rigging & shock loading

[frans]

Spidey was kind enough to give me these figures on the weights generated with rigging out wood:

"Re: shock loading
i got my rigging calculator back from guy i gave my computer to. Installed and been playing with it and your numbers. Was trying to make a spread sheet to show some points, but rather multidimensional of what rope, what distance from brake to 1st hitch, what load, height of CG (then doubled by doubling effect of folding over), which is then doubled by pulley etc.; to place on a 2 dimensional spreadsheet.

But, 2500# with a CG 5' from pulley/pivot (that gets doubled on fold), with only 1' of 3/4 StableBraid from brake to 1st hitching and no run creates 62k force in 12k line (guess ya can figure out what happens there); then gets doubled for force on pulley and mount for 124k.

Drop CG to only 1'; line tension drops to 30k, 60k at pulley. 10' of line from brake to 1st hitching drops force to 10k, 20k at pulley.

Place CG back to 5'; keep 10'line gives 22k, 44k at pulley. 30' of line gives 13k. Usually increasing this factor, places you higher on spar, so the lessened force; can take more leverage over the spar length; so is a double edged sword. Remeber our rigs like this don't sit inline with spar, but s-lightly off to side!

Now dropping the load in half; you'd expect the forces to drop in half, butttt; @ 1250# line tension drops from 13k to 9.8k. This is because as the SWL goes up the elasticity goes down; so the halved loading automatically gets less buffering!!

When we doulbe legs to load for a 2/1; we have higher SWL, but lower elasticity. So a 2500# load hanging from pulley places 5k (for a 2x factor) on mount, but rig in 2/1 places only 3750 (for a 1.5x factor) statically; but if load dropped in each system the math comes out after a bit(not at 1" drops, but perhaps at 2'+ drops) to show that the 2/1 in a dynamic situation gives more loading to the 2/1 support when it gets less loading in static situation.

Changing ropes in previous scenario, whereby the 12k 3/4" gets 14k peak tension, the 8k 9/16 gets 12k peak tension. the 1/2" 6k gets 10.6k; because as the tensile goes down the same loading has lower SWL, therefore higher elastic buffering. Those are all for same materials and construction of StableBraid at different strengths. Alter materials and construction to a 'premium polyester' 5/8" of 7k tensile, gives 12k; like SB 8k line.

Lots of considerations elasticity of certain materials and construction of line, ratio of that tensile to laoding , amount of line taking the hit(brake to 1st hitching on load), height of CG etc. All these numbers are without run.

So, height of hitching on load above block squeaks some extra line buffering in there by increasing line (rubber band) length that takes the hit. And, if we can get laod to tip and flex over on hinge, it doesn't accelerate as fast/ drop as far before tearoff. These 2 factors combine with well tensioned line; whereby the greater line pre-tension grabbing up higher on the laod gives more tension at a higher leveraged position, to flex hinge more!!

etc. etc. isn't this fun??
-KC "

I was trying to figure out what loads are generated in the load line assuming that there is NO 'running of the rope", the load line being static, and the load dropped into the rigging from 4 feet above the pulley with a piece of wood weighing 2,500 lbs. Say the load line is 3/4" high strength static line.

I am not smart enough to figure this out from the figures that spidey provided me (see above).
Can someone explain this to me in language that an American grade school student could understand?

thanks
frans

 

[treesandsurf]

eer... huh....

jp

 

[Mark Chisholm]

There are other variables to consider first Frans. For example, you say a 2500# log dropped 4' above the block into a static line. Be specific about "drop", the amount of rope in the entire system, and also the term "static". 3/4" dbl braid or do you mean more like KMIII?

Spidey is also quoting center of gravity.

 

[TheTreeSpyder]

This Picture shows outputs of ArborMaster/Sherrill calculator. Altering 1st length of line taking shock from brake to 1st hitching; this shows energy absorbtion by more line in system (but not for redundant pulls on load etc.). Also shows how different lines abosrb energy of such shock. The first set in yellow shading are all same type manufacture and materials; so also shows how line increasing line tensile gives more shock.

Then altering how foar the Center of Gravity is above pulley, then how that is doubled in fall length when load tears off and hangs upside down. This force is then doubled by pulley. Also showing various line constructions and tensile strengths.

Then the weight of load itself is altered, and showing half weight gives more than half forces. This is actually similair to our running primary example in yellow boxes of playing with various tensile strength to load weight ratios. Only here we vary the same ratio by varying the load itself. But still line tensile to load force is part of elascticity that absorbs shox. So, is length of line, materials and construction of line/braid.


Notice last category; with only 10' of line in system; Load weight must drop to 1/4th to drop output force to 1/2; becasue of elasticicy is partially dependant on load weight; as load wieght goes down; elastic buffering reduces!

 

[Treespotter]

Sorry Treespider, This is all I got from the link. See Attachement.

 

[TheTreeSpyder]

Sorry; didn't know the permissions were set like that at TH. Here is same pic as attachment above, to use space here too for same pic. i log in automatically when i go there; so didn't ever get that message.

i forgot to expound on one of my rules of thumb expressed in chart purposefully

; meant to do a chart section on it

and got in hurry to go to dinner; remebered it on way home. It takes 4x the length of rope from 1st hitch to brake, to drop rope tension in half. Calculator actually shows this tendency specifically in lengths greater than 1';sharp increase in rope stiffness at real low lengths / immediate friction strategies(1st hitch then friction). But anyway; the tendency in the rope length variable section(rope lengths in red); where the 40' lengths have 1/2 the tension of the 10' lengths is rather consistant.

 

[Colin]

I didn't know they were set like that, either.

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