Cracked Cast Iron Shaft - Opinon Needed
Cracked Cast Iron Shaft - Opinon Needed
(OP)
Sorry I cross posted this...but this seems like the better board.
http://home.thezone.net/~bpriddle/shaft.jpg
http://home.thezone.net/~bpriddle/shaft_closeup.jpg
This is a cast iron drive shaft on my new snowmobile. It is 1 inch thick. It cracked for no reason that I can see. But it has a weird centre core, that is actually off-centre. Can anyone explain or be able to determine by looking at it if this could have been a faulty shaft? The area circled in
the close-up pic also sticks out alittle.
Thanks for any help
http://home.thezone.net/~bpriddle/shaft.jpg
http://home.thezone.net/~bpriddle/shaft_closeup.jpg
This is a cast iron drive shaft on my new snowmobile. It is 1 inch thick. It cracked for no reason that I can see. But it has a weird centre core, that is actually off-centre. Can anyone explain or be able to determine by looking at it if this could have been a faulty shaft? The area circled in
the close-up pic also sticks out alittle.
Thanks for any help





RE: Cracked Cast Iron Shaft - Opinon Needed
Why do you think the shaft is cast iron? If it is, is it ductile iron or what?
The shiny ring area is probably the fatigue-cracked part, and it may have taken quite a while to get to the last, sudden break (the rough/raised part in the center).
How far out in the woods were you?
RE: Cracked Cast Iron Shaft - Opinon Needed
It doesn't appear to be Cast Iron in the photo.
Make sure you don't damage the fracture surface as you might be able to have failure mode investigated.
Are you near an academic or industrial setting where you might be able to find someone to look at the fracture surface?
Have you check the makers website or contacted them as to recalls or other problems. You might pose this problem on one of the Snowmobile websites, preferably one that covers racing.
RE: Cracked Cast Iron Shaft - Opinon Needed
Luckily I was next to a road for easy pickup. The day before would have been a different recovery story!
As for cast iron, I am probably mistaken. It _is_ a hex though. I only have a 70HP sled, so torque is not an issue.
Over the summer I'm getting a 4130 CroMo shaft 1.25 inches thick versus the stock 1 inch. I'll feel better then.
Thanks again...I'll post more info when I get it.
Brad
RE: Cracked Cast Iron Shaft - Opinon Needed
Hi MountainCat1M
I am surpised the shaft is Cast Iron, looking at the pictures its hard to tell for sure but it looks like a
fatigue failure to me.The area that you have ringed does it by any chance have small contours kind of circular a bit like you sometimes see on the beach when the tides going out. Another clue to the type of failure:- If you push the shaft back together do the two parts go together like a jigsaw piece or is there that much distortion they won't fit
snuggly.
regards desertfox
RE: Cracked Cast Iron Shaft - Opinon Needed
Looked at the pics. Looks like the failed end of the hex bar remaining on the hub has a sharp radius. Something as simple as having a generous radius will improve fatigue life. Check to make sure you have a radius or smooth transition (no sharp corners) after repairs.
RE: Cracked Cast Iron Shaft - Opinon Needed
RE: Cracked Cast Iron Shaft - Opinon Needed
RE: Cracked Cast Iron Shaft - Opinon Needed
If you plan on conducting any further failure analysis, do NOT push the fractured faces back together as this will tend to erase/obscure valuable clues about the failure mechanism.
Jesus is THE life,
Leonard
RE: Cracked Cast Iron Shaft - Opinon Needed
I shall summarise the various points suggested by the members. You can then make a new shaft,with the help of these suggestions.
a) The broken shaft material is not castiron,( probably a steel). Do not attempt to repair the broken shaft discard it.
b) Hexagon cross section to be converted into a circular one.
c) At the region where the shaft enters the hub, a smooth transition to be provided.
d) You can use 4130 grade steel material, properly heat treated,and also introduce compressive stresses on the skin by shot peening.
Happy and safe sledding
RE: Cracked Cast Iron Shaft - Opinon Needed
Because this is a new machine it shouldn’t be a fatigue problem because that takes time for those to develop. My guess is that your tracks were frozen down and you gunned it to get going and weakened the shaft and it broke in normal use. I have seen that before.
I would be very careful with the idea of modifying a machine without knowing the reason why it is designed the way it is. The shaft could be designed to be the weak link in the chain on purpose. If you make it stronger, then if you overstress the drive, something else would have to break and cost you more, or put your life at risk.
Happy sledding!
RE: Cracked Cast Iron Shaft - Opinon Needed
First, it is probably steel, not iron. My mistake.
Second, the hex is by design. The sprockets that fit onto the shaft stay in place with this design.
Third, I was doing 10mph at the time, no sudden accelerations etc. There was only 200 miles on this shaft.
I haven't gotten my sled back yet, when I do I will take more pics of the shaft.
Cheers
Brad
RE: Cracked Cast Iron Shaft - Opinon Needed
RE: Cracked Cast Iron Shaft - Opinon Needed
I'd give you star here if we knew the actual failure mechanism because now that you mention it, even with the relatively poor quality photo the fracture face, it resembles one in the low energy examples of impact test samples showing a small core of fibrous/tougher separation and a larger perifery of granular brittle fracture.
It will be interesing to see MountainCat's better photo's.
Very likely no beach marks (only 200 miles)and maybe the typical largely brittle vs small ductile appearance will be evident.
Jesus is THE life,
Leonard
RE: Cracked Cast Iron Shaft - Opinon Needed
The central zone of Charpy impact specimens is where you find the brittle fracture, surrounded by ductile shear fracture.
The shaft could easily have a few hundred thousand revs. in 200 miles, so beach marks would be expected.
RE: Cracked Cast Iron Shaft - Opinon Needed
Do correct me if I'm wrong.. Al alloys usually don't have endurance limit right? If yes, then unless that the shaft is subjected to high stresses, it would take millions of rev before it fractures, if, the shaft has high fatigue strength, which I would liked to believe that designers of the shaft will design in such a way.
Indeed this is an interesting case, which the fracture surface looks like, or may be fatigue fracture. But would would there be any other way that the crack could initiated?
RE: Cracked Cast Iron Shaft - Opinon Needed
Al, steel and Ti can fracture via fatigue in just a few thousand cycles (low-cycle fat.) as well as millions. Depends on quite a few factors, including surface finish, stress concentrations, etc.
RE: Cracked Cast Iron Shaft - Opinon Needed
haha.. sorry.. my mistake...
RE: Cracked Cast Iron Shaft - Opinon Needed
nick
RE: Cracked Cast Iron Shaft - Opinon Needed
New photos
http://home.thezone.net/~bpriddle/crack1.jpg
http://home.thezone.net/~bpriddle/crack2.jpg
There were also line fractures in the middle of the shaft. Like someone bending a piece of wood and it starts to crack. The main thing is warranty covered it. Sorry for late reply, but only got it back today. And thanks for the interest!
Brad
RE: Cracked Cast Iron Shaft - Opinon Needed
That ~central area that looks different than the outer parts of the hex area is all that was driving your sled when it snapped.
What is the large round unrusted area that surrounds the fracture?
RE: Cracked Cast Iron Shaft - Opinon Needed
MountainCat1M,
did you ran over some hard rocks or something like that? It looks like there's a large scratch through the wheel (rim?) which is in contact with the hex shaft. Could this impact be the cause of the failure, where the crack(s) started? If the shaft is ductile enough, the snowmobile may still travel some distance, until the c.s.a reduced so that it finally fails.
RE: Cracked Cast Iron Shaft - Opinon Needed
Thanks for correcting me on my reversal of ductile/brittle zones. The improved photo's clarify and negate the transition temperature/brittle theory asw noted below. Also the cup cone comment is very relevant and the central final failure area appears somewhat fibrous all pointing to dutile failure. Additionally the periferal failure area appears to have some beach marks and the shininess of this area indicates rubbing action of the mating surfaces during fatigue. Also the unusual pattern on the shoulder of transition from hex to larger round section suggests rough machining (or?) marks which could be gross stress risers contributing to fatigue drack initiation.
Excellent photo's MountainCat. Thanks for the feedback.
Jesus is THE life,
Leonard