Cracked Cast Iron Shaft - Opinon Needed

[MountainCat1M]

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

 

[Metalguy]

Hard to tell from those photos, but it appears that you MAY have a common fatigue-craking failure. There are probably multiple crak-initiations locations around the extreme OD of the fracture, but I'd need a much better photo to say for sure.

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?

 

[unclesyd]

Is that a change in section at failure point? It looks like a hex in the picture.
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.

 

[MountainCat1M]

Thanks for the replies. I've scoured sled forums and others with the same shaft have had it crack. I figured who better to ask but professionals in the area. My dealer doesn't know if it is a bad shaft, just that it should not crack. Right now the shaft is at the dealer, but when I go back to get the sled I'm taking better pics and a closer look. I can post an update then. I'm paranoid now to trust the sled as it might not get me home. I am just the type that needs a solid answer to why this happened, and also a solid shaft haha

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

 

[desertfox]

Hi MountainCatM

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

 

[metengr]

MountainCat1m;
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.

 

[TVP]

I'm somewhat surprised to see that the driveshaft has a hexagonal section, as that will result in an irregular stress distribution under torsion. When you get a chance, definitely post some additional photos-- images from the side as well as along the axis of the shaft will help complete the view for us. I would assume that fatigue is involved, but the images need to be higher quality to determine for sure. Changing to a round shaft will probably be a significant improvement.

 

[Salvatio]

hm... this drive shaft does look weird... I'm not really into mechanical or automotive, but why is it in hex shape? such that it fails faster and that ppl buy new ones? hah..

 

[metman]

MountainCat1M,

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

 

[arunmrao]

Mountaincat1m ,
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

 

[aviat]

The reason that the shaft has a hex is that the pulley is a variable speed drive and it has to move side-to-side as well as rotate. These machines are extensively tested before going to market, but like everything else mechanical, there can be defects or design problems. The dealer should have sent the broken shaft back to the OEM, because if there were a problem with other machines, they would become aware of it.

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!

 

[MountainCat1M]

Thanks for the replies.

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

 

[Salvatio]

hm.. I do agree with aviat. If it's a new snowmobile, how could the shaft break in short usage time? But from those pictures, could it be a ductile-brittle fracture? Since that it does looks like a fatigue fracture, with both bright and rough fracture surface, ductile-brittle fracture sometimes too looks like that. Maybe the temperature while MountainCat1M was riding fell just a few deg C below the transition temperature?

 

[metman]

Salvatio,
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

 

[Metalguy]

Metman,

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.

 

[Salvatio]

Hmm.. but if you'd looked closely, doesn't the pics showed the crack initiated from one of those hex corners?

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?

 

[Metalguy]

Is the shaft Al? Yes, Al and its alloys do not have an actual "fatigue limit" like steel and Ti do. The good news is that Al fracture surfaces hold fatigue striations very well, so that any decent met. lab. can tell you a LOT about the fracture--assuming someone hasn't rubbed the fracture surface with something.

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.

 

[Salvatio]

Oh OH Oh... sorry... almost forgot! the shaft is not Al alloy! would it?
haha.. sorry.. my mistake...

 

[NickE]

Notice the red-rust on the hex-shaft in the closeup photo, definately not Al. I also dont think that an Al shaft of this size would hold up to the torque. (Or have the required stiffness)

nick

 

[MountainCat1M]

Hey all...

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