Understanding Lateral 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 Lateral Vibration?

Lateral vibration (also called radial vibration or transverse vibration) refers to the motion of a rotating shaft perpendicular to its axis of rotation. In simple terms, it is the side-to-side or up-and-down motion of the shaft as it spins. Lateral vibration is the most common type of vibration in rotating machinery and is typically caused by radial forces such as unbalance, misalignment, bent shafts, or bearing defects.

Understanding lateral vibration is fundamental to rotor dynamics because it represents the primary mode of vibration for most rotating equipment and is the focus of most vibration monitoring and balancing activities.

Direction and Measurement

Lateral vibration is measured in the plane perpendicular to the shaft axis:

Coordinate System

Horizontal Direction: Side-to-side motion parallel to the ground
Vertical Direction: Up-and-down motion perpendicular to the ground
Radial Direction: Any direction perpendicular to the shaft axis (combination of horizontal and vertical)

Measurement Locations

Lateral vibration is typically measured at:

Bearing Housings: Using accelerometers or velocity transducers mounted on bearing caps or pedestals
Shaft Surface: Using non-contact proximity probes for direct shaft motion measurement
Multiple Orientations: Measurements in both horizontal and vertical directions provide complete picture of lateral motion

Primary Causes of Lateral Vibration

Lateral vibration can arise from numerous sources, each producing characteristic vibration signatures:

1. Unbalance (Most Common)

Unbalance is the most frequent cause of lateral vibration. An asymmetric mass distribution creates a rotating centrifugal force that produces:

1X (once per revolution) vibration frequency
Relatively stable phase relationship
Amplitude proportional to the square of speed
Circular or elliptical shaft orbit

2. Misalignment

Shaft misalignment between coupled machines creates lateral forces:

Primarily 2X vibration (twice per revolution)
Can also excite 1X and higher harmonics
Often shows high axial component as well
Phase relationships differ from unbalance

3. Bent or Bowed Shaft

A permanently bent or bowed shaft creates geometric eccentricity:

1X vibration that may appear similar to unbalance
High vibration even at slow roll speeds
Difficult to correct by balancing alone

4. Bearing Defects

Rolling element bearing defects produce characteristic lateral vibration:

High-frequency components (bearing fault frequencies)
Modulated by lower frequencies creating sidebands
Often requires envelope analysis for detection

5. Mechanical Looseness

Loose bearings, foundations, or mounting bolts create:

Multiple harmonics (1X, 2X, 3X, etc.)
Non-linear response to forcing
Erratic or unstable vibration

6. Rotor-Stator Rub

Contact between rotating and stationary parts generates:

Sub-synchronous components
Sudden changes in vibration amplitude and phase
Possible thermal bowing

Lateral Vibration vs. Other Vibration Types

Rotating machinery can experience vibration in three principal directions:

Lateral (Radial) Vibration

Direction: Perpendicular to shaft axis
Typical Causes: Unbalance, misalignment, bent shaft, bearing defects
Measurement: Accelerometers or velocity sensors on bearing housings; proximity probes on shaft
Dominance: Usually the largest amplitude vibration component

Axial Vibration

Direction: Parallel to shaft axis
Typical Causes: Misalignment, thrust bearing issues, process flow problems
Measurement: Accelerometers mounted axially
Dominance: Typically lower amplitude than lateral, but diagnostic for certain faults

Torsional Vibration

Direction: Twisting motion about shaft axis
Typical Causes: Gear mesh issues, motor electrical problems, coupling problems
Measurement: Requires specialized torsional vibration sensors or strain gauges
Dominance: Usually small but can cause fatigue failures

Lateral Vibration Modes and Critical Speeds

In rotor dynamics, lateral vibration modes describe the characteristic deflection patterns of the shaft:

First Lateral Mode

Simple bending shape (single arc or bow)
Lowest natural frequency
Most easily excited by unbalance
First critical speed corresponds to this mode

Second Lateral Mode

S-shaped deflection with one node point
Higher natural frequency
Second critical speed
Important for flexible rotors

Higher Lateral Modes

Increasingly complex shapes with multiple nodes
Relevant only for very high-speed or very flexible rotors
May be excited by blade passing or other high-frequency excitations

Measurement and Monitoring

Measurement Parameters

Lateral vibration is characterized by several parameters:

Amplitude: The magnitude of motion, measured in displacement (µm, mils), velocity (mm/s, in/s), or acceleration (g, m/s²)
Frequency: Typically 1X running speed for unbalance-dominated vibration, but can include harmonics and other frequencies
Phase: The timing of maximum displacement relative to a reference mark on the shaft
Orbit: The actual path traced by the shaft center as viewed end-on

Measurement Standards

International standards provide guidance for acceptable lateral vibration levels:

ISO 20816 Series: Vibration limits for various machine types based on RMS velocity
API 610, 617, 684: Industry-specific standards for pumps, compressors, and rotor dynamics
Severity Zones: Define acceptable, caution, and alarm levels based on equipment type and size

Control and Mitigation

Balancing

Balancing is the primary method for reducing lateral vibration from unbalance:

Single-plane balancing for disc-type rotors
Two-plane balancing for most industrial rotors
Modal balancing for flexible rotors operating above critical speeds

Alignment

Precision shaft alignment reduces lateral forces from misalignment:

Laser alignment tools for accurate shaft positioning
Thermal growth consideration in alignment procedures
Soft foot correction before alignment

Damping

Damping controls lateral vibration amplitudes, especially at critical speeds:

Fluid-film bearings provide significant damping
Squeeze-film dampers for additional control
Support structure damping treatments

Stiffness Modification

Changing system stiffness moves critical speeds:

Shaft diameter increases raise critical speeds
Bearing span reduction increases first critical speed
Foundation stiffening affects overall system response

Diagnostic Importance

Lateral vibration analysis is the cornerstone of machinery diagnostics:

Trending: Monitoring lateral vibration over time reveals developing problems
Fault Identification: Vibration frequency and pattern identify specific fault types
Severity Assessment: Amplitude compared to standards indicates problem severity
Balancing Verification: Lateral vibration reduction confirms successful balancing
Condition-Based Maintenance: Vibration levels trigger maintenance actions

Effective management of lateral vibration is essential for reliable, long-term operation of rotating machinery, making it a primary focus of vibration monitoring programs, predictive maintenance strategies, and rotor dynamic design considerations.

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What is Lateral Vibration in Rotating Machinery?

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