The yaw and pitch bearings let a turbine turn into the wind and feather its blades. Each bearing is a large slewing ring bolted between two structures, and the rings of bolts that hold them carry combined tension, shear and overturning moment while the bearing rotates beneath constantly changing load. They are among the most carefully engineered bolted joints on the machine.
Yaw and pitch bearing bolts (M20–M36, grade 10.9 or 12.9 depending on OEM) are among the most carefully engineered joints on the wind turbine because they carry combined loads: bolt tension from preload, shear from in-plane forces, and bending from out-of-plane moments. Pitch bearing bolts experience very high cyclic count — every pitch actuation event cycles the joint. OEM drawings specify exact bolt grade, length, and preload target; any deviation must go through engineering change approval. Inspection interval is typically 6 months for the first 2 years, then annually.
- Best for
- O&M engineers and procurement teams sourcing replacement yaw or pitch bearing bolts and understanding why dimensional and grade compliance to the OEM drawing is mandatory
- Not suitable for
- Substituting a higher grade (12.9 for 10.9 or vice versa) without OEM engineering approval — the joint preload and friction coefficient assumptions change with grade and coating combination
- Sourcing steps
- 1 — Obtain OEM drawing for the specific bearing bolt → 2 — Confirm diameter, length, thread pitch, and grade → 3 — Request EN 10204 3.1 MTC → 4 — Verify dimensions on receipt → 5 — Install per OEM torque procedure with calibrated tool
- RFQ information
- OEM drawing reference, bolt diameter and length, thread pitch, grade, coating, MTC level, quantity, turbine model
§ 01 Yaw bearing vs pitch bearing
- Yaw bearing — the large slewing ring between the tower top and the nacelle, allowing the whole nacelle to rotate horizontally to track wind direction.
- Pitch bearing — one per blade, between the hub and the blade root, allowing each blade to rotate about its own axis to control power and load.
Both are slewing (large-diameter rolling) bearings bolted by two rings of fasteners — one ring securing each race to its mating structure.
§ 02 What the bearing bolts do
Each bolt ring clamps a bearing race to its structure tightly enough that the joint transmits the full load — tension, shear and a large overturning moment — without the race lifting or slipping, while the bearing rotates. Unlike a static flange, the load distribution around the ring shifts continuously as the rotor turns and the blade pitches, so individual bolts cycle through high and low load every revolution.
§ 03 Why they are so demanding
Several factors combine to make bearing bolts a special case:
- Compact, high load — bolt diameter is constrained by the bearing geometry, so each bolt works hard; some joints use class 12.9.
- Uneven ring loading — the moment load means bolts are most stressed on one side and rotate through peak load as the bearing turns.
- Bearing performance depends on clamp uniformity — uneven preload distorts the race and shortens bearing life, so preload accuracy is critical.
§ 04 Grade, coating and tensioning
Bearing bolts are high-strength — class 10.9, or class 12.9 where the compact joint needs more capacity per diameter. Because 12.9 must not be hot-dip galvanized (hydrogen embrittlement), bearing bolts typically use zinc-flake (Geomet) coatings; the grade trade-off is in grade 10.9 vs 12.9. To achieve uniform, accurate preload around the ring, these joints are usually hydraulically tensioned rather than torqued — see tensioning vs torquing.
§ 05 Maintenance
Bearing bolt rings are on the inspection and re-torque schedule like other critical joints, because preload settles and the cyclic duty is severe. Loss of preload here is doubly serious — it threatens both the bolt and the bearing. Re-check intervals follow the maintenance plan, in line with how often to re-torque wind turbine bolts, and any sign of movement should be investigated promptly (signs of bolt loosening).
Evidence and decision boundary
- Direct evidence
- ISO 3800 defines axial-load fatigue test conditions for threaded fasteners. DNV-RP-C203 provides fatigue-assessment methods for offshore steel structures and components; neither source proves the fatigue life of an unspecified turbine bolted joint.
- Engineering inference
- Assess load spectrum, preload, load sharing, stress concentration, thread runout, contact geometry, corrosion, installation scatter and inspection strategy for the actual blade-root or yaw/pitch joint.
- Typical or indicative value
- Cycle counts, preload ratios and service intervals are indicative unless supported by OEM load cases, joint analysis and approved test evidence.
Primary sources checked
- ISO 898-1:2013 - carbon/alloy-steel fastener property classes
- ISO 16047:2005 - torque/clamp-force testing
- ISO 3800:1993 - axial-load fatigue testing of threaded fasteners
- DNV-RP-C203 - fatigue design of offshore steel structures
Related commercial route: Compare wind-turbine clamp systems and RFQ inputs.