Build My Own FeedQC LAB: Can a hot belay device melt my rappel slings?
Before any piece of Black Diamond gear makes it on to the shelves, it spends months, sometimes years getting put through the wringer by our Director of Quality, Kolin Powick, and his team of Quality Assurance engineers. Through extensive and meticulous testing, both in the lab and in the field, KP and his team help ensure that you can count on your BD gear to be as durable, reliable and as strong as possible every time you head into the mountains or out to the crags. Our QC Lab posts aim to answer some of climbing's most common gear-related questions.
This month KP and the crew tackle a question that has been floating around the office here lately—can a belay device get hot enough during rappels to actually melt the sling you are clipping into the anchor with?
It's happened to us all before—you go zipping down a rap line, get to the ledge, and grab the belay device to remove it from the rope, only to burn the crap out of your hand. Belay devices can get hot, but can they get hot enough to melt your rope, or maybe the sling you're using to anchor in with? This is the question that keeps Black Diamond's Content Manager, Jonathan "JT" Thesenga, constantly hovering around the QC lab, asking if we've determined whether or not a hyper-hot ATC could get him killed when rapping off a multi-pitch route. I answer with what every climber should know: you should always be clipped in with two independent slings, and use nylon if you're paranoid, since it has a higher melting temperature than Dyneema/Spectra/Dynex.
But his questions got us thinking, so we decided to put a quick battery of tests together to get some real data to help put him at ease, and more importantly keep him out of the lab.
History
Now I've never heard of a case where someone's hot belay device melted through the sling they were anchored in with, however, I have heard of the following:
- Burning your hands when touching the belay device or carabiner after a long rappel—we've all done it
- A few instances where people were rappelling quickly, stopped at the end of their rope to fiddle at the next anchor station, and the belay device melted into the sheath of their ropes
- Rappelling so quickly that parts of the sheath of the rope were "glazed"
However, I have never heard of a sling being burned through by a hot belay device. So given this, we ultimately decided that were trying to learn two things:
- How hot can a belay device get when rappelling?
- Can a hot belay device melt a sling?
The Testing
Any of these tests will start with a few certain facts and assumptions:
- For argument's sake, Dyneema, Spectra and Dynex are all the same thing. Basically different brand names for UHMWPE (Ultra High Molecular Weight Polyethylene), which has a melting temperature of around 145 °C (293 °F)
- Nylon has a melting temperature of around 245 °C (473 °F)
- If something is in the neighborhood of 70 °C (158 °F), it's basically too hot to touch
- A typical "hot knife" used at climbing shops to cut accessory cord usually tops out at over 650 °C (1202 °F).
- Skin burns at 100 °C (212 °F)
- An old assumption in climbing is to spit on your belay device. If it sizzles, the assumption goes, then it's hot enough to melt your rope, slings, etc.
As in any QC Lab test, there are a lot of factors to consider. What kind of belay device? Rapping on what diameter of ropes? Single or double line rappels? How heavy is the person, with/without haul bags? Single or multiple raps? What material is the sling you are using to anchor yourself? How much tension is on the sling? The permutations and combinations are endless, and in order to get a general idea we had to narrow it down, make a few assumptions, and then finally perform a few quick tests. Ultimately we settled on four different tests to help us wrap our brains around this situation.
First, however, we wanted to find out exactly when spit actually sizzles. Seeing as this is known as good, simple indicator of temperature, we decided to prove it out. We cooked belay devices in our environmental chamber in increments of 5 °C to try to determine when spit actually sizzles.
Knowing this, we could use 120 °C (248 °F) as a baseline for how "hot" is hot.
TEST #1: How hot can we get a belay device? (simulated rappelling)
A few of the crack crew of Quality Engineers at BD spent a few hours out in the parking lot setting up a simulated rappel. Let's just say it involved 125 feet of several different rope diameters, a load cell, several belay devices, some thick gloves, an infrared thermometer... and a pickup truck. Our test setup was definitely not OSHA approved or recommended, but it did get us some good results.
The idea was to drive the truck at a reasonable rate representing a high-speed rappel (125 feet in 10 seconds), as the belayer was keeping the pre-determined load on the load cell (representing different masses of climbers and/or haul bags combined). The chart below shows each rope/gear configuration and the temperature achieved.
We attained quite the range of temperatures with our simulated high-speed rappels, from a reasonable temperature of 133 °C (271 °F) for a 250-pound climber, up to 256 °C (493 °F) for 400 pounds of a combined climber/haul-bag weight.
Some of these temperatures do enter the danger zone when compared to the estimated melting temperature of UHMWPE, and even nylon, but let's remember this was a pretty extreme, not overly realistic test.
TEST #2: How hot can we get a belay device? (actual rappelling)
Given the range of values we obtained in the parking-lot test, we then headed to a local cliff, set up a 150-foot rap (unfortunately NOT free hanging) using a 10 mm cord, and had our lightest and "not-lightest" Quality Engineers rap the line. We then took a temperature reading at the bottom with our infrared thermometer. Each guy did two rappels—one at a moderate rate and one at a high-speed-Kamikaze-not-recommended speed.
This showed us two things:
- Rappelling faster resulted in a hotter belay device
- A heavier climber can get a belay device hotter
We could have guessed both of these, but it's always nice to get some empirical evidence. These were single raps, however, and we were wondering if the belay device got progressively hotter as you continued to rappel, as one would while descending a multi-pitch route with multiple rappels, eventually getting hot enough to burn through your anchoring UHMWPE sling (JT's biggest concern). For the sake of expediency, we rallied five of us to the same cliff at lunch one day and brought a fishing rod. Yes, a fishing rod. Each one of us zipped down the line at a reasonable/fast rate, took a temperature reading at the bottom, clipped the belay device onto the fishing line and reeled it back up to the top for the next guy to use.
In our limited test, this shows that the belay device didn't really get progressively hotter, and we basically peaked out at about 135 °C.
Check out the video below for a look at both Test #1 and Test #2:
TEST #3: What temperature does a belay device need to be to melt through various tensioned polymers?
The point of this test was to determine to what degree a hot belay device compromises the strength of slings. We pre-tensioned the slings with 250 pounds to simulate a climber with all his weight hanging off the sling, heated the belay devices in our environmental chamber in 25 °C increments, and pushed the flat side (realistic case) of the ATC as hard as we could to see if we could get the sling to cut.
This test shows that yes, with a tensioned sling, and pushing the belay device hard against the sling, it's possible to get it to cut but ONLY if the belay device is at high temperatures—higher than we could obtain in the field.
This also shows that the flat webbing and Kevlar cord could withstand higher temperatures than the 10mm Dynex, with the nylon supertape performing even better still. So the fears of having a warm belay device just come in contact with a sling causing it to sever are starting to fade.
TEST #4: At what temperature does a Dynex sling actually start to melt?
Seeing as we weren't really having much luck melting slings, we decided to slowly increase the temperature on our oven until we could visibly see a Dynex sling start to melt. A few things to remember:
- The literature says that this material's melting point is 145 °C (293 °F).
- Dynex slings aren't actually made of 100% UHMWPE. There usually is nylon included to add some color.
We didn't really notice any visible signs of melting until the oven was stable at about 160 °C (320 °F):
- 160 °C - Dynex sling started to twist and contract
- 165 °C - actual melting
After we let the sling cook for a while at 165 °C (329 °F), we decided to break it in the tensile tester and attained a result of 540lb (about 11% of the rating 22kN [4946 lb]). Better than we expected, actually, considering the melted mess that it was.
Conclusions
As usual this is far from comprehensive. There are many more tests we could dream up and investigate, but what did we learn from our limited testing?
- Spit sizzles at about 120 °C (248 °F)
- In real world use, we couldn't get a belay device over 135 °C (275 °F)
- In simulated extreme conditions we could get a belay device over 170 °C (338 °F) and even into 250 °C (482 °F) with extreme loads
- The speed of the rappel definitely affected the temperature of the belay device (faster rappel = hotter belay device)
- The mass of the load/climber drastically affected the resulting temperature of the belay device after a rappel (heavier load = higher temperature)
- We did manage to cut through tensioned slings with hot belay devices:
- Dynex at 250 °C (482 °F)
- 9/16" nylon Supertape at 325 °C (617 °F)
Bottom Line
The bottom line is that we couldn't get a belay device in a realistic scenario to a temperature where it would melt a sling. JT is more at risk of burning his hands than melting through the single 10mm sling that he's anchored to while rapping a multi-pitch route.
However, a few things you can do to reduce your stress level:
- Rappel slower—this keeps the temperature of the belay device lower
- Rappel with lower loads—this also keeps the temperature of the belay device lower
- Use nylon slings to anchor in with—they have a higher melting temperature
- Always anchor in with two slings—this is just good practice
- If unsure, the spit test can give you an idea of approximate temperature
Be safe out there,
KP
United States / English 
9 Nov 2012, 2:23AM
Hey folks,
thanks for the tests! Could you give some more explanation for the people that are not so familiar with all the mentioned product types? What are flat webbings, Kevlar cords, Dynex slings and nylon supertapes? How to identify them if not labled in an online store?
Thx!
29 Oct 2012, 1:26PM
I'd also be curious to hear how the state of the rope affects the heating. Specifically, a dirty rope with embedded grime seems to heat up belay devices (due to greater friction) more than a clean, new rope. Obviously how "dirty" a rope is is not quantifiable but some testing around this would be enlightening.
Thanks for an interesting article.
24 Oct 2012, 6:09PM
Thanks for this - I have often wondered as I do the hot potato with my rap device.
23 Oct 2012, 4:08AM
Interesting stuff. Can you please perform one more part to these tests to satisfy my curiosity / fears. You heated a sling to 165 degrees and it failed at 11% of its rating (540lb) I weigh 110kg (240lb) any sort of shock load on that melted sling of yours could have almost definitely caused a vital failure for me (I know you shouldn't shock load your slings but still).
Mark or note a spot on some slings and heat them as if by ATC, repeating in say ten degree increments. Test the slings failure percentage v's rating loads and see what comes of it, noting if failure takes place at the heated part of the slings. Please.
17 Oct 2012, 1:56PM
Does the state of the rope effect temperature increase ?
That is, does a dirty, or an old furry rope, or small diameter rope, heat up more ?
16 Oct 2012, 11:17AM
To expand on Phil Stone's question:
Why you didn't address the melting points of ropes in this test. Are there published figures you can site, or have you done similar tests as Test#3 with ropes?
16 Oct 2012, 9:46AM
Thanks guys for risking your lives rappelling without helmets in order
to gather the data for this experiment. :)
11 Oct 2012, 8:36PM
I'd be curious to know at what point the strength of the sling is compromised by the heat. I'm not worried about it melting through, but if a 275 degree belay device touches a dyneema sling, does that weaken it at all?
11 Oct 2012, 3:22PM
Did y'all get any results for a belay device melting onto the sheath of a rope? Would it be similar to your results for a nylon sling?
11 Oct 2012, 3:20PM
Great work guys. I would really like to see similar testing to see how hot a prussik can get when used as a rappel backup. A lot of people seem to think that you could damage a spectra runner if you use it that way.
11 Oct 2012, 3:04PM
According to you video all your tests were through a single rope which would be representative of rapping a fixed line. I think It's safe to say that the majority of us will be rapping with two strands of rope in the ATC so that we won't be leaving fixed lines on every rappel we do. Does this make the ATC considerably hotter then just a single strand? I never really thought the temperature could be an issue, but interesting food for thought. Does the rate at which we rappel effect how quickly the teeth in the ATC-guides will be worn down?
Thanks guys! Keep it up.
11 Oct 2012, 10:11AM
Thanks for that, as a climber or many years.. okay a few decades, this question has nagged me a bit. Your tests are very reassuring. However, no matter how hot I've seen a belay device get it's never really looked like it was actually going to melt through the rope - but what about progressive weakening?
Say I rap on the rope a thousand times over its useful life, what effect does this repeated heating have on the structure and strength of the material??