A spindle that ran noisy after 6 months. It was an angular contact ball bearing set (7014C) in a back-to-back arrangement. The customer regreased it. The noise didn’t go away. The issue: the preload was lost — the bearing locknut had loosened, and the bearing set ran with clearance. This is about angular contact bearing preload arrangements.

The three mounting arrangements

Angular contact bearings take axial load in one direction. For a spindle that sees load both ways, you use two bearings. The arrangement determines rigidity and thermal expansion behavior:

  • Back-to-back (DB): the outer ring backs face each other. Good moment rigidity. The contact lines diverge outward. Thermal expansion of the shaft reduces preload. Use for fixed-position arrangements where the shaft grows axially.
  • Face-to-face (DF): the inner ring faces each other. Contact lines converge inward. Lower moment rigidity. Thermal expansion increases preload. Can overheat if the shaft grows. Use for short shafts with minimal thermal growth.
  • Tandem (DT): all bearings in the same direction. High axial load capacity in one direction. Must pair with another bearing set for the opposite direction. Use for heavy axial loads.

The preload amount

Preload eliminates clearance and increases rigidity. Too little preload: the spindle wobbles and the bearings rattle. Too much preload: the bearings run hot and wear fast. The preload is typically 5-15% of the dynamic load rating.

For a 7014C bearing (C = 25.5 kN), light preload is 1.2 kN. Medium is 2.5 kN. Heavy is 4.0 kN. For a grinding spindle running at 6000 RPM, I use light preload (1.2 kN). For a milling spindle at 3000 RPM with heavy radial loads, medium (2.5 kN). The preload is set by shimming the inner ring spacing or by a spring-loaded cap.

What I changed

1. Reset the preload with shims. The locknut had loosened. I removed the bearing cover, measured the endplay with a dial indicator (0.08 mm play), and added shims to set the correct preload. The preload was reset to 1.2 kN (about 0.02 mm negative clearance). The noise disappeared. The spindle ran smooth.

2. Used a spring-preloaded arrangement. For high-speed spindles, I use a spring-loaded bearing set (disk springs apply constant preload). As the shaft grows thermally, the springs compress — the preload stays constant. No need for locknuts that can loosen. The spring arrangement costs more but maintains preload for life.

3. Selected the correct arrangement. The original was DB (back-to-back) for a 300 mm long spindle. At 6000 RPM, the shaft grows about 0.03 mm from heat. The DB arrangement reduces preload as the shaft grows. That’s fine — the springs compensate. If I’d used DF (face-to-face), the thermal growth would increase preload and the bearings would overheat. For long spindles, DB is correct.

The temperature check

After setting preload, I run the spindle at full speed for 30 minutes. I measure the housing temperature near the bearing. If it rises above 45°C (15°C above ambient), the preload is too high. I reduce it by one shim. If the spindle wobbles at low speed, the preload is too low. Temperature is the best indicator of correct preload.

The arrangement I use: back-to-back DB for most spindles, with spring preload for high-speed. Set preload with shims, then verify by temperature rise. The noisy spindle wasn’t worn — the locknut loosened and the bearing ran with clearance. Reset preload and it’s quiet.