Small wind / Buyer questions
Mechanical interfaces
Why must a rotor thrust value include its line of action?
The short answer
Because a force applied away from a reference point also creates a moment about that point. A rotor thrust number alone cannot describe every load at a bearing or nacelle interface. The review needs its direction, application line and the point about which interface moments are reported.
Keep axial thrust and offset geometry separate
Thrust aligned with the shaft and passing through the chosen reference point produces no moment from that force about that point. A lateral offset changes the result even if thrust magnitude is unchanged. Do not use the axial distance along the force line as a bending lever arm; only the perpendicular offset contributes to the magnitude of the moment generated by that force.
Use consistent reference transformations
For an explicitly hypothetical 2 kN force acting on a line 0.1 m perpendicular to a stated reference point, the associated moment magnitude is 0.2 kN m. Its direction follows the vector cross product of offset and force. A report already giving force and moment at that point must not have the same offset moment added again. This is a load-reference example, not a product load limit.
Ask for a complete interface load definition
Request the three force components, three moment components, coordinate axes and reference location for the relevant rotor load case. If the supplier changes the reporting point, ask for the transformation rather than compare moment numbers directly. OpenFAST illustrates frame-dependent load reporting, not MARSTON capacity. Keep the question focused on physical load transfer: neither nominal turbine kW nor rotor thrust alone establishes bearing reactions, mounting strength or allowable substitution of another rotor on the same mechanical interface. A load package should name whether its moment includes the force-reference transformation already. This prevents an apparently conservative review from double-counting the same lever arm and obscuring the actual governing design case.
Sources
Examples are illustrative. Confirm product-specific performance, compatibility and supply scope in the applicable documents.