Diagnosis Guide: Abnormal Noise and Excessive Vibrations in Industrial Gearboxes

1. Problem Description and Scope

This diagnostic guide addresses the symptoms of abnormal noise and excessive vibration in industrial gearboxes, critical elements for power transmission in machine tools. These phenomena indicate potential degradation of internal components, such as gears and bearings, or alignment and lubrication problems, which, if not identified and addressed promptly, can lead to catastrophic failures, costly downtime and operational safety risks.

The methodology is applicable to a wide range of gearboxes, including parallel axis, bevel, worm and planetary gearboxes, typically used in machining centers, lathes, milling machines and other high-precision machines. The severity of the problem is classified as follows:

• Critical: Sudden and significant increase in noise/vibration, rapid rise in temperature, emission of smoke or burning odors. Requires immediate machine shutdown.
• Major: Constant noise and vibration above acceptable limits, slight variation in operating temperature, presence of visible metal particles in the oil. Requires short-term intervention planning.
• Minor: Intermittent noise or slight increase in vibrations, detectable only with specific instrumentation, without immediate impacts on performance. Requires monitoring and diagnostic planning.

The reference for vibration limits is the UNI EN ISO 10816-3 standard, which classifies the severity of mechanical vibrations in non-rotating industrial machines.

2. Safety Precautions

CRITICAL WARNING: Before carrying out any inspection or maintenance on the gear reducer, it is MANDATORY to implement the LOCKING AND LABELING (LOTO - Lockout/Tagout) procedure in compliance with UNI regulations EN 1037 and Legislative Decree 81/08. Make sure all energy sources (electrical, hydraulic, pneumatic, mechanical) are deactivated and blocked. Check the absence of residual or stored energy (e.g. pre-loaded springs, pressure in the hydraulic/pneumatic circuits, suspended masses).

MANDATORY PPE: Always wear protective gloves (EN 388), safety glasses (EN 166), hearing protection (EN 352) and safety shoes (EN ISO 20345).

RISKS: Be careful of hot surfaces (over 50°C), pressurized fluids, sharp edges and moving components in case of partial tests under tension.

3. Necessary Diagnostic Tools

The following table lists the essential tools for an accurate diagnosis:

4. Initial Assessment Checklist

Before proceeding with the in-depth diagnosis, it is essential to collect preliminary information and observations:

5. Systematic Diagnostic Flow

The diagnostic process must follow a logical path to isolate the root cause efficiently:

1. Preliminary and Security Check:
   1. Carry out the initial checklist (Section 4).
   2. Implement LOTO procedures (Section 2) if the intervention requires access to internal parts or exposure to hazards.
2. Acoustic Inspection (with machine running, if safe):
   1. Use a mechanical stethoscope to locate the area of ​​origin of the noise.
   2. IF high frequency noise/squealing: POSSIBLE bearing or gear problem. PROCEED to 3b and 3c.
   3. IF low frequency noise/rolling: POSSIBLE misalignment, coupling problems, or foundations. PROCEED to 3a and 3e.
   4. IF intermittent noise/knocking: POSSIBLE excessive play, damaged gears or imbalance. PROCEED to 3d.
3. Vibrational and Thermal Inspection (with machine running, if safe):
   1. Vibration Measurement: Position the accelerometer on strategic points of the gearbox crankcase (near bearings, gears) according to UNI EN ISO 10816. Record RMS values of speed (mm/s) and acceleration (g).
   2. Vibration Spectrum Analysis: Use the analyzer to identify the predominant frequencies.
      • SE peaks at the meshing frequencies (GMF - Gear Mesh Frequencies) or their harmonics: INDICATES gear problems (wear, pitting, backlash).
      • IF peaks at bearing characteristic frequencies (BPFI, BPFO, BSF, FTF): INDICATES damage to bearings (inner race, outer race, balls/rollers, cage).
      • IF peaks at 1X or 2X the shaft rotation frequency: INDICATES imbalance or misalignment.
   3. Thermal imaging: Scan the entire gearbox with the thermal imaging camera. Look for anomalous hot spots (differences > 10°C from adjacent areas or > 20°C above ambient temperature).
4. Oil Analysis (with machine off and LOTO):
   1. Take an oil sample according to standard procedure ISO 14597.
   2. Send for laboratory analysis:
   3. SE viscosity out of specification or low TBN (for engine oils): INDICATES oil degradation.
   4. IF presence of water (> 0.1%): INDICATES contamination.
   5. SE high code ISO 4406 (e.g. 21/18/15): INDICATES particle contamination.
   6. SE high concentration of Fe, Cr, Cu, Al (ICP): INDICATES wear of gears (Fe, Cr), bearings (Fe, Cr), bushings/cages (Cu, Al).
5. Measuring Backlash and Games (with machine off and LOTO):
   1. Remove inspection covers.
   2. Use the dial indicator to measure the backlash between the gear teeth (backlash) and the axial/radial clearances of the shafts. Compare with OEM specifications.
   3. IF values ​​out of specification: INDICATES gear wear, bearing seat wear, or incorrect assembly.
6. Internal Inspection of Gears and Bearings (with machine off and LOTO, partial/total disassembly):
   1. With the aid of an endoscope (if possible) or by dismantling the cover, visually inspect the gear teeth: pitting, scuffing, shouldering, breakage.
   2. Inspect the bearing running surfaces: pitting, corrosion, false brinell.
   3. IF visible defects: CONFIRM gear wear or bearing damage.

6. Fault-Cause Matrix

The following table summarizes common symptoms, probable causes (ordered by frequency of occurrence), diagnostic tests to be performed and expected results to confirm the cause.

7. Root Cause Analysis for Each Fault

7.1 Wear and Damage to Gears

Explanation: Gear wear manifests itself in various forms such as pitting (fatigue pitting), scuffing (surface seizing), abrasion or broken teeth. It is often caused by inadequate lubrication, continuous overloading, misalignment, or oil contamination. pitting is surface fatigue failure due to cyclic stress, while scuffing is adhesive wear caused by breakdown of the lubricating film and metal-to-metal contact.

Confirmation: The vibration spectrum analysis will show distinct peaks at the meshing frequencies (GMF) and their harmonics. Oil analysis will reveal a high concentration of ferrous (Fe) and sometimes chromium (Cr) particles if gears are bonded, with morphology and size indicative of gear wear. Direct visual inspection (if possible via endoscope or after disassembly) will reveal surface damage on the teeth.

Damage if not resolved: Progressive wear leads to increases in noise, vibrations and temperature. In the most serious cases, the breakage of one or more teeth can cause the gearbox to completely block, with extensive damage to other components and potential cascading damage to the entire machine.

7.2 Damage and Wear of Bearings

Explanation: Rolling bearings are subject to fatigue wear (pitting), abrasion, corrosion, and false brinelling. The main causes include insufficient or contaminated lubrication, incorrect assembly (excessive or insufficient preload), overloading, shaft imbalance or misalignment, and eddy electrical currents. pitting on the track or rolling elements is the most common sign of fatigue.

Confirmation: Vibration spectrum analysis is the most effective method, detecting peaks at specific frequencies associated with inner race (BPFI), outer race (BPFO), rolling element (BSF) and cage (FTF) defects. Thermography will identify a localized rise in temperature. Oil analysis will show an increase in ferrous particles, chromium, manganese, and sometimes copper (if the cage is bronze), indicating bearing degradation.

Damage if left unresolved: Bearing damage progresses from small pitting to extensive surface cracking, which increases friction, noise, vibration and temperature. Bearing failure can cause shaft binding or severe misalignment, damaging gears and seals and leading to gearbox collapse.

7.3 Misalignment of Shafts and Couplings

Explanation: Misalignment occurs when the axis of rotation of the gearbox shaft is not coaxial or parallel to the drive shaft or driven machine shaft. It can be angular, parallel or a combination. Causes include assembly errors, foundation failure, uncompensated differential thermal expansion, and stresses caused by piping connections or machine structure.

Confirmation: Vibration spectrum analysis will typically show high peaks at 1X, 2X and sometimes 3X the rotational frequency of the shaft, often with greater amplitude in the radial direction. The use of a laser alignment system is the most accurate method to measure and quantify misalignment.

Damage if left unresolved: Misalignment generates additional and cyclic radial and axial loads on bearings and gears, accelerating wear and dramatically shortening their operating life. It can also cause overheating of bearings and premature failure of elastic joints.

7.4 Inadequate or Degraded Lubrication

Explanation: Insufficient lubrication or degraded oil are among the most common causes of mechanical failure. Inadequacy can result from a low level of lubricant, from using an oil with the wrong viscosity or specifications for the operating conditions, or from degradation of the oil itself due to oxidation, contamination (water, dust, other substances), or mixing with incompatible oils. An insufficient or compromised lubricating film is unable to separate metal surfaces, increasing friction and wear.

Confirmation: Oil analysis is the primary diagnostic tool. It will detect out-of-spec viscosity, a high acid number (TAN) or low total base number (TBN) (for engine oils), a high water content (e.g. > 100 ppm), a high cleanliness code ISO 4406, and the presence of oxidation products (FTIR). Low level can be confirmed visually or via dipstick/sensor.

Damage if left unresolved: Metal-to-metal friction accelerates the wear of gears and bearings, generating excessive heat that further degrades the oil. This vicious cycle leads to rapid deterioration of components, overheating, and ultimately blockage or failure of the gearbox.

8. Step-by-Step Resolution Procedures

For each identified root cause, follow the detailed resolution steps:

8.1 Resolution: Wear and Damage to Gears

1. SAFETY WARNING: Implement full LOTO on the gearbox and connected machine.
2. Draining the lubricating oil. Inspect the drained oil for large metal fragments.
3. Disassembly of the gearbox casing and internal components to access the damaged gears. Document the location and orientation of all components.
4. Thorough inspection of all gears, not just those that are visibly damaged. Use a non-destructive liquid penetrant (NDT) to detect micro-cracks invisible to the naked eye.
5. Replacement of damaged gears with original or equivalent spare parts with the same specifications (module, number of teeth, material, heat treatment, UNI ISO 1328 precision class).
6. Thorough cleaning of all internal components and the crankcase to remove metal residues and contaminants.
7. Verification of gear fit (tooth contact pattern) using impression varnish. The contact impression must be centered and cover at least 60-70% of the tooth surface.
8. Measurement of backlash (backlash between the teeth) with dial indicator. The value must be within OEM tolerances (typically 0.10 - 0.25 mm depending on module and size). Adjust if necessary.
9. Reassembly of the reducer, tightening all the bolts to the torques prescribed by the manufacturer, using a certified torque wrench.
10. Filling with new lubricating oil, of the type and viscosity grade (ISO VG) specified by the manufacturer (e.g. EP gear oil, ISO VG 220), up to the correct level.
11. Idle test for 30 minutes, monitoring noise, vibration and temperature.
12. Test under gradual load, continuously monitoring the parameters. The temperature must not exceed 70°C, and the vibration values must fall within the 'A' or 'B' zone of the UNI EN ISO 10816-3.

8.2 Resolution: Bearing Damage and Wear

1. SAFETY WARNING: Implement full LOTO.
2. Disassembly of the gearbox and extraction of damaged bearings. Use appropriate extractors to avoid damage to shafts and seats.
3. Inspection of the bearing seats on the shafts and in the crankcase for wear, corrosion or chafing. Dimensional tolerances must be within ±0.01 mm.
4. Thorough cleaning of all components and bearing seats.
5. Installation of new bearings. Use induction heaters or ovens to heat the internal bearings before mounting on the shaft, to facilitate mating. Never exceed 120°C. For external bearings, use presses or mounting sleeves, applying force only to the appropriate ring.
6. Check the internal clearance (if it is not a pre-loaded bearing) with a dial gauge.
7. Reassembly of the gearbox following OEM procedures, ensuring correct alignment and preload of the bearings. Torque tightening.
8. Filling with new and correct lubricant.
9. Test at no load and under load, with vibrational (spectrum) and thermographic monitoring to confirm correct operation and the absence of new anomalies. The vibration values should be in the 'A' zone of the UNI EN ISO 10816-3.

8.3 Resolution: Shaft Misalignment

1. SAFETY WARNING: Implement full LOTO.
2. Disconnect the coupling between the gearbox and the motor/load.
3. Use a precision laser alignment system (e.g. PRUFTECHNIK, SKF) to measure angular and parallel misalignment between shafts. Record the initial values.
4. Adjust the position of the gearbox (or motor) using calibrated shims under the fixing legs. Make gradual adjustments and repeat laser measurements until the misalignment values ​​are within tolerances specified by the joint or OEM (typically < 0.05mm for parallel and < 0.01mm/100mm for angular).
5. Tighten the gearbox and/or motor anchor bolts to the prescribed torque, checking that the alignment does not alter during final tightening.
6. Reconnect the coupling.
7. Test the machine at no load and under load, monitoring the vibration spectrum to verify the reduction of peaks at 1X, 2X and 3X rotation frequency. The bearing temperature should stabilize.

8.4 Resolution: Inadequate or Degraded Lubrication

1. SAFETY WARNING: Implement full LOTO.
2. Complete draining of old oil.
3. Visually inspect the inside of the gearbox (if accessible) for sludge or deposits. If present, consider an internal flush with a compatible flushing oil.
4. Filling with new lubricating oil, of the exact type and degree of viscosity specified by the manufacturer (e.g. synthetic PAO or mineral, EP additive), up to the correct level indicated by the dipstick or level window.
5. Check the operation of the cooling system (if present) and the cleanliness of filters and fans.
6. Test the reducer, monitoring the operating temperature. A correct lubricating film will reduce friction and the temperature should stabilize at normal values ​​(typically 50-70°C).
7. Schedule oil analyzes at regular intervals to monitor oil condition and component wear.

9. Preventive Measures

The adoption of a preventative and predictive maintenance strategy is essential to minimize failures:

10. Spare parts and components

When replacing critical components, it is essential to use quality spare parts that meet the original specifications. UNITEC-D offers a wide range of gearbox components.

For purchasing and advice on spare parts, visit our e-catalog: www.unitecd.com/e-catalog/

11. References

• UNI EN ISO 10816-3: Mechanical vibrations - Evaluation of machine vibrations by measurements on non-rotating parts - Part 3: Industrial machines with rated power exceeding 15 kW and rated speed between 120 rpm and 15,000 rpm when measured in situ.
• ISO 4406: Hydraulic Fluid Cleanliness Code - Method of coding the level of solid particle contamination.
• ISO 14597: Guidelines for sampling lubricating oils.
• UNI EN 1037: Machinery safety - Prevention of unexpected start-up.
• Legislative Decree 81/08: Consolidated Law on health and safety at work.
• Technical Manuals of Gearbox Manufacturers (e.g. Flender, Bonfiglioli, Rexroth).