Sheave block mechanical advantage?

[Industrial Toys]

Just a question that I can't get my head around.

For argument, let's say you are lifting a load. You have a block and tackle (hook) attached to the load.

One end of the line goes to your winch or vehicle frame.
The Other goes to your winch drum.

So in theory you have multiplied the pulling force of the winch many times.

But really you have the weight of the load divided by two on your frame and the other half on the drum.

So, how can it be easier to pull, when the Static weight is still there?

Clearly, I must be missing something.

 

[GLyford]

The pulley block attached to the load acts as a lever.

At any given moment, one side of rim is not moving (anchored line). For every inch the hub center moves (hook attached to load), the far side of the rim is moving two inches (winch drum side). And the real joy of the system is, every time it moves, you don't have to re-anchor the lever, it keeps being a "new" lever in the correct perfect position as it rolls.

Now if you really want to have fun, we can throw in parbuckling. This is where the line is anchored, goes out past the load (log) and returns on the other side back to the winch. The log itself becomes the pulley. Obviously, this works better with things like bucked logs or barrels on their side, and not so well with things like trees with tops still attached that will keep them from rolling.

 

[GLyford]

I should also note that the total weight on all of the lines is still the same, so if you have one end attached to the bumper and the other to a winch, the total load on the vehicle is the same. Half the load is on each point. This gets important if your tow vehicle is about to slide. You can improve this by putting your anchored line on a rock or tree (if there is one) so then only half the load is pulling on the tow vehicle.

 

[bigtiller]

Here is a pic of my 3 to 1 block & tackle. Notice one end of the rope is attached to the lower pulley with a single wheel. The top pulley has 2 wheels. I pull on the end of the rope you see beside the 2x4.

 

[/pine]

Some may remember this video from a ways back...35/1 advantage.:

Big Stump, Little Tractor - YouTube

 

[Industrial Toys]

So that is kind of my point.

The winch I just got is drum pull rated at 2000#

So, in some manner I rig up a block and tackle and lift a 6000 # load.

But, there is still only two attachments points to the drum and frame, which means there is 3000# on each.

So, how can the winch pull 3000 # which is the Static load?

 

[bcp]

If your arrangement has only those two lines anchored, it will only pull 4000 lbs, not 6000 lbs. (minus friction losses, etc)

If you are pulling 6000 lbs, you have another line you are not telling us about.

Bruce

 

[GLyford]

Also, there is a difference between what the winch pulls, and what the tractor pulls. If your anchored line (or one of the pulley sets in a multiple pulley arrangement) is also on the tractor, then it is seeing more of the pull than just what is on the winch.

 

[jaxs]

Doesn't this make you wonder what conversations between Ancient Egyptians must have sounded like as they discussed how the Great Pyramid would be constructed?

 

[flyerdan]

To get around the limiting factor of the winch (or vehicle) capacity, you'd have to anchor to a tree or some other sturdy object. Take Bigtiller's picture above, a 200 pound person could lift a 300 pound weight, because the line he's pulling (winch line) has only 100 pounds of force on it. That's how heavy recovery trucks snake big rigs up from an embankment they've went over.