Causes of vibration and noise in main gearing

Causes:

• Gear tooth profile errors/misalignment of the toothed wheels causing impacting (hammering) of teeth.
• Backlash (excessive clearances between meshing teeth) causing rattle/impact noise.
• Gear tooth pitch errors and eccentricity of the gear wheels producing a once-per-revolution torsional and radial forcing.
• Resonance of the gear train/natural frequency coinciding with the meshing frequency (tooth frequency) or its harmonics.
• Wear, pitting, cracked or distorted teeth, foreign particles and poor lubrication raising impacts.
• Misalignment of the main engine, coupling and shafting, causing the shaft to bend and run out; non-uniform loading.
• Shaft whirling (critical speeds), misaligned bearings and thrust causing radial forces.
• Torsional vibration of the shaft system / misfiring of engine cylinders exciting the gear.
• Thermal distortion, looseness of the gear casing, and resonance of the gear case radiating airborne noise.

Detrimental effects on machinery

• Accelerated wear, fretting and pitting of teeth and bearings; cracking and failure of teeth, keys and gear rims.
• Fatigue failure of gear teeth, shafts and couplings due to cyclic stressing.
• Loosening of bolts, casing fastenings and connecting flanges.
• Overheating of bearings and lubricating oil due to the extra dynamic loads and friction.
• Reduced gear efficiency and premature overhaul.
• Secondary damage if a tooth breaks (consequential damage to the whole train).

Detrimental effects on personnel

• High noise level causing hearing loss/damage and masking warning alarms; annoyance and fatigue.
• Whole-body or localised vibration causing discomfort, fatigue, reduced concentration, and health complaints; possible effect on hands (vibration white finger) and on balance.
• Reduced communication in the machinery space.

How to detect the source

• Vibration analysis using accelerometers/velocity pick-ups mounted on the gear casing, bearing housings and the gear train; spectral/FFT analysis to relate the dominant frequencies (gear meshing frequency = teeth x rpm, shaft/engine order and multiples) to components.
• Torsional vibration measurement on the shaft (strain gauge/optical encoder) to find excitation frequencies.
• Sound level/frequency analysis (noise surveys) to locate noise origin and its path.
• Observing operating conditions (load, rpm), comparing at no-load/load, and correlating with running data.
• Routine visual/inspection, temperature and metallic (particle) monitoring of lubricating oil, and vibration trending to detect deterioration.

How it may be reduced

• Correct alignment of engine, gearbox and shafting; accurate gear tooth geometry/tooth crowning and profile.
• Correct backlash and meshing clearances; using torque/teeth with lower excitation.
• Damping: fitting torsional dampers; elastic couplings on the flywheel; flexible mounting of the gear casing to isolate structure-borne noise; vibration isolators and sound-deadening casing with acoustic lagging.
• Lower gear meshing frequency may be achieved by higher tooth count or design; balancing gears.
• Reduce excitations at source (engine torsional irregularities/tuning) and stay away from resonant critical speeds where possible.
• Good lubrication (correct oil grade, pressure, temperature) and maintaining tooth/gear condition.
• Routine preventive maintenance, alignment checks and vibration monitoring to detect changes early, with repairs such as replacing worn gears/thrust washers or fitting resilient mounts.