Nut coming off Petzl Stop

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Postby ian mckenzie » Jan 18, 2006 7:56 pm

Hey NZcaver, as you can see I've deleted my post as my Stop is not built the way I said it was, it only looks that way... when I took it apart, what looks like a normal thin nut is in fact the top of a longer threaded bit inside, which you just can't see... apologies for the misleading info... maybe edit out the quote from your post so others don't get misled as well.
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Postby NZcaver » Jan 19, 2006 2:14 am

ian mckenzie wrote:Hey NZcaver, as you can see I've deleted my post as my Stop is not built the way I said it was, it only looks that way... when I took it apart, what looks like a normal thin nut is in fact the top of a longer threaded bit inside, which you just can't see... apologies for the misleading info... maybe edit out the quote from your post so others don't get misled as well.

But if I edited my post, that would destroy all the evidence... :laughing:

I'm still really curious about the original post on ukCaving. Did it really happen the way it was described? If so, HOW? Not that I'm an unbeliever, but I like to know the nuts and bolts of things (pun intended). :wink:

I saw you posted this reply on the UK forum -
ian mckenzie wrote:I've seen it happen, once, during a cave rescue seminar. The thing just came apart in the caver' hands. It was the nut that had come off...

And then another voice added to the chorus -
Joel Corrigan wrote:Er, happened to me years ago so guess it's not that unusual...

So assuming it isn't a big conspiracy, there must be some truth to this issue - right? :question:
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Re: Nut coming off Petzl Stop

Postby paul » Jan 19, 2006 3:09 am

NZcaver wrote:That original post on the ukCaving forum states -
One Stop; irregularly used for the past five years. Checked often. Never had bobbins replaced nor springs. The thing decided to part company of itself in as much as the retaining nut came away from the bolt (lower bobbin section)!!

To me, this statement implies that this particular Stop is about 5 years old - in which case it would use the threaded sleeve design. However, if it was acquired after being used by someone else - perhaps not.

I hate to give up any piece of gear as much as the next guy, but I wonder if there comes a time when you should just retire that obsolete item? Equipment designs usually change for a reason... :wink:


You can buy seconhand Stops from a local caving shop - apparently ex- military or fire service and very little used. They have the original metal clip so must be of some age...

Since these retail for about 25 pounds as opposed to about 45 pounds for brand new Stops, there's a lot of incentive to stick with the older design!
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Re: Nut coming off Petzl Stop

Postby NZcaver » Jan 19, 2006 5:11 am

paul wrote:You can buy seconhand Stops from a local caving shop - apparently ex- military or fire service and very little used. They have the original metal clip so must be of some age...

Since these retail for about 25 pounds as opposed to about 45 pounds for brand new Stops, there's a lot of incentive to stick with the older design!

Hi Paul - I know what you mean. I'm no stranger to seeking good deals on gear - new or otherwise. :woohoo:

Between yard sales/auctions at caving events, outdoor equipment sales, and eBay - I do OK. My first Stop was bought from a friend (used) in the early 90's - and it had the steel safety catch. It was an 80's model, and I believe it had the full threaded sleeve through the lower bobbin. See 'version D' on Gary Storrick's website here - http://storrick.cnchost.com/VerticalDev ... in435.html By the way, I no longer have that Stop any more - I upgraded to the newer version (E).

I suppose that initial post on the UK forum could have referred to a '5 year old' Stop that was purchased AFTER it had sat on a shelf somewhere for a couple of decades. Any chance you can persuade someone to take apart one of those Stops from the local caving shop? Stripping down and photographing the actual descender that started that thread would be even better. I'm curious... :?


PS - (General announcement) - On the subject of that old version of the Stop with the steel safety catch, please make sure you attach it with a locking carabiner - and NOT an oval Maillon Rapide screw link! This also applies with the current version of the Stop, and any versions of the Petzl Simple (bobbin).
This advice might just save your life... :shock:
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Postby potholer » Jan 20, 2006 10:22 am

I haven't taken apart a *really* old Stop, but my first one dates from 1984, shortly after the design changed to include the pin at the top, (rather than having the plates bent to come together over the rope running over the top bobbin), and my first is essentially the same design, down to the nuts, as my recent one, made in 2002.
The only differences between my Stops are the metal->plastic change to the gate clip, and the back plate design - the old model had a flat backplate, whereas the new one has the backplate bent for stiffness around where the lower cam bolt is attached, which I think was done to reduce the chance of the backplate folding around the bolt- hole if the Stop was loaded without the front-plate being closed.

Though I absolutely wouldn't advise anyone else to disregard any safety warnings, the maillon-causing-a-Stop-to-open scenario requires specific moves to replicate deliberately when using a 10mm long maillon on an old metal-gated Stop, and is harder on a plastic-gated newer version.
Also, though certainly more difficult, it is possible to do the same thing on a new Stop with the narrow end of some asymmetric-D krabs, and again, that is relatively easier on an old metal-gated one.
At least with a 10mm long, it does require the Stop to be twisted in a fashion which seems unlikely to happen in actual practice, and next-to-impossible while a Stop was vaguely loaded or while lowering onto one with a hard-lock on, where the rope would essentially prevent the Stop from moving on the maillon at all.
In practice, (and from experience) if using a 10mm maillon, a larger risk may be of the maillon gate unscrewing itself when moving through a cave, with the possibility that if not checked and noticed, the Stop could detach itself from the harness completely if not under tension, though that could be mitigated against to some extent by ensuring the gate faced the user and was therefore easier to check when attaching the rope, and that would be a risk common to using a maillon to attach any other kind of descender.

It does seem that smaller-diameter maillons are much more of a risk for accidental gate opening than a 10mm, but if choosing a crab in the hope of avoiding the problem entirely, one with a tightly-curved end (specifically a small, narrow-ended asymmetric D with a thin cross-section) would be worth avoiding.

Certainly, as a general rule, 'avoid maillions' does make some sense, especially if that's what Petzl recommend, since it is easy to remember, but things aren't quite as black-and-white as that.

Whatever, when it comes to 'getting on the rope' risks, leaving a long cowstail clipped in until a descender is loaded and checked would possibly help avoid many descending accidents when using any descender.
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Postby David_Campen » Jan 20, 2006 12:21 pm

Certainly, as a general rule, 'avoid maillions' does make some sense, especially if that's what Petzl recommend, since it is easy to remember, but things aren't quite as black-and-white as that.

No, the rule to not use a maillion on a Stop makes a lot of sense, it is much more than "a general rule". You have confounded the facts with over analysis.

Whatever, when it comes to 'getting on the rope' risks, leaving a long cowstail clipped in until a descender is loaded and checked would possibly help avoid many descending accidents when using any descender.

That is certainly nice general advice akin to looking both ways before you cross the street but does it have anything to do with the topic under disussion?
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Postby potholer » Jan 20, 2006 2:20 pm

I think there's probably a difference of interpretation over what I meant by 'general rule', which is probably my fault, and which I'm more than happy to clarify.

My intention was to say that since the possible problem (of catch operation in rare circumstances) isn't necessarily entirely avoidable by using a crab, the 'don't use maillons' rule isn't entirely comprehensive.

For someone looking for the simplest possible rule, and the least likely to be misrembered or misapplied, 'Don't use maillons' is probably the best and most concise one, but not sufficient to be a guarantee of safety in respect of safety catch opening.

In deliberate experiments, manipulating the stop and attachment link with hands which can apply tension and torque in precise directions, on a Stop with a plastic catch, which seems consistently more secure than the earlier type, catch opening opening is pretty easy to replicate with a 7mm long maillon in essentially one manouvre, very much harder with a 10mm long one (requiring much wiggling around, and/or an application of force in precise directions), and still possible with at least some krabs.
Not having many different krabs to experiment with at I couldn't be sure that there are no krabs around that might not be similar to or even worse than a 10mm long maillon in respect of catch opening.
It's possible that Petzl do somewhere also make a point of warning against the lesser risk from narrow-ended karabiners, though it doesn't seem like they actually do so in the literature that accompanies a Stop.

That's what I was trying to convey, and my apologies if I was unclear.

You have confounded the facts with over analysis.


Again, I am sorry, I didn't aim to confuse the fact that there was one school of thought (supported by Petzl) that maillons are a bad idea by going off on a rant.

That is certainly nice general advice akin to looking both ways before you cross the street but does it have anything to do with the topic under disussion?


Actually, I think it might, though I didn't explain the details for fear of going on at too much length.
Since the only feasible scenarios for the catch opening and the front plate rotating in caving use rather than manual manipulation seem to be when the Stop is unloaded, the only two scenarios I can think of are when:
a) Someone unloads the rope esentially completely part-way down a pitch and starts thrutching around, (possibly in a *very* tight pitch?)
b) Someone is getting onto a section of rope at a pitchhead or rebelay, and somehow manages to get things twisted in a bizzare manner before lowering their weight onto the Stop

In the second scenario, a long cowstail would likely prevent any fatal consequences even if a catch did open, so it is a specific action which although justified for many other reasons could also reduce risks from the possible situation which lead to the 'no maillons' rule. For someone who was unlikely ever to unload their descender pretty much entirely mid-descent, and who would therefore seem unlikely ever to run up against possibility a) habitual use of a long cowstail would likely reduce their already slight chances of injury from catch opening somewhat further.
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Postby NZcaver » Jan 21, 2006 12:03 am

potholer wrote:...My intention was to say that since the possible problem (of catch operation in rare circumstances) isn't necessarily entirely avoidable by using a crab, the 'don't use maillons' rule isn't entirely comprehensive...

Potholer - this I agree with. Thanks for providing some fascinating insight in your previous posts. :grin:

I can also see where David was coming from when he objected to your "general rule" comment. The rule is - those maillons should NEVER be used to attach a Stop. However, using a carabiner instead does not absolutely guarantee a similar event will never occur - it's just much less likely with most carabiners. Right? :nuts: Confused?

David_Campen wrote:...You have confounded the facts with over analysis...

I'm not sure it is possible to over-analyze any important safety issues like this. If it is - then I'm certainly guilty of it! The original topic was about a Stop "falling apart", so I think discussing other safety precautions/concerns with the same descender is entirely appropriate.

In the spirit of discussing similar things, check out this accident report and analysis (with photos) involving a Petzl Simple bobbin with a similar style of locking catch -
http://www.bstorage.com/speleo/LoriCoriAccident/
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