Background
Crankshaft and main bearing design is governed by the IACS Unified Requirement M53, supplemented by the rules of each classification society and by the standards of engine designers including MAN Energy Solutions, Wartsila, Caterpillar, MTU, and Mitsui. The design must accommodate the firing pressures of the working cycle, the inertia of the reciprocating components, the torsional vibration spectrum of the shafting installation, and the manufacturing and operational tolerances that arise in service. The bearing selection in turn must accommodate the journal load, surface speed, oil film thickness, and the operating temperature regime.
This article describes the principal aspects of marine engine crankshaft design and main bearing engineering: crankshaft materials and forging, IACS UR M53 calculation, bearing types and metallurgy, common failure modes, monitoring, the hydrodynamic theory underlying journal bearing operation, oil clearances, deflection measurement procedures, and shaft alignment practice that integrates the engine into the propulsion shafting system.
Crankshaft Design
A marine crankshaft consists of a series of crank throws separated by main bearing journals, with the crank pin at each throw offset from the main journal axis by the radius equal to half the engine stroke. Each crank throw comprises two webs joined by a crank pin. At one end of the shaft is the flywheel and the coupling to the propeller shafting; at the other end are the gear drives for camshaft, lubricating oil pumps, and other auxiliary drives.
For slow-speed two-stroke engines, crankshafts are built up from individual forgings: the throws are forged separately, machined, and then shrunk-fit onto the journals. The shrink fit transmits torque by friction. The advantage is manufacturability, since a single forging for a 12-cylinder slow-speed crankshaft would be impossibly large. For medium-speed four-stroke engines, the crankshaft is normally a single solid forging.
The crankshaft material is typically a low-alloy steel with controlled chromium, nickel, and molybdenum content, with carbon levels selected to allow surface hardening at the journals and pin areas. The forging is heat-treated to develop the required core strength, and the journals and pins are induction-hardened or nitrided to provide the wear-resistant running surface against the bearing white metal.
Counterweights are bolted to or integral with the webs and are sized to balance the rotating moment of the crank pins and connecting rod big-ends, plus a fraction of the reciprocating mass. The remaining unbalanced reciprocating force gives rise to free moments at engine orders that drive the longitudinal vibration response of the ship, addressed by mounting and by tuned mass dampers.
IACS UR M53: Crankshaft Calculation
IACS Unified Requirement M53 prescribes the calculation method for crankshafts of marine diesel engines. The method establishes minimum dimensions of journal and pin diameters and the web thickness based on the engine’s working pressures, the firing order, and the geometric parameters of the running gear.
The calculation proceeds in stages. First, the cyclic gas force at firing is computed from the maximum cylinder pressure, the bore, and the relevant crank angle relations. This gives the radial and tangential components of force at the crank pin. Second, the inertia force from the reciprocating mass is added, with both gas and inertia components combining to produce the net pin loading.
Third, the bending stress in the web (the most critical stress) is computed at two reference points. The first is the bending in the web at the fillet between web and crank pin, and the second is the bending at the fillet between web and main journal. Stress concentration factors at these fillets are taken from the M53 charts, which are functions of the geometric ratios of the fillet radius, web thickness, and journal diameter.
Fourth, the torsional stress is computed from the firing forces propagating along the shaft to the next throw and so on to the output flange.
Fifth, a fatigue analysis combines the bending and torsional stress amplitudes against the material’s fatigue strength reduced for the actual surface treatment, fillet radius, and dimensional effects. The result is an acceptability criterion expressed as a safety factor that must exceed the M53 minimum.
The torsional vibration calculation, performed under IACS UR M68, complements M53 by establishing barred speed ranges and acceptable stress amplitudes across the operating range.
Main Bearing Types
The main bearings of a marine engine support the crankshaft journals. Three principal bearing constructions are used:
White metal (Babbitt) bearings consist of a steel backing shell coated with a thin layer (typically 0.5 to 2 millimetres) of tin-based or lead-based white metal alloy. The white metal provides the soft, conformable, and embeddable running surface against the harder steel journal. Tin-based white metal (typical composition 87% Sn, 7% Sb, 6% Cu) has been the marine standard for over a century and remains the dominant choice for slow-speed and medium-speed engines.
Tri-metal bearings add a thin overlay (10 to 30 micrometres) of lead-tin or lead-tin-copper alloy on top of an intermediate copper-lead or aluminium-tin layer. The overlay provides excellent break-in and embedding behaviour for the first hundreds of operating hours; the intermediate layer provides higher load capacity than pure white metal once the overlay has worn. Tri-metal bearings are common in higher-speed medium-speed engines.
Polymer-coated bearings apply a polymer (PTFE or polyetheretherketone-based) overlay on a metal substrate. They offer dry-running capability, high load capacity, and good resistance to contamination. They are increasingly applied in modern medium-speed engines and in auxiliary engines.
The bearing shells are thin-walled (typical wall thickness 4 to 10 mm depending on bearing diameter) and are held in place by interference fit with the bearing housing, supplemented by a tang or dowel preventing rotation. The bearing back provides the heat conduction path to the housing.
Bearing Failure Modes
Several distinct failure modes affect main bearings:
Wiping is plastic deformation of the white metal layer, normally triggered by metal-to-metal contact between journal and bearing. Localised heating melts or softens the white metal, which is then smeared in the direction of journal rotation. Wiping is usually the consequence of inadequate oil supply, contaminated oil, or excessive load.
Fatigue is the formation of cracks in the bearing layer, typically perpendicular to the running direction, caused by cyclic loading at amplitudes exceeding the material’s fatigue strength. Fatigue cracks propagate through the white metal and may detach flakes that further damage the bearing surface. Fatigue is associated with prolonged operation at firing pressures higher than the bearing’s design rating.
Scoring is the formation of axial grooves in the bearing surface, caused by hard particles trapped between journal and bearing. The particles may be debris from upstream wear (other bearings, gears, the crosshead bearing), water-related corrosion products, or contamination introduced during overhaul. Scoring of the journal surface is a more serious finding because the journal cannot be replaced without major overhaul.
Cavitation erosion is the pitting damage caused by the collapse of vapour bubbles in the oil film during pressure fluctuations. It typically appears as a series of small pits on the bearing surface in the unloaded zone. Cavitation is associated with high-speed engines and with bearings operating near the limits of their hydrodynamic capacity.
Corrosion of the white metal occurs when the oil acidity rises, particularly through fuel-derived contamination or from moisture. Modern crankcase oils with appropriate additive packages (see marine lubricating oil systems) prevent corrosion in normal service.
Bearing Temperature Monitoring
Bearing temperature is the single most reliable real-time indicator of bearing condition. Modern marine engines fit thermocouples or RTDs (resistance temperature detectors) in each main bearing housing, with continuous logging through the engine room automation system.
The normal operating temperature is engine-specific but typically lies in the 70 to 90 degree Celsius range for slow-speed engines and slightly higher for medium-speed. A trending rise of 5 to 10 degrees above normal is an alarm condition; a rise of 20 degrees is a slow-down or shut-down condition under most engine builders’ service letters.
The bearing temperature responds to changes in oil supply, oil viscosity, journal load, and clearance. A rapid rise typically indicates wiping in progress and demands immediate slow-down. A gradual rise across multiple bearings indicates oil quality deterioration or oil cooler fouling.
Lubricating oil sampling, with metallurgical analysis tracking tin, antimony, and copper levels, complements the temperature data and detects bearing wear before temperature rise becomes apparent.
Oil Film Theory
The hydrodynamic journal bearing operates on the wedge principle. As the journal rotates within the bearing, the eccentricity between journal centre and bearing centre creates a converging wedge of oil. The oil’s viscosity resists shearing, and the result is a pressure distribution that supports the journal load.
The dimensionless Sommerfeld number S = (μN/P)(R/c)^2, where μ is dynamic viscosity, N is rotational speed, P is load per unit projected area, R is journal radius, and c is radial clearance, characterises the operating regime. High Sommerfeld numbers correspond to thick films and lightly loaded conditions; low Sommerfeld numbers correspond to heavily loaded conditions approaching boundary lubrication.
Minimum oil film thickness in normal operation is typically several micrometres for marine main bearings. Below this threshold, asperity contact between journal and bearing surface increases, leading to wear and ultimately wiping.
The Reynolds equation governs the pressure distribution in the film and is the foundation of bearing design software. Modern engine bearing design uses computational fluid dynamics to evaluate the film under combined steady and dynamic loading representative of the actual cyclic gas force on the journal.
Oil Clearances
Bearing clearance is the diametrical gap between journal and bearing in the unloaded condition. Typical clearances for marine bearings are 0.1% to 0.15% of journal diameter, so a 600 mm journal would carry roughly 0.6 to 0.9 mm of clearance.
Excessive clearance produces a thin oil film at the loaded zone, knocking sounds at start-up, and increased oil consumption (oil leaving via the bearing ends in greater volume). Insufficient clearance produces high temperature, risk of seizure on starting, and inadequate volume of cool oil flowing through the bearing.
Clearance is measured at survey by feeler gauge or by lifting the journal with a hydraulic jack and measuring the rise with a dial indicator. The measurements are taken at the top of the bearing (where running clearance is greatest) and at the side. Side clearance check verifies the housing alignment.
The ratio of length to diameter (L/D) of the bearing affects performance. Marine main bearings typically have L/D of 0.5 to 0.7, balancing load capacity (favouring longer bearings) against end leakage of oil (favouring shorter bearings).
Deflection Measurement
Crankshaft deflection is the alignment metric for the main bearings of a slow-speed marine engine. With the engine cold and stopped, a dial gauge is mounted between the webs at each crank throw, and the engine is rotated through a full revolution. The change in distance between the webs at top dead centre, bottom dead centre, exhaust side, and fuel side is recorded. The differences indicate misalignment of the bearing housings.
The acceptable deflection range is engine-specific and typically published in the engine builder’s manual. MAN B&W deflection limits depend on engine type and bore; for a typical slow-speed engine the limit is in the order of 0.05 mm per metre of crank radius.
Deflection readings outside the limit indicate bearing wear, hull deflection in way of the engine seating (particularly in lightship versus loaded condition), or hot-spot growth following thermal cycling. Corrective action ranges from re-bedding the engine on its chocks to replacing main bearings to address vertical alignment.
Deflection should be measured periodically: at delivery, after major repairs, after grounding events, and at intervals defined in the planned maintenance schedule or under continuous machinery survey.
Alignment
Crankshaft alignment is integrated with the broader propulsion shafting alignment. The main engine output flange must be aligned with the intermediate shaft, gearbox, or thrust shaft so that no excessive bending moment is transmitted from one component to the next.
The alignment procedure follows the calculation produced by the engine and shafting designer, which specifies the desired bearing reactions at every bearing in the system from the aft stern tube bearing forward to the main engine. The reactions are measured by jacking each bearing in turn, recording the load against jack lift, and converting to the bearing reaction.
The alignment is checked at delivery in three conditions: ship in dock with all weights aboard, ship afloat at lightship, and ship afloat at loaded condition. The shafting deflection differs between these conditions and the engine alignment must accommodate the range.
In service, alignment is rechecked after grounding, propeller damage, hull damage, or any incident that might have changed the shafting geometry. The check is also performed at major surveys where the propeller and tail shaft are drawn for examination.
Related Wiki Articles
- Marine Diesel Engine
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- Marine Engine Cylinder Liners and Pistons
- Marine Engine Camshaft and Valve Train
- Marine Engine Combustion Analysis
- Marine Engine Performance Monitoring
- Marine Lubricating Oil Systems
- Marine Propulsion Shafting and Stern Tube
- Marine Reduction Gears
- Continuous Survey of Hull and Machinery
- Marine Engine Room Automation and Monitoring
References
- IACS Unified Requirement M53, Calculation of Crankshafts for Internal Combustion Engines
- IACS Unified Requirement M68, Dimensions of Propulsion Shafts and Crankshafts
- IACS Unified Requirement M51, Type Testing of Diesel Engines
- ISO 3046-1, Reciprocating Internal Combustion Engines - Performance
- ISO 7146-1 and 7146-2, Plain Bearings - Appearance and Characterisation of Damage
- ISO 4378-1, Plain Bearings - Terms, Definitions, Classification and Symbols
- MAN Energy Solutions Service Letter SL2018-666, Main Bearing Maintenance
- MAN Energy Solutions Operation Manual for MC and ME engines
- Wartsila Service Bulletins on bearing inspection and maintenance
- DNV Rules for Classification of Ships, Pt.4 Ch.3, Rotating Machinery - Drivers
- Lloyd’s Register Rules and Regulations for the Classification of Ships, Part 5 Chapter 2
- ABS Rules for Building and Classing Marine Vessels, Part 4 Chapter 2