The Rail That Was Too Stiff: When Preload Creates Noise, Not Precision
A linear guide rail on a dispensing axis. We installed it with maximum preload (for maximum stiffness). The carriage moved smoothly but made a high-pitched whine at high speed. The customer complained about noise. The preload was too high. A rail doesn’t need maximum preload — it needs the right preload for the load.
What preload does
A linear guide’s preload is the interference between the balls and the raceways. Higher preload = stiffer carriage (less deflection under load) but more friction. Maximum preload (Z4) doubles the friction compared to no preload (Z0). The extra friction generates heat and noise at high speed.
| Preload class | Friction | Stiffness | Best for |
|---|---|---|---|
| Z0 (no preload) | Low | Low | Light loads, low noise |
| Z1 (light preload) | Medium-low | Medium | General automation |
| Z2 (medium preload) | Medium | High | Standard positioning |
| Z3 (heavy preload) | High | Very high | Heavy loads, moment loads |
| Z4 (max preload) | Very high | Maximum | Only where specified |
What we found
The axis carried a 2 kg dispensing head. The load was light. We had specified Z4 preload because “stiffer is better.” The Z4 preload on a light load means the balls are over-compressed. The friction force is about 10 N instead of 3 N. At 500 mm/s, that friction generates heat and noise. The whine was the balls rolling under excessive preload.
The fix
We swapped the carriage to Z1 (light preload). The friction dropped. The whine disappeared. The stiffness was still more than enough for the 2 kg load. The carriage deflection under load was 0.01 mm — well within the 0.05 mm requirement.
For a rail carrying a 50 kg load (like a gantry axis), Z3 preload is appropriate. The heavy load needs the stiffness. But for a 2 kg dispensing head, Z1 is overkill in the other direction. Match the preload to the actual load, not to “maximum stiffness.”
The noise sources on linear rails
- High-pitched whine: Excessive preload. The balls are over-compressed. Switch to lighter preload.
- Grinding noise: Contamination. Dirt in the ball path. Clean and re-lube.
- Rough feel: Worn balls or raceway. The rail is at end of life.
- Clicking sound: Ball recirculation. Normal, but if it’s loud, the end caps are worn.
The lubrication matters too
A rail running dry is noisy and wears fast. We greased the carriage with the manufacturer’s specified grease (every 100 km or every 6 months). The grease reduces friction and noise. A rail that’s not lubricated makes noise regardless of preload. We now add a grease fitting to every rail and include lubrication in the PM schedule.
The preload rule: Match preload to load. Light loads (under 5 kg) use Z0-Z1. Medium loads use Z2. Heavy or moment loads use Z3. Don’t spec Z4 unless the catalog says it’s required. A stiff rail that whines is over-preloaded, not better. The noise is the balls complaining.
