Ball screws often have trouble delivering extremely fine movement and slow motion, like moving 1 m m/sec. Motion this slow requires consistent friction torque. Intermittent short strokes or frequent oscillations cause balls in regular ball nuts to back into each other, increasing inter-ball friction and idling torque, and potentially rendering the servoloop control unstable. To eliminate these concerns, Steinmeyer Inc., Bedford, Mass. (www.steinmeyer.com), developed elastic spacers reliable enough for ball screws. Until now, using spacers to build flexible ball chains would work for linear guides but fail in ball nuts because of higher speeds and morecomplex motion path.

The new plastic spacers resemble back-to-back Belleville springs, and the slight compression of each spacer between adjacent balls ensures close guidance during recirculation inside the nut. The spacers also have holes in the middle, like donuts. If a ball chain jams, putting excessive force on the balls, adjacent balls can touch each other. Thus, spacer compression is limited and they won't crack due to overloads. The spacers also store lubricant, so they extend ball-screw life despite little or no maintenance. Using spacers does not reduce the number of balls needed, because the distance between balls with spacers is less than the gap inherent in ball nuts without spacers.