[Seasparrow]

In trying to learn about snatch blocks, engineering generally describes a pulley as a device that can change direction of force but the travel of the rope remains 1:1 and the lifting force of the rope remains 1:1 with a single fixed pulley. The rope is attatched through the fixed pulley then to the object.

Some offroaders believe a single snatch block on a tree to a winch increases force, Engineers do not ( I think) . I admit that I may be missing something and that is what I am asking. That would mean the pulley would have to be directly attatched to the other vehicle right (movable pulley) in order to multiply force, not through a pulley on a tree ????

Is a snatch block and a tree a movable pulley or simply a fixed pulley ??
Powerful Pulleys - Lesson - TeachEngineering
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[patkelly4370]

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[Inv4drZm]

The picture on the left shows the proper use of a snatch block if you want to increase the force, 5kg force to move 10kg weight. The pulley in this situation is attached via rope to the stuck vehicle while the winch vehicle's rope goes around the pulley and anchors back to the bumper. Winch line speed is halfed, so therefore half the work is being done in the same amount of time = double the force. Or something like that.

A fixed pulley does nothing to increase force and just redirects the rope to allow you to pull from some place useful.

So both have their advantages.

[Seasparrow]

Thanks, that was what I was looking at and some videos / blogs showed a tree method a used the words "more" to describe the force.

[JuniorPigeon]

Originally Posted by Inv4drZm

The picture on the left shows the proper use of a snatch block if you want to increase the force, 5kg force to move 10kg weight. The pulley in this situation is attached via rope to the stuck vehicle while the winch vehicle's rope goes around the pulley and anchors back to the bumper. Winch line speed is halfed, so therefore half the work is being done in the same amount of time = double the force. Or something like that.

A fixed pulley does nothing to increase force and just redirects the rope to allow you to pull from some place useful.

So both have their advantages.

I think I understand your question OP, and if so then this would be correct. When the pulley is attached to the vehicle needing recovery - picture it as doubling the force with two ropes effectively. Minus the slight loss in friction that you will have in the pulley itself (which is probably negligible).

The fixed pulley for winching yourself is only going to redirect and very slightly increase force (or maybe just efficiency) by allowing the spool to run out further and decrease the drum radius. Same friction loss in the pulley applies. The location of the pulley in relation to the force/moving object is what changes things.

As an engineer and off-roader lol. No need to over-think it like I do, just make sure recoveries are safe and efficient haha, pulley or not.

[mistervague]

Imagine a 10 kg weight sitting on a playground swing. That is what you have on the left. Each side of the rope is holding 5 kg.

In the right-hand image, it is the same as if you were just lifting the weight straight off the ground. The only difference is the direction of force is changed 90 degrees by the pulley. So, 10 kg is needed to move the weight.

So on the left side, if you imagine the left rope is attached to an immovable object (tree), the blue box is the stuck vehicle, and the right rope is attached to a winch on another vehicle, you have cut in half the force you need to apply to the line to move the stuck vehicle. Note that when you pull, the pulley moves.

This also means that you have effectively doubled your winch’s capacity, all other things being equal.

On the right side, however, if the black line is an immovable object, the blue box is the stuck car and the left rope is attached to a winch, the winch has to pull 10 kg to move the weight – there is no advantage. Note the pulley does not move -- it is fixed.

[TommyG]

Snatch Blocks

https://www.youtube.com/watch?v=M2w3NZzPwOM