To obtain accurate and reliable crankshaft deflection readings, it is essential that certain ideal conditions are satisfied. These conditions ensure that the readings truly reflect the alignment of the crankshaft and are not influenced by external factors.

• Firstly, the ship should be afloat and on an even keel, meaning there should be no significant trim or list. If the vessel is inclined, hull distortion can affect the crankshaft alignment and lead to incorrect readings.
• Secondly, weather and sea conditions should be calm. Excessive rolling or pitching can disturb the dial gauge and introduce errors during measurement.
• The loading condition of the ship should be consistent with previous measurements (whether in ballast or loaded condition). This allows for meaningful comparison of readings over time, as hull deflection varies with loading.
• The engine temperature should be stable, preferably in a cold or ambient condition. This is important because temperature variations can cause thermal expansion of engine components, which will influence deflection values.
• Finally, the turning gear should be properly engaged to rotate the engine slowly. However, when taking the actual readings, care should be taken to ensure that no unnecessary load or backlash from the turning gear affects the measurements.

Accurate readings are achieved by following correct procedures and ensuring proper handling of measuring instruments.

• To begin with, the dial gauge must be properly calibrated and checked for smooth operation before use. Any fault in the instrument will directly affect the accuracy of the readings.
• The gauge should be positioned exactly between the designated center punch marks on the crank webs. This ensures that all measurements are taken at a consistent radius, which is essential for reliable comparison.
• Before starting, the dial gauge is set to zero at the bottom dead centre (BDC) position, usually just after passing the connecting rod. This serves as the reference point for all subsequent readings.
• The crankshaft should be rotated in one continuous direction, typically in the ahead direction. Reversing direction can introduce errors due to backlash in bearings or the turning gear.
• After completing a full 360-degree rotation, the reading should return to the initial zero (or very close to it). If there is a significant deviation, it indicates possible error in the measurement process, and the readings should be repeated.

Purpose

The main purpose of taking crankshaft deflection readings is to assess the alignment of the main bearings and the crankshaft. Over time, uneven wear of bearings or structural deformation of the ship’s hull can cause misalignment. This leads to bending (deflection) of the crankshaft during rotation.

Regular monitoring helps in early detection of abnormalities, thereby preventing serious issues such as crankshaft fatigue, web cracking, or bearing failure.

Interpretation of Readings

Deflection readings are used to evaluate both vertical and horizontal alignment of the crankshaft:

• Vertical Alignment is determined using the difference between the Top (T) and Bottom (B) readings.
  • A positive (+) value indicates that the crank webs are opening at the top. This usually suggests that the middle bearing is higher than adjacent bearings, or that the adjacent bearings have worn down.
  • A negative (–) value indicates that the webs are closing at the top, suggesting that the middle bearing is lower or excessively worn.
• Horizontal Alignment is assessed using Port (P) and Starboard (S) readings. These values indicate any side-to-side misalignment of the crankshaft.

Finally, all readings must be compared with the manufacturer’s specified allowable limits. If any value exceeds these limits, corrective actions such as bearing adjustment, replacement, or engine re-alignment (re-chocking) must be carried out to restore proper alignment.