Having taken crankshaft deflections the practical work might be finished, but a theoretical analysis and interpretation of those results is certainly required to take any meaningful decisions regarding the main bearings based on the recorded data.
We learnt the need and method of taking crankshaft deflections in our previous article. But taking the readings is one thing and it is equally important to properly record, analyze and interpret those readings. This is exactly we are going to learn in this article in the following sections.
Tabulation of Results
The results are tabulated in a manner shown in the example table shown below which represents the readings taken from a real engine. You will note that the numbers in the first row represent the unit or cylinder number and the first column shows the position at which those readings are taken. These positions had been graphically illustrated in the previous article so feel free to turn back if you have forgotten about them. The final row shows the difference between top and bottom readings which gives the vertical misalignment of the shaft. The values of the vertical misalignment need to be compared to the maximum permissible limits specified by the manufacturer of the engine.
Plotting the Deflection Curve
A majority of the people find that the graphical representation of data is more appealing and easily understood, rather than a list of numbers hence we will see how we can use the above data to plot the deflection curve. Take a look at the deflection curve diagram first so that you can correlate to what you read next.
- As you can see above on the top is the crankshaft which shows the 7 units and the vertical misalignments written on top of each unit (these values are taken from the last row of the table above).
- A straight line is drawn beneath the crankshaft and the center lines from each unit are drawn downwards through this line. This now acts as our basic infrastructure upon which to start plotting the deflection curve.
- Since the first deflection is -5.2 units (remember they are normally mm unless otherwise specified), we plot this distance downwards from the reference line on the center line of unit 2 (not unit 1) and make the line ab which is at an angle proportional to the deflection at a. This line is further extended to intersect the center line of the next unit.
- The next step is to measure the deflection from this point of intersection (downwards for negative value and upwards for positive value) and join the point from the previous point, in this case which gives rise to the line bc.
- This step is repeated till the last unit
A smooth curve is drawn between these points and the position of this curve with respect to the base line XY gives an idea about the state of various bearing. For example in this particular case you can easily make out that bearing of units 1 and 2 are too far away from the base line and hence need attention.
You must have realized by now that why only comparing the values of vertical misalignment against the manufacturer’s specifications is not sufficient? This is so because even if the values are within limits, the relative wear and tear might be sufficient to cause too much misalignment hence this is a method to be double sure about the crankshaft being aligned properly.
In our next article we will learn about the modes of crankshaft failure and points to be kept in mind during the maintenance of crankshafts so that they have a long life.
Images of Crankshaft Table & Crankshaft Deflection Curve: Sanyal, D.K. (1998) Principles & Practice of Marine Diesel Engines. Mumbai: Bhandarkar Publications.