I think, on the one hand, it is likely that the quality of the material, due to the material is not hard enough, or too hard, does not meet the application of the material, it is easy to bend, deformation, and so on. I once encountered this problem, and later replaced the product with better materials, and only later did not break.
Broken crankshaft !?
- Thread starter minh-thanh
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Help Support Home Model Engine Machinist Forum:
Hi Tony Steam Hobb: your comment: "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? ": As I have not spotted a reply from Mechanicboy yet - and I'm sure he'll help and correct me where I go wrong... - here's my understanding of what stress raisers/reducers do:
To look at a "sharp cornered journal" of 10mm diameter, on a shaft of 12mm diameter: Assuming the "sharp corner" is 0.25mm radius: The stress raiser would be extracted from a standard table using D/d, and r/d, where D = Larger shaft diameter, d is smaller shaft diameter, r = radius of tool/corner.
So D/d = 12/10 = 1.2, and r/d = 0.0025
For Bending: stress concentration factor (SCF) is something like 2.25~2.3.
For torsion, the SCF is something like 1.9.
And CSA ( area that is stressed = Pi x d-squared /4) = 78.5sq.mm.
Now if we undercut the corner by 1mm to make a radius of r=0.5mm, and d = 9mm, r/d becomes 0.0556, D/d becomes 1.33.
This moves us along the graphs of SCF values so we get:
Bending - SCF = 1.9, Torsion SCF becomes 1.65. So the SCF reduction is about 17.4%, Torsion SCF reduction about 13%.
The CSA of 63.6 sq.mm has an effect of increasing the stress by about 25%.
However, if you repeat the sums with a root diameter of 9.5mm, by taking the 0.5mm radius into the side wall a touch, then the D/d becomes 1.26, and r/d becomes 0.0526, which are small changes, yet the stress increase from the 10dia to 9,5dia is only about 11%.
Therefore there can be a selection of dimensions where the stress is increased - by reducing the diameter locally to permit a stress-reducing corner radius. And there is a selection of dimensions where the stress is reduced by choosing a significant change of SCF reduction against the stress increase of reducing the root diameter.
Correct selection of dimensions - by such calculations - are what make a good bit of Engineering when designing parts.
Ignorance is bliss, but sharing knowledge and understanding is (IMHO) simply beautiful and leads to more long-lived happiness. (Fewer failures).
"Here endeth today's lesson".
I hope it helps?
To look at a "sharp cornered journal" of 10mm diameter, on a shaft of 12mm diameter: Assuming the "sharp corner" is 0.25mm radius: The stress raiser would be extracted from a standard table using D/d, and r/d, where D = Larger shaft diameter, d is smaller shaft diameter, r = radius of tool/corner.
So D/d = 12/10 = 1.2, and r/d = 0.0025
For Bending: stress concentration factor (SCF) is something like 2.25~2.3.
For torsion, the SCF is something like 1.9.
And CSA ( area that is stressed = Pi x d-squared /4) = 78.5sq.mm.
Now if we undercut the corner by 1mm to make a radius of r=0.5mm, and d = 9mm, r/d becomes 0.0556, D/d becomes 1.33.
This moves us along the graphs of SCF values so we get:
Bending - SCF = 1.9, Torsion SCF becomes 1.65. So the SCF reduction is about 17.4%, Torsion SCF reduction about 13%.
The CSA of 63.6 sq.mm has an effect of increasing the stress by about 25%.
However, if you repeat the sums with a root diameter of 9.5mm, by taking the 0.5mm radius into the side wall a touch, then the D/d becomes 1.26, and r/d becomes 0.0526, which are small changes, yet the stress increase from the 10dia to 9,5dia is only about 11%.
Therefore there can be a selection of dimensions where the stress is increased - by reducing the diameter locally to permit a stress-reducing corner radius. And there is a selection of dimensions where the stress is reduced by choosing a significant change of SCF reduction against the stress increase of reducing the root diameter.
Correct selection of dimensions - by such calculations - are what make a good bit of Engineering when designing parts.
Ignorance is bliss, but sharing knowledge and understanding is (IMHO) simply beautiful and leads to more long-lived happiness. (Fewer failures).
"Here endeth today's lesson".
I hope it helps?
Yes that does help. I randomly picked some diameters and various radii (to make something which looked like Minh’s crank) and then modeled them in Freecad FEA. I was getting various results, but not what I expected. I was mostly expecting the stresses to move away from the radius but it did not…As I mentioned, I’m not formally trained in mechanical engineering and I “learn as required”. That’s why I visit the forum as my first order of business daily. Minh’s question has opened up a whole new area of learning for me!
Unfortunately, Freecad only let me model the bending stress and not the torsional stress, as seen above. Back to the drawing board…
Thanks for the detailed reply.
Unfortunately, Freecad only let me model the bending stress and not the torsional stress, as seen above. Back to the drawing board…
Thanks for the detailed reply.
Thanks Tony. Makes my day when someone says I helped. (A rare event?).
Ask if you need more semi-accurate drivel from me!
Your CAD modelling is way ahead of my knowledge. I just know simple equations and numbers.....
K2
Ask if you need more semi-accurate drivel from me!
Your CAD modelling is way ahead of my knowledge. I just know simple equations and numbers.....
K2
Hi All !
An update :
I already know the cause of the broken shaft.
I want to learn more, so
I followed the suggestions
One test / in 3 time tests....the cause is probably machining
I am looking for more reasons
I forgot to take a picture when it broke, so it rusted a little
An update :
I already know the cause of the broken shaft.
I want to learn more, so
I followed the suggestions
One test / in 3 time tests....the cause is probably machining
I am looking for more reasons
I forgot to take a picture when it broke, so it rusted a little
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Minh-Thanh,
I have attached a file on steel manufacture.
I wrote this specifically for customers who bought "cheap" welding wire - it's cheap for a reason and that reason is generally not good.
There are many ways ranging from cheap and nasty to excellent.
Look at the difference in performance between continuous cast, ingot cast and electroslag refined.
With each improvement in quality, the strength and fatigue resistance improves but machinability get tougher.
Logical conclusion, free machining steels are not necessarily your friend here.
The problem is a steel manufacturer has hundreds of corners he can cut - to find out which can be difficult.
If your steel supplier can sell you continuous cast or billet cast - go for billet cast. More likely he will ask you where you parked your spaceship as they mostly have no idea.
Continuous cast bar tends to have at it's core long coarse epitaxial grain structure - lousy in torque and fatigue resistance if you machine down into it - I think I see evidence of this in your photos.
Regards, Ken I
I have attached a file on steel manufacture.
I wrote this specifically for customers who bought "cheap" welding wire - it's cheap for a reason and that reason is generally not good.
There are many ways ranging from cheap and nasty to excellent.
Look at the difference in performance between continuous cast, ingot cast and electroslag refined.
With each improvement in quality, the strength and fatigue resistance improves but machinability get tougher.
Logical conclusion, free machining steels are not necessarily your friend here.
The problem is a steel manufacturer has hundreds of corners he can cut - to find out which can be difficult.
If your steel supplier can sell you continuous cast or billet cast - go for billet cast. More likely he will ask you where you parked your spaceship as they mostly have no idea.
Continuous cast bar tends to have at it's core long coarse epitaxial grain structure - lousy in torque and fatigue resistance if you machine down into it - I think I see evidence of this in your photos.
Regards, Ken I
Last edited:
In terms of machining, I'm sure you're already thinking about stress risers caused by sharp corners. I would also be looking at how much flexing the part is experiencing - either due to insufficient bearing support, or to something not-quite-aligned, or so on.
Hi Minh-Thanh. This is almost a text book failure by torsion. The near circular witness marks showing the line between sheared crack and failed core shows circular because of the torsion,with a bit of distortion in the region where additional bending stress took the total stress beyond the strength of the steel. The initial crack probably developed at the sharp crier between journal and web just at the point where the bending stress was maximum. It is a bit out of focus so I have to guess a bit. My camera is not as good as yours, so I understand the limitations of photography! (I once had to buy a video camera to fit onto a microscope to present magnified sections to a group of people by TV, instead of each looking down the lens individually! The weakness was the camera and TV for the required resolution.).
What more do you need to know?
You design needs a re-think to reduce the stress raiser of the sharp corner.
Not a machining problem, but a design that will always break at that point given enough cycles of fatigue.
"Halve" the stress, by use of a lower stress concentration factor (change 0.1mm rad to 1mm radius in the corner), and you will get 10 times the durability before failure, putting it simply. I understand fatigue is a logarithmic function, but I need an expert in fatigue to correct my inaccurate knowledge of a proper relationship.
Have fun.
K2
What more do you need to know?
You design needs a re-think to reduce the stress raiser of the sharp corner.
Not a machining problem, but a design that will always break at that point given enough cycles of fatigue.
"Halve" the stress, by use of a lower stress concentration factor (change 0.1mm rad to 1mm radius in the corner), and you will get 10 times the durability before failure, putting it simply. I understand fatigue is a logarithmic function, but I need an expert in fatigue to correct my inaccurate knowledge of a proper relationship.
Have fun.
K2
Hi !
Thank you for the comment.
I will ask the seller for more information ( When I bought it they said it was the best - and also the grade ) I am using C45 steel
EN 1.0503 Material C45 Steel Equivalent, Properties, Composition
Thank you for the comment.
I will ask the seller for more information ( When I bought it they said it was the best - and also the grade ) I am using C45 steel
EN 1.0503 Material C45 Steel Equivalent, Properties, Composition
It is not an sharp angle, it has a radius of about 0.8 mm.
That's the same thing I'm thinking
Bentwings
Well-Known Member
Our race car hemisphere have substantially guilt radius enough that bearing shells have to be reverse trimmed I made a fixture to do this in a 6” lathe tedious work as there was a lot of hand polishing when done . Cranks regularly crack you can easilyvtestvbyvtappingvcranknwith hammer and listen for ring full mean toss it . Fork over another 10 grand for new crank . Might as well toss rods pistons and pins . Add another 8-10 grand . Hope you win or sponsor comes through LOL I NEVER BROKE A MODEL CRANK ONLY RODS AND PISTONS LOTS IF BITRO
Bentwings
Well-Known Member
I forgot about built up cranks . These may not ring very well most model engine built up cranks are heavy press fit So ring after assembly my opinion if you make model cranks from 4130 4340 or 8620 you should not have issues. I would not use leaded steels they machine nicely but having to make a new one to replace one would not be fun I don’t think I’d bother with cast iron for a high performance engine . Save it forvthe steamer people
Richard Hed
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In the Philippines there is a brand of welding rod called wipweld which will not weld at all. I do not know what itis made from but it is sold all over the place and people buy it. I've told many businesses to stop carryng it, but it's the Philippines where the only thing that matters is to separate one's $$ frm their pocket. Even if I put my welder on it's highest setting I cannot strike an arc. If I can't strike an arc, I know others would have a very difficult time doing it, so why is that wipweld still in existence? Very strange ?!
If that was happening in this country, it would be out of business the first week.
Indeed.
I look for beach marks and micro polishing from the beginning of crack propagation. Sometimes you can see how many fatigue cycles it took to propagate in order to find out if it was catastrophic or from long-term fatigue. It sort of looks polished along the outer rim. The failure looks brittle too without the typical plastic deformation swirly down the middle. The stress concentration on those sharp edges and the hard, unpolished material would contribute. If the metal is torn more than cut when machined, the residual tensile stress will be additive to the loads.
I often will grind a small radius to my HSS cutters, so there's always a smooth transition. Parting tools are the worst offenders.
The original break looks to me more like a brittle failure, rather that a fatigue one.
Mechanicboy
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Hi Minh-Thanh...
This steel is best suited for crankshafts, can you get hold of this steel AISI / SAE 4340 ALLOY STEEL?
It is possible to ask the material dealer if they can order the steel for you in the desired quantity.
https://steelforge.com/alloy-steel-4340/
I have searched for information about that steel , there are quite a few companies supplying that steel in Vietnam . but right now I can't contact them due to the traditional Tet holiday / Lunar New Year
I think it's probably not the steel (still my guess)
But I will keep that steel in mind when I make a crankshaft or camshaft next time
Thank you !
Mechanicboy
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Steamchick..
There are some other properties in bolts that are exposed to stretching and twisting.
The crankshaft is exposed to bending and twisting.
The undercut must not be deep and rounded from edge to edge.
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