SRT Base Tied TIP Forces

Bart_

Well-Known Member
Location
GTA
I don't have any X rigging rings, but based on a perhaps 1" diameter for the rope to slide over I'm guesstimating a tension ratio of 1.6 to 1.8 range. So that means 100 lbs hand side for lowering a 160/180 lb log which means a 60 or 80 lb difference up at the rigging tip. 260/280 lbs total at the tip instead of theoretical 320/360 lbs at the tip. Theoretical because a pulley runs about 1.1 tension ratio or 145/164 lbs hand side for the same 160/180 lb log, not right on double - for lowering - for raising it makes the total more than double.

Bet that's the first time anyone's quantified the tip friction load change for a rigging ring.

Edit - Crow eating time. Dr. Kane measured the tension ratio on X Rings and did a writeup:
Nov 2019 http://digimag.tcia.org/publication/?m=54984&i=629325&p=34&pp=1

He measured the forces using the Donelli 1999 method of hanging weights on both ends of the rope and then measuring how much more force started the rope sliding. He got 1.5:1 and 0.7:1 which inverts to 1.43:1 (he slid the rope in the opposite direction) so pretty repeatable. I believe 1/2" line on large rings.

He also measured during a log drop and got a ratio of 1.2, but I question the influence of dynamics i.e. is it during a net acceleration or a net deceleration etc which is very hard to pin down on an instantaneous basis even from high speed video, which I think wasn't done.

So punchline my estimate looks high at 1.6 to 1.8 but I don't have slippery hard anodise coating data to work from.
 
Last edited:

Bart_

Well-Known Member
Location
GTA
The I'm bored Biner Double Whip Corollary

Sling on log with rope in biner on sling.

Was using double whip the other day and pondered that friction was at play as I lowered the pieces. A rope doing a 180 around a biner has an about 1.8 tension ratio, so the stationary overhead anchored side of the rope has 1.8 times the tension than in the rope you're spooling down to the biner. So (1.0 + 1.8) x spooling rope tension = log weight. Or spooling tension = log weight / 2.8. Rope wrench territory. So if you bare hand the rope, you feel log weight/ 2.8.

If you throw the moving rope 180 degrees u turn over a natural crotch and then hold it, the natural crotch does it's magic. Let's say it's not super rough at 2.5 tension ratio, but average at 2.0 tension ratio. Now the (log weight/ 2.8) you would have bare handed gets reduced to (log weight/ 2.8)/2.0 = log weight/ 5.6. This is BMS Belay spool 1 1/2 wrap or Morgan block territory. On a rough bark tree this would be log weight/ 7.0. And if you want more advantage throw in another 1/4 to 1/2 wrap natural redirect down where you are situated cutting the base of the log, where it's convenient to tie off the rope before/while you cut off the log (assume rigging tip above, not -ve rigging - don't shock load biners).

The eureka for me was how beneficial the seemingly poor choice of biner and sling was friction wise, for such a simple setup. And climber releasable lowered pieces from aloft until you run out of biners and slings.
 

Bart_

Well-Known Member
Location
GTA
The I'm Still Up Pulley System Mechanical Advantage Corollary

Browsing some threads about n:1 pulley systems and recalled how once I tried to partially right a root failed cherry tree - some success, pruned it weird, had a 45 degree trunk on a prop and produced nice eating cherries that were super easy to pick - it was a bitch moving it despite a many to one pulley system. Future me lightbulb on - friction did me in all those years ago.

Take a 4 or 5:1 system with 4 rotating pulleys, 2 at each end where the rope does a 180 turn.
Now add: W47 big blue rollgliss kit pulley, bronze bushing, 1.97" groove dia x 0.58" wide groove x 0.19" deep groove, tension ratio 1.20 equivalent mu 0.06 - typical of such used pulley.

Each pass around the pulley ups the effort by 1.20:1. 1.20x1.20x1.20x1.20 = 2.07

So you set up 4:1 but get screwed by 2.07 from pulley friction and end up at 2:1 in reality. Don't even think about small pulleys. Maybe real good ball bearing pulleys will get you an effective 3:1.

DOH!!!!!!!

Edit - I found the Sheehan 2004 reference from the HSE chapter 8 - about pulley friction as above -
 
Last edited:

Bart_

Well-Known Member
Location
GTA
Donzelli 1999 arborist block friction: (I can't get the link to copy, it automatically downloads the paper)

Donzelli was prompted into some analysis perhaps by:

"If the block were frictionless, and both parts of line were parallel and directed vertically, a balance of forces predicts the reaction force at the block to be twice the line pull measured at the base of the tree. In 3 trials, the reaction force exceeded twice the line pull by 7.5%, 10.5%, and 9.3%."

So I did some ciphering to convert this to a tension ratio:

logT > baseT because the friction through the block drops the basal/porty rope tension

TipT = (1.075 to 1.105)x 2xbaseT
TipT = baseT + logT they're both hanging at 180 degrees around the block

baseT + logT = (1.075 to 1.105)x 2xbaseT call it (1.1) x 2 x baseT i.e. pick a nominal value

logT = (1.1) x 2 x baseT - baseT = (1.1x2 - 1) x baseT

logT/baseT = (1.1x2 - 1) = 2.2 - 1 = 1.2 !!!!!!!!


Holy crap he got the same tension ratio as my W47 blue pulley. At least it's a vote for nominal consistency amongst typical pulleys. I'd take it to the bank as a rule of thumb. His impending motion hanging weights tests corroborated the about 20% difference in tensions except for one block that was probably better that had about 10% or a 1.10 ratio.
 
Last edited:

New threads New posts

Kask Stihl NORTHEASTERN Arborists Wesspur TreeStuff.com Kask Teufelberger Westminster X-Rigging Teufelberger Tracked Lifts Climbing Innovations
Top Bottom