Let’s sort it out: Ball recirculation is the single most important design detail when it comes to ball screw performance. Why? Because it determines how smoothly the balls recirculate as well as running performance and product lifetime. And there is still no generally accepted nomenclature for this characteristic. Result? Engineers use hands and feet, lengthy explanations and quickly drawn sketches to make sure everyone understands what they are talking about.
Steinmeyer, as the only maufacturer in the world who builds all known ball returns, (well OK – except the one with tubes that we won’t use unless we really have to) started some time ago to at least categorize their return systems as either internal or external ball return. But what does that mean? Internal ball return includes all designs where the ball is not lifted off the screw shaft. Balls are merely guided sideways, climbing over one outside edge of the thread groove. Because they will necessarily “fall” into the next groove after crossing the outside diameter of the screw, this is always a thread-to-thread, cross-over, or flop-over ball return. It is not always, however, a “ball deflector” system, which suggests a piece that handles just one ball circuit. This cross-over system can also be built with several ball return grooves in-line in the same piece, which is called a “multi-liner”.
Got it so far? Internal means that the balls are forced sideways and guided into the adjacent thread. So they are normally used in single-start ball screws, and after crossing into the next thread groove the circuit is completed. One ball return (and if there are individual deflector pieces you can typically see and count them) serves one circuit with one trip around the shaft, so the number of “circuits” is equal to the number of “turns”.
That is different with all “external” ball returns. Steinmeyer calls these external because here the balls are lifted off the screw shaft and moved into a channel. This channel guides them all the way to the other end of the nut (we will talk about the limitations of this later), where they are placed back into the thread groove. Because several threads can be crossed this way, each circuit may consist of several turns. The way the pick-up of balls is designed, and the shape of the ball return channel determine if the number of turns is a whole (integer) number, or if it includes a half turn as you normally see in those external returns that use some sort of tube.
Performance and reliability (or up-time as machine tool folks call it) depends largely on how the ball return is built. That holds true for both the internal return and the external one. However, the external return offers significantly more ways to “screw” up. It needs some sort of “scooper” that guides balls into the return channel, and this scooper is necessarily quite thin as it has to reach into the ball track and under the ball to capture it. If it breaks, loss of all balls from that circuit (which, in safety relevant applications, is hopefully not the only one!!) would be the result. So it is desirable that the ball path at this critical point be tangential, so there are no mass forces that the scooper has to overcome.
One requirement is common to external and internal returns: The balls have to follow their prescribed path with minimum effort and no jamming. Smooth transitions, a perfect fit, and maximum radii in all bends of that path are good starting points for designing such ball returns. Close guidance of balls is another one – because excessively wide channels or grooves allow balls to push each other sideways. Materials of inserts and ball channels is important in two ways: The material has to be both durable enough, and offer a low coefficient of friction.
Unfortunately, the maximum number of balls that can be in one circuit is limited: Manufacturing tolerances cause balls to travel at slightly different speeds, and fast ones catch up to slower ones. It’s pretty much like a two-lane highway during rush hour – there is no way to pass, so faster ones follow the slow one in front. With too many balls in one row, this can cause a significant amount of extra friction from the balls rubbing each other.
In aerospace applications, there are even more crazy requirements: What happens if you fill the ball return channel or groove with water and then freeze it? How about “digesting” dirt? What about the jamming risk once balls have worn down?