calculating shock load

[igetbisy]

So just how technical do you get while calculating shock load? Is there a simple way to do it, if so, I sure would like to know. In fact, unless you're a mathematical genius, as well as a master at guesswork, I think overkill is about the only way to be sure about the really big stuff. what do ya all think about this?

 

[MasterBlaster]

I don't know what Kenny might say, but I don't calculate nothing. Its either a 1/2", 5/8", or 3/4 bullrope. Nuttin' fancy here.

 

[Lumberjack]

Originally posted by MasterBlaster
I don't know what Kenny might say, but I don't calculate nothing. Its either a 1/2", 5/16", or 3/4 bullrope. Nuttin' fancy here.

5/16"? Really? Do ya mean 5/8"? I thought that I was the only (about) one that used 1/4".

 

[MasterBlaster]

You are correct, sir. Duly edited.

 

[wiley_p]

I do a lot of rigging the rule of thumb I use is 1' = 1 unit of weight so say the peice is an estimated 600# top falling 3' before loading the block which about normal with a negative block angle, so anyway take weight x distance in feet + 1 unit, in this case 2400# I talked to a physics proffessor at the UW and he said this crude calc was fine inside of 6' of drop, after that other factors needed to be calculatd

 

[Lumberjack]

Originally posted by wiley_p
I do a lot of rigging the rule of thumb I use is 1' = 1 unit of weight so say the peice is an estimated 600# top falling 3' before loading the block which about normal with a negative block angle, so anyway take weight x distance in feet + 1 unit, in this case 2400# I talked to a physics proffessor at the UW and he said this crude calc was fine inside of 6' of drop, after that other factors needed to be calculatd

An important note is that it is how far the COG moves, not just the butt end, but normally the COG falls in a long arc, so the shock load is delivered over a longer time.

Moving your knot down on the peice doesnt change how far the drop is, but moving your pulley up does.

 

[TheTreeSpyder]

i just try to make informed guesses; and strategize on how to have as lil force loaded against me; while loading as much force on my side.

i mite spout out some numbers, and there are those that outrun me there; but i just try to find the patterns and balances when i look at the numbers. That and all the amazing ways that the puzzle of a few numbers can be assembled in; like Nature gives you the tools to do almost anything; if ya can figure it out!

The C.o.G./C.o.B. observation i really like; after i track out if a hinged item has a clear fall and rest for it's size/footprint i might consider a lot of things, some might say too much; but at that point one thing i do do, that i think makes it simpler. As i work the line and saw to fell or rig; i don't look at/consider the whole tree or limb, just the C.o.G./C.o.B. i believe in the tapered hinge etc.; cuz it leverages over the C.o.G./C.o.B./Heavy Side; and if that don't fall; i kinda find the rest won't! So all i watch is that one point and how it reacts to the line and saw; so even if more complicated on some fronts; i guess i 'cheat' here.

After guessing the shockloading, you have to then guess how it is recieved by the support, what direction that force is exerted. i think you have to develop a feel and the math, if ya get in deep enough, can make apparent patterns that it might take years to notice smacking ya in the head otherwise! Also, make a habit out of minimizing load force and maximizing support, by not falling directly into gravity, using tension in wood and nylon fibers etc.

OverKill for UnderKill is Good!
:alien:

 

[murphy4trees]

Also a good ground man can spread the load out by letting it run... Reduceing the load, I think, by about 15-20%.. The Center of Balance or center of gravity is important to keep in mind. To expand on the above... the calculated # of feet dropped is the distance the COB moved from start to finish... I've also wonderred how the arc effects the shock load... Never seen any science on it though...

Based on the above math I think:
There are plenty of people out there that are dropping big pieces, many feet, far exceeding the 10% SWL of their lines and getting away with it...

Other disciplines use a 20% SWL, which I can live with if used occasionally, and it saves time and energy... just retire the ropes a little early.... I had a 240' high Vee get "retired " early a few years ago when it fell out the truck in a tupperware container with two old saddles.... Maybe it was for the best because I had dropped some heavy pieces on that line.

 

[Lumberjack]

Originally posted by murphy4trees
Also a good ground man can spread the load out by letting it run... Reduceing the load, I think, by about 15-20%.. The Center of Balance or center of gravity is important to keep in mind.

The further the groundy can let the load run, the less the shock load. IMO he can reduce it by more than 15-20%, if conditions all for the load to be ran farther.

To expand on the above... the calculated # of feet dropped is the distance the COB moved from start to finish... I've also wonderred how the arc effects the shock load... Never seen any science on it though...

COB from start till the it is to the point it would be if you locked off the rope. If you take the total distance that the COB moved with the amount it ran, your calculations will be way off.

The arc, IMO, streads the load out (somewhat) over more time, reduceing shock loading to a point. The reason I think that is because if the piece fell straight down, the load is applied on the rope directly. In the arc it is a more gradual loading, after the hindge breaks. This would be more easily explained if you watched the pulley when blocking down, on a video, in slow mode.

Based on the above math I think:
There are plenty of people out there that are dropping big pieces, many feet, far exceeding the 10% SWL of their lines and getting away with it...

Other disciplines use a 20% SWL, which I can live with if used occasionally, and it saves time and energy... just retire the ropes a little early.... I had a 240' high Vee get "retired " early a few years ago when it fell out the truck in a tupperware container with two old saddles.... Maybe it was for the best because I had dropped some heavy pieces on that line.

Double Braid Poly rope can withstand a load of up to 40%, without damage to the rope. Over 40% the rope is permenatly damaged from the stretch takin set on the rope (past the elastic limit of the rope).

It would be nice to have a dyno meter to read the forces on the ground. This could easily be done by having a hydraulic cylinder (bigger the better but over 3" and a short tiny stroke) filling with fluid, and screwing in a pressure gauge with a resetable needle (stays at highest reading till reset). Take the PSI recorded, and multiply it by the surface area of the piston.

Just some random thoughts.

 

[TheTreeSpyder]

i think that running the load to gradually brake it, deaccelerate slowly; or to control it so it doesn't accelerate can reduce shock loading from the movement; but running it to stop suddenly could increase loading.

Running it can also move it lower, so that if 2 lines or DWT is on it the legs of line aren't as leveragednor the supports.

The more exposed line before braking friction, can allow more dampening from the line; but early/high braki9ng friction reduces the force as the trade off; also the isolated line length(by high friction) can be used to reduce the amount of stretchable line area; make it easier to load line before tearoff in Overhead Rigging. i think that the arc that the C.o.B. takes would leverage on the hinge, but just catch the increased speed x weight on the hitch etc.. Whereby, if you forced/muscled the load over slowly on the hinge with a pull from the top of the log; the hinge would incur the force of the arcs of the line pull and C.o.B.; while the eventual loading on the hitchings/supports would prolly be reduced; due to this being slower than a straight drop; though the C.o.B. commited to the same arc; in the case of the hinge it would be incurred, but not on the hitchings/supports. So that example splits those hairs and renders me the definitions/principals that seperate these things IMLHO.

Orrr something like that...........
:alien:

 

[Lumberjack]

Is there a differance in COB and COG? If so what is it?

 

[xtremetrees]

Man this board is the bomb!
How many times have you folks ever broken a rope while rigging!!??

I've broken 1 while in the tree
The piece just cleared the house about a 40 ft popular limb about40 feet off the ground,25 inch thru.. It was a one cut shot, i knew the rope was gonna break when the tip hit the ground first, fishpoling the weight to the butt that was tied off and once the slack was gone CRACK. the rope broke. CRACK the limb broke in half. All the force was directed away fromt he house limb and all peice landing well clear.
It was a planned bomb, the rope did break just as I expected it to.

 

[xtremetrees]

It was the only way to do the limb.

The other climber running the guys freighttrain walks up and says. "Man your about the best climber I ever seen." He was a good tree dude and it meant alot. The owner of the train says, " See just how I planned it." He did not plan it that way but I just smilled and gave a kool kinda nod..

The 5 groundies looked at me with wonder and amazment as they lugged the limb from the back yard.

All the while I was thinking. "I just broke a rope! After 10 years of everday. CRACK I broke my first rope!. God dont ever let that happen again!" jJesus cCrist I broke a rope! Burned alot but never..... I kept thinking..

$800 for 1 cut. The other option There was none.

 

[topnotchtree]

This may be hard to explain, but if I have the room, I always cut my roped off limbs "down and around".Lots of guys like to cut thier limbs twards the direction the lowering rope is set.Causing the rope to slacken a little as the piece is going over. I usually cut the limb away from that direction, then turn the saw to steer the limb around where it needs to go. This will apply tension to the rope gradually.

 

[TheTreeSpyder]

i been meanig to post something like that; when i can't get enough line tension before cutting to serve right; i will have the hinge pull it left; this leverages the line more; as anything that moves directly away from the support etc.

So the line is pulling right and the hinge pulling left; load caught in the trap between both of the forces; can tiptoe around an aluminum antanae somethimes.

i call it 'Rock-Around-the-Clock'; for starting at 10-11 c'clock to finish hanging under 1-2 oclock; i bring it 11-10-9-8-7:30; then across to 4:30 with the hitchpoint, then cut free. Lining the hitchpoint under the support point before tearoff takes the swing dynamics out of it. The line should stay tight the whole time.


Very nice strategy TopNotch! i guess some would ay it is couynter-intuitive to go left to go right..........

Mostly i like to sweat the limb in so hard that it lifts, then hinge down; by the time it reaches approx. original position; line is real tight, and have needed that strategy less.

Another one is to have friction support over right shoulder, so that as you cut down, load moves directly away from support leveraging line; that also pulls back into the hinge, done right ya need less stretched hinge fiber to maintain needed 'stick' or compression, as the rope fiber takes over this for the wood fiber.

Any of these high leverage deals can overload, so i give the teeniest bit of releif in line, hinge face, Nature will take the easiest way out; so i feed her that path and releif; and watch her follow it like a heat sinking missile home!



Orrrrrrrrrrrrrrrrrrrrrrrrrrrr something like that!
:alien:

 

[igetbisy]

Originally posted by topnotchtree
This may be hard to explain, but if I have the room, I always cut my roped off limbs "down and around".Lots of guys like to cut thier limbs twards the direction the lowering rope is set.Causing the rope to slacken a little as the piece is going over. I usually cut the limb away from that direction, then turn the saw to steer the limb around where it needs to go. This will apply tension to the rope gradually.

I share that opinion with you, topnotch. also, I've been waiting for someone to suggest the flex in the support as the factor decreasing the shock load. often, I will go through a couple of crotches of slightly smaller limbs way up in an adjascent oak, in a manner that puts the tension on the limbs in a direction not so likely to break them. you get an incredible amount of dampening from this.

 

[topnotchtree]

I guess to answer the first question of the thread, I just make experienced guesses. I have had the rare oppertunity to work with some incredible teachers. And half inch ropes is what we are supplied with, so thats all we use. I have seen half inch ropes stretched to look like pruner chord! And in my learnings, how the ropeman runs the rope makes all the difference in the world. On a slightly off-topic note, I have made mention of a gin rope, or a gin. And nobody seemed to know what I was talking about. It was wierd because that is the language used in this part of the country. A gin is a crotch you set your lowering line in. And a gin line would be the work line. Is this foreign terminology to everybody but me?

 

[ROLLACOSTA]

CUT SIT BACK HOPE ,DONT DO RISKY WORK WITH HARD WORKED OR OLD ROPE SIMPLE

 

[RescueMan]

I have made mention of a gin rope, or a gin. And nobody seemed to know what I was talking about.

A gin pole is a single leg derrick, guyed off with at least three ropes. And I know what gin is.
- Robert