Can anyone explain what caused this chain to break?

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Brucey
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Re: Can anyone explain what caused this chain to break?

Postby Brucey » 18 Feb 2015, 11:47pm

resurrecting this topic....

I've never seen anything like this! Without wishing to point out the obvious, there is clearly something different about the inner side plates on one side of the chain. This could be the loading conditions (how is/was the chainline?) but it could be something else like the plates themselves and how they are made.

When the chain is manufactured each link is assembled on an automated line. SIde plates, rivets, rollers etc are fed from separate hoppers and assembled by ingenious machines so that chain comes out at high speed. There are most likely four different side plate hoppers; two each for inner and outer side plates, such that each hopper feeds either left or right plates into the finished chain.

My suspicion is that one of these hoppers got filled with defective side plates, that were either cracked already or were so brittle that they were likely to crack in service. For the side plates to fail in this fashion there would have to be a distinctive pattern of cracking near the ends of the side plate so that a central section of the side plate can just fall out.

If this idea is correct then it perhaps has happened to others. To test this idea I did a little google search for PC68 chain and I found this;

Image

and although I don't speak Polish it appears to be a chain that was lubed with finish line and had done about 3000km or so.
The thing that I find interesting is that the chain has cracked in a similar location, and that there are multiple cracks lying parallel to one another. This smacks of poor steel quality or similar; steel with a high inclusion content could easily crack in this way, for example.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

geocycle
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Re: Can anyone explain what caused this chain to break?

Postby geocycle » 19 Feb 2015, 3:44pm

Good find Brucey, it was/is certainly a mystery about how the middle section of the plate just disappeared so your manufacturing argument may well be correct. The chain line was perfect (rohloff) and the only possibility I mentioned in the thread above was that it could have been loose so managed to impale itself on the chainring but I've no evidence for that. In hindsight I should have done some more investigation at the time before discarding it. The miracle was it got me home with just the strength of the side plates!

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NATURAL ANKLING
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Re: Can anyone explain what caused this chain to break?

Postby NATURAL ANKLING » 19 Feb 2015, 4:17pm

Hi,
You say that the (picture above) side links could be defective.
What about the pins :?:
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Brucey
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Re: Can anyone explain what caused this chain to break?

Postby Brucey » 19 Feb 2015, 4:41pm

the pins are just worn in the usual way. They are badly worn though!

cheers
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danhopgood
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Joined: 20 Jan 2015, 5:16pm

Re: Can anyone explain what caused this chain to break?

Postby danhopgood » 19 Feb 2015, 4:53pm

SRAM PC850 with 900 miles on it did this in three places last month:
Used on a hub geared bike used for commuting.

https://drive.google.com/file/d/0BySMW7YA0TAgQVZNd0FjMTV1Yzg/view?usp=sharing

Chain snapped before I noticed any defects in the waisted section opposite one of the failed links.

I returned to the dealer that supplied the bike to discuss with SRAM - no response from them yet.

It's a manufacturing defect as far as I'm concerned - probably the tolerance between pins and plates causing the "at rest" stresses in the plates being too high. That and possibly the steel being brittle.

I've seen internet posts about other people with this kind of problem. Doesn't seem to be confined to SRAM chains.

Replaced with identical chain - this seems to be OK at the moment.

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Can anyone explain what caused this chain to break?

Postby Brucey » 19 Feb 2015, 5:20pm

that is the more commonplace type of chain failure. The residual stresses from the swaging operation may well have something to do with the failure as you say.

The swaging operation is usually carried out by passing the chain through a pair of rollers. The rollers swage a 'stripe' each side of the rivet ends so that they are

a) a tighter fit on the diameter and
b) there is an overhanging part on the end of the pin

In the simplest terms, if they overdo the swaging, or the steel is a bit brittle, or the service conditions are unfavourable, the chain can crack. I've had similar failures in chain that has seen nothing but fair weather use, and not much of it either. It just takes a small error in QA for this to happen.

Here is a similar failure;

Image

cheers
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reohn2
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Re: Can anyone explain what caused this chain to break?

Postby reohn2 » 19 Feb 2015, 7:29pm

FWIW,I've used Sedis and latterly Sram chains for donkeys years on 5,6,7,8 & 9sp dérailleur systems,in all that time I've only ever had two issues,both when an outer plate cracked from the rivet similar to Brucey's photo above.Both times the chain held together.
One was on a tandem timing chain approx 8 years ago, the other a solo dérailleur chain approx 4 years ago,in both incidences I replaced the link with a Powerlink by the roadside and carried on.
The dérailleur chain was a 971 9sp,the other an 8sp PC58.
I can't offer any reason for the faults only that both seem to be random events :?

I still use Sram 971's and don't plan to change as a result.
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Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Can anyone explain what caused this chain to break?

Postby Brucey » 19 Feb 2015, 9:59pm

that is interesting because (like R2 I guess) I'd describe the few failures I've had on my own bikes as 'isolated events' that have not heralded multiple subsequent failures in the same chain.

This appears to be different from what some other folk have seen which is that the same chain can have multiple links that fail in a similar fashion.

The former seems like a random event of some kind and the latter suggests that something systematic is going on in some cases.

cheers
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mercalia
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Location: london South

Re: Can anyone explain what caused this chain to break?

Postby mercalia » 19 Feb 2015, 10:15pm

these are all frightening pics!never had anything like it

tim-b
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Re: Can anyone explain what caused this chain to break?

Postby tim-b » 20 Feb 2015, 7:24am

Hi

Here's mine. It stayed on and got me 10 miles home, it was pouring with rain and I couldn't be bothered to investigate the rhythmic "tick". I didn't realise that it had broken until I got home and ran a cloth over the wet chain, which snagged on the damaged link

The break is on the inside plate, and the outside plate had a bend (result of the unequal forces??) and is just beginning its bid for freedom

Refunded by the retailer under warranty. My first broken chain in more than 35 years of "proper" cycling

Regards
tim-b
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