Doubling effect at pulley block

Eric Frei

Common scenario where a block is installed up the tree and you are lowering sections out.

The force at the block is double whatever the weight of the section is ... that's not taking into consideration friction or any dynamic loading if the section was falling.

Imagine you cut off a 100lb section. To stop it hitting the roof you need to apply force to the other end of the lowering rope ... if you apply 100lb then the section is stopped. So you have a 100lb on one side to stop it coming down and you have the weight of the section which is another 100lb. Total is 200lb on the pulley block.

Here's a quick sketch to illustrate.

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Attached Files

	Pulley MA Systems.pdf (65.3 KB, 238 views)
	Loads_and_Forces_on_Anchor_Poinst_TCIA_Magazine.pdf (348.7 KB, 66 views)
	Detter_Kinematics_Arborist_News.pdf (542.3 KB, 56 views)

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Sean Freeman

Very good diagrams Ekka, this is a crucial concept in basic physics that absolutely every one who has ever thought about using ropes in tree work has to understand....Yours and others lives often depend upon understanding the forces we put onto the rope systems we use.

SF

chris girard

I would definitely agree. As tree workers we don't need to be engineers but we do need to know about the physics of engineering and how they apply to our work.

This is a subject that I find fascinating and always love learning more about. I am sure alot of you guys (and girls) feel the same way.

TreeCo

Here's a way I rig quite a bit. I'm able to set ropes and pulleys from the ground with throw lines. The limb sees four times the load so you've got to watch it.

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Eric Frei

Graeme McMahon often natural crotch rigs taking his wraps at the top of the tree to eliminate the doubling effect.

Wrap from the top down or the outside of the limb to inside so the piece can come down not get caught in it's wraps.

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Drouin Tree Service

Its alot easier on groundguys climbers who natural crotch IMO

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Drouin Tree Services | Excavator Hire - Drouin and SE Gippsland | Landclearing Melbourne

Sean Freeman

Do you think because there's less hardware, or simply because the greater friction is such an aid in lowering the pieces?

We've a portawrap II but hardly ever use it.

SF

Drouin Tree Service

Less hardware/friction and actual time in getting the tree down,heres a small limb just ran thru a fork.

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Drouin Tree Services | Excavator Hire - Drouin and SE Gippsland | Landclearing Melbourne

Drouin Tree Service

A couple of diagrams out of "handbook of rigging practices"

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Drouin Tree Services | Excavator Hire - Drouin and SE Gippsland | Landclearing Melbourne

TreeCo

Looks like a good book. I've not read it.

Eric Frei

And natural crotch on many trees prevents you having to climb to the top and then back down to rig the low branches first ... you can rig on the way up.

Here's another diagram on sling config angles.

Notice when slings are at 120 degrees to the lifting hook each sling (or leg of sling) experiences the same weight as the load.

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Notahacker

There is certainly a place for natural crotch lowering of branches.--No doubt.

However, what I am about to say has not been addressed, but I am sure you all know this.

Lowering large wood in a natural crotch is not always a good choice.

Remember: If you use a natural crotch as a rigging point there is added (unnecessary) friction in that crotch. So, this adds unnecessary force to that rigging point. Ultimately, this could result in a failure of the rigging system.

And, the frictin with natural crotching is inconsistent. A lowering device is much more predictable with the amount of friction needed to lower something (granded you know how to use it properly).

But, you all know that, right?

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Notahacker

Like Dan illustrated, I like to do that same technique too. It is great to hit a throw shot from the ground to install a retractable rigging point. Fast way to set up and tear down.

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Eric Frei

You better check them crotches and if it's a co-dom with included bark then you could bust it out.

I know some dudes were lowering out a large gum via natural crotch method and the fork they were using gave way ... no damage to property but the climber had to change his undies as both sections came smashing down and hung up.

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jim1nz

I want to point some of the negatives with lowering over natural crotches.

- Unknown amount of friction
- Undue ware on ropes
- Chance of using a unsuitable rigging just because it is there
- Unable to tip tie/cradle brances because of low rigging point
- Less chance of being able to let branch "run" for a smooth rig

Angus

Like most technique choices in tree work, overlooking the possible benefits of one technique, takes a backward step in decision making.
This in turn reduces your flexibility as a climber, and doesn't leave room for thinking outside the square. There is a time and place for natural crotch lowering. IMHO
Angus

TREE-SURFER

When doing a TD, I always use a natural crotch for lowering. Depending on the weight of the section or limb, is how many times to wrap around the trunk. The rope I use, is a used tow rope from a life boat (RNLI) rated at about 6ton (used to be 1 of the crew) so it's old rope for money,(donations). When taking down limbs on a live 1, I use strops and steel biners to lower, but not too much weight for the groundies to handle. Thats when the winch (I got the 1 on the 4X4 or a portable 5 ton winch ran from a 12v battery) comes in handy.

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Isle of Man Tree Surgeon| All Aspects of Tree Work

What experts say about TOPPING

jim1nz

Angus you are right. So treesurfer you are limited to how much weight you lower all the time, you have to try to calculate each rig to be similar weights because its not as easy as putting another wrap on a porta wrap is it.

TREE-SURFER

A couple of turns on the trunk is usually enough for what we do if doing a TD.
When we do a limbing job we generally use the strops and old climbing rope to lower smaller limbs if over a greenhouse or garden shed etc. Touching wood, never had one do any damage. Probably takes a little bit longer without the mechanicals though. Bit like the tortoise and hare.

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Isle of Man Tree Surgeon| All Aspects of Tree Work

What experts say about TOPPING

Jeff Darby

Hi folks,
I think you use all your knowledge when it comes to lowering limbs and chunking down. natural crotch is handy to use but you have more friction, a porty' basic but very handy works well and reasonably versatile, piece of pipe made up from steam pipe welded to a backing plate and tied to the tree for straight lowering jobs is pretty cool, but you cant go past a GROC or similar devise. Just to name a few but like always we usually stay to what we know, but trying new techniques can save a lot of hard back breaking work as long as they are applied correctly...dont be afraid of new lowering systems just make sure everything is rated for the intended job at hand. Go forward...learn...we always take notice when someone says have you seen this method! And something else I feel I should mention is a little knowledge in tree defects goes a long way as well.
All The Best
JayD

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Eric Frei

I natural crotch a lot of the small/medium trees, I'd say max wt cut off is perhaps 150kg. Also depends on the tree, spreading crown trees are easier for natural crotch as you can shove the rope thru someting above what you're cutting rather than having redirect pulleys.

But larger timber, pulleys thanks, unless of course your doing some far out Regnans or something.

Angus, got any videos of some big lowering off natural crotches?

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Angus

Not to be argumentative, but 150kg is a fairly definitive cutoff and I think regardless of tree species to give a top figure for natural crotch lowering disregards your diagrams above. Why limit yourself to 150kg? The total weight on the high point is much greater with a pulley. Does that mean you will only put 75kg load on a pulley?

Regards
Gus

TREE-SURFER

I use the same method as Ekka for blocking down.

http://http://www.treeworld.info/attachment.php?attachmentid=417&d=1172261504

I do have a fabricated lowering post that straps to the trunk with ratchet straps, but, to be honest, have only used it a couple of times.

The lowering system I use for limb takedowns over some fellas shed or greenhouse is very effective. If the limb that the system is attached to for lowering seems to be too week, then I would divert to the trunk with another strop. Yes this system takes time to set up, but priced accordingly because of the nature of the obstacles.
I just know I'm gonna get criticised for this 1. But what would be a better system than the 1 I use. Up to now, it has been effective and the customer has always looked on, just to make sure you're not glazing instead of sawing

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Isle of Man Tree Surgeon| All Aspects of Tree Work

What experts say about TOPPING

Eric Frei

Angus, that's just me, I have a crappy 3 strand rope for natural crotch work but my nice double braid gets smoked thru crotches so I dont use it.

So for my "limit" it's more the rope. I know Lopa uses some bad azz 1" rope for big azzed lowering down Drouin countryside.

I also know what you guys are up against down there, in fact I recall Graham saying that rope on some of his dismantles is considered a disposable item where as us arbo types tend to keep the rope for years. Then he showed a video that frankly had about 75 people sitting there gob smacked by what the rope had to do.

Some of the limbs he cut off were like 80' to 100' long probably weighing in at 1000kg. And how could you forget a shot taken at 180' out of an EWP bucket where Graham knocked out around 120' head of tree above him and hung it off the stem ... the whole tree bent like a banana whilst he was swinging around in mid air suspended from another tree. Awesome footage.

Tree-Surfer, that technique for getting limbs and turning them we call slewing. Darned valuable. Having the "A-Frame" set up like you got there is good to make sure the butt doesn't fall. Very good technique.

How do you attach one of the A-Frame legs to the lowering line?

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TREE-SURFER

I use a two wraps each end of the 'Bridle' (A-Frame) and terminated with a carbine hook that is then shackled to a multi drilled plate and another shackle connected to a steel 35kN biner onto the lowering rope.

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What experts say about TOPPING

Angus

You've seen that footage??? When?

DBS

I found a quick online calc for snatch block loads.

Thought I'd share.

Snatch Block Rigging Calculator

-Daryl

redstree

It is because of the doubling of the load on the suspended block that I always recommend using rope or webbing slings that are double the SWL of the load bearing line. Sometimes this is not easy due to the construction of some of the blocks. They are not wide enough at the suspension point to fit the larger size rope, especially one covered with and extra layer to help prevent wear.

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Eric Frei

The alternative which I do is a loop of rope, doubled rope.

Or have a really large eye spliced so when cow hitching to tree two parts of rope are at work.

I dont use whoopie slings but there's an option too.

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Vector Tree

Forces are vectors, since they have a definite size and direction.
The direction of a vector determines which way a force is
acting. Since a rope can only support a load along its length,
the rope indicates which way the force is acting. If we turn a
rope through a pulley, the force is now acting in two directions?
the direction of each leg of the rope. The resulting force
on the pulley now depends on these two component forces and
the angle the rope enters the pulley. These two component
forces create a single force that would cause the pulley to move
toward the middle of this angle if it were free to move, much
in the way a bow and arrow works .
Why is this important in tree rigging? Well, if we can manage
to direct the force exerted on the block to act along the
length of a tree leader, as opposed to across the grain horizontally,
we utilize the natural columnar strength of the tree, which
can give us a greater working load limit. Everyone should understand
how a 2-inch diameter limb is much easier to break if
we step on its middle instead of standing it upright and stepping
on it end to end. It would be nearly impossible to break.
We can mimic this in the tree by incorporating more than one
block into our rigging.
In order to calculate the load on a block, we need to determine
two things: the load on the rigging line and a
block-loading factor. This loading factor is determined by the
angle by which the rope is deflected by the block. The angle
that we are referring to is not the angle between the two legs of
rope, but the angle between an imaginary line drawn straight
through the block from the load, and the leg of rope entering
the block from the friction device.
The greater the angle created by the block, the greater the
resultant force placed on that block. For example, a block that
turns a rope 180 degrees will see a load equal to twice that of
the load on the rope itself. Conversely, a line that enters the
block at 0 degrees or runs straight through the block would
create a resultant force of zero on that block. In the middle of
the road, we have a 90-degree angle. The block-loading factor
of a right angle would be 1.41 times the weight of the load.
How can this be useful in our day-to-day operations? We
can create lesser pulley angles on smaller rigging points or
structurally compromised parts of the tree and use a second
pulley to create a larger angle at a more beneficial spot to support
more of the load. This can be very useful when dealing
with co-dominant stems and other structural flaws. The rigging
line itself can act as a temporary cabling system to lace the
stems together.

Angle of Deflection Load Multiplier
30 o .52 x Load (L)
45 o .76 x (L)
60 o 1.00 x (L)
75 o 1.22 x (L)
* 90 o 1.41 x (L)
105 o 1.59 x (L)
120 o 1.73 x (L)
135 o 1.85 x (L)
150 o 1.93 x (L)
160 o 1.97 x (L)
180 o 2.00 x (L)
* Example: 90 o Rope
angle with 500 lb. load = 705 lbf. on Block
RESULTANT FORCE
ON BLOCK...