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The Importance of Wind Dampeners

Introduction

Anyone who has longlined in high winds knows that the wind can cause subject serious vibrations and sinusoidal oscillations to the line, especially without wind dampeners. Although the oscillations generated without wind dampeners can appear to wreak devastating loads on a longline, very little research exists about how much force the wind can subject.

Wind Dampeners Wind Dampeners

Methods

The method in which I recorded the oscillation phenomenon is quite simple. I rigged two 250’ longlines, one lightweight polyester and one super low-stretch high-tech webbing, with a dyno scanning at 300 Hz. in line with the anchor, and I recorded the load readings for a 5-minute sample, both with and without wind dampeners. I then combed the data and I will show you the highest recorded load for each sample pool. While the samples were not subjected to a whole gale, NOAA reported winds in the area averaging 14 MPH with 33 MPH gusts, so the lines were subjected to continuous, moderately strong wind for the entire duration of the test. I recorded three samples: one high-tension high-tech line, one moderate-tension high-tech line and one high-tension polyester line, all at 250’. I did not stand on the line during any period in the five-minute test window.

The following 40-second video will display specifically what I was recording.

 

 

Observations

Force Comparison Force Comparison
High Tension vs Low Tension High Tension vs Low Tension

High-Tech (high tension):

  1. Pre-test resting tension: Approximately 1890 lbf
  2. Peak load without dampeners: 2039 lbf. (+149 lbf)
  3. Load cycles per second (mean): 2.75

High-Tech (moderate tension):

  1. Pre-test resting tension: Approximately 1217 lbf
  2. Peak load without dampeners: 1407 lbf. (+190 lbf)
  3. Load cycles per second (mean): 2.75

Lightweight Polyester

  1. Pre-test resting tension: Approximately 1860 lbf
  2. Peak load without dampeners: 1942 lbf. (+82 lbf)
  3. Load cycles per second (mean): 13
  4. Peak-to-peak load (maximum): 115 lbf

High-Tech (high tension) with wind dampeners:

  1. Pre-test resting tension: Approximately 1890 lbf
  2. Peak load without dampeners: 1915 lbf. (+25 lbf)

 

Conclusions

Although it may appear that the wind was subjecting the line to extreme loads, the load increase caused by the wind was actually quite small. In fact, standing in the middle of the line subjected the line to more force than the wind did. This can be explained easily by examining the amount of sag the wind subjects to the line. While the wind was causing peak-to-peak oscillations of over 4’, the downward sag subject to the line was only around 2’. When I stood on the line I subjected nearly double that. The following graph shows how the load fluctuations are relatively small compared to known high-loading events such as a highline fall.

 

Other Comparisons Other Comparisons

However, there are some very legitimate concerns regarding the vibration and cyclic loading presented to the pulley system and anchor components when the line is allowed to flop in the wind. The extreme vibration, as seen in the video, caused every screwgate steel carabiner in the system to unlock within a matter of seconds. In addition, if allowed to propagate and continue for an extended period, the vibration will cause severe wear to any aluminum-to-steel contact point, such as the contact point between a shackle and aluminum rigging plate. Last, the alarmingly high number of cyclic loads (13 per second for Green Magic), brings into question the long-term fatigue suitability of the tensioning system, especially aluminum components.

According to Hairer (2012), at least one type of large AL-7075 aluminum carabiner will fail at 240,000 cycles if subjected to 225 lbf load cycles with a mean tension of 1,350 lbf. With an increase to 1,800 lbf of tension and a load-cycle range of 450 lbf, a large carabiner can fail with less than 35,000 cycles. At a rate of 13 cycle per second, the anchor would see 35,000 cycles in just under 45 minutes.

Considering the cyclic-loading nature of a longline in the wind, it would be most advantageous to use wind dampeners under all conditions, and if a longliner were to leave a line up for an extended period, she or he would be well off to softpoint the system and remove any aluminum component, especially aluminum carabiners.

 

Reference and Further Reading

  • Hairer, F. (2012). Aluminum climbing carabiner under constant load . n.d., n.d.: n.d..
  • Analysis of fatigue failure in d-shaped carabiners. (2002). Retrieved from http://web.mit.edu/sp255/www/reference_vault/Fatigue_Presentation.pdf

Reference articles available upon request.

Article written by Sayar Kuchenski,



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6 thoughts on “The Importance of Wind Dampeners”

  • I would think that the Hairer numbers relate to completely loading and unloading the biner? But that is not what is happening here. The load is to first approximation constant, and it seems like this would change things quite a bit.

    I think it would be interesting to actually try and break the biners using the wind loading! You could use the software to count the number of cycles.

    • Sayar Kuchenski - January 29, 2014 at 12:03 pm

      I dont know what a Hairer number is, but when we think of a cycle load it is true that we normally think of a complete load and unload cycle. However, in reality a cycle load is a sliding scale. It is not an absolute by which we say anything over xx% of a change in load is a load cycle and anything under it is not. Think of an automobile collision. What speed is required to cause enough damage to an automobile to make it inoperable? 10 MPH? Probably not. 30 MPH? Possibly. 50 MPH? Certainly. What about 10 collisions at 10 MPH? Then yes, possibly. The same thing applies with fatigue. A complete unload and reload will cause more damage faster, but it is possible for a item under constant load to fail from fatigue even if the item is never fully unloaded. Look at the references I posted. In the tests conducted in those articles, none of the samples were ever fully unloaded, buy they all failed from fatigue.

      So, yes, completely unloading and reloading a device will cause it to fail from fatigue much faster, but a complete unload is not necessary for fatigue to occur.

    • I do believe that Hairer numbers apply only to a situation where the item is being cyclically loaded and unloaded. Slacklines are unique in that they have a base tension and then more tension is added with various events. It would be very interesting to see how many cycles it would take the various hardware pieces to fail in this sort of scenario.

      • For some reason until now I hadn't thought of the cyclical loading effects on anything but the carabiners and attachment points, but it is also important to remember that this effects all components whether Aluminum or Steel. While quality aluminums (i.e. 7075, 7050, 6061) are designed to have excellent fatigue strengths, there is a threshold at which they will no longer be able to stand up. Things like pulleys, linelockers, quicklinks, shackles and anything else that is cyclically loaded can and will fail with enough cycles (and high enough tension). I think it is important to remember that gear that has been loaded will not be as strong as when it was new, and caution should be applied before taking gear with close to or above the WLL.

  • Excellent information, thank you for posting. I was wondering about this after rigging in the wind last weekend. https://plus.google.com/112124086986973637768/posts/NC7WFGs7tNB
    Good to hear I rigged according to most of your suggestions (softpoint and only aluminum was a rigging plate). I was a little concerned about my steel line locker and was thinking of replacing it with a AWL3.0 but that increases the amount of aluminum in the system and I want to keep confidence in my AWL3.0 for highlines.

  • It would be interesting to see the stats for the same test with someone standing on the line....
    Also screw gate carabiners could be substituted for auto-locking steel biners, which I assume would solve the issue of them coming unscrewed...?

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