Broken crankshaft !?

[grahamgollar]

Does the engine have a thrust bearing to mitigate end float?

G

 

[Steamchick]

Minh-Thanh, perhaps I mis-understand the mixed messages in this thread, but if some part has failed, it seems crazy to expect a smaller part (smaller journal diameter) to work..??? And then you suggest it will be longer... (even worse!). And you are ignoring advice to use material with better Fatigue resistance...??
I think (from my experience designing stainless steel cranks on much bigger sizes) that the greatest benefit after ensuring everything is true and in-line, should be corner radii to reduce stress raisers. But use a better (stronger and more fatigue resistant steel) as well. Making 1 decent part is surely better than making something that is breaking with fatigue . ..
K2

 

[Steamchick]

Oh... Something not mentioned.... In bending (which propagates fatigue), the crank is working against a fixed end - the flywheel. As it rotates, the flywheel resists the bending of the crank. Dynamically, it is many orders of magnitude stiffer than the last main bearing. Car makers resolve this by making flexible flywheels, with the outer mass bonded to the centre boss with an elastomer joint. This permits a saving of 1mm or so on the crank journal diameter, which is a huge cost saving and fuel consumption improvement. The elastomer allows the crank to flex without fighting against the stiff flywheel. I have seen it as a cure for the main shaft holding the crank, or the first crank journal failure....
So maybe you should try a lighter flywheel? What will the calculations permit?
K2

 

[Steamchick]

Minh, just odd ramblings... I would have expected to see the crank journals much bigger, maybe the same diameter as the mains?
The crank webs could possibly be a bit thinner, but no thinner than the smallest journal diameter...?
Your "sharp" corners may be 0.010 in. radius... or 0.25mm ... with a 6mm journal, the stress raiser is something like 2.6... maybe more, but increase the corner rad to 0.5mm and the stress raiser drops to maybe a 1.9... (my tables are not exact for these parameters). As I don't know your sizes this is just a guide of how to reduce the stress on the journal by about 25%.

So do the sums and check fatigue life related to your material before you cut any metal.
Cheers!
K2

 

[minh-thanh]

. And you are ignoring advice to use material with better Fatigue resistance...??

No, I'm not ignoring any advice!
As I said :" there are many useful comments that I need to think about
But something is wrong and very strange and I don't understand why!? "
I make it even smaller just to find out what I doubt and don't understand
About materials: I think the same materials people are using here are good enough

 

[minh-thanh]

Does the engine have a thrust bearing to mitigate end float?

G

I don't understand what you mean, but here is a picture of the assembled crankshaft

 

[Steamchick]

Ok Minh Thanh, I am sorry you have had to resort to "bold type" to reiterate your opinion. Please "learn and be patient". What seems strange to you seems less strange to many who have written their advice and opinions here. Some of us have done these jobs - and learned these lessons already - in our professional careers, and have realised the solutions (obtainable from mathematics and trial and error !) are easier than you expect.
see stress concentration table attached:
Using D = the diameter of your crank web, and d = the diameter of the crank pin, calculate D/d.
Then take the radius of the cut at the smallest diameter of the journal as it starts to bend around to meet the crank web. If you have a sharp pointed tool this radius can be VERY small, but looking at your photo in post #1 I think you need to appreciate that a single cut-mark is where the cracks start to propagate in fatigue failures. Do you not round-off the corners of your tools when you grind them? - Especially the tool for the finishing cuts of these journals.
Using the TOOL radius as "r" calculate r/d.
Now refer to the diagram, and for a line relating to d/D, find the Kt for your r/d. now consider a larger "r" = corner radius - say 1mm? - then for that r/d you can get a different Kt. That way you can SEE how the stress will drop (dramatically?) with just a small change of "r". It may mean that you should make the crank webs 1mm thinner, so you'll have to check the stress calculations for these, and STIFFNESS relative to the STIFFNESS of the journals, But I doubt there will be an issue with the thinner crank webs.

WHEN you do machine a new crankshaft (whatever design you decide to use) I recommend you polish the journal and corner radius so there are no visible machine marks. The easy way to do this is to not have "sharp-cornered" tools - avoiding the tool marks apparent in your photo. Stress always finds the point where the "skin" of the material is "damaged", so it can start a crack and fatigue the part to failure.
I hope you find some understanding here, and I have not offended you. That is not my intent.
K2

 

[minh-thanh]

Hi K2 !
Thank you information !

I hope you find some understanding here
K2

Sure !!!


I've had material problems before
The first time, I made the cylinder out of steel, everything was perfect, until I tried compressing it, it has compressed only about 1/3 of the length of the cylinder - from TDC
I don't understand why, but when I fully immersed the cylinder in the oil, I discovered a leak in the cylinder body. An error
The second time, I made the cylinder out of cast iron, but this time there was no compression at all. repeat the test, A long crack, hard to see
I want to make it smaller because I want to know if it's not the material or not. If I do the same with the broken one, I won't be able to identify anything.

 

[Mechanicboy]

Minh- Thanh,.. To understand bending and torque of the crank shaft, see this picture..

 

[Steamchick]

Excellent Jans. That says it in a couple of pictures. But as well as torsional bending - especially against the torsional inertia of the flywheel - a light flywheel will permit the flywheel to "wobble" on its axis, and not concentrate stress on the first and second journals (beam loaded with hinged ends). Whereas the massive flywheel forms a stress concentration at the adjacent 2 journals like a cantilever loading from a fixed end (the massive flywheel). Hence my description of the car engine flexible flywheel.
Hope this helps? Your pictures help me (even though I have done the calcs decades ago). Just one add en dum, I would make the webs at least as thick as the diameter of the thickest journal, but it doesn't need to be much thicker.. just what the bending calcs require - with consideration for stress concentrations as sections change profile, etc. - and with considerations for stress limits being reduced according to fatigue stress versus lifetime for the material.
Fun this, isn't it?
K2

 

[Mechanicboy]

Steamchick

As a general rule, the diameter is the same for both the main pins and the crank pins so we do not have problems with bending and twisting that lead to breakage. With a slightly smaller diameter of the crank pin, the length should not be longer until it can bend the crankshaft at forces from the combustion pressure and flywheel.

Damping discs used on one side of the crankshaft are used to reduce the load on the crankshaft (I have been a car mechanic since 1993).
State-of-the-art vibration dampers either have a spring coupling (rubber damper) or no spring coupling (viscosity damper).

Old engines often had 2 equal flywheels to dampen vibration, turning the crankshaft since the crankshaft did not have a large diameter in both the crank pin and the main shaft.

New descendants still have a lot to learn that we give to them here in the forum based on our experiences.

 

[minh-thanh]

Hi Jens , K2 !
Thanks !
With Small Engine, Designing the crankshaft to fit everything in the block engine, including the connecting rod's turning diameter, it's not as easy to do everything as the big engine.
It's not impossible, but the point is that it will change the shape of the engine that I aim for
But, I will keep this in mind. With my design V12 it has a 19mm cylinder - more powerful - and it has space to make things bigger
With little experience , I am sure with 5mm diameter rod journals , It is fine .
I tend to make things bigger than what I know : ok, it's ok with 5mm , I'll make it bigger : 6mm
As I said:
"But something is wrong and very strange and I don't understand why!?"
I have probably identified more than 90% of the causes of shaft fractures
I need to check more

 

[minh-thanh]

New descendants

???????????????????????????????????????????

Are you sure ???

 

[Steamchick]

Hi Jens, I was taught that cranks are essentially beams, supported at the mains and loaded at the big-ends, and the bearings suffer different loadings so need to be greater than different minimum sizes. On all full sized "modern" engines I have seen larger mains than big-ends. But I understand this is for bearing sizing rather than stiffness, yet the big-ends are "reduced" to their minimum because of reciprocating and rotating mass constraints. On many modern full-sized engines the webs are a complex shape - rather than the simple "single thickness" of our models.
And the whole is "tuned" using FEA modelling. But I don't have the computer power, nor brain, nor training, to go there.
But on our models, we should be aware that beam stiffness is as much a feature of length as diameter, so a short slightly smaller Big-end journal can be as stiff as a longer main that is slightly larger (yes, I know there is a cube rule of the radius for stiffness).
But in doing so, the big-end journals still have to be capable of the combined bending and torsional stresses, so an analysis of each journal is necessary.
The "Big-one" that I experienced, was when we received an expensive design change on an upgraded engine, as the (gyroscopic inertia) mass of the flywheel became too "stiff" so the "bending beam with hinged ends" analysis was superceded by the model using the flywheel as a "fixed" end and the crank as a cantilever beam from that point. (perhaps I should describe this as a beam fixed at the flywheel end, and free to rotate on the far end?) The flexible flywheel wasn't just like a mass damper (that you describe) that absorbs and smooths the torsional variation, but to "put a hinge" at the end of the "cantilever" model thus making it the beam supported freely at both ends. Thus reducing the stress concentration that caused the long durability fatigue failure.
Probably not relevant, but in the interests of helping descendants learn, I thought it was a useful anecdote? (Sorry my skills of pencil sketching are "flippin' poor" on a keyboard! (MS have changed Windows so many times my "working" scanners are all obsolete!).
K2

 

[minh-thanh]

New descendants still have a lot to learn that we give to them here in the forum based on our experiences.

One more thing I want to tell with you - It's something I really hate to say !!
With My Questions, Get Answers
There are things I know, there are things I don't know, there are things I know and forget, there are things I don't know well..
But I always appreciate the input, because I have learned more, more understanding and more importantly what I already know but it will help others.
I am a person who is very comfortable with opinions and at the same time I have to have my own inferences
"Your experience" I'm not so sure !

 

[Steamchick]

Hi Minh Thanh.
As you still seem confused as to what has happened to your crank: (Maybe you think we are the cranks, and you may be right...? - But we think after decades of working in industry and having met these problems before, then we have an opinion that is worth presenting). Your choice - of course - as to what you do with it. But please be patient and learn from those who have really had this problem and know what they did to fix it?). And perhaps we can only answer with hypothesise, or opinions or even just open ideas, as not all the information is available for us to check the design, material, machining process, running conditions, etc. (e.g. if the engine was pre-igniting the forces would not be as "designed" - but we cannot know that unless you tell us).
You want direct answers, but if we say "You made it wrong" or "you used the wrong material", or "There was another fault in the engine", that does not help. You are asking "WHY? (did it fail)", an entirely different question, which needs us to explain the possible causes of failure. We cannot give exact cause of failure because we do not have all the information to do so. So we express our ideas from our experience. Take it or leave it ("Be patient and learn"?). We are not forcing you to do anything.
Did you examine the cracks under a good lens or microscope to see if the cracks propagated from a tool mark or sharp corner? (40 x magnification of the picture loses resolution - a cheap toy microscope can do 100x magnification to help, if you can borrow one from a child?).

I can't see clearly from just this picture. But the amount of (inconsequential) scoring on the crank-web suggests a sharp-cornered tool has been used there. If the crank-pin is 5mm diameter, then I GUESS the sharp tool has a "radius" of less than 0.2 mm, but as I cannot see the side view of the journal I am guessing at what I am trying to measure. The radius could be as small as 0.06 - at a guess - from what I can see and try and measure off the picture.
But a radius in the corner of 0.2 mm for a 5mm pin would give a stress concentration factor of more than 1.9.
AND a radius of 0.06mm would have a stress concentration factor of in excess of 3: - That means an increase of >50% of the real stress seen by the pin, at some point where there is also a small tool mark that can propagate cracks easily. A real killer for fatigue failures of this type.
In fact, due to the stiffness of the crank web varying in different directions, these SCFs may be way more than "my guess" - even by as much as a factor of 10.
I think that others, as well as I, think the fatigue failure you have is that you are working in the "sloping" zone of the fatigue and endurance limit file picture: I.E. the higher stressed zone where fatigue occurs. You need to be working in the lower stressed zone (below the horizontal line).

My hypothesis is that the "normal" stress on this design of crank is OK (from what you intimate that it has worked before: "I made 4 similar crankshafts, but this is the first time it broke "?) but that a sharp corner, machining mark, or other stress raiser has raised the stress by more than 3 times to induce the fatigue failure. Reducing this stress raiser is not hard, just a small change to tooling and process.
So again I encourage you to find a way of getting a smooth transition by a larger radius from the crank-pin to web with a larger radius on the cutting tool (0.4mm or more?) - and NO SCORING.
And a more fatigue resistant material will be a benefit as well. A steel with twice the tensile strength will have something like twice the resistance to fatigue - perhaps moving you from the sloping zone to below the flat line?
I hope you find some patience and learning from us?
K2

 

[minh-thanh]

Thanks .K2
My way: I learn from everyone, I don't care what engines they have done.
I consult everyone's opinions
The causes of shaft breakage are many: as everyone said , and more : too much power, flywheel, jammed gearbox, misalignment of cylinder and crankshaft, or piston that expands when it heats up and gets stuck in the cylinder....
I need to determine the exact cause, otherwise when I make a new crankshaft it will break and it will cause much worse. and again made a new and broken crankshaft , again ...
I'm very patient, but if someone insults me I won't let it go , and i really hate that .

 

[Steamchick]

Sorry if I wrote something that caused offence. None intended. But reading your replies to many people it seems to me you are rejecting many ideas - without explanation of why their opinions appear wrong to you?
e.g. You asked about "why the crankshaft would break?" - the direct answer was "fatigue and excess stress". Then you appeared to reject suggestions to "reduce stress" and "Increase fatigue resistance". Maybe if you explained your list of possible causes, of the excess stress on the crank, with a comment box of what you think of each cause, and ask for direct comment on any or all of those, or maybe just those you cannot explain, then we can give more direct advice? And perhaps a little more detail of your investigation so far will help us understand your problem, so we can give clearer answers?
And I stand by my suggestion to improve the machining with radiused corners of the crank pin to reduce stress. I know it worked for the hundreds of parts in industry that were OK when made with a larger radius corner on the crank pin. (Equipment that lasts 40 or more years in service must be reliable - and it is still in service after nearly 40 years! - THAT is experience.).
K2

 

[minh-thanh]

K2 !

Sorry if I wrote something that caused offence. None intended.

I'm not talking about you, I'm talking about someone else .

You asked about "why the crankshaft would break?" - the direct answer was "fatigue and excess stress". Then you appeared to reject suggestions to "reduce stress" and "Increase fatigue resistance".
K2

That's all I said :

TonyM !

Yes, there are many useful comments that I need to think about
My test is only for learning more...
But something is wrong and very strange and I don't understand why!?

These are the problems I'm trying to figure out :

Thanks .K2

The causes of shaft breakage are many: as everyone said , and more : too much power, flywheel, jammed gearbox, misalignment of cylinder and crankshaft, or piston that expands when it heats up and gets stuck in the cylinder....
I need to determine the exact cause, otherwise when I make a new crankshaft it will break and it will cause much worse. and again made a new and broken crankshaft , again ...

Maybe my wording is incorrect, I'm sorry

 

[Mechanicboy]

Minh-Thanh,.. Learn, Practice and Patience !! You asked questions and we answed as best we can .. there is a lot one can not know everything without learning from others .. painful, but its truth..

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