, lost motion for this type of drive is less than two minutes of arc, or 2/60ths of a degree. In addition, the hysteresis curve of transmitted torque versus rotated angle for a Fine Cyclo drive is substantially narrower than the corresponding curve for a conventional gearbox and does not have the discontinuity at zero torque displayed by the conventional system.
Henny Lemmers, Sumitomo's location manager for the Benelux countries, explains, "There are other methods of achieving zero backlash, but the cyclo design gives a rugged zero backlash unit that is far more compact than a conventional gearbox." He adds that a zero-backlash cyclo drive is also more economical than a play-free conventional solution. And, with more than 30% of each of the three cycloidal disks in contact with the rollers on the housing, the drive can withstand peak shock loads of more than 500% of the normal torque load.
Additional details...Contact Dean Johnson, Sumitomo Machinery Corp. of America, 4200 Holland Blvd., Chesapeake, VA 23323; Tel: (757) 485-3355.