What is Crankshaft Deflection and slippage? | Marine Engineering

Crankshaft Deflection and slippage? | Marine Engineering

The crankshaft of the ship's diesel engine is a marine engine part which transmits the power of a combustion cylinder to the rotary impeller (or alternator when a generator). Therefore, it must remain as straight as possible during operation, otherwise it can lead to serious engine damage.

As a marine engineer working on a board ship, you must know key reasons which can lead to a misalignment of the crankshaft. It is mainly based on:
  • Wear of main bearings
  • Distortion of engine bedplate
  • Grounding of the ship
  • Damage to the ships hull
 It can be detected by measuring deflections of crankshaft webs for each unit of the engine. If misalignment exists the crank webs will open and close slightly as the engine is rotated. This is measured by means of dial gauge fitted between adjacent webs at a point in line with the outside of the journals furthest from crank pin. A spring extension rod will hold this in position.

The first measurement is taken with the engine just beyond BDC position with gauge close to the side of connecting rod. The gauge is set at zero at this position. The engine is now rotated with turning gear and stopped at each quarter turn where the gauge readings are recorded as plus or minus values.

The final reading is taken near BDC with the connecting rod on the opposite side of the gauge to the first reading. The first and last reading  are averaged for the BDC reading. The procedure is repeated for each unit in turn.
Crankshaft Deflection

All the readings are recorded and this should be compared to the previous values preferably with the ship in similar load condition and at similar temperatures.
Total deflection vertically and horizontally is calculated for each crank. The vertical total will be proportional to misalignment between bearings due to wear down. The horizontal total indicates side wear in the bearings.

By plotting all vertical deflections it is possible to obtain which bearings are misaligned. This may be assisted by bridge gauge readings of the bearings but these do not take possible distortion of bedplate into account.

Limiting values for maximum deflection are set my engine makers .They indicate permissible limits at which renewal of bearings and realignment is necessary.
Excessive misalignment will cause bending of crankshaft and webs and possibility of fatigue failure of shaft. It will set up vibrations and cause damage to main bearings.
Crankshaft Slippage :

When a crankshaft is built up by shrink fitting, reference marks are made to show the correct relative position of web and journal. These marks should be inspected during crankcase inspection. If for some reasons, the web slips with respect to the journal then it is termed as “crankshaft slippage”.

The most common causes for slippage are :

* If starting air is applied to the cylinder when they contain water or fuel inside the unit or when the turning gear is engaged.
* If an attempt is made to start the engine when the propeller is constrained by for example ice or log.
* If during operation the propeller strikes a sunken object.
* If the engine comes to a rapid unscheduled halt.

Repairs :

The repairs can be carried out in place which requires thorough planning and specialized knowledge (by experts provided by engine makers) under supervision by classification society.
Turning of shaft is carried out by cooling the shaft piece and heating of surrounding crankshaft throw to get the marks back in alignment. Cooling is carried out by liquid nitrogen until the shaft is cooled to -50 to -100 C. The crank throw is heated by means of large special burners until the shrink fit is relieved to a level which makes it possible to turn the two parts of crankshaft to right position by hydraulic jacks.
For slippage upto 5 deg retiming of affected unit can be carried out. Above it there are chances of uneven loading leading to excessive vibrations and stresses.

*Cover Image Credit :http://forgingmagazine.com

Author Amit


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