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Technical Discussions
Crankshaft deflection MarineTalk Discussion Forums
All dates are given in mm/dd/yyyy format. Technical Discussions
  Crankshaft deflection
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  Member: kanungos Post Date: 3/23/2006  
Should a crank web deflection be carried out in drydock
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  Member: temana Post Date: 3/25/2006  
FYI - Deflections should only be taken while the vessel is in the water and properly trimmed.
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  Member: kanungos Post Date: 3/25/2006  
Thanks for the reply ,but seek a reason for such an answer.
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  Member: ashwin_ocean Post Date: 3/26/2006  
in drdock the whole weight of the ship is borne by the keel, being resting on keelblock,reaction forces act on it,thus affecting the crankshaft deflection to large extent.hence its only in water natural reaction forces act on the keel and it takes the actual shape in water.the ship should be evenly trimmed or trim to what the original deflections were taken during trials.
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  Member: Maneesh Jha Post Date: 3/28/2006  
It will be a good practice to take deflection readings just before dry dock in afloat condition and just after the dry dock in similar condition and compare results.
As rightly stated by Ashwin, taking readings when vessel is on the blocks will not be useful.
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  Member: rob Post Date: 4/18/2006  
I don't think the actualk deflection is dependant on whether or not the ship is on blocks or water borne. The deflection is dependant mainly on bearing clearance. The manner is support is more of importance on alignment.
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  Member: aqualove Post Date: 4/21/2006  
rob.. delections are affected by trim. and placing the ship on blocks gives it a trim which is diff frm tht during afloat condition. secondly on blocks.. the supported length.. just doesnt do justice to give us the idea abt bearing foundation status during normal state.


maneesh.. u are right.

Thanks.
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  Member: Ritinkar Sen Post Date: 8/11/2006  
Crankshaft deflections can alter if the main bearing clearances change. The reason for this is the reduction of the "crown thickness" of the bearing bottom half that will lower the particular journal in question and the crankshaft will no longer be straight.

Similar situation can arise if there has been any 'fretting' at the mating surfaces of the underside of the bottom half bearing against the bearing housing on the transverse girder of the bedplate. Frettinmg is rubbing wear that can take place if for some reason or the other, the bearing is not tight in its housing.

The main reason for change in the deflection readings is however not due to above.

When a ship is afloat, the hull is seldom straight. There will almost always be some 'hogging' or 'sagging'. If one reads the ship's draughts forward, aft and amidships - the degree of deflection of hull will be immediately apparent. If mid-ship draught is more than the mean draught forward and aft it means the ship is sagging and if less then hogging. The greater the diffrenec between actual mid-ship draught and the mean draught, the more is the hogging or sagging.

As the hull deflects, so does the double bottom tanks, so does the bedplate which is firmly secured to the tank top, and so does the crankshaft.

Depending on the loading or ballasting condition, the carnkshaft deflection will change. If the deflection is too high, the first things to check are the ship's loading condition. This is why it is always recommended to record the ship's draughts when taking the crankshaft deflction. For similar reasons, once a set of deflections have been taken and it is desired to compare with the previous readings, such comparison must be made with an earlier reading taken under the same draught condition. A high deflection reading observed under an adverse loading or ballast condition will in most cases immediately turn normal when the conditioins are changed.

Deflections should be taken from time to time under different loading and ballast conditions.

It is recommended that deflections are also taken in drydock when the ship's bottom is straight. If the readings are high this will almost certainly be indicative of problems with bearings and the shaft line straightness.

Best wishes

Ritinkar Sen
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  Member: tatta seshur Post Date: 10/18/2006  
Dear Mr. R SEN,

Very nice to see your explanation about crankshaft deflection for main engines. One more point to add is the temperature of the crankcase at the time of deflections also another parametr will be added.

Can you please more emphasize on D/G deflections varriations(if any) with changing trim/list/temp variations etc.,

ANOTHER PRACTICAL APPROACH: Reading the deflection readings withthe dial gauge when turning and viewing through the mirror and interpretations tend to move away from the real observations.

With present technological uptrend some developments has taken place in recording and logging correct readings without dial gauge use and remote reading of deflections in the computer screen and very easy to record the readings.

The following are available in market and those who are interested can try with the following companies.

1. Prisma technik
box 5, 543 21 tibro, Sweden , e mail: info prismateknik.com

www. prismateknik.com

2. Bosung engg co
1189 - choryang - 3 dong, dong ku, busan, korea,

e mail: master@bosung.com

Appreciate Mr. R sen's contributions in this site and request his more involvement in some practical diffculties in maintenance of marine machinery.
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  Member: sasakura Post Date: 5/25/2008  
Took readings today on a GE 12 cylinder marine engine. There are only provisions to take the readings at the drive end web. I don't understand the point of taking the readings from only one web?
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  Member: Service Eng Post Date: 7/20/2008  
Deflection should be taken under warm conditions i.e after running the engine at load, all webs.
Cold deflection is just a rough giude and does not really tell much.
Use of electronic deflection instrument is recommended since it is faster and more precise as well. The DI-4 made by Prisma Teknik is to reccomend.
But if you do deflection once in a blue moon, the cost might of the instrument might be hard to justify.
Also refer to engine makers manual about max permissable deflection.
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  Member: Sten Post Date: 7/31/2008  
The deflection should only be taken as reference in fully ballast and one in loaded condition, be cousion with the sunlight and turn the engine in direction were you have the turning gear.
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  Member: gaurav Post Date: 8/31/2008  
can u tell me how we can be sure that the crank shaft deflection readings we have take are correct or not? is there any thuumb rule?
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  Member: Service Eng Post Date: 9/11/2008  
Align the dial gauge properly and try to measure at the same spot each time. Meaning, same spot between the webs. Some engine makers have a punch hole as a guide for you to put the gauge. Turn crankshaft in its normal running direction. If you take hot deflection, do not open up all crankcase covers at once, but only the one you measure and move on to next when done. That is to avoid cooling down of crankshaft to fast in case you have a ventilation fan blowing at your engine. Also good practice so that you do not get any rubbish inside if all is open.

Best result would of course be to use electronic deflection instrument. Prisma teknik DI4 is to prefer. Easy to use.
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  Member: Jyotindra Trasi Post Date: 9/29/2008  
I used to think we should take deflections while the vessel is afloat,but now the Japanese practice is to take deflections in dry dock. I had done 3 dry docks with Japanese Suprentendents in Singapore Hitachi Dry Docks, They had Taken the readings always
in Dry Docks. The reason they had given was all the stresses on the Main Bearings are relieved and on the Engine too. But at sea I always took it with vessel fully loaded and at Even Keel.
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  Member: Jyotindra Trasi Post Date: 9/29/2008  
Japanese practice now is to take readings in dry dock.
On inquiry explanation given was all stresses on the Engine components are relived in dry dock.
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  Member: Jyotindra Trasi Post Date: 9/29/2008  
Hiachi Shipyard in Singapore had taken deflections in dry dock.The reason given was all stresses are relived as the vessel sits on the blocks.
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  Member: paul Post Date: 11/1/2008  
The first thing to understand is cs deflection makes true sense only from the veiw point of trend analysis ie. for comparision hence now you can define the periods "when you need to take it to serve some usefule purpose" dry-dock is not one of them
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  Member: rec Post Date: 12/2/2008  
how is initial alignment done on the engine during assembly of the engine.
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  Member: kanungos Post Date: 12/2/2008  
In my opinion vessel in drydock i.e on the blocks is more of encouraging point loads and hence undue stress concentration.A floating vessel underside is in contact with water at all points and hence upthrust is same on the underside ,proportional to draught. This is more of a udl and hence components are less stressed.
[This message has been edited by kanungos (edited 12/2/2008).]
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  Member: mwbridger Post Date: 3/11/2009  
Seems to me there are more factors involved such as how the engine in question is connected to the hull, whether the bedplate is integral to the ships structure or is a stand alone construction, The size and type of engine is important too, after all there was no point at all in taking crankshaft defelctions on opposed piston engines or medium speed mitsubishis as the defelctions are so small they are hardly measurable.
Finally I suspect the question was related to the fitting of a new shaft or bearings at drydock and whether the adjustments to bearing pockets should be made prior to floating. I personally would make a "draft adjustment" and then check several times when afloat in the ballast condition...
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  Member: mwbridger Post Date: 3/11/2009  
Seems to me there are more factors involved such as how the engine in question is connected to the hull, whether the bedplate is integral to the ships structure or is a stand alone construction, The size and type of engine is important too, after all there was no point at all in taking crankshaft defelctions on opposed piston engines or medium speed mitsubishis as the defelctions are so small they are hardly measurable.
Finally I suspect the question was related to the fitting of a new shaft or bearings at drydock and whether the adjustments to bearing pockets should be made prior to floating. I personally would make a "draft adjustment" and then check several times when afloat in the ballast condition...
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  Member: doublebottom Post Date: 3/25/2009  
i have one question ....
After taking crankshaft deflections is there any way/method by which we can deduce/confirm that the deflections readings taken are correct.
some kind of thumb rule etc...
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  Member: vimatech Post Date: 5/9/2009  
...
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  Member: rob768 Post Date: 5/18/2009  
one way to make sure the readings are correct is to have them done by two individual persons, under the sam e conditions. Readings should be more or less the same..
If you mean whether or not the results are acceptable, you may contact the engine builder, or may be the classification surveyor involved with your ship.
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  Member: Ricardo Flores Post Date: 6/14/2009  
Of course, the logically needed classification society approval depends on that measuring data which has to be finally submitted (after chocking and holding down bolts) The first measurings are used as a well accepted as regular basis for the beginning of chocking job, and the practice seriously indicates to be done with the ship afloating. Final approval which is to be issued by the Class Society will be the one and legal basis for any insurance policy application. No classing, no deals. No adequate measurings, no classing. Then... let's be obvious and effective.
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  Member: helmyj Post Date: 6/17/2009  
Do you know how to take the deflection? From which point to which point? I am using a dial guage.. I saw the punch hole web and set up the gauge but don't know which readings i should take.. Hope you guys can help me
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  Member: anonymous Post Date: 11/2/2009  
Exactly! Couldnt agree more!
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  Member: naidu_mariner Post Date: 1/29/2010  
crankshaft deflection to be taken in calm weather and when the ship is on even keel. punch marks are provided on the webs for placement of the dailgauge or else if not provided the dail auge to be placed on the centerline of web from the 1/3 length of the free end of the web, as the deflection is high on the free end.+ve and -ve values will be recorded resulting due to the expansion on contrction of the webs due to the varying loads on the bearings.starting with the 1st unit to the last unit values to be recorded. 5 readings will be recorded. port(P)(left) and stbd(S)(right) readings. top(T) and bottom(B=(B1+B2)/2)(two readings will be recorded as the pistonrod or the connecting rod will be in-way)readings.if T-B=P-S(approximately)for each unit then the readings taken are said to taken correct orelse once again the above said procedure to be done.temp. of the engine to be recorded while taking the readings. values recorded to be filed for future reference and to be checked with earlier recordings. preferabily these values are taken onboard a vessel when it is unloaded to avoid undue stresses.
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  Member: Pracheer Singh Post Date: 1/4/2011  
Dear Mr. Naidu,
Thank you for your info as you correctly said as to how to ascertain wether the set of crankshaft deflection readings are true or not. Here I would like to add on a bit.

The turning gear should also be considered. In turning the engine, it must overcome the static friction of the running gear. With the today's modern flexible shafts, there is a large possibility that the crank system will be "wound up"a little. This twist could distort the readings adjacent to the turning gear. To check for this, stop the turning gear at each reading and then back it off the teeth on the flywheel. Any change in the deflection guage indicates that the shaft was being twisted, and the readings with the turning gear backed off are the valid ones! Secondly, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing. To prove this you can use a "swedish"type feeler guage and check for any clearance underside. If its not seating, then remove the main bearing shims and pull the bearing keep down as each deflection reading is taken. The reading taken when the shaft is pulled down is the valid one!
More over:
Crankshaft deflection readings in a Dry Dock have ABSOLUTELY NO MEANING as then the vessel is not in its natural lay on water, as then the vessel is kept on Keel blocks which will have reaction forces wich will be very dissimilar to the forces when the vessel floating.

Thanks and Regards,
Pracheer singh,
NYK
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  Member: shippyrupaankar Post Date: 1/8/2011  
Mr. Pracheer Singh

Thanks for your reply but I need some explanation of those part where you mentioned that, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing.In case main engine , if you don't have that type feeler gauge then how one can come to know about the proper sitting of the shaft.
Secondly by seeing the reports how one can come to know that his deflection reading is correct.

Thanks & Regards

shippy
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  Member: shippyrupaankar Post Date: 1/8/2011  
Mr. Pracheer Singh

Thanks for your reply but I need some explanation of those part where you mentioned that, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing.In case main engine , if you don't have that type feeler gauge then how one can come to know about the proper sitting of the shaft.
Secondly by seeing the reports how one can come to know that his deflection reading is correct.

Thanks & Regards

shippy
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  Member: shippyrupaankar Post Date: 1/8/2011  
Mr. Pracheer Singh

Thanks for your reply but I need some explanation of those part where you mentioned that, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing.In case main engine , if you don't have that type feeler gauge then how one can come to know about the proper sitting of the shaft.
Secondly by seeing the reports how one can come to know that his deflection reading is correct.

Thanks & Regards

shippy
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  Member: shippyrupaankar Post Date: 1/8/2011  
Mr. Pracheer Singh

Thanks for your reply but I need some explanation of those part where you mentioned that, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing.In case main engine , if you don't have that type feeler gauge then how one can come to know about the proper sitting of the shaft.
Secondly by seeing the reports how one can come to know that his deflection reading is correct.

Thanks & Regards

shippy
  Reply | Email Edit/Delete  

  Member: shippyrupaankar Post Date: 1/8/2011  
Mr. Pracheer Singh

Thanks for your reply but I need some explanation of those part where you mentioned that, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing.In case main engine , if you don't have that type feeler gauge then how one can come to know about the proper sitting of the shaft.
Secondly by seeing the reports how one can come to know that his deflection reading is correct.

Thanks & Regards

shippy
  Reply | Email Edit/Delete  

  Member: shippyrupaankar Post Date: 1/8/2011  
Mr. Pracheer Singh

Thanks for your reply but I need some explanation of those part where you mentioned that, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing.In case main engine , if you don't have that type feeler gauge then how one can come to know about the proper sitting of the shaft.
Secondly by seeing the reports how one can come to know that his deflection reading is correct.

Thanks & Regards

shippy
  Reply | Email Edit/Delete  

  Member: shippyrupaankar Post Date: 1/8/2011  
Mr. Pracheer Singh

Thanks for your reply but I need some explanation of those part where you mentioned that, it is also possible for getting incorrect readings when the shaft does not sit on the main bearing housing.In case main engine , if you don't have that type feeler gauge then how one can come to know about the proper sitting of the shaft.
Secondly by seeing the reports how one can come to know that his deflection reading is correct.

Thanks & Regards

shippy
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  Member: Pracheer Singh Post Date: 1/16/2011  
Dear shippy,

Firstly, you have to have that kind of feeler guage which is equipped with the standard measuring instruments for main engine, otherwise you have to order for that type of feeler guage!!
If you want to make sure still, then you have to jack up the shaft and put a dial guage on top, base resting on the bearing cover. Once you start pumping, you will see a very slow change in the reading, and at a point suddenly there will be a steep and rapid change. This will happen only if the shaft is lying well flush on the bearing otherwise from the start itself, there will be a steep increment in the dial guage reading.

Secondly, the readings can be judged that they are correct when your T-B = P-S ( APPROXIMATELY ), so if not then again the readings have to be checked and taken again properly considering every aspect and then recheked. If still they are not roughly equal, then please check the bearing clearances, foundation, foundation bolts, tank top, check for any cracks in the bed plate etc.

Thanks and Regards,
Pracheer Singh,
NYK
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