Forces involved with snatching using a block

[arbmark]

im not a loler inspector but more rope will surely allow more fall before engagement of the friction, and i would say that it is better to half hitch lower to reduce the kinetic energy achieved by the piece. if the centre of gravity of the chog then swings around the half hitch point/fulchrum then all well and good, the centripetal force to hold the swing will be lower than direct 'snatch effect' and another plus is that energy can be lost into the trunk/bark etc when it impacts the tree......just my take

[Badgerado]

Thanks for all the replies, glad I could start a discussion

 

My experience came only last year after being in the business for 9 years, when I was groundying for a guy who put his half hitches higher (which I'd never seen before) he wasn't keen to change his ways so.. I ended up with another wrap on the port-a-wrap and a little movement of the body in order to let his bits run.. Wasn't smooth in comparison..

 

Thanks Jules and Mark, you described perfectly what I was taught in Arb school

[Wooden Hand]

I am interested in the physics involved with snatching wood, the shockload in my experience feels like more when the lead is longer i.e. half hitch is further away from the block..

I have been told however that in theory the load should be exactly the same unless you take the half hitch above the halfway point of the bit your topping down..

The way I see it that allows it to drop more before you're able to 'catch' it on the rope and let it run, there are other factors involved of course, can people elaborate on this for me (we are talking about a piece that is let run by the groundy not a snuffed off system)..

 

This was the only literature I found on it (not that I looked too hard)

 

http://www.hse.gov.uk/treework/articles/rigging-research-3.pdf

 

Hopefully my lame drawing clarifies what I'm describing

 

By the time we get on to dynamic rigging there are many layers of interaction.

 

1. Length of section - As has been stated, the centre of gravity determines the incoming force.

 

2. Rope type and length within the system - let us assume in your drawing that you are decelerating the cut section and stopping before ground impact. We have a broad spectrum of rope choice from fully steel like static to rock climbing dynamic. Most arborists in England use a polyester/polyester double braid though a more stretchy polyester/nylon is becoming known. In fact choice of sling type determines peak loading too. The cut section can depart the stem at different angles depending on the type of cut and wood type. Ordinarily it will project outwards keeping the rope at least moderately taught as it does so. Ground rigger interaction begins here. Do we choose to begin running the rope immediately or pausing? The outcome begins to be determined.

3. Position of half hitch - This is always the point in contention. The placement of it determines the final hanging position of the cut section, butt tie hangs vertically, balance point tie hangs horizontally and all angles in-between. Each angle will have a different peak load. I agree with Mike and Pete that lowering the half hitch toward the cut DOES NOT decrease peak loading. As long as it is tied beneath the centre of gravity the cut section falls EXACTLY THE SAME DISTANCE. The half hitch should never be allowed to slip of the bottom of the cut section.

 

4. Position of the block - The block should be tied as close to the cut as possible. This is where climbers misunderstand half hitch placement. It is the block placement that determines extra kinetic increase.

As an aside, climbing line and lanyard should be tied, with one choking the stem, above the block and sling.

 

5. Dynamic movement at the anchor points - different cuts will influence the play of hinge released induced dynamic movement, so to will the decision of the ground rigger. And of course lets look at the tree in question. A 2M wide 15M tall beech tree will react differently to a 30cm wide 20 M tall red pine.

 

6. Energy transference - Where do you want to move energy to? I think relaying on the trunk to dissipate energy as the cut section impacts it is highly suspect, hazardous even, I do not recommend it.

[Loler UK]

Its quite sad that our industry training falls flat at the first hurdle, one of our industries veterans and i were talking about this just the other day. Training courses teaching people this, actually quite basic, level of understanding are few and far between with cs 41 falling WAAAAY short. Even when 10's of thousands of pounds are spent on producing & peer reviewing rr668 you have people on AT disagreeing with its findings, sigh. I really feel for the students and professionals that come on AT to help further their understanding having to try and decipher who's posts are correct.

 

Id love to post a load of green-screen annotated photos from my training literature to help illustrate and explain these forces. A great amount of time and resources that went into producing them and my clients have paid accordingly to benefit from them and would quite rightly feel pretty miffed if I plastered the original literature up here for all to access for free.

 

Hopefully the new guide to good rigging practice will illustrate the theory well enough for all 'on the ground' to understand that the forces DO NOT increase relative to the attachment point unless that attachment point causes the centroid to fall further.

 

Length of active & semi-active rope, recovery time between sections, holding power of the hinge, absorption properties of the rope & anchor, wind resistance of the section etc are all factors but the most basic to understand and most important to quantify is the distance of the fall (that means distance of the CENTROID'S fall) we can reduce this in a number of ways:

 

1 tie the impact block as high (close to the cut) as possible

2 place the attachment point no higher than the centroid

3 cut shorter sections

4 take as much slack out of the system as possible without significantly loading the rope/s (never pre tension lines that will then be dynamically loaded as the split second in which they become unloaded does not allow the rope sufficient time to recover!!!)

 

as we work down the stem everything starts to work against us, the active rope is less, the sections become wider making it harder to reduce them in length for fear of the AP slipping off and we have less distance to 'let it run'

 

A few years ago i was discussing with Chris Cowell the potential for 'more dynamic' ropes (more dynamic ropes than we currently use in arb) to absorb the forces when snatching, he scowled and said he believed that instead of looking for bigger, stronger/ more dynamic ropes to solve this problem we should look to other solutions, crash matts, cranes, air bags maybe? that way we can eliminate the need to 'torture' (as he put it) our ropes & rigging systems. It was a welcome angle but occasionally rope is the only/ most practical solution and with a good understanding of the physics we can rig more productively, safely.

 

The age old rule of thumb of original mass x distance of fall (OF CENTROID) in ft + the original mass was found in rr668 to be pretty bob on as field calculations go, this is the start point, until we understand this we're fish out of water and when we do get it we're in the water but not yet swimming.

 

if you've read this far then thanks for taking the time to read my poorly constructed, rushed response (typed rapidly whilst i should be working) better get back to some paid work now, peace out xxx

[scotspine1]

there's no great science to where you tie the half hitch despite what you read in this thread

 

basically you place it high enough to stop it rolling off the butt-end of the timber as the section inverts and is caught by the rigging

 

the relative distances between the block, cut and half hitch is not as important as some people think, the 3 critical positions will nearly always fall within safe parameters mainly due to the work position of the climber ie the climber is not going to tie the block 6ft below the cut or tie the half hitch 6ft above the cut because to do so would be very time consuming and pretty damn stupid. The main thing is you tie the half hitch and running bowline well so they dont fail when the timber is rigged.

 

on the science being discussed in this thread, someone could research then write up a 40,000 word thesis on half hitch placement when rigging down a tree trunk. This research however would not be able to give a definative answer on the optimum position of the half hitch because we're working with non uniform objects, large organic structures of infinite variety and structural strength.

 

 

 

 

 

.

Edited June 13, 2015 by scotspine1

[bumble B]

there's no great science to where you tie the half hitch despite what you read in this thread

 

basically you place it high enough to stop it rolling off the butt-end of the timber as the section inverts and is caught by the rigging

 

the relative distances between the block, cut and half hitch is not as important as some people think, the 3 critical positions will nearly always fall within safe parameters mainly due to the work position of the climber ie the climber is not going to tie the block 6ft below the cut or tie the half hitch 6ft above the cut because to do so would be very time consuming and pretty damn stupid. The main thing is you tie the half hitch and running bowline well so they dont fail when the timber is rigged.

 

on the science being discussed in this thread, someone could research then write up a 40,000 word thesis on half hitch placement when rigging down a tree trunk. This research however would not be able to give a definative answer on the optimum position of the half hitch because we're working with non uniform objects, large organic structures of infinite variety and structural strength.

 

 

 

 

 

.

Good point re climbers position. Because we don't have any means of weighing what we lower our experience counts for a lot and a safety factor gives us a more than adequate margin. Regarding climbers experience of the actual lowering part experiment with cuts and how your groundy lowers. The calculations and paperwork are great I'm just glad I knew what I did back then from the 10 week course lowering off pegs and wraps around the stem.

[scotspine1]

I'm just glad I knew what I did back then from the 10 week course lowering off pegs and wraps around the stem.

 

the first thing all trainee climbers should learn is how to dismantle a tree with just a single rope for rigging, that includes vertical sections

 

it's the best way to understand how friction is the most important part of the rigging system

[Stephen Blair]

All very technical, basically the further up the stem or out the limb gives a higher chance of getting hit by the butt end.

If you are working to your equipments limits then it's because you don't know your equipment or wood. It's a guessing game if you don't have experience imo.

I couldn't tell you how heavy a branch is, but I have never broke a lowering rope.

If in doubt either get a bigger rope or cut a smaller bit.

Edited June 13, 2015 by Stephen Blair

[Wooden Hand]

the first thing all trainee climbers should learn is how to dismantle a tree with just a single rope for rigging, that includes vertical sections

 

it's the best way to understand how friction is the most important part of the rigging system

 

I agree, friction is the starting point. A rigging system cannot reduce further than the rope and tree, thus it is the best starting point. The basic models that are taught, pulley to reduce friction and friction device to apply it, are definitely not the only way to find solutions.

[Badgerado]

By the time we get on to dynamic rigging there are many layers of interaction.

 

1. Length of section - As has been stated, the centre of gravity determines the incoming force.

 

2. Rope type and length within the system - let us assume in your drawing that you are decelerating the cut section and stopping before ground impact. We have a broad spectrum of rope choice from fully steel like static to rock climbing dynamic. Most arborists in England use a polyester/polyester double braid though a more stretchy polyester/nylon is becoming known. In fact choice of sling type determines peak loading too. The cut section can depart the stem at different angles depending on the type of cut and wood type. Ordinarily it will project outwards keeping the rope at least moderately taught as it does so. Ground rigger interaction begins here. Do we choose to begin running the rope immediately or pausing? The outcome begins to be determined.

3. Position of half hitch - This is always the point in contention. The placement of it determines the final hanging position of the cut section, butt tie hangs vertically, balance point tie hangs horizontally and all angles in-between. Each angle will have a different peak load. I agree with Mike and Pete that lowering the half hitch toward the cut DOES NOT decrease peak loading. As long as it is tied beneath the centre of gravity the cut section falls EXACTLY THE SAME DISTANCE. The half hitch should never be allowed to slip of the bottom of the cut section.

 

4. Position of the block - The block should be tied as close to the cut as possible. This is where climbers misunderstand half hitch placement. It is the block placement that determines extra kinetic increase.

As an aside, climbing line and lanyard should be tied, with one choking the stem, above the block and sling.

 

5. Dynamic movement at the anchor points - different cuts will influence the play of hinge released induced dynamic movement, so to will the decision of the ground rigger. And of course lets look at the tree in question. A 2M wide 15M tall beech tree will react differently to a 30cm wide 20 M tall red pine.

 

6. Energy transference - Where do you want to move energy to? I think relaying on the trunk to dissipate energy as the cut section impacts it is highly suspect, hazardous even, I do not recommend it.

 

Thanks for the reply Wooden Hand, good call to point out how close to the block the cut is made..

I will be starting another thread regarding what you say about having both climbing line and lanyard above the block, In my mind it is ideal to have your lanyard (steel core) above the block and your chocked off climbing line nearer to your knees, allowing for downward movement of the topping strop is neccesary but so is distance between your lines in case of cutting or the possibility of flicking your line off (I know climbing line is chocked but still) hopefully you can follow that thread and give me your views on that