CMC fatality

A few screen shots from All Access Equipments website.
 

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The "new stiff boom technology" makes me wonder if they traded flexibility for stiffness at the expense of increased brittleness.

Reminds me of years ago dealing with pneumatic vibrators. If they were bolted to the C-channel "backing plate" on a hopper with grade 8 bolts, they'd break off. Grade 5 bolts did not break; although weaker, they're not as brittle. Also, the C-channel had to be stitch welded to the hopper. A continuous weld would end up cracking.
 
Dan Cobb said:
The "new stiff boom technology" makes me wonder if they traded flexibility for stiffness at the expense of increased brittleness.

Reminds me of years ago dealing with pneumatic vibrators. If they were bolted to the C-channel "backing plate" on a hopper with grade 8 bolts, they'd break off. Grade 5 bolts did not break; although weaker, they're not as brittle. Also, the C-channel had to be stitch welded to the hopper. A continuous weld would end up cracking.
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I’m curious too, but I’m not sure the upper boom is the stuff boom, as it flexes pretty well. Maybe a little less than some smaller, lighter lifts I’ve run, but it still bends a lot. The lower boom not so much, maybe that’s the stiff part? Could be part of the problem though, for sure.

I’m hoping whatever the problem, CMC will figure it out and announce it. Knowing what is happening is worth a lot, even if there’s not a solution yet.
 
Reach said:
I’m curious too, but I’m not sure the upper boom is the stuff boom, as it flexes pretty well. Maybe a little less than some smaller, lighter lifts I’ve run, but it still bends a lot. The lower boom not so much, maybe that’s the stiff part? Could be part of the problem though, for sure.
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Could be part of the problem for sure...of that lower boom doesn't absorb any of the movement, ALL of the flex is concentrated to one spot rather than spread across multiple.
 
Reach said:
I’m not sure what you’re asking exactly? I’ll take a guess, but if you can clarify I’ll try to better answer your question.

When you’re doing cut-and-chuck, you’re cutting a piece and holding the weight of the piece yourself, which adds weight to the lift. If you’re taking larger pieces, you’re catching the weight as it drops, which adds a dynamic load to your body and by extension, to the lift basket.

When you’re handling logs that way, a 50+ pound log being cut and thrown off the top of a spar can easily add 80-100 pounds of force to the basket. If you are at full extension, where the LMI on the basket stops you from extending further with a 250 lb load in the basket, that extra 80-100 pounds of force while you’re throwing logs is considerable, especially if you multiply that by the 35’ of boom extended out the side.

Your capacity will increase with a higher boom angle, like in a crane, hence my hesitation to add any dynamic loading at full horizontal extension - it’s the point where the boom is most heavily loaded and therefore weakest.
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We used a dynomometer to drop a 25lb piece of wood 1 foot. The output reading was over 300lbs. This was a straight drop on rope...not in a rigging through a pulley scenario.
 
Phil said:
We used a dynomometer to drop a 25lb piece of wood 1 foot. The output reading was over 300lbs. This was a straight drop on rope...not in a rigging through a pulley scenario.
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That is interesting.

I was under the impression that for every foot an object falls, it gains a unit of its own weight plus one.

So a 25 lb log should have a force of 50 lbs…

300 lbs doesn’t sound right to me but maybe I am misunderstanding something here
 
MA Arborist said:
That is interesting.

I was under the impression that for every foot an object falls, it gains a unit of its own weight plus one.

So a 25 lb log should have a force of 50 lbs…

300 lbs doesn’t sound right to me but maybe I am misunderstanding something here
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Physics be wild sometimes.
 
The interesting thing to me about the fall calculations is that a lot of the parameters are squares - for example gravity speeds your fall at 9.8 m/sec in one second, then at two seconds you're at 19.6 m/sec and so on. The energy when you hit the deck is something like (1/2 * the mass * the velocity squared) - why the foot above hurts so. It builds up quickly - also why we don't wear fall protection equipment in arb and ANSI/ other global standards limit arb "falls" to about 20" only. All this stuff is why I get the whillies when I see someone rigging/ lowering off their basket. It is really easy to build up forces with a swing, say, when the equipment design basis was really only dead weight or "quiet" lifting.
 
On a dynamometer reading from dropping something into catching it, the elasticity of the catch "rope" determines most of the story. Eg static vs dynamic rock climbing lines. Imagine pulling the slack out of a chain when you pull your buddy out of the mud - bang. Now use a nylon tow strap - boing. 25 lbs to 300 lbs is 12G - that's a hell of a spike only achievable with a rigid system. It's like energy under the force curve for rigging rope or crash helmets. The DOT spike level and duration is a bit different than the SNELL waveform. One is pointier and shorter and the other is lower and more spread out. :) Whatever scrambles your noggin! :)

Dan, I like those nuggets you periodically release from your knowledge reservoir.
 
The rock climbing analogy must also consider the concept of “fall factor" - how much rope is in the system, etc. High fall factor falls can be really dangerous. Typically, arb ropes are considered to be static save for some rigging ropes like Dynasorb etc. But these are still not dynamic in the sense of alpine gear with a 30 - 60 meter length of rope in the system (without zipper slings).
Fascinating topic

Addenda: These variables are part of the calculations performed using Wesspur’s/ Samson's Log Impact Force Calculator wheel - great tool.
 
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Bart_ said:
On a dynamometer reading from dropping something into catching it, the elasticity of the catch "rope" determines most of the story. Eg static vs dynamic rock climbing lines.
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True, the rope makes a difference. However, the static rope will show that there is in fact a tremendous amount of force built up in a short 1 foot drop. It's the definition of what max peak load we might see while cut and chucking 25 lb pieces while horizontally extended in a spider lift. Now the question becomes, how do those additional forced get absorbed, or not, down the length of the boom.
 
I'd be speedlining over CnC from a spider lift, amap.

I'll peel limbs to hanging vertical before cutting free, too, pre-loading.

I rent a tow-behind, squirt-boom every year or two, and tread lightly.
 
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