Machining or welding a large crankshaft 1

[21121956]

Hello everybody

We are planning to rebuild a large Diesel engine.

One of the parts subject to repair (or change) is the crankshaft; with main journals of 380 mm and connecting rod journals of 360 mm.
We have received two offers:

1. Basically, to perform welding on indented or cracked
areas of the journals, straighten and polish.
2. Basically, straighten, grind off damages areas to an
extend of 5 mm per side on some of the journals and
polish.
The crankshaft will be delivered with undersized
journals and will have to be installed on oversized
journal bearings.

I have some questions: with the crankshaft with reduced diameters, is it possible to await that the engine will give its nominal output, 17000 kW? Because of the welding process or reduced diameters, how about the possibility of a broken crankshaft? How about the need to acquire a crankshaft brand new?

Any comments will be greatly appreciated. Thanks

 

[EgyDesign]

hi ,

in the matter of fact your question is a complex one
first: if you do welding it must be by an expert....because the heat induced in the shaft while welding will exceed the recrystallization tempreture of the shaft ...if any inadequate
cooling happened it will raise the hardenesse of the shaft and thought it will increase the brittlness....so the shaft will have cracks soon after the first shock.....add to this that welding a part from a shaft could give this specific part different initial stresses and properties so if this part has high fatigue for examplpe it will more dangerous to make this.
welding could be used but with caution and by experts in the building up condition of a crankshaft which is a very sensitive place in the engine.

second: the second solution a risk....u see, reducing the diameter of a crank shaft could make a huge problem.....anyway ofcourse the designers made factors of safety on the diameter...but u cant count on this....if u are going to reduce the diameter so dont get near the corner fillets....and make it very little reduction and its not safe totally....

the first solution is good if done by experts....i mean the conditions of welding, the HAZ "heat affected zone" should be well know and so on, but it will work.

 

[wes616]

EgyDesign Brings up some very good points.

In order to determine the better of the two options that you have presented, I would like to know how the power is created. Is the shaft high torque or high angular velocity.



Wes C.

 

[wes616]

and what is the span of the shaft?

Wes C.

 

[21121956]

Wes C. :

Basically, in a reciprocating engine, the combustion is achieved into a combustion chamber constituted by the cylinder liner, the piston with the piston rings and the cylinder head.

The high pressure created by the combustion (in our engine 175-180 bar, generating a Mean Effective Pressure (mep) of 21-22 bar) develops a force that push the piston down in a linear movement.

This force is equal to: Cylinder area X mep
In our engine is aprox. 24.000 kg on each of the eighteen cylinders.

That linear movement is converted in a circular one by the crankshaft, creating big torsional forces and stresses.

The velocity of the engine is 600 rpm and the span of the crankshaft is 6.5 meters.

Ricardo

 

[EgyDesign]

even if we know the power and the angular velocity still wer not sure about the material.........the yeild strength i mean......and the stress concentration points, but there is a solution.....first we calculate the materials yeild in the normal state of the engine with combined stresses anaylsis, then we try the new diameter after machining on this yeaild strength......maybe it works.........and by the way there is a third solution ......using the welding principale....u can heat all the shaft....then weld the nautch by an oxyacetylene neutral flames.....then leave it to cool down normally the a hot medium....thus this could lower the hardeness some how.....anyway im trying to do my best. .regards

 

[wes616]

EgyDesign,

True about the materials selection. I guess I was assuming that it would just be a Med Carbon steel.

Assuming this, I was just trying to determine if we should be more concerned with cracking from brittleness, or from shaft whirl (balance) from reduction in area of shaft.

Normalizing.




Wes C.

 

[EgyDesign]

yeah wes c thats a good point........if we use welding it will be very hard to rebalance the shaft again.

 

[minerk]

I have seen the second option successfully employed on large diesel engines (not as large as the OP's though). I am familiar with a company here in the US that specializes in exactly that type of repair, and they may have done shafts of this size. I would be very apprehensive to have the crankshaft weld repaired, the risk involved is too great. Of course the second option must be done by a machine shop that has the skill to properly perform the repair. Have they done similar work? Will they stand behind their repair? Have you visited their shop?

 

[EgyDesign]

by the way....most crank shafts have three times allowance of maching......every oone 100 micronsdecrease in diameter....so the second solution will be good if u make it in this interval.

 

[fsmyth]

It would be preferable to grind the shaft
undersize, but only if that eliminates ALL
the faults. This is standard procedure for
a wide range of motor sizes, and works fine
if properly implemented.
For broken or badly damaged journals, your
only option is welding to build up and/or
re-join the shaft parts. There are folks
very experienced at this, and I would
carefully select (by talking to past
customers) the shop. The shaft will never
be the same as an originally cast, but can,
IN SOME CASES, be successfully used.
And in some very rare cases, can exceed the
original specs (remember terms like X-ray,
magna-flux, shot-peened, stress-relief,
grain orientation, cryogenics - all as
applied to engine machining).
Tread carefully here.

Experience counts in both cases. Try to find someone
you can trust that has a proven track record.
It is more aggravating to do the job a second time,
and there is usually not enough left at that point
to salvage, necessitating a new or replacement part.

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