Understanding Shaft Whip in Rotating Machinery

Devices

Portable balancer & Vibration analyzer Balanset-1A

Parts

Optical Sensor (Laser Tachometer)

Complete vibration diagnostics and balancing kit from Vibromera, including four vibration sensors with cables, a signal interface module, laser tachometer with magnetic stand, digital scale, USB cable, and a carrying case. The system is designed for field rotor balancing and condition monitoring on industrial equipment.

Devices

Dynamic balancer “Balanset-1A” OEM

Definition: What is Shaft Whip?

Shaft whip (also called oil whip when occurring in hydrodynamic bearings) is a severe form of rotor instability characterized by violent self-excited vibration that occurs when a rotor operating in fluid-film bearings exceeds a critical threshold speed, typically about twice the first critical speed. Once whip occurs, the vibration frequency “locks” onto the rotor’s first natural frequency and remains there regardless of further speed increases, with amplitude limited only by bearing clearances or catastrophic failure.

Shaft whip is one of the most dangerous conditions in high-speed rotating machinery because it develops suddenly, grows to destructive amplitudes within seconds, and cannot be corrected by balancing or other conventional methods. It requires immediate shutdown and bearing system modifications to prevent recurrence.

The Progression: Oil Whirl to Shaft Whip

Stage 1: Stable Operation

Rotor operates below instability threshold
Only normal forced vibration from unbalance present
Bearing oil film provides stable support

Stage 2: Oil Whirl Onset

As speed increases past approximately 2× the first critical speed:

Oil whirl develops—sub-synchronous vibration at ~0.43-0.48× shaft speed
Amplitude is initially moderate and speed-dependent
Frequency increases proportionally with shaft speed
May be intermittent or continuous
Can coexist with normal 1X vibration from unbalance

Stage 3: Whip Transition

When oil whirl frequency increases to match the first natural frequency:

Frequency Lock-In: Vibration frequency locks at natural frequency
Resonant Amplification: Amplitude grows dramatically due to resonance
Sudden Onset: Transition from whirl to whip can be instantaneous
Speed Independence: Further speed increases don’t change frequency, only amplitude

Stage 4: Shaft Whip (Critical Condition)

Vibration at constant frequency (first natural frequency, typically 40-60 Hz)
Amplitude 5-20 times higher than normal unbalance vibration
Shaft may contact bearing clearance limits
Rapid heating of bearings and oil
Potential for catastrophic failure within minutes if not shut down

Physical Mechanism

How Oil Whip Develops

The mechanism involves fluid dynamics in the bearing oil film:

Oil Wedge Formation: As shaft rotates, it drags oil around the bearing, creating a pressurized wedge
Tangential Force: The oil wedge exerts a force perpendicular to the radial direction (tangential)
Orbit Motion: Tangential force causes shaft center to orbit at approximately half shaft speed
Energy Extraction: System extracts energy from shaft rotation to sustain orbital motion
Resonance Lock: When orbit frequency matches natural frequency, resonance amplifies vibration
Limit Cycle: Vibration grows until limited by bearing clearance or failure

Diagnostic Identification

Vibration Signature

Shaft whip produces characteristic patterns in vibration data:

Spectrum: Large peak at sub-synchronous frequency (first natural frequency), constant regardless of speed changes
Waterfall Plot: Sub-synchronous component appears as vertical line (constant frequency) rather than diagonal (speed-proportional)
Order Analysis: Fractional order that decreases as speed increases (e.g., changes from 0.5× to 0.4× to 0.35×)
Orbit: Large circular or elliptical orbit at natural frequency

Onset Speed

Typical Threshold: 2.0-2.5× first critical speed
Bearing-Dependent: Specific threshold varies with bearing design, preload, and oil viscosity
Sudden Onset: Small speed increase can trigger rapid transition from stable to unstable

Prevention Strategies

Bearing Design Modifications

1. Tilting Pad Bearings

Most effective solution for preventing shaft whip
Pads pivot independently, eliminating destabilizing cross-coupling forces
Inherently stable across wide speed ranges
Industry standard for high-speed turbomachinery

2. Pressure Dam Bearings

Modified cylindrical bearing with grooves or dams
Increases effective damping and stiffness
Less expensive than tilting pad but less effective

3. Bearing Preload

Applying radial preload to bearings increases stiffness
Raises threshold speed for instability
Can be achieved through offset bore designs

4. Squeeze Film Dampers

External damping element surrounding bearing
Provides additional damping without changing bearing design
Effective for retrofit applications

Operational Measures

Speed Limitation: Restrict maximum operating speed to below threshold (typically < 1.8× first critical)
Load Management: Operate at higher bearing loads when possible (increases damping)
Oil Temperature Control: Lower oil temperature increases viscosity and damping
Monitoring: Continuous vibration monitoring with alarms set for sub-synchronous components

Consequences and Damage

Immediate Effects

Violent Vibration: Amplitudes can reach several millimeters (hundreds of mils)
Noise: Loud, distinctive sound different from normal operation
Rapid Bearing Heating: Bearing temperatures can rise 20-50°C in minutes
Oil Degradation: High temperatures and shearing degrade lubricant

Potential Failures

Bearing Wipe: Bearing babbit material melts and is wiped away
Shaft Damage: Scoring, galling, or permanent bending
Seal Failure: Excessive shaft motion destroys seals
Shaft Breakage: High-cycle fatigue from violent oscillation
Coupling Damage: Transmitted forces damage couplings

Related Phenomena

Oil Whirl

Oil whirl is the precursor to whip:

Same mechanism but frequency hasn’t locked onto natural frequency
Less severe amplitude
Frequency proportional to speed (~0.43-0.48×)
May be tolerable in some applications

Steam Whirl

Similar instability in steam turbines caused by aerodynamic forces in labyrinth seals rather than bearing oil films. Exhibits similar sub-synchronous vibration locking onto natural frequency.

Dry Friction Whip

Can occur at seal locations or from rotor-stator contact:

Friction forces provide destabilizing mechanism
Less common than oil whip but equally dangerous
Requires different corrective approach (eliminate contact, improve seal design)

Case Study: Compressor Shaft Whip

Scenario: High-speed centrifugal compressor with plain cylindrical bearings

Normal Operation: 12,000 RPM with vibration of 2.5 mm/s
Speed Increase: Operator increased to 13,500 RPM for higher capacity
Onset: At 13,200 RPM, sudden violent vibration developed
Symptoms: 25 mm/s vibration at 45 Hz (constant), bearing temperature rose from 70°C to 95°C in 3 minutes
Emergency Action: Immediate shutdown prevented bearing failure
Root Cause: First critical speed was 2700 RPM (45 Hz); whip threshold at 2× critical = 5400 RPM was exceeded
Solution: Replaced plain bearings with tilting pad bearings, allowing safe operation to 15,000 RPM

Standards and Industry Practice

API 684: Requires stability analysis for high-speed turbomachinery
API 617: Specifies bearing types and stability requirements for compressors
ISO 10814: Provides guidance on bearing selection for stability
Industry Practice: Tilting pad bearings standard for equipment operating above 2× first critical speed

Shaft whip represents a catastrophic failure mode that must be prevented through proper bearing selection and design. Recognition of its distinctive sub-synchronous, frequency-locked vibration signature enables rapid diagnosis and appropriate emergency response, preventing expensive damage to critical high-speed rotating equipment.

← Back to Main Index

What is Shaft Whip? Severe Rotor Instability Explained

Published by admin on October 28, 2025 October 28, 2025