What size snatch block?

davecla · 11-03-2009, 01:45 AM

I've got a 14mm bull rope and want to purchase a snatch block for a redirect.

Heard somewhere that there is a relationship between the minimum size of the pully and the rope diameter. If the pully is too small the rope compresses (on the inside) and stretches (on the outside) too much. All sounds very complicated.

What do you guys think?

TreeWürx · 11-03-2009, 07:42 AM

You want your sheave to be at least 4x the size of your rope. Bigger is better. I use a 3/4" Block for all my rigging no matter the rope I put through it.

treeoperations · 11-04-2009, 08:27 PM

but then you dont want the groove in the pulley block to be to wide as that will cause the rope to flatten out as it goes around under load

squad143 · 11-05-2009, 06:04 PM

"If the pully is too small the rope compresses (on the inside) and stretches (on the outside) too much" - You would be speaking of sheave diameter and what happens to the rope as it goes around the sheave.

When using a block (or pulley), in relation to your rope diameter you need to consider two things. Diameter of the sheave and width of the sheave.

The tighter the bend, the greater stresses it puts on the rope. Treewurx is correct in stating that the sheave diameter should be a minimum of 4x your rope diameter. 1/2" (close to 14mm) rope should be run in a sheave that is at least 2" in dia. The larger the diameter, the less stress on the rope.

Ideally the width of your sheave should be matched to the size of your rope. IMO going 1mm either way should not make too much of a difference. - As long as your not stressing the rope out to it's max.

When using your block, ensure your sling size & running line match your block. Oversize rope/sling will flatten out under load and exert side pressure on the side plates of the block, possibly damaging the block, rope or both.

Here is a good site for tree rigging concepts. Although it does not go into block diameters, it is a good read.

http://www.treemettlenexus.com/article4.html

11-03-2009, 01:45 AM	#1
davecla Junior Member Join Date: Feb 2009 Location: Wellington, NZ Posts: 16	What size snatch block? I've got a 14mm bull rope and want to purchase a snatch block for a redirect. Heard somewhere that there is a relationship between the minimum size of the pully and the rope diameter. If the pully is too small the rope compresses (on the inside) and stretches (on the outside) too much. All sounds very complicated. What do you guys think?

11-03-2009, 07:42 AM	#2
TreeWürx Member Join Date: Oct 2009 Location: Wyoming farm country. Posts: 93	You want your sheave to be at least 4x the size of your rope. Bigger is better. I use a 3/4" Block for all my rigging no matter the rope I put through it. __________________ *Never underestimate the power of human stupidity.* If you are ever feeling down and depressed and want to feel better about yourself, just take a walk through Wal-Mart.

11-04-2009, 08:27 PM	#3
treeoperations Senior Member Join Date: Jun 2008 Location: living in paradise Posts: 208	but then you dont want the groove in the pulley block to be to wide as that will cause the rope to flatten out as it goes around under load __________________ nothing howls like a husky. 1x 338xpt, 1x 350, 1x346xp, 1x 365, 1x 385xp coming soon to my hands a 3120xp not enough climbing and rigging gear, always wanting more. bandit 65 aw, isuzu elf 250 chip truck, bobcat t190, drill gear, pallet forks and soil conditioner, bobcat 323, drill gear, grapple.

11-05-2009, 06:04 PM	#4
squad143 Senior Member Join Date: Jul 2007 Location: Toronto Canada area Posts: 455	"If the pully is too small the rope compresses (on the inside) and stretches (on the outside) too much" - You would be speaking of sheave diameter and what happens to the rope as it goes around the sheave. When using a block (or pulley), in relation to your rope diameter you need to consider two things. Diameter of the sheave and width of the sheave. The tighter the bend, the greater stresses it puts on the rope. Treewurx is correct in stating that the sheave diameter should be a minimum of 4x your rope diameter. 1/2" (close to 14mm) rope should be run in a sheave that is at least 2" in dia. The larger the diameter, the less stress on the rope. Ideally the width of your sheave should be matched to the size of your rope. IMO going 1mm either way should not make too much of a difference. - As long as your not stressing the rope out to it's max. When using your block, ensure your sling size & running line match your block. Oversize rope/sling will flatten out under load and exert side pressure on the side plates of the block, possibly damaging the block, rope or both. Here is a good site for tree rigging concepts. Although it does not go into block diameters, it is a good read. http://www.treemettlenexus.com/article4.html __________________ Bravery is the capacity to perform properly even when scared half to death. -Omar N. Bradley Last edited by squad143; 11-05-2009 at 06:08 PM.

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