Pull-Testing at the MRA Intermountain Recert - 7/00
At the MRA Intermountain Recert in July we did a pull-test session using a vehicle winch and our group's load cell. The usual disclaimers apply here - these are all quick look tests, a single pull on each setup, and there's no control over the rate of pull.
A quick word on safety - since the picture looks a bit sketchy. Everybody in the photo is 10-15 feet back from the winch cable and the sample - something you can't really see from this angle. The whole system is steel except for about two feet of Nylon static rope. That means the energy storage in the system is very small. (If you do this with dynamic rope, be careful -things fly around more.) The forces we were generating were well under the strength of the cable and shackles we used to connect to the sample. A blanket was placed on the line at the sample to minimize the recoil. (In most of the pulls, we had one on each side of the sample.) The people closest to the sample were all wearing safety glasses or plastic sunglasses. When the sample failed - in every case things traveled down and close to the line of pull. The best way to do this kind of testing is to make a lexan shield to put over the sample. That way everyone can see through it, but breaking parts are contained.
Test #1: Pull a Gibbs (dimpled,
"rescue" verson) on used 11 mm static rope.
Result: Stripped the sheath off the rope at 2560 lb. Inside the core of the rope was a tracer that said "New England Rope, 1997, 11mm"
Test #2: Test 5.5 mm Vectran.
The load was applied to a loop tied with a triple fisherman's
Result: Cord broke at TFK at 3580 lb.
Test #3: Test 5.5 mm Vectran.
The load was applied to a single strand with a bowline in one
end and a figure-eight on a bight at the other end.
Result: Cord broke at figure-eight knot (!!!) at 1200 lb.
Test #4: Pull an old, used 8mm
prusik to failure. The material was 8mm nylon cord, tied with
a double fisherman's knot and wrapped as a 3-wrap prusik on used
11 mm New England static rope, tied with a figure eight on a bight
at the end. The load was placed between the prusik and the 11
Result: Cord broke where it enters the prusik hitch at 1870 lb.
Note: This is very low for 8mm material.
Test #5: Test 5mm Technora (orange
sheath). The load was applied to a loop tied with a double fisherman's
Result: Sheath broke at 2340 lb. Core untied by pulling through knot.
Note: The breaking strength is very low for Technora (Tech Cord). I have no way to confirm that the material was actually Technora.
Test #6: Pull an old, used piece
of 7/16" goldline rope. The load was applied to a single
strand with figure-eight knots at each end.
Result: Broke at knot at 3090 lb.
Test #7: Test 5mm Technora (orange
sheath). The load was applied to a loop tied with triple fisherman's
Result: Cord broke at shackle at 3720 lb.
Test #8: Pull a 10 year-old homemade
1" sling to failure. 5 bar tacks. Load was applied to the
Result: Broke at shackle at 4240 lb.
Test #9: Pull a 5/8" sewn
runner with 6 bar tacks. Load was applied to the loop. Unknown
Result: Broke at shackle at 5050 lb.
Test #10: Test old, used 5.5mm
Vectran. The load was applied to a loop tied with a triple fisherman's
Result: Cord broke at shackle at 2520 lb
Test #11: Test the beer knot.
This is an alternative to the water-knot. It's tied as an overhand
follow-though, but open up the tubular webbing and do the follow
through inside the other piece. The load was applied to a loop
of used 1" webbing tied with the beer knot.
Result: Webbing broke at shackle at 4750 lb.
Test #12: Test the holding power
of a belayer using a Munter hitch. The Munter hitch was tied with
used 11mm static rope on an aluminum carabiner. The belayer did
not wear gloves, but he sat down, braced himself and held on for
all he was worth.
Result: Slipped at 1430 lb
Note: this doesn't represent a typical belay.
Test #13: Same test of a Munter
hitch under more normal belay conditions. Belayer standing, wearing
Result: Slipped at 670 lb
Note: Without measuring the force applied by the belayer, this is a very subjective test. In drop tests conducted by others on a Munter hitch belay with a 200 kg load, factor 1/3 drop on static rope, the belayer has always been unable to keep the load from hitting the ground.