Rotor Design & Construction
Starting With What Rotates
Track 1 covered the thermodynamics of what happens to steam as it passes through a turbine. Track 2 shifts focus to the physical machine itself, starting with the rotor — the single rotating shaft assembly that carries every stage's moving blades and transmits shaft work out to the generator. Everything about how a turbine is built, from casing design to bearing selection, exists to support and control this one rotating structure.
Solid vs. Built-Up Construction
Rotors are built one of two fundamentally different ways. A solid (monoblock) rotor is machined from a single large steel forging, with blade attachment features cut directly into that forging along its length. A built-up (disc) rotor instead uses individually forged discs — each carrying one stage's worth of blading — shrunk onto and keyed to a separate central shaft.
Solid construction eliminates the joints and interfaces a built-up rotor has between each disc and the shaft, which matters most in HP sections where high temperature and high stress make every additional interface a potential stress concentration and failure point. Built-up construction, by contrast, allows each disc to be forged, heat-treated, and inspected individually before final assembly, and becomes practically necessary in LP sections, where disc diameters get large enough that manufacturing a single forging that size stops being practical.
Solid rotors dominate HP applications for stress and temperature reasons; built-up rotors are common in LP sections for manufacturing and diameter reasons. Many real turbines actually combine both approaches across a single shaft line.
Forging and Heat Treatment
Rotor forgings begin as massive steel ingots, forged under enormous pressure to refine internal grain structure and eliminate porosity and internal defects that would otherwise become crack-initiation sites. After forging, the material is heat-treated to develop the specific combination of strength, toughness, and — critically for HP sections — creep resistance at operating temperature. This directly connects to Module 1.5's discussion of creep: material selection and heat treatment are the first line of defense against creep damage, chosen specifically to match the actual temperature each section of the shaft line will experience in service.
Dynamic Balancing
Once assembled, a complete rotor is spun on specialized balancing equipment that detects any residual mass imbalance and directs corrective weight addition or material removal until the rotor balances within a tight tolerance. Balancing is performed at or near actual operating speed because an unbalanced rotor produces vibration that scales with rotational speed — the same physical relationship that makes field vibration troubleshooting (a later track in this course) often trace back to a shift in rotor balance rather than a bearing or foundation problem.
The Full Shaft Line and Critical Speed
On a reheat turbine-generator, HP, IP, LP, and generator rotors are separate forgings, rigidly or flexibly coupled end-to-end into one continuous rotating shaft line, each section supported by its own set of journal bearings. Because the entire line behaves as one long, flexible rotating system, misalignment or vibration originating in one section can influence behavior elsewhere along the train — which is why alignment and vibration analysis during outages considers the whole shaft line, not just individual bearings in isolation.
Every rotor shaft also has natural mechanical resonant frequencies called critical speeds, where even small residual imbalance produces disproportionately large vibration. Startup and shutdown speed ramps are deliberately designed to pass through any critical speed bands quickly rather than lingering — sustained operation at a critical speed risks building resonant vibration to damaging levels, which is why operators are specifically trained not to hold speed within those bands during roll-up.
The danger from critical speed isn't "high speed is dangerous" — it's resonance at a specific band. A turbine can run perfectly smoothly well above its first critical speed once past it; the risk is specifically in lingering at the resonant band itself.