Understanding Shaft Bow in Rotating Machinery

Devices

Portable balancer & Vibration analyzer Balanset-1A

$2,353.83

Parts

Vibration sensor

$107.26

Parts

Optical Sensor (Laser Tachometer)

$147.78

Devices

Balanset-4

$8,107.90

Parts

Magnetic Stand Insize-60-kgf

$54.82

Parts

Reflective tape

$11.92

Devices

Dynamic balancer “Balanset-1A” OEM

$2,067.80

Definition: What is Shaft Bow?

Shaft bow (also called shaft bending, rotor bow, or simply “bow”) is a condition where a rotor shaft has developed a permanent or semi-permanent curvature, causing it to deviate from a straight centerline. Unlike temporary run-out that might be caused by a loose component or eccentric mounting, shaft bow represents actual deformation of the shaft material itself.

Shaft bow produces vibration symptoms that superficially resemble unbalance, but it cannot be corrected through conventional balancing procedures. This makes correct diagnosis critical to avoid wasting time attempting to balance a bowed shaft.

Types of Shaft Bow

Shaft bow can be categorized based on its cause and duration:

1. Permanent Mechanical Bow

This is plastic (permanent) deformation of the shaft material caused by:

• Mechanical overload or impact
• Improper lifting or handling during maintenance
• Dropping the rotor
• Excessive bending stress during operation
• Manufacturing defects or improper heat treatment

Once the shaft has yielded (permanently deformed), the bow remains even when the shaft is at rest and all loads are removed.

2. Thermal Bow (Transient)

Also called thermal bow or hot bow, this is a temporary condition caused by uneven heating of the shaft. The heated side expands more than the cool side, creating a temporary curve. Causes include:

• Asymmetric heat sources (hot process fluid on one side, cooling air on the other)
• Bearing friction heating one side of the shaft
• Rotor rubs generating localized heating
• Solar heating on outdoor equipment
• Improper warm-up procedures for large turbines

Thermal bow typically disappears when the shaft cools uniformly or when thermal equilibrium is reached. However, repeated thermal bow cycles can eventually cause permanent set.

3. Residual Stress Bow

Internal residual stresses from welding, heat treatment, or manufacturing processes can cause a shaft to slowly bow over time, particularly when subjected to operating temperatures or mechanical loads that cause stress relief.

Causes of Shaft Bow

Understanding the root causes helps prevent shaft bow and guide corrective actions:

Mechanical Causes

• Overload: Operating at loads exceeding design limits
• Improper Storage: Storing shafts horizontally without proper support, causing sag over time
• Mishandling: Lifting by the shaft instead of designated lifting points
• Accident or Impact: Dropping, collision, or foreign object damage
• Bearing Seizure: A seized bearing can cause the shaft to bend under the driving torque

Thermal Causes

• Uneven Heating: Non-uniform temperature distribution around the shaft circumference
• Rapid Temperature Changes: Thermal shock during startup or shutdown
• Hot Spots: Localized heating from friction, rubs, or process conditions
• Inadequate Warm-Up: Starting cold turbines or large machines too quickly
• Shutdown Procedures: Allowing a hot shaft to stop rotating before cooling (thermal sag)

Material and Manufacturing Causes

• Poor Material Quality: Inclusions, voids, or material inhomogeneities
• Improper Heat Treatment: Residual stresses from quenching or tempering
• Welding Distortion: Asymmetric welding creating residual stresses
• Machining Stresses: Stresses induced during manufacturing

How Shaft Bow Causes Vibration

A bowed shaft creates vibration through two mechanisms:

1. Geometric Unbalance

When a bowed shaft rotates, its curved centerline sweeps out a cone or other non-circular path. Even if the rotor’s mass distribution is perfectly balanced, the bowed geometry creates an eccentric rotating mass that generates centrifugal forces, producing 1X vibration (vibration at the shaft’s rotational frequency).

2. Moment Loading on Bearings

The curvature creates bending moments that are transmitted to the bearings, causing fluctuating bearing loads and vibration.

Detecting Shaft Bow

Distinguishing shaft bow from true mass unbalance is crucial for effective troubleshooting:

Symptom Comparison: Bow vs. Unbalance

Characteristic	Unbalance	Shaft Bow
Vibration Frequency	1X running speed	1X running speed
Phase Relationship	Consistent, same at all times	May change during warm-up
Slow Roll Vibration	Present (proportional to speed²)	Present and often significant even at very low speed
Response to Balancing	Vibration reduced by correct balancing	Minimal or no improvement; may get worse
Thermal Sensitivity	Relatively stable with temperature	Changes significantly during warm-up/cool-down
Run-out Measurement	Low when rotor at rest	High run-out even at rest (permanent bow)

Diagnostic Tests

1. Slow Roll Measurement

Rotate the shaft very slowly (typically 5-10% of operating speed) and measure run-out with a proximity probe or dial indicator. High run-out at slow roll indicates shaft bow or mechanical run-out, not unbalance (which produces force proportional to speed squared).

2. Shut-Down Phase Shift

Monitor vibration phase angle as the machine shuts down. True unbalance maintains constant phase regardless of speed. A bowed shaft may show phase changes, particularly as it cools.

3. Thermal Bow Test

For suspected thermal bow, monitor vibration during startup and warm-up. Thermal bow typically shows increasing vibration as the machine warms up, then may stabilize or decrease as thermal equilibrium is reached.

4. Off-Machine Run-Out Check

Remove the rotor, support it on V-blocks or a lathe, and rotate it slowly while measuring radial run-out with a dial indicator. Significant run-out (typically > 0.001″ or 25 µm) confirms permanent bow.

5. Visual Inspection

For large shafts, visual sighting down the shaft length or using optical methods (laser alignment) can reveal obvious bow.

Correction Methods

The appropriate correction depends on bow severity and type:

For Permanent Mechanical Bow

1. Shaft Straightening

For mild to moderate bow (typically < 0.005" or 125 µm), the shaft can sometimes be cold or hot straightened using hydraulic presses. This requires specialized equipment and skilled technicians. The shaft is supported and carefully loaded to plastically deform it back toward straight.

2. Thermal Stress Relief

Heat treat the shaft to relieve residual stresses, potentially reducing or eliminating bow from stress-related causes. This requires proper furnace equipment and process control.

3. Shaft Replacement

For severe bow or in critical applications, replacement is often the most reliable solution. The cost of a new shaft must be weighed against downtime and the risk of straightening attempts failing.

4. “Balancing Around the Bow”

In some cases, particularly for large turbines, correction weights can be calculated and installed to counteract the bow’s effect. This doesn’t fix the bow but minimizes vibration. This approach has limitations and is typically a temporary solution.

For Thermal Bow

1. Operating Procedure Changes

• Implement slow warm-up procedures
• Maintain continuous turning gear operation during shutdown to prevent thermal sag
• Control steam admission or process fluid temperatures more carefully
• Ensure symmetric heating/cooling

2. Design Modifications

• Add insulation to reduce thermal gradients
• Install heating jackets for uniform warm-up
• Improve cooling system to ensure even temperature distribution

3. Turning Gear Operation

For large turbines, operate the turning gear (slow-speed rotational drive) during warm-up and cool-down to rotate the shaft and prevent thermal bow from developing.

Prevention Strategies

Preventing shaft bow is far easier than correcting it:

Design and Manufacturing

• Use proper heat treatment procedures to minimize residual stresses
• Design adequate shaft stiffness for the application
• Specify appropriate materials for the thermal environment

Installation and Maintenance

• Always lift rotors using designated lifting points, never by the shaft
• Store spare rotors with proper support to prevent sagging
• Avoid mechanical shock during handling
• Check shaft straightness periodically (annual or per manufacturer schedule)

Operation

• Follow manufacturer warm-up and shutdown procedures
• Avoid rapid temperature changes
• Monitor for signs of thermal bow during startups
• Investigate any unexplained changes in vibration phase

Impact on Balancing Procedures

Attempting to balance a bowed shaft is generally futile and can be counterproductive:

• Ineffective Corrections: Balance weights calculated for mass unbalance won’t correct geometric bow
• Masking the Problem: Partially successful “balancing” of a bowed shaft may reduce vibration temporarily but leave the underlying problem unaddressed
• Wasted Time: Multiple balancing iterations without success indicate the need to check for bow
• Potential Damage: Adding large correction weights to a bowed shaft can increase stresses and cause further damage

Best Practice: Always check for shaft bow before beginning balancing procedures, particularly if the rotor has a history of handling, thermal events, or unexplained vibration issues.

---

← Back to Main Index