Patrick Scott
Product Manager, Ballscrews
Bosch Rexroth Corp.
Linear Motion and Assembly Technologies Charlotte, N.C.

Usually, component choices are determined by which achieve the highest performance with the lowest total enduser cost of ownership. Mistakes can be costly, either in redesign losses or in poor machine performance.

Engineers are most often concerned with travel accuracy, which might be why so many published norms are available on this single topic. Chief among these are DIN 69051, ISO3408, JIS B1191, and ANSI-B5.48; they cover everything from material specifications to geometric tolerances. The sheer preponderance of norms makes it difficult to identify the most critical indicator of travel accuracy. Let's focus on the single-most cited factor. Common to all of these specs is the measurement of lead error, the excess or insufficient distance traveled along a screw. It's expressed in mm per 300 mm or in. per ft and determines a ball screw's accuracy rating ... say P1 or T7, for example.

In many applications, a single ball nut preloaded with the ballselect method can achieve much of the performance of other systems at reduced cost.

But let's back up. What do these classifications mean? According to established conventions,a lower number means less lead error and therefore better accuracy. In other words, a classone screw has a substantially higher accuracy than a class-seven screw. Of course, for a high accuracy rating a class-one screw is costlier and may take up to ten weeks longer to produce. When selecting a ballscrew, designers should use accuracy ratings as the design starting point for overall slide accuracy. However, designers should also weigh accuracy requirements against leadtime and cost requirements.

Ballscrews and rails minimize the total number of parts and enable Sunnen Products Co. (St. Louis) to include the same technology in several machine types.

Myth number one: The specification for accuracy grade also dictates the manufacturing method for the screw material — so high-accuracy screws must be ground screws.

Fact: Although many designers believe that high-accuracy screws can only be achieved by grinding, none of the specifications cited earlier actually dictate a production method for a given class of ballscrews. They do, however, differentiate between precision screws and transport screws. To reiterate, lead error is represented by mm of error per 300-mm travel segment. Transport T-Class lead error is allowed to accumulate in a linear fashion over multiple travel segments. (In the past, rolled or cold-formed screws fell into the T Class.) By contrast, precision P-Class accuracies keep 300-mm lead error down and limit error accumulation over more extended lengths. Ground ballscrews used to be the only type capable of holding these exacting P-Class tolerances. Today however, new technologies enable manufacturers to extract P-Class accuracies from precision-rolled screws. Rolling has now evolved into a CNCcontrolled tight-tolerance process with P3 accuracy capability, near perfect roundness, and tolerances well within the DIN control limits. So with the vast overlap in manufacturing capabilities, it's now possible to obtain screws of virtually any accuracy with any given technique, with huge benefits for machine designers.

The ballscrew motion control in Sunnen's SV-Series honing machines produce true vertical stroke, for in bore size and finish.

Beyond the ballscrew itself
To begin this section, let's address another myth.

Myth number two: Ballscrew accuracy equals axis accuracy. On the surface, this seems true. But, are accuracy and repeatability the same thing? And, do other machine components affect the total tolerance stack-up? Repeatability is a screw's capability to return to a defined point time after time.

Fact: Many factors contribute to overall repeatability. Drive connections, guide mechanisms (rail or shafting) and the machine's structure all contribute to repeatability. It seems logical that poor performance of surrounding components can jeopardize both the accuracy and repeatability of even a 'perfect' ballscrew. However, the pressure to save money often results in lessthan-satisfactory overall machine performance once testing begins. If this happens, designers may achieve shortterm savings only to lose them and more by having to redesign the machine ... and this does not consider lost revenue from a missed market introduction or additional maintenance and repair costs suffered by the enduser.

Preload

Let's assume our machine structure is properly designed. Then we can focus on the motion-related components. For optimum performance, it's critical to eliminate any and all lost motion. Commonly called backlash, most of this motion occurs between the ball nut and screw. A range of nut designs can remove backlash by means of preloading. Preloading ball nuts means that neither axial or radial freedom exists. Instead, the ball nut is matched to the screw by adjusting the dimensions of a variable component.