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

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?


 

Can a hot belay device melt rappel slings?

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.

 

Spit test

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.

Test #1

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.

Test #2

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.

Test #2

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.

Test #3

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.

Measuring the ATC's temperature while pressing it to Kevlar cord

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.
Cooking a Dynex sling in the oven
 

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