Installing a block

All the time with a rigging rope over a solid crotch anchored to the trunk. You have be choosy about rigging points with that method because it doubles the load, but it saves a lot of time over climbing to the rigging point.
 
[ QUOTE ]
All the time with a rigging rope over a solid crotch anchored to the trunk. You have be choosy about rigging points with that method because it doubles the load, but it saves a lot of time over climbing to the rigging point.

[/ QUOTE ]

This will quadruple the load. 2x at the block. Then double that at the crotch.

Edit: maybe that's what you mean... double the already doubled load? Just wanted to clarify.
 
[ QUOTE ]
[ QUOTE ]
All the time with a rigging rope over a solid crotch anchored to the trunk. You have be choosy about rigging points with that method because it doubles the load, but it saves a lot of time over climbing to the rigging point.

[/ QUOTE ]

This will quadruple the load. 2x at the block. Then double that at the crotch.

Edit: maybe that's what you mean... double the already doubled load? Just wanted to clarify.

[/ QUOTE ]

I was talking about the anchor line only since the block will be 2x weight of the work no matter what. Shoulda made that clear.
 
The efect of the doubling or 'quadding' potential can be minimized with frictions, like a more friction or one way block(and also more open angle of legs). And/or also if line is holding pulley (potentials of 2x2 or 4xLoad), the line holding pulley is given more fricton at and around the mounting point(over a larger limb and around under, maybe even then lacing back upwards to another, stronger point or more horizontally than vertically etc. around main trunk).
 
[ QUOTE ]
Jamin, you missed the part in his post where Norm said " you can install the rigging FC from the ground"

[/ QUOTE ]

blush.gif
Oops!
blush.gif
 
[ QUOTE ]
The efect of the doubling or 'quadding' potential can be minimized with frictions, like a more friction or one way block(and also more open angle of legs). And/or also if line is holding pulley (potentials of 2x2 or 4xLoad), the line holding pulley is given more fricton at and around the mounting point(over a larger limb and around under, maybe even then lacing back upwards to another, stronger point or more horizontally than vertically etc. around main trunk).

[/ QUOTE ]

Good post. This shows how knowledge and situational awareness can be applied to make an adjustment so that a mathematically implausible technique can then become useful. Friction and angles can work both against us and for us.

Dave
 
[ QUOTE ]
...Friction and angles can work both against us and for us...


[/ QUOTE ]

There is a detail here that needs mentioning. Just because you are using a natural crotch, the bark is rough, and the rope is loaded doesn't mean there is any "friction." The frictional force, as in relates to the diagram, is only guaranteed to be present when the loaded rope is moving. Once everything stops moving, it is entirely possible that both the crotch and the pulley will be perfectly neutral so that both rope legs from the pulley have equal tension, and likewise for both rope legs from the crotch. In that case the 4X calculation applies.

However... when the load drops and stretches the rope, everything should be moving. When maximum load is reached at maximum stretch, the frictional force will also be at a maximum and would lower the maximum downward force at the crotch.
 
Moray, evidently, there is a dynamic in this situation for which I do not know the proper terminology. As you state, friction is created from movement, so what is the term of the force that is applied to the drum of the bollard/lowering device that enables the groundman to hold a 500 lb log with one hand, stationary without movement?
confused.gif


Thanks.

Dave
 
Dave, your terminology is fine. Your example is an instance of static friction, as I'm sure you know. Let me elaborate on my point using your example:
Your groundman is holding the load using 50 lb force. He could now pull a lot harder, say 500 lb, and still everything would be "stationary without movement." He could even pull with 4000 lb and still everything would be at rest. If we say, for the sake of the argument, that he is pulling in the same direction as the load, then the total load on the bollard would be 550 lb in the one case and 4500 lb in the other! In the tree diagram earlier in the thread, you don't know, when everything is at rest, if friction is helping to reduce the downward load or not. It could even be much worse than the 4X we would have if no friction were involved.

For example, in a simple crotch setup (no pulley), if 500 lb drops and causes a maximum dynamic load to the rope of 2000 lb, perhaps the anchor leg would see 1500 lb. When the load bounces and comes to rest, tension in the load leg will drop to 500 lb. But because of all the friction up at the crotch, the anchor leg may not give up all that 1500 lb. It may end up with 1000 lb. In effect, the static friction in the crotch allows the anchor leg to store energy in the form of tension, and the total rest load may end up being considerably greater than the theoretical 2X. It also could be smaller. The crotch friction prevents the two legs from equilibrating their tensions unless the difference in tensions is sufficiently great. Just like your bollard.

The one thing I think you can say for sure is friction at a crotch (or pulley) will reduce maximum downward dynamic load at the crotch.
 
[ QUOTE ]
[...] But because of all the friction up at the crotch, the anchor leg may not give up all that 1500 lb. It may end up with 1000 lb. In effect, the static friction in the crotch allows the anchor leg to store energy in the form of tension, and the total rest load may end up being considerably greater than the theoretical 2X. [...]

[/ QUOTE ]

Thanks for the nicely rendered explanation. That's pretty interesting about the residual load on the anchor leg.

It makes me wonder what happens when a second dynaic load occurs befor the anchor line is completely unloaded... like when hanging a tree from a crane tree and sectioning it out from the bottom.

When I'm doing that by myself the anchor line inevitably takes several dynamic loads before it's fully unloaded because nobody is there to let the tree drop to the ground before locking it off. Granted, each one is less than the one before but will that residual load actually add to the stress on the rope? Or does the friction at the crotch deal that out of the picture?
 
[ QUOTE ]
...Granted, each one is less than the one before but will that residual load actually add to the stress on the rope? Or does the friction at the crotch deal that out of the picture?

[/ QUOTE ]

A good question. There is a limit to how big the disparity can be between the two legs, depending on the friction element that separates them. Take 2 or 3 wraps around a limb and hang 100 lb from one leg. Say you can just hold it with 10 lbs on the other leg. The maximum disparity for this setup would be 10:1. It would mean, equally, that you can increase your 10 lb to 1000 lb before slippage would occur. It also means, anytime the disparity is less than 10:1, that the system is locked right there--the high-tension side has no way to unload some tension to the low-tension side.

When I did some measurements a few months ago with various pulleys and rings and sections of limbs, I found for small loads the maximum disparity for a polyester rope over a limb was about 2:1. There is your safety factor. Assuming something close to that 2:1 ratio applies to your rigging scenario, you could be sure, whenever everything is at rest, that the tension on one side of the crotch can never be more than twice the tension on the other side. More than that and the rope will slip, bringing the ratio down to 2:1. Sort of like a safety valve.
 
Another weigh to go is to hang a load from frictional redirect, and leverage the control leg, and let loaded leg pull the purchase thru. Then, we could trap more tension on the control leg side of the friction buffer IMLHO. i think it all comes down to observing which side is presently the initiating force.

Similarily, drop force could be releived on load side of frictional buffer, but trapped in control leg...
 
You must log in or register to reply here.

New threads New posts

Back
Top Bottom