Vertical Rescue (VR) Equipment Load Testing Report

Transcription

1 Vertical Rescue (VR) Equipment Load Testing Report Steve Cliffe (Wollongong SES) & Alan Sheehan (Oberon SES) NSW SES VR Subject Matter Advisory Group (SMAG) Introduction This paper describes the results of some equipment load testing that was carried out during a session at the Vertical Rescue Professional Development workshop held at Kiama on May The main purpose of this testing was to give the participating Vertical Rescue Trainers an appreciation of the load that various equipment and knots can withstand, and the way in which they fail. Setup The testing was conducted at the Kiama SES Unit s headquarters. A load cell provided by our State Headquarters was used to give an accurate load reading at the point of failure with it s peak hold function. The load cell was a Straightpoint NIP/5T 5tonne full scale load cell with remote display unit. It has a display accuracy of +/- 0.5kg, and a measurement accuracy of +/- 2 kgs over the full scale range according to the calibration data. Two trees approximately 25m apart used as anchor points. At one tree the load cell was anchored and then attached to a length of old 11mm Bluewater static rope via a double figure-8 loop. At the other tree a Tirfor winch was anchored with it s cable setup with a SWR snatch block to give a 2:1 mechanical advantage. The snatch block was attached to another length of old 11mm Bluewater static rope via a double figure-8 loop. The nonanchored ends of the 11mm rope were then joined with knots or equipment under test. For safety, anchored lengths of 9mm rope were attached to the 11mm main rope via prussik loops either side of the equipment or knot being tested to catch the rope after a failure. A diagram of the setup used appears below:

2 Test 1: 6mm triple-wrap prussik loop For this test one 11mm rope was connected to a prussik loop formed from 6mm static cord via a steel screwgate karabiner. The loop was then attached to the other 11mm rope with a triple-wrap prussik loop. The prussik cord snapped at 600 kgf away from both knots indicating a possible weak or damaged spot in the cord. No photo available. Test 2: 6mm double-wrap prussik loop The same cord from Test 1 was re-tied into a loop and then attached via a double-wrap prussik knot. The prussik cord snapped at 891 kgf. Some slippage and heat damage was evident in the 11mm rope near the prussik knot. Note the necking and removal of fuzz on the 11mm rope to the left of the prusik. Careful examination shows the cord failed in the knot, and friction from slippage in the knot created enough heat to weld the loop to the rope before the knot slipped fully undone. Test 3: 8mm triple-wrap prussik loop For this test one 11mm rope was connected to a prussik loop formed from 8mm static cord via a steel screwgate karabiner. The loop was then attached to the other 11mm rope with a triple-wrap prussik loop.

3 The prussik cord snapped at 1392 kgf at the prussik knot after slipping approximately 20cm on the 11mm rope. The 20cm length of 11mm rope suffered heat damage and was visibly thinner and stiffer than the rest of the rope. The prussik knot itself had fused with the resultant heat. Note the melted prusik loop material in the interstices of the rope sheath on the LHS of the knot. Note also the melted prusik loop sheath material extruded between the wraps of the knot. Test 4: Double figure-8 knot - incorrectly tied The two ends of 11mm rope were joined with a double figure-8 knot so that both ropes entered the knot from the same end. This method of joining ropes it not used in Vertical Rescue but has become popular with canyoners so we were curious as to it s strength and behaviour. The abnormally tied figure eight bend. Note the length of the short tail prior to loading.

4 Approximatel y 3-4cm slippage was observed at the knot. Note the length of the short tail underload. One of the 11mm ropes snapped at the knot at 1237 kgf (LHS) and peeled half a bend out of the knot. WARNING: The author has observed the abnormally tied figure eight bend slipping under normal abseil loads, and as such expressly warns against this knot s use for any life support activity.

5 Test 5: Double fisherman s knot For this test we compared the results of the previous test with the method we normally use to join two ropes, namely the double fisherman s knot. One of the 11mm ropes snapped at the knot at 1443 kgf (LHS). No noticeable knot slippage was observed. Test 6: CMI Hand Ascender For this test a CMI hand ascender was connected to one of the 11mm ropes via a steel screwgate karabiner. The ascender was then attached to the other 11mm rope via it s cam. The ascender failed at 779 kgf with the rope and cam being pulled through the ascender. The 11mm that was attached via the cam suffered sheath and some core damage. The body of the ascender showed stress damage in the area around the cam.

6 Note the cam pulled completely through on the ascender. This is somewhat unusual for a statically loaded ascender. Note the crack in the side of the J groove. The rope damage is also a little unusual, showing a second chunk out of the sheath joining the original sheath damage. This suggests the ascender tore the sheath and gripped again at the edge of the original damage the resulting shock load destroying the ascender. End view of the damaged ascender. The cam pivot has bent causing the misalignment of the cam. The J groove was also cracked. This type of failure of the ascender is not the expected failure mode. Usually the ascender tears a chuck out of the rope, and slips until the load is relieved then grips again. We suspect this ascender gripped again before the load was relieved, and the resulting shock load has resulted in the damage to the ascender itself. Test 7: Alpine Clutch One of the 11mm ropes was attached to two steel screwgate karabiners. The other 11mm rope was then attached to the karabiners via an alpine clutch.

7 The 11mm rope snapped at the alpine clutch at 1404 kgf. This photo shows the alpine clutch with some load applied. This photo shows the rope ends after the rope failed in the alpine clutch. We did not expect the alpine clutch to perform as well as it did. We expected the rope to be cut at lower loads.

8 Incidental Samples: A double fishermans knot in the system following the 8mm triple wrap prusik test. I.e. this knot had been loaded several times including loaded to 1392 kgs. Figure 8 loop that survived all the load tests including several load applications in excess of 1400kgs. Another Figure 8 loop that survived all the load tests including several load applications in excess of 1400kgs.

9 This prusik knot was one of the safeties to prevent the rope from recoiling dangerously. This prusik was dynamically loaded only as a result of the recoil from one of the tests that let go at about 1400 kgs. Note the prusik slipped approx. 30mm (LHS of knot) before welding to the rope. Disclaimer The information provided in this paper is presented in good faith. While every effort has been made to eliminate mistakes and false information from the information included in this paper, errors may occur. The authors, and The New South Wales State Emergency Service, its employees, volunteers and Units do not accept responsibility for any errors contained in this paper or for the results of the application of this information correct or otherwise. Vertical Rescue is a hazardous activity and requires appropriate quality equipment, and sound initial and ongoing training, teamwork, discipline, protocols and procedures to be executed safely. Check your vertical rescue protocols before applying the information provided in this paper.