Maintenance and Repairs

Operating With or Realigning a Slipped Crankshaft

 
 

 

A semi built crankshaft is made up of forged crankthrows with the main journals shrunk into holes bored in the crankwebs.

See: The semi built crankshaft and 2 stroke engine crankshaft

 

The shrink fit is very carefully calculated so that the stresses are not high enough to crack the crankweb or to allow slippage during normal operation. The crankshaft journal which is to be located in the web is is between 1/570 and 1/660 larger than the diameter of the crankweb bored hole. This will give a compressive load of about 77MN/m2

 

After the main journal is fitted in the crankweb, witness marks are marked in journal and web. These should be inspected during crankcase inspections.

Witness Marks on Main Journal and Crankweb

 

If however, an exceptional load is put on the crankshaft, for instance if the engine is suddenly stalled when running, by jamming the propeller when running aground, or if trying to start the engine with a cylinder full of liquid then it is possible for the journal to slip in the web. Slippage can also happen when a disaster within the engine occurs. (for example if the bottom end fails and comes apart and the con rod jams against the crankpin.)

 

The number of units affected will depend on which unit the slippage occurs, and whether the slippage occurs on one or two webs. If for instance the slippage occurs on one web, closest to the timing chain situated at the aft end of the engine then all units will be affected. In another example, if the slippage occurs on the after web on No 2 unit then only  Nos 1&2 units will be affected. If both webs on a unit slip an even amount, then only that unit will be affected.

 

When slippage occurs, the exhaust valve and injection timing will be altered on the affected units. Air start timing will also be altered and because the air flow into the cylinders is now altered, surging of the turbocharger may occur. The crankshaft may also be subject to vibration, where non was apparent before the slippage occurred.

 

What should be appreciated is that if slippage does occur, the metal at the surface of the journal and hole may tear and so reduce the tightness of the shrink fit.

 

 

 

If slippage does occur then two options are available. If the slippage is small ( up to about 5), then the fuel pump and exhaust timing can be altered on the affected units by hydraulically expanding and rotating the cams on the camshaft. The air start timing will be slightly out, but depending on the number of cylinders in the engine (and thus the size of the air start overlap) this should not be a problem. As mentioned the turbocharger may be liable to surge and care must be taken not to operate the engine at any speed where excessive vibration occurs. The slip must be monitored to ensure that it does not move.

 

If the slippage is excessive then there are two choices. Change the crankshaft or try and jack it back to the original position.

The following describes the sequence of operation to jack a slipped crankshaft back into the correct position:

    Chill the crankpin using liquid nitrogen, dry ice (VENTILATION!!). This will take several hours.

    Mount jack on battens to prevent localised load on bedplate.

    Warm up the web using a broad flame. (Usual Precautions)

    Lock journal against rotation (remove bearing shims & tighten down keep).

    Jack web back into position.

 

Great care has to be taken over the operation. If the crankthrow does not move back gradually as the pressure is applied, then as the jacking pressure is increased the webs may jump back suddenly and go past the witness mark, thus slipping the crankshaft the other way. Normally if this occurs then the shrink fit is deemed to be destroyed due to the tearing of the metal, and the crankshaft must be replaced.

 

The picture opposite shows the jacking procedure in operation. The bottom end bearing has been removed and the con rod palm is sitting on the crankpin

 

 

See Cracked Piston Crown - Slipped Crankshaft

 

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