Guide to Diagnosing Excessive Vibrations on Rotating Equipment

1. Description of the Problem and Scope

This technical guide is intended for maintenance technicians and reliability engineers experiencing abnormal or excessive vibrations on industrial rotating equipment. The goal is to provide a structured methodology to diagnose the root cause of vibration, minimize unplanned shutdowns and prevent catastrophic failures, while optimizing asset life.

Typical equipment affected includes, but is not limited to:

• Centrifugal and positive displacement pumps
• Fans and blowers
• Electric motors (AC/DC)
• Compressors (reciprocating, centrifugal, screw)
• Turbines (steam, gas, hydraulic)
• Motor pump groups, motor fans, and other rotating assemblies

Symptoms of excessive vibration include noticeable machine movement, increased noise, overheating of bearings, rapid degradation of seals, leaks, and in severe cases, emergency shutdown or mechanical destruction.

Severity Classification:

• Critical: Vibration level exceeding the alarm limits set by the NF EN ISO 10816-3 standard (or OEM specific), requiring immediate shutdown to avoid irreparable damage, safety risks or major loss of production.
• Major: Vibration level meeting or exceeding warning limits, indicating significant performance degradation, accelerated component wear, or risk of medium-term failure. Requires rapid planned intervention.
• Minor: Increase in vibration levels compared to the baseline, or presence of specific frequencies indicating the beginning of degradation (e.g.: incipient bearing defect). Requires increased monitoring and condition-based maintenance planning.

2. Safety Precautions

Before any intervention on industrial equipment, personnel safety is critical. Failure to follow procedures could result in serious injury or death.

IMMEDIATE DANGER:

• LOCKING/LABELING (LOTO): Before any physical contact with the machine, ensure that all energy sources (electrical, hydraulic, pneumatic, mechanical) are isolated, locked and tagged in accordance with the internal procedure and standard NF C 18-510.
• RESIDUAL ENERGY: Discharge any accumulated energy (capacitors, live springs, hydraulic/pneumatic pressure, inertia of rotating parts).
• HOT SURFACES: Let the equipment cool before handling or use Personal Protective Equipment (PPE) suitable for heat.
• ROTATING PARTS: Never work on a moving machine. The risk of entrainment is extremely high.

WARNING:

• PERSONAL PROTECTIVE EQUIPMENT (PPE): Always wear safety glasses, hearing protection (NF EN 352), protective gloves (NF EN 388, NF EN 407), safety shoes (NF EN ISO 20345) and appropriate work clothing.
• CHEMICAL RISKS: In case of contact with lubricants or fluids, consult the Safety Data Sheets (MSDS) and use the required PPE.
• STABILITY: Make sure the equipment is stable and secure before beginning diagnostics.

Normative References: NF C 18-510 (Operations on electrical installations), NF EN ISO 12100 (Safety of machines – General design principles), NF EN 60204-1 (Safety of machines – Electrical equipment).

3. Required Diagnostic Tools

An accurate diagnosis of vibrations relies on the use of calibrated and adapted tools. Selecting the appropriate tool is essential for collecting reliable data.

4. Initial Assessment Checklist

Before initiating an in-depth diagnosis by vibration analysis, a rigorous visual and contextual assessment is essential. These initial observations can guide the search for the cause and save valuable time.

5. Systematic Diagnostic Flowchart

This process guides the technician from detecting the initial symptom to confirming the root cause, leveraging spectral vibration analysis.

1. Symptom: Excessive vibration detected
2. Initial Check (Refer to section 4)
   • Complete the initial assessment checklist.
   • Record operating parameters (speed, load, temperature).
   • WARNING: If the vibration is critical and/or rapidly increasing, stop the machine and perform LOTO.
3. Acquisition of Vibration Data
   • Install the vibration analyzer on the machine bearings and at strategic points (NF EN ISO 10816-1/3).
   • Acquire speed (mm/s RMS) and acceleration (g RMS) spectra over a wide frequency band (up to 100x RPM and beyond for bearings).
   • Measure the vibration phase if possible.
4. Spectral Vibration Analysis
   • Identify the Dominant Frequencies:
     1. If dominant peak at 1X RPM (rotation frequency):
        • Check the phase:
          • If phase stable and shifted of approximately 90-180° between opposite points: Unbalance.
          • If unstable or variable phase, or offset of 0° or 180° at adjacent points: Misalignment or Soft Foot.
     2. If dominant peak at 2X RPM (double the rotation frequency):
        • Check phase:
          • If offset of 0° or 180° axially between the two sides of the coupling: Misalignment (parallel).
          • If phase stable radially and 180° between the bearings: Mechanical Loosening (e.g.: soft foot, significant play).
     3. If harmonics of the RPM (3X, 4X, 5X...):
        • Presence of multiples of 1X RPM: Misalignment (combined angular and parallel), Mechanical Loosening, Friction.
     4. If peaks at non-synchronous shaft frequencies:
        • Check the bearing fault frequencies (BPFI, BPFO, BSF, FTF):
          • If these frequencies are present: Bearing Fault. Locate the defective bearing.
        • Check the meshing frequencies:
          • If peaks at the meshing frequency or its harmonics: Gear Fault.
        • If broadband spectrum (random):
          • If turbulence or cavitation in the pumps/fans: Hydraulic/Aerodynamic problem.
          • If metal-on-metal friction: Friction.
     5. If the amplitude of the vibrations varies significantly with speed:
        • Check the natural frequencies of the machine:
          • Perform a speed up/down test or a shock test (bump test).
          • If a natural frequency is close to 1X, 2X RPM or an excitation frequency: Resonance.
     6. If peaks at mains frequencies (50 Hz, 100 Hz in Europe):
        • Electrical Problem (phase imbalance, eccentric rotor, broken rotor bars).
5. Root Cause Confirmation (Refer to section 7)
   • Use specific tools to confirm the fault (laser for alignment, thermography for friction/overheating).
6. Resolution (Refer to section 8)

6. Fault-Cause Matrix

This matrix relates the characteristics of the vibrations observed (mainly via spectral analysis) to the probable causes and confirmation tests.

7. Root Cause Analysis for Each Major Defect

Unbalance

Explanation: Imbalance occurs when the center of mass of a rotor does not coincide with its axis of rotation. This creates a variable centrifugal force with each rotation, inducing vibration at 1X RPM (shaft rotation frequency). It can be static (center of mass shifted on a single plane), dynamic (shift on two distinct axial planes), or torque (center of mass on the axis but mass poorly distributed angularly on two planes).

How to confirm:

• Spectral Analysis: Dominant and stable peak at 1X RPM in the speed spectrum (mm/s), mainly in the radial direction.
• Phase Analysis: The vibration phase at 1X RPM is relatively stable and varies from approximately 90° to 180° between opposite measurement points (e.g. up/down or left/right) on the same bearing, and also between bearings on the longitudinal axis.
• Rev Up/Down Test: The amplitude of the 1X RPM increases proportionally to the square of the speed (V = k * RPM²).

Damage if not resolved: Accelerated wear of bearings and shaft seals, fatigue of shafts and foundations, loose bolts, increased power consumption, and potentially shaft breakage or catastrophic bearing failure.

Misalignment

Explanation: Misalignment occurs when the axes of rotation of the coupled machines are not collinear. There are two main types: parallel misalignment (the axes are parallel but offset) and angular misalignment (the axes intersect at an angle). The misalignment generates significant cyclic stresses at the coupling and bearings.

How to confirm:

• Spectral Analysis: Dominant peaks at 2X RPM, often accompanied by 1X RPM and sometimes 3X RPM, particularly in the axial (angular misalignment) or radial (parallel misalignment) direction.
• Phase Analysis:
  • Parallel misalignment: Radial phase 0° or 180° (in opposition to phase) between the bearings, and little or no axial phase.
  • Angular misalignment: Axial phase 0° or 180° (in opposition to phase) between the two sides of the coupling, and little or no radial phase.
• Laser measurement: Use of a laser alignment system to precisely measure misalignment values ​​(in µm or thousandths of an inch).
• Thermography: Localized overheating at the coupling or bearings, due to internal friction and stress.

Damage if not resolved: Premature wear of bearings, couplings (rubber, grid), seals, bending of the shaft, fatigue of foundation bolts, increased energy consumption. May lead to coupling or shaft failure.

Bearing Defects

Explanation: Bearing defects are one of the most common causes of vibration and can affect the inner race (BPFI), outer race (BPFO), balls/rollers (BSF) or cage (FTF). These defects create repetitive pulses each time a defective element contacts another surface of the bearing.

How to confirm them:

• Spectral Acceleration Analysis: Appearance of peaks at non-synchronous frequencies, specific to bearing defects (BPFI, BPFO, BSF, FTF), calculable as a function of the geometry of the bearing and the speed of the shaft. These frequencies are often accompanied by their harmonics and sidebands around other frequencies (1X RPM).
• Enveloping Analysis:critical technique for detecting bearing defects at an early stage. It filters low frequencies and demodulates the high frequency signal to highlight repetitive pulses from bearing faults.
• Temperature: Abnormal increase in bearing temperature (measured by IR thermometer or thermal camera).
• Noise: Audible grinding, clicking, or roaring sounds coming from the bearing.
• Oil Analysis: Presence of metallic particles (ferrography) or contamination.

Damage if not resolved: Bearing overheating, seizure, rotor lock, secondary damage to shaft and housing, catastrophic machine failure. Can also induce a secondary misalignment or imbalance.

Mechanical Looseness

Explanation: Mechanical looseness represents excessive play between the components of a machine, or between the machine and its foundation. It can be structural (weak foundation, loose base bolts, soft foot) or mechanical (excessive clearance between bearing and shaft, clearance in a coupling, loose rotor on the shaft).

How to confirm:

• Spectral Analysis: Presence of numerous harmonics of the rotation speed (2X, 3X, 4X RPM, etc.), often accompanied by high background noise (broadband noise). Peaks may be unstable in amplitude and phase.
• Phase Analysis: The vibrational phase may be inconsistent or vary greatly with slight variations in load or speed.
• Rev Up/Down Test: May reveal non-linear behavior, with peaks appearing and disappearing erratically.
• Visual Inspection: Checking the tightness of all foundation and fixing bolts. Using thickness gauges to detect soft foot.
• Bump Test: To assess the integrity of the structure and identify natural frequencies that could be excited by loosening.

Damage if not resolved: Fatigue of components, broken bolts, premature wear of bearings, couplings and shafts due to abnormal stresses and movements. May generate secondary misalignment or imbalance.

Resonance

Explanation: Resonance occurs when the excitation frequency (e.g. shaft rotation frequency, meshing frequency) coincides with or is very close to a natural vibration frequency of the machine or one of its structures (base, frame, piping). Even a low excitation force can then result in drastic amplification of vibrations, with amplitudes that can be twenty times greater than normal.

How to confirm:

• Run-up/Coast-down Test: The vibration amplitude increases sharply at a specific rotational speed (critical speed) then decreases after having exceeded it. The Bode or Nyquist diagram is critical for visualizing this phenomenon.
• Shock Test (Bump Test or Hammer Test): A calibrated percussion on the machine or its foundations with an instrumented hammer makes it possible to excite the natural frequencies. FFT analysis of the vibration response identifies these frequencies. If one of them matches an operating frequency, resonance is confirmed.
• Spectral Analysis: A very high peak at a constant frequency, even if the machine speed varies slightly (if the excitation is speed related), or at a frequency that is not a direct multiple of the rotational speed.
• Modal Analysis: For complex cases, modal analysis makes it possible to determine the shapes of vibration modes and the natural frequencies of the entire structure.

Damage if not resolved: Rapid structural fatigue, cracking of foundations, broken shafts, repeated failures of bearings and seals, damage to ancillary components (piping, instrumentation). Resonance is a major cause of premature failure.

8. Step-by-Step Resolution Procedures

Once the root cause is identified, specific procedures are required to correct the defect. Always respect the LOTO before any physical intervention.

Correction of Imbalance

1. SECURITY: Perform the LOTTO.
2. Cleaning: Clean the rotor thoroughly to remove any accumulation of dirt or debris.
3. Checking: Make sure that all bolts are tight, that keys and pins are in place and that the rotor is not damaged (e.g. broken blades on a fan).
4. Dynamic Balancing:
   • Use portable dynamic balancing equipment.
   • Carry out a test in one or two planes (depending on the type of imbalance and the NF ISO 21940-11 standard).
   • Add or remove balancing weights (screws, welds, grinding) at locations and in quantities indicated by the balancer.
   • Tolerances: Reduce the vibration to 1X RPM below the limits specified by the NF EN ISO 10816-3 standard, generally class II or III for industrial machines (e.g.: 2.8 mm/s RMS or less for medium-sized machines).
   • Repeat tests until vibration levels are acceptable.
5. Post-Repair Check: Start the machine and check the vibration levels. Save the new spectra.

Correction of Misalignment

1. SECURITY: Perform the LOTTO.
2. Soft Foot Check:
   • Loosen each foundation bolt one by one and measure the clearance with a feeler gauge.
   • If the clearance is greater than 0.05 mm, install stainless steel shims under the affected tab. (Standard NF E 29-901).
   • Tighten all bolts to specified torque.
3. Laser Measurement and Correction:
   • Install the laser alignment system on the coupled shafts.
   • Take initial measurements to determine existing misalignment (angular and parallel).
   • Use the shims provided with the laser system to adjust the height and horizontal position of the moving machine.
   • Tolerances: Aim for misalignment of less than 0.03 mm (30 µm) in parallel and 0.03 mm/100 mm mating distance in angular for critical equipment. Respect OEM tolerances or reference standards (e.g. NF EN ISO 14686).
   • Check the final alignment with the laser system.
4. Post-Repair Check: Start the machine, allow it to stabilize and resume vibration measurements.

Replacement of Defective Bearings

1. SECURITY: Perform the LOTTO.
2. Preparation: Disconnect the coupling, dismantle the casings and protections.
3. Bearing Removal:
   • Use a suitable bearing puller (mechanical or hydraulic) to avoid damage to the shaft or housing.
   • Carefully clean the shaft and housing. Check the condition of the surfaces (absence of burrs, scratches, ovalization).
4. Inspection: Check the tolerances of the shaft and housing (NF EN ISO 286).
5. New Bearing Installation:
   • Heating: For interference fit bearings, heat the bearing evenly in an induction oven (max temperature 110°C, depending on OEM and lubricant specifications) or a clean oil bath (NF ISO 281). NEVER HEAT WITH A FLAME.
   • Mounting: Slide the heated bearing onto the shaft until it stops. For cold bearings, use a press-fit tool to apply force only to the appropriate ring (inner for fit on shaft, outer for fit in housing).
   • WARNING: Never strike directly on the bearing or on the wrong ring.
   • Lubrication: Fill the bearing housing with the specified lubricant (type, quantity) (NF EN ISO 15330).
6. Reassembly and Checking: Reassemble the casings and the coupling. Start the machine gradually. Monitor vibration levels and bearing temperature.

Correction of Mechanical Loosening

1. SECURITY: Perform the LOTTO.
2. Full Inspection: Check all foundation bolts, support legs, housings, bearing supports, coupling bolts.
3. Tightening: Tighten all bolts to the torque specified by the manufacturer using a calibrated torque wrench (NF E 25-030).
4. Soft Foot Correction: Use the method described in the "Misalignment Correction" section.
5. Clearance Check: Check the clearance between the bearings and the shaft with a comparator or feeler gauges. If the clearance is excessive, replace the bearings or adjust the adjustment shims (for plain bearings).
6. Post-Repair Check: Start the machine, check vibration levels.

Resonance Attenuation

1. SAFETY: Perform LOTO before any structural modification.
2. Identification: Confirm the exact natural frequency via a shock test or modal analysis.
3. Mitigation Strategies:
   • Modification of Rigidity:
     • Add structural reinforcements to the base or frame (e.g. stiffeners, spacers).
     • Change the base material to a more rigid one.
     • Increase the diameter of the shaft (if possible).
   • Mass Modification:
     • Add mass (e.g. concrete blocks, counterweights) to vibrating structures to lower their natural frequency.
     • Remove ground to increase natural frequency (less common).
   • Dampening:
     • Install vibration dampers (isolators, elastomer anti-vibration supports) between the machine and its foundation.
     • Use viscoelastic cushioning materials.
   • Changing the Excitation Frequency:
     • If possible, modify the operating speed of the machine to move away from the critical natural frequency.
4. Post-Modification Verification: Carry out new shock tests and vibration measurements in operation to confirm the effectiveness of the correction.

9. Preventive Measures

Prevention is critical to maintain the integrity of rotating equipment and avoid recurrence of defects. A robust predictive maintenance program is essential.

10. Spare Parts and Components

Having spare parts that conform to original specifications is essential for a quick and lasting repair. UNITEC-D offers a complete range of high quality industrial components.

To order your certified and compliant spare parts with industrial standards, visit our E-catalog UNITEC-D.

11. References

• Standards:
  • NF EN ISO 10816 (Mechanical vibrations – Measurement and evaluation of machine vibrations on non-rotating bases).
  • NF EN ISO 15242 (Vibrations – Measurement and evaluation of bearing vibrations).
  • NF EN ISO 21940-11 (Balancing – Balancing requirements for rigid mass rotors).
  • NF EN ISO 14686 (Alignment of shaft couplings).
  • NF C 18-510 (Operations on electrical installations).
  • NF EN ISO 12100 (Machine safety – General design principles).
  • NF EN ISO 15330 (Lubricants – Guide for the lubrication of plain bearings and bearings).
• Original Equipment Manufacturer (OEM) Service Manuals.
• UNITEC-D internal technical guides on predictive maintenance and equipment reliability.
• Technical documentation from suppliers of bearings (SKF, FAG, Timken), couplings (KTR, Flender) and laser alignment systems (Easy-Laser, Pruftechnik).