Rope and Gear Testing

 Pull-Testing 11/18/98 and 11/23/98

Salt Lake County Sheriff's Search and Rescue

 

Test Methods

We used a vehicle winch on a Hummvee to apply forces. A second Hummvee was initially used as an anchor. However, with an end-to-end pull, and with all four wheels locked, we were able to drag both vehicles across the concrete floor with 5000 lbs force. (good number to know if you use vehicles as anchors.) We ended up anchoring one vehicle to a tank (yes - really) and the other to eyebolts mounted in the wall. A Sensotec load cell was used to measure forces. It is calibrated internally with a shunt resistor.

Results

11/18 Test #1:

Pull an old 9/16" tubular web sling to failure. This sling was found girth hitched around a tree on Mt. Olympus and had obvious burn marks where ropes had been pulled through it. The sling was loaded with an end-to-end pull on the loop.

Result: Material failure at one of the burn marks at 4410 lbs.

11/18 Test #2:

Pull an old 1" tubular web sling to failure. This sling was found tied around a tree on Mt. Olympus. It had no obvious burn marks on it, but it has to be assumed that at least one rope had been pulled through it. The sling was cut off and retied with a water knot. It was loaded with an end-to-end pull on the loop.

Result: Material failure in the middle of the sling (presumably at an unseen burn) at 4300 lbs.

11/18 Test #3:

Pull a new 1" tubular sewn web sling to failure. One of our members sewed the sling on his home machine with a random stitching pattern (for testing purposes only!) The sling was loaded with an end-to-end pull on the loop.

Result: Failure of the stitching at 5260 lbs.

11/23 Test #3

Repeat of 11/18 Test #3

Result: Failure of the stitching at 5920 lbs.

11/18 Test #4

Pull a new 1" tubular sewn web sling to failure. The sling was sewn professionally with 5 bar tacks. The sling was loaded with an end-to-end pull on the loop.

Result: Failure of the stitching at 4720 lbs.

11/23 Test #2

Repeat of 11/18 Test #4

Result: Failure of the stitching at 4730 lbs.

11/18 Test #5

Pull a new 1" tubular web sling to failure. The sling was tied with a water knot and loaded with an end-to-end pull on the loop.

Result: Material failure in the knot at 4980 lbs.

11/18 Test #6

Pull a new 1" tubular web sling to failure. The sling was tied with a double fisherman's knot and loaded with an end-to-end pull on the loop.

Result: Material failure at the bend over the shackle at 6210 lbs.

Discussion:

The relatively high breaking strength of the two old and burned slings surprised me. Nevertheless, don't expect to see me rapping off a single old sling any time soon. I was also surprised that the home sewing job outperformed the professional bar tacking - and that the bar tacked slings were so weak. I have always been told that sewn slings are stronger than tied ones.

The sling tied with a water knot failed at 4980 lbs. If the two sides are loaded equally, each side carries 2490 lbs of tension. The CMC Rope Rescue Manual claims a 36% strength reduction for a water knot in webbing. Mountain Search and Rescue Techniques also lists a 36% strength reduction, although in nylon rope. If these are correct, the webbing has a material strength of 3890 lbs - very close to the 4000 lb rating usually assumed for 1" webbing.

The sling tied with a double fisherman's knot was the strongest of all slings. It is notable that the sling did not fail at the knot.

11/18 Test #7

Pull a prusik to failure. The prusik loop was made of new 7mm Teufelberger cord, manufactured in 7/93. It is the cord used by the local FEMA team. The prusik was tied with a double fisherman's knot and wrapped as a 3-wrap prusik on old 11 mm Blue Water rope, tied with a figure eight on a bight at the end. The load was placed between the prusik and the 11 mm rope.

Result: Material failure of the 7mm cord as it enters the prusik hitch at 2840 lbs. The prusik had started to slip on the 11 mm rope at almost the same load.

11/18 Test #8

Pull 7mm cord (same manufacturing information as test #7) to failure to determine the actual material strength of the cord. A loop was tied in the cord with a double fisherman's knot. The loop was loaded with an end-to-end pull between two winch pulleys (diameter ~4"). The knot was unloaded by placing it in the middle of a double wrap around one of the pulleys.

Result: Material failure in the middle of the 7mm cord at 4840 lbs. Material strength is 2420 lbs.

11/18 Test #9

Test the strengths of an overhand on a bight and a figure eight on a bight. A very old (retired) 11 mm Blue Water training rope was used with the knots to be tested at each end. The load was applied to a single strand of rope.

Result: Material failure in the middle of the rope at 3290 lbs.

Discussion: Aieeee! The rope did not fail at the knot. It did not have any obvious weak spots in the section we tested, but clearly it had at least one spot that we didn't see that was significantly weaker than the rest of the rope. This is more than a 50% reduction in strength from new rope!!!

11/18 Test #10

Repeat Test #9 with new rope. Brand new 11 mm Blue Water rope was used.

Result: Material failure at the overhand knot at 4660 lbs.

Discussion: The CMC Rope Rescue Manual lists an overhand on a bight as having a 15% strength reduction and a figure eight on a bight as having a 20% strength reduction. Our testing (on one sample only) contradicts this, since the overhand failed first.

11/18 Test #11

Test the strengths of a figure eight on a bight and a double figure eight on a bight. New 11 mm Blue water rope was used. The load was applied to a single strand of rope.

Result: Material failure at the double figure eight loop at 5250 lbs.

Discussion: We were surprised to see the double figure eight on a bight fail first.

11/23 Test #1

Pull an Omega locking carabiner to failure. The carabiner was used, but not visibly damaged. Its rated strength in an end-to-end pull is 26 kN, or 5800 lbs. The load was applied between the shackle on the load cell and the hook on the winch cable. The carabiner was tested with the gate locked.

Result: Gate self-locking occurred at ~300 lbs. Material failure at 7280 lbs. The carabiner failed at the notch where the gate pin engages.

11/23 Test #4

Test the strength of a directional figure eight on a bight. New 11 mm Blue Water rope was used. Two knots were tied - one "correctly" with the rope coming out the same hole as the bight, and the other "incorrectly", with the rope coming out the other hole. The load was applied between the two knots, on a single strand of rope.

Result: Material failure at the "correctly" tied knot at 4950 lbs.

11/23 Test #7

Repeat of test #4.

Result: Material failure at the "incorrectly" tied knot at 4760 lbs.

Discussion: For this knot, the conventional wisdom - that the knot should be tied with the loop and the rope coming out the same hole - appears not to matter.

11/23 Test #8

Test the strength of the rope with a directional eight tied in it. Once again, two directional figure eight on a bight knots were tied - one "correctly", the other "incorrectly". This time the load was applied not to the knots, but to the rope - by a regular figure eight on a bight knot at either end.

Result: Material failure at the "correctly" tied knot at 4030 lbs. The "correctly" tied knot had also slipped considerably by the time it failed. (The bight had grown much smaller.) The "incorrectly" tied knot had distorted quite a bit, but it did not fail.

Discussion: Wrong again - the conventional wisdom seems either to be in error or not to matter for this knot.

11/23 Test #5

Pull an old prusik loop to failure. The loop was made of 7mm cord and tied with a double fisherman's knot. It was used, but not obviously damaged. The loop was loaded with an end-to-end pull.

Result: Material failure in the middle of the loop at 3080 lbs.

11/23 Test #6

Pull a clove hitch to failure. The clove hitch was tied around the shackle on the load cell. The other end of the rope was tied with a figure eight on a bight. New 11 mm Blue Water Rope was used.

Result: Material failure at the clove hitch at 5110 lbs.

Discussion: The clove hitch did not slip! We were all very surprised at this. Before drawing any further conclusions, I would like to test this again - on a carabiner instead of the shackle. I suspect the rough surface of the shackle added extra friction to the knot.

11/23 Test #9

Pull a Ferno litter to failure. The litter was an old and very abused litter. It did not have the eyelets that the new litters have. The litter was loaded end-to-end with new 1" tubular webbing. One side was tied as we normally set up the litter for "scree evac", wrapping through the second set of handles where we would normally wrap through the eyelets. The other side was tied similarly, except that the 1" webbing did not wrap around the litter rail at the second set of handles.

Result #1: Plastic deformation at the webbing at the incorrectly tied side at ~2000 lbs. Litter was untensioned and retied correctly.

Result #2: Aluminum rail cracked at 4120 lbs. Continued pulling to failure at the rivets where the inserts connect to the aluminum rail without reaching 4120 lbs again.

Discussion: Good news! I had feared much worse.

11/23 Test #10

Test the maximum force a belayer can hold with a sticht plate belay device. New 11 mm Blue Water rope was used. The loaded end of the rope passed through the sticht plate, around the shackle on the load cell, and back through the sticht plate to the belayer.

Result - bare hands: Slipping at 880 lbs

Result- gloves: Slipping at 1060 lbs

11/23 Test #11

Test the maximum force a belayer can hold with a Münter hitch belay. New 11 mm Blue Water rope was used. The Münter hitch was wrapped around the shackle on the load cell.

Result - bare hands: Slipping at 1550 lbs

Result- gloves: Slipping at 1800 lbs

Discussion: As in test #6 on the clove hitch, I now think the results from these tests should not be believed until they are repeated using an aluminum carabiner instead of the steel shackle. I think the roughness of the shackle probably caused the peak loads to be higher than a belayer could hold with a carabiner. In addition, this represents an absolute maximum for a static pull, with a braced belayer using both hands.

11/23 Test #12

Pull a Kleimheist hitch to failure. The Kleimheist was constructed from a loop of new 1" tubular webbing tied with a water knot. It was wrapped seven times around a piece of new 11 mm Blue Water rope.

Result: Slipping at 1690 lbs

11/23 Test #13

Same as test #12, except that a four-wrap Kleimheist was used.

Result: Slipping at 2050 lbs. Test was stopped at this load and then continued.

Result #2: Material failure in the webbing at the Kleimheist knot at 3460 lbs.

Discussion: As often happens in prusik testing, the slipping probably caused some melting, which fused the Kleimheist to the rope when the test was interrupted.

11/23 Test #14

Pull a bowline knot to failure. A bowline was tied in each end of a piece of new 11 mm Blue Water rope. One knot was tied "correctly". The other was tied "incorrectly", with the tail of the rope outside the loop formed by the bowline. The load was applied between the two bowline knots, on a single strand of rope.

Result: Material failure at the "correctly" tied knot at 4840 lbs.

Discussion: Conventional wisdom bites the dust again. The CMC Rope Rescue Manual lists the strength reduction for a bowline knot as 33%. Mountain Search and Rescue Techniques lists it at 35%. This would make the material strength of the Blue Water rope between 7200 and 7400 lbs. Similar estimates from the testing on figure eight on a bight knots put the material strength of the rope at between 6500 and 6800 lbs. We have not yet directly tested the material strength of the rope as we did for the 7mm cord.