Snatch Block Ratings - help?

[HarryO]

After my recent fun and games in the mud I have been reading up on the use of snatch blocks for winching but I can not find any details on how to rate the snatch block.

I have a 9000lbs winch and in theory if I use the double line shown below I should generate about 18000lbs of pull (with rope on first row of drum). There is some debate on the forums about whether it's double or less but for the sake of this question lets assume it doubles the pulling force.

To technical for my head but my question is then - in the double line configuration what do I rate the snatch block at 10000lbs (for the 9000lbs winch rating) or 20000lbs (for the 18000lbs pull) ? Even more for 3 line configuration at about 27000lbs:o

I would assume that the snatch blocks need to be rated for the 18000lbs but when I look at the 4wd suppliers most blocks are only rated 10000lbs.

http://www.arrivealive.co.za/images/4x4/24.jpg

[Blknight.aus]

correct, the snatch block is rated for the maximum pull it can handle.

Ie a 10000LB snatch block would be used in conjunction with a 5000LB winch and would need a shakcle and single line rated for 10000LB minimum. to attach it to something.

in your 3:1 scenario both blocks would need to be rated to 10000lb if the winch was pulling 5000lb.

What gets frustrating is when they change the specification units.

I have a winch rated for 9000lb pull and my snatch block is rated for 10000kg. so Im good to use it.

[PTC]

I am confused by your wording. Though my understanding of this from my experiences at work each day. Is that by adding in a pulley into the system you are halving the amount of weight you are moving. Making it a whole lot easier for us to haul gear between our teams.

Say if the car in your picture was 1000kg. Then the first car is putting 1000kg onto the tree. The second car going through pulley back to its self. The Pulley is holding the 1000kg. Though both sides of the system are only feeling the effect of 500kg on them. So the winch will work better. In the last photo of the 3 to 1 you are reducing the weight the winch is having to pull by even more. Though the actual weight on the pulley and strap is the same.

Using more pulleys it makes it a whole lot easier and less stress on the winch. Though it does use a lot more rope.

[Blknight.aus]

the question is all about how much each item is rated for.

if you use a 5000lb winch and you want to run maximum pull through a snatch block you need to use a 10000lb snatch block.

[HarryO]

Edited this post because I was 100% wrong as pointed out by Blknight above - Thanks for the reply's - everyone.

Thsi site helped explain it.

http://www.ropebook.com/information/pulley-systems

[Blknight.aus]

And you would be off the mark.

If you had a 9000lb winch and a 9000lb snatch block you could only use the winch to 4500lb of pull because at 2:1 you would be putting 9000lb of pull on the attachment point of the snatch block. (it's got two 4500lb lines hanging from it)

You might as well just run the winch on a single line pull.

Let's look at what would happen if you needed to run a 15000lb pull from your 9000lb winch. What's size snatch block would you need?

Via the modern day equivalent to smoke signals fromsome place other than the cave where my hat hangs.

[HarryO]

Blknight was right but I like to understand why - Finally found it in "for dummies" english and easy enough for me to understand. Remember my question was "What should my Snatch Block be rated at if I have a 9000lbs winch?"

Figure 1
When you take an 4x4 with a 9000 pound winch, the winch pulls on the cable, pulling towards the 4x4 towards the tree with a force of 9000 pounds. That's the action. The tree that supports the other end of the cable is consequently being pulled towards the 4x4 with a force of 9000 pounds. Remember that for every action (force), there is an equal but opposite reaction (force).

Figure 2
This time however, instead of attaching the cable to the tree, attach a snatch block to the tree and use it to redirect your cable back to the front of your 4x4. Now is where it really gets complicated, because stuff gets all turned around. When you pull with the winch, it applies 9000 pounds of force on your 4x4 towards the tree. The counter force has been redirected via the snatch block, and is now also applying 9000 pounds of force to your 4x4, and again, towards the tree. That's 18000 pounds of force towards the tree. Hence the "double your pull" statement.

The snatch block however, being the reaction member in this set-up, now has the vector sum of two identical forces being applied to it, or double the force. The same 18000 pounds that you are applying to the 4x4, you are also applying to the tree (i.e.your snatch block).



None of this is my own work it is merely the sum of all the sites that I plagiarized to find the info I needed in a “for dummies” explanation that I could understand. Most of it came from a post on www.highlifter.com (http://www.highlifter.com) It makes some sense to me now but that is no guarantee that it is right.

Which raises the question of the rating on the Dyneema rope when using snatch blocks... My head hurts an I am going home !!

[Blknight.aus]

If your winch is rated at 9000lb then your snatch block should be rated to 18000lb as should the shackle you use to connect the snatch block to whatever your hook up to.

And no, I'm not going to go into conducting the hook up and working out the math on the anchoring side of business, that can wait till after New year.

Via the modern day equivalent to smoke signals fromsome place other than the cave where my hat hangs.

[Hymie]

Snatch Blocks are rated at the Head Fitting, ie, where the shackle hooks onto the body of the Block.
The 9000Lb load is measured there, thus on a 2:1 there will be maximum of 4500 Lb on each rope fall in the Block.