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Thread: Metal on Metal

  1. #1
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    Default Metal on Metal

    The other day, I was connecting a pulley to the AZ vortex head with a carabiner. One of my co-workers stopped me and said " You can't connect like that, metal on metal" Now I know this not to be true as I have seen carabiners being connected directly to the head on RTR videos.

    I can see metal on metal being not a good thing like connecting ropes using two carabiners as this would have torsional forces. Like in this image:



    But something like this should be okay right?



    So where does this metal on metal rule come from and how can I explain the exceptions to this rule. If you have any written reference to this, that would be great.

    Thanks

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    I think that both of those are probably ok. My general "rule" is that if there is something in the rigging to absorb the torsional force, it's probably okay to use. When you are attached to a rope to be lowered there is no soft link between the metal D ring of your harness and the biner on the end of the rope. If you were to spin around while in the air, the carabiner is not going to be torqued against the D ring to the point of failure. The rope is just going to twist.

    You are able to put carabiners and pulleys right to the head of the AZV if you have something in your system to absorb the torsion; like hooking a block and tackle to the head in tripod configuration. Even if there is nothing to absorb the a twist, you're still good to go as long as you make sure your rope coming into the AZV head is lined up with your pulley so it wont bind. If it's not, your torquing problem will resolve itself pretty quickly when your AZV topples over once the system is loaded.

    I think "No Metal on Metal" is a poorly defined, often quoted, poorly understood rule. To the point where it isn't really even a rule. Some quick point where we have metal on metal:

    Biners attached to rigging plates.
    Biners attached to harness.
    Biners attached to MPD's, racks, 8, etc...
    Biners atached to MPD's, racks, 8, etc... and then attached to a rigging plate!
    Biners atached to MPD's, racks, 8, etc... and then attached to a harness for a rappel.
    Block and Tackle attached to to tripod head.
    Metal D ring of a pickoff strap with a biner on it.
    Biner attached to a pulley.

    The list could go on and on.
    MichaelXYZ likes this.
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    Quote Originally Posted by Rescue 2 Training View Post
    I think "No Metal on Metal" is a poorly defined, often quoted, poorly understood rule. To the point where it isn't really even a rule.
    You are absolutely correct! I think the "rule" came from the old days when people were using aluminum non-locking carabiners linked together to form a crude brake bar rack. Just like many other "rules" that somebody heard one time and accepted it as the gospel without ever investigating or trying it out to see if there was any truth to the "rule".

    Mike Dunn

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    Quote Originally Posted by Rescue 2 Training View Post
    I think that both of those are probably ok. My general "rule" is that if there is something in the rigging to absorb the torsional force, it's probably okay to use.
    I agree with Kelly 100%.

    Michael in your pic you have 2 ropes connected together via 2 carabiners, I don't think there is a torsional concern in that situation; it's just bad, redundant rigging. Ditch one of the carabiners or tie the ropes together depending on their intended use and it will be a lot prettier.

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    Best Practice is where possible, reduce hardware to hardwear contact. Side or torsional loading on a beiner can cause damage at great pressure. While not likely, why risk it?
    ~Drew
    Firefighter/EMT/Technical Rescue
    USAR TF Rescue Specialist

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    drew,
    not sure if i agree with you regarding the use of the term best practice in this instance. maybe the best practice would be to eliminate torsional loading not hardware to hardware (h2h) contact.

    the addition of soft links/swivels/other devices designed to eliminate torsional loading increase the gain of the system, add unnecessary gear, and slow the operation down significantly. we do need to address torsional loading where appropriate but not every time there is a h2h connection.

    if the definition of best practice of hardware connection is to eliminate h2h contact then we'd most likely find many rescue practitioners adding unnecessary soft links to every connection just because they were taught to reduce every likelihood of h2h contact. i think the emphasis should be on teaching the elimination of torsional loading and/or the potential for rollout.

    my .02
    My opinions posted here are my own and not representative of my employer or my IAFF local.

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    Quote Originally Posted by stickboy42 View Post
    drew,
    not sure if i agree with you regarding the use of the term best practice in this instance. maybe the best practice would be to eliminate torsional loading not hardware to hardware (h2h) contact.

    the addition of soft links/swivels/other devices designed to eliminate torsional loading increase the gain of the system, add unnecessary gear, and slow the operation down significantly. we do need to address torsional loading where appropriate but not every time there is a h2h connection.

    if the definition of best practice of hardware connection is to eliminate h2h contact then we'd most likely find many rescue practitioners adding unnecessary soft links to every connection just because they were taught to reduce every likelihood of h2h contact. i think the emphasis should be on teaching the elimination of torsional loading and/or the potential for rollout.

    my .02
    Ha, very literal translation of what I said. Possibly when addressing hard linking we should then say; Where possible, limit h2h contact without adding additional software or hardware into the system. ???

    Smart ***.
    ~Drew
    Firefighter/EMT/Technical Rescue
    USAR TF Rescue Specialist

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    Hey Drew, LTNS, I was wondering what happened to ya. Glad to see your back

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    Quote Originally Posted by FiremanLyman View Post
    Ha, very literal translation of what I said. Possibly when addressing hard linking we should then say; Where possible, limit h2h contact without adding additional software or hardware into the system. ???

    Smart ***.
    I don't think he was being a smart***, I think it was a legitimate comment on your response. I agree with Stickboy that if we are going to call anything in this discussion "Best Practice", it would be to eliminate torsional loading rather than attempt to stretch the "Metal on Metal" rule even further past its poorly defined edges.
    Last edited by Rescue 2 Training; 05-06-2013 at 09:12 PM.
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    Quote Originally Posted by Rescue 2 Training View Post
    I don't think he was being a smart***, I think it was a legitimate comment on your response. I agree with Stickboy that if we are going to call anything in this discussion "Best Practice", it would be to eliminate torsional loading rather than attempt to stretch the "Metal on Metal" rule even further past its poorly defined edges.
    FiremanLyman, I'll have to agree with Rescue2T. For me though, I prefer to simply look at it this way... Rig cleanly as possible as time allows, and whenever possible, rig gear for use and not abuse. Something tells me that most of you guys in this forum are doing this already, some with different perspectives on methods, as there are nearly always options in variation to be chosen. And now we see there are different ways of verbalizing cleanliness in rigging, as there are also different degrees of cleanliness.

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    It seems as though I will have to live with this rule as every time I try to change the thinking of the higher ups I always get "That's the way we do it in the fire service". Despite my efforts to point out otherwise, I am just simply told, "That's the way we do it". So I find myself tying a figure 8 follow through to connect a guy line on a tripod or a webbing to connect a 4:1 on the AZV head.

    I have looked for papers on this subject to change the thinking but have not found any. Grrr.

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    Michael,
    Why don't you show your supervisors any current or book written in the last decade or so that is from a reputable source the fire service uses, or for that matter many departmental training manuals. The metal-to-metal connections are prevalent in many. I haven't seen an ITRS paper in the past decade that specifically addresses it but many texts do with illustrations of direct hardware connections. Look into Rope Access and Arborist rigging examples as well as part of your case.

    If you need a list of books that show m-t-m connections let me know.

    -mike
    My opinions posted here are my own and not representative of my employer or my IAFF local.

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    The idea of rigging cleanly certainly seems to apply in this case and represent best practice. I know the pictures Michael attached are for illustration purposes only but in both of those I would wonder why we're using HW/'biners at all.

    Joining 2 ropes via a knot is much cleaner (less gear, lower profile/less chance of jamming (assumes the connection may be moving e.g., a lower/raise).

    A friction wrap around the "tree" could work well and reduce the need for any hardware and minimize loss of strength in the anchor rope.

    Rigging for no metal-to-metal contact in all elements reflects dogmatic adherence to "rules" and can create other problems (excessive gain, etc.) as others have noted. Familiarity and experience help a lot but that is a luxury some agencies don't have. They just don't have enough frequency to develop the skills.

    That being said torsional/twisting forces and risk of cross loading is very real and needs to be understood and managed.

    Finding credible sources to cite is certainly good advice. Thinking evolves and time often helps clear some dogmatic thinking.

    Good luck and keep asking questions.

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