Connecting Rod

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jrvakil
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Connecting Rod

Post by jrvakil »

Please provide what should I check before purchasing it from scrapyard ? Also advise what process and tests should I do on them in my workshop to ensure they are ready to use ??
Big Pete
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Re: Connecting Rod

Post by Big Pete »

Hi again jrvakil,

You are making full commercial use of this site!!
Maybe you should consider making a donation to Martin's costs to run the site? (or employ me as a Consultant?)

Connecting Rods:

Check all the surfaces of the rod for dents, cuts, scores or any other form of mechanical damage, check around the top end and Bottom End bearings for any discolouration of the metal. If a bearing has siezed, this will generate heat, and steel will usually turn blue if this happens. This heating can effect the crystalline structure of the metal molecules, reducing the ductility and fatigue resistance of the rod, and making it unfit for further use.

Check that the rod is straight, if the engine has sufferred from seizure or other major damage it is possible that the con. rod will be bent.
With a forged steel con rod it may be possible to forge it straight and heat treat it to remove the stresses. But for you, so long as you can meet demand, there is no point in carrying out this work, it will only eat into your profit margin.
If you are honest about the repair, then it is unlikely that a buyer would want to pay as much for a rod that has been straightened, as for one that has not.

Check the top end where the rod is connected to the piston or crosshead. There will usually be a bush type bearing fitted here. Check the internal diameters according to the maker's manuals. Usually it must be within limits for circularity and diameter. If it is outside limits a new bush can be obtained from the maker at areasonable cost. The bushes are either heat shrunk or hydraulically pressed into position. This can be done by any reasonable competent Engineering Workshop.
Because of the oscillating motion of the gudgeon pin in the bush it is very hard to generate the "Hydrodynamic wedge Lubrication" that usually keeps rotating shafts seperated from their bearings. In theory, this is the hardest bearing surface in the engine to lubricate adequatly. However, the engine designers have paid great attention to the design, and unless the oil feed to the bush is interrupted, these bushes will easily run for the life of the engine.

Bottom End (Big End)
Personally, I think the old bearing shells should be removed and scrapped. I would not like to re-use shell bearings from an unknown source. Modern shell bearings are electro plated with a very soft & thin, run in layer, any dirt that gets into the bearing during assembly is supposed to be pushed into this so that no sharp edges protrude to damage the shaft. If dirt is partly embedded in the soft white metal, the bearing surface then acts like emery tape and wears away the "hard" shaft. The running in layer is enough to run the bearing in once, but not a second time on a different crankshaft!!
Bearing shells also have a fatigue life (for the steel backing plate), and many engine makers do not allow these bearings to be re-used if they have been opened up. There is another potential problem with water. If the tin (Stn) in the bearing alloy gets wet and oxygen (in the air) is present, it forms a white powdery oxide layer that is easily brushed off. If the tin is exposed to water, in the absence of Oxygen, it forms a Hydroxide of Tin. If the hydroxide of Tin is subsequently exposed to air/Oxygen, then it further oxidises to a different Oxide of Tin (One is Stannous Oxide the other Stannic Oxide. I forget which is which). This is BLACK & EXTREMELY HARD, (harder than a crankshaft). If a bearing with this Black Tin Oxide layer is used in any machinery it is probable that it will completely destroy the shaft that is running in it. It is possible to remove the Black Oxide layer with scrapers and nylon scouring pads, but this is delicate and time consuming work.


Once the shells have been removed, the bottom end should be tightened according to the maker's instructions. The diameters should be measured at various points in accordance with the maker's manual, to check that the bottom end is within limits for circularity, and the correct size. The inner faces shoulsd also be inspected to make sure that there is no damage. If the B.E. is oval, it may be possible to correct this by maching the mating faces of the bottom end, to reduce the diameter in that axis, and then re maching the opening to the correct size circle. Again, this cost is probably not worth your while.

The mating faces at the B.E. should be cleaned and closesly examined for damage. If the faces are serrated they can suffer damage due to incorrect tightening and many Maker's introduced revised tightening Instructions in Service Bullettins over the lifetime of the engine. You should particularly look for signs of cracking, scuffing, and fretting.
Again, some makers permit the mating surfaces to be machined to remove surface damage and then re-bore the bearing housing, but the cost is not worth while for you, especially if you have to machine a serrated B.E.!!

You should carry out some form of crack detection, either using aerosol Dye Penentrant Sprays, or Magnetic Particle Imaging, to the mating faces of the Bottom End (especially if serrated), the bolt holes and the Bolts.

I think that about covers it, but someone will probably add something to the list.

Good Luck and Happy Diesel Ducking.

BP
It is always better to ask a stupid question than to do a stupid thing.
jrvakil
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Re: Connecting Rod

Post by jrvakil »

Thanks my friend again.

I may hire you as technical adviser, bring me some business and you will have share :P

Thank you very much for your help... :)

Now can you provide details for liner ?? :-|
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JK
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Re: Connecting Rod

Post by JK »

Much of the information you are asking about can be found in the engine mfr manuals that go with the engine. These can be purchased from the Mfr.
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The Dieselduck
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Re: Connecting Rod

Post by The Dieselduck »

Hi again jrvakil,

You are making full commercial use of this site!!
Maybe you should consider making a donation to Martin's costs to run the site? (or employ me as a Consultant?)
Hehehe, Yes, I do agree you should hire Big Pete as consultant ! But then again we may not have such good answer on the forum. But, like JK said the manuals are the easiest thing to have a look at, and chances are they are right on the ship the engine is coming out. So it should be pretty straight forward, well of course if you have some reasonably competent mechanic types.

Anyways, thanks Big Pete, I am always learning something new with your insightful and simply put posts.
Martin Leduc
Certified Marine Engineer and Webmaster
Martin's Marine Engineering Page
http://www.dieselduck.net
Big Pete
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Re: Connecting Rod

Post by Big Pete »

Hi Martin,

Thanks for the compliments!
I will let some else reply to liner question, I appear to be hoggong all the posts at present.

BP
It is always better to ask a stupid question than to do a stupid thing.
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offshoresnipe
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Re: Connecting Rod

Post by offshoresnipe »

Yes, thanks BP.
I always look for you and JK's replies to the questions. Much help for me.

Just spent the last week rebuilding a 60 ton A/C water chiller compressor. The crank was shot but the bearings did not look that bad, would have thought that we would have found chunks of bearings in the oil sump. The crank had wasted away a good 1/8 of a inch in one spot, not all the way around. We are wondering if could of had the same situation that you talk about in a closed system.
Anyway keep up the post's, something to look forward to each day when I get off watch.
Big Pete
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Re: Connecting Rod

Post by Big Pete »

Hi Offshore snipe,
I remember something I heard from a School friend who worked in the Auto Industry about 40 years ago that might possibly relate to your problem, even if it doesn't it is interesting.

A new car had a gearbox failure after a short period. The suppliers could not find any reason for the failure and replaced the entire gearbox, the new gearbox failed and was replaced. Eventually, after several gearboxes, they found the reason, an earthing strap from the gearbox casing to the chasis had been ommitted on the assembly line. In consequence of this, D.C. electric current (Earth return for the electrical system) was actually passing through the gears and there were arcs between one gear and the next one. This caused small surface pits and as the gears ran on each other the pits were transferred back and forth between the gears and became worse until the gear box failed.

It might be worth checking for any Earth faults on any electrical equipment mounted on the compressor, (crankcase heaters, unloading solenoids, instrumentation, alarms etc., D.C. Earths are always a pain) there is also a possibility that the crankshaft, which is a rotating conductor, may be in a magnetic field caused by an adjacent motor Alternator or transformer and therefore acting as a generator itself.
(all conductors moving in the Earth's Magnetic field will generate electricity, we just don't think about it happening in shafts, pistons, crossheads etc!)
Usually the electrical energy just causes eddy currents and the energy is dissipated as heat, but sometimes it can cause a problem for us Engineers.

Either way the resulting current flowing through the compressor could either cause Galvanic corrossion or sparking as described above. The gaskets and oil films will act as insulators, forcing the current to flow in certain directions.

It is possible that the compressor had been left stopped for a long time so that the oil had drained out of the bearing so that the bearing and shaft had good electrical contact at one point, explaining why the damage was localised.

Something else for you to read, and think about when you get off watch!!!

I have just edited this post so maybe it is not as concise as before, but I thought some more of my ramblings might make things a bit clearer to some people!
BP
Last edited by Big Pete on Sun Jun 27, 2010 11:55 am, edited 1 time in total.
It is always better to ask a stupid question than to do a stupid thing.
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JK
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Re: Connecting Rod

Post by JK »

Hey BP, you just answer so darn well ! I always enjoy reading your very concise posts, that is an art in itself.
Very interesting post about the AC chiller.
Thank you Offshore Snipe, I am very happy that my posts are a help to someone.
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