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.
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 class-one 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.
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 CNC-controlled 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.
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 less-than-satisfactory overall machine performance once testing begins. If this happens, designers may achieve short-term 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.
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.
One method of preloading a ball nut is to use a double nut system. Double nuts consist of engaged nut body pairs wedged against a spacer and locked in place. In household applications, this technique is also utilized with simpler “jam nuts” used on nut-and-bolt mounts. In this case, the second nut wedges against the first to lock it in place. A common misconception about the double-nut method is that capacity is doubled as well, since there are two nut bodies. In fact, each nut body takes load only in its respective direction. The opposing nut body is deflected out of load sharing during operation. So, no doubling of the capacity occurs. Additionally, the assembly method for double nuts is one of the most difficult.
Another preload method is the lead-shift, which consists of a manufactured raceway spiral µm offset halfway down the nut. This changes the angle of engagement for the balls similar to that of the double nut. Lead shift nuts perform similarly to the double nut for ball contact patterns, but typically have much fewer active grooves, thus reducing load-carrying capacity, and therefore life expectancy.
Finally, in the ball-select method, balls are intentionally selected a few microns larger than perfect fit. Because the balls are larger than the groove, they force a contact with all raceways of the nut and screw, causing a four-point contact scenario to eliminate the backlash. This is a difficult method because the raceway tolerance requirements are very tight for both nut and screw. For this reason, only a few manufacturers preload ballscrews this way.
Myth number three: The double nut is the best (some say only) preloading method for ballscrews.
Fact: For many applications, the ball-select preload method achieves most of the performance at considerably lower cost. In addition, this preload method offers a compromise between the previous two options. The oversized balls make each groove in the nut an active one, while taking loads in both directions.
Now that backlash is eliminated, the only remaining lost motion is lengthwise deflection. A double nut has only a 5 to 10% benefit in rigidity over the ball-select nut, while the lead-shift version drops off dramatically. When considering the effect of the screw length, as shown in the graph, the ball-nut styles have even less differentiation. As one can see, in many applications, the single ball nut with the ball select preloading method can achieve much of the performance at substantially reduced cost.
The meaning of life
So let's assume we have a well-designed screw assembly, capable of all performance expectations. What happens during actual operation where the ballscrew must operate for extended periods? Lubrication failure is enemy number one, so we should consider a ‘lubed-for-life’ assembly. But what does lubed for life mean?
Myth number four: Lubed-for-life options on screw assemblies guarantee 10,000 km of travel in every application. One common misconception is that ‘lubed for life’ means exactly that: No maintenance required ever. (As if finally, we have found the fountain of youth for ballscrews; just add a couple mLs of oil in some type of reservoir attached to the nut, and watch it go.) In fact, many published claims indeed state 10,000 km of travel reached in tests. But for purchasers of ballscrews, it's critical to know the test conditions and how they relate to real-world conditions. In some cases, the 10,000 km are achieved with no applied loads in a cleanroom environment.
Fact: Real-world applications rarely operate without load in clean environments. Let's look at this from an everyday perspective. Anyone who maintains their own car understands that oils (synthetic and natural) degrade with usage. The equivalent of a cleanroom here would be a car always garaged. How long might the oil in such a situation last? Probably indefinitely. But in a Nextel Cup car, the oil would barely last one race — if that. Ballscrew applications are no different: For a ballscrew operating inside a machine tool with heavy loads and high speeds, lubricating for life isn't possible.
Therefore, one must understand how advertising claims relate to specific applications. If the test isn't transparent or easily understood, or if the claim seems too good to be true, get more information so the final design isn't compromised. When comparing products from different suppliers, always gather enough information for an apples-to-apples comparison in the application. So-called lubed-for-life ball/nut designs do extend service intervals because they seal in and preserve the lubrication where it is needed — inside the ball nut. Their effective seals also keep environmental contaminants out. However, each manufacturer interprets “life” to mean different things.
For more information, visit www.boschrexroth-us.com.
TOOLING EXAMPLE: Honing in on perfection
Ballscrews help new honing machines deliver precise and consistent size, surface finish, roundness, straightness, and surface texture.
Honing is the removal of small amounts of material after boring high-precision parts such as fuel injectors. “The honing process is often considered a mystery because many people don't understand how it works,” says Jose Martin, senior mechanical engineer at Sunnen Products Co.
St. Louis-based Sunnen has long been a leader in industrial bore sizing and finishing machines, catering to many U.S. customers, including the Big Three automakers, Caterpillar, and Cummins. Recently, what started out as an internal research project for Sunnen concluded with a significant shift to using ballscrews and rails for the tool stroking process in their new machines — a major divergence from traditional cam drives. Improvements in machine speed, control, and accuracy in part come from ballscrews from the Bosch Rexroth Corp.'s Linear Motion and Assemblies Technology Group (Charlotte.)
During bore sizing and finishing, a diamond or CBN cutting tool passes through the bore while the tool rotates and removes material. Sunnen has further refined the honing aspect of this process with its tool stroking system. Tool stroking works like this: The part — a fuel injector, for example — is presented to the cutting tool on a spindle. Then either the part or the spindle reciprocates back and forth. As the part is stroked the tool expands, removing small amounts of material. However, because part geometries are becoming increasingly complex, bore non-uniformities along the bore axis need special motion control afforded by this stroking system to generate the highest precision finished product. To achieve tight tolerances, the part must be allowed to move or float within three axes.
Given the challenges of using ballscrews in a honing application (namely, the short cycling motion and high number of strokes per minute) many thought their control and productivity goals were impossible.
But ballscrews are suitable because they can achieve multiple gs of acceleration with very high linear cycle speeds to meet the rapid-reversal move profiles. Their high rigidity means the strokes and dwells of Sunnen's heavy honing mechanism are controlled as needed.
“Ballscrews reduced part quantity by a factor of ten,” says Martin. As a result, assembly time is reduced and field repairs are simplified with only three areas to troubleshoot: the ballscrew assembly, the coupler, and the servo system.
The new design also allows the machine to dwell in any part of the bore, end-to-end, selectively removing stock for the straightest, most precise bore possible.
|Shown here are the key advantages and disadvantages of different preload options.|