Snatch block recovery

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x factor

x factor

Joined
Aug 1, 2007
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Germantown, IL
After a bunch of research and coming up with conflicting accounts of when a snatch block will "double" the pulling power I decided to do some testing in the garage.
Using 2 small blocks, a 30# chunk of lead and a fish scale I was able to confirm that when a single snatch block is attached to a fixed object it only changes the direction of the pull. The amount of force to lift the weight by itself was the same as when I put a block on the "fixed" point (a chain on my rafter).
When I attached the block to the lead and made the chain on the rafter the dead end it then lessened the amount of force required to lift the weight to about half or 16#.
The final test was with 2 blocks and changing the dead end from the lead weight to the the chain. with the dead end on the chain the applied force dropped to around 10# or in effect tripled the pull power and with the dead end on the weight it took around 16# again or doubled.

In conclusion: with 1 snatch block and attaching it to a tree you DO NOT double the pull power but only change the direction of the pull. If you are attached to another tree or object. Does double if attached back to the rig in effect making it the moving block.

If you are pulling out a buddy with 1 snatch block attached to his rig and you doing the winching you will have doubled the pull power.


And now having gone through this and being proven wrong I found the easiest testing method to be a comealong with the hook on the end of the of the wire rope that can be attached or removed from the main body and one pulley for doubling. Attachment points can make a difference.
 
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your wording is misleading here. If you put the snatch block on the tree... run the cable to it from your winch, and back to your bumper/tow hook/some other attachment on your rig... it doubles the force, does not just change the direction of pull.

Easiest way to look at it is... how may lines are pulling on the load.


This is really rock simple stuff.


Mark...
 
you get almost 2 to one ,you have to take into account friction of the cable and the friction of the snatch block pin.with 2 snatch blocks the friction increaces and you never get quite 3 to one.this info is from the us army sub course number 86 vehicle recovery manual .good luck
 
congrats on doing the experiment to find out by yourself. Good for you! Best way to go.


But Demerits for having slept through High School (or is Junior High?) Physics! :D
 
I didn't have phyisics in high school.

Block and tackle rigging states that the block must be attached to the moving object to have any effect on reducing the force needed. This is what I tested and proved in my testing. This is also why I tried to give examples that were easier to understand in that self recovery would require 2 blocks to reduce the required force vs. 1 block if someone else was doing the pulling (in effect the dead end) and the block would be attached to the moving object who ever was stuck.
 
x factor said:
I didn't have phyisics in high school.

Block and tackle rigging states that the block must be attached to the moving object to have any effect on reducing the force needed. This is what I tested and proved in my testing. This is also why I tried to give examples that were easier to understand in that self recovery would require 2 blocks to reduce the required force vs. 1 block if someone else was doing the pulling (in effect the dead end) and the block would be attached to the moving object who ever was stuck.
Click to expand...

You're missing something. If you're stuck and have a winch, you can run your cable directly to a tree (1X) OR you can run it to the tree (thru a snatch block) and back to your vehicle, thus doubling the power (2X).
 
x factor said:
I didn't have phyisics in high school.

Block and tackle rigging states that the block must be attached to the moving object to have any effect on reducing the force needed. This is what I tested and proved in my testing. This is also why I tried to give examples that were easier to understand in that self recovery would require 2 blocks to reduce the required force vs. 1 block if someone else was doing the pulling (in effect the dead end) and the block would be attached to the moving object who ever was stuck.
Click to expand...

I thought that you had just stated your results poorly. It looks like you are misunderstanding. Or I am misunderstanding you.



In any event... this is fact and true:

If you run the cable of your winch to a pulley attached to a tree, and back to a hook on your rig. You will have to pull 4 feet of cable in to move 2 feet forward toward the tree. And (excepting losses from friction) you will have doubled the pulling force moving your rig forward.

If you rig is anchored solidly and instead of a tree, the pulley is attached to the front of another rig that you are extracting... it is the same. No difference

Double line pull doubles the available pulling force.






This is really about as simple a thing as you will come across. I can not understand how there can be debate or misunderstanding or any need to "test" to prove it.


Mark...
 
Everyone do as bigndn said and take a look at Warn's page 16. The reason you get help in pullling force with a snatch block is because you are using more cable. The lesser winds of cable on your winch the more force you will have. If you do this test with a pull scale you will not see the same results because you will not get the same mechanical advantages as cable on a spool.

bigndn See page 16.

http://www.warn.com/corporate/images...PN62885-A2.pdf
 
Nope.

That does help. can help a lot depending on the winch and the cable load on the drum


But that is not how the applied force is doubled

Come on people.... is it that hard? This is simple leverage. Look at how much cable has to be reeled in to move the vehicle "X" amount. A double line pull doubles your applied force and requires you to spool in twice as much cable. A triple line triples it and calls for 3 times the cable to be pulled in. It does not matter if the pulley is on the rig that is moving or the anchor that it not.

If you have two lines attached to the front of the rig... going straight to and from a pulley... and each line has 8000 pounds of force on it from your 8000 pound winch... that equals 16000 pounds of pull on the front of the rig.

Have you never rigged or used a winch?



Mark...
 
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Mark W said:
This is really about as simple a thing as you will come across. I can not understand how there can be debate or misunderstanding or any need to "test" to prove it.


Mark...
Click to expand...

x2. :D
 
Pulley - Wikipedia, the free encyclopedia

Under "Types of Systems" -

A cable going through a snatch block and back to your bumper will act as a moving pulley and give a mechanical advantage of 2, doubling the force.
 
I think this is where the misunderstanding is coming.

If I understand what this sentence states he is talking about using a tree as a dead man when winching another vehicle. In this case the snatch block only changes the direction of pull without any doubling effect. ie 1' of line in on the winch moves the other vehicle 1'.
x factor said:
.....

In conclusion: with 1 snatch block and attaching it to a tree you DO NOT double the pull power but only change the direction of the pull.

..
Click to expand...


And here the OP makes a statement that agrees with the other posters.
x factor said:
.....

If you are pulling out a buddy with 1 snatch block attached to his rig and you doing the winching you will have doubled the pull power.
Click to expand...
 
go find a 250 tonne crane with a 8 part line , it has been working on them for 100's of years .
 
If you are willing to try the experiment that I have done, you will
1) directly lift a weight with only a fish scale (mine was 30#s of lead)
2)attach a sigle pulley to ceiling, attach cable/string to the lead weight, run cable through pulley and attach to the fish scale, pulling down on the fish scale to lift the lead off the floor took 30# of force to lift the weight (NO DOUBLING EFFECT)
3)attach one end of the cable to the ceiling, run cable through the pulley that is now attached to the weight directly and the other end to the fish scale and lift, it now only took 16# of force to lift the weight (REDUCED THE FORCE REQUIRED BY HALF)

This was the start of the experiment and does explain the fixed verses moving block I spoke of.

As I stated before I have read conflicting reports on this subject and did the experiment in real world application with the affor mentioned results. This said I feel reading more contradictory statements without real world proof redundant.

Block and tackles have been used for a very long time and what I found from the research I did on those systems is what compelled me to do the testing. Say what you will but results are results.
 
That made it worse for me.:hhmm:

In your example, the equivalent regarding a vehicle extraction would be to fix a line at the ceiling, have a pulley attached to your load, and lift the free line end with your scale. The load as measured by the scale will be half because half the load is being carried with the fixed line.

In a vehicle extraction with a snatch block at the dead man position, you will need to wind in twice the length of line but your pulling force will be doubled.

This is not rocket science If I can understand it.


You can also use snatch blocks to change the direction of pull, but that's a different subject.

To muddy it further for the original poster--We recovered a truck last year using the truck to the rear of the truck being recovered forward. The winch line went out, past the stuck truck, then doubled back through a pulley, pulling the truck in front. Because the ends were attached to different objects, the pulley block only changed the direction but not the force of the pull.
 
Fish scale = winch
weight = vehicle being extracted
 
x factor said:
If you are willing to try the experiment that I have done, you will
1) directly lift a weight with only a fish scale (mine was 30#s of lead)
2)attach a sigle pulley to ceiling, attach cable/string to the lead weight, run cable through pulley and attach to the fish scale, pulling down on the fish scale to lift the lead off the floor took 30# of force to lift the weight (NO DOUBLING EFFECT)
3)attach one end of the cable to the ceiling, run cable through the pulley that is now attached to the weight directly and the other end to the fish scale and lift, it now only took 16# of force to lift the weight (REDUCED THE FORCE REQUIRED BY HALF)

This was the start of the experiment and does explain the fixed verses moving block I spoke of.

As I stated before I have read conflicting reports on this subject and did the experiment in real world application with the affor mentioned results. This said I feel reading more contradictory statements without real world proof redundant.

Block and tackles have been used for a very long time and what I found from the research I did on those systems is what compelled me to do the testing. Say what you will but results are results.
Click to expand...

Your explanation is exactly correct.

In your example (#2) with the pulley attached to the rafter, you are essentially lifting the entire weight, but pulling down on the rope vs pulling straight up on the weight. No mechanical advantage here. This is a straight pull, using the pulley to simply alter the direction of pull.

In #3, fixing the end of the rope to the rafter, down thru a pulley on the weight, back up to the rafter, effectively gives 2X mechanical advantage. This is a compound pull, doubling your pulling power.
 
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