I was wondering if any of you could enlighten me as to why a broken crankshaft is pretty much the end of an operating locomotive's life. Can they not be successfully welded back together again? Or is it just the nuisance factor of trying to remove the crankshaft. One of the guys I work with used to work on the big alcos, and said they never gave any thought to replacing them, and so when they broke they got out the torches and cut holes in the carbody to extract them rather than disassemble the entire engine, granted they weren't museum pieces, just every day working engines in the mid 1970s.

If the crankshaft is the only piece that is failed, it can be replaced, albeit with a great deal of effort. In many cases, there is often collateral damage to other pieces that creates enough damage to render a prime mover uneconomical to repair.

To answer another part of your question: no, it's generally not practical to weld a broken crankshaft back together. Replacement would be required.

Why would anybody cut holes in the carbody?

You simply take off the top hatch or the hood. That's why they are removable in the first place.

Pull the engine out, remove the damaged crank ("broken" covers a lot of territory),
inspect the crankcase and pan for other damage, put in a new, reconditioned, or second-hand crank with new main bearings, button it up, and you're ready to get back to work.

When a crankshaft breaks most of the time it does allot of other damage too. Parts try to exit out of the side of the block, stuff gets throw around inside the engine doing damage.

It is not practical to weld a crankshaft because;

1) A welded repair cannot be made as strong as the original forged material (thickness of the weld and the basic strength of the weld filler metal)

2) It would be nearly impossible to keep it straight because of the heat, all welding processes cause some warping/twisting of the metal parts

3) The heat from the welding process will change the hardness of any nearby journal surfaces

Impossible, no, but the fixtures, re-machining (assuming there is enough metal to machine back to a useful shape) and the heat treating required would likely cost more than a replacement part.

If a replacement part is not available the cost of making a new forging and machining it into the correct shape is probably more than the entire engine is worth.

So, yeah it's dang near impossible to fix a broken crank by any means except replacement which is very costly by itself.

Cheers, Kevin

dump the engine, put a trolley pole or pantagraph on, put some wire up, and yer good....


you know with all this computerized gizmos making things you'd wonder if you could cut a chunk of metal into form replicating the crankshaft.

Since any diesel is really a generator to power the electric trucks (every diesel is "Traction" to all youse interurban nerds) if your run into a broken shaft, perhaps unless you can locate a replacement, consider a new engine that fits, whatever can turn that generator.

A sample of what is involved with making a crankshaft plus the connecting rods, pistons and cylinders. When a crank breaks, as stated above, it can damage all of these.

viewtopic.php?f=1&t=35945&p=207290#p207290

As the article states, the tolerances on the crank journals were + .000/-.001"

For anyone familiar with od grinders and large work pieces, that requires a very large od grinder (as the caption in the article stated at the time - one of the largest of its kind in the world) and I would imagine a temperature controlled room. Having ground punch press cranks with 6" od journals, I can attest it is not an easy undertaking. Temperature changes play havoc with results. BTW - a human hair is about .002" in diameter so the total tolerance on these cranks is half that.







The last photo with the dial indicator on the tail stock shaft is for removing taper. Behind are the counterweights. On cold mornings I could watch the grind go heavier in each pass from heating up. Final size had to wait for the machine/coolant to stabilize or I couldn't hold size from one end of the journal to the other. Eliminating taper was critical before you went under size and that had to be done with the machine/coolant stable - up to operating temperature. The larger the piece - the more sensitive it is to temp changes.

Another concern is slippage between work piece and chucks. I constantly had to check this since the force of the wheel, vibration and thermal expansion could cause this to happen. Both head and tail stock had pins that would lock the heads in the vertical. If the head locked but the tail didn't, the piece had slipped and the journal would have to be centered again.

Steel has a thermal coefficient of expansion of around .0000067 in/F-degree.

http://www.engineeringtoolbox.com/linea ... -d_95.html

If the crankshaft is 16 feet long and it sees a 10 degree increase in temperature, it will expand (.0000067)(10 F-degrees)(16 feet)(12 inches/foot) = .0129 inches. The grinder having a much greater mass and also being insulated since the grinding wheel effectively insulates the machine from the heat buildup of the grinding won't heat up as much as the work piece. The work piece will expand and slip in the chuck jaws while subjected to the torque and vibration of grinding.


Steve

Just a note to the conversation:

Shay #5 at Cass Scenic Railroad has a "built from scratch" all welded crankshaft. It was built in a jig at Cass shop many years ago, and is in operation today.

Are these large engine crankshafts Forged? I have seen cranks fluxed and then cracks repair welded and/or brazed - but usually a total failure is too much a compromise in strength - and as others have said it can cause huge collateral damage that might total the components.

Steve, I apologize as this may seem argumentative, but I feel it's important to inform all readers with another perspective. I found your post very accurate with my experience grinding journals. With all due respect however, I might add that the machine pictured appears to be near its maximum size limit and this may have impacted the work as you described. I ground journals similar in size on a much larger machine - I think approx 30" dia wheel and 1" face. However, it had centers, support ways, and much larger chucks with 4 or 6 jaws. It could easily grind 1 to 6in journals to +/-.0005 and probably better but never had the need as usually -.001 was plenty, I wish I recalled the feed graduations and bed width. You did have to be mindful of temperature differences especially on first 'touch' and first job in the morning with cold coolant, but this was usually all evened out in 10 minutes or so, and the feed was so fine/minute that you could hear the swoosh of coolant and part/wheel contact and very slowly ease into the grinding over several part revolutions. (The giant wheel dia also helped keep temp down) Anyways, my point is that what you describe is a bit extreme IMHO - there are machines made specifically to grind these and although much skill is involved, it is much less perilous if you have the right one. On automotive crank grinding machines I have seen and larger machines, such as that pictured of Baldwin, I am sure the task of finish grinding to an acceptable tolerance journal size is an easier task. My .02

Respectfully,
Sully

For really big crankshafts:

http://ajw.asahi.com/article/sci_tech/t ... 1308050117




Steve

OK, now you're in my territory. My first 'railroad' job was at National Forge in Irvine, PA, the makers of those big crankshafts and also where the 20-throw SD45 cranks were first developed. Wow, were they proud of those. We did EMD, GE and Alco in all sizes.

The crank starts as one long piece - longer than the final crank. It's then put into the 'worlds largest' close-form die press (Loewy) at Irvine, and done one 'throw' at a time to distort the straight shaft, step by step, into a "u" shaped throw in rough form. All the throws, and the geometry, and done in rough form. So the metallurgical flow of the crank itself in continuous through the part. It's all one flow. And metallurgically, it's quite a wonder. The cranks are annealed, heat treated, finish machined on lathes that can rotate on both the axis of the throw and the axis of the crank (which is amazing to watch).

When I worked at the plant in the late 70's you had to be REALLY CAREFUL driving around the plant roads because if a forklift was moving a 20-throw crank blank sideways in the forks, it was taking up the entire road both sides, and you had to yield and sometimes hit an emergency U-turn to get out of the way.

The other fascinating thing they did at Irvine was to 'layer' on rare metals on the bearing surfaces, at different depths, so that you could do spectrographic analysis on the lube oil, which would tell you how much the crank surface was worn.

Some nice shots here:
http://www.ellwoodgroup.com/Companies/Forging/ENC.aspx

What you can't see is the SOUND of that Loewy press operating, it's so loud doing a hydraulic press like that that it seemed to me that's what the gates of hell would sound like.

To work there in management you had to take - and pass - the ASM ferrous metallurgy course, no matter what your function was. Everybody had to understand steel and metallurgy at what was a Masters level. At the time, as a solid "C" student in college chemistry, it seemed like a ridiculous burden. Today I look back and it was a gift. As you can see on the 'glowing red crankshafts' shot, the heating and control-cooling is part of the process to keep the strength.

I can't imagine any way to weld one of this size. Bluntly, one of the inherent problems of locomotive cranks is that they are possible to break even when perfect and new, particularly if any of the bearing surfaces overhead and compromise the annealed strength surfaces and grain flow. I think there were some pretty good stories on the original development of the SD45 cranks involving stiffening of the locomotive bed to prevent crank breakage. Essentially, you're pushing the edge of metallurgy to start with, and a compromised/repaired part just can't take it.

I see your thought and raise you one. Nice work. different operating circumstances and designs. A broken shaft in the diesel may have been a missed flaw or slow metal breakdown, you better have one solid metal piece than 2 pieced together, as noted in the cautions above. The Shay shaft is extrernal vs internal in a diesels, broken shavings parts may not be so much a hazard.

cheers.

You are probably right. The shop where this old Van Norman crank grinder sat in had very poor insulation and the heat would be turned down every night. By morning it was around 40 degrees during winter. This grinder had .001 graduations which I would split. One additional complication was our journals were built up with chrome plating. The chrome would load up the wheel incredibly fast as opposed to plain steel. Before a finish pass I would have to dress the wheel, blue up the journal, touch off and go to size. After removing .001 of chrome, the loaded up wheel would not grind round/straight to .0005 from one end to the other on a long journal. Another machinist that ran a large OD grinder had the experience of an outside door near his machine. In winter if someone opened it and the cold air hit his machine, his pass would go south and he'd have to stop and wait for the door to close. Another shop I worked in with a much smaller Brown & Sharp No.5 OD grinder also turned the heat down over night and the next morning I would spend at least 2 hours chasing my dimensions as the machine came up to temperature. Those were smaller parts but tighter tolerances, typically around .0003 to .0005.

Steve

"Shay #5 at Cass Scenic Railroad has a "built from scratch" all welded crankshaft."

Different question; the original question was "why can't you just weld a broken (diesel) crankshaft back together ?

Because it is a forging and difficult to fix for the reasons mentioned.

Can you fabricate a crankshaft as a welded piece ? Yes, but to do it for the tolerances and strength required for an internal combustion engine is probably not practical. It would probably have to be too large (for the necessary strength) to fit inside an engine block already designed for a forged crank.

The Shay crankshaft is turning quite a bit slower, has only three "throws" and has looser tolerances. It is a very nice piece of work.

Cheers, Kevin.