Gas-bearing rotor dynamics guide

The rotor model begins with the bearing state

A gas-bearing coefficient is not a portable material property. It belongs to a specific geometry, clearance, supply state, load, equilibrium position, coordinate system and calculation method. Those data must travel with the coefficient into the rotor model.

Without that operating-point passport, a numerically complete rotor calculation may be based on a support state that does not represent the actual machine.

Geometry, mass and support placement

The shaft model requires geometry, material properties, disks, tools, couplings and the location and orientation of each support. Support reactions and load paths should be consistent with the machine layout used in the bearing calculation.

Rigid-body modes, shaft-bending modes and support-dominated modes can then be separated and interpreted rather than treated as an undifferentiated list of frequencies.

Natural frequencies and Campbell diagrams

Natural frequencies at zero or fixed speed are only the start. Rotational speed introduces gyroscopic effects and changes the relationship between excitation and modal families. A Campbell diagram maps those families against speed and shows where synchronous excitation intersects them.

A critical speed is a system result. Its engineering meaning depends on damping, excitation, mode shape, operating dwell and the separation from the intended speed range.

Critical-speed separation

Separation margin is a useful screening measure when the critical family and operating range are defined consistently. It should be reported with the actual operating speed, critical speed and model basis.

The public AURA spindle case carries a journal-bearing state into the rotor model at 20,000 rpm and identifies the first critical family at 29,780 rpm, giving 48.9% separation for that controlled case.

Unbalance response

Unbalance response moves the analysis from a speed crossing to a predicted motion. The model needs an excitation basis: residual unbalance magnitude, distribution, phase, correction planes or a stated trial case.

Response may be reported as displacement, orbit, force, stress or another machine-relevant quantity. The response of the selected shaft-bearing system should remain traceable to the bearing state used in the model.

Stability and damping

Gas bearings can introduce direct and cross-coupled dynamic coefficients. Stability work therefore requires a consistent coefficient model and a clear damping basis. Where damping provenance is weak, the result should be used to plan the next calculation or test rather than silently treated as a final stability claim.

For high-speed systems, the analysis programme may also include decay rates, logarithmic decrement, threshold speed, transient response and sensitivity to support parameters.

A practical decision sequence

1. Define the shaft, disks, tool and support positions.
2. Attach bearing coefficients with their operating-point passport.
3. Identify modal families and critical-speed crossings.
4. Check separation from the operating range.
5. Calculate unbalance response where the excitation basis is defined.
6. Record what additional damping, thermal or test evidence is needed.