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Directional pull line for felling trees

  1. Jan 20, 2005 #1
    Hi

    Lets say you have a 100' tree and you have the pull line going through a fork up the top. You have 2 options of where to tie it ...

    A/ Tie to the top fork (running bowline etc)

    B/ Tie off on trunk at base

    My question is, is it the same?

    Before you answer please consider that we are trying to settle a debate on which is better and we want it qualified with fact. Some trees such as Norfolk Pines etc are extremely difficult to isolate the top due to the volume of branches. The rope passing down the trunk makes it easier but do you get the same amount of force onto the tree because energy is transfered via the rope down the trunk, also the top fork may create a vector... I've searched and searched for some facts but cant find any.
     
  2. jcsd
  3. Jan 20, 2005 #2
    The physics of the situation are quite complex, but not the way you think.

    As far as the force being at the knot vs. being at the fork if the rope passed through the fork then led down the trunk and tied off, the entire force will be at the first place the rope touches a part of the tree that will not bend, except for the force that stretches the rope. Thus, once the rope is stretched, it doesn't matter where you tie the knot; it matters where the rope first touches the tree trunk or the fork. But there are a couple of other considerations also.

    The first is that the force must be applied in line with the center of the trunk of the tree. If the force is off-center, then you will twist the tree, which could make its fall unpredictable; and property could be damaged or someone could be hurt or killed if the tree does not fall as you want it to. Unless your fork is centered over the center of the trunk (an equal fork left and right, not a branch sticking out one way or the other with the trunk continuing on up), you will not apply the force in line with the center of the tree, and it will twist when you try to pull it down. Thus, it might be safest to use a bowline at the fork, and ensure that the rope emerges centered on the trunk in the direction you will pull.

    This brings up a very important point. Note that if the tree is not symmetrical, its center of gravity will not be over the trunk; this is unusual, but not impossible, especially for oaks. In that case, the pull must either be in the direction of the weight, or it must be over the center of gravity (not the center of the trunk) for the tree to fall in the direction of the pull. You should always ensure that the entire area is clear for the height of the tree and a safety margin around an asymmetrical tree before felling it! You can't know for certain that you can control it with the rope! With an asymmetrical tree, it is often best to cut things off the top until it is symmetrical.

    The second is that ropes stretch, and they stretch as a percentage of their length. The rope led over the fork and tied to the trunk lower down will therefore stretch more than the rope tied to the fork. But once it is done stretching, all the force will come on the fork.

    The third is that you don't want the fork to break. If it does, and the rope is tied to the bottom of the tree, you might wind up pulling the bottom of the tree toward you instead of the top and that might make the tree fall in the opposite direction; property damage or personal injury or death could ensue, just as in the scenario where the pull is not centered and the tree twists.

    I personally would not use a rope to try to control the direction of felling of a tree unless I had good confidence that the force would be centered on the center of the trunk, that the rope would not stretch significantly, and that the fork had been carefully selected to be strong enough to support the pull and not break.

    This third cannot be guaranteed unless someone climbs up and checks it out; and once they're up there, a bowline is a small additional task. The bowline also makes sure that the force is centered on the center of the trunk, and it shortens the rope lessening the stretch. For all of these reasons, I recommend the bowline, but you are of course free to do as you choose (and you assume all liability for doing it). This is a dangerous occupation, and I recommend getting professionals to engage in it, or being extensively trained by professionals before you try to do it yourself. Even professionals are sometimes injured or killed, or cause property damage, when felling trees.
     
  4. Jan 21, 2005 #3
    To Ekka

    The higher up the tree the tie occurs ( any vertical rope is useless) the more leverage occurs in pulling the tree whilst it is ankored at the base by the remaining wood. But the effect is to bend the tree as well , to avoid this it is probably adviseable to ankor lower down , how far may be species dependant some taper and some do not. However a single rope may not control twist , that is the tree may swing in an arc unless for instance two pull lines are used.
    However my points are only theoretical thoughts and if I were you I would enquire from a professional Government source associated with forestry management , ( although they may use different techniques and machines , they are often helpfull to individuals interested in safety)
    Yours Ray.
     
  5. Jan 22, 2005 #4
    Hmm,

    I am an aborist and run a tree business. I have taken on many "big" trees and am well experienced. I have never (touch wood) had anything go wrong and ended up with an insurance claim.

    But, the reason I'm seeking the answer in physics is because a debate has been going on for years that there may be a difference.

    Just change the way you think about it from being a tree to a vertical steel pole. Imagine you have a 100' pole and on the very top of it is a "Y" fork, you can tie off to that fork with a knot or you can send the rope down the back of the pole to the base and tie off. The angle of the rope pulling is the same ... imagine a truck tied to the rope pulling on the pole from 150' away (we never try to pull at an angle under 45degrees), we use high quality ropes with less than 3% stretch.

    Is there a difference? Is one way better than the other?

    Thanks
    Ekka
     
  6. Jan 22, 2005 #5
    Ok Ecca now I see what you are saying. Well yes there is a difference if you assume little friction at the fork ( not sure).
    The difference is that for a given pull on the rope ( which determines the rope tension ) , that if you are ankored at the notch then the force direction is as per the rope.
    However if the rope decends the tree (say to the base) then the force on the tree is directed more downwards , it is both bending and compressing the tree.
    The result depends upon your angle of pull ( length of rope) -- I would have no clue what difference in result occurs -- it seems to me (uninformed ) that compressing the tree is futile --- and that bending it until the base cracks is all that matters.
    Sorry Ecca still theoretical -- Has any body ever found a difference in practice or are you just theorising ????????
    Ray.
     
  7. Jan 22, 2005 #6
    To Rayjohn01

    I have noticed a difference, you seem to get more force and a better pull tied to the top but I cant prove it. Most people say the only difference is stretch in the rope as you need a lot more rope if you're going to tie off at the base. Some say that the force is equal, 30kn is 30kn and it's all applied to the first point of contact (the fork) regardless of where you tie off.

    What about running the math if we assume, no friction and the angle of pull from the fork to the truck is 60 degrees and 30kn of force is applied?

    Will a new vector be created if the rope runs down the trunk of the tree? Would it be 30 degrees?

    Wouldn't there be (assuming no friction) 30kn of force also applied to the knot no matter where you tie it?

    I've tried to attach a diagram to assist, I hope it helps.
     

    Attached Files:

  8. Jan 24, 2005 #7
    To Ecca

    Hi -- well I am getting myself highly confused and dont have a ready answer yet --- if you ignore friction you can consider the 'touch point' as a pulley wheel .
    So here's a thought , if you brought the rope back to the pulling point and anchored it in the ground -- then you would have twice the pulling advantage . Since for every foot pulled the tree top only moves half -- that is a lever.
    If you ankored it an equal distance on the other side of the tree you would have zero pull, just compression.
    In the simple (tied to the top case ) if the rope makes a 30deg angle to the gnd , then there is ~26 kn horizontal levering force and 15kn in compression.
    I had convinced my self that a tie off at the base , resulted in a force of 52kn directed at 30 deg to the tree which if resolved appears to yield
    15kn levering (horizontal ) ,and 45 compression.
    The only problem is that the tie off height, does not seem to matter
    but , But if you tie off at the top your back to square one ???
    It has something to do with the rope lengths , and in which direction the pulley is trying to move.
    I.e. if I pull 1 foot and the pulley moves one ( then there is no advantage )
    and this is what happens if tied to the top ( approx) --- but if tied to the base the pulley will move less hence greater force ( the question then is what direction ) .
    So my 15kn stills seems right , but if I can I will try to resolve the tie off height issue .
    Ray
     
  9. Jan 24, 2005 #8
    Hey Rayjohn01

    I don't know if you got to open the diagram but the angle of the rope going from the tree to the ground, where it's connected to a truck is 60 degrees (the pulling angle), we never try to pull at less than 45 degrees becase that means we are trying to get more downward force than horizontal force.

    We generally tie off at about head height on the trunk as we need to make our cuts under the tie off point. (don't want to cut the rope when felling the tree).

    I hope this helps.
     
  10. Jan 24, 2005 #9

    DaveC426913

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    Gold Member

    Is it possible that the reason for tying off at the base has less to do with the initial force and more to do with what happens subsequently? such as (shots in the dark):
    - what happens when the tree tries to fall and the forces/angles begin to change
    - where the base of the trunk will fall
     
  11. Jan 24, 2005 #10
    To Ekka , forgive me for the ecca , are you Greek by any chance ?
    OK so I gave it a little more thought -- I think you have a very difficult question here which would probably be better solved with a complex simulation -- why ?
    well I do not think you can ignore friction -- tree bending -- and the spin of the earth ( the latter is just a joke ).
    Here are my latest thoughts which are reasoned on energy ( which invokes a problem ) .
    Lets assume the following
    a) the rope has reached it's limit of stretching ( wasted work )
    b) the tree is really stiff ( oak like )
    c) the pull angle ( wrt to the ground ) is low like 30 degs.

    Then I think the following applies :-
    The tree is forced to 'lean' from the cut , but as a radius
    this means that the string length ( to the grnd is fixed) , this means that the string length from 'touch' to puller also does not change.
    This implies that if the puller moves 1 foot horizontally then so does the top of the tree .
    Now if there is NO loss of energy then work = f.d = f. ( 1 foot)
    The tree top almost moves horizontally also and by the same distance ( so no loss of energy means ) equal force .
    Clearly if friction is important ( and I think it cannot be ignored) then that's the same as the rope being nailed to the top.
    If however the tree bends then we have a complex situation , the down tree rope length changes ( depending on the ankor point ) , the direction of the tree reaction is now different , and it also assumes slippage of the rope over the touch point so now we have to deal with 'motional friction' ( not static ) ---
    EKKa were you sent into this world to create Havoc and confusion -- I mean this just 'logging' right .
    The problem with your statement that ' you noticed a difference' is that I doubt that you can quantify this --- obviously each tree is different , it may have flaws , it may be flexible , and I am quite sure you did not keep exact data .
    But as I said it's an intuiging question My guess is that there is not a lot of difference ( and I do not think any one is going to put a pulley at the tree top ) .
    However if you had a real slippery rope , then you should throw it over the top and ankor it at the pulling site -- this gives you a 2:1 advantage . ( In pull that is you will still expend as much gas ).
    My point on energy loss was that wood is not a perfectly elestic material so that work in does not equal the force x distance equation --- this is very dependant on the tree considered -- I guess that Yew is different from some critter ridden pine .
    I suppose my final point here is that the question although very interesting probably cannot be solved -- the variables are too variable -- and in any event the amount of work (i.e. cost ) if not lossed in the rope , is the same regardless so the question in moot.
    The question of cost is interesting , my guess is that you spend a lot of money just moving dirt to get traction , a lot more selecting correct trees ,
    and even more just getting to the site.
    The real question is how to avoid tearing them down to begin with -- that way we would have more fresh air and a whole lot less advertising circulars.

    Ray.
     
    Last edited: Jan 24, 2005
  12. Jan 25, 2005 #11
    Hi again ... to both of you

    I'm an Aussie, and this question is a long debate in our industry. Once the log is falling the rope does nothing, it's there to get it going.

    We fell trees with a directional scarf, when doing the back cut the rope is used to pull the tree over on the hinge. Some trees such as Norfolk Pines have a lot of branches and for us to isolate the main stem it's very difficult so we run the rope down the backside and tie it off.

    Trees are a renewable resource, the rate of planting in the urban environment far exceeds the rate of removal. Also did you know that trees also add co2 to the environment. They reverse their cycles and now studies are being made to find out how much 02 vs co2 trees put out ... wouldn't it be funny to discover they polute the environment?

    Anyway, thank for trying to answer, we are going to set up a model and try to figure it out.
     
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