Ball-screw basics: Debunking the myths

Misconceptions can lead to overspending and premature failure. Linear Motion and Assembly Technologies Ball screws come in a myriad of types and styles. Look at factors such as accuracy, preloading, and lubrication to help match the right product to a given application. With so many different ball screws on the market and the attendant flood of data from manufacturers, designers can have a tough time sorting out what truly affects ball-screw performance. Information often focuses on whiz-bang features that bring few real benefits, while more mundane but critical factors are missed completely. To help cut through the confusion, here's a look at four factors critical to successful ball-screw applications, as well as some related myths and misconceptions. Typically, an engineer's first and foremost concern is travel accuracy, which is possibly why so many standards address this single topic. Chief among them are DIN 69051, ISO 3408, JIS B1191, and ANSI-B5.48, which cover topics ranging from material specifications to geometric tolerances. The different norms can confuse the important issues. But common to all the specs is that lead error is the most critical indicator of travel accuracy. Lead error refers to the travel error over a given length as the nut travels along a screw. It is expressed in "mm per 300 mm" or "inches per foot" and determines a ball screw's accuracy rating. According to convention, a lower rating number means less lead error and, therefore, better accuracy. In other words, a Class 1 screw has a substantially higher accuracy than a Class 7 screw. Of course, the high-accuracy Class 1 screw also costs more and may take up to 10 weeks longer to produce. Designers should use accuracy ratings as the starting point for overall slide accuracy, but also weigh performance requirements against lead time and cost. MYTH #1: Accuracy