High 1X Vibration Does Not Automatically Mean Rotor Unbalance. A turbine develops high vibration at running speed. The vibration spectrum shows a dominant component at *1× RPM*. The immediate reaction may be: “The rotor needs balancing.”
That conclusion can be expensive — and sometimes completely wrong.
A strong 1× component can certainly be produced by rotor unbalance. But similar vibration behaviour may also result from:
- Shaft bow
- Thermal bow
- Coupling problems
- Misalignment
- Rotor rub
- Mechanical looseness
- Eccentricity
- Bearing problems
- Foundation or support resonance
- Oil-film instability interacting with other conditions
- Mechanical runout
- Electrical or magnetic forces
- Structural resonance
Turbine balancing should therefore never begin simply by attaching a trial weight because 1× vibration is high.
Diagnosis must come before correction.
What Is Rotor Unbalance?
An ideal rotor would have its mass distributed so that its mass centre coincides perfectly with its rotational axis. In reality, manufacturing tolerances, repairs, deposits, erosion, blade differences, coupling eccentricity and material variations can shift the mass centre away from the rotational axis.
When the rotor turns, this eccentric mass generates a centrifugal force.
Conceptually:
Unbalance force ∝ Mass × Eccentricity × Speed²
The important point is the speed-squared relationship.
A relatively small residual unbalance that creates acceptable vibration at low speed can produce significant force at turbine operating speed. This is one reason balancing high-speed turbomachinery requires considerable care.
Static, Couple and Dynamic Unbalance
Rotor unbalance is not always concentrated in one plane.
Static Unbalance
The principal mass axis is displaced approximately parallel to the shaft axis. A single correction plane may sometimes correct a rotor exhibiting predominantly static unbalance.
Couple Unbalance
Equal unbalance forces in different planes can create a couple or moment. The rotor may appear statically balanced while still having significant dynamic unbalance.
Dynamic Unbalance
Most real industrial rotors contain a combination of static and couple components. Correction may therefore require two or more balancing planes. For long turbine rotors, the situation becomes considerably more complex because rotor flexibility and modal behaviour must also be considered.
Rigid Rotor Versus Flexible Rotor Behaviour
This distinction is fundamental. A rotor does not become “rigid” simply because it is made from a solid steel forging.
The terminology refers to its dynamic behaviour within the operating-speed range.
ISO 21940-11 establishes balancing procedures and unbalance tolerances for rotors exhibiting rigid behaviour. ISO 21940-12 addresses rotors exhibiting flexible behaviour and recognises the need to consider rotor configuration and balancing requirements accordingly.
Rigid-Behaviour Rotor
A rotor behaves essentially rigidly throughout the relevant balancing and operating speed range. Its unbalance condition can generally be represented adequately using limited correction planes.
Flexible-Behaviour Rotor
A flexible rotor changes shape dynamically as its rotational speed changes. As the rotor approaches bending critical speeds, relatively small residual unbalance distributions may excite significant vibration.
For such rotors, correcting vibration at one operating condition without understanding modal behaviour can sometimes make the response worse elsewhere.
Why Turbine Rotors Require Special Attention
Steam and gas turbine rotor trains may contain:
- Multiple bearings
- Long flexible shafts
- Several turbine stages
- Couplings
- Generators
- Compressors or driven equipment
- Multiple potential correction planes
- One or more critical speeds
The vibration response at one bearing can therefore be influenced by unbalance located some distance away. A correction weight added near one turbine stage may affect several bearings differently. This is why turbine balancing requires analysis of both amplitude and phase.
Amplitude Alone Is Not Enough
Suppose the turbine shows:
Bearing 1: 80 µm at 1×
Bearing 2: 95 µm at 1×
These numbers alone do not provide enough information to calculate a reliable correction. Phase tells us where the vibration response is occurring relative to shaft rotation.
A turbine balancing assessment may therefore examine:
- 1× amplitude
- 1× phase
- Shaft relative vibration
- Bearing pedestal/casing absolute vibration
- Speed
- Bearing location
- Operating load
- Temperature
- Historical response
Amplitude indicates how much the rotor system is responding.
Phase provides information about where that response occurs in the rotational cycle.
Together they make vector-based balancing possible.
Understanding the Heavy Spot and High Spot
The point of maximum measured vibration — sometimes referred to as the high spot— is not necessarily identical to the physical heavy spot on the rotor.
The relationship between applied force and measured response depends on the dynamic characteristics of the rotor-bearing-support system.
As speed changes, especially around a resonance or critical speed, phase relationships can change dramatically.
This is why simply placing weight opposite the vibration peak is not a technically reliable balancing method for critical turbomachinery.
The Influence Coefficient Method
One of the established techniques used for field balancing is the influence coefficient method.
In simplified form:
Step 1 – Establish the Baseline
Measure the original vibration amplitude and phase.
Step 2 – Add a Known Trial Weight
A trial weight of known mass and angular position is temporarily installed in an approved correction plane.
Step 3 – Operate the Machine
The machine is returned to the required balancing condition.
Step 4 – Measure the New Response
The change in vibration amplitude and phase is measured.
Step 5 – Calculate Rotor Influence
The system determines how strongly that correction plane influences the measurement location.
Step 6 – Calculate the Correction Weight
Using vector mathematics, the required correction mass and angular position are calculated.
For multi-plane balancing, the process involves multiple measurement locations and correction planes.
Selecting the Trial Weight Is Critical
A trial weight must be large enough to produce a measurable change in vibration. But it must not be so large that it creates an unsafe operating condition. Before installing a trial weight, engineers should consider:
- Rotor speed
- Correction radius
- Existing vibration
- Machine trip limits
- Rotor mass
- Available correction locations
- Fixing method
- Centrifugal force
- OEM restrictions
- Maximum allowable correction mass
For critical turbine rotors, trial-weight location and attachment should always comply with the rotor/OEM-approved balancing arrangement.
Bode Plots During Run-Up and Coast-Down
A Bode plot displays vibration amplitude and phase as functions of rotational speed. For turbine diagnostics, this can be extremely valuable. During run-up or coast-down, engineers can observe:
- Critical-speed amplification
- Phase changes
- Rotor response through resonance
- Changes after balancing
- Possible thermal bow
- Rotor dynamic behaviour
A true unbalance response often has characteristic amplitude and phase behaviour as the rotor passes through a critical speed.
This is very different from evaluating a single vibration measurement at full operating speed.
Polar Plots
A polar plot displays vibration amplitude and phase as a vector. This provides a visual representation of how the rotor response changes during:
- Speed variation
- Thermal changes
- Load changes
- Balancing corrections
For complex turbines, polar and Bode plots can provide much more diagnostic information than a conventional spectrum alone.
Mechanical Runout Must Be Checked
Eddy-current proximity probes measure the distance between the probe tip and the shaft surface. The indicated signal can therefore contain contributions from:
- Actual mechanical shaft movement
- Shaft surface irregularity
- Electrical runout
- Metallurgical variations
Before interpreting shaft vibration, mechanical/electrical runout should be considered. Otherwise, an engineer may attempt to balance a vibration component that is partly measurement artefact rather than true rotor motion.
Thermal Bow Can Be Mistaken for Unbalance
Some turbine rotors may develop temporary or persistent bow due to uneven temperature distribution. A thermally bowed rotor can produce significant 1× vibration.
If balancing weights are added without understanding the thermal condition, the rotor may operate acceptably under one thermal state and poorly under another.
The diagnostic questions should therefore include:
- Does vibration change during warm-up?
- Does vibration change after soak time?
- Is phase stable?
- What happens during coast-down?
- Does vibration return after a hot or cold restart?
- Is there evidence of rub or uneven cooling?
Resonance Can Amplify a Small Unbalance
Every rotor-bearing-structure system has natural frequencies. When running speed or another excitation frequency approaches a natural frequency, vibration can increase significantly. In this case, a relatively small amount of residual unbalance may produce a large measured vibration.
Balancing may reduce the forcing input, but the structural or rotor-dynamic resonance still exists.
This distinction is important:
Balancing reduces unbalance force. It does not remove resonance.
Shop Balancing Versus Field Balancing
These are complementary activities.
Shop Balancing
A rotor may be balanced in a dedicated balancing facility before installation. Depending on the rotor design, balancing may be carried out at low speed or, for specialised applications, in a high-speed balancing facility.
Field / In-Situ Balancing
Field balancing corrects the behaviour of the assembled machine system.
This can account for influences such as:
- Couplings
- Assembled components
- Site conditions
- Bearings
- Supports
- Installed rotor train
- Repairs or deposits
Field balancing can therefore sometimes solve vibration that remains after an individually balanced rotor is installed.
However, it should not be used to disguise another mechanical problem.
When Should a Turbine NOT Simply Be Balanced?
Before balancing, investigate signs of:
- Misalignment
- Shaft bow
- Rotor rub
- Loose components
- Bearing damage
- Foundation problems
- Coupling defects
- Abnormal runout
- Steam-path problems
- Resonance
- Cracked components
- Thermal instability
If the underlying problem is mechanical, adding balance weights may reduce one vibration reading while leaving the actual defect unresolved.
How Should Success Be Evaluated?
A successful turbine balancing job is not simply: “Vibration reduced.”
The engineer should review:
- Overall vibration
- 1× amplitude
- 1× phase stability
- All relevant bearing locations
- Shaft relative vibration
- Casing vibration
- Behaviour during run-up
- Behaviour during coast-down
- Critical-speed response
- Load-related response
- Thermal stability
The final condition must also comply with the applicable OEM requirements, plant acceptance criteria and relevant machinery standards. ISO 21940 provides important balancing principles and procedures, but ISO itself notes that the flexible-rotor standard is not intended to act as a universal acceptance specification for every specialised rotor. OEM and machine-specific requirements remain important.
VIBSpECTRUM Turbine Balancing Capability
VIBSpECTRUM provides advanced field vibration analysis and dynamic balancing services for critical rotating machinery, including turbine-generator and compressor trains.
Our engineering approach can include:
- Multi-channel vibration measurement
- Shaft relative vibration
- Casing absolute vibration
- 1× amplitude and phase analysis
- Bode plots
- Polar plots
- Run-up and coast-down analysis
- Critical-speed evaluation
- Influence-coefficient balancing
- Single-plane balancing
- Two-plane balancing
- Multi-plane balancing
- Rotor-dynamic investigation
- Post-balancing verification
The objective is not merely to reduce one vibration number. It is to establish whether unbalance is truly present and achieve a stable, technically defensible rotor response.
Frequently Asked Questions
Does high 1× vibration always indicate rotor unbalance?
No. Misalignment, bow, resonance, eccentricity, rubs and other conditions can also generate strong 1× vibration.
Can a turbine be balanced while installed?
Many turbines can be field-balanced using OEM-approved correction locations and suitable procedures, subject to machine design and operating requirements.
What is multi-plane balancing?
It involves calculating corrections at several axial rotor locations to control the dynamic response of more complex rotor systems.
Why is phase required?
Phase provides the angular relationship of the vibration response relative to shaft rotation and is essential for reliable vector balancing.
What is the difference between rigid and flexible rotor balancing?
Flexible rotors undergo significant dynamic deformation within their relevant speed range and therefore require balancing techniques that consider modal behaviour. ISO 21940-11 addresses rigid behaviour, while ISO 21940-12 addresses flexible behaviour.
