TS TIME BOMB

[GTV-074]

Wow thats really taken a beating.....

Cracks on the gear and the shaft looks all chopped up...

Nasty...

[Colin Edwards]

Hi MD,

Great photos.  Now we are getting somewhere!!

The fractures explain how it failed or at least the cause of the loss of interference fit.  Just now need to explore why did the sprocket boss fracture?

The design in the two most recent images is significantly different to that in the earlier photo.  The sprocket boss wall thickness appears much thinner.  Are these sprockets from similar engines?  Which is the earlier / later design?   Both designs feature "lobes" 180⁰ apart..........curious!   The failed sprocket "lobes" are significantly smaller than those in the earlier photo.

All other things being eaqual, the failed sprocket is definately more likely to fail exactly as it has than the one in the earilier image.  The location of the fractures is no coincidence! 
Implementing a purely interference fit in this instance is not the issue.  The realitively thin wall thickness of the boss is very likely the issue and possibly the root cause of failure.  The interference fit causing the relatively thin wall to stretch beyond its elastic limit. 

The combination of wall thickness variation and direction of rotation probably determined where the stress is concentrated and where the likely fractures would occurr.  Now if the boss featured a consistant wall thickenss and say the OD was that measured at the lobes............................................!?!

[Duk]

The problem could be a result of stacked tolerances.
Where both the bore of the sprocket and the diameter of the shaft are both within their respective tolerances, but if you did the numbers for the fit, that'd be incorrect.
The broken shaft boss on the sprocket could well have been a result of a fit that was too tight.
That is, the shaft was machined on the high side of its diameter and the bore in the sprocket on the small side of its tolerance.
With the sprocket blank looking like it was cast rather than forged, the grain structure is less dense and more likely to crack.

[GTV-074]

MD, whats the flip side of the larger failed gear look like?

There is a new one for sale on evilbay at the moment and i'm just trying to see what the other face looks like.

Cheers,

Paul.

[GTV-074]

OK.... Don't worry... i got which side is which now...

Cheers.

[festy]

Hey MD, your sprocket looks fine to me - but I'll swap you for my slightly used Nord one if you like? 

[bteoh]

What would cause them to chip like that? 

[festy]

What would cause them to chip like that? 

The sprocket rotating on the shaft forced the woodruff key to split the hub.
The arrows in the pic below show where the shaft and sprocket *should* line up:


Amazingly, the engine was still running like this. I only discovered the damage when I found the missing chunk of the sprocket hub in the sump pan, and compared it to all the misc spare engine parts I had lying around until I found a match so knew where to go looking