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Thread: Broken / Snapped Crankshaft.
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14-06-2008, 11:34 AM #1
Broken / Snapped Crankshaft.
Hi Everyone
A few months back i had a request from a friends son for info on compressor failures , causes etc. I have sent loads of info , including photos and several "links" found on this site which i thought had answered his questions .
However the one thing i am struggling to answer to his satisfaction is this ---
When he stripped a failed compressor in their workshop he found the crankshaft broken but all rods and valves intact with no wear or damage evident. He's asking for a detailed explanation as to how this could happen.
Can anyone help with a link or similar that i could send to him ???
Many thanks in anticipation
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14-06-2008, 12:10 PM #2
Re: Broken / Snapped Crankshaft.
It depends where crank was broken.
Had crank been ground previosly or worked on? No true answer can be given withuot more details,as each failure may have differant circumstances.
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14-06-2008, 12:21 PM #3
Re: Broken / Snapped Crankshaft.
Hi Ranger1 - i totally agree.
Having not seen the crank i can not answer any questions. However i believe he is just looking for general information to satisfy his own curiosity.
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14-06-2008, 02:47 PM #4
Re: Broken / Snapped Crankshaft.
You can get the situation where the crank will snap leaving the attached con rods etc intact.
When a bearing surface, be-it at either end or in some cases in the middle, picks up and seizes.
The resultant inertia cannot dissipate through the con rods etc as they are designed to "move with the flow" so to speak.
The only place the energy can dissipate is at some point between each end of the crankshaft and the point of seizure.
I hope you understand what I am trying to explain.
But basically all the rotational forces have to dissipate somewhere and usually its at the drive end of the crank.
But as is in the situation being described not ALLWAYS!
Incidental I have seen the end of a 3.5 inch crankshaft twisted like a Chinese burn on a plasticine sausage.
Due to the external electric motor seizing.
The flexible steel rubber steel drive disk in between the motor and compressor tore itself apart.
But not before the end of the crank twisted as described above.
Sadly I did not take any photos of it.
Cheers Grizzly
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14-06-2008, 03:29 PM #5
Re: Broken / Snapped Crankshaft.
albion
In the case of the smaller air-cooled compressors, a busted crank is usually the result of a liquid slug. Since liquid cannot be compressed, pressures up to 10,000 psi could be seen in the head. With these pressures something is going to give ie: rod, valve or crank.
I have some good pictures but I'm 2000 miles from home at this time. I'll try to post them when I get home.
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14-06-2008, 04:40 PM #6
Re: Broken / Snapped Crankshaft.
Hi - Grizzly , as far as i'm aware there was no seizure. In fact no damage apart from the busted crank.
As Powell has stated the obvious cause of failure is liquid slugging - but the question he is asking me is why just the crank ,why no damage to the rods , valves etc.
He's not happy with the explanation " it's what ever gives first "
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14-06-2008, 05:32 PM #7
Re: Broken / Snapped Crankshaft.
I have seen such cases before. Usually liquid in the discharge is the cause, especially if the condenser is above the compressor and there is no NRV on the discharge line, when the compressor is at rest, liquid is going down and fill the discharge side of the compressor head or heads and "holds" the pistons > braking the shaft.
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14-06-2008, 08:23 PM #8
Re: Broken / Snapped Crankshaft.
My answer is not infallible as Powell and Chemi cool point out.
Their explanations are just as plausible.
In fact as they point out on smaller machines liquid slugging is more likely!
On larger machines there is usually buffer springs to eliminate this.
Only you don't state size.
Even Rangers option is feasible a inherit weakness anywhere could fail.
So like you we can only surmise.
Cheers Grizzly
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14-06-2008, 09:03 PM #9
Re: Broken / Snapped Crankshaft.
Depending on place where it is broken it could be over tensioned belts if belt drive.If crank was ground and radius in corners is not copied it can snap clean through.
If you ask a car or truck engine reconditioner they may be able to give some more clues.
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14-06-2008, 11:44 PM #10
Re: Broken / Snapped Crankshaft.
Could some liquid have splashed on the hot crankshaft causing a thermal stress fracture?
"If Hannah was an air handler, I would be a condensing unit so I could open her TXV and pump my refrigerant through her coils." - a HVAC friend of mine
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15-06-2008, 03:51 AM #11
Re: Broken / Snapped Crankshaft.
I have seen them snap due to generator testing. Generator was phased opposite of incomming line. Puts a heck of a strain going from full speed one direction to full reverse.
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15-06-2008, 08:14 AM #12
Re: Broken / Snapped Crankshaft.
I don't think so, because the crankshaft is on the suction side and isn't hot at all.
MoOst bigger machines are oil pressure injected via the oil pump, so they have the same temperature as the oil.
If that should be true, then we should have a lot of broken compressors when charging liquid.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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16-06-2008, 10:56 AM #13
Re: Broken / Snapped Crankshaft.
Another possible cause could be too much oil in the crankcase. The crank will hit the oil and over a period of time the continual pounding into the oil can cause cracks at the ends of the big end journals.
Paul
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16-06-2008, 09:25 PM #14
Re: Broken / Snapped Crankshaft.
i would love a few pics etc of compressor failures as it was something that i never done much on when comin up
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16-06-2008, 11:25 PM #15
Re: Broken / Snapped Crankshaft.
Many thanks for everyones input
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20-06-2008, 02:34 AM #16
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