Marine Engine Crankshaft & Bearings: Guide for MEO Class 4
Introduction
The crankshaft is one of the most highly loaded components of a marine diesel engine. It converts the reciprocating motion of the piston into rotary motion and transmits the developed power to the engine's driven system.
Its reliable operation depends on the condition of the main bearings, crankpins, crank webs, bedplate and associated structural components. In large two-stroke marine engines, the A-frame and tie rods also play an important role in maintaining the strength and alignment of the engine structure.
1. Marine Engine Crankshaft: Function and Construction
The crankshaft converts the reciprocating movement of the piston into rotary movement. The force produced during combustion is transmitted through the connecting rod to the crankpin, producing torque that rotates the crankshaft.
Main functions
Converts reciprocating motion into rotary motion.
Transmits engine power to the driven system.
Carries forces resulting from combustion and the inertia of moving components.
Supports the crank throws through which connecting rods transmit force.
Works with the main bearings and bedplate to maintain the required alignment.
Main parts of a crankshaft
Part | Function |
Main journal | Runs in the main bearing and supports the crankshaft. |
Crankpin | Connects the crankshaft to the connecting rod through the bottom-end bearing. |
Crank web | Connects the crankpin and main journal to form the crank throw. |
Fillet radius | Provides a smooth transition between sections and helps reduce stress concentration. |
Oil passages | Supply lubricating oil to designated bearing surfaces, where provided by the design. |
Shaft ends | Connect to associated engine components or shafting, depending on the arrangement. |
2. Types of Marine Engine Crankshafts
Crankshafts may be classified according to their construction.
A. Semi-built crankshaft
In a typical semi-built crankshaft, the crank webs are forged integrally with their associated journal sections, while the crankpins are manufactured separately and fitted into the webs using an interference fit.
The crankpin-to-web joint must withstand the forces and stresses experienced during engine operation.
Important points:
Individual components can be manufactured and handled separately.
The crankpin-to-web joint requires accurate manufacture and assembly.
The interference fit must be maintained within the manufacturer's specifications.
The condition of the crankpin, webs and fitted joints is important during inspection.
B. Fully built-up crankshaft
A fully built-up crankshaft is assembled from separate components, including crank webs, journals and crankpins, according to the design.
The components are joined using engineered fits and assembly procedures. Correct positioning, alignment and joint integrity are essential.
C. Solid or one-piece crankshaft
A solid crankshaft is manufactured as one integral piece rather than assembled from separate crankshaft components. Its use depends on engine size, manufacturing requirements and design.
3. Crankshaft Stresses and Inspection
The crankshaft is subjected to repeated loads throughout the operating cycle. These include bending and torsional stresses, as well as stresses associated with the inertia of reciprocating and rotating components.
Stress concentration can occur at changes in section, especially around crankpin and journal fillets. Correct design, surface condition and inspection are therefore important for avoiding fatigue damage.
Common inspection concerns
Cracks around fillets, journals or crank webs.
Scoring and damage to journal surfaces.
Abnormal wear or changes in bearing clearances.
Changes in crankshaft deflection readings.
Evidence of misalignment or uneven bearing loading.
Crankshaft deflection readings should be taken and assessed using the engine manufacturer's procedure. Permitted values and the interpretation of readings depend on the particular engine.
4. Main Bearings
Main bearings support the crankshaft main journals and carry the loads transmitted through the rotating assembly. They are fitted into the main-bearing supports associated with the engine bedplate.
Functions
Support the crankshaft.
Maintain the correct running clearance.
Carry the loads acting on the crankshaft.
Allow rotation with minimum friction under the intended operating conditions.
Maintain the correct crankshaft alignment.
Construction of main bearings
A typical large marine-engine main bearing consists of upper and lower bearing shells supported by a bearing housing or pocket. The shells have a backing material and a bearing lining selected for the engine's operating conditions.
The exact bearing materials and layer arrangement vary by design. Bearing constructions may use steel backing shells and bearing linings such as white metal or other specified materials.
The bearing cap or keep secures the upper portion of the bearing assembly. Correct seating and tightening are essential for maintaining the required geometry and clearance.
Lubrication of main bearings
Lubricating oil is supplied through the engine's designed oil passages. During normal operation, relative movement between the journal and bearing creates a hydrodynamic oil film that separates the metal surfaces and carries the load.
The oil film depends on correct clearance, oil supply, viscosity, operating conditions, surface condition and alignment.
A breakdown of the oil film can result in overheating, wiping, scoring and serious bearing damage.
5. Main Bearing Clearance: Measurement and Importance
Main bearing clearance is the space between the crankshaft journal and the bearing surface. It must remain within the limits specified by the engine manufacturer.
Incorrect clearance can affect oil-film formation, oil flow, bearing temperature and load distribution.
Why is clearance measured?
To assess bearing wear.
To identify changes from previous measurements.
To help investigate abnormal crankshaft deflection or alignment.
To determine whether maintenance or bearing replacement is required.
Methods of measuring bearing clearance
1. Bridge gauge method
A bridge gauge is placed over the crankshaft journal with the bearing keep and upper shell removed as specified by the procedure. The gap between the gauge and journal is measured using a feeler gauge or depth-measuring arrangement, depending on the method.
The reading is compared with the original or previously recorded measurement to assess changes in wear-down.
2. Special feeler gauge method
Some engine designs allow clearance to be checked using a specified feeler gauge between the upper bearing shell and journal. Care must be taken not to force the blade into the gap or scratch the surfaces.
3. Dial-type depth gauge
Some engine designs provide access for a dial-type depth gauge through a designated opening in the bearing keep. The reading is obtained according to the manufacturer's procedure.
4. Lead-wire method
Lead wire is a traditional method used when appropriate. Wire is placed at specified locations, the bearing is assembled and tightened according to the procedure, and the compressed wire thickness is measured after dismantling.
The wire must be positioned and handled carefully so that it does not damage the bearing or obstruct proper assembly.
Important: These methods are not interchangeable in every engine. Use the specified method, tools, clearance limits and tightening procedure for the particular engine. Numerical examples from study notes must not be treated as universal clearance values.
6. Crosshead Bearings
Crosshead bearings are used in crosshead-type marine diesel engines. They allow relative movement between the crosshead pin and connecting rod while transmitting forces from the piston assembly towards the crankshaft.
Functions
Transfer forces through the crosshead mechanism.
Accommodate relative movement at the crosshead pin.
Maintain the required running clearance and lubricating oil film.
Reduce friction and wear under cyclic loading.
Common problems
Insufficient lubricating oil supply.
Excessive clearance.
Overheating or wiping.
Misalignment and uneven loading.
Damage to the bearing lining.
A damaged bearing or displaced bearing material can interfere with lubrication. Any serious signs of bearing distress require investigation under the vessel's procedures and the engine manufacturer's instructions.
7. Bottom-End Bearings
The bottom-end bearing, also called the big-end bearing, connects the connecting rod to the crankshaft crankpin.
It permits relative movement between the connecting rod and crankpin while transmitting forces to the crankshaft.
Functions
Transfer connecting rod forces to the crankpin.
Maintain the designed clearance.
Support the lubricating oil film.
Reduce friction and wear.
The bearing shells, fasteners and contact surfaces must be correctly fitted. Incorrect assembly, insufficient lubrication or excessive clearance can contribute to serious damage.
8. Comparison of Main, Crosshead and Bottom-End Bearings
Feature | Main bearing | Crosshead bearing | Bottom-end bearing |
Location | Main journal and bearing support | Crosshead pin and connecting rod | Crankpin and connecting rod |
Main function | Supports the crankshaft | Transfers force through the crosshead mechanism | Transfers connecting rod forces to the crankshaft |
Key concern | Alignment, clearance and oil film | Lubrication, clearance and loading | Lubrication, clearance and correct assembly |
Typical arrangement | Crosshead and trunk-piston engines | Crosshead-type engines | Crosshead and trunk-piston engines |
9. Common Main Bearing Defects and Failures
Recognising bearing defects and understanding their possible causes helps engineers investigate failures systematically.
A. Abrasive wear
Abrasive wear occurs when hard particles pass between the bearing and journal surfaces and remove material by cutting or scratching.
Possible causes:
Contaminated lubricating oil.
Ineffective filtration or purification.
Rust, scale or particles entering the oil system.
Prevention: Maintain oil cleanliness, inspect filtration equipment and investigate contamination.
B. Corrosive wear
Corrosive wear occurs when chemical attack damages the bearing material. Acidic lubricating oil, oxidation, water ingress and contamination may contribute.
Prevention: Monitor oil condition, investigate water contamination and follow the specified oil-management procedures.
C. Adhesive wear, scoring and wiping
Adhesive damage can occur when the lubricating film breaks down and the surfaces make damaging contact. Material may be torn or smeared across the bearing surface.
Wiping is a serious form of bearing-lining damage and can be associated with overheating, poor lubrication, excessive loading or misalignment.
D. Fatigue cracking
Repeated loading can cause cracks to develop in the bearing lining. Local stress concentration, excessive temperature, poor bonding between lining and backing, or uneven loading may contribute.
Cracks can grow and lead to pieces of bearing material becoming detached. A cracked bearing must be assessed and dealt with according to the manufacturer's instructions.
E. Cavitation erosion
Cavitation damage can occur when pressure changes cause bubbles in the oil to form and collapse near the bearing surface. The resulting local forces can cause pitting.
Possible preventive measures include investigating oil flow, maintaining the specified oil viscosity and addressing design or operating conditions that promote cavitation. Any change must be approved for the particular engine.
F. Misalignment and uneven loading
Misalignment can concentrate the load on particular areas of the bearing. It may be associated with a distorted bedplate, problems with adjacent bearings, structural movement or uneven cylinder loading.
The result may include local overheating, wiping, fatigue cracking and abnormal wear.
G. Fretting
Fretting is surface damage caused by small relative movements between loaded contact surfaces. It can occur at bearing support or joint surfaces when the required clamping force, fit or structural rigidity is inadequate.
Bearing defect summary
Defect | Possible cause | Key preventive action |
Abrasive wear | Hard particles in the oil | Maintain oil cleanliness and filtration |
Corrosion | Acidic oil or water contamination | Monitor oil condition and investigate contamination |
Wiping or scoring | Oil-film breakdown, roughness or misalignment | Maintain lubrication, clearance and alignment |
Fatigue cracks | Repeated stress, high temperature or poor bonding | Inspect the bearing and investigate loading |
Cavitation | Local pressure changes and bubble collapse | Investigate oil-flow and bearing-design conditions |
Fretting | Relative movement at loaded contact surfaces | Check fit, clamping and structural condition |
10. Bedplate: Construction and Function
The bedplate forms the foundation of the engine and supports the main-bearing structure. It must be sufficiently rigid to carry engine loads while maintaining the alignment of the crankshaft and associated components.
Main functions
Supports the main-bearing assemblies.
Provides a foundation for the engine structure.
Maintains crankshaft and engine alignment.
Transfers engine loads to the ship's supporting structure.
Provides the required structural rigidity.
Construction
Large propulsion-engine bedplates may be fabricated from mild-steel plates and steel castings welded together. Smaller engines may use cast-iron bedplates.
The choice of construction depends on engine design, manufacturing requirements and the loads the structure must carry.
Cast iron: Has useful vibration-damping characteristics.
Fabricated mild steel: Can provide a strong structure suited to welded construction.
Hybrid construction: May combine fabricated mild-steel sections with cast-steel transverse members.
Longitudinal girders and transverse members
Longitudinal girders extend along the engine and provide structural strength. Transverse members or cross girders connect the structure and support the main-bearing locations.
The bearing pockets and welded junctions are important inspection areas in fabricated structures. Weld failures around bearing pockets and at junctions can result in costly repairs.
11. Bedplate Chocking and Holding-Down Bolts
The bedplate must be secured to the ship's foundation while maintaining the correct engine and shafting alignment.
What are chocks?
Chocks are fitted between the bedplate and the engine foundation to provide the required support and compensate for unevenness in the foundation surface.
Different arrangements include metal chocks, resin chocks and resilient chocks.
Metal chocks: Provide support through accurately fitted contact surfaces.
Resin chocks: Use a synthetic resin to provide contact between the bedplate and foundation. Their installation requires suitable preparation, curing and approval.
Resilient chocks: Flexible mounts used in certain medium-speed or smaller-engine installations to reduce vibration transmitted to the supporting structure.
Resilient chocks are not a universal arrangement for large two-stroke propulsion engines. The appropriate type depends on engine design and installation requirements.
Functions of chocks
Compensate for unevenness in the foundation.
Support the bedplate and maintain alignment.
Help distribute the engine's load.
Work with the holding-down arrangement to reduce unwanted movement.
What are holding-down bolts?
Holding-down bolts secure the bedplate to the foundation through the specified chock and foundation arrangement.
Their functions include:
Keeping the bedplate in position.
Providing the required clamping force.
Helping resist forces transmitted between the engine and ship structure.
Supporting the required engine and shafting alignment.
Long, necked holding-down bolts may be used in some designs. The reduced-diameter plain section provides an elastic length, while the design aims to avoid abrupt stress concentration near the threaded section.
Problems caused by loose holding-down bolts
Loose bolts can allow relative movement between the bedplate, chocks and foundation. This may cause fretting, changes in vibration, movement of the engine and deterioration of alignment.
Retightening bolts without investigating existing fretting or distortion may not restore the original alignment.
Problems caused by overtightening
Excessive tightening can distort the bedplate or alter the alignment of the crankshaft and bearings. In severe cases, bolt damage or failure may occur.
Maintenance point: Inspect holding-down bolts and chocks at the intervals specified by the vessel's maintenance plan and engine manufacturer. Use the approved checking and tightening procedure rather than relying on an assumed torque value.
12. A-Frame and Tie Rods
In a large crosshead-type engine, the A-frame is located above the bedplate and supports the upper engine structure.
Functions of the A-frame
Supports the cylinder block or entablature.
Provides mounting points for the crosshead guides.
Helps carry side-thrust loads through the guide arrangement.
Contributes to the stiffness of the engine structure.
Supports equipment and attachments specified by the engine design.
A-frames may be fabricated from steel plates welded together, with full-penetration welds in critical locations where specified by the design.
What are tie rods?
Tie rods are long structural fasteners that clamp the bedplate, engine frame and cylinder structure together in many large marine engines.
During combustion, high gas pressure produces forces that act on the piston mechanism and the upper structure. These forces create cyclic stresses in the engine structure.
Tie rods hold the major sections together under compression, helping limit the tensile stresses caused by firing loads. This allows the engine to maintain structural strength without making every part excessively heavy.
Why are tie rods positioned near the crankshaft centre line?
Positioning tie rods close to the crankshaft centre line helps reduce bending moments in the transverse girders.
Tie-rod maintenance
Check for looseness or signs of failure.
Follow the manufacturer's specified tightening or hydraulic tensioning procedure.
Avoid overtightening.
Investigate abnormal vibration or structural movement.
Inspect for damage in accordance with the maintenance schedule.
13. Main Bearing Keep and Jack Bolts
The main-bearing keep or cap secures the upper part of the main-bearing assembly.
Some engine designs use jack bolts or hydraulically tensioned fasteners to secure the bearing keep. The exact arrangement varies by manufacturer.
Placing jack bolts above the keep can allow tie rods to be positioned closer to the crankshaft centre line in some designs, reducing bending stresses in the transverse girder.
Correct tightening and bearing-cap seating are essential. After bearing work, the specified clearance and assembly checks must be completed before the engine returns to service.
14. Main Bearing Inspection and Maintenance
During inspection, engineers should assess both the bearing and the crankshaft journal.
Points to check
Record bearing clearance and compare it with previous readings.
Examine the bearing metal for wiping, scoring, cracking, pitting or squeezing.
Check the journal surface for scratches, roughness or abnormal wear.
Look for signs of overheating or uneven loading.
Check for evidence of oil contamination.
Investigate abnormal crankshaft deflection readings.
Confirm that the bearing cap, fasteners and shell seating are correct.
If a new bearing shell is installed, record the relevant measurements in the engine's adjustment or maintenance records as required.
Thin-wall bearings used in many modern engines may not be designed for adjustment or reconditioning. If a bearing is damaged or reaches its specified wear limit, follow the manufacturer's replacement instructions.
15. MEO Class 4 Practice Questions
Q1. What is the function of a crankshaft?
It converts the reciprocating motion of the piston into rotary motion and transmits power to the driven system.
Q2. What is the difference between a main journal and a crankpin?
The main journal is supported by a main bearing. The crankpin connects to the connecting rod through the bottom-end bearing.
Q3. Explain the construction of a semi-built crankshaft.
In a typical semi-built construction, the crank webs are forged integrally with associated journal sections, while the crankpins are separately manufactured and fitted into the webs using an interference fit.
Q4. Why is main bearing clearance measured?
To assess bearing wear, monitor changes over time and help investigate abnormal bearing or crankshaft conditions.
Q5. Name methods used to measure main bearing clearance.
Depending on the engine design, methods include a bridge gauge, a specified feeler gauge, a dial-type depth gauge and the traditional lead-wire method.
Q6. What causes abrasive wear in a main bearing?
Hard particles in the lubricating oil can scratch and remove material from the bearing surface.
Q7. What is bearing wiping?
Wiping is damage in which bearing lining becomes smeared or displaced, commonly associated with oil-film breakdown, overheating or abnormal loading.
Q8. What is the function of a bedplate?
It provides the engine foundation, supports the main-bearing structure and helps maintain the required alignment.
Q9. What is the purpose of chocks?
Chocks provide support between the bedplate and foundation, compensate for surface unevenness and help maintain alignment.
Q10. What can happen if holding-down bolts become loose?
Relative movement and fretting can occur between the bedplate, chocks and foundation. This can affect engine alignment and vibration.
Q11. Why are tie rods fitted in large marine diesel engines?
They clamp the bedplate, frame and cylinder structure together, helping the engine withstand cyclic firing forces while limiting tensile stresses.
Q12. Why should holding-down bolts not be overtightened?
Overtightening can distort the bedplate, change crankshaft and bearing alignment, and damage the bolts or supporting structure.
Conclusion
The crankshaft, bearings and bedplate form an interconnected system that must withstand heavy loads while maintaining correct alignment and lubrication. Main bearings support the crankshaft, connecting-rod bearings transfer forces, and the bedplate, chocks, holding-down bolts and tie rods maintain the required structural integrity.
For marine engineers, understanding these components is essential for MEO Class 4 examinations and for recognising the early signs of mechanical problems onboard.
Understand the construction. Monitor the clearances. Protect the oil film. Maintain alignment.
