Diagnosis Guide for Excessive Vibration in Rotating Equipment: From Spectral Analysis to Root Cause

1. Problem Description and Scope

This technical guide is designed to assist engineers and maintenance technicians in diagnosing and troubleshooting excessive vibration problems in industrial rotating equipment. Vibration is a critical indicator of machine health, and abnormal levels can lead to premature failures, unscheduled downtime and lost production. We will address symptoms of vibration in machines such as pumps, electric motors, fans, compressors, reducers and transmission systems.

1.1 Vibration Severity Rating (NBR ISO 10816-3)

Vibration severity can be classified according to the ABNT NBR ISO 10816-3 standard (Measurement and Assessment of Mechanical Vibration Severity in Industrial Machines Based on Non-Rotating Parts), which establishes operating zones:

• Zone A (Good): Vibration at very low levels. Machine in ideal conditions.
• Zone B (Satisfactory): Machine considered acceptable for continuous operation.
• Zone C (Unsatisfactory): Machine can operate temporarily, but corrective interventions are necessary in the medium term.
• Zone D (Unacceptable): Machine operating with risk of failure. Requires immediate corrective action to prevent catastrophic damage.

Typical RMS Speed Limits (mm/s) for Large Machines (Ex: Motors > 300 kW, Pumps > 100 kW):

• Zone A/B (Limit): 2.3 mm/s
• Zone B/C (Limit): 5.6 mm/s
• Zone C/D (Limit): 11.2 mm/s

Exact values depend on machine class, foundation and power. Always consult the NBR ISO 10816-3 for the specific limits of your application.

2. Safety Precautions

CRITICAL SAFETY WARNING: Before starting any diagnosis or intervention on rotating equipment, it is imperative to strictly follow the plant's safety standards, in particular NR-10 (Safety in Electrical Installations and Services) and NR-12 (Safety at Work in Machinery and Equipment).

• LOCKING AND TAGGING (LOTO): Ensure that all energy sources (electrical, hydraulic, pneumatic) of the equipment and its associated components are isolated and locked out. Verify the absence of power by testing.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Always wear a helmet, safety glasses, ear protectors, safety gloves and safety shoes. Additional PPE may be required depending on the environment and task (e.g., arc flash face shield).
• STORED ENERGY: Discharge any stored energy (springs, capacitors, pressurized hydraulic systems).
• HOT/SHARP COMPONENTS: Be aware of hot surfaces and sharp edges. Avoid contact.
• UNEXPECTED MOVEMENT: Never place parts of the body in areas at risk of crushing or cutting, even with the machine blocked, without confirming mechanical stability.

3. Required Diagnostic Tools

Correctly identifying the root cause of vibration requires the use of specific and calibrated tools. Below is a table of essential tools:

4. Initial Assessment Checklist

Before connecting any diagnostic instrument, a visual inspection and collection of operational and historical data are crucial. This saves time and directs the investigation.

5. Systematic Diagnosis Flowchart (Spectrum Analysis)

Vibration spectral analysis is the most effective method for identifying the root cause. This flowchart will guide the technician through the most common vibration patterns.

1. High Vibration Detection:
   • Action: Record global vibration levels (RMS speed in mm/s). Compare with NBR ISO 10816-3.
   • IF Level > Zone B/C: Continue with diagnosis.
   • ELSE IF Level < Zone B/C: Monitor.
2. Vibration Data Collection:
   • Tool: Vibration analyzer with accelerometer.
   • Action: Collect vibration spectra at strategic points (motor bearings, coupling, driven machine bearings) in the radial (horizontal and vertical) and axial directions.
   • Configuration: Frequency range up to 100x RPM, 1600 lines of resolution, average 4 to 8.
3. Vibration Spectrum Analysis:
   1. Peak in Rotation Frequency (1X RPM):
      • Symptom: Dominant peak (largest amplitude) in rotation frequency (1X RPM) in all directions (radial and axial).
      • Probable Cause: Unbalance or slight angular misalignment.
      • Additional Diagnosis:
        • IF Dominant 1X RPM spike, especially on radial and without significant multiples:
          1. Most Likely Root Cause: Imbalance.
          2. Action: Check accumulation of material (dirt, corrosion), loss of counterweights, rotor eccentricity. Perform dynamic balancing.
        • IF Dominant 1X RPM peak with presence of 2X RPM, and high radial and axial amplitudes:
          1. Most Likely Root Cause: Angular Misalignment.
          2. Action: Check coupling laser alignment.
        • IF Dominant 1X RPM peak with presence of 2X RPM, and high amplitudes mainly in radial:
          1. Most Likely Root Cause: Parallel Misalignment.
          2. Action: Check coupling laser alignment.
        • IF 1X RPM peak with multiples of 1X RPM (2X, 3X, 4X...) and broadband noise (frequencies not synchronized):
          1. Most Likely Root Cause: Mechanical Backlash.
          2. Action: Visual inspection of loose screws, cracks, worn bearings, flexible bases.
   2. Spikes at Multiples of Rotation Frequency (2X, 3X, 4X RPM):
      • Symptom: High spikes at 2X RPM, 3X RPM or more.
      • Probable Cause: Misalignment, Mechanical Play or lubrication problems.
      • Additional Diagnosis:
        • IF 2X RPM dominant (greater than 1X RPM), without many other multiples:
          1. Most Likely Root Cause: Parallel Misalignment (if stronger in radial) or Angular Misalignment (if strong in axial and radial).
          2. Action: Perform laser alignment.
        • IF Various multiples of RPM (2X, 3X, 4X+) with wideband noise:
          1. Most Likely Root Cause: Mechanical Backlash.
          2. Action: Detailed inspection of fixings, bearings, bushings, bases.
   3. High Frequency Spikes / Unsynchronized Frequency Bands:
      • Symptom: Spikes at high frequencies, often accompanied by frequency bands around 1X RPM or multiples, or spikes with clear harmonic spacing.
      • Probable Cause: Failure in bearings, gears, electrical problems, friction.
      • Additional Diagnosis:
        • IF Peaks spaced by rolling element passing frequencies (BPFO, BPFI, BSF, FTF):
          1. Most Likely Root Cause: Bearing Failure.
          2. Action: Calculate bearing failure frequencies. Inspect lubrication. Monitor temperature.
        • IF Clearly spaced spikes that do not correspond to bearings, and multiples of the gear frequency (GMF):
          1. Most Likely Root Cause: Gear Failure.
          2. Action: Inspect teeth, reducer lubrication.
        • IF Peaks in motor pole crossing frequencies (Fp = RPM * Np / 60) or mains harmonics (120 Hz, 180 Hz in Brazil):
          1. Most Likely Root Cause: Electrical Problems in the Motor (broken rotor bars, eccentric air gap, phase problems).
          2. Action: Electrical check (multimeter, current analysis).
        • IF Unsynchronized broadband vibration (noise) without clear peaks:
          1. Most Likely Root Cause: Excessive Friction or Cavitation.
          2. Action: Inspect contact surfaces, lubrication, fluid flow conditions.
   4. Spikes at Subsynchronous Frequencies (below 1X RPM):
      • Symptom: Spikes at 0.5X RPM, 0.4X RPM, or non-integer RPM frequencies.
      • Probable Cause: Lubrication instability (oil whirl/whip) in plain bearings, friction, excessive clearances.
      • Additional Diagnosis:
        • IF Peak at 0.42-0.48X RPM, common in plain bearings:
          1. Most Likely Root Cause: Oil Whirl / Oil Whip.
          2. Action: Check oil pressure and viscosity, bearing clearance.
   5. Low Frequency Spikes / Natural Frequencies:
      • Symptom: High amplitude spike at a frequency that is not a multiple of the RPM, and that may be equal to or close to the RPM.
      • Probable Cause: Resonance.
      • Additional Diagnosis:
        • Action: Perform bump test to identify natural frequencies of the structure. Change the stiffness or mass of the structure, or modify the RPM of the machine, if possible.

6. Failure and Cause Matrix (Symptom x Probable Cause)

This table summarizes the most common vibration patterns and their likely causes, assisting in quick diagnosis after spectral analysis.

7. Root Cause Analysis for Each Major Failure

7.1. Imbalance

Why it happens: It occurs when the center of mass of a rotor does not coincide with its axis of rotation. This creates an uneven centrifugal force that rotates with the rotor, resulting in vibration. Causes include dirt accumulation, corrosion, loss of counterweights, manufacturing errors, inadequate repairs or thermal deformations.

How to confirm: The main symptom is a dominant peak at a frequency of 1X RPM in the vibration spectrum, generally more pronounced in the radial direction. The vibration amplitude increases with the square of the rotational speed. This can be confirmed with a dynamic balance test.

Potential damage: Premature wear of bearings and housings, structural fatigue, shaft breakage, loosening of fixings and degradation of product quality.

7.2. Misalignment

Why it happens: It refers to the condition where the axes of coupled machines are not on the same geometric line. There are two main types: parallel misalignment (axes parallel but offset) and angular misalignment (axes at different angles). Causes include improper installation, uneven foundation, differential thermal expansion, loose fixings, or deformation of the foundation.

How to confirm:

• Parallel Misalignment: Dominant peaks at 2X RPM (and sometimes 1X RPM and 3X RPM), with high amplitudes in the radial directions.
• Angular Misalignment: Dominant peak at 1X RPM and 2X RPM, with a strong axial presence, which may be greater than the radial.

Confirmation is done with precision alignment tools, such as laser aligners, which indicate deviations between axes.

Potential damage: Overheating of the coupling, premature wear of bearings, seals and gears, breakage of shafts due to fatigue, increased energy consumption and cracks in the structure.

7.3. Mechanical Backlash

Why it happens: It results from excessive play between components, such as worn bearings, loose bushings, loose fixing screws, cracks in the foundation or casing. These gaps allow uncontrolled movements that cause vibration. It can be classified into type A clearance (non-linearity of the mass), type B (fixing/bearing clearance) and type C (cracks in the structure).

How to confirm: The vibration spectrum shows a series of RPM harmonics (2X, 3X, 4X, etc.) with or without 1X RPM, and often a high “broadband noise”. The pattern can vary drastically with load. Confirmation involves visual inspection, torque testing on screws, use of a dial indicator to measure clearances in bearings and checking for cracks.

Potential Damage: Uneven vibration, excessive noise, accelerated component wear, structural fatigue, and catastrophic failure if play leads to structural component failure.

7.4. Bearing/Loading Failure

Why it happens: Bearings can fail due to inadequate lubrication (type, quantity, contamination), misalignment, unbalance, overload, incorrect installation or material fatigue. In plain bearings, the failure may be due to instability of the oil film (oil whirl/whip).

How to confirm: The vibration spectrum generally displays peaks at frequencies calculated based on bearing geometries (BPFO - Outer Race Ball Passing Frequency, BPFI - Inner Race Ball Passing Frequency, BSF - Ball Rotation Frequency, FTF - Cage Fundamental Frequency). In early stages, high frequency bands can be observed. Thermography may indicate overheating.

Potential damage: Bearing seizure, excessive heating, shaft or housing breakage, lubricant contamination and abrupt equipment stop.

7.5. Resonance

Why it happens: It occurs when an excitation frequency (vibratory force) of a machine approaches or coincides with one of the natural frequencies of its support structure or its components. Small forces can generate large vibration amplitudes.

How to confirm: The vibration spectrum will show a high amplitude peak at a specific frequency, which may or may not be a multiple of the RPM. A “bump test” on the structure can be performed to determine its natural frequencies. If a natural frequency coincides with a machine operating frequency, resonance is confirmed.

Potential damage: Fatigue cracks in the structure, damage to components, loosening of screws and catastrophic failure of the machine or its foundation.

8. Step-by-Step Resolution Procedures

8.1. Resolution for Unbalance

1. SAFETY WARNING: Lock and tag (LOTO) the equipment.
2. Visual Check: Inspect the rotor for accumulation of material (dirt, corrosion, scale), loss of counterweights or structural damage. Clean or repair if necessary.
3. Dynamic Balancing: Use a portable dynamic balancer. Add or remove mass in specific rotor planes to compensate for eccentricity.
4. Tolerance: The objective is to achieve a balancing quality in accordance with NBR ISO 21940 (e.g. G6.3 for most industrial machines). Reduce vibration from 1X RPM to acceptable levels (< 2.3 mm/s).
5. Verification: Reconnect the equipment (after LOTO is undone and safety verified) and collect a new vibration spectrum.

8.2. Resolution for Misalignment

1. SAFETY WARNING: Lock and tag (LOTO) the equipment.
2. Alignment Measurement: Use a laser aligner to measure the angular and parallel misalignment between the axes. Record the initial values.
3. Correction: Adjust the position of the mobile machine (usually the engine) through shims on the feet and lateral movements (jack bolts) until alignment tolerances are reached.
4. Tolerance: For flexible couplings, a residual misalignment of 0.05 mm for parallel and 0.05 mm/100 mm for angular is a good target. For rigid couplings, the tolerance is even smaller.
5. Retightening: After alignment, retighten all base and coupling screws with the torque specified by the manufacturer.
6. Verification: Reconnect the equipment (after LOTO is undone and safety verified) and collect a new vibration spectrum, focusing on reducing 1X and 2X RPM.

8.3. Resolution for Mechanical Backlash

1. SAFETY WARNING: Lock and tag (LOTO) the equipment.
2. Detailed Inspection:
• Check all fixing screws (base, bearings, housings). Tighten to the specified torque.
• Inspect the foundation and base of the machine for cracks, corrosion or deformation. Repair or reinforce if necessary.
• Check bearings and bearings: measure radial and axial clearances with a dial indicator. If they are out of tolerance (e.g. radial clearance > 0.1 mm for small bearings), plan for replacement.
• Inspect bushings and coupling pins.
3. Verification: Reconnect the equipment (after LOTO is undone and safety verified) and collect a new vibration spectrum. RPM multiples should be significantly reduced.

8.4. Resolution for Bearing/Loading Failure

1. SAFETY WARNING: Lock and tag (LOTO) the equipment.
2. Disassembly: Remove the damaged bearing or bearing, following the manufacturer's procedures.
3. Contamination Inspection: Check the presence of contaminants in the housing and lubricant. Clean completely.
4. Replacement: Install new bearing/housing, using appropriate tools (inductive heater for bearings, extractors).
5. CAUTION: Never force the bearing by hitting the inner/outer race directly, as this will cause damage.
6. Lubrication: Apply the correct lubricant (type and quantity) as specified by the manufacturer.
7. Verification: Reconnect the equipment (after LOTO is undone and safety verified). Monitor temperature and high frequency vibration levels.

8.5. Resolution for Resonance

1. SAFETY WARNING: Lock and tag (LOTO) the equipment.
2. Natural Frequency Identification: Confirm the natural frequency of the structure or component using a bump test with the equipment stopped.
3. Correction Strategies:
• Mass Change: Add mass to the structure (e.g. reinforcement with beams) to reduce the natural frequency.
• Change in Rigidity: Reinforce the structure or base to increase the natural frequency.
• Changing the Excitation Frequency: If possible and feasible, change the machine's operating speed to move away from the natural frequency.
• Damping: Add vibration dampers, although this is usually a stopgap solution.
4. Verification: Reconnect the equipment (after LOTO is undone and safety checked) and collect a new vibration spectrum to confirm the reduction in amplitude at the resonance frequency.

9. Preventive Measures

Prevention is key to the longevity and reliability of rotating equipment. Implementing a predictive and preventive maintenance program is essential.

10. Spare Parts and Essential Components

The availability of quality replacement parts is essential for agility in maintenance and for restoring equipment reliability. UNITEC-D GmbH offers a wide range of components.

For a complete selection of industrial spare parts and components, visit the UNITEC-D e-catalog: https://www.unitecd.com/e-catalog/

11. References

• ABNT NBR ISO 10816-3: Mechanical vibration – Assessment of machine vibration through measurements on non-rotating parts – Part 3: Industrial machines with rated power above 15 kW and rated speeds between 120 r/min and 15,000 r/min when measured on site.
• ABNT NBR ISO 21940: Mechanical vibration – Requirements for balancing rigid and flexible rotors.
• ABNT NBR ISO 1940-1: Mechanical vibration – Quality requirements for balancing rigid rotors.
• ABNT NBR ISO 7919-3: Mechanical vibration of machines with nominal power above 15 kW – Assessment of machine vibration through measurements on rotating axes.
• ABNT NBR 10082: Alignment of rotating machines.
• Operation and maintenance manuals from manufacturers (OEMs) of specific equipment.
• Other UNITEC-D Maintenance Guides: Consult our guides section for complementary topics such as lubrication, oil analysis and thermography.