We learnt in the previous articles about the process of taking crankshaft deflections in marine diesel engines and how to go about interpreting and analyzing the readings. We know that having an overly deflected crankshaft could spell danger for the engine and/or the vessel as a whole and therefore we need to know certain important points to be kept in mind related to crankshaft maintenance which have been discussed in this article as follows.
Modes of Crankshaft Failure
A crankshaft consists of several parts such as the crank-web, crank-pins and journals and therefore there could be a fault with either of these parts. There could be various modes of a crankshaft during service. The failure could be due to crack developed in any section such as the web, pin or journal and the piece could get completely broken in the worst case. Also there could be a case of slip shrinks which would be problematic.
Apart from that there could be web failure due to fatigue which is due to the presence of deflections which are beyond permissible limits and hence cause bending of the crankshaft causing fractures due to fatigue.
There are other defects which might develop in the crankshaft but are not directly related to the forces on it and such defects include corrosion, scoring and ovality of the cross-section of the crankshaft.
Observations to Prevent Failure
Since there are so many ways in which a crankshaft might fail, it is important to know various checks and observations which need to be done regularly so that such a situation does not arise and given below are a few such important points.
Firstly as described in the previous articles a routine check on the crankshaft alignment is a must and this helps to detect any major defects that are in the process of development.
If the differences in the bearing wear are substantial this means that the cause for the same needs to be rectified. Sometimes this may be due to the use of main bearings having different material which could be due to a different manufacturer etc.
Since lubricating oil is an important ingredient of the lubricating system, care should be taken to ensure that only clean and proper grade oil is used which could otherwise lead to bearing damage and ultimately affect crankshaft alignment. Apart from being cleaned thoroughly by the lub oil purifier, the oil should also be regularly checked for its pH value to ensure that it is not acidic in nature.
Since the bedplate and the foundation bolts acts as a base for the proper support of the crankshaft both these should be kept in order. The bolts should be properly tightened while a rigid bedplate should be used which is of course more of a design issue rather than in the hands of the engineer on duty.
Finally since resonance can give rise to excessive forces, care should be taken to avoid operating the marine diesel engine at such speeds which gives rise to critical frequencies.
Having learnt so much about crankshafts in previous few articles, it is now time to move on to the next component of the marine diesel engines namely pistons.