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Re: [TowerTalk] rope strength

To: "''" <>
Subject: Re: [TowerTalk] rope strength
From: Gedas <>
Date: Mon, 2 Dec 2019 10:02:32 -0500
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Hi Grant and tnx for the additional input & considerations. You are absolutely right and that would be a much better approach to what I have here.

But based on the 20 years I have been using a similar system w/o any pulleys at all at the bottom and never once had a break or any signs of rope fraying I think by me adding pulleys it will only help the situation. But I understand what you are saying and it is sound advice.

I don't know if you remember from an earlier post but for all these years I have simply been wrapping the bottom rope around one of the round aluminum cross support members. I think they are about 3/8" in dia and smooth. I figured if the rope did not break after probably >100 raisings/lowerings then having a pulley should be ok as well.

The rope has a rated breaking strength of 750 pounds and if I assume ~50 to ~100 pounds as the real max that I could ever have as a tension I think I will be ok.

At this point with all the pulleys bought and installed and the entire system going on-line in a few days I am not going to re-engineer the setup. Also, one final detail......the loop once pulled up to the top of the tower will be holding another pulley that the end of the dipole rope etc will be going through and brought down to ground level. So once hoisted up there (and the dipole totally loose) there will be no real tension in the lines going up & the tower.....the only tension will arise once at the top and I start pulling on the end of the dipole rope to get it's end up near the tower top.

Gedas, W8BYA

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Light travels faster than sound....
This is why some people appear bright until you hear them speak.

On 12/1/2019 11:40 PM, Grant Saviers wrote:
I looked at your 2x4's with pulleys jpgs.  If rope strength is a concern, then consider that going around a sheave (pulley) significantly compromises strength in two ways. I thought a comment would be appropriate given the discussion about knot strength.

The rope fibers are compressed inside and stretched outside in the sheave wrap so the strength can be degraded 50% or more.  The pulleys you appear have a tiny radius.   Repeated passage is cumulative for damage. Check rope specs for the recommended sizes. (might be for wire rope, same problem, bigger loads and life safety = bigger concern)

The second problem is there is a lot of friction in the pulleys. I've seen tests for hardware store ones where the output tension is only half of the input tension.  I use a good sailing block, the best have polymer ball bearings.  It's amazing the difference low friction makes.

Sailboat pulleys are also made to minimize chafe, so if the load moves or the rope stretches in the wind you will be much happier in the long run spending $20 for the good ones.  I figure a tower climb costs much more than $20 if the rope breaks or the pulley fails.  Harken, Lewmar, Ronstan, are all great.  Here is the one I am currently using

Also, I went back to your original post to understand the goal. You mention 30# load.  The actual load in a rope pulled from the side is greatly affected by the angle the rope makes with the a line between the ends.  i.e. at halfway actual rope tension is load/sine(angle).  That means at zero degrees the tension multiplier is infinite for a load 50% between the support points. Obviously, that never happens since the rope stretches or something fails.  When your hoist point gets to the top pulley then that pulley will take all the load and the other line part can be slack, so not a problem.  Be aware that when hoisting,there might be a larger tension then expected halfway up if the hoist is a tight loop.  And the loop force on the supports is twice the tension. So a better solution is to put a lot of slack in the hoist loop.  I always use a loop through my permanent sailboat blocks in trees, just to never lose an end, but they have a lot of slack, also to reach the antenna on the ground partway out.

Grant KZ1W


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