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post #1 of (permalink) Old 10-06-2006, 09:09 PM Thread Starter
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Mechanical advantage?

I as many of you use a snatch block alot. I was noticing while winching a piece of farm equiptment onto the trailer today that on single line the entire system (ie mount, winch and cable) seemed o be at its limits. As soon as I doubled the line the pull became easier and nothing seemed strained at all. The mount I have for my winch has a bit of flex in it. I am going to fix that but it seemed that there was not near the flex in the mount after switching to double line.
So my question is if I am pulling with a 7500 lb winch at max capacity and I run it through a block that esentaly doubles it's ability to pull. So does that mean that my winch plate is now haviing 15000 lbs exerted on it or is it actually still only holding 7500?

I am redoing my winch mount because I don't like the way it strains the trailer. I want a straighter pull, but am looking for an explanation as to why everything seems to have less strain on it in double line pull.
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post #2 of (permalink) Old 10-06-2006, 09:30 PM
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Re: Mechanical advantage?

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post #3 of (permalink) Old 10-06-2006, 11:43 PM Thread Starter
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Re: Mechanical advantage?

Here is what confuses me now. In the photo below is basicly what I am doing with a single block. Why will my winch pull much easier by simply adding a block at the item being pulled? Acording to the author I have changed nothing, but the world tells me I have doubled my capacity.
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post #4 of (permalink) Old 10-06-2006, 11:51 PM Thread Starter
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Re: Mechanical advantage?

After looking at that photo I think what I have is actually diffrent. Here is my rendetion.
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post #5 of (permalink) Old 10-07-2006, 01:10 AM
 
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Re: Mechanical advantage?

Yes, you have effectively doubled your pulling ability. No your winch mount does not have 15000 poulds of stress on it. Here's why...

In your diagram, your winch is pulling on the cable with a force, say 7500 pounds. Effectively, the stationary object is also applying that force on the piece of farm equipent, resulting in a total force of 15000. How? well, your winch line has 7500 pounds of tension in it. Therefore, it has that much tension from one end of the line to the other, regardless that it crosses a pulley in the middle. So! There is effectively 7500 pounds on either side of the snatch block, resulting in a total pull of 15000. It that makes no sense, blame it on being 215am! Hope that helps! [img]/ubbthreads/images/graemlins/smile.gif[/img]
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post #6 of (permalink) Old 10-07-2006, 07:04 AM
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Re: Mechanical advantage?

Here is my modified pic version of the last answer.
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post #7 of (permalink) Old 10-07-2006, 07:38 AM
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Re: Mechanical advantage?

Looking at the diagram -- the winch mount "sees" 7500 lbs, the "stationary" sees 7500 lbs.
But - if the "stationary" point is also the winch mount, ie. bumper etc, then the mount "sees" both stresses - 7500+7500 = 15000.
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post #8 of (permalink) Old 10-07-2006, 07:52 AM
 
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Re: Mechanical advantage?

[ QUOTE ]
Here is what confuses me now. In the photo below is basicly what I am doing with a single block. Why will my winch pull much easier by simply adding a block at the item being pulled?

[/ QUOTE ]

Because half of the load is on the stationary object, the trade off is now the load loads at half the speed; Or in LEVE's diagram, 3/4 of the load is on the stationary object and the load moves at 1/4 speed (In reality you have to calculate drag from the pullys which gets multiplied also). Another thing to consider is, when you start doubling up you get closer to the bare drum where your winch pulls the hardest. In other words simply running out more cable gives you a big mech advantage.
An 8000lb winch may easily pull over 10,000 lbs. on a single line bare drum pull. The winch rating is NOT its maximun pull.

You have to always keep in mind, the increase in force at the load. Make sure your block(s) and rigging can handle it, as well as the load itself. You don't want to start pulling off pieces of the equipment or whatever you're trying to load.
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post #9 of (permalink) Old 10-07-2006, 10:30 AM
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Re: Mechanical advantage?

Here's a good calculator:

Snatch Block Rigging Calculator
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post #10 of (permalink) Old 10-07-2006, 10:47 AM
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Re: Mechanical advantage?

[ QUOTE ]
if the "stationary" point is also the winch mount, ie. bumper etc, then the mount "sees" both stresses - 7500+7500 = 15000.

[/ QUOTE ]

Pay very close attention to this point. Before the restoration I almost pulled the rear cross member off the Jeep while winching a tree out of the ground. I had several sheaves chained to the tree and several more chained to the Jeep's snow plow push frame. The Jeep was restrained by the drawbar chained to another tree. After a while I heard popping noises coming from in back of the Jeep and looked to find the drawbar pulled down and back, and the aft six inches or so of the frame rails twisted and almost pulled off.

Now any time I use a multi-line pull I make sure that nothing is attached to the Jeep except the winch. The return line and all of the sheaves on that end are attached to whatever I'm using as the anchor. That way there's no strain passing through the Jeep except for the direct pull the winch is putting out.
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