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Cracked crankshaft repair?

Just a Sparky

Hot Rolled
Joined
May 2, 2020
Location
Minnesota
I got the worst possible news earlier today from the crankshaft shop grinding my compressor crank. Magnafluxing revealed a crack in the flywheel-side bearing seat, about half an inch outboard of the bearing race itself.

Is there any way to save and use this crankshaft? These haven't been made since probably the 1950s and there aren't many of them around. Aside from this one defect the compressor is ready to run and probably has several decades of life left in it. I've invested a lot of time, money and energy getting it this far.

Would it be worth attempting a surface repair by undercutting the affected area and building it back up with weld? Or would I be better served by cutting off the affected region and drilling + tapping the stubbed crank for a new undercut shaft extension which can be threaded and then welded in place? I don't know if this crank is cast or forged.

:wall:

I can get a photograph later, but in lieu of that, here's a drawing - approximate location of the crack in dotted red:

AcroRd32_qKVGyD5dsy.jpg
 
if the pulley drops off,its not a disaster...there is no chance of internals being smashed by loose rods or anything like that,and the drive will simply stop.......consequently ,Id run it as is,might go for years ......when it breaks ,then you can bore out and insert a piece......weld up the crack,the crank will warp ,and it will turn into a major job
 
Think I should just take it back to the shop and tell them to grind it anyways and accept the risk?

Would it be possible to simply bore the crank and expand it for a shrink fit extension without welding?

I think the flywheel got hit at some point in it's life. It wobbled about half an inch while running prior to disassembly. Hence the crack just outboard of the bearing.
 
I would expect it is regular cast iron no special forging. I assume this an old school slow speed piston compressor. Weight saving and quick rpm changes are the reasons to use a light weight forging.
Bill D
 
First, does it run true right now as is?

We worked in the crank department way back, we were "helper" so we polished finished cranks but straightened camshafts and the welded crankshafts.

If welded it will move all over the place and may not survive the whole process.

Second, commercial or home shop?

If home then duty cycle light and we assum it has been this way for years and it MAY out live you but safety is a concern.

So step outside the box and walk under the shade tree...

You could place in your lathe and with support of steady rest drill from the end of the flywheel end into and through the web of the crank.

You will need to thread the area both sides of the Crack to insure no additional stresses.

If you can thread the whole thing but that limits your next part.

Use a grade 10 bolt to thread through the threaded part, if there is clearance on the inside a thin lock nut.

On the outside the bolt head needs to be smaller than shaft.

You can make good all thread maybe.

Last step it threadlock or epoxy, I have some HVAC grade epoxy made for fixing the coils, very strong stuff.

This could be added to the inside to fill any gaps between the bolt and threads so nothing can move.

You could create something that fits over the end of the shaft that will allow you to add compressed air.

This would push the epoxy through the threads and if you had nut and seal on back side then it may force it into the Crack.

Lots of stuff to think about, the threaded insert if correct will retain everything if it let's loose, the epoxy should limit any movement that may worsen the Crack.

Others sharper than I will likely have other ideas.

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Agreed ,if it was made here ,crank would be a grey iron casting......but made in the USof A in 1950?....anyhooo,if its cast iron ,then the crack likely wont spread,and can be ignored.....if the compressor is of immense sentimental value,then a new crank can be plasma cut from steel plate and machined........Ive done this several times to make Henderson cranks.
 
The folks at the regional Quincy dealership told me one time that Quincy cranks usually can't be successfully welded.......FWTW.
They told of a few attempts with cracking failures as the result. So........???

Many years ago I had a Quincy 390 that needed the rod journals welded and ground. I saw an ad in the paper from a guy that 'fixed' cranks. The ad was actually impressive. He came to me (I didn't go to his "shop".......mistake) and after his convincing sales pitch, he took the crank to "fix" it. Turns out he didn't have a crank welder, he just gobbed it up with 7018 or some such mess, and couldn't straighten (it warped like a pretzel) or grind it. I learned a lesson. I suspect he welded it in his driveway with his buzz box. Oh and he wanted to get paid after ruining the crankshaft. Beware.
 
I did time in the crank dept, we did lots of welding to build up journals, iirc the cast iron required submerged arc, steel used gas, might have been other way around. After welding/grinding they always had to be straightened, do not recall ever welding a cracked crank. One of the regular customers built engines for the big multi engine tractor pullers, I frequently found cracks in his cranks, more than a few times after showing him the crack, he would say "thats not too bad, it will make at least 3 more runs", so who knows, maybe on a compressor it will last another lifetime.
 
My only concern is that the thing was fluxed after straightening, so I don't know if it was there before I took it to the shop or not.

On the plus side, the crack is invisible to the naked eye. I had to ask the guy at the shop where exactly it was and that caused him to pause, scratch his head and go flux it one more time just to be able to put a sharpie mark on it for me. Supposedly it runs all the way around the shaft "pretty deep" because it gave a strong indication. That would be consistent with gray iron I suppose given how little the shaft has actually been deflected.

The pump is located beneath a work bench with a guard on it so the worst that will happen if it fails is one hell of a ruckus and a few sparks until I can hit the E-stop on the 'Start/Stop, Off, Continuous Run' station.

I guess if it is cast iron then I can expect a fairly clean break if it does go so it likely won't trash the crank in that case. I could then resort to one of the options discussed above for repair.

Any thoughts on the boring and shrink fitting option? Baking the crank in the oven and soaking the extension in dry ice from a 5 lb fire extinguisher for a while? Would a shrink-fit extension grip tight enough to do the job on a ~1-3/4" shaft?

EDIT: I suppose one way of looking at it is that the straightening process exerts much greater loading on the shaft than it will ever see in actual service, so the only way it will actually become a problem is through fatigue over time. And now that it has been straightened to remove the wobble...
 
Fit of 001 per inch is all I would do with cast iron.....and I would press it together ....unless it is steel ,then it can withstand a lot more force .......what HP is the motor?...5/10/15hp ?....how much force can the motor apply to the pulley...not much.
 
In books they show a press used to bend cranks straight.

In our shop it was hammer and chisel, a few hits on the low side relieves stresses and crank comes towards you.

On a main bearing surface who knows, one could cut out a good size groove all the way around then take it to a crank shop with correct welder to fill in and re-grind but removing material may result in crank falling apart.

Run a bolt through the middle to create the inside support and run it

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Here are some pictures of the alleged crack:

20211116_052418.jpg20211116_052343.jpg20211116_052409.jpg

I don't see a damn thing. If it held up to the hydraulic press used to straighten it out, it ought to hold up to spinning in circles, no? Albeit it will be transmitting pulsating torque because it's between the flywheel and crank, but even so...

I brought it back to them today and told them to grind it anyways. The guy across the counter raised an eyebrow and looked at me funny but grudgingly agreed to do it after a little convincing... about a week after Thanksgiving, even though it was originally going to be ready tomorrow before they red-tagged it.

I'll run it as-is and keep the belt guard on it when it's running.

Re: above

It's a 10 horse pump. Quincy 244. 4" bore x 3.5" stroke duplex, runs at 900 RPM. Weighs about 300-400 lbs. They killed that product line some time in the 50's afaik. Mine's a '48, ROC 11. Just stumbled across a rusty ROC 35 in Texas for $200... but the crankshaft changed from 1262 to 1466 at ROC 25 and the two are not interchangeable. 1262 supports a gear pump at the end of the shaft, 1466 a plunger pump in the middle. Different bearings too I think.
 
Put a bolt thru it like Tony suggested. C bore the hole and thread only the bottom portion so that that bolt puts that portion of the shaft in compression. That way, even if it breaks, you will probably never know. Because its overhung you have a case of fully reversed bending outboard of the bearing. Putting that part in compression will reduce the effect of the bending moment. Pretty quick and simple solution.
 
the though bolt from pulley end to cheek will give you some peace of mind and why you are at it you could add a stub at the pulley end of the shaft with a pillow block baring to help support the flywheel . the shop that i do some work in grinds cranks they are cheeked for straightness before grinding and if they need to be straighten they are wet maged and black light before and after.the straightening can be with heat the blunt chisel and the press . and after that they will need a grind as they won't be completely concentric and they will have some brilnilling aka flat spots to them . i would not drill any holes to try to stop the crack and as far as welding the only time i have seen it done was to repair a worn surface on a crank not to stop or repair a crack . i would give you the Oklahoma guaranty on that one if it brakes you own both half's
 
What sort of fastener are we talking here? I imagine I'll want as small a diameter bore as possible to achieve a healthy compression preload without excessively compromising the remaining undamaged cross-section.

3/8"-16?

As far as actually pulling the operation off - I'll have to use an extended length drill bit. Not sure how to get a tap that deep though. We're talking about 5 or 6".

Suggestions?
 








 
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