Recheck Your Gut Feelings

Patrick Scott, Product Manager, Ballscrews, Bosch Rexroth

Engineers can be stubborn people. And mostly, as a society, we want them to be stubborn. We want engineers to be natural skeptics, study alternatives, gather first-hand experience, test their conclusions, and THEN offer a well-considered judgment. Nowadays, however, technology advances so quickly, thanks to these same engineers, that it's impossible for them to stay abreast of changes that may run counter to long-held opinions.

Advances in ball screw rolling technology are a case in point. Machine designers are very opinionated about rolled ball screws, with some even claiming that rolled screws simply aren't acceptable in critical applications; instead, a good designer will insist on using a ground screw.

This is an outdated opinion that, simply put, drives me crazy. Rolling technology has developed into a highly precise process capable of matching and, in some cases, outperforming ground screws of an equivalent accuracy class. Our customers, and those of other high-quality rolled screw manufacturers, have demonstrated this. A high-quality rolled screw not only gets the job done, but it gets it done just as well for substantially less money with much shorter lead times to boot.

Perhaps it's the narrowing performance gap that leads to such an emotional attachment to ground screw technology. As the differences get smaller, a long-held opinion gets harder and harder to defend.

Historically, the two products were intended for different purposes. Grinding has always been routinely accepted for precision processing. Rolling, or cold-forming, however, was not originally intended for precision, but rather for high throughput and cost efficiency. So if a machine required precision motion, the default choice was a ground screw. Rolled screws were chosen only when the specification wasn't critical. Unfortunately, this perception of non-accuracy for rolled screws has persisted, and still often carries over to the machine designer's perception of how his machine will perform.

But the rolling process has matured substantially since its introduction, and is in fact a viable alternative for the manufacture of high-performance ball screws. A virtual army of specifications—DIN, ABEC, JIS, and others—all specify plenty of numbers, but none dictate which manufacturing method should be used to achieve them. A P5 ball screw is a P5 ball screw, whether it's rolled or ground.

Apparently, engineers aren't the dispassionate evaluators of objective data they like to think they are. They hold grudges like the rest of us. And somewhere, sometime in the past, many engineers must have gotten some really terrible rolled ball screws that put them over the edge, delayed their project, or hurt their careers. Or they don't understand the difference between a lead screw and a rolled ball screw.

In our experience, 80 percent of screw applications require travel lengths of less than 800 mm. So in the range of applications requiring less than 1m of travel, the debated gray area tends to be applications requiring positional accuracy around 0.012 to 0.025 mm/300 mm, along with high expectations for repeatability and other performance criteria, all of which can be easily met with today's rolled screw technology.

It really is time for designers to take a fresh look at this and other technologies they may have "gut" feels for. Because if they don't, they may be costing their customers additional, unnecessary money and time.

Reach Scott at patrick.scott@boschrexroth-us.com.