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    Default Anchor Forces During A Skate Block Operation

    Has anyone here done any load force testing on the anchors during a skate block operation. If so what were the force readings and what was there peak point.
    Thanks,
    Mike Donahue
    "Training Prepares You...For Moments That Define You

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    Quote Originally Posted by ProgressiveRescue View Post
    Has anyone here done any load force testing on the anchors during a skate block operation. If so what were the force readings and what was there peak point.
    Thanks,
    Mike Donahue
    Mike,
    Maybe someone with a dyno will do a few set-ups at various load/angles. It's a hard question to answer due to the many variables involved. In Rick Lipke's Technical Rescue Riggers Guide, Second Edition, there is a reference to tensioning guiding lines. It states: Adjust a loaded guiding line with a pulley system. Do not exceed a factor of 12 when tensioning (4 pullers on a 3:1, 2 pullers on a 6:1, 3 pullers on a 4:1 etc)." You may have already seen this. I just mention it because it's the first time I've seen a specific pretension for guide lines/skate blocks listed in print. Sorry I could not address your specific question.

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    I'm curious because you're not pre-tensioning these systems like you would a tensioned highline. The tension in the system comes once it's loaded by the rescuer and or victim. You do put tension on the system first when lifting the victim by vectoring or a simple haul however it's the forces put on the system mid-span generated by the human load that I'm curious about.
    Mike Donahue
    "Training Prepares You...For Moments That Define You

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    Mike -

    I assume by "skate block" you mean a low-tension guiding type line. If you are talking about a sloped tesnioned line than a completely different response will be required

    If so here is my response. When we were outlining topics for my book we wrestled with application of low- and high-tension systems. We decided it best for the training community and from an applicaiton stand-point to seperate the two.

    The guiding line system aka: skate block, is a low-tension technique. With that being said, we recommend that tension be applied with personnel only (2 people with no MA), or with a minimal MA with minimal personnel (I wouldn't use more than a 3:1 with 1 or 2 people). The only purpose of tensioning the guiding line is to clear the obstacle. The term skate block is generally used in tower applications; in that environment tension would only be applied to clear the tower and the fence. This should amount to very little tension in the system.

    To be honest, I wouldn't be overly concerned with the forces in this application. First, if a handpull is used to tension, the pullers will feel the reactive forces and will not be able to overload the system. If an MA is used again it should be used sparingly.

    There are too many variables to tell you what the forces would be. You would need to know the load, the angle (distance of the anchor from the base) and then it would be dependant on how much tension was actually needed.

    One other note...As with any other time you tension a rope, you only tension as much as needed to get the job done.

    If we are talking about two different things feel free to delete my post

    Jeff

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    Good explanation Jmatt. That's how we play. Tensioned track line stays close to structure on the way down for belay purposes and close to ground during "floating" over grade issues. Skate block for apron towers or landing outside the fence on cell towers if needed. Both have a top belay and both have a pendulum effect with the skate being of more concern.
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    Hey Jeff,
    I'm not concerened about the initial forces applied during the pulling force to clear the victim of the tower I was more curious about the forces on the topside directional pulley and the bottom side directional pulley when the load is about midspan. The angle the victim creats between the two anchors is close to 120 degrees. Granted there's remedeys for this such as choosing or building a anchor point on the ground that's closer to the structure your victim is being lowered from or extending the distance of the chest pully or "guide pully" by means of prusick cord. I'm going to have to pick up two dynos and grab some numbers.
    Thanks,
    Mike Donahue
    "Training Prepares You...For Moments That Define You

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    Quote Originally Posted by ProgressiveRescue View Post
    Hey Jeff,
    I'm not concerened about the initial forces applied during the pulling force to clear the victim of the tower I was more curious about the forces on the topside directional pulley and the bottom side directional pulley when the load is about midspan. The angle the victim creats between the two anchors is close to 120 degrees. Granted there's remedeys for this such as choosing or building a anchor point on the ground that's closer to the structure your victim is being lowered from or extending the distance of the chest pully or "guide pully" by means of prusick cord. I'm going to have to pick up two dynos and grab some numbers.
    Thanks,
    Mike Donahue
    Mike,

    I think you're actually thinking "anchor angles". The angle at the skate block pulley is creating a "directional angle" in the rope. It's just that the "directional pulley" here and the skate block pulley are one and the same. Notice that if you, say, put the lowering mechanism (rack, I'D, etc.) to your harness instead of to a fixed anchor and you stood way out from the wall so as to offset the package as it comes down, you're going to be resisting skid force with your feet. Now walk toward the wall. The angle in the rope at the skate block pulley is getting flatter, but the skid force resistance required of your feet is far less. What's going on? Consider the relationship between anchor angles and directional angles. They go different ways on the graph, intersecting at 120. If you walk with the lowering mechanism toward the wall/tower until the skate block pulley becomes unloaded per a 180 degree angle in the rope, it's very easy to see then what you're doing to the anchor holding the redirect pulley atop the operation. Considering ideal mechanical advantage (not practical), you're doubling the load up there. Now (bear with me here as I continue with my example) climb the tower with the lowering mechanism still on your harness and run the exactly same rigged operation from atop the tower. The skate block set up just became a z-rig that you're lowering with. You've bottomed out the angle at the "skate block" pulley to zero degrees and you're holding a third of the load.

    One could maybe even say that lowering with a skate block is like lowering with a z-rig with really bad parallax in the system. Ha.

    So in a standard skate block lowering op, aside from having the loading doubling awareness at the upper redirect, depending on whatever constitutes the anchor up there, it ought to be rigged to accommodate the load angle changing outward.

    And, notice that with a skate block, your bottom end anchor can be as close as you want to the wall / plumb line (until you've no offset action). But with a tracking line offset, your bottom end anchor must be way out from wall, so as to keep the angle reduced at the tracking pulley, or else end up having a high line.

    With the skate block, it's the fact that the rope is running through the top redirect pulley and back to the load that prevents the upper anchor from being overloaded. The pulley acts a limiter of sorts because it is what allows the load to move. And the load would simply reverse course and go up if you overcame the load's weight at the bottom end by pulling (ignoring pulley friction).

    Regarding the term "guiding line", in the Ropes That Rescue paradigm, the term indicates a minor offset similar to a tracking line, except the bottom end is only held in the hands, which affords lateral movement as well.
    Last edited by EricUlner; 07-29-2011 at 02:50 PM. Reason: additional info

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    Hey Eric,
    Sorry for the delay responding to your post. You made a few great points and set on track a bit. Do you have any other info you could email me?
    Thanks,
    Mike
    "Training Prepares You...For Moments That Define You

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    ----------------
    Last edited by EricUlner; 08-06-2011 at 01:23 AM. Reason: duplicate

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    Quote Originally Posted by ProgressiveRescue View Post
    Hey Eric,
    Sorry for the delay responding to your post. You made a few great points and set on track a bit. Do you have any other info you could email me?
    Thanks,
    Mike
    Mike, I'm not sure exactly what info you are interested in receiving. But if there's any way I am able to assist, I'll do what I can.

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    Hey Eric,
    I looking to get my hands on maybe some diagrams showing the info you described.
    Thanks,
    Mike
    "Training Prepares You...For Moments That Define You

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    Quote Originally Posted by ProgressiveRescue View Post
    Hey Eric,
    I looking to get my hands on maybe some diagrams showing the info you described.
    Thanks,
    Mike
    Mike, I'll send you a response via email.

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