Module 11 • Marine Engineering Oral Preparation

Torsional Vibration Study Recap

A Chief Engineer-level infographic covering resonance, critical speeds, barred speed ranges, dampers, detuners, Class TVA evidence, failure symptoms and operational fault response.

BSR TVA Dampers Fatigue

Main risk

Fatigue

Avoid

BSR dwell

Evidence

TVA

Exam level

Chief

Why this module matters

Torsional vibration is not normal visible shaking. It is alternating angular twist in the crankshaft and shaft line. The danger is that repeated cyclic torsional stress can accumulate fatigue damage until a crankshaft, coupling, gear train or shaft component fails.

Explain resonance, critical speed, nodes and antinodes clearly.
State why barred speed ranges must be passed quickly and never used for steady operation.
Compare viscous dampers, spring/oil dampers, flexible couplings and detuners.
Know when Class needs TVA review after modifications.
Diagnose gear chatter, fretting, fatigue cracking and BSR passing problems.

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CoreWhat it is ModesNodes & antinodes BSRCritical speeds DampersAbsorb energy TVAClass & trials SymptomsFailures ChangesPTO / propeller CE responseFault actions OralModel answer

Fundamental definition

What Torsional Vibration Is

Angular oscillation

The shaft twists and untwists about its own axis. It is not axial movement or lateral bending.

Exciting torque

Diesel engines deliver torque in pulses due to firing pressure and reciprocating inertia forces.

Fatigue danger

The alternating stress may be invisible but can damage crankshafts, couplings, gears and shafting.

Simplified mass-elastic shaft model

Engine
masses

Damper
free end

Coupling
/ gears

Shaft
line

Propeller
inertia

TVA models this as rotating inertias connected by torsional stiffness. The wrong combination of individually acceptable components can still create a dangerous system.

Oral exam focus

Define it as angular twist due to fluctuating torque, then immediately link it to resonance, fatigue and the need for barred speed ranges or damping.

Mode shapes

Nodes, Antinodes & Resonance

Natural frequencyThe frequency at which the shaft system naturally wants to oscillate when disturbed.

Exciting orderA repeated torque pulse from firing order, cylinder number or reciprocating mass effects.

NodeZero angular displacement, but typically high torsional stress.

AntinodeMaximum angular movement. Useful location for a damper because there is movement to absorb.

Critical point: resonance occurs when the exciting torque frequency coincides with the natural torsional frequency. Stress can rise sharply even if the visible vibration seems small.

Critical speed operation

Barred Speed Range (BSR)

What it means

A barred speed range is an rpm band around a critical speed where torsional stresses are too high for continuous operation. It is a Class-approved operating restriction.

How to operate

Pass through the barred range as quickly and safely as possible. Do not manoeuvre, slow steam or hold rpm inside the barred range.

PlanKnow BSR limits before manoeuvring.

PowerEnsure enough margin to accelerate through.

PassDo not dwell at critical rpm.

LogRecord abnormal BSR problems.

EscalateCall Class/OEM if limits cannot be met.

Exam trap

“Stay in the BSR until stable” is wrong. The purpose of the BSR is to prevent stress accumulation by avoiding continuous running at the critical speed.

Control methods

Dampers, Detuners & Flexible Couplings

Component	Main purpose	How it works	Chief Engineer trap
Viscous damper	Absorbs vibration energy.	Inertia ring lags behind casing and shears silicone fluid, converting energy into heat.	Runs warm normally; abnormal temperature or failed fluid sample is significant.
Spring/oil damper	Combines detuning and damping.	Steel springs alter stiffness; pressurised oil provides hydrodynamic damping.	Low oil pressure may disable damping and damage the unit.
Detuner	Shifts critical speed.	Changes system stiffness/elasticity so resonance moves away from operating rpm.	It is not the same as a pure damper.
Flexible coupling	Reduces transmitted torsional peaks.	Adds controlled elasticity and damping between connected machines.	Wrong replacement changes TVA assumptions.

Viscous damper checks

Inspect casing for dents, overheating and leakage.
Follow fluid sampling interval and viscosity limits.
Check replacement is approved for the engine/TVA.

Geislinger-type checks

Confirm clean pressurised lube oil supply.
Respond to low-pressure alarms.
Watch for leaks, overheating and unusual noise.

Class & evidence

Torsional Vibration Analysis (TVA)

Predicts

Natural frequencies, exciting orders, critical speeds, stress levels and required BSRs.

Verifies

Sea trials or measurements confirm the real system agrees with approved calculations.

Controls

Approved operating limits, BSR passing time, power margin and modification restrictions.

Change	Why TVA may need review	Exam answer
New propeller / re-pitch	Changes propeller inertia and load curve.	Check Class/OEM; do not assume old TVA remains valid.
PTO / shaft generator	Adds rotating inertia and electrical torque disturbances.	Require TVA review and protection logic checks.
Different damper/coupling	Changes damping, stiffness or inertia.	Must be approved against the original torsional design.
Engine derating / slow steaming	May move normal operation nearer to a critical speed.	Check approved operating envelope and BSR proximity.
Cylinder cut-out / misfire	Unbalances turning moment and changes excitation pattern.	Follow maker restrictions and Class/company guidance.

Symptoms & damage

What Torsional Vibration Looks Like in Service

Gear chatter

Alternating twist makes gear teeth slap, especially camshaft/reduction gear trains.

Fretting dust

Microscopic rubbing at flanges/couplings produces reddish-brown dust and bolt fatigue risk.

Fatigue cracks

Crankpins, webs, coupling bolts and shaft sections can crack under repeated shear stress.

Damper heat

Heat can be normal energy dissipation, but abnormal change suggests damper or loading issues.

Chief Engineer warning: torsional vibration may not be visible externally. Do not dismiss gear chatter, coupling fretting, repeated bolt loosening or BSR passing difficulty.

Operating profile changes

Propeller Load, CPP, PTO & Dual-Fuel Operation

Propeller and hull condition

Heavy-running conditions from fouling, damage or high sea state can reduce acceleration through the BSR. Light-running conditions after cleaning may improve passing performance but still need approved operation.

CPP operation

A controllable pitch propeller can keep rpm near a critical speed while changing pitch/load. Automation and procedures must prevent prolonged running in the barred range.

PTO / shaft generator

Electrical load changes add torque disturbances and rotating inertia. Retrofitting a shaft generator is a classic TVA review trigger.

Dual-fuel / gas mode

Different combustion pressure rise and misfire/knock behaviour can alter excitation. Maker limits and approved modes must be followed.

Fault response

Chief Engineer Response Guide

Finding	Likely concern	Correct response
Gear chatter at one rpm band	Critical speed / BSR issue.	Do not dwell; check BSR, damper, cylinder balance and operating records.
Fretting at flange	Micromovement and bolt fatigue.	Stop/slow if required, inspect bolts/flanges, preserve evidence and notify company/Class.
Cannot pass BSR in time	Power margin or load condition problem.	Investigate hull/propeller condition, engine performance, CPP logic and seek OEM/Class guidance.
Damper fluid out of limits	Reduced damping effectiveness.	Follow maker instruction, replace/repair if required and avoid assuming continued protection.
Cylinder cut-out required	New harmonic excitation risk.	Check maker restrictions, rpm limits and safe operating envelope before continued running.

Practical answer

Reduce risk, avoid the critical range, preserve evidence, inform Master/company, follow maker/Class limits and do not normalise abnormal vibration symptoms.

Exam-ready structure

Model MCA Oral Answer

“Explain torsional vibration.”

It is angular twisting/untwisting of the crankshaft or shaft line.
It is excited by cyclic combustion torque and reciprocating masses.
If excitation coincides with a natural frequency, resonance occurs.
Resonance creates high alternating torsional stress and fatigue risk.
Class-approved TVA identifies critical speeds and BSRs.
Dampers, detuners and flexible couplings control the response.
Watchkeepers must pass barred ranges quickly and not dwell.
Changes to mass, stiffness, propeller or PTO require TVA review.

Never say

The flywheel eliminates torsional vibration.
A barred speed range is safe for continuous operation.
A misfire only affects exhaust temperature.
A damper can be replaced with any similar-looking unit.
Torsional vibration is always visible as external shaking.

Final one-line memory aid

Torque pulses → natural frequency → resonance → barred range → damping/detuning → fatigue control.

Reference basis

Accuracy & Source Notes

DNV torsional-vibration analysis guidance: used for component interaction, engines, propulsion systems and generator sets.
DNV BSR/fatigue guidance: used for barred speed range passing time and shaft fatigue risk.
Geislinger technical material: used for steel-spring tuned dampers and hydrodynamic oil damping.
TST Module 11 quiz and flashcards: used to align the infographic with the current course module and oral-exam traps.

Use the ship-specific approved TVA, engine-maker manuals and Class conditions for real operating limits. This recap is for oral preparation and revision structure.