Doubling effect at pulley block

Ekka · 22nd February 2007, 11:33 PM

Common scenario where a block is installed up the tree and you are lowering sections out.

The force at the block is double whatever the weight of the section is ... that's not taking into consideration friction or any dynamic loading if the section was falling.

Imagine you cut off a 100lb section. To stop it hitting the roof you need to apply force to the other end of the lowering rope ... if you apply 100lb then the section is stopped. So you have a 100lb on one side to stop it coming down and you have the weight of the section which is another 100lb. Total is 200lb on the pulley block.

Here's a quick sketch to illustrate.

Sean Freeman · 23rd February 2007, 12:40 AM

Very good diagrams Ekka, this is a crucial concept in basic physics that absolutely every one who has ever thought about using ropes in tree work has to understand....Yours and others lives often depend upon understanding the forces we put onto the rope systems we use.

SF

chris girard · 23rd February 2007, 08:42 AM

I would definitely agree. As tree workers we don't need to be engineers but we do need to know about the physics of engineering and how they apply to our work.

This is a subject that I find fascinating and always love learning more about. I am sure alot of you guys (and girls) feel the same way.

TreeCo · 23rd February 2007, 09:08 AM

Here's a way I rig quite a bit. I'm able to set ropes and pulleys from the ground with throw lines. The limb sees four times the load so you've got to watch it.

Ekka · 23rd February 2007, 12:12 PM

Graeme McMahon often natural crotch rigs taking his wraps at the top of the tree to eliminate the doubling effect.

Wrap from the top down or the outside of the limb to inside so the piece can come down not get caught in it's wraps.

a_lopa · 24th February 2007, 04:14 AM

Its alot easier on groundguys climbers who natural crotch IMO

Sean Freeman · 24th February 2007, 04:43 AM

Quote:

Originally Posted by a_lopa

Its alot easier on groundguys climbers who natural crotch IMO

Do you think because there's less hardware, or simply because the greater friction is such an aid in lowering the pieces?

We've a portawrap II but hardly ever use it.

SF

a_lopa · 24th February 2007, 04:46 AM

Less hardware/friction and actual time in getting the tree down,heres a small limb just ran thru a fork. http://www.youtube.com/watch?v=fAmSTCCtXBk

a_lopa · 24th February 2007, 04:53 AM

A couple of diagrams out of "handbook of rigging practices"

TreeCo · 24th February 2007, 05:47 AM

Quote:

Originally Posted by a_lopa

A couple of diagrams out of "handbook of rigging practices"

Looks like a good book. I've not read it.

Ekka · 24th February 2007, 12:12 PM

And natural crotch on many trees prevents you having to climb to the top and then back down to rig the low branches first ... you can rig on the way up.

Here's another diagram on sling config angles.

Notice when slings are at 120 degrees to the lifting hook each sling (or leg of sling) experiences the same weight as the load.

Notahacker · 24th February 2007, 03:31 PM

There is certainly a place for natural crotch lowering of branches.--No doubt.

However, what I am about to say has not been addressed, but I am sure you all know this.

Lowering large wood in a natural crotch is not always a good choice.

Remember: If you use a natural crotch as a rigging point there is added (unnecessary) friction in that crotch. So, this adds unnecessary force to that rigging point. Ultimately, this could result in a failure of the rigging system.

And, the frictin with natural crotching is inconsistent. A lowering device is much more predictable with the amount of friction needed to lower something (granded you know how to use it properly).

But, you all know that, right?

Notahacker · 24th February 2007, 03:34 PM

Like Dan illustrated, I like to do that same technique too. It is great to hit a throw shot from the ground to install a retractable rigging point. Fast way to set up and tear down.

Ekka · 24th February 2007, 05:24 PM

Quote:

Originally Posted by Notahacker

There is certainly a place for natural crotch lowering of branches.--No doubt.

However, what I am about to say has not been addressed, but I am sure you all know this.

Lowering large wood in a natural crotch is not always a good choice.

Remember: If you use a natural crotch as a rigging point there is added (unnecessary) friction in that crotch. So, this adds unnecessary force to that rigging point. Ultimately, this could result in a failure of the rigging system.

And, the frictin with natural crotching is inconsistent. A lowering device is much more predictable with the amount of friction needed to lower something (granded you know how to use it properly).

You better check them crotches and if it's a co-dom with included bark then you could bust it out.

I know some dudes were lowering out a large gum via natural crotch method and the fork they were using gave way ... no damage to property but the climber had to change his undies as both sections came smashing down and hung up.

jim1nz · 11th March 2007, 12:52 PM

I want to point some of the negatives with lowering over natural crotches.

- Unknown amount of friction
- Undue ware on ropes
- Chance of using a unsuitable rigging just because it is there
- Unable to tip tie/cradle brances because of low rigging point
- Less chance of being able to let branch "run" for a smooth rig

Angus · 18th March 2007, 05:40 AM

Like most technique choices in tree work, overlooking the possible benefits of one technique, takes a backward step in decision making.
This in turn reduces your flexibility as a climber, and doesn't leave room for thinking outside the square. There is a time and place for natural crotch lowering. IMHO
Angus

24th February 2007, 04:14 AM	#6 (permalink)
a_lopa Astronaut Join Date: Jan 2007 Location: Drouin Posts: 837	Its alot easier on groundguys climbers who natural crotch IMO __________________ Free Tree Industry Link Directory Drouin tree lopping, land clearing and stump removals

24th February 2007, 04:46 AM	#8 (permalink)
a_lopa Astronaut Join Date: Jan 2007 Location: Drouin Posts: 837	Less hardware/friction and actual time in getting the tree down,heres a small limb just ran thru a fork. http://www.youtube.com/watch?v=fAmSTCCtXBk __________________ Free Tree Industry Link Directory Drouin tree lopping, land clearing and stump removals

24th February 2007, 03:31 PM	#12 (permalink)
Notahacker Gettin' motoring Join Date: Feb 2007 Location: Longmont, CO Posts: 17	There is certainly a place for natural crotch lowering of branches.--No doubt. However, what I am about to say has not been addressed, but I am sure you all know this. Lowering large wood in a natural crotch is not always a good choice. Remember: If you use a natural crotch as a rigging point there is added (unnecessary) friction in that crotch. So, this adds unnecessary force to that rigging point. Ultimately, this could result in a failure of the rigging system. And, the frictin with natural crotching is inconsistent. A lowering device is much more predictable with the amount of friction needed to lower something (granded you know how to use it properly). But, you all know that, right? __________________ "Quality costs as much as it saves." My great uncle Simon Murphy

24th February 2007, 03:34 PM	#13 (permalink)
Notahacker Gettin' motoring Join Date: Feb 2007 Location: Longmont, CO Posts: 17	Like Dan illustrated, I like to do that same technique too. It is great to hit a throw shot from the ground to install a retractable rigging point. Fast way to set up and tear down. __________________ "Quality costs as much as it saves." My great uncle Simon Murphy

11th March 2007, 12:52 PM	#15 (permalink)
jim1nz Afterburner is shakin' Join Date: Jan 2007 Location: Tauranga, New Zealand Posts: 296	I want to point some of the negatives with lowering over natural crotches. - Unknown amount of friction - Undue ware on ropes - Chance of using a unsuitable rigging just because it is there - Unable to tip tie/cradle brances because of low rigging point - Less chance of being able to let branch "run" for a smooth rig __________________ SPEED ALWAYS FOLLOWS TECHNIQUE

23rd February 2007, 12:40 AM	#2 (permalink)
Sean Freeman PDF King & Arborist Extrodinaire Join Date: Feb 2007 Location: Townsville Nth Queensland & Gold Coast Sth Queensland Posts: 1,521	Very good diagrams Ekka, this is a crucial concept in basic physics that absolutely every one who has ever thought about using ropes in tree work has to understand....Yours and others lives often depend upon understanding the forces we put onto the rope systems we use. SF

23rd February 2007, 08:42 AM	#3 (permalink)
chris girard Break'n the ice Join Date: Feb 2007 Location: Gilmanton, N.H. USA Posts: 9	I would definitely agree. As tree workers we don't need to be engineers but we do need to know about the physics of engineering and how they apply to our work. This is a subject that I find fascinating and always love learning more about. I am sure alot of you guys (and girls) feel the same way.

18th March 2007, 05:40 AM	#16 (permalink)
Angus Gettin' motoring Join Date: Mar 2007 Location: Victoria, Australia Posts: 16	Like most technique choices in tree work, overlooking the possible benefits of one technique, takes a backward step in decision making. This in turn reduces your flexibility as a climber, and doesn't leave room for thinking outside the square. There is a time and place for natural crotch lowering. IMHO Angus Last edited by Angus : 18th March 2007 at 05:42 AM. Reason: IMHO needed to be added ;)