Understanding Radial Vibration 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 Radial Vibration?

Radial vibration is the motion of a rotating shaft perpendicular to its axis of rotation, extending outward from the center like radii of a circle. The term “radial” refers to any direction radiating from the shaft centerline, encompassing both horizontal (side-to-side) and vertical (up-and-down) motion. Radial vibration is synonymous with lateral vibration or transverse vibration and represents the most commonly measured and monitored form of vibration in rotating machinery.

In practical applications, radial vibration is typically measured in two perpendicular directions—horizontal and vertical—at each bearing location to provide a complete picture of the shaft’s motion perpendicular to its axis.

Measurement Directions

Horizontal Radial Vibration

Horizontal vibration is measured in the side-to-side direction:

Perpendicular to the shaft axis and parallel to the ground/floor
Often the most accessible measurement location
Typically shows effects of gravity, foundation stiffness asymmetry, and horizontal forcing functions
Standard measurement orientation for most vibration monitoring programs

Vertical Radial Vibration

Vertical vibration is measured in the up-and-down direction:

Perpendicular to the shaft axis and perpendicular to the ground/floor
Affected by gravity and weight of the rotor
Often higher amplitude than horizontal due to rotor weight creating asymmetric stiffness
Critical for detecting problems in vertically oriented machines (vertical pumps, motors)

Overall Radial Vibration

The total radial vibration can be calculated as the vector sum of horizontal and vertical components:

Radial Total = √(Horizontal² + Vertical²)
Represents the actual motion magnitude regardless of direction
Useful for single-number severity assessments

Primary Causes of Radial Vibration

Radial vibration is generated by forces acting perpendicular to the shaft axis:

1. Unbalance (Dominant Cause)

Unbalance is the most common source of radial vibration in rotating machinery:

Creates centrifugal force rotating at shaft speed (1X)
Force magnitude proportional to unbalance mass, radius, and speed squared
Produces circular or elliptical shaft orbit
Correctable through balancing procedures

2. Misalignment

Shaft misalignment between coupled machines creates both radial and axial vibration:

Primarily 2X (twice per revolution) radial vibration
Also generates 1X, 3X, and higher harmonics
High axial vibration accompanies radial vibration
Phase relationships between bearings diagnostic for misalignment type

3. Mechanical Defects

Various mechanical problems produce characteristic radial vibration patterns:

Bearing Defects: High-frequency impacts at bearing fault frequencies
Bent or Bowed Shaft: 1X vibration similar to unbalance but present even at slow roll
Looseness: Multiple harmonics (1X, 2X, 3X) with non-linear behavior
Cracks: 1X and 2X vibration with changes during startup/shutdown
Rubs: Sub-synchronous and synchronous components

4. Aerodynamic and Hydraulic Forces

Process forces in pumps, fans, and compressors create radial forcing:

Blade passing frequency (number of blades × RPM)
Hydraulic unbalance from asymmetric flow
Vortex shedding and flow turbulence
Recirculation and off-design operation

5. Resonance Conditions

When operating near critical speeds, radial vibration amplifies dramatically:

Natural frequency coincides with forcing frequency
Amplitude limited only by system damping
Potential for catastrophic vibration levels
Requires adequate separation margins in design

Measurement Standards and Parameters

Measurement Units

Radial vibration can be expressed in three related parameters:

Displacement: Actual distance of movement (micrometers µm, mils). Used for low-speed machinery and proximity probe measurements
Velocity: Rate of change of displacement (mm/s, in/s). Most common for general industrial machinery, basis for ISO standards
Acceleration: Rate of change of velocity (m/s², g). Used for high-frequency measurements and bearing defect detection

International Standards

ISO 20816 series provides radial vibration severity limits:

ISO 20816-1: General guidelines for machinery vibration evaluation
ISO 20816-3: Specific criteria for industrial machines > 15 kW
Severity Zones: A (good), B (acceptable), C (unsatisfactory), D (unacceptable)
Measurement Location: Typically on bearing housings in radial directions

Industry-Specific Standards

API 610: Centrifugal pumps radial vibration limits
API 617: Centrifugal compressors vibration criteria
API 684: Rotor dynamics analysis procedures for radial vibration prediction
NEMA MG-1: Electric motor vibration limits

Monitoring and Diagnostic Techniques

Routine Monitoring

Standard vibration monitoring programs measure radial vibration:

Route-Based Collection: Periodic measurements at fixed intervals (monthly, quarterly)
Overall Level Trending: Track total vibration amplitude over time
Alarm Limits: Set based on ISO or equipment-specific standards
Comparison: Current vs. baseline, horizontal vs. vertical

Advanced Analysis

Detailed radial vibration analysis provides diagnostic information:

FFT Analysis: Frequency spectrum showing vibration components
Time Waveform: Vibration signal over time revealing transients and modulation
Phase Analysis: Timing relationships between measurement points
Orbit Analysis: Shaft centerline motion patterns
Envelope Analysis: High-frequency demodulation for bearing defect detection

Continuous Monitoring

Critical equipment often has permanent radial vibration monitoring:

Proximity probes for direct shaft motion measurement
Permanently mounted accelerometers on bearing housings
Real-time trending and alarming
Automatic protection system integration

Horizontal vs. Vertical Differences

Typical Amplitude Relationships

In many machines, vertical radial vibration exceeds horizontal:

Gravity Effect: Rotor weight creates static deflection, affecting vertical stiffness
Asymmetric Stiffness: Foundation and support structures often stiffer horizontally
Typical Ratio: Vertical vibration 1.5-2× horizontal is common
Balance Weight Effect: Correction weights placed at bottom of rotor (easy access) preferentially reduce vertical vibration

Diagnostic Differences

Unbalance: May show more strongly in one direction depending on unbalance location
Looseness: Often shows non-linearity more pronounced in vertical direction
Foundation Issues: Vertical vibration more sensitive to foundation deterioration
Misalignment: May appear differently in horizontal vs. vertical based on misalignment type

Relationship to Rotor Dynamics

Radial vibration is central to rotor dynamics analysis:

Critical Speeds

Radial natural frequencies determine critical speeds
First critical speed typically corresponds to first radial bending mode
Campbell diagrams predict radial vibration behavior vs. speed
Separation margins from critical speeds prevent excessive radial vibration

Mode Shapes

Each radial vibration mode has characteristic deflection shape
First mode: simple arc bending
Second mode: S-curve with node point
Higher modes: increasingly complex patterns

Balancing Considerations

Balancing targets reduction of radial vibration at 1X frequency
Influence coefficients relate correction weights to radial vibration changes
Optimal correction plane locations based on radial mode shapes

Correction and Control Methods

For Unbalance

Field balancing using portable analyzers
Single-plane or two-plane balancing procedures
Precision shop balancing for critical components

For Mechanical Problems

Precision alignment to correct misalignment
Bearing replacement for bearing defects
Tightening loose components
Foundation repairs for structural issues
Shaft straightening or replacement for bent shafts

For Resonance Issues

Speed changes to avoid critical speed ranges
Stiffness modifications (shaft diameter, bearing location changes)
Damping enhancements (squeeze film dampers, bearing selection)
Mass changes to shift natural frequencies

Importance in Predictive Maintenance

Radial vibration monitoring is the cornerstone of predictive maintenance programs:

Early Fault Detection: Changes in radial vibration precede failures by weeks or months
Trending: Gradual increases indicate developing problems
Fault Diagnosis: Frequency content identifies specific fault types
Severity Assessment: Amplitude indicates problem severity and urgency
Maintenance Scheduling: Condition-based rather than time-based maintenance
Cost Savings: Prevents catastrophic failures and optimizes maintenance intervals

As the primary vibration measurement in rotating machinery, radial vibration provides essential information about equipment condition, making it indispensable for ensuring reliable, safe, and efficient operation of industrial rotating equipment.

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What is Radial Vibration in Rotating Machinery?

Published by admin on October 31, 2025 October 31, 2025