Shock load

[moray]

[ QUOTE ]
...I know that a final equation for this is beyond the power of my brain...

[/ QUOTE ]

Not at all! You clearly have quite a knack for the math.

I would respectfully suggest there is no "equation" for this. You just have to plug in some reasonable assumptions for the way the stopping force is applied and then figure the answer based on the energy of the falling object. For the two simple cases I cited (uniform force or linearly increasing force from zero to max) you don't need any fancy math like calculus. A simple diagram and some high school geometry will get you the answer.

Now of course in the real world, no one is ever going to stop a load precisely according to one of my two calculated cases. It seems very likely to me, though, that a competent groundie will easily fall between my two cases, so those numbers should be of some value as a rule of thumb.

 

[Tom Dunlap]

On a ski trip with Pete Donzelli I remember talking on a couple of lift rides about this issue. He was baffled too. The large parts of the equation can be calculated. There are so many small variables that would have to be calculated that he said it wasn't likely to be solvable.

Pete had an idea about having a rope made with micro load meters woven into the fibers. That way he could track loads at variable points in space and time and get a very accurate comparison. The micro load meters are a dream but that would be about the only way to accurately picture what is going on during a log drop.

 

[joe]

Dave: The work to calculate the energy absorption of an object on a rope has been done.

http://www.yalecordage.com/html/usage_arb.html

What is found through the yale cordage site is most likely broken down so "we" can understand how to work with the energy absorption capacity of a particular rope.

Put the equations and concepts from the Yale site, along with the information from the HSE site generously provided by Andreas D., into a spreadsheet would satisfy your current quest for better information.

Joe

 

[moray]

I love the Yale Cordage site--their graphs of force vs. stretch for the various ropes are simply excellent and should help anyone to understand how the rope absorbs energy.

The rope, all by itself, absorbs energy more or less according to my second scenario: the stopping force starts at zero and increases almost linearly till the load has stopped, at which point the force has reached a maximum.

The rope can never be as efficient as a good groundie because it can never apply a uniform stopping force. But this is a minor advantage. The HUGE advantage is that the groundie can let the load run, and this could easily reduce the peak load by a factor of 10, 20 or more.

 

[Fred_Berkelhammer]

Guying the tree is a simple way to make it more reliable above a weak point.

 

[Blinky]

[ QUOTE ]
[...]

The rope can never be as efficient as a good groundie because it can never apply a uniform stopping force. But this is a minor advantage. The HUGE advantage is that the groundie can let the load run, and this could easily reduce the peak load by a factor of 10, 20 or more.

[/ QUOTE ]

Now you're talking. Calculating loads is interesting and everything but a good groundie with a friction device can minimize shock loading throughout the system. I can't think of many situations in which I would have the ground man lock a heavy load off. I like to see it run right to the ground in a controlled fall without ever fully loading the rope.

 

[eljefe]

Hi, Very interesting. I would like to point out that many measuring instruments of all kinds, weight, force, voltage, RPM, whatever, are accurate only for a static measuring condition. Damping, I think, is the term for this. Underdamped and the instrument will read momentarily higher than the real value. Critically damped, I think is the term for an instrument that would read correctly for a peak value.
To see this in action, take a steady source of DC voltage, a car battery is fine, and connect an analog voltmeter to it. The old Simpson 260 or the Triplet 630 were pretty well damped but still read 50%, at least, higher than the steady state voltage. You will see a momentary reading of maybe 19 volts with the meter fairly quickly settling to say 13.0 volts. The voltage was never 19 volts, it was always 13.0 volts. You are seeing an underdamped system in action. The same phenomenon is in action when trying to measure an instantaneous load with a system not designed to measure an instantaneous value-peak value- accurately.
I have seen real underdamped meter movements swing easily more than 100% above the steady state value. Digital equipment can easily do the same and they can suffer from Crest Factor issues, as well. The measurement device must be capable of measuring accurately the Instantaneous Peak Value. I would guess the 360 plus page HSE report people already know this and the measurements they made are accurate. I am not saying anybody's measurements are not accurate, but I am trying to point out the requirement for the proper instrument to make these measurements accurately. Except for fish scales which are made for steady state loads, ie dead fish. And that's no fish story. eljefe

 

[jim454]

[ QUOTE ]
[ QUOTE ]
[...]

The rope can never be as efficient as a good groundie because it can never apply a uniform stopping force. But this is a minor advantage. The HUGE advantage is that the groundie can let the load run, and this could easily reduce the peak load by a factor of 10, 20 or more.

[/ QUOTE ]

Now you're talking. Calculating loads is interesting and everything but a good groundie with a friction device can minimize shock loading throughout the system. I can't think of many situations in which I would have the ground man lock a heavy load off. I like to see it run right to the ground in a controlled fall without ever fully loading the rope.

[/ QUOTE ]

Good groundy's, IMHO, are totally underrated. Check out the guys Reg has working his ropes, very impressive.