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.
§ 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).