redirect forces

KevinS

KevinS

Branched out member
Location
ontario
There are many good threads about force, momentum, swing, re-directs, etc.
My question is with re-directs, fishing pole, smart rigging with re-directs through the crown to help hold the weight.
At what weights or sizes do you start worrying about such things? I've used them before but I'm wondering if my rigging point can handle its own top rigged into itself it can handle say double that top size. So does the load have to be 3,4,5,6 times that original tops size before you tend to worry about such things?
 
Did you read the Mark's "Engineering a Tree Removal" in the articles section? That may help in your thinking.

It's not quite so simple as multiples.
 
I have read through that and a few others, just trying to get a practical hand on it. Trying to rig smarter and bigger when I can.
 
Ok, that gives us a baseline. When I use multiple blocks, which is most of the time I'll look at the anchor points to get a sense of their relative strength and which one would be best to carry the largest load. If you look to create an angle of 120 deg. then you'll reduce the load on the anchor point to 1 x the load on the rope. If you're using a single point with the two parts of the rope through the block parallel to each other then the load is double on the anchor point of the block.

It's all about vectors and the potential loads that'll be swung into the system. The top often doesn't come out until the end before I reduce the system to a single block.
 
Thanks. Depending what I'm doing I do a few different things. I usually use as solid of a rigging point as I can, but sometimes to get it to a lz off a roof or something I use a limb as a redirect.

Sometimes I leave that limb until last or when I'm putting heavier loads into it I shave it and pop off its top, so no force can reach any further length for any extra leverage. I do this to hopefully help keep my rigging redirect limb from busting.

Does any one else do this or is it a silly waste of time?
I'm sometimes torn between leaving the top and branches for dampening the spring( not sure if it would ever need that) or just a stub cause that puts all the force on the very end instead of half way out the limb,but its the same size and distance so there's less load.

By dampening I only think of that cause everyone has seen blocks rigged into a tree and block rigged into a topless spar( I call them wiggle sticks)

Sorry for the ramble but I think I got it all down. Thanks
 
Look at the angle your line creates through the redirect. That will help you establish the amount of force on the limb. Making sure it's below your main point will reduce the load and direct more of the force in compression along the limb or laterally toward the main point.

It's good that your thinking about all of this. Start to learn more about the mechanical strength of the different species so your doing things from a knowledge perspective instead of hopeful.
 
I like to use multiple rigging points. You can a lot of times select a rigging point that is lower in the crown and work your way up thus distributing the forces through out the canopy. sometimes I will have 4 or more pulleys in a tree. I try to keep the system as static as possible by use redirects and spider leg balancers or by tip tying butt tying. So then your not creating a big shock by having a lot of swing or movement. hard to do in a top down rigging situation. Top down take smaller pieces or let it run as much as you can add more rope to the system.
 
Using the math from Mark's article assuming a 50 lb log the angles are:
42 degrees = 38 lbs
160 degrees = 98.5 lbs
210 degrees = 96.5 lbs
90 degrees = 70 lbs

That's the load of each angle so if it's anchors through a porty and the load is suspended. This spreads the weight out through out the crown which is good but in turn does that equal a 606 lbs load on the rope?
I understand each limb (rigging point) sees less but if you used a single rigging point that would be: 180 degrees = 100 lbs which is far less strain on the rigging line.

Am I really over thinking this or am I on the right track?
Yes I know I took out all swing and shock load forces, it just makes for easier starter math.
 

Attachments

You're on the wrong track. Those figures are the loads on the anchor point of each block based on the 50 lb. load on the rope. I.e., the rope carries the 50 lbs. through out as a constant. Got it?
 
To add to this. By having the redirects, as mentioned above, you have more rope in the system. By doing this you dampen the load by absorbing energy in the rope as the piece falls into the system. All the above calculations are oversimplified and assume a static line with 0% stretch. In fact the rope depending on whether it is a "static" or dynamic design will reduce the shock load by absorbing the energy over the distance the rope is stretched. Typical dynamic ropes have 3% (dynasorb) whereas the static (stable braid) is about 1%. More rope is the system will increase the stopping distance and reduce the net effective load on the blocks. Of course it will fall farther before stopping, which means something else to consider when making your rigging decisions.
 
KevinS said:
There are many good threads about force, momentum, swing, re-directs, etc.
My question is with re-directs, fishing pole, smart rigging with re-directs through the crown to help hold the weight.
At what weights or sizes do you start worrying about such things? I've used them before but I'm wondering if my rigging point can handle its own top rigged into itself it can handle say double that top size. So does the load have to be 3,4,5,6 times that original tops size before you tend to worry about such things?
Click to expand...

Good question. That I know of, as a poster to this thread, there is no research which gives a rule of thumb which relates a limb's length to how much of that piece can be rigged on itself. The Tree Fund gives money to people who try to answer these guestions.

2 more links which can give a better idea with the different scenarios you'll encounter when rigging are posted below. The simplest link can be quite an eye opener if you're unfamiliar with levers and mechanical advantage. The 2nd link is intense and will give quite a bit of information directly related to tree rigging and how the trees react to the rigging activities.

http://enginemechanics.tpub.com/14037/

http://www.hse.gov.uk/research/rrhtm/rr668.htm

Joe
 
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Check out "The Art and Science of Practical Rigging". Hosted by Ken Palmer and Rip Tompkins of ArborMaster Training, Inc. as well. Very good 2 disc dvd set that covers: 1. Introduction to techniques and methods
2. Equipment and ropes
3. Rigging knots
4. Basic limb removal
5. Advanced limb removal
Disc 2
6. Compound rigging techniques
7. Understanding forces in rigging
8. Top removal and rigging heavy wood
Number 7 on the video introduces Peter Donzelli who was a mechanical engineer and tree climber. They actually do an experiment dropping a piece of wood of known weight and calculate the forces using an electronic dynamometer. Dropping a 650 lb. piece generated 4554 lbs. of force at the rigging block and half of that at the portie. Very awesome dvd set and a must have for anyone interested in proper rigging techniques as well as understanding the forces involved.
 
So if you are using just 1 block (no redirect) and a portie and say for instance dropping a piece that weighs 100 lbs. you would have 100 in the lead and 100 in the fall of the rope equaling 200 lbs. at the load point. By using redirects and keeping your rope angle close to 120 deg you can take a 100 lb. piece and your load point should see closer to 100 lbs. of force instead of double. So yes you can take pieces that are double the weight of your load point (in some instances) by using redirects and trying to achieve angles close to 120 deg. Let the rope run when you can add more rope to the system and when in doubt take it smaller. Using these techniques and experience will dictate how much you can take.
 
what kinds of forces and such are we dealing with when Rigging down a tree using an Equalizing type method, Now Im not sure on the equalizing term I used but lets say you are taking down a tree that is smack in the middle of 2 other trees and you run a pulley in each one of those outside trees with 1 rope passing between them. from there you rig the center tree out off the midline of that rope. How are forces and such distributed when done this way?

I suppose this could be done on a smaller scale when working multiple leaders out of a tree as well.

Also note I have never done this and I am interested in learning what I need to be aware of before I attempt it. ;)
 
If you keep them at equal distance and equal angles then it will distribute the load evenly. Once you drift to one side or the other then the load will increase on the side you're drifting toward. How that load is distributed on each tree will be dependent on the lean of those trees.
 
awesome thanks tree humper. I was thinking a little too hard there that makes sense to me now.

This sparked a new question for me also, how does the rope handle being loaded on its side like that versus the usual way?
 
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