Several years ago in this column I questioned whether linear guideways had become a commodity, noting key engineering differences between various designs and their effect on performance and reliability.
George A. Jaffe
President Steinmeyer Inc. Bedford, Mass.
Steinmeyer is a maker of motion control equipment. www.steinmeyer.com
Perhaps it's time to give ball screws a similar level of scrutiny.
Ball-screw catalog data appear remarkably similar, as do the products themselves when inspected at trade-show displays. It's no surprise design engineers often bring their product analysis to purchasing and declare (with some frustration): "pick any one of these three or four ball screws because they're all basically the same."
Well, are they? Have ball screws attained the status of " commodity?" To Steinmeyer, the answer is a definite "No," especially for such demanding applications as machine tools and precision positioning equipment. The reasons go beyond simple comparisons of load-carrying capacity, product dimensions, performance ratings, and accuracy grades, and instead focus on fundamental design differences.
All ball screws convert rotary motion to linear motion with a nut and recirculating balls that travel on a threaded shaft. Balls recirculate in one of two ways: through external tubes that transfer the balls from one end of the nut to the other; or with internal deflectors that divert balls to the next thread or down the entire length of the nut.
Ball screws with external-tube return systems cost less to make than equivalent internal-deflector types but also have speed ratings up to 40% lower. (Speed rating by this definition equals the product of screw diameter and maximum rpm). This is because each ball exiting the thread must push forward several balls ahead of it in the tube, which raises friction. As speeds rise, tubes often jam with balls, causing ball skidding and excess wear.
Stiff competition among ball-screw makers has some of these companies boosting speed ratings of tube-return models, citing only minor changes in material, geometry, or both. However, the basic design and its inherent limitations remain unchanged.
Machine designers traditionally select ground screws for precision applications such as machine tools and inspection equipment. Pick-and-place and other general automation systems typically use rolled screws. But recent advances in rolling and whirling hard cutting, for example, have manufacturers touting lower-cost rolled screws as "equivalent to" ground threads. We say, caveat emptor. Rolled screws may give a similar lead error. But they also have reduced load capacity, higher friction variation, more noise, lower speed ratings, as well as higher concentricity errors.
Wipers are another important design metric. Effective wipers help ball screws last longer by keeping dirt out of the ball nut and lubricant in. Most designs use a simple plastic labyrinth wiper, which in our view is essentially useless because it does not contact the thread. Still other designs augment labyrinth wipers with a brush-type wiper, which again is not particularly useful because dust can easily bypass the bristles.
A more effective design combines a friction-type contacting wiper on the outside and a felt wiper inside the nut. The outside wiper touches the threads and expels dirt and dust. The felt, saturated with oil, distributes lubricant from a refillable reservoir as the nut traverses the screw. In contrast, some "lube for life" ball screws use a plastic wiper plate impregnated with lubricant. These depend on heat generated by increased friction to release and (hopefully) reabsorb lubricant.
It is important engineering details such as these that truly differentiate ball screws. Remember, all ball screws regardless of design will work for routine applications. But for more precise and demanding machinery, be sure to look beyond the "commodity" label and evaluate the differences.