Crankshaft is the main element of the motor mechanism and it sums up the rotational motion of all cylinders and transmits it to the consumers, which in the case of marine engine systems can be a propeller or a generator. For this reason, from a mechanical point of view, it has the higher load and stress factor of mobile parts of the engine.
The defects of the crankshafts are quite rare but very dangerous because in case of a failure, it will take the engine out of service and the repairing process is very expensive and time consuming. The most frequent defects are:
- cracks which can lead to crankshaft breakage with disastrous consequences for the engine. The cracks appears usually on the web and pin connection due to the phenomenon of fatigue that is favored by the existence of tension concentrators and the faulty bearing’s alignment.
- crankshaft deformation by bending or torsion caused by various accidents like: connecting rod studs failure, high Pmax on some of the cylinders, engine overload etc.
- crankshaft breakage
- bearing’s failure
Due to bearings number their alignment, coaxiallity and load uniformity are very important and must be continuously kept. Coaxiallity deviation is mainly caused by bearing’s weariness, assembling errors or vessel load. This deviation, if exceeds allowable limits, produces crankshaft deformation and leads to additional stress which sum up to already existing stress during engine operation. Bearings wear and alignment are interconnected and a wrong alignment caused by excessive bearing’s wear will cause crankshaft bending and will affect its lubrication. Thus causing and increasing, most of the time, uneven bearing’s wear which will negatively affect the shaft alignment. Excessive clearance in one main bearing can shift the majority of the load to another main bearing and flexing the crankshaft under load may result in crank journal fatigue and ultimately breakage.
Therefore the change in distance between crank webs, measured during one rotation of the crankshaft is known as crankshaft deflection and is an indirect indication of the crankshaft loading condition.
There is a good maintenance need of periodically measuring the crankshaft deflections to ensure that the shaft alignment remains within allowed limits. Normally the crankshaft deflection is to be checked into the following situations:
- Under normal running circumstances, once or twice a year.
- In case the ship grounded or touched bottom
- After replacing main bearing shell and again after approximatively 100 hrs.
- Before and after engine overhaul in case of auxiliary engines.
- If there is any signs of damaged main bearings
Before commencing the measurements the following conditions must be met:
- The vessel must be afloat and even keel, as much as possible into the water.
- The crankshaft must rest on all main bearings
- Indicator valves must be open
The measurements can be affected by:
- the engine temperature (so it might be stated if the engine is cold or at working temperature)
- vessel load condition
- ambient air and water temperature
The measurement is done by a dial indicator placed at a predefined location between crank webs. The crankshaft is then rotated in one direction and readings are taken at the defined angular locations. Normally, deflection should be measured at four points on the crank, namely the top, bottom, and two sides. However, in practice, the bottom reading is omitted due to the possibility of fouling by the connecting rod, and instead readings are taken on either side of the bottom position, yielding a total of five readings from each crank web at the positions specified.
To determine the crank deflection, the crankshaft must always be rotated in such a way that, regardless of the engine’s normal rotation direction, the flywheel 1 and pinion 2 of the turning gear rotate in the direction indicated by the arrow.
With the running gear in place, the crank to be measured has to be turned towards (before or after) B.D.C. until the dial gauge can be fitted next to the connecting rod at the position indicated and then pretension the dial gauge slightly and set it to zero. After the dial gauge is set turn the crankshaft with the turning gear, reading the dial gauge in the crank positions which are indicated and noting down the values at B.D.C. – FUEL SIDE – T.D.C. – EXHAUST SIDE – B.D.C. The last value at B.D.C. is for checking and if everything has been carried out correctly it should be nearly zero.
The data are summarized in the form depicted in the example table below, which shows actual engine readings. You’ll notice that the first row’s numbers correspond to the unit or cylinder number, whereas the first column indicates the position of the readings. The final row displays the difference between the top and bottom measurements, which indicates the shaft’s vertical misalignment. Vertical misalignment values must be compared to the manufacturer’s declared maximum allowed values.
After measurements have been taken, these data numbers need to be interpreted in order to see and asses the condition of the crankshaft and bearings. A full explanation of how to interpret the same can be found into the following link: https://www.iims.org.uk/engine-crankshaft-deflection-measurement/
Where values are measured which lie above the maximum permissible limits, the cause has to be found and the necessary remedial measures taken (defective main bearing, engine stay altered due to hull deformation, loose holding-down bolts, defective propeller shaft bearings, etc.).
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