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
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