[TowerTalk] rope strength - Knot Expert

[Gedas]

Grant TU for the very useful write-up and description. Yesterday before 
solid rain and below freezing temps set in today my wife and I managed 
to put everything in place and raised up both towers. Each now has my 
2x4 with pulleys arrangement attached & threaded and I really like the 
way it turned out. Running up the loop with attached SS Carabiner clips 
up & down worked out so well.

I provided a couple of URL's showing the 99% finished setup. Images do 
not show the SS pulleys attached to those clips. The images may look a 
little confusing but I tilt over my 70' Universal towers (with 22' mast) 
using a system I designed and have been using for over 35 years. The 
black poles are a pair of 6"x6"x18' PT timbers bolted together and set 
into 7' deep concrete (same as the HD tower bases). A double pulley 
system and geared winch allows me to tilt up & down the towers in about 
10 minutes since I do not climb.

The images show I used a simple square knot to join the rope ends but 
these will be replaced with a double fisherman's knot in the next day or 
so. Thank you again to everyone who helped out and provided input.

https://drive.google.com/open?id=1VLLuLA9e7lridHe3BgwfX34fbRPZeKti

https://drive.google.com/open?id=12aRNdm-8hnw31shSPDEoO-Lz7QGm4J-R

https://drive.google.com/open?id=1hn2NeCGb5lYBqMVq4dqbvomNpWM0836T

Gedas, W8BYA

Gallery at http://w8bya.com
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. 
> https://www.mazzellacompanies.com/portals/0/Images/Page206_3.png 
> (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
> https://www.amazon.com/Ronstan-Ball-Bearing-Orbit-Block/dp/B002NSSCF2/ref=sr_1_6?dchild=1&hvadid=3527191554&hvbmt=bp&hvdev=c&hvqmt=p&keywords=ronstan&qid=1575260910&sr=8-6 
>
>
> 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
>
>