Machining or welding a large crankshaft
Machining or welding a large crankshaft
(OP)
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
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





RE: Machining or welding a large crankshaft
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.
RE: Machining or welding a large crankshaft
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.
RE: Machining or welding a large crankshaft
Wes C.
RE: Machining or welding a large crankshaft
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
RE: Machining or welding a large crankshaft
RE: Machining or welding a large crankshaft
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.
RE: Machining or welding a large crankshaft
RE: Machining or welding a large crankshaft
RE: Machining or welding a large crankshaft
RE: Machining or welding a large crankshaft
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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