# Life Expectancy for Single vs. Double Nuts

Life expectancy equations provided by some manufacturers only cover nuts having two-point contact. But what about four-point nuts? And what about the effects of preload?

The fundamental determining factor of life expectancy is duty cycle, which includes load, thrust, speed, and duration of the travel. Calculating these forces is commonly understood. But preload is a very important factor in ball screw life calculations and must always be considered. In cases where the applied external load is low, preload can determine as much as 90% of the life.

Steinmeyer engineers have developed a preload graph that consists of two curves: each representing the force-deflection curve of one ball nut in a double nut configuration. It shows how deflections increase with greater force (and vice versa). It also displays how the forces in the opposite nut decrease as soon as deflection results in an axial displacement. For example, with double nuts, the higher the thrust carried by one nut, the more likely it will be the first to fail if not properly preloaded.

Since single nuts have four-point contact there are twice as many load/unload cycles for each given spot on the ball surface. The preload penalty of the four-point contact configuration is anywhere between a 50% life reduction (if the only force the nut sees is due to preload), and no reduction at all if there is zero preload. Real-world applications fall somewhere in between.

Lastly, if thrust is high enough to cause sufficient deflection, one set of balls in a double nut may run unloaded, which is generally unacceptable and potentially catastrophic. The preload must be high enough to ensure unloading will never occur. This isn’t the case for single nuts since there is no ball set to be unloaded, and exceeding the limits in the preload graph above is not a problem. Therefore, the life penalty depends on preload – life increases as preload is reduced

Contact Steinmeyer engineers to the see if which nut design is right for your application.

Does a Single Nut Have the Same Life Expectancy as a Double Nut?

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Key Takeaways:

- Differences between single and double nuts
- Determining the life of a ball screw
- Calculating impact of usage/application
- Understanding the effects of preload