3:1 DWT with RnW and OmniBlock Pulleys
JeffGu

3:1 DWT with RnW and OmniBlock Pulleys

The RnW would actually be close to the climber, and further away from the pulleys, but scale is an issue trying to draw things with Paint.
Because a MA setup doesn't add any load to the anchor... if it's picking up a 100-lb. log, the anchor still sees the 100 lbs., the MA just spreads the load across more sections of rope, it doesn't multiply the weight.

So, this setup just has two legs that are anchored, regardless of any MA within a leg. So the load on the anchors is the same as if the MA wasn't in there. If you got rid of the end pulley and just tied the end of the rigging line to that end of the branch, you'd have the same setup except it would be 2:1 instead of 3:1 mechanical advantage. For a given load, the anchor loading would still be the same as it was.

The loading on the anchors in either setup is the same as if you just had the mobile pulley on a rope with both ends tied to a limb. We are, of course, ignoring things like friction, rope angles, movement, etc. that will affect the loading... that's all variable stuff that we're not likely to care much about, because our safety factor will cover that. Also, the calculations will just make your brain hurt.
 
there is a shift in the way the force on the anchor limb is distributed in the 2:1. vs 3:1

with three legs as pictured half the force is on the tip of the anchor limb and half the force is
on the but.

with 3 legs, one third is on the tips and two thirds on the but
 
Is this correct? It seems like there would be 2/3 the load at the tip of the limb anchor, and 2/3 of the load at the redirect back to the climber, with the climber seeing a 3:1 mechanical advantage (disregarding friction and angles, of course).



Because a MA setup doesn't add any load to the anchor... if it's picking up a 100-lb. log, the anchor still sees the 100 lbs., the MA just spreads the load across more sections of rope, it doesn't multiply the weight.

So, this setup just has two legs that are anchored, regardless of any MA within a leg. So the load on the anchors is the same as if the MA wasn't in there. If you got rid of the end pulley and just tied the end of the rigging line to that end of the branch, you'd have the same setup except it would be 2:1 instead of 3:1 mechanical advantage. For a given load, the anchor loading would still be the same as it was.

The loading on the anchors in either setup is the same as if you just had the mobile pulley on a rope with both ends tied to a limb. We are, of course, ignoring things like friction, rope angles, movement, etc. that will affect the loading... that's all variable stuff that we're not likely to care much about, because our safety factor will cover that. Also, the calculations will just make your brain hurt.
 
Dookie balls. Unintended mistake on pick (should be "2/3x load" - two thirds - not "2x load")...

 
there is a shift in the way the force on the anchor limb is distributed in the 2:1. vs 3:1

with three legs as pictured half the force is on the tip of the anchor limb and half the force is
on the but.

with 3 legs, one third is on the tips and two thirds on the but
For some reason it won't let me edi that post and the typo.

The point is that with the third leg of the line tied off to the load, making for a 3:1 system instead of a 2:1 system, there is relatively more weight on the tip if the limb, rather than at the but.
 
Sorry, I've been distracted... @LordFarkwad yes, your drawing is correct... the two fixed pulleys are change of direction, but equalize their legs and have the 2x multiplier to the anchor, so 1T input is 2T on the anchor. The mobile pulley equalizes the 1T input to all four legs and has the 2x multiplier applied to the load, so 2T plus the 1T from the last leg in the system gives you the 3T at the load.

I can't fix a typo on media comments... grrr... that is so frustrating that the edit function is disabled here... but...

In the 2:1 config, where the end of the rigging line is fixed to the outer end of the limb and that fixed pulley is eliminated, the order is as such... the 1T input is equalized through the mobile pulley to 1T on all three legs, 2x multiplier on fixed pulley so anchor is 2T and end of rope anchored to limb is 1T and load sees 2T. So in the 2:1 config the outer anchor point (end of rigging line) is actually only seeing 1x the load and the fixed pulley anchor point is seeing 2x of the load.

Sorry about the delay. Ignore Daniel, he's been drinking, again.
 
In other words, you actually had it right... the fixed pulley anchor points see 2x the input force, or 2/3 of the load... depends on how you like to look at it.
 
Damnit... I'm really tired... again, we're going to assume the rope legs are all parallel and avoid discussing friction and rope angles.
 
For completeness, to illustrate your last response, @JeffGu...

Going by the same convention as your magnificent original drawing, "anchor A" is tip of limb and "anchor B" is butt of limb, at/towards the trunk.

 
Yes... and it looks a bit more refined with a vector drawing program than my .bmp doodling with a shaky, wireless mouse that's on its last leg.
 

Media information

Album
Rigging Setups
Added by
JeffGu
Date added
View count
5,916
Comment count
30
Rating
0.00 star(s) 0 ratings

Share this media

New threads New posts

Kask Stihl NORTHEASTERN Arborists Wesspur TreeStuff.com Kask Teufelberger Westminster X-Rigging Teufelberger Tracked Lifts Climbing Innovations
Top Bottom