Broken crankshaft !?

[minh-thanh]

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 ?

 

[TonyM]

Looks very much like the corner is too sharp causing a stress raiser. You need to radius the corner more to prevent this. I remember doing the exact same many years ago on a 40mm diameter vertical shaft. It had a 200Kg drum on it that was rotated by hand and abruptly stopped every couple of minutes. After a few days it snapped like a carrot. Luckily no one was hurt and luckily for me that the design drawings had been checked twice by my supervisor.

 

[minh-thanh]

Looks very much like the corner is too sharp causing a stress raiser.

I made 4 similar crankshafts, but this is the first time it broke
Do you think it's due to the material?
Looking at the break, it looks like the material is not uniform

luckily for me that the design drawings had been checked twice by my supervisor.

 

[TonyM]

I had six of the vertical trolleys made but only one broke.
Still looks like a stress fracture to me.

 

[minh-thanh]

stress raiser : You are correct !
Today I checked everything again, I discovered a problem causing stress on the crankshaft : gear assembly or something wrong leading to the transmission not smooth - I usually check things when assembling to sure they're smooth - but this time, something seems to be wrong, I'll double-check everything
Thanks for the help !

 

[gbritnell]

Those rod journals look really small to me!

 

[Bentwings]

Those rod journals look really small to me!

Yeah I was going to say I thought it was small my little steamer has only 6 mm shaft and rod journals even though it well supported, it doesn’t have much torque unless it got a massive load of stem . It interesting my early hot tod model airplane enginges all had pretty smal cranks too. I blew a few up but 20k rpm takes its toll on 30 % nitro and high compression . Blew the top of the block off one.
I’d put a larger radius on all the journals . Our race car enginge about 1/4” radius. Big enough that the bearing shells need to be relieved.
Just looking at your pictures it almost looks like it just twisted until it failed . Either a flaw in the material or lack of stress relief. I’ve seen stainless steel machine shafts break in similar fashion that had work hardened in machining then cracks developed . Yours almost looks like a brittle crack. I’ve seen custom boring bars break like that due to over hardness and not tempering properly . Yours may just need larger journals and radius .

Byron

 

[Noel Gordon]

Interesting, what exactly was the material that you used?? It almost looks like a shaft was hardened and not enealed.

 

[Mechanicboy]

All rounds on corner of the shafts make the shaft strong against stress and load.

There is 2 difference rounded corners, see the next picture. The other rounded corner is used in the larger shaft.

 

[jmille7916]

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
The only time that I have ever seen crystallization like that appears to be was years ago on the rear axle on a 1948 chevy, which parted, looking very much like those photos. Also due to bad steel at the time(1948 was not a good year for non-military material such as steel.

 

[Ken I]

That looks like a combination of a fatigue fracture from too sharp a corner radius plus torque or flexing.

Since you were only running trials - where did the stress originate - was it difficult to turn or is there a misalignment in the mains - such that you are running the crank "bent" - this effectively flexes it with every revolution and quickly leads to fatigue failure.

It is also possible that due to stresses in the original stock the crank "bent" during machining - leading to the same problem of flexing.

If you can, roller burnish the corner radii as well this vastly improves its fatigue resistance (the surface is then under compression and the greatest tensile stress then occurs below the surface thereby inhibiting crack propagation from surface defects.)

Also what material and what hardness condition.

Regards, Ken

 

[Gedeon Spilett]

this seems to me he result of a twisting force, heavy flywheel and sudden slowing or increase in speed.
and yes, the journal looks flimsy as compared to the whole crankshaft, and the radius are "mandatory" too.

 

[Badhippie]

This is caused by a couple issues as mentioned above #1 issues I see is the rod pins are to small compared to the mains. So any stress that happens to be transferred through the crankshaft is going to the weak spot thus the rod pins.
#2 Need to get rid of the sharp 90% corners on the main pins and the rod pins. This is where any stress in the crank will find the weakest spot on the crank.
#3 I don’t think this is the issue here but it needs mentioned. Line bore make sure it’s good. But as I said I don’t think it’s a issue
#4. This is not a rotating issue you can see it’s a clean break. There are no signs of this happening with rotational force or twisting.
#5 I don’t think this is a material issue either I work with a lot of different stainless steels on a daily basis. And I see this grain structure on 90% of broken shafts. It’s also not from work hardening of the stainless. Most of the stainless steels have to be cooled in order to be hardened but not all have to be. But I don’t think you could work that small of a piece hard enough to cause hardening and you would also see the stains on the stainless from the heat.

I personally think #1&2 are the biggest issues you have with #1 being the biggest issue followed by #2
Thanks Tom

 

[minh-thanh]

Thanks for the comments .
Thanks a lot .!!
More information :
My engine : cylinder 16mm stroke 16mm
rod journals : diameter is 6mm
material that I used : If I remember correctly it is equivalent to 12L14 steel, not hardened.
The motor only runs for about 3-4 minutes
With about 3-4 minutes of running , I think it does not cause problems with the material - but I not sure
I do a little test
With quite a bit of my force ( my engine can never make it ), With 8 turns : left - about 25 degrees and right - about 25 degrees, , I couldn't break it

And after 14 turns : it broke
It has quite a large difference in the surface at the fault location

 

[Bentwings]

Thanks for the comments .
Thanks a lot .!!
More information :
My engine : cylinder 16mm stroke 16mm
rod journals : diameter is 6mm
material that I used : If I remember correctly it is equivalent to 12L14 steel, not hardened.
The motor only runs for about 3-4 minutes
With about 3-4 minutes of running , I think it does not cause problems with the material - but I not sure
I do a little test
With quite a bit of my force ( my engine can never make it ), With 8 turns : left - about 25 degrees and right - about 25 degrees, , I couldn't break it

View attachment 135956


And after 14 turns : it broke
It has quite a large difference in the surface at the fault location

View attachment 135957

The material you noted is a leaded free machining steel, not noted for extreme strength . A remake even with cold rolled low carbon steel would be an improvement. Any 41 series or 86 seekers would be huge improvement . Much more difficult to machine but much stronger.
Byron

 

[Mechanicboy]

Use high tensile steel or steel from driveshaft from the car is strong enough against fatigue of the stress/load of the engine.

 

[SmithDoor]

All rounds on corner of the shafts make the shaft strong against stress and load.

There is 2 difference rounded corners, see the next picture. The other rounded corner is used in the larger shaft.

View attachment 135943

View attachment 135944

I agree

Dave

 

[minh-thanh]

Use high tensile steel or steel from driveshaft from the car is strong enough against fatigue of the stress/load of the engine.

I don't think it's necessary, it's fine to use the same steel as everyone is using here
The problem is that my engine only runs for about 3-4 minutes and crankshaft broke
I use the same material for the crankshaft on a diesel engine ( rod journals with larger diameter: 8mm )- it's 2x more powerful than this engine, With hours of testing and running and it's fine

 

[TonyM]

Hi Mihn Thanh
There is a lot of good advice here albeit fragmented in all the posts. To summarise(I think) The first thing to get the fracture is stress. No stress no fracture. You are using three main journals So alignment of the bearings(line boring) and shaft straightness and bearing diameter alignment is first step. A small misalignment or offset will cause flexing. The flexing stresses the weakest part at its weakest point. Normally a 6mm big end in mild steel would be ample for that size engine. Although it is the weak point with the sharp corner being the weakest.
Your test is not representative. A very small movement of thousandths of a mm are enough if repeated often enough. Remember that your engine ran for 5 mins @ 2,000 RPM means 10,000 bending movements.
So - Line bore crankcase bearings, Straight shaft, Aligned crankshaft bearing diameters, Radiused corners. Smooth running crank. Should prevent the problem in future.
edit. Just to throw a spanner in the works.
I did notice the two large ''unsupported'' sections in the crank. Have you used these in other cranks. They look like they could be problematic causing an imbalance if the shaft was slightly bent.

 

[minh-thanh]

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!?
And I really hate that feeling
I plan to make a similar crankshaft - same material -but smaller rod journal diameter - maybe 4 mm but will use with v4 engine - it's easy to change - With flywheel bigger and with rod journals 2 times longer it would be a real test .