Ball screws are used in a wide variety of applications, but some of the most challenging are those on the extreme ends of the performance spectrum – from large diameter, large lead screws for machine tools, to screws with small diameters and very fine leads for optical and medical applications. For very small, high-precision movements, designers and engineers often turn to miniature ball screws.
While there’s no industry standard for what classifies a screw as “miniature,” most manufacturers apply the designation to screws with a diameter smaller than 16 mm, while others include 16 mm screws in their “miniature” product line. To further segment the range of sizes, screws with a diameter smaller than 6 mm are sometimes referred to as “sub-miniature” or “ultra miniature.”
Some manufacturers offer a “compact” range of ball screws. This term generally indicates that the ball nut uses internal recirculation, giving it a smaller outer diameter. (Hence, the term “compact.”) Despite their name, compact designs often include screws with diameters up to 25 mm – much larger than traditional “miniature” ball screws.
In some regards, miniature ball screws are the same as their larger, “standard” counterparts. For instance, miniature ball nuts are offered in many of the same styles as standard ball nuts, including flanged, cylindrical (also referred to as “compact” or “slim”), or with a threaded end for easy mounting into a carriage or table assembly. Ball recirculation can be done inside the ball nut or with external recirculation methods. Lead accuracies and preload classes follow the same designations regardless of the screw diameter. And sizing parameters, such as L10 life calculation, buckling load, and critical speed, are the same for both miniature and standard ball screws.
Miniature ball screws, like standard versions, can be manufactured by either rolling or grinding the screw threads. Screw diameters below 8 mm are commonly produced by grinding, although some manufacturers offer screws as small as 6 mm diameter in rolled versions. In some miniature screw sizes, the journal diameter of the screw is too small to accommodate an appropriately sized end bearing. To address this, manufacturers can friction weld a larger journal onto the screw end, providing a sufficient journal for the support bearing.
To learn more about the differences between rolling and grinding methods for ball screws, check out this article.
In addition to small screw diameters, miniature screws offer an advantage over standard versions with their option for very fine leads. The smallest miniature screws have a lead of just 0.5 mm, which means every rotation of the screw produces 0.5 mm of travel. This is a significant benefit in applications that require fine adjustments, such as positioning semiconductor wafers or optical equipment, or driving small medical pumps and dispensing equipment.
Because miniature screws use very small balls for load carrying, preload options are more limited, with only a light preload of 1 to 2 percent typically achievable, versus up to 5 percent with standard ball screws using the oversized ball preload method. However, miniature screws offer more customization options and more standard variations in some respects. For example, combination left- and right-hand screws (where one segment of the screw has left-handed threads and another segment has right-handed threads) are commonly available in miniature designs.
Screws and nuts made from stainless steel are also readily available in miniature screw offerings, where they are rarely available for standard sizes. This can be attributed to the fact that miniature screws typically carry smaller loads, whereas stainless material would not be able to withstand the high loads that larger ball screws often transport.
Due to their small leads, miniature ball screws can execute very fine movements, and in vertical applications, these small leads make backdriving nearly impossible. Because ball screws operate with metal-to-metal contact, they’re not good for oscillating applications, even in miniature sizes where stroke lengths are typically small. For applications with oscillating-type motion, voice coil or piezo actuators may be a better choice.