Rigging off another tree

[benjo75]

Check out Johnny Pro's video called "Crane Swing". Not that I recommend doing that but it does show a little physics about the pendelum effect.

If the rope is relatively tight so there's not much vertical drop then there's not too much shock load. It takes a while for the the energy to bleed off in the form of swing. It will put some unnatural forces in the form of side pull on the tree. It would be a good idea to make sure everyone is clear of the area in case something lets loose.

Have you thought about a speedline? You can put a port a wrap in the tree you're in and have a controlled descent. If you're not catching too big of a piece, the porty could also be an option for the swing too. If it was allowed to run pretty good.

 

[benjo75]

Or if you're just wanting numbers, set up a demo swing. Divide all the numbers by 1/3 and try it. 15 meters away, 33 kg. block. Find a suitable scale like the Rock Exotica Load Cell Scale. It has an app for your phone which will chart and record the measurements at the scale. Do the experiment then scale everything back up.

 

[treebilly]

Depending on the mass of the piece and the size of the drop zone this might be a good situation for span rigging as someone else stated. As also stated if the area closer to the rigging piont doesn't matter a good ripeman will hit the ground before striking the tree. If it is an open swing the forces on the rigging point can be huge.

 

[rbtree]

The load will end up at least 15 meters below the rigging point. If it doesn't hit the tree, which would be with a LOT of force, it will swing back and forth until it stops or is lowered to the ground. I'm not sure what the peak loads would be, but it would be great to test the scenario with a load cell! Since there is no true vertical free fall involved, as in negative rigging, I'm going to guess that the peak loading might be under 400 kg, just to throw out a wild guess. Preloading the branch prior to cutting would limit the additional drop and forces.

Of course, as stated, a butt line should be used to limit the wild swing, which would create controlled drift line rigging

 

[TheJollyLogger]

I guess I'm a little confused by all the hypothetical advice being given, is this not a common practice where y'all are from? There are no wild oscillations, the forces are pretty straight forward. Its fairly common to rig off another tree, especially if the tree you're taking down is inherently unsound for rigging. Granted, 15 meters is a fairly large swing, but as long as the clearances are good, what's the big deal? The physics are sound, and it's only a 200 pound pick.

 

[treebilly]

I can agree to a point JL. Is linear velocity an absolute and if not how does one calculate it. It's been 20 years since I slept through AP physics so I'm a bit rusty on it. I know this doesn't generate the same amount of force as negative rigging but I have a hard time with only seeing theee times the weight on the rigging piont.

 

[TheJollyLogger]

Gravitational constant is 9.8 m/s^2. The rope in this situation is merely converting a linear force to an angular force.

Take your throw line, and wrap around your finger. Swing it back and forth, and you'll feel a little more force at the bottom of the arc, but not too bad. Now pick up your weight and drop it. Finger hurts, doesn't it?

Another example of linear versus rotational acceleration is a big shot versus a hand throw. Set a line 40 ' up with both methods. The throwball accelerates to velocity from the Big shot over a distance of maybe 2 feet, creating a violent release. The hand throw accelerates over an arc of 7 or 8 feet, creating a much smoother release. Both weights need to achieve the same velocity to reach the target, but the hand throw accelerates much slower.

 

[Griff93]

We rig off other trees pretty frequently. We quite often pick up a whole tree or large spar when doing this using a tracked mini skid steer, truck, or tractor. We have taken 50 to 60 ft whole pine trees using this method. As long as your rigging tree is of a suitable size and room is available, I don't see anything wrong with doing it this way or the way the op is discussing. Putting a rope on the piece to slow the swing/slam into the rigging tree is a good idea. It also helps with getting the rig rope back to the climber. Lets the ground guy pull it up to him instead of making the climber repeatedly haul it on their tail.

 

[BuckmasterStumpGrinding]

Well that was a fun physics refresher. I went through all the math and it turns out that all pendulums with a point mass, dropped from 90°, create 3 g's of force regardless of the rope length or weight.

 

[BuckmasterStumpGrinding]

A pendulum changes potential energy into kenetic energy.
Pe=Ke
MGH=(MV^2)/2
After finding your velocity at bottom plug it into the formula for centrifugal force.
MAc=(MV^2)/R
At this point you will have 2g's centrifugal force as the piece reaches the bottom. Add 1g for the weight of the piece and you have the total of 3 g's regardless of the weight or the length of rope.

 

[rbtree]

A pendulum changes potential energy into kenetic energy.
Pe=Ke
MGH=(MV^2)/2
After finding your velocity at bottom plug it into the formula for centrifugal force.
MAc=(MV^2)/R
At this point you will have 2g's centrifugal force as the piece reaches the bottom. Add 1g for the weight of the piece and you have the total of 3 g's regardless of the weight or the length of rope.

Good figuring..... note, that a few posts, up I guessed that the forces would be about 400kg max... which is 4x.... that said, if the load slammed the trunk, it would be with far far more force than 4x..... which could be a problem.