Wind Turbine Bearing: Large Part Turning

The Scale Challenge of Wind Turbine Bearings

Wind turbine main bearings, pitch bearings, and yaw bearings are among the largest precision-machined components in any industry. A main shaft bearing for a 5 MW offshore turbine has an outer diameter of 2.0-2.5 meters, weighs 5-8 tonnes, and must achieve IT5-IT6 tolerances on bore and OD surfaces. The scale alone creates unique challenges for tooling, machine rigidity, and thermal management that are absent from conventional machining operations.

Bearing Materials and Specifications

Wind turbine bearings are manufactured from through-hardening bearing steels or case-hardening grades:

• 100Cr6 (AISI 52100, DIN 1.3505): The standard bearing steel. Through-hardened to 58-64 HRC. Used for balls and raceways in pitch and yaw bearings.
• 42CrMo4 (AISI 4140): Used for bearing rings that are induction-hardened on the raceway surface to 55-62 HRC at 2-4 mm depth while maintaining a tough, ductile core (280-320 HB).
• 18CrNiMo7-6 (DIN 1.6587): Case-hardening steel for main shaft bearings. Carburized to 58-62 HRC at 1.5-2.5 mm case depth with a core hardness of 330-380 HB.

Turning Operations: Roughing and Semi-Finishing

The turning operations on wind turbine bearing rings are performed on large vertical turning lathes (VTLs) or horizontal boring mills with turning capability:

Rough Turning of Forged Rings

Bearing ring forgings arrive with 8-15 mm of stock allowance per surface. Rough turning parameters in the annealed or normalized condition (200-260 HB):

• Machine: Vertical turning lathe with 3-6 meter table diameter, 100-250 kW spindle power.
• Insert grade: P25-P40 carbide with Al2O3-TiCN CVD coating for steel roughing.
• Cutting speed: 80-140 m/min. Lower speeds than conventional turning due to the large diameter (a 2.5 meter OD at 120 m/min requires only 15 RPM, making chip breaking more difficult).
• Feed rate: 0.5-1.0 mm/rev for heavy roughing.
• Depth of cut: 5-10 mm per pass, removing the forging scale and most of the stock allowance in 3-5 passes.
• Tool life: 2-4 rings per edge (40-80 minutes of cutting time).

Semi-Finishing

• Cutting speed: 120-180 m/min with P15-P25 grade inserts.
• Feed rate: 0.3-0.5 mm/rev.
• Depth of cut: 1.5-3.0 mm.
• Surface finish achieved: Ra 3.2-6.3 um, adequate for the subsequent grinding operation.

Raceway Machining

The raceway is the most critical surface on a bearing ring. For pitch and yaw bearings, the raceway is a large-diameter groove machined into the inner or outer ring:

• Raceway profile: Gothic arch (two-point contact) or circular arc (radius 51-54% of ball diameter). For a 50 mm ball, the raceway radius is 25.5-27.0 mm.
• Raceway turning: CNC-contoured form tool or single-point interpolation. Cutting speed 60-100 m/min, feed 0.1-0.2 mm/rev.
• Induction hardening: After turning, the raceway is induction-hardened to 55-62 HRC at 2-4 mm depth. Hardness must be verified by portable Rockwell testers or ultrasonic methods.
• Raceway grinding: After hardening, the raceway is finish-ground using vitrified CBN or aluminum oxide grinding stones (600-1,200 mm diameter). Wheel speed 25-35 m/s, infeed 0.01-0.03 mm per pass.
• Final raceway finish: Ra 0.2-0.4 um, with a waviness (Wt) of less than 0.010 mm across the raceway width.

Bolt Hole Machining

Pitch and yaw bearings are bolted to the turbine structure using 40-120 bolt holes per bearing, arranged on a bolt circle:

• Hole diameter: M30-M52 (33-56 mm drilled diameter for tapped holes).
• Drilling: Indexable insert drills (30-50 mm diameter) at 60-100 m/min, 0.15-0.30 mm/rev feed. Through-coolant at 20-40 bar for chip evacuation.
• Tapping: Solid carbide or indexable insert taps at 8-15 m/min. For M42 threads in 42CrMo4 (280-320 HB), tap life is 30-60 holes per tap.
• Thread quality: Class 6H internal threads per ISO 965-1, verified with go/no-go thread plug gauges.

Mounting Surface Machining

The bearing mounting faces must be flat and parallel to ensure uniform load distribution:

• Face milling: 400-600 mm diameter face mill with P20-P30 carbide inserts, 100-150 m/min, 0.3-0.5 mm/tooth feed.
• Flatness achieved: 0.05-0.10 mm across the full face diameter (2.0-2.5 m).
• Surface finish: Ra 1.6-3.2 um for gasketed mounting surfaces.

Thermal Management During Machining

Large bearing rings are susceptible to thermal distortion during machining due to the heat generated by cutting:

• Ambient temperature control: The machining area should be maintained at 20 +/- 2 C to minimize thermal expansion effects on dimensional measurements.
• Soaking time: After rough machining, rings should rest for 8-24 hours before finish machining to allow residual stress relaxation and temperature equalization.
• Coolant temperature: Coolant must be maintained at 20 +/- 1 C using a chiller unit, especially during grinding operations.

Measurement and Inspection

Inspecting a 2.5-meter-diameter bearing ring requires specialized equipment:

• Portable CMM: Articulated arm CMM (such as Faro or Hexagon ROMER) with 3-4 meter reach, accuracy of 0.030-0.050 mm at full extension.
• In-machine probing: Renishaw or Heidenhain touch probes integrated into the VTL for in-process diameter and roundness measurement, accuracy of 0.005-0.010 mm.
• Roundness measurement: Large-diameter roundness testers (Taylor Hobson or Mahr) or laser tracker-based methods for roundness within 0.020-0.050 mm.

Conclusion

Wind turbine bearing machining demands a unique combination of heavy-duty metal removal capability and fine precision finishing on workpieces that weigh several tonnes. The key to success lies in managing the interaction between machine rigidity, thermal stability, and tooling selection. As turbine sizes continue to grow (15 MW offshore turbines require bearings over 3 meters in diameter), the challenges of large-part machining will only increase, driving innovation in both machine tool and cutting tool technology.