QC LAB: CAN A HOT BELAY DEVICE MELT MY SLINGS?

Our QC Lab posts aim to answer some of climbing's most common gear-related questions. This time—how hot do belay devices really get?

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.

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:
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