Rotors are often subjected to torsional loads. Therefore, the rotor transfers the torsional load while it is rotating about its own axis. The bearings that support the rotor do not constrain the axial rotation of the rotor. Since you analyze the rotor in the corotating frame during the rotordynamic analysis, you need a cross section on the rotor as a reference about which torsional vibration can be studied. The Fixed Axial Rotation node is a default node in the Solid Rotor interface that provides the method to constrain the axial rotation of the rotor at a given cross section. In general, each cross section along the rotor can have an axial rotation that differs from the rigid body rotation due to twist in the rotor. The nonrigid part of the axial rotation is constrained by this feature.

Therefore, taking the cross product by X − Xax on both sides and integrating over the area, gives