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QC LAB: Can a hot belay device melt my rappel slings?

Discussion in 'Tech Tips and Gear' started by Jimmy Olsson, May 16, 2013.

  1. Jimmy Olsson

    Jimmy Olsson

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    TannerT, Deerchaser and Mountaineer like this.
  2. ratagonia

    ratagonia

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    Sweet! Thanks for bringing this forth, Jimmy.

    Tom
  3. TannerT

    TannerT

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    Very cool. Thanks Jimmy.
    And remember, you can always do a spit test. Good words of advice
  4. Kuenn

    Kuenn

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    Good info.

    I've personally witnessed smoke trailing from rappel device on more than one occasion. However these were very long raps (> 200m) and mostly free hanging. It may not melt the rope through but it will unnerve the crap out of you.

    Their field testing (in video) was not free hanging so the ability to achieve elevated temps would be challenging. The simulated test were certainly more effective for that.

    I have to take a jab at their safety standards....speed rappels without helmets? :disagree:
  5. Jimmy Olsson

    Jimmy Olsson

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    Kuenn: You're right about that. Also, weather conditions and air temperature will probably have an effect on how fast the rap-device gets cold. If there's chilly winds or burning hot like in death valley... Otherwise it is one more reason why to bring at least 2 gallons of water during your trips ; )

    However, I don't see any point in rapelling down in super speed in reality? Chillax and watch the great view during your rappel. :)
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  6. Kuenn

    Kuenn

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    Couldn't agree more; I'm a big believer in enjoying the journey and taking in the view. There is, however, a direct correlation between the enjoyment experience and gradual numbness in the legs - at which point it's time to speed'r up!
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  7. Kuenn

    Kuenn

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    Shown only to demonstrate that it is possible to heat up a rappel to the point of rope smoker.


    For your viewing pleasure/criticism.
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  8. ratagonia

    ratagonia

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    Jimmy - you have not heard of this thing, the Flash Flood?

    There are many times when rappelling "expeditiously" is a good thing to do.

    Tom
  9. Jimmy Olsson

    Jimmy Olsson

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    Tom: Yeah, you're right. That might be an exception. :)
  10. Benjamin Pelletier

    Benjamin Pelletier

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    The URL seems to have changed to this:
    http://www.blackdiamondequipment.com/en/qc-lab-can-a-hot-belay-device-melt-my-rappel-slings.html

    But just in case it disappears, here are my notes:
    * Nylon melts at 245 C
    * Dyneema/Spectra/Dynex melts at 145 C
    * Too hot to touch at ~70 C
    * Hot knives (for cutting ropes) are 650 C
    * Spit sizzles around 120 C
    * Using a pickup truck to pull 125' ropes through an ATC (at 12.5 ft/s) of a stationary climber who adjusted braking to maintain a certain tension in the rope, an infrared thermometer measured 133 C for a 250 pound climber and 256 C for 400 pounds
    * Having unladen climbers rappel quickly down a non-freehanging drop on 10mm cord, reel the belay device quickly back up the cliff, and repeat, the maximum ATC temperature measured with an infrared thermometer was 135 C [I disagree with their characterization that the temperature basically leveled off at this point though]
    * Rappelling faster and being heavier both resulted in hotter ATCs in their real-world test
    * Pressing a 250 C belay device against a sling tensioned to 250 pounds caused it to break
    * Pressing a 325 C belay device against tensioned 1/2" flat webbing caused it to break, 300 C left a mark. Similar results for nylong 9/16" supertape & 5.5mm Kevlar cord (!)
    * Dynex slings were observed to start melting in an oven at 160 C
    Ram likes this.
  11. Benjamin Pelletier

    Benjamin Pelletier

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    Summarized from a thread on canyoneering.net (now protected :facepalm:):

    Mark notes that a Pirana is 0.09 kg of aluminum, which has a specific heat of 905 J/kg/C. The potential energy an 90 kg (200 pounds) rappeller has at the top of a 60m rappel is about 53,000 J (m*g*h), so if all that energy were transferred into the Pirana and none of it escaped, the device would get to be 650 C above ambient. Devices don't get this hot because 1) they're being cooled by the air 2) they're radiating heat away and 3) some of the potential energy is used to bend the rope rather than heat the device.

    Some good papers on how friction works while rappelling are:
    http://www.jrre.org/att_frict.pdf
    http://www.paci.com.au/downloads_public/PPE/19_Belay_Device_Theory.pdf

    The latter paper suggests that only about 40% of the potential energy is used to heat an ATC. The other 60% goes into bending and unbending the rope, and we don't really care about that part because it's spread out over the whole rope so it can't do much damage.

    I measured the heat dissipation ability of an ATC and figure 8 and they're both around 0.2 C/W with no/little wind and 0.55 C/W with a fan blowing (graph of data below). So, based on this data, an ATC can conservatively dissipate only around 21 W without getting hot enough to melt anything. If the 40/60 split is correct, a 200 pound rappeller could only descend at about 0.15 ft/s indefinitely without getting the ATC hot enough to melt Spectra, or 0.3 ft/s without melting nylon. This result was surprising to me (since I'm pretty sure people don't take 15 minutes to descend the last rappel in Heaps, yet still don't melt their nylon ropes), but I haven't had a chance to do some better tests.

    [​IMG]
    Ram likes this.
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