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
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
