Can rotor weight alone determine the loads on two main bearings?

The short answer

No. Bearing reactions depend on where rotor weight acts relative to the supports. An overhung rotor can create a reaction greater than its weight at one bearing and an opposite reaction at the other. Dividing total weight equally ignores the moment that the support pair must resist.

Draw the support geometry first

Identify the rotor centre of gravity, bearing reaction locations and spacing in a common shaft coordinate. A simplified two-support beam model can illustrate the effect, but real bearing arrangements may include distributed supports, moments or housing flexibility. The rotor’s centre of gravity is not necessarily at its mounting face. Nacelle mass supported through another path should not automatically be assigned to the same shaft bearings.

Check a hypothetical equilibrium example

Let a 1 kN downward load act 0.2 m outside the nearest bearing, with the second bearing 0.5 m farther inboard. In an ideal simply supported model, equilibrium gives a 0.4 kN downward reaction at the far support and a 1.4 kN upward reaction at the near support. These are forces exerted on the shaft; bearing loads use the corresponding opposite actions. They balance the original force and its moment.

Use the example to request the missing inputs

The arithmetic is not a MARSTON bearing calculation. Actual rotor thrust, gyroscopic effects, torque transmission and combined operating cases can change the reactions. Request the shaft layout, mass-centre location and designer’s load distribution rather than select bearings from rotor mass alone. NASA supports mass-centre interpretation, while the example uses elementary statics. Review the support arrangement before comparing catalogue capacities, because even a correct bearing rating cannot repair an incorrect allocation of applied loads. For an overhung shaft, the farther bearing can carry a reaction opposite to the applied weight. A selection sheet that lists only unsigned load magnitudes should still preserve which direction each housing must resist.