Why does a yaw slewing bearing need a defined mounting-surface flatness?

The short answer

Because the mounting structure supports and can distort the bearing rings. A bolt pattern that lines up does not prove an acceptable support surface. If a small-wind yaw arrangement uses a slewing bearing, its specified flatness and connecting-structure conditions need assessment before assembly acceptance.

Trace local support into raceway shape

A slewing ring has finite stiffness. Pulling it against a surface with high and low areas can change ring shape and internal load distribution. The attachment can therefore be geometrically compatible in plan view while unsuitable through its supporting face. Flatness is distinct from the overall inclination of the assembly: a flat surface can be tilted, and a nominally level surface can still be locally uneven.

Use the bearing maker’s assessment boundary

Rothe Erde explicitly checks connecting-surface evenness during installation. Its requirements depend on bearing size and type, so a number from another installation is not a universal tolerance. The support’s loaded deformation can also matter in addition to unloaded manufacturing flatness. A measurement should identify the contact annulus and reference method rather than combine unrelated surfaces into one favorable average.

Request the actual mounting-surface requirement

Ask the yaw-assembly supplier for bearing identity, required support geometry, measurement boundary and the connecting structure assumed in its assessment. For MARSTON equipment, a slewing-ring design itself must first be confirmed. Do not infer flatness from a photograph, compensate with ad hoc shims or pull a ring flat by additional tightening. The concrete decision is whether the supplied bearing and its actual support satisfy their documented interface conditions together. Ask whether the flatness result was obtained before or after the support’s final attachment. A connecting surface can change shape as the surrounding structure is assembled, so measurement state matters alongside the inspected annulus.