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/ What's wrong with metal on metal anyway?

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Pay Attention - on 13 May 2018

Recently I clipped the screw gate on my belay sling directly into the carabiner on a cordelette as it was quick and easy.  I was told I shouldn't clip metal on metal - but why?

i do it all the time with ice screws. I believe sports climbers do it on bolts.

"don't clip metal on metal" is a warning I've heard over the years but it doesn't make sense to me.  Is there science behind it?

Trangia on 13 May 2018
In reply to Pay Attention:

To reduce the risk of twisting forcing the gate of a krab open?

Pay Attention - on 13 May 2018
In reply to Trangia:

Good point!

captain paranoia - on 13 May 2018
In reply to Pay Attention:

Can cause damage to the rope-running surface, which, in turn, can damage/cut your rope.

Sport climbers usually have a 'bolt end' and a 'rope end' to their QDs.

Mark Stevenson - on 13 May 2018
In reply to Pay Attention:

Using a karabiner as a "master point" is fairly common in some contexts especially aid climbing or when guiding. It uses an extra krab but generally works really well.

Back in the day before quickdraws were common and chocks on rope/cord made up the bulk of racks, wires were generally just clipped directly to the rope with a single krab or occasionally a couple of snapgate krabs linked together. In the later case, krabs occasionally unclipped from the rope or were loaded awkwardly so the practice came to be depreciated and climbers moved to carrying extensions, or what are now quickdraws. This was probably part of the historical reason for the mantra of "no metal on metal" but with modern racks that particular issue really doesn't exist any more.

Offwidth - on 13 May 2018
In reply to Pay Attention:

As posters have explained there are good reasons in some circumstances. However in my experience when cragging  most people telling others off are clueless and regard it falsely as a mantra (like saying always using screwgates for belay pro) . Metal on metal is quite normal for Aid climbing. 

john arran - on 13 May 2018
In reply to Pay Attention:

As with many generalised 'rules' (e.g. i before e except after c), there are good reasons for them in many situations but many other occasions where the 'rule' simply doesn't apply. 

Snaplinks unclipping is a genuine concern, especially when there's limited chance of rotation and one krab can be exerting a rotational force on another. The same, or similar, rotational forces can also put sideways strain on a screwgate.

Where the 'rule' doesn't make sense is in clipping bolts, where metal on metal is very much necessary (with the proviso mentioned above that sharp hangers can lead to rope-damaging burrs on the inside of krabs, so such krabs should be reserved for that use. Clipping wires is a very similar situation. Also, there are many other pieces of metal gear that are correctly clipped directly with a krab, such as grigris, ascenders, etc. Then the main issue is one of preventing krab rotation and ensuing cross-loading.

Know the reasons for the rule and you will be well placed to apply it when it makes sense.

Incidentally, the opposite 'rule', that of avoiding fabric-on-fabric, is similarly only partially valid. In that case the main factor is to what extent the nylon components can move with respect to each other. Any expected movement will likely lead to nylon damage through burning, whereas no significant movement (e.g. harness tie-in) won't.

Oceanrower - on 13 May 2018
In reply to Pay Attention:

I think the "metal on metal" thing was much more important (from a sport climbing sense) when the predominant system was the Petzl Coeur type of hanger. These really did put nicks into your carabiners hence sport climbers using one particular end of the QD for the hanger and one for the rope.

With the newer style of rounded bolts I really don't worry too much any more.

 

(Note: Obviously there are other reasons for using one end of a carabiner predominantly for the rope.)

 

Post edited at 09:28
john arran - on 13 May 2018
In reply to Oceanrower:

> I think the "metal on metal" thing was much more important (from a sport climbing sense) when the predominant system was the Petzl Coeur type of hanger. These really did put nicks into your carabiners hence sport climbers using one particular end of the QD for the hanger and one for the rope.

> With the newer style of rounded bolts I really don't worry too much any more.

I agree with that but I'm struggling to see what avoiding metal-on-metal has to do with it. Whatever hangers are clipped, it's still metal-on-metal, but with Coeur-type hangers, the thing to avoid is then using the same krabs for metal-on-rope!

Oceanrower - on 13 May 2018
In reply to john arran:

Agreed. I think it was more a case of don't use the metal on metal equipment for anything else.

If we didn't use metal to metal, sport climbing wouldn't exist!

jonny taylor on 13 May 2018
In reply to john arran:

> Incidentally, the opposite 'rule', that of avoiding fabric-on-fabric, is similarly only partially valid

At the risk of taking things off on a tangent... I recently heard second-hand about some interesting results from Lyon on the weakness of larksfooted slings. The implication seemed to be that, whether it was the small slippage under tension or possibly the tighter radius of the sling, it was weakened significantly. I don't know the full details, and I do find it intuitively surprising. However, it seems they do have drop test results showing this, so that has made me a bit more wary of thinking I "know the reasons" for that particular "rule".

[I don't know what sort of harness tie-in you had in mind, but I would previously have imagined that a properly oriented and dressed larks-foot around a post would fall into the "no significant movement" category...]

john arran - on 13 May 2018
In reply to jonny taylor:

I always thought that a larks-foot was about the weakest way to join slings and reduced strength by around 50% or so. While that'a a fair reduction, the gear is so overstrong in the first place that I don't see it as being serious in the same way as the terminal consequences that friction-melting can cause.

timjones - on 13 May 2018
In reply to Offwidth:

> As posters have explained there are good reasons in some circumstances. However in my experience when cragging  most people telling others off are clueless and regard it falsely as a mantra (like saying always using screwgates for belay pro) . Metal on metal is quite normal for Aid climbing. 

One of the great things about aid climbing is that it really challenges you to think about a lot of the oversimplified mantras that are so readily accepted elsewhere in climbing.

timjones - on 13 May 2018
In reply to Pay Attention:

When someone parrots this sort of thing it's always worth asking "who told you that" ;)

Pay Attention - on 13 May 2018
In reply to timjones:

There was no expert around to support the expert opinion so I decided to get the opinion of the experts in this forum. 

 

I've found all the contributions interesting and helpful.

steveriley - on 13 May 2018
In reply to Pay Attention:

Anyone who’s ever placed a wire has clipped metal on metal

timjones - on 13 May 2018
In reply to Pay Attention:

I wasn't expecting that there was a handy expert to pop out and answer the question, but if you ask the person who quotes the rule where it came from and what the justifcation is you can often formaulate a good answer on  the spot.

Lusk - on 13 May 2018
In reply to Pay Attention:

Next time you're with the individual concerned and he's about to lead a bolt route, tell him he'll have to thread each bolt.  Can't clip metal on metal, mate!

Pay Attention - on 13 May 2018
In reply to Lusk:

If he's going to climb with me he's not going to be on a bolt route. ;-))

Big Ger - on 13 May 2018
In reply to Pay Attention:

https://www.youtube.com/watch?v=jDC2e6i6Quw

This is all that is wrong with Metal on Metal....

Dell on 13 May 2018
In reply to timjones:

> I wasn't expecting that there was a handy expert to pop out and answer the question....

 

Oh don't worry, there's plenty of experts on here! 

 

springfall2008 - on 13 May 2018
In reply to Pay Attention:

If you think about it a carabiner clipped to a carabiner can cause a torsional loading (twist) which may not apply all the force in the strongest direction and could even load or open the gate.

Also as metal rubs against metal it's going to wear it down and you would prefer to replace cheap slings not more expensive metal components.

Of course its unavoidable when clipping bolts or wires but at least they are designed to keep the load in the correct direction.

 

timjones - on 13 May 2018
In reply to Dell:

> Oh don't worry, there's plenty of experts on here! 

Let us know when you've worked out what we're all experts on ;)

 

gravy - on 13 May 2018

It's a rule of thumb to prevent one metal part twisting or levering open another. 

As long as you're aware of this and don't leave yourself at risk then metal on metal is ok.

the main exception to the rule is quickdraw to protection (bolt, piton wire) but the tape of the QD provides the flex you need most of the time (but we've all seen a dodgy bolt where the carabiner can still lever the bolt)...

rgold - on 13 May 2018
In reply to Pay Attention:

I think it is an example of taking a very specific and problem and issuing a generic proclamation which, although it does cover the specific problem, is capable of far more extensive and usually  worthless applications.

So: don't chain carabiners when the nature of the loading might cause one to load the gate of the other,  but there is nothing in principle the matter with metal on metal and numerous incidences when it is commonplace and unavoidable.

 

john arran - on 13 May 2018
In reply to rgold:

Funny how people are happy to clip chains, which are possibly the most extreme example of metal-on-metal in climbing!

jonny taylor on 13 May 2018
In reply to john arran:

> I always thought that a larks-foot was about the weakest way to join slings and reduced strength by around 50% or so. While that's a fair reduction, the gear is so overstrong in the first place that I don't see it as being serious in the same way as the terminal consequences that friction-melting can cause.

Agreed. To be fair this was in a rescue context, where 50% reduction becomes problematic for the safety margins.

I think was surprised me about that was that the scenario was a lot "gentler" than a larks foot to join two slings (larks foot around a post, not another sling), and intuitively I would *not* have expected such a reduction in that context. Made me wonder what the root cause of the weakening actually was.

springfall2008 - on 14 May 2018
In reply to john arran:

Chains twist, and they also don;t have the risk of opening as it's one ring!

jkarran - on 14 May 2018
In reply to Pay Attention:

> Recently I clipped the screw gate on my belay sling directly into the carabiner on a cordelette as it was quick and easy.  I was told I shouldn't clip metal on metal - but why? "don't clip metal on metal" is a warning I've heard over the years but it doesn't make sense to me.  Is there science behind it?

Nothing inherently wrong with it short term in the right places. Long term when exposed to weather you can get galvanic corrosion which is of interest mainly to people fixing bolts.

I've always understood the warning to stem from the risk of using a single krab on fixed gear which being quite rigidly supported a rope can easily unclip from or two linked krabs as a proto-extender as happened in the early days of modern equipment development. These being similar shapes are prone to twisting together then splitting apart as they open each other's gates.

There is also the problem of hard sharp kit like bolts damaging softer alloy krabs, this doesn't really fall into the 'no metal on metal' rule, you just need to be aware of it so you don't use the roughed up krabs on the rope, they can fray or even split the sheath.

Basically it's a mostly harmless rule, just stick with it except where convention dictates exceptions or understand the roots of the concern then feel free to mostly ignore it as nonsense, there are after all far more exceptions than applicatiopons! Either option works.

jk

Post edited at 09:01
jkarran - on 14 May 2018
In reply to springfall2008:

> Also as metal rubs against metal it's going to wear it down and you would prefer to replace cheap slings not more expensive metal components.

If I wanted to saw through a krab in the wild (as opposed to the workshop where I have saws) I wouldn't use another krab, I'd use a wet sling loaded with sand. Neither wear each other appreciably in normal use anyway and slings cost more than krabs last time I bought them which is to be fair a while ago.

jk

Jim Walton on 14 May 2018
In reply to jonny taylor:

 

> To be fair this was in a rescue context, where 50% reduction becomes problematic for the safety margins.

  1. A standard 11mm Dyneema (Or Nylon for that matter) sling has a minimum rating of 22kN,
  2. A Carabiner has a minimum rating of 20kN
  3. A Harness has a minimum rating of 15kN
  • A Rescue Load is usually defined as 200kg.  (80kg casualty, 80kg rescuer and 40kg excess of equipment or 2 x 100kg casualties).
  • The maximum steady loading on any component is usually defined as 8kN.  Based on 4:1 with 200kg from mechanical advantage of pulleys including friction etc.

Edelrid has done some tests on girth-hitching/larks-footing slings to the tie-in-loop  on your harness.

  • 8mm Dyneema failed at ~11kN
  • 6mm Aramid failed at ~ 12kN (if larks-foot into belay loop it will fail at less because the aramid sling cuts the belay loop)
  • 12mm TechWeb failed at ~13kN

In their conclusions they wrote

  • Slings according EN566, used as Girth Hitch self belay slings, are not built to absorb energy.
  • Girth hitch sling are safe - as long as they are loaded only stactically
  • A girth hitch can weaken a tie-in-loop for more than 50%
  • Self belay slings with higher energy absorption capacity are recommendable
springfall2008 - on 14 May 2018
In reply to jkarran:

I think you would find it wouldn't work and the sling would break first!

Last time I ordered quickdraw slings they were around £2-£3 each with the cheapest snap-gate coming in at around £5. But the point is a good crab will last 10+ years but a sling needs to be replaced much more frequently anynow

Offwidth - on 15 May 2018
In reply to springfall2008:

Slings simply do not need to be replaced much more frequently than 10 years, where did you get this information from? They do really need to be replaced when visibly damaged, but a hardly-used 10 year old sling that is in very good condition  and has been stored in the dark and dry will be OK.

https://www.thebmc.co.uk/tech-skills-retiring-textile-equipment

Post edited at 12:34
jkarran - on 15 May 2018
In reply to springfall2008:

> I think you would find it wouldn't work and the sling would break first!

I very much doubt it, the wear develops over a significant length of the sling and the grit barely moves relative to the sling carrying it. The moving grit acts constantly under pressure on a small area of the krab. If I've got a tired old sling (they're all 10+ years) at home I'll demonstrate. Anyway, we digress.

jk

john arran - on 15 May 2018
In reply to springfall2008:

With respect, that's precisely my point!

jimtitt - on 15 May 2018
In reply to jkarran:

> I very much doubt it, the wear develops over a significant length of the sling and the grit barely moves relative to the sling carrying it. The moving grit acts constantly under pressure on a small area of the krab. If I've got a tired old sling (they're all 10+ years) at home I'll demonstrate. Anyway, we digress.

> jk

I think it is going to be touch and go which is destroyed first!

I just did a project on wear of top anchors to settle a long-running dispute about titanium relative to stainless steel. The text books tell us that using ASTM G65 (which involves rubbing sand onto the metal using a rubber wheel) titanium wears over twice as fast as stainless steel. 6061 T6 aluminium (the grade there is information for) wears over twice as fast again as titanium and roughly five times faster than stainless.

I tested by pulling a loaded (very dirty) rope through and it suffered badly, it had to be changed every 320m as it started to get core-shots and the sheath disentigrated. A sling is going to have a hard time of it because the grit will get completely in the weave, not just the sheath of a rope.

springfall2008 - on 16 May 2018
In reply to Offwidth:

> Slings simply do not need to be replaced much more frequently than 10 years, where did you get this information from? They do really need to be replaced when visibly damaged, but a hardly-used 10 year old sling that is in very good condition  and has been stored in the dark and dry will be OK.

This one is quite vague, but it's saying to retire harnesses after 5 years (manufacturers guideline) and I wouldn't expect a sling to be quite as durable as a harness.

Of course I agree if it's not used and kept well it should last a long time, but I'm talking about in use here not sat in a cupboard!

 

jkarran - on 17 May 2018
In reply to jimtitt:

> I think it is going to be touch and go which is destroyed first!

> I tested by pulling a loaded (very dirty) rope through and it suffered badly, it had to be changed every 320m as it started to get core-shots and the sheath disentigrated. A sling is going to have a hard time of it because the grit will get completely in the weave, not just the sheath of a rope.

Interesting, thanks Jim. You say every 320m you had to change the rope but I'm struggling to understand quite what you mean by that, was it a 1m rope that made 320 passages over the bar? Presumably under near constant tension the wear in the sand loaded rope occurs only where the rope is flexed so for any given spot on the rope it's the number of flexing cycles that matter? The wear on the bar is presumably determined by the bar properties, the load, the grit and the number of linear meters that pass over it?

jk

Offwidth - on 17 May 2018
In reply to springfall2008:

The link was more to illustrate about when and why you might need to retire a sling early (rather than maximum lifetimes.. which are way more than manufacturer data indicates for 'as new' webbing). At a Peak area BMC meeting Dan Middleton demonstrated a new sling rubbed hard on a lump of gritstone across its full width was weaker than a new sling cut half through across its width. In practice rope elasticity takes most of the shock load energy and its rare for UK lead falls to even be Fall Factor 1, so people get away with their old furry slings. You can pretty safely assume the quoted breaking force will be more than halved in practical terms on old furry slings (so a sling with a weight equivalent to a climber will very likely snap if you fall from the level of the attachment to the full sling length below). New slings normally pass single FF1 drop tests but normally fail at FF2.

Post edited at 11:15
jimtitt - on 17 May 2018
In reply to jkarran:

It was 3.2m of rope that was pulled through backwards and forwards so was changed every 100 strokes when I measured the wear on the metal. The sheath just wears through in the end, goes furry at first then you start to see the white of the core showing through, if you keep going (I did the first time) in the end the sheath just lets go altogether.


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