Ardingly show Part 2.

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Peter Morris
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Re: Ardingly show Part 2.

Post by Peter Morris »

Mick D wrote:Hi



Any chance you could post some good photos of the break surfaces of the sprocket? How're the crank shaft keyways?

Regards Mick
Right here you go Mick,
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Peter Morris
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Re: Ardingly show Part 2.

Post by Peter Morris »

The centre of the sprocket is quite scored.
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Re: Ardingly show Part 2.

Post by Peter Morris »

The woodruff key was still in it's keyway, and both are fine, so the sprocket could not have spun on the axle.
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Re: Ardingly show Part 2.

Post by Peter Morris »

...........but the axle, directly opposite the keyway, is scored along the shaft, very strange!
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clanger9
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Re: Ardingly show Part 2.

Post by clanger9 »

Very interesting failure! It looks like the sprocket has been chattering on the shaft and eventually fractured in two.
I wonder if the retaining nut wasn't fully tight?
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Peter Morris
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Re: Ardingly show Part 2.

Post by Peter Morris »

clanger9 wrote:Very interesting failure! It looks like the sprocket has been chattering on the shaft and eventually fractured in two.
I wonder if the retaining nut wasn't fully tight?
Thanks for your reply Paul. The rotor nut was tight, (It had only just been put back together, and not run). I guess I could have done something wrong but this is FAR from the 1st time I have had this side of the bike apart and I've never had problems before.

I'm not sure the sprocket could have been chattering as the key and keyway are not damaged.

What I cannot fathom is the scoring on the axle, directly opposite the keyway, running along the shaft, and the scoring on the inside of the sprocket going in the other direction (round the sprocket).... I have had the sprocket off a fair few times and never noticed the scoring, but I weren't looking for it, so I guess it could already have been there.
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Re: Ardingly show Part 2.

Post by Mick D »

Hi Peter

Thanks for the detailed photos, very thought provoking.

My initial thoughts are that the keyway in the sprocket acted as a stress raiser and the damage initiated there, it looks like the scoring to the sprocket bore has been caused as the woodruff key rotated inside the expanded bore - are there witness marks on the key to support this?
With the crack opening up, (under pressure from the drive train), the sprocket the chain would no longer have engaged and whilst riding over the sprocket the chain has knocked the holes into the cases followed shortly afterwards by the chain tension fracturing the split link pins.
At some point after the crack propagated to completion on the keyway side of the sprocket the drive train forces have caused the sprocket to fracture on the opposite side, I think this fracture is most likely to account for the damage to the crank shaft.

As an aside, did the gearbox move in it's mount? was there any change in the secondary chain tension?

Look on the bright side - it's all fixable and you'll know the quality of the replaced components - do you know a good alloy welder?

Regards Mick
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clanger9
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Re: Ardingly show Part 2.

Post by clanger9 »

That makes sense, Mick - the shaft has presumably rotated within the (fractured) sprocket when it finally failed.
As Peter says, those witness marks on the shaft are proper strange. :?
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Peter Morris
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Re: Ardingly show Part 2.

Post by Peter Morris »

Thanks for your replies Mick and Paul,

Mick, That all sounds feasible, I'm tending to think the sprocket gave way first.

The woodruff key has suffered very light damage to one edge please see pics below, so I guess that could have caused the scoring to the centre of the sprocket. Oddly, one half of the sprocket is scored more badly than the other half. :headbang:

The key was still in the keyway when I dismantled the primary side, in fact it was fairly well stuck in there.

Mick D wrote:

Look on the bright side - it's all fixable and you'll know the quality of the replaced components - do you know a good alloy welder?

Regards Mick
I have bought the new parts from Andover Norton so the quality is definitely good. I have given the cases to Exactweld Ltd, East Grinstead, who say it is no problem to repair them. They repaired a fork slider on my G3C a while ago, building up the alloy around a bottom clamp stud, and did an excellent job.

Peter
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Re: Ardingly show Part 2.

Post by Peter Morris »

The more I look at it, the more it looks like the scoring to the sprocket was caused by it running over the woodruff key.
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