Nevada Space Grant
| We use small aluminum "carabiners" to connect
payloads together (actually they are key chains - not to be used for
rock climbing!!!). We have used steel locking chain links from
Home Depot, but they weigh a lot.
During NBS-07-07 we lucked out because one of the carabiners actually broke. It was holding the bottommost payload, which was lying only a few feet away from the rest of payloads. We are not sure when the carabiner broke, but we think it must have been during on landing. It appears that the carabiner snagged some sagebrush during landing, causing the 'biner to fail and separate that payload from the rest of the payloads.
After that experience we decided to take a closer look at the
construction of the carabiners. Unlike real rock climbing carabiners,
the screw gate is not actually attached to both ends of the opening. If
you pry the carabiner open, the gate simply slips off the unthreaded
end (while staying attached to threaded end).
We decided to test a variety of
carabiners to see which ones are strong enough to use on future
missions. We used a
digital force gage to measure the maxium load the carabiner can hold
before opening. All of the carabiners were aluminum, except for the
stainless steel ones as noted below. The photo below shows the results
of the tests from lowest to highest failure load (from left to right):
From our tests it appears that the larger the carabiner, the
failure load. This makes sense from a strain standpoint becuae the
strain required to open the carabiner would be lower for a larger size.
It also appears that the location of the gate makes a difference. If
the gate is closer to the "small" end of the carabiner, it seems to
have a higher failure load. Again, this makes sense from a strain point
of view. This is probably well documented somewhere for acutal rock
Although the stainless steel carabiners failed at a fairly low
lbs), the failure mode was good. Whereas the aluminum carabiners fail
suddenly (the carabiner "pops open"), the stainless steel carabiners
slowly yeild open as the load
is increased (typical of steel yeilding). It took about 30 lbs to open
the steel carabiners wide enough for a string to pass through and about
double that to open it to the point shown in the photo.
We have also found zipties to
get brittle in cold temperatures, so we do not use them either.
While shopping at Walmart, we came across a package of
aluminum carabiners (sold in pairs) for less than $2. The nice thing
about these locking rings is that both sides of the gate are threaded.
This means the threads have to physically fail in order for the whole
structure to fail. We tested it closed and it supported 100 lbs (the
maximum rating of the load cell used in all the tests) without failure,
more than any other carabiner tested. We then opened (unscrewed) the
gate and tested it again. The photo below shows what 100 lbs of force
did. The carabiner bent but did not break.
At 2.9 grams each, these seem to be a good balance between
weight, cost and strength. The come in both red and green.
Conclusions: The Walmart screwlocks are more than adequate for our use.