Guide to the Diagnosis and Resolution of Noise and Abnormal Vibrations in Gearboxes

1. Problem Description and Scope

This diagnostic guide addresses the symptoms of excessive noise and abnormal vibrations that can occur in industrial gearboxes, with particular reference to machine tools used in the Italian manufacturing sector. The presence of such anomalies indicates a degradation of internal operating conditions and, if not resolved promptly, can lead to critical failures, unexpected machine downtime, production losses and high repair costs. The severity of these symptoms is classified as critical, as it directly impacts the reliability and safety of the system.

Affected equipment includes spur, bevel, worm and planetary gear reducers, typically used in spindles, motion axes, transmission systems and conveyors.

2. Safety Precautions

ATTENTION: Before any inspection, diagnosis or maintenance on a gear reducer or on machinery connected to it, it is essential to rigorously implement the BLOCK/TAGOUT (LOTO – Lockout/Tagout) procedures in accordance with the EN ISO 14118:2018 standard. Make sure all energy sources (electrical, hydraulic, pneumatic, mechanical) are turned off and locked in a safe position.

Stored Energy Hazard: Gearboxes may contain stored energy in springs, suspended weights or static loads. Release these energies before proceeding. Hot oil can cause severe burns; let the system cool down or wear suitable PPE (thermal gloves, face protection).

PPE (Personal Protective Equipment): Always wear safety glasses, oil- and cut-resistant gloves, protective clothing and safety footwear. In noisy environments, use hearing protection compliant with Directive 2003/10/EC. For inspections with a thermal imager, consider the risk of laser beams to the eyes.

Crushing/Shearing Hazard: During visual inspection or manipulation of components, pay maximum attention to moving parts or those that may move unexpectedly. Keep hands and other objects away from gears, belts and chains.

Hazardous Substances: Used lubricating oils must be disposed of according to local environmental regulations and UNI EN guidelines ISO 14001. Avoid prolonged contact with the skin.

3. Diagnostic Tools Required

The accurate diagnosis of noise and vibrations requires the use of specific instrumentation, calibrated according to current regulations and with CE certification.

4. Initial Assessment Checklist

Before undertaking any invasive diagnostic procedure, it is essential to gather information on the operating context of the gearbox. This preliminary phase helps narrow down potential causes and guide the diagnosis.

5. Systematic Diagnosis Flow

The following flowchart guides the technician through a logical process to isolate the root cause of abnormal noise and vibration. Proceed with caution and document each step.

1. Initial Symptom: Abnormal Noise and/or Vibration
   1. Perform Initial Evaluation Checklist (see Section 4).
      1. IF obvious external problems (loose bolts, serious leaks): Fix these problems immediately and recheck.
      2. ELSE (no obvious external problems): Proceed with the instrumental analysis.
   2. Vibration Measurement and Analysis (with Vibration Analyzer)
      1. Place the accelerometers on the bearings of the gearbox, on the input and output shaft.
      2. Record acceleration (g), velocity (mm/s RMS), and displacement (µm RMS) data.
      3. Analyze the FFT spectrum.
         1. IF peaks at 1x, 2x the rotational frequency (RPM) of the shaft: Probable Cause: Imbalance or Misalignment.
         2. IF peaks at high frequencies with sidebands around gear mesh frequencies (GMF - Gear Mesh Frequency) or harmonics: Probable Cause: Gear Wear/Damage.
         3. IF peaks at specific frequencies of bearing defects (BPFI, BPFO, FTF, BSF): Probable Cause: Bearing Damage.
         4. IF overall vibration speed values exceed ISO 10816-3 thresholds (e.g. 4.5 mm/s RMS for long-term operation): Confirm Anomalous Vibration.
         5. ELSE (no clear dominant peak or values below threshold): The vibration could be transmitted from other sources or have a different origin. Proceed.
   3. Thermal Graphic Analysis (with Thermal Camera)
      1. Scan the entire surface of the gearbox, focusing on the bearings and gear casings.
      2. Record surface temperatures.
      3. IF localized hot spots (>10-15°C above ambient temperature or adjacent areas): Probable Cause: Excessive friction, lack of lubrication, overloading bearings/gears.
         1. CAUTION: Temperatures > 90°C on the gearbox housing may indicate severe oil degradation or imminent failure of internal components.
   4. Noise Analysis (with Sound Level Meter)
      1. Measure the sound level in different positions around the reducer.
      2. Compare with historical values ​​or manufacturer specifications.
      3. IF significant increase (> 5 dB) compared to baseline: Confirm acoustic anomaly. The sound level meter can help locate the main source.
   5. Lubricant Analysis (with Sample and Laboratory Kit)
      1. Take an oil sample from the designated point (if available).
      2. Send to laboratory for analysis (elemental spectroscopy, viscosity, particle counting, water).
         1. IF presence of wear metals (Fe, Cr, Ni, Cu, Sn, Pb) above the alarm limit values: Probable Cause: Wear of gears or bearings.
         2. IF high particle count (> 5 µm): Probable Cause: Contamination or wear.
         3. IF viscosity out of specification (±10%): Probable Cause: Lubricant degradation.
         4. IF presence of water (> 100 ppm): Probable Cause: Contamination, degradation, acid formation.
   6. Internal Inspection (with Endoscope, if possible)
      1. Insert the endoscope through inspection holes or vent/fill plugs.
      2. Visually examine gear tooth surfaces, bearing cages and rolling elements.
      3. IF pitting, scuffing, spalling, corrosion, broken gear teeth or obvious damage to bearings: Confirm Internal Damage.

6. Cause-Fault Matrix

This matrix maps observed symptoms to probable causes, relevant diagnostic tests, and expected results for confirmation.

7. Root Cause Analysis for Each Fault

7.1. Wear or Damage to Gears

Explanation: Gear wear manifests itself in various forms such as pitting (surface pitting), scuffing (seizing), spalling (flaking), scoring (deep scratches) or broken teeth. These conditions are usually caused by:

• Operational overload: Loads exceeding the rated capacity of the reducer.
• Lack or degradation of lubrication: Insufficient oil film to separate contact surfaces, or oil that has lost its protective properties.
• Oil contamination: Abrasive particles (metallic, dirt) entering circulation.
• Installation/alignment error: Uneven load distribution on the teeth.
• Manufacturing defects: Non-compliant material or inadequate heat treatments (less frequent).

How to Confirm: Vibration analysis will reveal significant peaks at the meshing frequency (GMF) and its harmonics, with possible sidebands. Endoscopy or direct inspection (after disassembly) will show visible signs of wear or breakage on the teeth. Oil analysis will show an increase in iron, chromium, nickel and other metals related to the gear alloy.

Damage if Not Resolved: Gear degradation progresses rapidly, increasing noise, vibration and generating excessive heat. This leads to increased contamination of the oil with metal debris, accelerating the wear of all internal components (bearings, other gears) up to catastrophic breakage of the teeth and blocking of the gearbox.

7.2. Bearing Damage or Failure

Explanation: Rolling bearings are critical components subject to fatigue, brinelling (static indentations), false brinelling (indentations due to micro-movements), corrosion and contamination. The main causes include:

• Fatigue: Exceeding the fatigue limit of the material due to prolonged load cycles.
• Lack or degradation of lubrication: Insufficient oil film, incorrect choice of lubricant or contamination.
• Incorrect installation: Excessive preload, use of inadequate force during assembly, shaft misalignment which induces abnormal loads.
• Contamination: Infiltration of dirt, moisture or metal debris that damages the raceways and rolling elements.
• Eddy electric current: In the case of electric motors, passage of current through the bearings which causes electrical pitting.

How to Confirm: Vibration analysis is the most effective tool, revealing peaks at frequencies characteristic of bearing defects (BPFI, BPFO, FTF, BSF) long before the damage is visible. Thermography can identify overheated bearings. Oil analysis will show increased iron, chromium, copper (for brass cages), and other metals related to the bearing alloy, as well as a high particle count.

Damage if Not Resolved: A damaged bearing generates increasing friction, heat and vibration. Internal clearance increases, compromising the precision of the gears and causing further wear. Final bearing failure can lead to shaft seizing or destruction of the gearbox.

7.3. Tree misalignment

Explanation: Misalignment occurs when the connected shafts (e.g. motor-gearbox, gearbox-load) are not perfectly coaxial or parallel. There are two main types:

• Parallel misalignment: The axes are parallel but not coincident.
• Angular misalignment: The axes meet but form an angle.

The causes can be:

• Incorrect installation: Inaccurate assembly.
• Foundation failure: Base not stable, excessive vibration, loose anchor bolts.
• Structural deformations: Differential thermal expansion between components.
• Support wear: Excessive play in the external support bearings.

How to Confirm: Vibration analysis will show dominant peaks at 1x and 2x the rotational frequency (RPM) of the affected shafts. The use of a laser alignment system will provide a precise measurement of angular and parallel misalignment, with typical alarm thresholds at 0.05 mm/m for angular and 0.05 mm for parallel.

Damage if not Resolved: Misalignment imposes additional loads and abnormal bending forces on bearings and couplings, drastically reducing their useful life. It increases power consumption, operating temperature and overall system vibration, leading to premature failure of bearings, gears and seals.

7.4. Imbalance

Explanation: Imbalance occurs when the center of mass of a rotating component does not coincide with its axis of rotation. It can be caused by:

• Manufacturing defects: Components not originally balanced.
• Dirt/debris buildup: On fans, couplings, or other rotating items.
• Damage: Breakage of fan blades, deformations.
• Missing Components: Balance mass removed or lost.

How to Confirm: Vibration analysis will detect a dominant peak at 1x the rotational frequency of the unbalanced component. Dynamic balancing in situ or on a balancing machine is the confirmation and correction method.

Damage if not Resolved: Imbalance generates centrifugal force causing high-amplitude vibrations at 1x RPM. This induces structural fatigue, reduces bearing and coupling life, and can damage other adjacent equipment. Typical alarm values ​​for RMS vibration velocity due to imbalance can vary, but values ​​above 2.8 mm/s are often indicative of a significant problem.

7.5. Lubrication problems

Explanation: Lubrication is vital to the health of the gearbox. Common problems include:

• Insufficient level: There is not enough oil to create a protective film or to dissipate heat.
• Incorrect lubricant type: Incorrect viscosity or additives for the application, resulting in inadequate protection.
• Oil degradation: Oxidation, contamination (water, particles), loss of additives.
• Clogging of oil passages: Clogged channels or filters preventing oil from reaching lubrication points.

How to Confirm: Visual inspection of the oil level and condition (color, turbidity, foam). Analysis of the oil in the laboratory will provide precise data on viscosity, acidity index, water content and presence of additives. Thermography will detect abnormal overheating due to increased friction.

Damage if Not Resolved: Inadequate lubrication is a major cause of premature bearing and gear failure. It leads to a drastic increase in friction, heat and wear, accelerating all other degradation mechanisms and leading to irreversible failures.

8. Step-by-Step Resolution Procedures

The following procedures should be performed only after the root cause has been identified through diagnosis. Always remember the safety precautions in Section 2.

8.1. Resolving Gear Wear or Damage

1. BLOCK/TAGOUT (LOTO).
2. Drain the lubricating oil: Collect the used oil for proper disposal.
3. Reduction gear disassembly: Follow the instructions in the OEM manual for progressive disassembly of the crankcases and shafts.
4. Detailed inspection: Examine all gears, identifying any damaged ones.
5. Gear replacement: Replace the damaged gear (or set of gears) with an original UNITEC spare part, checking the part number and specifications (e.g. module, number of teeth, material, precision class UNI 787).
6. Check backlash: After assembly, measure the backlash between the gear teeth with a dial gauge. Adjust the spacers or eccentric bushings (if applicable) to obtain the clearance specified by the OEM (typically 0.05 – 0.20 mm depending on the module and application).
7. Reassembly: Reassemble the reducer, applying the specified tightening torques on the bolts (e.g. M10 class 8.8: 50 Nm; M16 class 8.8: 200 Nm).
8. Filling lubricant: Fill with the specific lubricant indicated by the OEM (e.g. ISO VG 220, compliant DIN 51517-3) to the correct level.
9. Functional test and verification: Perform a test without load and then under progressive load. Monitor noise, vibration and temperature with diagnostic tools.

8.2. Resolving Bearing Damage or Failure

1. BLOCK/TAGOUT (LOTO).
2. Drain the oil (if necessary for access).
3. Disassembly: Access the damaged bearings according to the OEM manual. Use appropriate extractors to avoid further damage to trees.
4. Bearing replacement: Replace all suspect or damaged bearings with original UNITEC-D spare parts (e.g. SKF Explorer, FAG FAGOR, NSK bearings, with C3 or C4 specification if required for high speed/temperature applications).
5. Correct assembly: Fit the new bearings using specific tools (induction heaters for shrink-fit bearings, cold thrust bushes on the appropriate ring) to avoid brinelling. Check the correct fit with the shaft and the housing.
6. Preload/clearance check: Make sure the preload or axial/radial clearance conforms to OEM specifications by measuring with a dial gauge or feeler gauge (typically radial clearance 0.02 – 0.05 mm for standard ball bearings).
7. Reassembly, lubrication and functional test: As for the gears.

8.3. Resolving Tree Misalignment

1. BLOCK/TAGOUT (LOTO).
2. Foundation inspection: Check the flatness and integrity of the mounting base and the absence of loose bolts (tightening to the specified torque).
3. Laser Alignment: Use a laser alignment system to align shafts. Correct the parallel and angular misalignment by acting on the feet of the motor/gearbox with calibrated shims and/or micrometers, until reaching values ​​lower than 0.03 mm for parallel misalignment and 0.03 mm/m for angular misalignment.
4. Soft foot check: Eliminate soft foot (soft foot) by adjusting the base supports, ensuring that all feet rest firmly with a maximum variation of 0.02 mm.
5. Final tightening and verification: Tighten the anchor bolts and recheck alignment.
6. Functional test: As described above.

8.4. Imbalance Resolution

1. BLOCK/TAGOUT (LOTO).
2. Component identification: Through vibration analysis, identify the unbalanced component.
3. Cleaning/Inspection: Thoroughly clean the component (e.g. fan) of accumulated dirt or debris. Inspect for physical damage.
4. Balancing:
   • In-situ balancing: If possible, use the vibration analyzer to perform a dynamic balancing in the field, adding or removing masses at the calculated points until the vibration is reduced to 1x RPM below 1.0 mm/s.
   • Balancing in the workshop: If the component can be dismantled, send it to a specialized center for dynamic balancing.
5. Reassembly and functional test.

8.5. Lubrication Problem Solving

1. BLOCK/TAGOUT (LOTO).
2. Drain the oil completely: Dispose of the used oil.
3. Flushing: If the oil was heavily contaminated or degraded, perform an internal flush of the gearbox with a flushing oil or new short-term lubricant.
4. Filter Replacement: Replace all oil filters (if equipped) and breather filter elements.
5. Inspection and replacement of gaskets/oil seals: Check the integrity of all gaskets and oil seals. Replace any that show leaks or signs of hardening to prevent future contamination.
6. Filling lubricant: Fill with the specific OEM approved lubricant (e.g. PAO ISO VG 320 synthetic oil, CLPF class DIN 51502) to the correct level.
7. Check temperatures: After restart, monitor the operating temperature to ensure it is within normal limits (typically < 80°C on the casing).

9. Preventive Measures

The implementation of a predictive and preventive maintenance program significantly reduces the probability of the occurrence of abnormal noises and vibrations.

10. Spare parts and components

The availability of original spare parts that comply with technical specifications is essential for an effective and long-lasting repair. UNITEC-D offers a wide range of gearbox components, all CE certified and compliant with quality standards ISO 9001.

To purchase original spare parts and to consult our complete catalogue, visit our E-Catalog UNITEC-D.

11. References

• UNI EN ISO 10816-3:2009: Evaluation of machine vibrations through measurements on non-rotating parts. Part 3: Industrial machines with rated power exceeding 15 kW and rated speeds between 120 r/min and 15 000 r/min when measured in situ.
• UNI EN ISO 14118:2018: Machinery safety – Prevention of unexpected starting.
• DIN 51517-3: Lubricants – Lubricating oils – Gear lubricants – Part 3: Gear oils with EP (Extreme Pressure) additives.
• API 610: Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries (contains guidelines for alignment and vibration).
• Gearbox manufacturer specific OEM Maintenance Manuals.