/ Cam failure - BMC article
A sensible response to the failure; a reminder that our gear won't last forever and should be inspected routinely.
Seems fair enough.
Maybe the wire should be joined to the stem much closer to the cams.
perhaps that way there would be less shearing force?
I believe this is not to do with the length of the rigid section of stem but flex fatigue where the cable is crimped into that section. Any crimped product will be stressed at the crimp point, concentrating that stress and preventing the natural movement seen in a steel rope. Eventually with enough flexure a cable will fail at that point...
> Eventually with enough flexure a cable will fail at that point...
...IF the wire is repeatedly stressed beyond its fatigue limit.
The cam may have been repeatedly and extensively bent (i.e. over an edge) causing fatigue failure of the wire rope in the relatively highly stressed region at the end of the rigid section, or there was another issue with the design/material/corrosion etc. I'm not sure I buy a relatively simple fatigue failure with a cam - I just don't think it would see enough loading cycles* over it's lifetime (even 10 years) to cause a fatigue failure. The fatigue limit for the wire rope should not be exceeded by, for instance, placing in a vertical crack and sitting/falling on it, or by stuffing in a bag. It may be exceeded by placing in horizontal cracks/over and edge, but I'm struggling to believe this would have happened enough times to cause a fatigue failure. I'd be very interested in seeing the failed cam...
*Not falls, just being 'bent' beyond the fatigue limit.
I've replace cams because stem wires have snapped exactly where the pictured cam had failed wires (only WC ones though!) and only after a few years irregular use too in one case (and this wasn't due to abrasion), pretty sure if I'd have left it more and more of the wires would have snapped
Just for information, this was taken from a fairly well known document on wire rope forensics:
"A visual and tactile examination of a steel wire rope will have to rely on the condition of the outer wires. In most ropes, these represent about 40% of the metallic cross section. The outer wires are only visible for about half of their length [due to the weave of the rope]. Therefore a visual and tactile inspection of a steel wire rope will have to rely on the condition of about 20% of the metallic cross sectional area only. Visual rope inspection = 20% evidence + 80% hope"
Not the best wording, but it implies that a visual/tactile inspection of the external surface of a wire rope is optimistic at best.
I don't think that accounts for Mikes point that the most likely failure point is the first place the wires can bend after the crimp. Also in this case the outer wires are most likely to break as they are placed under the most tension (on one side) and bent the most (on the other).
I've found wires are most likely to break this way on soldered micro nuts and also the WC super lights.
You're right, it wasn't meant to be a potential reason for any cam failure in particular. More just a reminder that if you've just observed a broken wire on the outside of the rope then it's possible that there are many more inside that are also damaged.
The vicinity of the joint is the most likely place to break due to the relatively high stresses as you implied. The outer wires are only more likely to break (compared to the internal wires) if the failure mechanism was something like fatigue. If corrosion played a part then that could easily be reversed. As I'm skeptical it was (purely) fatigue for the reasons I stated earlier, I thought it worth mentioning that internal wires can break before external wires.
Anyway, it's all moot really as I'm sure a full investigation by another metallurgist has been done!
Sorry, I'm going to call you up on that one, as everyone on this thread is taking about failure through bending the steel wire over an edge.
The highest bending moments and therefore the highest stresses always occur in the furthest fibre of any member. This means that although you can only see 20% of the cross section, you are seeing the worst affected fibres (in terms of bending).
However, I do agree that corrosion could play a big part in failure of internal fibres, thus don't rely on the fact that the outer fibres that you can see are all intact.
Quite right, if you only consider stress on the individual wires due to bending.
Corrosion usually plays a part in many failures, and so I was pointing out that more wires can be broken towards the core than you may expect (based on the external visual condition) and simply to be aware of it. You quite rightly implied that if 'bending the steel wire over an edge' was the only factor, then yes, you'd most likely notice the external wires fracturing first. I guess I was talking more generally about some failure modes of wire ropes, rather than specifically of bending that cam over an edge. Well spotted.
Fair enough but should the manufacturers put a plastic sheath thing over the most likley point of failure to make it harder to inspect?
owner of a 0 WIld Country Technical friend that failed like this....
I think you are barking up the wrong tree wrt to corrosion, if this had been the mode of failure I think BMC / WC would have picked it up.
I've had a size 00 wc tech friend also have broken wires near the crimp which required me to retire it.
I'd be interested if anyone has ever retired a cam of another make because of this?
I'm wondering if the problem isn't in some way magnified by the manufacturing process / design.
I had a.chat with the bmc bod, other cams have same issue but not the other major brands in UK market. It'll be intetesting if the small heliums have same issue
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