Depending on nut construction, space constraints, and precision requirements, several mechanical methods are utilized to preload ball screw pairs. These are broadly categorized into single-nut preloading and double-nut preloading methods.
Figure 1: Single-Nut and Double-Nut Ball Screw Preload Configurations
1. Single Nut Preload Methods
- Oversized Ball Preload (Four-Point Contact Mode): Steel balls with a calibrated diameter slightly larger than the raceway groove space are assembled. Each ball maintains four-point contact with the screw and nut raceways. Highly compact, cost-effective, and suitable for light preloads (≤ 0.03 C).
- Lead Offset Preload (Two-Point Contact Mode): The thread lead of the nut is offset by a few microns (ΔP) at the midpoint of the raceway. Steel balls maintain two-point contact, achieving uniform preloading with less friction than four-point contact.
2. Double Nut Preload Methods
- Tension Preload (Spacer Shim Mode): A precision ground spacer shim is inserted between two nuts. Tightening the nuts pulls them apart in tension, causing steel balls in Nut 1 to press against one flank and Nut 2 against the opposite flank. Highly rigid, standard for precision machine tools.
- Compression Preload: Two nuts are compressed towards each other using an internal ring or adjustable threaded collar under spring tension. Suitable for absorbing thermal elongation.
3. Comparison and Selection Guide
| Preload Method |
Nut Type |
Max Preload |
Rigidity |
Compactness |
| Oversized Steel Ball |
Single Nut |
0.03 C |
Moderate |
Very Compact (Short length) |
| Lead Offset |
Single Nut |
0.05 C |
High |
Very Compact |
| Tension Spacer |
Double Nut |
0.10 C |
Very High |
Longer Axial Length |
| Spring Preload |
Double Nut |
0.02 C |
Constant |
Thermal Expansion Absorbed |
