qustion

I coming in kind of late on convo, but top hopper is 100% right when you ease into it and put the brakes on slow. That is the best way to cut back the shock load. If you preload it big time in other words take all the stretch out of the rope and let in run slow and smooth and stop it slow is good but if you preload and slam the piece into it that has to be the worse thing to do.
 
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I just sitting wondering... if you preload and drop, you're taking as much stretch out of the rope as possible, right?
If you do that, the rope has less ability to absorb shock.

To me, in this case, if you have the distance to let the load run and come to a relatively smooth stop, you should run it because that way the line will never see the amped up loads from a sudden stop.

For my money, this is where an experienced, intelligent groundie makes all the difference. Somebody who knows their friction device, be it hobbs or porty or whatever can get the load on the ground with a minimum shock load.

It's not always possible but I prefer the ground to stop the load, the rope just gets it there in a controlled way.

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I agree with everything Blinky said here. I even noticed how he used "amped up" to describe the situation. I believe there is plenty of research that will agree 110%
 
this is the link that shows what shock loading does to lifting devices
http://www.treebuzz.com/forum/showflat.php?Cat=0&Number=42912&an=0&page=3#42912
man i finally found it!!! i've been looking for it all night!
tongue.gif
 
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I just sitting wondering... if you preload and drop, you're taking as much stretch out of the rope as possible, right?

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With all due respect Blinky, I believe you're wrong.

The 1500 pounds of preload is released at the point the wood drops off the trunk and begins it's fall. I believe the purpose of the preloading is to limit the loads fall distance, and a shorter fall means less momentum.

jomoco
 
If what I've said in the above post is valid, then the ability to suck up slack at the moment the load starts it's fall and limit it's fall distance would be an advantage, particularly in close vertical proximity to a target below.

Which leads me to think that a heavy duty rotary recoil spring being added to the inner capstan mechanism of the hobbs might be capable of sucking up slack fast enough to limit the loads fall distance even further in the fractions of a second that the load is in freefall.

This just might minimize fall distance and the loads momentum faster than a groundie is humanly capable of doing. The stronger the rotary spring is, the more slack that can be taken up, the less momentum the load can attain.

The logic is sound, but engaging the rotary spring as an option may require a bit of clever engineering.

Okay guys, tear my hobbs 3 theory apart if you can!

jomoco
 
If you are but hitching a 750# piece and preloade it to 1500#. Will you then be sending it into the rope with more force.You are building up energy that has to go somewhere.
 
An excellent question Allmark.

And I suspect that you're correct, atleast initially.

The pertinent question would then be which trade off yields the most benefit, increased initial acceleration and a shorter fall distance, or natural freefall acceleration and a longer fall distance?

In terms of dynamic shock loading, one would expect the latter choice would be less. But in terms of target avoidance in a vertical sense, the first choice would yield the better outcome, provided the rope can hold the load without damage to it.

These questions can be answered, and quite possibly already have, and I'm just unaware of it?

If I had a 20K Dillon dynamometer or two I could answer them myself and post the results.

An excellent thread guys.

jomoco
 
uummm wouldn't these 2 basically equal the same. Example log falls 5 foot gains 15mph. You preload log it falls 2.5 feet and gains 25 mph cause of the force pulling it down.

Besides that you have taken your dynamic line and now made it a static line by sucking all the strech out of it. This in turn will make the rope easier to break / fail?
 
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If you are but hitching a 750# piece and preloade it to 1500#. Will you then be sending it into the rope with more force.You are building up energy that has to go somewhere.

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I think that 1500# would be lost taking up slack rope before the butt hitched piece loaded the rope. There is that second or so of time right in the middle there as the bull rope goes slack........and then another second or to as the rigger says......damn! it held again. I knew it would.

If there is clearance for the piece by far the best way to reduce shock load in a system is letting it run. Ever see the stuff Dar Robinson the late great stunt man used when jumping off buildings being slowed down and caught using skinny little steel wires....
 
I believe you're making the same mistake that Blinky made Hollenreich.

The preload is lost the moment the load goes over the edge and begins to fall. Allmark is correct that the preload accelerates the load's fall rate. At the point the load is stopped by the rope, all it's dynamic properties are there and working.

To me this means that perhaps the only benefit of preloading the rope is in limiting the fall distance to a minimum.

Have any of you guys ever priced a 20K Dillon dynamometer?

jomoco
 
If preload is lost the moment it leaves the cut and begins to fall, whats the point in preloading? You cant say it limits the distance of fall, because that would be lost also at the moment the piece leaves the cut. I dont buy your preloading theory.
 
Hey Treeco, wasn't Dar Robinson the dude that got blown out the upper floor window by Burt Reynolds in Sharky's Machine?

That was one helluva stunt!

jomoco
 
You can suck the slack out of the system when the section tips over and then immediately begin letting it run while applying brake to slow it down. It's the "Method of rapid slack removal." Sorry, no springs needed.

Pretensioning a line is akin to stretching a rubber-band. And that's all good when you're picking your work, but catching it?? You better be careful. I most certainly wouldn't want to be the climber in your scenario.
 
I'm still just thinking, I'm no authority on this stuff at all. But rope elongation from the preload won't return instantly, at least not all of it. I doubt it matters much either way though.

To me, the sudden stop is the real problem, that's where the shock load peaks. Faster stop equals higher peak load from deceleration. If the distance is available, running the load and stopping it slowly won't ever generate those high shock loads.

Not sure if it was Dar Robinson but it was indeed a helluva a stunt.
 
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You can suck the slack out of the system when the section tips over and then immediately begin letting it run while applying brake to slow it down. It's the "Method of rapid slack removal." Sorry, no springs needed.

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Yes, it is as simple as that.

Dave
 
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I'm still just thinking, I'm no authority on this stuff at all. But rope elongation from the preload won't return instantly, at least not all of it. I doubt it matters much either way though.

To me, the sudden stop is the real problem, that's where the shock load peaks. Faster stop equals higher peak load from deceleration. If the distance is available, running the load and stopping it slowly won't ever generate those high shock loads.

Not sure if it was Dar Robinson but it was indeed a helluva a stunt.

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When you have a target say 10-15 feet below you Blinky, letting it run is not an option. When the wood you're catching has a dia much over 3 feet and is green, even a little 4 foot section can weigh enough to snap a 3/4 inch bull line, particularly wet euc. If you cut much shorter than 4 feet you run the very real risk of losing the piece all together!

I've already made the decision to upgrade to bigger bull lines and blocks for that very reason, and it aint cheap.

jomoco
 
You originally said the target was 50 feet below, hence the reason I said let it run. Now the target is 10 or 15 feet below? Thats a whole nother scenario! In that case I may agree to some extent. Although I wouldnt call it preloading, I would take out as much rope stretch as possible before making the cut. I would try to take as small as piece as possible without risking it rolling out of the line. Then yes it may be necessary to snub off the lowering line and stop it dead in the block. In which case you would be putting one hell of a shock load in the system. Fortunalty I dont encounter that scenario often, and when I do I may opt to cut and toss out poker chips as much of a pain as that can be, or simply crane it out assuming thats even possible.
 
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The preload is lost the moment the load goes over the edge and begins to fall

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Yes this is true.

Now, one has to remember basic laws of physics. Or should I simply say laws of motion. Stored energy has to go some where? It just does not disappear. The preload in the rope might some what go away, but, it will release it's enregy into the log being pulled over, in turn making it come off much faster than just pushing it off. This makes the short distance you are tring to obtain the same as if it was not preloaded.

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dia much over 3 feet , If you cut much shorter than 4 feet you run the very real risk of losing the piece all together!

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very little chance of this, almost none, when using a half hitch and running bow line. Logs can be taken as small as 3 foot.
 
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