The crankshaft is an integral part of the engine block. This engine part is attached to the pistons with crankpins and connecting rods. Crankshafts in marine engines induce propelling action in the propulsion system.
In this guide to crankshafts, we will discuss about:
What is a marine engine crankshaft
Types of crankshafts
Common crankshaft problems
Crankshaft maintenance and repairs
What is a Marine Engine Crankshaft?
The crankshaft is a component in marine diesel engines to regulate the propelling action in the watercraft. It may be attached to the flywheel on one end to reduce vibration and regulate power delivery. The rotating crankshafts are coupled with the propeller using an electric motor or alternator in diesel engines.
Crankshafts in marine diesel engines are mainly built of alloy steel. The tensile strength may vary from one model to another, ranging around 800 to 900 n/mm². Its role is to develop rotational motion from the reciprocal movement of the moving pistons. This energy generated from the rotational motion is transmitted to the propulsion system to regulate the propelling operation of the vessel in water.
Types of Marine Diesel Engine Crankshafts
There are two types of crankshafts in marine engines operating on diesel:
Completely or Fully Built
The cheeks or parts of a fully built crankshaft are separately produced and shrink-fitted to form a shaft. These heavy items comprise additional metal parts for bearing the shrink forces. The prime components fabricated separately by forging include – webs, journal pins, and crank pins. These parts are then specifically assembled employing shrink-fitting. Dowel pins are eliminated to prevent stresses despite offering stability to the journals and crank pins.
Partly or semi-built
Partly built crankshafts are designed for gigantic marine engines. An example of such installation is found in cross-head 2-stroke engines. The chief integrands in this type of shaft include crank-throw, pins, and webs. These parts are made by forging and later assembled by shrink fitting.
Common Problems in Marine Diesel Engine Crankshafts
Crankshafts are exposed to severe stresses, often exceeding 8.6 tonnes (19,000 pounds) for each cylinder. The continuous operation imposes crankshafts to cyclic loading, extreme forces, high pressure, and fluctuating temperatures.
One of the causes of crankshaft breaking and failure is deflection. Deflection is the term used to describe conditions such as bending, misalignment between the engine and the driven unit, etc. This fault occurs due to acting forces and vibration during the crankshaft operation.
However, the seamless operation in crankshafts may also be affected by weakened bearings, absence of oils, high oil temperature during operation, improper clearance between the bearings and the journals, etc. Adequate lubrication on the borings and journals prevents wear and tear during high-pressure movement.
Crankshaft Maintenance and Repairs
It is a strenuous challenge for marine engineers to keep the crankshaft in exact position. Even minute misalignment of dispositioning can damage the entire shaft or the big end bearings.
Moreover, this happens when the engine is running and the individual shaft parts are exposed to extreme pressure and stresses.
The best practices to keep the crankshaft in a precise position include:
Conducting Crankshaft Measurement
Marine professionals use micrometers to measure the roundness of the shaft and pins. Differences in the measurement depict the early signs of crankshaft dispositioning. The crankshaft deflection measurement may be necessary if there is abnormal wear and tear in the engine bearings.
Maintenance
Basically, the periodic maintenance includes inspection of the entire installation, lube oil treatment, etc. Checking the quality of the lubricant is vital to ensure no residues or pollutants in the oil. These particulates can contribute to the wear and tear of the bearings.
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