what is the physics of using snatch ring/block?

[Zandcwhite]

If you are using the pulley to simply change the direction of force then there is no mechanical advantage.

No mechanical advantage if used like this

But if the recovered vehicle has the pulley mounted to it, and the vehicle with the winch has the winch line connected back to it, it is still a double line pull to the stuck vehicle. In this scenario you'll usually have to anchor the vehicle with the winch to another vehicle or tree as it will usually want to slide toward the stuck vehicle. Even in your example, you gain mechanical advantage the more oblique the angle of pull becomes. You could connect to a tree right next to the front of your jeep, around a snatch block at the stuck vehicle, and back to your winch and still see nearly double the pulling force at the stuck vehicle with only half of the pulling force on your Jeep.

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[txj2go]

If you are using the pulley to simply change the direction of force then there is no mechanical advantage.

No mechanical advantage if used like this

The direction of the cables becomes important. In that example picture if one rectangle is your vehicle and the other rectangle is an anchor such as another vehicle or a tree, then the pulley (blue dot) will see about 40% more pull force than the force in the line. Physics and trigonometry come into play here, or in engineering we call it "statics". It's all in how the pieces are rigged and the angles of the cables. OTOH if one rectangle is your vehicle and the other rectangle is the other vehicle, then the force on each vehicle is the same. This assumes that blue dot is connected to a tree or big rock in which case the tree or big rock sees higher combined force than the tension in the cable.

 

[Valpo Jeep]

The direction of the cables becomes important. In that example picture if one rectangle is your vehicle and the other rectangle is an anchor such as another vehicle or a tree, then the pulley (blue dot) will see about 40% more pull force than the force in the line. Physics and trigonometry come into play here, or in engineering we call it "statics". It's all in how the pieces are rigged and the angles of the cables. OTOH if one rectangle is your vehicle and the other rectangle is the other vehicle, then the force on each vehicle is the same. This assumes that blue dot is connected to a tree or big rock in which case the tree or big rock sees higher combined force than the tension in the cable.

Yup when you can rig to a tree to pull yourself out or attach the pulley to the stuck vehicle and anchor back to your rig or another fixed object then the pulley will double your recovery force.

 

[Austin23]

pulley diagrams - Search (bing.com)


Mechanical Advantage Pull x4 – Portable Winch US
Full Winch kit and Advanced 4x4 winch rigging. (4-wheeling-in-western-australia.com)

lots of good stuff. you can find some old army field manual pdf's also.

This video is essential viewing for anyone who owns a winch. Thanks for posting.

 

[Ratbert]

A single pulley just redirects the load. Changes direction.
unless
the rope is attached to your Jeep at both ends. At which point you get twice the pulling power.

look at the various diagrams on the web for details.

The rope doesn't have to be attached to your Jeep at both ends to get a mechanical advantage. E.g.:

 

[CaJLMetalHead]

Some more info about the physics of pulleys:

 

[J0E]

The rope doesn't have to be attached to your Jeep at both ends to get a mechanical advantage. E.g.:

Or much quicker to rig, you get the anchor for free. You don't need a tree or some other anchor, no tree saver. It takes the same time to attach to a 2nd rig as it does to the winch rig. We use this approach several times per month

In the following video, with both ends attached to one recovery rig, the recovery rig is dragged and stuck rig doesn't move.

 

[Vinman]

If you are using the pulley to simply change the direction of force then there is no mechanical advantage.

No mechanical advantage if used like this

I just winched out a Jeep last weekend using the above principle. No way to get my Jeep in position for a straight-line pull.

 

[TX_Ovrlnd]

Just go to a gym with cable machines and watch the little dudes believe they're lifting heavy, all courtesy of pulley systems like a snatch block.

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