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Deflections

Deflections are readings obtained from between the webs of individual crank throws as the crankshaft is rotated. Standard procedure is to fit a dial gauge between the webs, usually as close to the shaft circumference as possible (at opposite side to throw), and set to zero when crank throw is as close to BDC as possible. Turning the crankshaft slowly and taking a reading at every 90 deg thereafter will provide top and bottom readings indicating the state of the shaft alignment in the vertical plane, and port and starboard readings indicating the state of alignment in horizontal plane.

It should be noted, however, that the readings from one unit alone do not enable the shaft alignment to be assessed. It is only from an overview of the interrelationship of all units that the lie of the shaft can be interpreted. The readings of an individual unit may point to a problem being present but they will not determine the cause. Also, although a set of deflections will indicate, when correctly translated, where misalignment is occurring, it is imperative that a set of wear down readings is also taken. An excessively worn bearing will inevitably give shaft misalignment. Only when the wear down readings have been taken, and satisfactory adjustments to the worn bearings have been made, should any attempt be made to rectify any misalignment indicated by deflections.

A simple but effective check on the validity of a set of deflection gauge readings is the ‘complemental check’. That is, the top plus bottom readings should be roughly equal to the port and starboard readings added together. Any large discrepancy in this should prompt the taking of another set of readings from the unit(s) concerned. The error in the recorded values may have come from a misread gauge or wrong transcription.
The following steps should be taken before attempting to take the readings.

  1. Check that no-one is working on the propeller or in the vicinity. This does not imply that readings should be taken in dry dock. On the contrary, readings taken there do not reflect the natural lie of the ship when she is afloat. The alignment at that time is enforced by the line of the ‘keel blocks’ and as such bears no resemblance to the natural deflection of the floating vessel. 
  2. Check that no-one is working inside the engine, on crossheads, or in cylinders etc.
  3. Check that all hanging bars or pins are removed and that lifting gear, chain blocks and engine room crane are not attached to any of the running gear.
  4. Open all indicator cocks.
  5. Whenever possible, use a gauge that shows negative readings under compression and positive on expansion.
  6. Watch amperage as the engine is turning to get an idea of resistance to turning. Any sudden changes in reading should be investigated immediately.
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