Question

(a) Explain why crankshaft deflections are taken. (4)

(b) Write a procedure for the taking of main engine crankshaft deflections. (8)

(c) Explain the action to be taken if some crankshaft deflection readings are outside acceptable limits. (4)

Answer 1

Part (a)

Crankshaft deflections are measured to detect the misalignment of main bearings. The misalignment occurs due to bearing wear or deflection of crankshaft and also to check the horizontal and vertical alignment of the crankshaft in bedplate, on a direct drive engine the alignment of the bedplate with the tail shaft. Because the hull of a vessel and therefore the engine bedplate move and flex, the alignment of the crankshaft will vary within given limits. It is when the alignment is outside these given limits that the crankshaft is subject to excessive bending stresses which may lead to failure.

Part (b)

The deflection of the crankshaft shall be represented by the value when the engine is cold, and since the values measured when the engine is warm sometimes differ significantly depending on the measured conditions, be minded not to use the value measured when the engine is warm as standard.

Stop the engine and engage the turning gear.
Start measuring deflections from the unit farthest to flywheel
Place the crank pin at the point of 30° (position ‘B’) past the bottom dead centre.
Install the deflection gauge in the pop point provided for this purpose.
Set the reading on the gauge to 0 (zero reading) at the position ‘B’ in the figure.
Slowly conduct turning of the engine in the normal direction of rotation, and measure the reading on the scale when the crankshaft is at the angle of ‘B’, ‘C’, ‘D’, ‘E’ and ‘A’ respectively, of which data shall be recorded. Before taking the measurements for each point the turning gear should be momentarily reversed to release and torsional stress that it is exerting on the crankshaft.

Calculating deflection (d): Calculate the deflection values as based not the measured values and in accordance with the following formula and record the calculated values.

Vertical (V) deflection: dV = D - A+B/ 2

Horizontal (H) deflection: dH = C - E

positive/ negative deflection: open downward (+), closing downward (-) A, B, C, D and E represent the measured values respective at each corresponding position shown in the figure above.

(c) Procedure to follow when Crankshaft deflection readings are outside the permissible limits

If the crankshaft deflection readings are found to be outside the acceptable limits, the following steps should be carried out systematically:

Repeat the deflection measurements with careful attention to all factors that may influence the readings. These include:
- Vessel’s loading condition
- Trim and list
- Weather conditions (e.g., heavy seas)
- Cargo operations underway
- Ambient temperature (e.g., cold sea or hot weather)
Confirm the dial gauge being used is properly calibrated and in good working condition.
Ensure proper tightness of holding down bolts and tie rod bolts.
Inspect the bedplate chocking arrangement for any signs of fretting or damage.
The most common cause of excessive crankshaft deflections is unevenly worn main bearings. This can be confirmed by measuring bearing clearances.
If worn bearings are identified, replacing them may restore deflections within permissible limits.
If the issue is due to worn or fretted chocks, or as a result of collision or grounding, then a full realignment of the engine may be required. This procedure must be carried out at a shore-based workshop.
If the engine must be operated with deflections still beyond the limits, it should only be done after consulting the engine builder.
Operation must be at a reduced load, under specific guidance.
Immediately inform the DPA (Designated Person Ashore) and the technical superintendent.
The engineer superintendent should initiate contact with the engine manufacturer for further instructions or support.

✓ Verified Model Answer (Text Solution)

The deflection of the crankshaft shall be represented by the value when the engine is cold, and since the values measured when the engine is warm sometimes differ significantly depending on the measured conditions, be minded not to use the value measured when the engine is warm as standard.

(c) Procedure to follow when Crankshaft deflection readings are outside the permissible limits