Double Nut vs. Single Nut
In the Steinmeyer view of ball screw design, a double nut is always associated with 2-point contact. The advantages of 2-point contact are numerous: lower friction, more consistent torque, higher stiffness, and longer life.
Steinmeyer achieves 2-point contact through one of two methods: pitch shift or our patented UNILOCK® system. Pitch shift is the well-known method of creating a lead offset in the middle of the nut. Though effective, it is more complex to machine and adds some cost. More info at: http://www.steinmeyer.com/en/technology/drive-technology/preload-and-rigidity/nut-designs/double-nut/
In much the same way, 2-point contact can be achieved in ball screws having 2 thread starts. In this case, there are 2 ball circuits, one in each thread start. The corresponding threads in the nut can be offset, thus creating the desired 2-point contact.
With UNILOCK, two separate nuts are preloaded against each other and locked together with special epoxy to form an oil tight connection. The result is a compact, cost-saving design that requires no spacers or shims and resists both moments and side forces. The UNILOCK double nut is nearly as compact and stiff as a single-piece design.
Steinmeyer defines single nuts as one-piece nuts without any shift or offset in their I.D. ball thread. They can only be preloaded by ball oversize and will always have 4-point contact. The shorter nut saves some space and cost. More info at: http://www.steinmeyer.com/en/technology/drive-technology/preload-and-rigidity/nut-designs/single-nut/
As a rough rule of thumb, if the Length/Diameter ratio is greater than 20, then double nuts are often preferred. However, the final choice must consider the details of your application and cost requirements. Please keep in mind that our proprietary ETA+ technology is not available for single nuts because it requires a minimum length. Steinmeyer engineers can recommend a variety of potential designs.