Forces involved with snatching using a block

[Badgerado]

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

 

Thanks for the in-depth reply Mike Interesting like you say that such a fundamental of our work can have different teachings throughout the industry.. I certainly wasn't trying to mislead anyone here on Arbtalk, I guess I'm wrong, still trying to work out why it felt so different though, he was definitely tying it below the centroid.. Anyway thanks for your time to respond to my thread

[Badgerado]

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.

 

 

 

 

 

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Thanks for your reply,

I understand what your saying with worker positioning but if you cut an extra foot above the block you would be adding an extra 2ft of shockloading, safety factors in my mind are to be adhered to i.e. Guestimating weights in your rigging system, 10:1 is used to give a little bit of leeway but if you start shockloading ropes past that then the life of that rope is reduced dramatically (stress turns into strain)

Also your right about variations due to natures different forms and weights, cuts etc.. Some of those are really only able to be judged or guessed by experience/intuition, but I think the more we understand the basic physics involved the better we can be at understanding our rigging systems

[Badgerado]

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

 

LOL and a three strand prussik loop to climb on

[Loler UK]

Thanks for your reply,

I understand what your saying with worker positioning but if you cut an extra foot above the block you would be adding an extra 2ft of shockloading, s

 

correct!

 

so in less than 40,000 words:

place leading point of attachment point below but only just below (id suggest 5-30cm with 30 allowing for a slight underestimation/ miscalculation) the height of the centroid, this will minimise the peak force, period. If you mess it up and tie 10cm higher than the centroid then you will increase the fall distance, if unsure of the centroids height then its better to tie a little lower than it is overcook it. The location of this leading point will not mean that neither the but, centroid or tip will be in ANY, ANY, ANYYYY!!!!!!!!! different position a resting position providing you tie below the centroid

(in fact if you want to get super anal and into microns the higher point will actually lead to a higher resting point wtf?? but that really is getting super technical and anal so disregard these brackets all together)

 

placement of the block in relation to cut and vica-versa is the more crucial element, block 1ft below the cut is an extra 2ft of fall (plus a little) which on a 200kg lump is an extra unnecessary 400kgf (plus a little)

 

If your still not with it give me LolerUK, tree partner or treevolution a ring and book your-self on some training (please note other providers may be available )