Failure analysis: Excessive vibrations in rotating machines

1. Problem description & scope

Excessive vibration in rotating machines is a critical indicator of potential failures and can lead to increased wear, reduced product quality, energy waste and unexpected downtime. This diagnostic guide focuses on the systematic identification of the root causes of abnormal vibration levels in industrial rotating machinery, such as pumps, fans, electric motors, compressors, gearboxes and generators. The aim is to enable maintenance technicians to accurately locate the source of vibration and implement effective corrective measures, in accordance with the NEN-EN-ISO 10816-1 standard for vibration assessment.

Vibration severity classification (According to ISO 10816-1)

Vibration severity is classified into zones to determine the urgency of action:

• Zone A (Green): New machines in good condition. Typically 0-1.8 mm/s RMS (root-mean-square).
• Zone B (Yellow): Machines suitable for long-term operation. May have limited defects. Typically 1.8-4.5 mm/s RMS.
• Zone C (Orange): Machines with defects that are not suitable for long-term, continuous operation. Corrective action is required. Typically 4.5-11.2 mm/s RMS.
• Zone D (Red): Vibration levels that can cause serious damage or immediate danger. Immediate shutdown and repair are necessary. Typically >11.2 mm/s RMS.

Note: These values are general and should be compared to OEM specifications and the specific machine class.

2. Safety measures

WARNING! Working on rotating machinery can result in serious injury or death. Always follow company procedures for safety.

• Lockout/Tagout - LOTO: Always perform a complete LOTO procedure before beginning any inspection, maintenance or repairs. Ensure that all energy sources (electrical, hydraulic, pneumatic, mechanical) are isolated and locked out.
• Personal Protective Equipment (PPE): Always wear proper PPE, including safety glasses (EN 166), hearing protection (EN 352), safety gloves (EN 388) and safety shoes (EN ISO 20345).
• Stored energy: Be aware of stored energy in springs, pressurized systems, or elevated components. Discharge this energy safely before working.
• Hot surfaces: Rotating machines may have hot surfaces. Be careful and use thermal gloves if necessary.
• Noise level: Machines can produce high noise levels during operation. Wear hearing protection when near running machines.
• Electrical hazards: Assume that electrical components are live. Only qualified personnel should perform electrical diagnostics.

3. Required diagnostic tools

The right tools are essential for accurate vibration diagnostics. Make sure all equipment is calibrated and in good working order.

4. Initial assessment checklist

Before beginning detailed vibration measurements, a thorough initial assessment is crucial to gather context and rule out obvious problems.

5. Systematic diagnostic flowchart (Flowchart)

This decision tree guides the technician through the process of vibration analysis, starting from initial detection to root cause isolation.

1. Start: Vibration alarm or unexpected increase in vibration level.
2. Measures overall vibration (mm/s RMS).
   1. Compare to ISO 10816-1 standards and OEM specifications.
   2. If < Zone C: Continue monitoring, no immediate action required unless rapidly increasing.
   3. If ≥ Zone C: Proceed to step 3.
3. Collect an FFT vibration spectrum.
   1. Use the vibration analyzer with accelerometer at strategic measuring points (bearings, housing).
   2. Ensure sufficient frequency range (up to at least 10x RPM) and resolution (e.g. 400-800 lines).
4. Analyze the FFT spectrum for dominant frequencies and patterns.
   1. Dominant peak at 1× RPM (rotational speed):
      1. Check for Imbalance:
         1. Diagnostic test: Phase analysis (with strobe and phase meter).
         2. Expected result: Constant phase angle around the center of gravity, larger amplitude on bearings further from the center of gravity, or amplitude dependent on rotational speed.
         3. Go to: Root Cause Analysis Imbalance (section 7.1).
      2. Check for Alignment Error (Parallel Offset):
         1. Diagnostic Test: Laser Alignment.
         2. Expected result: Deviation in parallel or axial alignment greater than 0.05 mm.
         3. Go to: Root Cause Analysis Alignment error (section 7.2).
      3. Check for Structural Looseness (Soft Foot):
         1. Diagnostic test: Measuring soft foot with feeler gauges and torque of foundation bolts.
         2. Expected result: Variation in foundation height > 0.05 mm when tightening bolts.
         3. Go to: Root Cause Analysis Looseness (section 7.5).
   2. Dominant peak at 2× RPM:
      1. Check for Alignment Error (Angle or Combined):
         1. Diagnostic Test: Laser Alignment.
         2. Expected result: Angular deviation greater than 0.03 mm/100 mm axis length.
         3. Go to: Root Cause Analysis Alignment error (section 7.2).
      2. Check for Mechanical Looseness:
         1. Diagnostic Test: Impact test with rubber mallet on housing while machine is running; check spectrum for change in harmonics.
         2. Expected result: Increase in harmonics (2x, 3x RPM) or change in vibration amplitude.
         3. Go to: Root Cause Analysis Looseness (section 7.5).
   3. Peaks at higher harmonics (3×, 4× RPM and beyond):
      1. Check for Serious Misalignment or Structural Looseness:
         1. Diagnostic Test: Laser alignment, check all foundation bolts, inspect housing for cracks.
         2. Expected result: Combination of significant misalignment and/or severe looseness.
         3. Go to: Root Cause Analysis Misalignment (section 7.2) or Looseness (section 7.5).
      2. Check for Gear Problems (if applicable):
         1. Diagnostic Test: Calculate Gear Mesh Frequency (GMF) and check for spikes. Visually inspect gears for wear.
         2. Expected result: Peaks on GMF and sidebands.
         3. Go to: Root Cause Analysis Gear problems (section 7.6).
   4. Peaks on Non-synchronous frequencies (not linked to RPM):
      1. Check for Bearing Defects:
         1. Diagnostic test: High-frequency enveloping, calculating bearing frequencies (BPFI, BPFO, FTF, BSF) and comparing with peaks in the spectrum. Use a stethoscope.
         2. Expected result: Peaks at calculated lower frequencies and their harmonics, or a wide bump at higher frequencies (early stage).
         3. Go to: Root Cause Analysis Bearing defects (section 7.3).
      2. Check for Electrical Problems (on motors):
         1. Diagnostic test: Current signature measurement (MCSA), rotor bar inspection, shaft voltage measurement (with multimeter).
         2. Expected result: Peaks at 1x line frequency and sidebands around 2x RPM (rotor bars), 2x line frequency (loose windings).
         3. Go to: Root Cause Analysis Electrical Problems (section 7.7).
   5. Wide band/high frequency 'noise' or random vibration:
      1. Check for Lubrication Problems:
         1. Diagnostic Test: Inspect lubricant for contamination, level; thermal camera for overheating.
         2. Expected result: Contaminated/insufficient lubricant, increased bearing temperature.
         3. Go to: Root Cause Analysis Bearing failures (section 7.3 - due to lubrication).
      2. Check for Cavitation or Turbulence (on pumps):
         1. Diagnostic test: Check suction and discharge pressure, flow.
         2. Expected result: Irregular pressure fluctuations, noise.
         3. Go to: Root Cause Analysis Process Problems (section 7.8).
   6. Sub-synchronous peaks (< 1× RPM):
      1. Check for Oil Whip (Oil Whip/Whirl):
         1. Diagnostic Test: Assess bearing type (sliding bearings), oil pressure and viscosity.
         2. Expected result: Peaks at 0.43-0.48× RPM (oil whirling) or 0.48-0.5× RPM (oil whip).
         3. Go to: Root Cause Analysis Oil Whip (section 7.9).
   7. Resonance:
      1. Check for Resonance:
         1. Diagnostic test: Run-up/coast-down test (accelerate/coast) and impact test.
         2. Expected result: Increased vibration amplitude at a specific critical speed (corresponding to a natural frequency).
         3. Go to: Root Cause Analysis Resonance (section 7.4).
5. End of Diagnostic Process. Go to Root Cause Analysis and Resolution.

6. Error Cause Matrix

This matrix ranks likely causes based on symptoms, including diagnostic tests and expected results to confirm the cause.

7. Root cause analysis for each defect

7.1 Imbalance

Why it happens: Imbalance occurs when the center of gravity of a rotating component (rotor, impeller, coupling, pulley) does not coincide with the axis of rotation. This can be due to improper manufacturing, uneven wear, dirt buildup, missing counterweights, or uneven material distribution. Static and dynamic imbalance are the most common forms. Imbalance causes a centrifugal force proportional to the square of the rotational speed, resulting in a vibration component at 1× RPM.

How to confirm: The primary indication of imbalance is a dominant vibration peak at 1× RPM in the frequency spectrum. Phase analysis (using a strobe and phase meter) will show a consistent phase angle across multiple measurement points, indicating a heavy point moving with the rotation. The amplitude of the vibration will generally increase with the square of the speed.

Damage if left unresolved: Long-term imbalance leads to increased stress on bearings, seals and shafts, resulting in accelerated wear, premature bearing failure, shaft fatigue, loss of seal integrity and structural damage to the machine housing or foundation. This can also lead to increased energy consumption and overheating.

7.2 Misalignment

Why it happens: Misalignment occurs when the axes of rotation of two coupled machines (e.g. motor and pump) are not co-linear. This could be a parallel offset, an angular error, or a combination of both. Causes often include inaccurate mounting, thermal growth, foundation problems, or improper shimming. Misalignment causes repetitive bending stresses and forces on the shaft and bearings.

How to confirm: Alignment errors often manifest as dominant peaks at 1× and 2× RPM. A parallel misalignment usually produces a higher 1× RPM radial vibration, while an angular misalignment often produces a higher 2× RPM radial vibration, and significant axial vibration at 1× and 2× RPM. A laser alignment system is the most accurate tool for quantifying the degree of misalignment. Tolerances for alignment are typically below 0.05 mm parallel offset and 0.03 mm/100 mm axial deviation.

Damage if left unresolved: Like imbalance, misalignment leads to increased stress on bearings, seals and couplings, resulting in accelerated wear and premature failure. Specifically, couplings can overheat and fail prematurely, shafts can suffer fatigue fractures, and seals can leak. This significantly increases energy consumption.

7.3 Bearing defects

Why it happens: Bearing failures are one of the most common causes of machine failure. They can be caused by inadequate lubrication, contamination (particles in lubricant), overload, incorrect mounting, current passage through bearings (in VFDs) or natural fatigue after a long period of use. Defects can occur in the outer ring (BPFO), inner ring (BPFI), rolling elements (BSF) or cage (FTF).

How to confirm: Bearing defects manifest as peaks at specific, non-synchronous frequencies in the spectrum, which correspond to the bearing's calculated defect frequencies. In early stages these are often visible as a broad bump or 'noise' at higher frequencies, detectable with high frequency enveloping (HFD) or shock pulse measurements. As the defect progresses, the peaks become sharper and more distinct. A stethoscope can help locate the sound.

Damage if left unresolved: Incipient bearing failure will quickly escalate, leading to serious damage to the rolling elements and raceways. This results in overheating, excessive vibration, bearing seizure, and ultimately catastrophic machine failure. Repairs become more complex and expensive if a defect is noticed too late.

7.4 Resonance

Why it happens: Resonance occurs when an excitation frequency (e.g. operating speed, unbalance) is close to or equal to a natural frequency of the machine system or one of its components (foundation, housing, shaft). This results in a drastic amplification of the vibration amplitude, even at relatively low exciter forces. It is a structural problem.

How to confirm: The most effective way to confirm resonance is a run-up/coast-down test, where the machine is slowly brought through its speed range. A peak in the vibration amplitude at a specific critical speed indicates resonance. An impact test (hammer test) can be used to identify the natural frequencies of the stationary system. If a natural frequency is close to the operating speed, resonance is likely.

Damage if left unresolved: Resonance can lead to extremely high vibration levels that quickly destroy the machine. This includes structural fatigue and cracking in the foundation, casing, and piping, as well as premature failure of bearings and other components due to excessive dynamic forces. It can also cause noise pollution and endanger safety.

7.5 Looseness (Mechanical & Structural)

Why it happens: Looseness can be mechanical (e.g. loose bearings in their housing, worn spigot joints) or structural (e.g. loose foundation bolts, cracks in the foundation or machine feet - soft foot). Mechanical looseness creates a non-linear response, where the shaft can move 'freely' within excessive clearance, while structural looseness reduces the stiffness of the system.

How to confirm: Mechanical looseness is often characterized by multiple rotational speed harmonics (2×, 3× RPM and higher) and a 'chaos' in the spectrum if the clearance is exceeded. The vibrations can also be unstable. Structural looseness (soft foot) can be identified by variations in alignment when foundation bolts are loosened or tightened, and by the increase in 1× and 2× RPM components when a soft foot is present. An impact test on the housing can also help locate loose components.

Damage if left unresolved: Looseness leads to unstable machine behavior, excessive movement of components, and accelerated wear of all involved parts, including bearings, shafts, couplings, and seals. It can also lead to secondary problems such as misalignment and imbalance due to changing geometry. Ultimately, this results in an increased risk of malfunction and unsafe situations.

7.6 Gear problems

Why it happens: Gear problems include wear (pitting, spalling), tooth breakage, improper backlash, eccentricity, and gear misalignment. These defects cause repetitive forces on the teeth with each mesh cycle.

How to fix: Gear problems are characterized by spikes on the gear mesh frequency (GMF = number of teeth × RPM of the gear) and its harmonics. Sidebands around the GMF peak, at the rotational speed of the gear in question, indicate modulation by specific teeth or eccentricity. Visual inspection of the teeth after disassembly is often definitive.

Damage if left unresolved: Continued operation with gear problems will lead to further wear and eventual breakage of teeth, damage to bearings that support the gear shafts, and possible spread of metal particles throughout the gear case, which can damage other components. This can result in complete gearbox failure and costly repairs.

7.7 Electrical Problems (Motors)

Why it happens: Electrical problems in motors that cause vibration include broken rotor bars, shorted windings, air gap eccentricity (static or dynamic), and power supply problems (phase imbalance). These defects create magnetic forces that induce mechanical vibrations.

How to fix: Electrical problems often appear as spikes at the line frequency (e.g. 50 Hz or 100 Hz for 50 Hz mains) and sidebands around 2× RPM or 2× line frequency, depending on the specific fault. Motor Current Signature Analysis (MCSA) is a powerful technique for diagnosing these problems by analyzing the motor current. Measurements of voltage on the shaft with a multimeter can reveal bearing erosion due to electrical current.

Damage if left unresolved: Unresolved electrical problems can lead to motor overheating, increased energy consumption, reduced power, and motor bearing failure due to electrical erosion. Broken rotor bars can unbalance the engine and cause additional mechanical vibrations, ultimately resulting in engine failure.

7.8 Process Problems (Cavitation, Turbulence)

Why it happens: Especially in pumps and fans, process-related problems such as cavitation (formation and collapse of vapor bubbles in the liquid), turbulence (irregular flow) or overvoltage (surge) can lead to excessive and often broadband vibrations. These are not related to the mechanical condition of the machine, but to the interaction with the medium.

How to confirm: Process problems often manifest as broadband 'noise' in the frequency spectrum, with few distinct discrete peaks, or as low-frequency, irregular pulses. Check process parameters such as inlet and outlet pressure, flow, temperature and liquid level. Listen to the pump for characteristic sounds of cavitation (gravel-like noise). Use a pressure sensor to monitor pressure variations.

Damage if left unresolved: Cavitation can cause severe erosion damage to the pump's impeller, housing and piping, leading to reduced efficiency and eventual structural failure. Turbulence and overvoltage can also lead to increased vibration and stress on machine components, resulting in increased wear.

7.9 Oil Whip / Oil Swirl (Sleeve Bearings)

Why it happens: Oil lash and oil swirl are instability phenomena that occur in plain bearings. Oil whirl (oil whirl) is a self-state in which the axis in the oil film rotates at a frequency of approximately 0.43× to 0.48× the rotational speed. Oil whip (oil whip) is a more serious phenomenon that occurs when the shaft speed is twice as high as a natural frequency of the system, and the oil turbulence forces the shaft to vibrate at that natural frequency, usually around 0.48× to 0.5× RPM.

How to fix: These problems are identified by distinct sub-synchronous peaks in the frequency spectrum, specifically around 43-48% of 1× RPM (oil swirl) or 48-50% of 1× RPM (oil whip). Checking oil pressure, viscosity, bearing clearance and bearing temperature is crucial for the diagnosis. Variation in operating speed can help distinguish between resonance and oil whip.

Damage if left unresolved: Oil whip and oil swirl can lead to extremely high vibration levels and cause rapid wear of the plain bearings, overheating and ultimately catastrophic failure of the bearing and shaft. This often requires a complete overhaul of the machine with replacement of bearings and possible shaft repair.

8. Step-by-step troubleshooting procedures

Once the root cause has been identified, correction follows. Always perform a LOTO before starting any repair work.

8.1 Solution Imbalance: Dynamic balancing

1. Security: Apply LOTO.
2. Preparation: Thoroughly clean the component to be balanced. Remove loose parts or dirt accumulations.
3. Balance: Use a portable field balance module of the vibration analyzer.
4. Test run: Place a test weight on the rotor. Run the machine at operating speed (if safe and possible) and record the vibration amplitude and phase angle.
5. Calculation: The balance module calculates the required correction weights and their positioning.
6. Correction: Add or remove weights at the calculated positions (e.g. using welding or drilling techniques, or with mounting weights).
7. Check: Perform a check spin and repeat if necessary until vibration is within acceptable limits (typically < 1.0 mm/s RMS at 1× RPM).
8. Verification: Collect a final vibration spectrum and save it for reference.

8.2 Solution Alignment error: Precise alignment

1. Security: Apply LOTO.
2. Preparation: Clean the machine feet and foundation. Check for soft foot and correct if necessary (section 8.5). Ensure clean, rust-free shims.
3. Initial measurement: Use a laser alignment system to measure the current alignment condition (radial and axial).
4. Calculation: The alignment system calculates the necessary corrections (vertical shimming and horizontal movement).
5. Vertical correction: Place the calculated shims under the machine feet. Provide the correct number of shims to create a rigid connection (maximum 4 shims, no more than 3.5 mm).
6. Horizontal correction: Move the machine horizontally using adjustable bolts or a hydraulic jack.
7. Check: Measure again with the laser alignment system. Repeat the steps until the alignment is within the specified tolerances (e.g. < 0.02 mm parallel and < 0.02 mm/100 mm axial).
8. Verification: Tighten all foundation bolts to the correct torque (NEN-EN 1515-4). Collect a final vibration spectrum.

8.3 Solution to Bearing Defects: Bearing Replacement

1. Security: Apply LOTO. WARNING! Plain bearings can be very hot. Rolling elements may have sharp edges.
2. Disassembly: Carefully disassemble the machine to gain access to the defective bearing. Use appropriate tools (bearing pullers, induction heaters).
3. Inspection: Inspect the shaft, bearing housings and seals for damage. Clean everything thoroughly.
4. New bearing: Heat the new bearing to 80-100°C (induction heater or oil bath) before mounting on the shaft, to make it slide easily. Never force hitting.
5. Installation: Carefully install the bearing on the shaft and in the housing. Ensure proper fit and axial positioning.
6. Lubrication: Fill the bearing housing with the correct amount and type of lubricant (ISO 3448). Avoid overfilling.
7. Finish: Reassemble the machine, check all bolts for correct torque.
8. Verification: After assembly, manually rotate the machine to check for resistance. Perform a short test run and collect a vibration spectrum. Monitor temperature.

8.4 Solution Resonance: Structural adjustment

1. Security: Apply LOTO.
2. Identification: Reconfirm the natural frequency via impact test.
3. Measures:
1. Increase stiffness: Add reinforcements (e.g. ribs, braces) to the foundation, housing or machine feet. Provide stiffer shims.
2. Adjust mass: Add or remove mass in strategic places to decrease or increase the natural frequency.
3. Add damping: Use vibration dampers or elastic elements between machine and foundation (if suitable).
4. Adjust operating speed: If possible, change the operating speed to move it away from the critical frequency. This is often a last resort.
4. Check: Repeat the impact test to check the new natural frequency.
5. Verification: Perform a run-up/coast-down test and a final vibration measurement.

8.5 Solution Looseness: Tighten and correct Soft Foot

1. Security: Apply LOTO.
2. Inspection: Check all foundation bolts, machine housing bolts, and rotating component bolts for proper torque.
3. Soft Foot Correction:
   1. Place a dial indicator on one machine foot and loosen the bolt.
   2. Measure the vertical displacement of the foot. If this is > 0.05 mm, this is a case of soft foot.
   3. Place one or more shims of the measured thickness under the foot.
   4. Tighten the bolts to the correct torque and check again for soft foot on all feet.
4. Mechanical Looseness: Identify the source of mechanical looseness (e.g., worn spline joint, excessive bearing clearance) and replace or repair.
5. Verification: Take a vibration measurement and compare with the previous results.

9. Preventive Measures

Prevention is crucial to extend the life of machines and minimize unexpected downtime.

10. Spare Parts & Components

The availability of the right spare parts is essential for quick and efficient repairs. UNITEC-D GmbH offers a wide range of industrial components.

For a complete overview of available parts and specifications, visit our e-catalog: www.unitecd.com/e-catalog/

11. References

• NEN-EN-ISO 10816-1: Mechanical vibrations - Assessment of machine vibrations by measurements on non-rotating parts - Part 1: General guidelines.
• NEN-EN-ISO 1940-1: Vibrations - Balancing of rotating bodies - Part 1: Determination of permissible imbalance.
• NEN-EN-ISO 15243: Rolling bearings - Damage and defects - Terminology, characteristics and causes.
• ISO 13373-2: Condition monitoring and diagnosis of machines – Vibration condition monitoring – Part 2: Processing, analysis and presentation of vibration data.
• OEM manuals for specific machine equipment.
• Related UNITEC-D maintenance guides (available on our website).