There’s much debate among manufacturers, and sometimes among users, regarding the superiority (or lack thereof) of ball screws with ground threads over those with threads that are formed by rolling. Ground ball screws have traditionally been the choice for high-precision applications, whereas rolled screws offered an economical solution for general industrial and automation applications. But manufacturing technologies have changed over the last several decades, and rolled ball screws are no longer the “pepper grinder” devices that some users experienced in the past. Regardless of which side of the “ground versus rolled” debate you identify most with, here are three facts to keep in mind when determining which to use in your machine or process.
DIN/ISO and JIS specifications distinguish between two types of ball screw accuracy: “P” (precision) and “T” (transport).
The lower the number, the better the accuracy, with precision classes ranging from P0 to P5 and transport classes from T5 to T9 (T10 for JIS). JIS specifications denote accuracy classes with the prefix “C” for precision and “Ct” for transport.
A common misconception is that the accuracy class specifies the manufacturing method, but the two are not intertwined. Rolled screws can be made in P5 and even P3 accuracy, and some ground screws only meet T accuracy requirements. The important thing to understand is whether, according to the manufacturer’s specifications, the lead error, v300, accumulates over the length of the screw. P accuracy classes do not allow lead error accumulation, while T accuracy classes do.
Geometric tolerances are also specified by DIN/ISO and JIS standards.
For ground ball screws, both thread grinding and journal grinding are done using the same reference centers, making it easier to minimize radial run-out and to keep the screw threads and end journals concentric.
When screws are manufactured by rolling, the end journals are machined and ground after the threads are rolled, so maintaining concentricity and run-out is more difficult. However, if a ball screw is manufactured to DIN/ISO or JIS standards, it will not only meet lead accuracy specifications, but also the geometric specifications, regardless of whether it was manufactured by rolling or by grinding.
The rolling and grinding processes produce different surface finishes.
A rough surface finish is problematic in ball screw assemblies because it can result in higher friction and more wear on the load-carrying balls.
The grinding process produces a very smooth surface finish, and in theory, the rolling process does as well. But in reality, screws that are rolled must also be polished in order to remove an oxidation layer that forms during the rolling process. So when considering surface finish, the comparison is not between the ground surface and the rolled surface; it is actually between the ground surface and the polished surface of the rolled screw. The quality of the polishing step, and not the rolling process, determines the surface finish quality of a rolled screw.
As with most design criteria, the decision regarding which type of ball screw to use comes down to performance requirements and cost. Ground screws are necessary when ball screw accuracy below P5 is required, as the rolling process cannot produce these accuracy classes. But for P5, and in some cases P3, accuracy, both rolled and ground screws can meet the required specifications. And for accuracy classes 7 and 9 (10 by JIS standards), either manufacturing method can produce ball screws that meet DIN/ISO or JIS standards.
Feature image credit: Thomson Industries, Inc.