Crankshaft deflections (the change in the distance between the crank webs at different crank angles) indicate the alignment of the main bearings. To find the actual degree of misalignment:

1. Take a complete set of crank web deflections at all cylinders, at several crank angles (e.g. every 15-30 deg), using a dial gauge fitted between the webs.
2. Compare the deflections with the maker's limits; a large deflection at a particular crank angle indicates that the main bearing at that position is low or high (misaligned).
3. To confirm, measure the main bearing clearances and the bearing heights (using a bridge gauge or by measuring the journal position relative to the bearing housing).
4. Check the shaft alignment (sag and gap at the couplings) and the bedplate level.
5. The actual degree of misalignment is determined by the magnitude of the deflection and the bearing height differences, which are compared with the allowable limits.

(ii) FOUR possible causes of the misalignment (4 marks)

1. Wear of the main bearings (uneven wear causing one bearing to be lower).
2. Settlement or distortion of the engine bedplate or the ship's structure (hull deflection).
3. Incorrect chocking or holding-down bolt tension (a bearing housing not correctly seated).
4. Thermal distortion of the engine (uneven heating) or a change in the hull loading (ballast/cargo) causing the bedplate to distort.

1. Hull loading: changes in the cargo/ballast distribution cause the hull to bend (hogging/sagging), which distorts the engine bedplate and changes the crankshaft alignment. A heavily loaded or unevenly loaded hull can cause the main bearings to be misaligned.
2. Sea water temperature: changes in the sea water temperature affect the hull temperature and the engine cooling, causing thermal expansion/contraction of the hull and the engine, which can change the alignment.
3. Deck temperature: the temperature of the deck (e.g. heated by the sun or by cargo) causes the hull to distort (the deck expands more than the bottom), which bends the hull and changes the crankshaft alignment.

These factors cause the bedplate to distort, changing the main bearing alignment and hence the crankshaft deflections.

Bearing wear may be a result of misalignment rather than a cause because: if the crankshaft is misaligned (e.g. due to hull deflection or bedplate distortion), the journal does not run concentrically in the bearing, causing edge loading - the load is concentrated on one edge of the bearing. This edge loading causes rapid, uneven wear of the bearing (wiping on one side), which is a result of the misalignment. The misalignment itself is caused by the hull/bedplate distortion, not by the bearing wear. So the bearing wear is a symptom of the misalignment, and correcting the misalignment (e.g. by re-chocking or adjusting the bearings) is necessary to prevent further wear.