Broken crankshaft !?

[minh-thanh]

. there is a lot one can not know everything without learning from others .. painful, but its truth..

Yes , that is correct . and no argument !!!

This is only for you : if I trust your knowledge and experience about mini diesel and denial of a diesel engine, I will definitely never be able to make a diesel engine

 

[awake]

Minh Thanh, the way I am reading this, you are seeking to test the advice given here through direct experimentation. Is that correct? If so, then I say, well done!

To all: A forum like this can generate an overwhelming amount of helpful advice ... not all of which is consistent, and some of which will run counter to one's own experience. I don't hear Minh rejecting anything, but rather taking it all under consideration. He is not a beginner, not by a long shot. I have learned from his previous experimentation with diesel injectors, and I am sure I will learn from his experimentation with crankshafts. Testing to failure is a time-honored practice!

 

[Steamchick]

Fair comment Andy. But in such modern times of engineering as we live, it is normal practice to test the design by calculation FIRST, before cutting metal. (At least, that was what I was taught in the 1970s, and by "modern" times, I suggest that is post Euclid, Pythagoras, and Archimedes).
So to check the effect of the stress concentrations versus machine tools used and scoring of the part can lead to a possible failure cause that has not been promoted by others.
Making another bit will not test the failed part. Study and calculations can...
Hence my approach to the problem.
If my approach is inconclusive, no metal has been cut and the engine not damaged further.
So from that perspective it seems a fair starting point, or at least that is my experience... though experience of some contributors is doubted by others.
Not my problem
K2

 

[lee webster]

I haven't built a model engine, yet! But I do own an Austin Seven of 1930 vintage. The A7 4 cyl engine only has two main bearings, one at each end. The bit in the middle is unsupported and it flexes. Later engines had a middle bearing. Depending on how the car is driven, the crank may flex a lot, as in the engines of those who race A7's. They are fully aware that the crank will flex and they relieve the big ends by scraping the bearings to allow for the flexing . Some A7's are still using the original crank, it might have been ground once or twice, who knows. The A7 crank is also held in place by the front pair of ball bearings therefore allowing end float at the back, where the flywheel pushes against the end of the crank introducing more flex. I wonder if the journals on this crankshaft could be too thin and also should there be a bigger radius between the journal and the web? And to finish I have to admit that many A7's have broken their crankshaft because of this flexing, and many A7's are driving around with a fractured crankshaft that could break at any time.

 

[Longboy]

Hello Minh. Can get overwhelmed with assistance / solutions by others and mixed in with such disappointments as time making a 4 cyl crankshaft to see a major part go away in a few minutes of run time makes a hard day. The cause unknowns make for a lousy day too. So I believe your design is routine solid. 3 bearing crank with adequate rod journal diameters. No binding by the rod bearings and no overloads upon the crank such as driven accessories or oversized flywheel. The machinist's advice is correct using the premise that the material was defective and fractured. The remedies of using different steel stock and blend a crank throw radius into the webs. Now sometime a premise after the fact is just a gut feeling, more so if this is the 1st incident of it's kind. You may just get a different batch of the same steel, remake the crank the same way and the break will never repeat......and we all need that kind of luck to blend in with our skills and knowledge.

 

[Klrskies]

Broken crankshaft !?
Last night, I just assembled the engine and did some tests...
And the crankshaft broke,
It seems that the cause is the material
Have you ever encounte

Broken crankshaft !?
Last night, I just assembled the engine and did some tests...
And the crankshaft broke,
It seems that the cause is the material
Have you ever encountered this case ?
View attachment 135937View attachment 135938

Hi Minh,
Before the days of computer analysis, small airplane engines had problems with breaking crankhafts as propellers got larger and heavier. Every time a cylinder fired, it produced a pulse that caused the crankshaft to twist a tiny bit as it tried to transmit it's force to the propeller. Acting like a spring, the crankshaft would flex a little as it tried to rotate the heavy propeller, then spring back the other way as the propeller sprang forward a little bit. With multiple cylinders generating this effect, the crankshaft could have an un-expectedly short life due to fatigue of springing forward and springing back small amounts.
The crank was acting as a spring, resonating like tuning fork, except it resonates along its axis, Torsional resonance. It can become critically severe if all the frequencies going on reach a frequency that resonates together and magnifies the effect...critical frequency.
The engineers at the time increased the journal diameters and corner Radii to combat the torsional frequency. Longer crankshafts with small journals are more at risk. changing from inline engines to "V" and horizontally opposed configurations shortened the cranks and bigger journals stiffened the crank to reduce the resonance...A fix by trial and error from a simpler time.
regards,
Ken

 

[Noel Gordon]

Hi Minh Thanh,
In my humble opinion you are 100% on the right track, you look,listen and learn.
You are sitting down and trying to work out what may have caused your crank to break. I have been an Engineer for 60 years and I still sometimes make a mistake BUT I still try to learn something new everyday.
You are heading in the right direction my friend keep trying. Noel.

 

[minh-thanh]

Noel Gordon !

Hi Minh Thanh,
In my humble opinion you are 100% on the right track, you look,listen and learn.
You are sitting down and trying to work out what may have caused your crank to break. I have been an Engineer for 60 years and I still sometimes make a mistake BUT I still try to learn something new everyday.
You are heading in the right direction my friend keep trying. Noel.

That's the path I'm on. Listen to everyone's opinion, eliminate the possibilities, find the real cause
Thank you !

I think this is one of the causes of the broken shaft
It also appears in the shaft part
scratches caused by two materials sticking together
Maybe the tolerance is too tight,

 

[minh-thanh]

lee webster !

I haven't built a model engine, yet!

Do not worry about that
I'm very comfortable with any opinion, even if it sounds crazy

 

[minh-thanh]

Hi Andy !

Minh Thanh, the way I am reading this, you are seeking to test the advice given here through direct experimentation. Is that correct? If so, then I say, well done!
Testing to failure is a time-honored practice!

Also, I want to check a few things, ...
and it gives me some basic information , maybe not exactly, but just enough
To make this project :
Diesel engine : Kromhout Gardner 4LW 1940

 

[Mechanicboy]

Hi Andy !



Also, I want to check a few things, ...
and it gives me some basic information , maybe not exactly, but just enough
To make this project :
Diesel engine : Kromhout Gardner 4LW 1940

Diesel engine with 3 bearing bearings is not a good idea as the crankshaft bends and twists. the crankshaft must be supported with 5 bearings in a 4-cylinder diesel engine. The crankshafts must be strong, not undersized as you did with your crankshaft.

 

[minh-thanh]

Hi All !
my idea to test...

Any comments are welcome !
Thanks .

 

[TonySteamHobby]

Although I haven’t broken anything yet, I see I have been turning my cuts way too sharp without a nice radius. This thread has sent me reading up on stress raisers, stress fractures, and material failures. Now I feel like I have just enough information to be dangerous. My research has raised a few questions.

I too did not see torsional stresses in the break, I remember what those looked like when my Austin Healey, rear wheel, shaft twisted off just outside the rear seal.
The edges of the broken journal show lines radiating, what appears to be, from the outside towards the middle. Are these the stress raisers, from flexing?

Are we assuming this is a “brittle fracture” failure, not a “ductile fracture”?

Mechanicboy suggested an undercut of the large diameter journal to create a smooth radius, I have done this when cutting screws for threading (when running threads to the head) and have wondered if the reduced diameter weakens the part at the head? So does the smooth radius spread out the stresses and negates the smaller dimension?

On a personal note, I have three different degrees, however I am sure I learned an equal amount, or maybe more important information from OJT,
(on the job training) from the “old farts” (no disrespect intended) I worked with and continue to hang out with. (Unfortunately, none of my degrees were in mechanical engineering…boohoo.)
When I was training the young guys that replaced me, it was hard, sometimes, to not sound condescending when trying to impart my years of experience. Now that I’m retired I hope they appreciate that I was just trying to help them out.

 

[minh-thanh]

Tony !
A lot of questions that I can't answer for you, hopefully someone more knowledgeable will answer .

Thanks !

 

[Longboy]

.......I think this is one of the causes of the broken shaft
It also appears in the shaft part
scratches caused by two materials sticking together
Maybe the tolerance is too tight,

View attachment 136022

Possible lubrication issue? Are you leaning to a dry bearing grabbing the throw and twisting it off?

 

[Steamchick]

Agree. That bearing seizure looks like inadequate lubrication. Remember, lubrication needs to be continuous and forced on running engines to COOL bearings (carry heat away in the hot oil from the bearing) as well as lubricate the bearing surfaces. There should be NO contact witness marks from an engine that is effectively new. The lubrication should prevent that.
There are many causes of innadequate lubrication, wrong oil (type), wrong pressure (feed), wrong flow (feed), wrong viscosity, comtamination, ageing of oil, wrong design, etc.
K2

 

[Mechanicboy]

Agree. That bearing seizure looks like inadequate lubrication. Remember, lubrication needs to be continuous and forced on running engines to COOL bearings (carry heat away in the hot oil from the bearing) as well as lubricate the bearing surfaces. There should be NO contact witness marks from an engine that is effectively new. The lubrication should prevent that.
There are many causes of innadequate lubrication, wrong oil (type), wrong pressure (feed), wrong flow (feed), wrong viscosity, comtamination, ageing of oil, wrong design, etc.
K2

See at the Briggs and Stratton are built without oil pump. The oil is splashing around in the crankcase by connecting rod with scop or by splashing wheel, no problems with the bearings when the lawn mower is working on the high grass. The old car engine in early era did not have the oil pump, same manner as the Briggs and Stratton engines. The O.S. 4 stroke engines from 1 cylinder to 4 cylinder and 5 cylinder radial engine are built without oil pump. Lubricated by oil in fuel where the oil is blown past the piston/cylinder into the crankcase as oil fog then all bearings are lubricated.

 

[Steamchick]

Hi Jens, If designed and kept supplied with good oil I agree.
But I don't know the design of the oil supply to Minh's engine. Just the prices for higher stressed modern engines...
2 strokes with oil mist passing through the crankcase are cooled by the airflow. And are not normally plain big-ends, which would be likely to overheat. (Which is how the plain bearing small ends fail from insufficient lube when they are tuned).
Designed with oiling to suit the bearings is a good idea, tested and proven by manufacturers. But Minh has not told us how his crank is lubricated.
The bearing looks like the lubrication is innadequate - to me. I cannot say "why?".
Just offering some experience and a related opinion, which is what he wants?
K2

 

[minh-thanh]

Longboy !

Possible lubrication issue? Are you leaning to a dry bearing grabbing the throw and twisting it off?

Lubrication problem: I think it's fine ( just based on experience )
Just use my hand, but it takes force to pull it out

Hello everyone to my engine debugging adventure
How I do my engines:
Finish the part , assemble and check if it is smooth with other parts , ...
.......
Last step : Clean and assemble the engine , I usually check the smoothness again , if there is anything abnormal I will check , re-align
But this time it seems I was overconfident and sloppy in the last step
Gearbox is stuck

 

[minh-thanh]

Hi Jens, If designed and kept supplied with good oil I agree.
But I don't know the design of the oil supply to Minh's engine. Just the prices for higher stressed modern engines...
2 strokes with oil mist passing through the crankcase are cooled by the airflow. And are not normally plain big-ends, which would be likely to overheat. (Which is how the plain bearing small ends fail from insufficient lube when they are tuned).
Designed with oiling to suit the bearings is a good idea, tested and proven by manufacturers. But Minh has not told us how his crank is lubricated.
The bearing looks like the lubrication is innadequate - to me. I cannot say "why?".
Just offering some experience and a related opinion, which is what he wants?
K2

Lubrication problem: I think it's fine ( just based on experience )
I usually put the oil level about 3 - 5 mm highter the bottom of the Con rod ( Again : just based on experience )

https://www.homemodelenginemachinist.com/threads/amount-of-lubricating-oil.33668/