International Technical Rescue Symposium Presented by John McKently

Transcription

1 Some of the factors affecting pulley efficiency in theoretical and real world rescue systems. International Technical Rescue Symposium Presented by John McKently 2011

2 It is generally accepted that pulleys with large diameter sheaves are more efficient than ones with small diameter sheaves. We wondered, Well, how much more efficient? for pulleys typically used in rope rescue. We also wondered What other factors should be considered in deciding on the right pulley for a given application? In other words, given all the other pros and cons, is a pulley with a bigger diameter sheave always a better choice?

3 No pulley is 100% efficient, as there are always losses due to friction. While testing pulleys in a lab can provide a reasonable estimate of their efficiency, variables such as line stretch, bending and unbending, and line/sheave friction will affect the results. Even the use of steel cable cannot completely eliminate these variables. In this exploration we were more interested in the relative efficiencies of the various pulleys tested, not their absolute efficiency.

4 While we acknowledge that the results obtained in this testing will differ somewhat from results using different pulleys and/or ropes, we believe they are representative of the results that would be obtained using pulleys, ropes and system configurations that are typical to rope rescue operations. As such, we find them valuable as a lead-in point for discussions on equipment selection.

5 Single sheave pulleys used (all with sealed ball bearings): CMC Rescue 1.5 tread diameter CMC ProSeries 2.25 tread diameter CMC ProSeries 3.75 tread diameter Test masses used: 300 and 600 pounds Ropes used: 1/4 steel cable 3/8 Static-Pro (100% polyester kernmantle) 7/16 River Rescue Lifeline (nylon/polyolefin, braid-onbraid) 1/2 Static-Pro (100% polyester kernmantle) 1/2 CMC Lifeline (100% nylon kernmantle)

6 Other pulleys tested in a 1:1 system: Steel carabiner with 0.5 diameter frame DMM Pinto sheave with Oilite bushing Petzl Rescue sheave with sealed ball bearing CMI RP sheave with bronze bushing CMI RP121A sheave with sealed needle bearing

7 For the 1:1 system tests, one end of the haul line was connected directly to the mass. The line was run up through the pulley to be tested, then down to a COD pulley anchored to the floor, and up to an electric winch used to lift the load.

8 A load cell was placed in-line between the pulley being evaluated and the COD pulley to record the force on the line at a rate of 200 times per second. The interior angle of the line through the pulley being tested was small, approximately 5-7 depending on the sheave size.

9 For the M/A tests, the COD pulleys were anchored above and the moving pulleys were connected to the mass. The load cell was placed between the end of the haul line and the winch. The mass was lifted 24 at a rate of 2 per second.

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11 As can be seen in the graph from the sample raw data, the force on the haul line leveled off and remained essentially constant after the mass was in motion and the stretch had been taken out of the line. For each set-up, two evolutions were recorded. The force on the line was averaged over a 4 second span after it had leveled off, and then averaged for the two evolutions. The following graphs are based on these averages.

12 700 1:1 system with 300 pound mass on 1/2" Static Pro Pounds Force Required " Steel Carabiner DMM 0.75" CMI 1.625" bushing Petzl 1.5" CMC 1.5" CMI 1.625" bearing Pulley Type CMC 2.25" CMC 3.75"

13 100 1:1 system with 300 pound mass on 1/2" Static Pro Efficiency (%) " Steel Carabiner DMM 0.75" CMI 1.625" bushing Petzl 1.5" CMC 1.5" CMI 1.625" bearing Pulley Type CMC 2.25" CMC 3.75"

14 800 Pounds Force Required " 2.25" 3.75" 600# 300# 1:1 system with 1/2 Static-Pro Efficiency (%) # 300# Observation: measured efficiency is slightly higher with a larger mass " 2.25" 3.75"

15 System configuration: one COD pulley anchored above, one moving pulley attached to mass Rope: 1/2 Static-Pro Lbf Required Actual M/A Efficiency (%) % M/A Lost

16 System configuration: two COD pulleys anchored above, two moving pulleys attached to mass Rope: 1/2 Static-Pro Lbf Required Actual M/A Efficiency (%) % M/A Lost

17 System configuration: two COD pulleys anchored above, one moving pulley attached to mass, second moving pulley attached via 8mm Prusik to input side of first moving pulley Rope: 1/2 Static-Pro Lbf Required Actual M/A Efficiency (%) % M/A Lost

18 600 pound mass on 1/2 Static-Pro Lbf Required M/A 3:1 5:1 9:1 Actual M/A M/A 9:1 5:1 3: Efficiency (%) M/A 3:1 5:1 9:1 % M/A Lost M/A 9:1 5:1 3:1

19 A larger diameter sheave is more efficient, by as much as 7% in the tread diameter range of 1.5 to 3.75 when hauling in a 1:1 system. The advantage in efficiency gained by the larger sheave is magnified somewhat in M/A systems. It was as high as 17% in the 9:1 M/A. The consistency of the results indicates that the test, as conducted, is repeatable.

20 Pounds Force Required :1 system with 300 Pound Mass Lifting Force Required 1.5" 2.25" 3.75" Rope Type 1/2" Static Pro 1/2" CMC LifeLine 3/8" Static Pro 7/16" River Rescue Lifeline 1/4" Steel Cable

21 Efficiency (% ) :1 system with 300 Pound Mass Efficiency Rope Type 1/4" Steel Cable 7/16" River Rescue Lifeline 3/8" Static Pro 1/2" CMC LifeLine 1/2" Static Pro 1.5" 2.25" 3.75"

22 With steel cable, the largest diameter sheave was only 1.4% more efficient than the smallest. For a given sheave size, a smaller diameter rope of the same type (Static-Pro) was as much as 3% more efficient. 1/2 CMC Lifeline, a softer rope with more elongation, was approximately 1.5% more efficient than 1/2 Static-Pro. River Rescue Lifeline, the softest rope with the most elongation tested, was the most efficient, by as much as 4%.

23 3:1 M/A System with 300 Pound Mass System configurations: #1 1.5 COD, 1.5 moving # COD, 1.5 moving #3 1.5 COD, 3.75 moving # COD, 3.75 moving All with 1/2 Static-Pro Lbf Required #1 #2 #3 #4 Actual M/A #1 #2 #3 #4 System configuration System configuration Efficiency (%) % M/A Lost #1 #2 #3 #4 0 #1 #2 #3 #4 System configuration System configuration

24 If building M/A systems with a mix of sheave sizes, placing the largest sheave at the moving pulley position and the smallest at the COD position provides more efficiency than the other way around. The difference in efficiency between an M/A with all small pulleys and a mixed system was not as significant as that between a mixed system and one with all large pulleys.

25 Lbf Required M/A 3:1 5:1 9:1 When building an M/A System, the total weight of the pulleys may be a consideration. A general market survey of rescue pulleys found an Extra force required to lift a 300 pound mass with 1.5 sheave vs sheave 3:1 5:1 9: lbs lbs lbs Extra weight of carrying 3.75 sheaves vs. 1.5 sheaves 3:1 5:1 9: lbs lbs lbs average of 8.8 oz. for 2 sheaves, 17.6 oz. for 3 sheaves and 26.2 oz. for 4 sheaves. In our test, the larger pulleys added more than 6 pounds to the 5:1 and 9:1 systems.

26 Let s go back to our original question of Is a pulley with a larger sheave the better choice? and carry it a step further: And if so, how much better?

27 True, there is a difference in pulley efficiency, and with all other things being equal the larger size is more efficient. However, that increase in efficiency needs to be tempered by the additional cost, weight, and bulk of the larger size. Those factors would be very important to a mountain rescue team that might carry or fly their equipment deep into the back country. Industrial or urban fire responders might downplay size and weight as significant factors, yet hauling an extra 6 pounds of pulleys up a tower crane will slow you down. Rope seems to be another factor. Should you use softer rope for haul systems? Is the stretch a concern for hauling as much as it might be for a belay?

28 Some of the results of these tests confirmed generally accepted theories, but they varied slightly from those previously stated. The relatively small difference between the 1.5 and 3.75 diameter sheaves when tested with cable also follows the results we obtained when testing anchors (ITRS-2007: McKently, Parker, Smith) which tend to disprove the idea that anything less than 4X the rope diameter adversely impacts rope strength and system efficiency, at least in a 1:1 system. Finally, while our results were measured with very precise electronic instruments, can the individual or team doing the hauling really tell the difference in the field?

29 Sheave tread width Sheave material Interior angle Haul team performance (grip strength, footing, fatigue, etc.) Rope construction (Kernmantle, braid-on-braid, three strand, etc.) Rope condition (icy, muddy, wet, old/fuzzy) System setup (pulling up vs. down, etc.)

30 For more information on this presentation, please contact John McKently International Technical Rescue Symposium Presented by John McKently 2011