Forces on Steam Turbine Rotor

Topic: Forces on Rotor (Steam Turbine / Turbo-generator Rotor)

The paragraph describes the various stresses and forces acting on a rotor during operation in a turbine or generator.

1. Centrifugal Stresses

When the rotor rotates at high speed, each particle of the rotor experiences a centrifugal force acting radially outward.

• This is the primary stress in the rotor and blades.
• It depends on rotational speed, radius, and material density.

2. Alternating Bending Stresses

These are cyclic (repeated) bending forces caused by:

• The weight of the rotor and blades, leading to sagging and bending.
• The rotational motion making this bending alternate with each revolution.

Result → Fatigue stresses can develop in the shaft due to continuous rotation.

3. Transverse Vibratory Forces

These are vibrations in the lateral (sideways) direction, caused by:

• Rotor unbalance (uneven mass distribution)
• Misalignment between turbine and generator shafts
• Bearing looseness or foundation vibration

Result → Leads to vibration amplitude and possible resonance if critical speed is approached.

4. Steady Torsional Stresses

• These arise due to twisting action along the shaft axis.
• Main sources:
  • Blading reaction forces
  • Coupling effects from upstream or downstream rotors

Result → Shear stresses along the rotor length.

5. Torsional Oscillations

• Caused by fluctuations in torque transmitted through the shaft.
• Often due to:
  • Electrical system interactions (generator-load disturbances)
  • Rotor misalignment
  • Sudden load changes

Result → Alternating torque waves along the shaft, can lead to fatigue failure.

6. Self-Excited Rotor Whirls

There are three main types mentioned:

(a) Oil Whip

• Happens due to fluid film instability in bearings.
• The oil film starts to oscillate and drives the shaft into a whirling motion.
• Common in journal bearings under certain conditions.

(b) Steam Whirl

• Caused by aerodynamic forces of steam flow on blades and seals.
• The uneven steam flow generates a tangential force that makes the rotor orbit or whirl.

(c) Friction Whirl

• Originates from internal friction in rotor materials or shrink-fitted parts.
• Also influenced by hysteresis energy (internal material damping).

7. Material and Damping Effects

• Shrink fits (like turbine discs or couplings) cause local stress concentrations.
• Material damping helps absorb vibration energy but cannot fully eliminate it.