Inconsistent ball-screw definitions can confuse engineers.
Alexander F. Beck, President
George A. Jaffe, Executive Vice President
Ball nuts are generally classified as either "singles" or "doubles." But what one vendor calls a single nut may be called a "double" nut by another. So what is a good definition of single and double nut, and what are the main differences?
If one takes the term literally, double nuts are made of two identical nuts. They can be preloaded against each other to remove play. Conversely, a single nut should then consist of only one nut, and it is not preloaded. This makes sense, and in the early days of ball screws, it was true. Today, however, even single nuts can have preloads.
Some experts point out that double nuts are made of two separate pieces, joined in some way, while single nuts consist of a single piece of steel. And at first glance, this seems a good definition. But then there are manufacturers -- like Steinmeyer -- that build what they call double nuts from a single piece of steel. Other companies say they have single nuts that are basically the same as double nuts, and with the same or better performance.
Preload is the key. It doesn't just eliminate play. It adds stiffness and keeps the nut centered on the screw. But what causes the preload makes a substantial difference. There are two ways to achieve preload. The nut can be permanently forced in one direction. In vertical applications, for example, gravity often provides the load. But no matter where the force comes from, it compresses the balls between the ball tracks of the screw and the opposite side of the nut's ball track, putting them into two-point contact.
Preload can also come from using oversized balls. Two-point contact requires an offset between ball tracks of the nut and screw, but the oversized balls align the ball tracks of nut and screw. However, the type of contact depends on the shape of the ball tracks.
Semicircular ball races result in line contact. That is, the ball touches the races in a full line, increasing friction and causing unpredictable results. More advanced ball screws employ a "gothic arch" where the thread profile has at least two arcs with a radius somewhat greater than the ball's and offset so that the ball touches the races in two points. This is true for the screw and nut, so there are a total of four contact points.
Regardless of what generates the preload force, the number of contact points of each ball determines how the balls move and transmit loads from nut to screw, and vice versa. Therefore, it makes sense to define ball nuts by whether they have two or four contact points. Under this rule, all nuts with four-point contact are "single nut," and those with two-point contact are "double nuts" -- even if they are not the split type. This definition describes nuts from a performance point of view, regardless of design details.
So what are the options? As we said, two-point contact requires some sort of axial offset between nut and shaft races. In many cases, two separate nuts with some play create this offset. A simple way to eliminate this play and establish the required offset is to just turn both nuts until they touch. After setting preload, couple the nuts so they won't come loose. But this raises a new problem. How do you set and maintain the correct amount of preload?
Several solutions have been developed. Some double nuts use a spacer or shim between the nuts. This removable shim is ground to a certain thickness that aligns keyways in the nuts. Once the correct preload is set and both keyways are aligned, inserting a key fixes the preload. Other designs use spacers with several holes for pins or setscrews that attach each nut to the spacer. Preload can then be adjusted by varying the width of the spacer, or by moving hole positions. But all of these require extra time and pieces.
Another method, called the Unilock system, locks two nuts together in any position they may be in after setting the preload. It doesn't use spacers or shims. Instead, the manufacturer places grooves with knurled bottoms on the face side of each nut. End users fill those grooves with durable epoxy resin, forming a toothed ring that prevents the nuts from rotating relative to each other.
Another nut design, though not used much anymore, houses both nuts inside a bore. Fitting keys keep them from rotating relative to each other and a ring screw compresses them, usually against an undersized spacer. After the balls touch the sides of ball tracks and start building resistance, the nut package can be compressed to the desired preload. This preload is referred to as "X" preload, based on contact lines, while the Unilock and oversized-spacer methods are known as "O" preloads. X preload is no longer used because it reduces angular stiffness in the nut package, and the housing represents an extra part with tolerances that takes up space.
A final approach is a special double nut and housing, Steinmeyer's Series 1510 and 1530 for mini ball screws. They use a Belleville spring instead of a rigid spacer. The spring maintains a constant preload regardless of nut position on the screw so idling torque can be kept constant despite manufacturing tolerances and wear. This is especially advantageous for small ball screws with fine pitches.