Crankshaft Deflection: Marine Diesel Engine Components

Written by: Ricky • Edited by: Lamar Stonecypher
Updated Mar 11, 2009

The crankshaft of the marine diesel engine is a huge bulky component which should remain as straight as possible during operation, otherwise it could lead to serious damage to the engine and the vessel. Learn about deflection in a crankshaft and how to keep the alignment within limits.

Introduction

In the previous article we studied about marine diesel engine crankshafts in terms of their construction, materials and so on. In this article we will proceed to learn about one very important task related to marine diesel engine crankshafts and that is learning to take their deflections. In the succeeding paragraphs you will learn about the importance and procedure of the same.

The Need to Measure Deflection

If you remember about the construction of the crankshaft from the previous article, you will recall that apart from other parts, it consists of crank-webs, crank-pins and journals along its length. The weight of the crankshaft is supported by the main bearings at the journals. Over a period of time as the engine keeps running, the wear in the bearings may not be uniform across the entire length of the crankshaft. This means that the crankshaft will not remain in the initial straight line but will get bent either upward or downwards to a slight degree which may not be visible with the naked eye but could be sufficient to cause dangerous levels of fatigue in the crank-webs. Hence there is the requirement of measuring crankshaft deflections at regular intervals to ensure that the alignment of the shaft is within permitted limits and these deflections can be measured as described in the next section.

How to take Crankshaft Deflections?

The figure shown below gives an idea of which measurement is taken to find the deflections of the crankshaft. As you can see from the picture a dial gauge is inserted between the crank-webs to find out the distance between them. If the deflection is measured after the specified interval, it is necessary that it is taken at the same point otherwise the reading will not give a real reflection about the degree of deflection. Normally a center punch is used to make markings so that each time the deflection is taken at the same point.

click to enlarge

Apart from using the same point on the crank-web for measuring deflection there are other factors which need to be kept in mind and these include load on the ship, trim, hog, sag etc. Ideally the deflection needs to be taken at four points of the crank namely top, bottom and the two sides. In actual practice however the bottom reading is not taken due to chances of fouling by the connecting rod and instead reading is taken on both sides of the bottom position, thereby in total 5 readings are taken from each crank-web at the positions shown by the following figure.

click to enlarge

We will learn more about recording, presenting and interpretation of the results in the next article.

References

Image of Dial Gauge in CrankWeb - Marine Engineering Knowledge by Brian

Edojonker Feb 8, 2012 10:26 PM

RE: Crankshaft Deflection: Marine Diesel Engine Components

Dear Sirs, i take the permision to make comments to your article when a diesel engine is too fast loaded and the topside (cooling water side) is too fast heated. -The top of the engine, heated by hot cooling water, expands faster then the bottom side including the crankshaft. -In practice, we take shaft deflection readings when the engine is cold. In fact these readings are not the readings the engine has during operation. -Please realise that a crankshaft with a journal diameter of a 20 cm takes appr. 2 hours to reached the operating temperature of 60 C. -Therefore large engines should be preheated hours before running to compensate for misalignment. Not especcially to heat the topside of the engine but in particulair the crankshaft. -It therefore is much better to keep the crankcase (crankshaft) between the operations on temperature to avoid misalignment. -When preheating of the crankcase (crankshaft) is not possible because the engine has to run directly after maintenance, the load of the engine should be gradually increased. -The crankshaft, still cold, should get time to warm-up untill operating temperature, appr. 60C, is obtained. -Please realize that it is the crankshaft that thermal hihgly will be loaded during the first running hours after a start. -The outside of the shaft is 60C and the core is at 30C. That gives high strains in the shaft and that is where you should take care for. -High thermal strains in the shaft makes the shaft bending in all kind directions and do not remains in the straight line as wanted by the engineer. -There is notting wrong to start and run a cold engine at normal operating speed as long as the load of the engine is low. That means, keep load on crankshaft and journals low. -But be assured that during this first engine loading process the crankshaft is absolutely NOT in line and therefore again the engine load should be kept restricted until the crankshaft temperature is equal.

edo jonker Feb 10, 2011 12:47 PM

cranckshaft alignment

The average lub oil temp at running condition is appr. 60C. So 40C is still cold. Edo

Alex Miranda Feb 8, 2011 10:59 PM

Hot Condition of Measuring Defection

40 degree cel. it is hot or cold condition when measure crankshaft deflecion?

Maint Engineer Feb 7, 2011 10:38 AM

crank deflection

dear sir kindly discuss in about the affects of readings of crank shaft

Devendra Dec 17, 2010 7:32 AM

crank-shaft deflection

Sir,
it is very informative article.

good luck for coming articles.

edo jonker Nov 19, 2010 12:00 PM

cranckshaft deflections

Dear mr Vinay naik,
I am willing to think with you about excessive deflection readings but explain what your real meanings are with this question.
Edo

vinay naik Nov 17, 2010 9:46 PM

maring engineering motor

three causes of excessive deflection reading

Tony Talley Nov 16, 2010 3:23 PM

Tools to measure deflection

For those who would like to measure deflection accurately and consistantly, there are tools available.

deba Aug 26, 2010 12:32 AM

Main engine cylinder head relief valve

please tell me the occassions at which the cyl. head relief valve gets acttivated.Please reply me as possible.

bayog Aug 25, 2010 10:02 AM

ehgine room

can you show me a set of equipments organized inside of the engine room?thank you!because it could help us as a pre-sea cadets or trainees to familiarize an engine room equipments and their location although its only an example that not exactly we will have on board. thank you

bayog Aug 25, 2010 9:56 AM

animation

do you have an animation of a marine diesel engine?can you show me?

Thet Khing Aug 23, 2010 1:50 AM

which parts?

Dear Sir,

Can u explain me about which parts are rotate and which are stable in crank shaft?

zaki mahmod Jun 17, 2010 9:49 AM

how to know measure between gab and piston in diesel engine

diesel engine

ahmed faris Jun 12, 2010 2:54 AM

crankshaft hot&cold deflection

how to take reading hot&cold deflection from crankshaft

Paresh Panchal May 27, 2010 6:36 AM

crank shaft deflection

Dear Sir,

Really it's very infomative information.

yogende varma May 20, 2010 8:54 AM

hull deflection

dear sir...its realy a wonderfull commnt on d tiopic.....
sir i m a naval architect ...n want to know abt deck breakage...for not any confusion it is a technique used to find out the deflection of the deck of a ship when it is drydocked or even whn it is docked immediately after drydocking.....basicaly we wll get the deflection of the deck with respect to the designed deck parameters....
my question is after getin the deflection wht all things can we find using this deflection parameters with us.....one thing is tht v can compare it with the permissible deflection of the ship deck....but apart from this wht all things can we find out?????
plz reply soon i m waitin for an answer

lisa Feb 12, 2010 11:37 AM

check rule

Of course one always must begin with the manufacturers specifications. However these may be in some degree inaccurate or improper. Understand that in operation a crankshaft floats on oil, not touching metal to metal. When taking deflections, obviously, crankshaft is stationary and resting on bearing metal. A rule would be that deflections must be less than the bearing oil clearance - quite a bit less, so that the shaft may turn without significant narrowing of the oil clearance - this is just a "rule of thumb" that the manufacturers specifications may be checked against.

edo jonker Feb 8, 2010 6:25 AM

Crankshaft deflections

No engine manufacturer will give you maximum permissible shaft deflections as they don't know what the maximum will be.

For good understanding, its are the main bearings out of line forces the crankshaft to bend.

Normally a crankshaft will not break apart due to excessive deflections. What will happen, when the engine runs at “excessive” crankshaft readings, is that the crankshaft load on the bearing surfaces will not equally divided over the full width of the bearing but concentrated at one side forming a parabola print. The wider the parabola print the extremer the load on the bearing. At that particular situation it will be hard to maintain the lubrication at that one side and metal to metal contact will start initiation a running hot bearing.

Are you in doubt about the deflections of your engine crankshaft, check the main bearings for wear patterns after a short test run of your engine at low load.

Edo Jonker

lisa lebowski Feb 6, 2010 9:44 PM

defection correction

I have taken deflections of Enterprise diesels used at nuclear power stations for emergency standby power. Good article.

In ships, determining the cause of unacceptable defections and determining proper corrective methods is of utmost importance. As the propeller shaft axis is difficult to change and deflection may be the result of a grounding raising the engine by hull and sole-plate distortion, proper engine to sole plate shimming at the initial construction phase is important. If this has been done properly one then has the option of shimming down where necessary. This is a matter for honest experts.

Yogesh Sahib Jan 26, 2010 1:12 PM

Maximum Deflection Permitted

Dear Sir,

Kindly explain how 2 know the maximum deflection permitted for the crankshaft.I am taking the readings at regular intervals but i couldnt able to find the max.value allowed from manufacturer.(MAK 6M 453 AK Engine).

anil kumar mehta Jan 25, 2010 10:04 AM

deflection of crankshaft in marine engine

basic fundamental of deflection given is execellent and helps in improoving performance.

aseemras Dec 21, 2009 6:01 PM

great article..helped in my xam prep!

basuka55 Oct 21, 2009 10:49 AM

CRANKSHAFT DEFLECTION

The article of crankshaft deflection is highly informative. The deviation in crank shaft plays a major role in the performance of the engine.
In the ''Need to measure crankshaft deflection'' , one thing is most important, whenever some steel plates are renewed in the ships hull or after any welding job is carried out at the tank top area , due to stress in the adjacent plates near the foundation of the engine,causing deviation in the crankshaft deflection.
So the checking of crankshaft deflection should be carried out immediately after any steel work is carried out to prevent any major breakdown of the engine.
Also in case of grounding the vessel, Immediately after the vessel is re floated, the crankshaft deflection should be checked for the allowed deviation as mentioned by the engine manufacturer.

All comments on these points are welcome

edo jonker Oct 13, 2009 11:03 AM

crankshaft deflections

Hard to say. It depends on several factors.
Large size crankshafts accepts larger deviations than small size shafts and a free spinning shaft accepts more then a high loaded shaft. And what the deflections are of the webs when the engine is under full load, we don't know. Therefore the mechanic has the duty to install the engine so that deflection readings of the manufacturer testbed are obtained which, with a sound engine is possible. To start, check the readings at first installation and compare readings with manufactures testbed readings. Readings should be more of less the same.
Take readings of a hot engine directly after jacket water and lub. oil are switch off. These readings are the most important because the engine has now the deflections existing during operation.
Take care the engine fundation is on operation temp. as well. Readings should may now deviate a few hundreds of a mm as a maximum.
Important is the crank deflection reading between one crank and the adjacent crank. It becomes harmfull when these deflections readings jumps from high pos. into high neg. Not more then 0.06 - 0.08 mm is permitted. Make allignment corrections when one crank reading is f.i. max. +0.04 and the adjacent crank reading is max. -0.04mm. Exact values can not be given because no one knows.

How to make corrections.
Large deflection readings in a small engine is a matter of misallignment of the main bearings or a crankshaft wrong machined.
Large deflection readings of a large engine,(medium speed or low speed) is a matter of misallignment, wrong machined crankshaft and by unsufficient bedplate support.
For sea going vessels take shaft deflection readings when the ship is free floating specific in morning time when the engine room hull is not influenced by sun radiation.

Govind B Rana Aug 24, 2009 5:33 AM

Dear sir,
Good day!!
Gone through your article and found very informative beside the deflection what is the maximum value allowed in deflection and how the deflection can be corrected.

regards
Rana

Crankshaft Deflection: Marine Diesel Engine Components

Introduction

The Need to Measure Deflection

How to take Crankshaft Deflections?

References

Comments