Crankshaft Deflection

[Pengze]

Hi Gentlemen,
Good day.
I would like to know what is the limit allowed or values that indicate a problem for crankshaft bearings after having taken a crankshaft readings?
Pls enlighten me on this if there is any norm to follow.
Appreciate your help, thanks.

[Big Pete]

Crankshaft deflections will not reveal any problems with the crankshaft or the bearings.
Even if the crankshaft is bent.
They will reveal a problem with the alignement of the main bearings, this can be caused by a failure of the chocking arrangements or distortion of the Hull. This is why, in order for the deflection readings to be comparable, they should always be taken with the ship afloat and in the same condition of loading.
Also why after collision or grounding deflections should be checked as soon as practical.

[Dieseldame]

Hey Big Pete, wonder if you could explain a bit more why deflections won't show any problems with crankshaft and bearings

DD

[Big Pete]

Hi Sorry I have not answered before,
As fitted the crankshaft is perfectly aligned with the bearings, usually deflections are small except where they are influenced by the mass of the flywheel, hanging off the end of the crankshaft which will cause the the end of the crankshaft to droop.
Try to visualise what is happening as you turn the crankshaft:-
The "deflections" being measured are the result of the crank throws moving in and out.
Even if the crankshaft was bent and the distance apart of 2 throws was permanently wrong, this would not show as a deflection, because the reading would not change as the crankshaft was turned.
Modern crank shafts are "short and stiff", designed to resist the load from the piston onto the crankshaft which is massively greater than that from gravity.
( Unlike say the old LB Doxford engines which were fitted with spherical crankshaft bearings which allowed the crankshaft to bend, and the bearings to align themselves with the crankshaft as it bent)
If the bearings or crankshaft journal become worn, that will only increase the clearance in the bearing, it will not put any force on the crankshaft, to cause a deflection. The crankshaft will just take up its normal line.
If however, the bedplate becomes distorted, the bearing housings will move out of line, either vertically or horizontally, this will cause the crankshaft to bend to fit the misalignement of the bearings. When the crankshaft is turned, the direction of the bend will continually change, causing the throws to move in & out, giving a deflection reading.
In practice it is difficult to monitor the condition of the bearing. If the LO pressure falls that could indicate excessive clearances. Opening the crankcase door, shortly after the engine has run, and measuring bearing temperatures with an Infra Red thermometer could give warning of trouble. Bearing clearances could be checked with special narrow feeler gauges. The bearings could be opened and examined and "leads" taken, however, with modern shell type bearings this can do more harm than good. Many engine makers worn that after relatively few running hours the metal backing of the shell loses it's elasticity and will no longer fit in the bearing housing properly if re-used. Modern shells usually have just a flash of electro plated running in material and do not have the capability of being run in a second timne after being disturbed. It is allways better to scrap bearing shells once they have been opened up.
I am sorry this is late, but I hope it helps. t

[Dieseldame]

Wow, thank you - that was an excellent explanation. I wonder when you have a moment if you could explain what taking "leads" are.

DD

[Big Pete]

Hi Diesel Dame,

Taking leads is a way of measuring bearing clearances.

Open up the bearing, and place lengths of lead wire (hence the name) across the bearing journal, refit the top half of the bearing and tighten down according to makers instructions.
The lead wire will then be flattenned down to the exact thickness of the bearing clearance.

(A modern alternative to lead is Plastigauge or similar which as it's name suggests is a plastic based material. Obviously it is essential that you have plastic deformation of the material you use for this. If the deformation is elastic it will bounce back to it's original dimension when the pressure on it is released, and also the material must be softer than the bearing material, I won't insult anyone by explaining that!!)

The bearing is then opened up, and the thickness of the lead wire meaured with a micrometer to give the bearing clearance.
This works well IF YOU ARE ABSOLUTELY CERTAIN THAT THE JOURNAL IS SITTING ON THE BOTTOM HALF OF THE BEARING WITH NO CLEARANCE BELOW. (Check with feeler gauges)
As I said before, modern crankshafts are short and stiff, so often they do not sit on the lower half of all the bearings.
In this case you would have to take leads below the crankshaft as well as above, and add the thicknesses to give a total clearance. The problem then is to get the lead wires in & out without scoring the bearing shell.

I hope this helps.

Pete

[JK]

You know the sad part of reading Diesel Dames question was knowing that I had actually have used the lead instead of the Plastigauge
But retirement is still so far away.

[Dieseldame]

Thanks Big Pete very through as usual - I'd like to be at that elbow too! I've used Plastigauge but I guess from JK's comments I would have to be an older salt to know the leads method

DD

[JK]

Using lead is the same...I'm just old.

[Dieseldame]

Time for a mid-life?

[Big Pete]

One thing I didn't mention earlier is that if the engine has a camshaft drive chain, the tension in that will try to lift the crankshaft and bend it. This bend will always be in the same direction in relation to the engine , but will vary its direction relative to the crankshaft as it turns.(same as the droop caused caused by the weight of the flywheel).
This is why you often have a high deflection value adjacent to the drive chain.
Also, this is why you should always turn the engine ahead when you take deflections, if you turn the engine astern, the tension moves to the other side of the chain and is acting at a different angle on a different place on the crankshaft, obviously this will give completely different deflections.
Even reversing the engine a couple of degrees in order to fit the clock gauge/ DTI will slacken the tension in the chain and give a false initial reading, and make all the rest of the readings for that unit useless.

BP

[Pengze]

Hi there,
Excellent explanation by BP but my question is still unanswered. What is the maximum allowable limit of deflection ?
I would also give my 2 cents worth of opinion, whereby nowadays in modern medium & high speed diesel engines, we come across shell-type bearings & not the olden babbitt pouring white metal bearings. And some manufacturers does not recommend taking of 'leads' for bearing clearances as it might give complications to the bearings. Usually, the bearing-halves are tightened up accordingly as per torque (of course out of the engine after overhaul) and taking inner diameter readings at different points with micrometer to compare with original dimensions. Also will take bearing shell thickness & ovality. Very rare nowadays to take 'lead wire' due to popularity & ease of maintenance for these types of bearings.
Any comments from you great guys.
Keep the engine running well.

[Big Pete]

All I can say is if you want precise limits for your particular engine, and your particular application, the best option is to email the engine maker's service department.
BP

[Dieseldame]

Hey JK, how about making that jail term a little more visible for the rest of us???
DD

[JK]

Martin has his Avatar size limits very tight....