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I dont want to start WWIII, but can someone explain to me the practice of running a winch cable from your truck, through a snatch block (anchored to a tree or whatever) and back to the truck and proceeding to winch the truck out of trouble. As I see it there is no Mechanical Advantage (MA) to this practice, in actual fact you are increasing the load on the winch because of the friction created by the snatch block pulley on the shaft. Admittedly there is less friction in the snatch block than running the wire around a tree trunk, the principle is the same though. To halve the load on the winch you would need to have a MA of 2, to achieve this you require 2 snatch blocks. MA is defined, in this scenario, by the number of parts of rope (wire, etc.) supporting the MOVING BLOCK. In the first case the Block (snatch) is not moving, but anchored and you are pulling to disadvantage, in other words the wire is being wound back onto the winch, therefore it is moving in the opposite direction to the load (truck).
If you ran your wire from the winch to the anchored snatch block and back to your truck and then through a second snatch block attached to the truck and back up to the anchor point, you will have 2 parts of wire supporting the moving block and you would be pulling to advantage thereby halving the load on the winch (+ the friction in the 2 blocks). would be interested in hearing from anyone on this subject. Frank.
 

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Not quite.

The formula is F=1/n x W, Where F is the force (on the winch, in this case), n is the numer of cables, and W is the weight of the load.

If your vehicle weighs 4000 lbs, and you use a single pull (number of cables is 1), the force on the winch is 4000lbs (4000=1/1x4000)

If you use a pulley, attached to a fixed point, and terminate the free end back to the vehicle, you now have 2 cables, so the force now equals 2000, 2000=1/2x4000.

Taking the formula one step further, if you run the cable out to a pulley attached to a fixed point, back to a pulley attached to the vehicle, then terminated it at the fixed point where the pulley is attached, or any fixed point, the formula is now F=1/3 x W (1333.33= 1/3 x 4000)
 
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